Overview of different types of spectrometers

By Hendrik Mathyssen


 

Summary

Summary. 2

Revision history. 4

1.      Introduction. 5

2.      Overview tables. 6

3.      Emission. 7

3.1         Emission spectroscopy. 7

3.1.1          X-Ray. 7

3.1.2          UV.. 10

3.1.3          Visual wave. 13

3.1.4          LIBS (Laser Induced Breakdown Spectrometry). 16

3.2         Luminiscence. 18

3.2.1          Fluorescence. 19

3.2.2          Phosphorensence. 25

3.2.2.1      General 25

3.2.2.2      Theory. 26

4.      Absorption. 27

4.1         Spectrofotometry. 27

4.1.1          X-ray. 27

4.1.2          UV / Visual 30

4.1.3          IR. 32

4.2         Photo-acoustic spectroscopy. 34

4.3         Laser absorption spectrometry. 36

4.3.1.         General 36

4.3.2.         Direct laser absorption spectrometry. 37

4.3.3.         Noise reducing modulated techniques (TDLAS). 37

4.3.4.         Laser absorption spectrometry using fundamental vibrational or electronic transitions. 38

4.3.5.         Cavity enhanced absorption spectrometry. 38

5.      NMR (Nuclear magnetic resonance spectrometry). 41

5.1         General 41

5.2.       The proton NMR spectrometer. 43

5.3.       The carbon NMR spectrometer. 46

5.4.       Electron spin resonance. 46

6.      Raman spectroscopy. 49

7.      Diffraction. 52

7.1.       X-ray. 52

7.2.       Electron diffraction. 55

8.      Mass spectrometry. 57

8.1.       Mass spectrometry. 57

8.2.       Gas chromatography – mass spectrometry. 60

 


Revision history.

Revision              Date                                     Description

A                            30/11/2012                        Creation of document

B                             8/04/2013                           LIBS added.


 

 

1.     Introduction

Modern technology provides several types of material analyzing instruments. Every method used by an instrument has its own application.

Some methods provide information about atomic structure, other give more information on molecular level or bond characteristics.

It should be noted that the analysis of a spectrum can be extremely complicated. Instruments are delivered with specific software and databases to support the technician in his work. It is recommended to give the analysis of materials in hands of specialized laboratory unless analysis is a day to day activity.

Instruments are available in all kinds of sizes depending on the method and accuracy. Handheld spectrometers are also present on the market.

A spectrum can be represented in different forms. Wavelength is often used but other types exist also like mass to charge ratio and angles.

This overview makes a distinction between emission, absorption and special types of spectrometers. In most of these cases, the theory is coming from quantum mechanics where energy levels of electrons related to their shells and nuclear spinning are studied.

The following picture gives a summary of the different kinds of waves which are used by most spectrometers.

http://library.thinkquest.org/10380/media/advanced-em-spectrum.gif

Figure 1 Different types of waves


 

2.     Overview tables

Name

Type

Output

Unit

Solid (S)

Liquid (L)

Gas (G)

Instrument

Emission

x-ray

Atoms

keV

S, L, G

Medium, handheld

 

UV

Signature

nm, keV

S, L, G

Medium, handheld

 

Visual

Signature

nm, keV

S, L, G

Medium, handheld

 

LIBS

Signature

nm

S, L, G

Medium, handheld

Luminescence

Fluorescence

Signature

nm, keV

S, L, G

Medium, handheld

 

Phosphorescence

Signature

nm, keV

S, L, G

Medium

Absorption

x-ray

Atoms

eV

S, L, G

Medium, handheld

 

UV-visual

Signature

nm

L, G

Medium, handheld

 

IR

Signature, structure

cm-1

S, L, G

Medium

 

Photo-acoustic

Signature

cm-1

S

Medium

 

LAS, TDLAS

Signature

nm

G

Medium,

small

NMR

P-NMR

Molecule structure

Relative

S, L, G

Very big

 

Carbon NMR

Molecule structure

Relative

S, L, G

Medium

Electron spin resonance

ESR - EPR

Signature

Magnetic field

S, L, G

Very big

Raman

 

Signature, structure

cm-1

S, L, G

Medium

Diffraction

x-ray

Structure

2 theta

S

Medium, handheld

 

electron

Image signature

-

S

Medium

Mass Spectrometer

General and

Gas Chromatography

Molecules, Isotopes, Fractions of molecules

m/z

S, L, G

Medium

Table 2.1.    Types of spectrometers

 

 

 

 

 

 

 

 

Material

Spectrometer technology

Steel

Optical Emission Spectrometer (spark)

x-ray Emission Spectrometer

Aluminium

x-ray Emission Spectrometer

Visual-UV Emission Spectrometer (plasma by laser)

Plastic

Mass Spectrometer

FTIR Spectrometer (FT = Fourier Transformation)

UV , Raman, NMR

Crystals

X-ray Diffraction

Absorption and Emission Spectrometers (x-ray, UV-Visual, IR)

Minerals

Raman

IR Spectroscopy

UV-Visual Emission Spectroscopy

Gas

TDLAS, Visual/UV absorption, IR

GC-MS, Raman

Table 2.2.      Materials and their Spectrometers

3.     Emission

3.1Emission spectroscopy

3.1.1       X-Ray

When an electron from the inner shell of an atom is lost due to some sort of excitation, it is replaced with an electron from the outer shell; the difference in energy is emitted as an X-ray photon which has a wavelength that is characteristic for the element (there could be several of characteristic wavelengths per element). Analysis of the X-ray emission spectrum produces qualitative results about elemental composition of the specimen. Comparison of spectrum of the specimen with spectra of standards of known composition produces quantitative results (after some mathematical corrections for absorption, fluorescence and atomic number). X-rays can be excited by a high-energy beam of charged particles such as electrons (as in electron microscope) or protons (see PIXE), or a beam of X-rays (see X-ray fluorescence, or XRF). These methods enable elements from the entire periodic table to be analyzed, with the exception of H, He and Li.

 

Like any other emission spectroscopic method, X-ray spectroscopy consists

of three steps:

a) excitation to produce emission lines characteristic of the elements in the material,

b) measurement of their intensity, and

c) conversion of X-ray intensity to concentration by a calibration procedure which may include correction for matrix effects (presence of other components other than the analyte).

The wavelength region of interest makes the instrumentation and requirements differ considerably, however, from other emission spectroscopic techniques.

 

The characteristic X-ray photons used for analysis are those photons which arise from transitions between inner electron energy levels in atoms . To generate characteristic emission the atom must first be ionized in, say, the K, L, or M shell. Ionization may be accomplished by any photon or particle whose energy exceeds the binding energy of the electrons in the particular shell. After ionization, the X-ray line emission occurs when the electron vacancy is filled by an electron from one of the outer shells. The energy, E, of the characteristic photon is equal to the difference in the binding energies between the two electron levels involved in the transition. Lines are called K series lines if the initial ionization is in the K shell, L series lines if it is in the L shell and so on.

 

http://www.spring8.or.jp/en/news_publications/press_release/2009/091001_fig/fig1.png

Figure 3.1.1.1.   Principle of x-ray spectrometry

The graph shows the abundances of elements in the Martian rock 'Jake Matijevic' (black line) and a calibration target (red line) as detected by the Alpha Particle X-ray Spectrometer (APXS) instrument on NASA's Curiosity rover.

Figure 3.1.1.2.    Spectrum of a Martian rock

 

 

http://www.eaglabs.com/cmss_files/imagelibrary/applications/compostion/xrf2.jpg

Figure 3.1.1.3.    Alloy composition

 

http://pimg.tradeindia.com/00413902/b/2/Handheld-X-ray-Spectrometer.jpg

Figure 3.1.1.4.    Handheld x-ray spectrometer

 

 

3.1.2       UV

Photoemission spectroscopy (PES), also known as photoelectron spectroscopy,  refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in a substance. The term refers to various techniques, depending on whether the ionization energy is provided by an X-ray photon, an EUV photon, or an ultraviolet photon. Regardless of the incident photon beam however, all photoelectron spectroscopy revolves around the general theme of surface analysis by measuring the ejected electrons.

X-ray photoelectron spectroscopy (XPS) was developed by Kai Siegbahn starting in 1957  and is used to study the energy levels of atomic core electrons, primarily in solids. Siegbahn referred to the technique as Electron Spectroscopy for Chemical Analysis (ESCA), since the core levels have small chemical shifts depending on the chemical environment of the atom which is ionized, allowing chemical structure to be determined. Siegbahn was awarded the Nobel Prize in 1981 for this work. XPS is sometimes referred to as PESIS (photoelectron spectroscopy for inner shells) whereas the lower energy radiation of uv-light is referred to as PESOS (outer shells) because it cannot excite core electrons.

In the ultraviolet region, the method is usually referred to as photoelectron spectroscopy for the study of gases, and photoemission spectroscopy for solid surfaces.

Ultra-violet photoelectron spectroscopy (UPS) is used to study valence energy levels and chemical bonding; especially the bonding character of molecular orbital. The method was developed originally for gas-phase molecules in 1962 by David W. Turner,  and other early workers included David C.Frost, J.H.D. Eland and K. Kimura. Later, Richard Smalley modified the technique and used a UV laser to excite the sample, in order to measure the binding energy of electrons in gaseous molecular clusters.

Extreme ultraviolet photoelectron spectroscopy (EUPS) lies in between XPS and UPS. It is typically used to assess the valence band structure. Compared to XPS it gives better energy resolution, and compared to UPS the ejected electrons are faster, resulting in a better spectrum signal.

The physics behind the PES technique is an application of the photoelectric effect. The sample is exposed to a beam of UV or XUV light inducing photoelectric ionization. The energies of the emitted photoelectrons are characteristic of their original electronic states, and depend also on vibrational state and rotational level. For solids, photoelectrons can escape only from a depth on the order of nanometers, so that it is the surface layer which is analyzed.

Because of the high frequency of the light, and the substantial charge and energy of emitted electrons, photoemission is one of the most sensitive and accurate techniques for measuring the energies and shapes of electronic states and molecular and atomic orbital. Photoemission is also among the most sensitive methods of detecting substances in trace concentrations, provided the sample is compatible with ultra-high vacuum and the analyte can be distinguished from background.

Typical PES (UPS) instruments use helium gas sources of UV light, with photon energy up to 52 eV (corresponding to wavelength 23.7 nm). The photoelectrons that actually escaped into the vacuum are collected, energy resolved, slightly retarded and counted, which results in a spectrum of electron intensity as a function of the measured kinetic energy. Because binding energy values are more readily applied and understood, the kinetic energy values, which are source dependent, are converted into binding energy values, which are source independent. This is achieved by applying Einstein's relation

 E_k=h\nu-E_B.

The h\nu-term of this equation is due to the energy (frequency) of the UV light that bombards the sample. Photoemission spectra are also measured using synchrotron radiation sources (= emitted electromagnetic radiation when charged particles are accelerated radially).

The binding energies of the measured electrons are characteristic of the chemical structure and molecular bonding of the material. By adding a source monochromator and increasing the energy resolution of the electron analyzer, peaks appear with full width at half maximum (FWHM) less than 5–8 meV.

http://upload.wikimedia.org/wikipedia/commons/8/83/ARPESgeneral.png

Figure 3.1.2.1.   Principle of UPS

http://www.xps.org/images/uv_spectrum.gif

Figure 3.1.2.2.   Types of UV radiations

http://www.me.berkeley.edu/ltl/research/ti_emission_spectrum.jpg

Figure 3.1.2.3.    Example of a UV-spectrum of titanium

Figure 3.1.2.4.    UV-Visual Spectrophotometer

 

3.1.3       Visual wave

See also 3.2.

 

In physics, emission is the process by which a higher energy quantum mechanical state of a particle becomes converted to a lower one through the emission of a photon, resulting in the production of light. The frequency of light emitted is a function of the energy of the transition. Since energy must be conserved, the energy difference between the two states equals the energy carried off by the photon. The energy states of the transitions can lead to emissions over a very large range of frequencies. For example, visible light is emitted by the coupling of electronic states in atoms and molecules (then the phenomenon is called fluorescence or phosphorescence). On the other hand, nuclear shell transitions can emit high energy gamma rays, while nuclear spin transitions emit low energy radio waves.

The emittance of an object quantifies how much light is emitted by it. This may be related to other properties of the object through the Stefan–Boltzmann law. For most substances, the amount of emission varies with the temperature and the spectroscopic composition of the object, leading to the appearance of color temperature and emission lines. Precise measurements at many wavelengths allow the identification of a substance via emission spectroscopy.

Emission of radiation is typically described using semi-classical quantum mechanics: the particle's energy levels and spacings are determined from quantum mechanics, and light is treated as an oscillating electric field that can drive a transition if it is in resonance with the system's natural frequency. The quantum mechanics problem is treated using time-dependent perturbation theory and leads to the general result known as Fermi's golden rule. The description has been superseded by quantum electrodynamics, although the semi-classical version continues to be more useful in most cases.

When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbitals. When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a photon. The wavelength (or equivalently, frequency) of the photon is determined by the difference in energy between the two states. These emitted photons form the element's emission spectrum.

The fact that only certain colors appear in an element's atomic emission spectrum means that only certain frequencies of light are emitted. Each of these frequencies are related to energy by the formula:

 

E_{\text{photon}} = h\nu,

 

where E is the energy of the photon, ν is its frequency, and h is Planck's constant. This concludes that only photons having certain energies are emitted by the atom. The principle of the atomic emission spectrum explains the varied colors in neon signs, as well as chemical flame test results (described below).

The frequencies of light that an atom can emit are dependent on states the electrons can be in. When excited, an electron moves to a higher energy level/orbital. When the electron falls back to its ground level the light is emitted.

 

File:Emission spectrum-H labeled.svg

Figure 3.1.3.1.    Visual spectrum of hydrogen

The above picture shows the visible light emission spectrum for hydrogen. If only a single atom of hydrogen were present, then only a single wavelength would be observed at a given instant. Several of the possible emissions are observed because the sample contains many hydrogen atoms that are in different initial energy states and reach different final energy states. These different combinations lead to simultaneous emissions at different wavelengths.

 

File:Emission spectrum-Fe.svg

Figure 3.1.3.2.    Emission spectrum of iron

Flame emission spectrum:

The solution containing the relevant substance to be analysed is drawn into the burner and dispersed into the flame as a fine spray. The solvent evaporates first, leaving finely divided solid particles which move to the hottest region of the flame where gaseous atoms and ions are produced. Here electrons are excited as described above. It is common for a monochromator to be used to allow for easy detection.

On a simple level, flame emission spectroscopy can be observed using just a flame and samples of metal salts. This method of qualitative analysis is called a flame test. For example, sodium salts placed in the flame will glow yellow from sodium ions, while strontium (used in road flares) ions color it red. Copper wire will create a blue colored flame, however in the presence of chloride gives green (molecular contribution by CuCl).

 

http://ars.els-cdn.com/content/image/1-s2.0-S0920379605006903-gr1.jpg

Figure 3.1.3.3.     Principle Optical Emission Spectrometer

 

http://ars.els-cdn.com/content/image/1-s2.0-S0042207X12002400-gr2.jpg

Figure 3.1.3.4.  Example of an optical spectrum

 

http://cfnewsads.thomasnet.com/images/large/458/458321.jpg

Figure 3.1.3.5.    Optical emission spectrometer

http://www.matech.nl/products/nr_2_ScrapPro%20vrijst.jpg

Figure 3.1.3.6.    Handheld Optical Emission Spectrometer

 

3.1.4        LIBS (Laser Induced Breakdown Spectrometry)

Laser-induced breakdown spectroscopy (LIBS) is a type of atomic emission spectroscopy which uses a highly energetic laser pulse as the excitation source. The laser is focused to form a plasma, which atomizes and excites samples. In principle, LIBS can analyse any matter regardless of its physical state, be it solid, liquid or gas. Because all elements emit light of characteristic frequencies when excited to sufficiently high temperatures, LIBS can (in principle) detect all elements, limited only by the power of the laser as well as the sensitivity and wavelength range of the spectrograph & detector. In practice, detection limits are a function of a) the plasma excitation temperature, b) the light collection window, and c) the line strength of the viewed transition. LIBS makes use of optical emission spectrometry and is to this extent very similar to arc/spark emission spectroscopy.

LIBS operates by focusing the laser onto a small area at the surface of the specimen; when the laser is discharged it ablates a very small amount of material, in the range of nanograms to picograms, which generates a plasma plume with temperatures in excess of 100,000 K. During data collection, typically after local thermodynamic equilibrium is established, plasma temperatures range from 5,000–20,000 K. At the high temperatures during the early plasma, the ablated material dissociates (breaks down) into excited ionic and atomic species. During this time, the plasma emits a continuum of radiation which does not contain any useful information about the species present, but within a very small timeframe the plasma expands at supersonic velocities and cools. At this point the characteristic atomic emission lines of the elements can be observed. The delay between the emission of continuum radiation and characteristic radiation is in the order of 10 µs, this is why it is necessary to temporally gate the detector.

LIBS can often be referred to as its alternative name: laser-induced plasma spectroscopy (LIPS). The term LIPS has alternative meanings that are outside the field of analytical spectroscopy, therefore the term LIBS is preferred.

LIBS is technically very similar to a number of other laser-based analytical techniques, sharing much of the same hardware. These techniques are the vibrational spectroscopic technique of Raman spectroscopy, and the fluorescence spectroscopic technique of laser-induced fluorescence (LIF). In fact devices are now being manufactured which combine these techniques in a single instrument, allowing the atomicmolecular and structural characterisation of a specimen as well as giving a deeper insight into physical properties.

http://upload.wikimedia.org/wikipedia/en/c/c8/Libs.jpg

Figure 3.1.4.1.  Principle of LIBS

 

 

http://www.stellarnet-inc.com/images/plasma_2.jpg

Figure 3.1.4.2. A portable LIBS-system

3.2Luminiscence

Luminiscence spectroscopy points to 3 different types of spectrometry :

-          Fluorescence spectrometry (see 3.2.1)

-          Phosphorescence spectrometry (See 3.2.2)

-          Chemiluminiscence spectrometry

 

Some words about chemiluminiscence spectrometry:

Reaction produces a molecule in electronically excited state

 

Example:

 

 

Used to detect NO from 1 ppb to 10 ppt

Intensity depends on rate of reaction of production of C*

 

 

 

Fig. 3.2.1.    Typical chemiluminiscence

 

Advantages:

(i) Simple instrumentation (no excitation hn)

(ii) High sensitivity (ppm to <ppb)

Disadvantages:

(i) Only valid for a few reactions

 

3.2.1       Fluorescence

Fluorescence spectroscopy also called fluorometry or spectrofluorometry, is a type of electromagnetic spectroscopy which analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of certain compounds and causes them to emit light; typically, but not necessarily, visible light.

Molecules have various states referred to as energy levels. Fluorescence spectroscopy is primarily concerned with electronic and vibrational states. Generally, the species being examined has a ground electronic state (a low energy state) of interest, and an excited electronic state of higher energy. Within each of these electronic states are various vibrational states.

In fluorescence spectroscopy, the species is first excited, by absorbing a photon, from its ground electronic state to one of the various vibrational states in the excited electronic state. Collisions with other molecules cause the excited molecule to lose vibrational energy until it reaches the lowest vibrational state of the excited electronic state. This process is often visualized with a Jablonski diagram.

http://upload.wikimedia.org/wikipedia/commons/f/fd/FRET-Jablonski-diagram.jpg

Figure 3.2.1.1.     Jablonski diagram

The states are arranged vertically by energy and grouped horizontally by spin multiplicity. Nonradiative transitions are indicated by straight arrows and radiative transitions by squiggly arrows. The vibrational ground states of each electronic state are indicated with thick lines, the higher vibrational states with thinner lines.

The molecule then drops down to one of the various vibrational levels of the ground electronic state again, emitting a photon in the process. As molecules may drop down into any of several vibrational levels in the ground state, the emitted photons will have different energies, and thus frequencies. Therefore, by analysing the different frequencies of light emitted in fluorescent spectroscopy, along with their relative intensities, the structure of the different vibrational levels can be determined.

For atomic species, the process is similar, however since atomic species do not have vibrational energy levels, the emitted photons are often at the same wavelength as the incident radiation. This process of re-emitting the absorbed photon is "resonance fluorescence" and while it is characteristic of atomic fluorescence, it is seen in molecular fluorescence as well.

In a typical experiment, the different wavelengths of fluorescent light emitted by a sample are measured using a monochromator, holding the excitation light at a constant wavelength. This is called an emission spectrum. An excitation spectrum is the opposite, whereby the emission light is held at a constant wavelength, and the excitation light is scanned through many different wavelengths (via a monochromator). An emission map is measured by recording the emission spectra resulting from a range of excitation wavelengths and combining them all together. This is a three dimensional surface data set: emission intensity as a function of excitation and emission wavelengths, and is typically depicted as a contour map.

When a primary x-ray excitation source from an x-ray tube or a radioactive source strikes a sample, the x-ray can either be absorbed by the atom or scattered through the material. The process in which an x-ray is absorbed by the atom by transferring all of its energy to an innermost electron is called the "photoelectric effect." During this process, if the primary x-ray had sufficient energy, electrons are ejected from the inner shells, creating vacancies. These vacancies present an unstable condition for the atom. As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process give off a characteristic x-ray whose energy is the difference between the two binding energies of the corresponding shells. Because each element has a unique set of energy levels, each element produces x-rays at a unique set of energies, allowing one to non-destructively measure the elemental composition of a sample. The process of emissions of characteristic x-rays is called "X-ray Fluorescence," or XRF. Analysis using x-ray fluorescence is called "X-ray Fluorescence Spectroscopy." In most cases the innermost K and L shells are involved in XRF detection. A typical x-ray spectrum from an irradiated sample will display multiple peaks of different intensities.

http://www.amptek.com/xrf_1.gif

Figure 3.2.1.2.    An electron in the K shell is ejected from the atom by an external primary excitation x-ray, creating a vacancy.

Figure 3.2.1.3.    An electron from the L or M shell "jumps in" to fill the vacancy. In the process, it emits a characteristic x-ray unique to this element and in turn, produces a vacancy in the L or M shell.

Figure 3.2.1.4.   When a vacancy is created in the L shell by either the primary excitation x-ray or by the previous event, an electron from the M or N shell "jumps in" to occupy the vacancy. In this process, it emits a characteristic x-ray unique to this element and in turn, produces a vacancy in the M or N shell.


Figure 3.2.1.5.   The excitation energy from the inner atom is transferred to one of the outer electrons causing it to be ejected from the atom.

Figure 3.2.1.6.  Example of a x-ray fluorescence spectrum

The characteristic x-rays are labeled as K, L, M or N to denote the shells they originated from. Another designation alpha (a), beta (b) or gamma (g) is made to mark the x-rays that originated from the transitions of electrons from higher shells. Hence, a Ka x-ray is produced from a transition of an electron from the L to the K shell, and a Kb x-ray is produced from a transition of an electron from the M to a K shell, etc. Since within the shells there are multiple orbits of higher and lower binding energy electrons, a further designation is made as a1, a2 or b1, b2, etc. to denote transitions of electrons from these orbits into the same lower shell.

Fluorometer

A fluorometer or fluorimeter is a device used to measure parameters of fluorescence: its intensity and wavelength distribution of emission spectrum after excitation by a certain spectrum of light. These parameters are used to identify the presence and the amount of specific molecules in a medium. Modern fluorometers are capable of detecting fluorescent molecule concentrations as low as 1 part per trillion.

Fluorescence analysis can be orders of magnitude more sensitive than other techniques.

There are two basic types of fluorometers, the fluorometer and the spectrofluorometer. The difference between them is the way they select the wavelengths of incident light. A filter fluorometer will use filters while a spectrofluorometer will use grating monochromators. Filter fluorometers are often purchased/built at a lower cost but are less sensitive and less resolute than spectrofluorometers.

Typically fluorometers utilize a double beam. These two beams work in tandem to decrease the noise created from radiant power fluctuations. The upper beam is passed through a filter or monochromator and passes through the sample. The lower beam is passed through an attenuator and adjusted to try and match the fluorescent power given off from the sample. Light from the fluorescence of the sample and the lower, attenuated beam are detected by separate transducers and converted to an electrical signal that is interpreted by a computer system.

Within the machine the transducer that detects fluorescence created from the upper beam is located a distance away from the sample and at a 90-degree angle from the incident, upper beam. The machine is constructed like this to decrease the stray light from the upper beam that may strike the detector. The optimal angle is 90 degrees. There are two different approaches to handling the selection of incident light that gives way to different types fluorometers. If filters are used to select wavelengths of light, the machine is called a fluorometer. While a spectrofluorometer will typically use two monochromators. However, some spectrofluorometers may use one filter and one monochromator, they are still called a spectrofluorometer.

Sources for fluorometers are often dependant on the type of sample being tested. Among the most common source for fluorometers is the low-pressure mercury lamp. This provides many excitation wavelengths, making it the most versatile. However, this lamp is not a continuous source of radiation. The xenon arc lamp is used when a continuous source of radiation is needed. Both of these sources provide a suitable spectrum of ultraviolet light that induces chemiluminesence. These are just two of the many possible light sources.

Glass and silica cells are often the vessels in which the sample is placed. The scientist must be very careful to not leave fingerprints or any other sort of mark on the outside of the cell.

http://staff.aist.go.jp/takeshi-saito/facility_files/Fluorescence_spectrometer_F4500.jpg

Figure 3.2.1.7.   Fluorescence Spectrometer

http://image.made-in-china.com/2f0j00YBvauQsoJGpm/X-Ray-Fluorescence-Spectrometer.jpg 

Figure 3.2.1.8.    Handheld x-ray Fluorescence Spectrometer

 

3.2.2       Phosphorensence

3.2.2.1General

Phosphorescence has been observed from a wide variety of compounds and is differentiated from fluorescence by the long-lived emission of light after extinction of the excitation source. The first analytical uses of phosphorescence were published in 1957 by Kiers et al. However, the technique has still not been widely accepted apart from a few selected areas such as pharmaceutical analysis and forensic science. The reluctance to use phosphorescence probably arises from the practical aspect of measuring the signal since cryogenic temperatures, using liquid nitrogen at 77 K, are normally required. Developments in room temperature phosphorescence (RTP) have given rise to practical and fundamental advances which should help stimulate interest in phosphorimetry.

The sensitivity of phosphorescence is comparable  to that of fluorescence and complements the latter technique by offering a wider range of molecules for study. As well as offering selectivity through the use of excitation and emission wavelengths, phosphorescence adds another dimension – that of time. Background interference from fluorescence and Rayleigh and Raman scattering can be rejected on a time basis. Fluorescence to phosphorescence ratios can be used to aid the identification of particular compounds and the purity of organic compounds can be determined by measuring their decay times.

The introduction of a microprocessor-based phosphorimeter using a pulsed xenon source will provide the analyst with much greater freedom and ease of operation which should stimulate greater interest in the technique.

 

3.2.2.2Theory

In molecules where the singlet S1 and triplet T1 energy levels are closely spaced, the molecule can drop into the lower energy T1 state through a process known as intersystem crossing (10-8 sec). The molecule can then return to the ground state by emission of radiation and this T1 to S1 transition is called phosphorescence. The latter has a long lifetime which can vary from 10-6 to 102 sec depending upon the structure of the molecule. Phosphorescence emission spectra occur at longer wavelengths

than fluorescence emission spectra because of the slight loss in energy which

occurs in going from the singlet to triplet state. Because of the long lifetimes, the molecule has a very high probability of losing its excess energy by radiativeless routes such as internal conversion, bimolecular collision and photodecompositions.

As a result, phosphorescence is not routinely observed in solutions at room

temperature. Quenching of the triplet state by oxygen is also effective in preventing phosphorescence, and thorough degassing of the solution is required before measurement.

Several methods have been used to enable the observation of phosphorescence, in other words, to restrict collisional deactivation. One of the most common techniques is to supercool solutions to a rigid glass state, usually at the temperature of liquid nitrogen (77 K). At these low temperatures, molecular collisions are greatly reduced and strong phosphorescence signals are observed.

 

http://www.jasco.de/uploads/pictures/20120113170413_phosphorescence.jpg

Figure 3.2.2.1.   Difference between the fluorescence and phosphorescence spectrum.

 

http://english.nanoctr.cas.cn/rh/fs/200907/W020090706492845133198.jpg

 

Figure 3.2.2.2.    Fluorescence and phosphorescence spectrometer

 

4.     Absorption

4.1Spectrofotometry

4.1.1       X-ray

X-ray absorption spectroscopy (XAS) is a widely-used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation sources, which provide intense and tunable X-ray beams. Samples can be in the gas-phase, solution, or condensed matter (i.e. solids).

XAS data are obtained by tuning the photon energy using a crystalline monochromator to a range where core electrons can be excited (0.1-100 keV photon energy). The "name" of the edge depends upon the core electron which is excited: the principal quantum numbers n=1, 2, and 3, correspond to the K-, L-, and M-edges, respectively. For instance, excitation of a 1s electron occurs at the K-edge, while excitation of a 2s or 2p electron occurs at an L-edge (Figure 4.1.1.1.).

There are three main regions found on a spectrum generated by XAS data (Figure 4.1.1.1.). The dominant feature is called the "rising edge", and is sometimes referred to as XANES (X-ray Absorption Near-Edge Structure) or NEXAFS (Near-edge X-ray Absorption Fine Structure). The pre-edge region is at energies lower than the rising edge. The EXAFS (Extended X-ray Absorption Fine Structure) region is at energies above the rising edge, and corresponds to the scattering of the ejected photoelectron of neighboring atoms. The combination of XANES and EXAFS is referred to as XAFS.

Since XAS is a type of absorption spectroscopy, it follows the same quantum mechanical selection rules. The most intense features are due to electric-dipole allowed transitions (i.e. Δ l = ± 1) to unfilled orbitals. For example, the most intense features of a K-edge are due to 1s → np transitions, while the most intense features of the L3-edge are due to 2p → nd transitions.

File:XASEdges.svg

Figure 4.1.1.1.   Transitions that contribute to XAS edges

http://images.iop.org/objects/jio/labtalk/3/3/14/figure1.jpg

Figure 4.1.1.2.   XAS of Cupper and Iron.

 

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Figure 4.1.1.3.    x-ray absorption spectrometer

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Figure  4.1.1.4.     x-ray absorption spectrometer

 

 

 

4.1.2       UV / Visual

A material's absorption spectrum is the fraction of incident radiation absorbed by the material over a range of frequencies. The absorption spectrum is primarily determined  by the atomic and molecular composition of the material. Radiation is more likely to be absorbed at frequencies that match the energy difference between two quantum mechanical states of the molecules. The absorption that occurs due to a transition between two states is referred to as an absorption line and a spectrum is typically composed of many lines.

The frequencies where absorption lines occur, as well as their relative intensities, primarily depend on the electronic and molecular structure of the molecule. The frequencies will also depend on the interactions between molecules in the sample, the crystal structure in solids, and on several environmental factors (e.g., temperature, pressure, electromagnetic field). The lines will also have a width and shape that are primarily determined by the spectral density or the density of states of the system.

 

Theory

Absorption lines are typically classified by the nature of the quantum mechanical change induced in the molecule or atom. Rotational lines, for instance, occur when the rotational state of a molecule is changed. Rotational lines are typically found in the microwave spectral region. Vibrational lines correspond to changes in the vibrational state of the molecule and are typically found in the infrared region. Electronic lines correspond to a change in the electronic state of an atom or molecule and are typically found in the visible and ultraviolet region. X-ray absorptions are associated with the excitation of inner shell electrons in atoms. These changes can also be combined (e.g. rotation-vibration transitions), leading to new absorption lines at the combined energy of the two changes.

The energy associated with the quantum mechanical change primarily determines the frequency of the absorption line but the frequency can be shifted by several types of interactions. Electric and magnetic fields can cause a shift. Interactions with neighboring molecules can cause shifts. For instance, absorption lines of the gas phase molecule can shift significantly when that molecule is in a liquid or solid phase and interacting more strongly with neighboring molecules.

Absorption lines are often depicted as infinitesimally thin lines, i.e., delta functions, but observed lines always have a shape that is determined by the instrument used for the observation, the material absorbing the radiation and the physical environment of that material. It is common for lines to have the shape of a Gaussian or Lorentzian distribution. It is also common for a line to be characterized solely by its intensity and width instead of the entire shape being characterized.

The integrated intensity—obtained by integrating the area under the absorption line—is proportional to the amount of the absorbing substance present. The intensity is also related to the temperature of the substance and the quantum mechanical interaction between the radiation and the absorber. This interaction is quantified by the transition moment and depends on the particular lower state the transition starts from and the upper state it is connected to.

The width of absorption lines may be determined by the spectrometer used to record it. A spectrometer has an inherent limit on how narrow a line it can resolve and so the observed width may be at this limit. If the width is larger than the resolution limit, then it is primarily determined by the environment of the absorber. A liquid or solid absorber, in which neighboring molecules strongly interact with one another, tends to have broader absorption lines than a gas. Increasing the temperature or pressure of the absorbing material will also tend to increase the line width. It is also common for several neighboring transitions to be close enough to one another that their lines overlap and the resulting overall line is therefore broader yet.

 

http://faculty.sdmiramar.edu/fgarces/LabMatters/Instruments/UV_Vis/Cary50_Pic/conventional-spectrophotometer.png

Figure 4.1.2.1.   Principle of visual absorption spectrometry

 

http://image.made-in-china.com/2f0j00KCnTHyFJckcv/UV752-UV-VIS-Spectrophotometer.jpg

Figure  4.1.2.2.    UV-VIS Spectrophotometer

 

http://www.hitachi-hitec.com/global/science/uv_vis/img/uv_basic/uv_basic_5_1_e.gif

Figure 4.1.2.3.    Example of an absorption spectrum

 

 

4.1.3       IR

The term "infra red" covers the range of the electromagnetic spectrum between 0.78 and 1000 mm. In the context of infra red spectroscopy, wavelength is measured in "wavenumbers", which have the units cm-1.

wavenumber = 1 / wavelength in centimeters

It is useful to divide the infra red region into three sections; near, mid and far infra red;

Region

Wavelength range (mm)

Wavenumber range (cm-1)

Near

0.78 - 2.5

12800 - 4000

Middle

2.5 - 50

4000 - 200

Far

50 -1000

200 - 10

The most useful I.R. region lies between  4000 – 670 cm-1.

 

IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states.

For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radiation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

 

The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR is the same as the vibrational frequency of a bond, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This can be achieved by scanning the wavelength range using a monochromator. Alternatively, the whole wavelength range is measured at once using a Fourier transform instrument and then a transmittance or absorbance spectrum is generated using a dedicated procedure. Analysis of the position, shape and intensity of peaks in this spectrum reveals details about the molecular structure of the sample.

This technique works almost exclusively on samples with covalent bonds. Simple spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra. The technique has been used for the characterization of very complex mixtures. Spectra issues with infrared fluorescence are rare.

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/Images/irspect.gif

Figure 4.1.3.1.    Different bonds and their absorption spectrum

 

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Figure 4.1.3.2.    IR absorption spectrum of bees wax

 

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Figure 4.1.3.3.    Fourier transform IR Absorption Spectrometer (FT-IR)

 

 

4.2Photo-acoustic spectroscopy

Photo-acoustic spectroscopy (PAS) is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. The discovery of the photo-acoustic effect dates to 1880 when Alexander Graham Bell showed that thin discs emitted sound when exposed to a beam of sunlight that was rapidly interrupted with a rotating slotted disk. The absorbed energy from the light causes local heating and through thermal expansion a pressure wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solar spectrum (i.e., the infrared and the ultraviolet) can also produce sounds.

A photo-acoustic spectrum of a sample can be recorded by measuring the sound at different wavelengths. This spectrum can be used to identify the absorbing components of the sample. The photo-acoustic effect can be used to study solids, liquids and gases.

Photo-acoustic spectroscopy has become a powerful technique to study concentrations of gases at the part per billion or even part per trillion levels. Modern photo-acoustic detectors still rely on the same principles as Bell’s apparatus, however to increase the sensitivity the following modifications have been made:

1.    Use of intense lasers instead of the sun to illuminate the sample since the intensity of the generated sound is proportional to the light intensity; this technique is referred to as "laser photo-acoustic spectroscopy" or "LPAS"

2.    The ear has been replaced by sensitive microphones. The microphone signals are further amplified and detected using lock-in amplifiers.

3.    By enclosing the gaseous sample in a cylindrical chamber, the sound signal is amplified by tuning the modulation frequency to an acoustic resonance of the sample cell.

http://www.ndt.net/article/v09n06/ramachan/fig1.gif

Figure 4.2.1.   PAS principle

 

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Figure 4.2.2.   PAS of methane in the presence of water

 

http://cires.colorado.edu/~gouw/Instruments/LPAS/files/LPAS.jpg

Figure 4.2.3.   Laser-PAS

 

4.3Laser absorption spectrometry

4.3.1.     General

 

Laser absorption spectrometry (LAS) refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry (AS).

Optical spectroscopic techniques in general, and laser-based techniques in particular, have a great potential for detection and monitoring of constituents in gas phase. They combine a number of important properties, e.g. a high sensitivity and a high selectivity with non-intrusive and remote sensing capabilities. Laser absorption spectrometry has become the foremost used technique for quantitative assessments of atoms and molecules in gas phase. It is also a widely used technique for a variety of other applications, e.g. within the field of optical frequency metrology or in studies of light matter interactions. The most common technique is tunable diode laser absorption spectroscopy (TDLAS) which has become commercialized and is used for a variety of applications.

4.3.2.     Direct laser absorption spectrometry

The most appealing advantages of LAS is its ability to provide absolute quantitative assessments of species. Its biggest disadvantage is that it relies on a measurement of a small change in power from a high level; any noise introduced by the light source or the transmission through the optical system will deteriorate the sensitivity of the technique. Direct laser absorption spectrometric (DLAS) techniques are therefore often limited to detection of absorbance  around 10−3, which is far away from the theoretical shot noise level, which for a single pass DAS technique is in the 10−7 – 10−8 range. This detection limit is insufficient for many types of applications.

 

There are three ways to improve the situation:

1.       to reduce the noise;

2.       to address transitions with larger transitions strengths and

3.       to increase the interaction length.

The first can be achieved by the use of a modulation technique; the second can be obtained by using transitions in unconventional wavelength regions, whereas the third by using external cavities.

4.3.3.     Noise reducing modulated techniques (TDLAS)

Modulation techniques make use of the fact that technical noise usually decreases with increasing frequency (often referred to as a 1/f noise) and improves on the signal contrast by encoding and detecting the absorption signal at a high frequency, where the noise level is low. The most common modulation techniques, wavelength modulation spectroscopy (WMS)  and frequency modulation spectroscopy (FMS), achieve this by rapidly scanning the frequency of the light across the absorbing transition. The most frequently used laser-based technique for environmental investigations and process control applications is based upon diode lasers and WMS and often referred to as tunable diode laser absorption spectroscopy (TDLAS). The typical sensitivity of WMS and FMS techniques is in the 10−5 range.

However, since these laser are mostly developed for the telecom industry, they emit in the near infrared (NIR) region, primarily in the 700 nm – 2 μm range. With light in this wavelength region, mostly only weak overtone transitions of molecules can be addressed. This limits the sensitivity of conventional TDLAS to detection of species down to the mid or high ppm m range (part-per-million concentrations times meter interaction lengths). This is still insufficient for a large range of applications, wherefore other actions have to be taken.

Tunable Diode Laser Absorption Spectroscopy instruments rely on well-known spectroscopic principles and sensitive detection techniques, coupled with advanced diode lasers and optical fibers developed by the telecommunications industry. The principles are straightforward: Gas molecules absorb energy at specific wavelengths in the electromagnetic spectrum. At wavelengths slightly different than these absorption lines, there is essentially no absorption. By (1) transmitting a beam of light through a gas mixture sample containing a (usually trace) quantity of the target gas, and (2) tuning the beam's wavelength to one of the target gas's absorption lines, and (3) accurately measuring the absorption of that beam, one can deduce the concentration of target gas molecules integrated over the beam's path length. This measurement is usually expressed in units of ppm-m.

4.3.4.     Laser absorption spectrometry using fundamental vibrational or electronic transitions

The second way of improving the detection limit of LAS is to employ transitions with larger line strength, either in the fundamental vibrational band or electronic transitions. The former, which normally reside at ~5 μm, have linestrengths that are about 2 to 3 orders of magnitude higher than those of typical overtone transition. On the other hand, electronic transitions have often yet another 1 to 2 orders of magnitude larger line strengths. The transitions strengths for the electronic transitions of NO, which are located in the UV range (at ~227 nm) are  about 2 orders of magnitude larger than those in the MIR region! If transitions with such large linestrengths can be used efficiently, a significant increase in sensitivity would result.

The recent development of quantum cascade lasers (QC) lasers working in the MIR region has opened up new possibilities for sensitive detection of molecular species on their fundamental vibrational bands. It is more difficult (although not impossible) to generate stable cw light addressing electronic transitions, since these often lie in the UV region. (cw = A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency). Despite the fact that transitions with larger transition strength can be reached by either MIR or UV emitting lasers, the limits of detection (LODs) of AS techniques using these transitions have not yet been improved as much as has been anticipated. The reason is that these types of laser have a number of unique properties that limit their practical applicability. Their full potential can only be used whence these limitations have been circumvented. This is a rapidly developing but still only partly explored field of science that can help overcoming some of the present limitations of the AS technique.

4.3.5.     Cavity enhanced absorption spectrometry

The third way of improving the sensitivity of LAS is to extend the interaction length. This can be obtained by placing the species inside a cavity in which the light bounces back and forth many times, whereby the interaction length can be increased considerably. This has led to a group of techniques denoted as cavity enhanced AS (CEAS). The cavity can either be placed inside the laser, giving rise to intracavity AS, or outside, when it is referred to as an external cavity. Although the former technique can provide a high sensitivity, its practical applicability is limited because of all the non-linear processes involved.

External cavities can either be of multi-pass type, i.e. Herriott or White cells, or be of resonant type, most often working as a Fabry–Pérot (FP) etalon. Whereas the multi-pass cells typically can provide an enhanced interaction length of up to ~2 orders of magnitude, the resonant cavities can provide a much larger path length enhancement, in the order of the finesse of the cavity, F, which for a balanced cavity with high reflecting mirrors with reflectivities of  about 99.99 to 99.999% can be about 104 to 105. It should be clear that if all this increase in interaction length can be used efficiently, this vouches for a significant increase in sensitivity!

 

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Figure 4.3.1.     Herriott cell – adjust D to change the number of passes

 

 

File:Etalon-2.svg

Figure 4.3.2.    A Fabry–Pérot etalon. Light enters the etalon and undergoes multiple internal reflections.

 

 

A problem with resonant cavities is though that a high finesse cavity has very narrow cavity modes, often in the low kHz range (the width of the cavity modes is given by FSR/F, where FSR is the free-spectral range of the cavity, which is given by c/2L, where c is the speed of light and L is the cavity length). Since cw lasers often have free-running linewidths in the MHz range, and pulsed even larger, it is non-trivial to couple laser light effectively into a high finesse cavity. There are though a few ways this can be achieved.

-          Cavity ring-down spectroscopy (sensitivity 10−7)

-          Integrated cavity output spectroscopy (sensitivity 10−7)

-          Continuous wave cavity enhanced absorption spectrometry (sensitivity 10−8)

-          Noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (sensitivity 10−13)

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Figure 4.3.3.   TLAS

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Figure 4.3.4.   Water Vapor Absorption Spectrum

 

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Figure 4.3.5.   TLAS

5.     NMR (Nuclear magnetic resonance spectrometry)

5.1General

Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.

Most frequently, NMR spectroscopy is used by chemists and biochemists to investigate the properties of organic molecules, though it is applicable to any kind of sample that contains nuclei possessing spin. Suitable samples range from small compounds analyzed with 1-dimensional proton or carbon-13 NMR spectroscopy to large proteins or nucleic acids using 3 or 4-dimensional techniques. The impact of NMR spectroscopy on the sciences has been substantial because of the range of information and the diversity of samples, including solutions and solids.

 

 

 

Theory:

Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as nmr, has become the preeminent technique for determining the structure of organic compounds. Of all the spectroscopic methods, it is the only one for which a complete analysis and interpretation of the entire spectrum is normally expected. Although larger amounts of sample are needed than for mass spectroscopy, nmr is non-destructive, and with modern instruments good data may be obtained from samples weighing less than a milligram. To be successful in using nmr as an analytical tool, it is necessary to understand the physical principles on which the methods are based.

The nuclei of many elemental isotopes have a characteristic spin (I). Some nuclei have integral spins (e.g. I = 1, 2, 3 ....), some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ....), and a few have no spin, I = 0 (e.g. 12C, 16O, 32S, ....).

 

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Figure 5.1.1.    Spin and magnetic field

 

Isotopes of particular interest and use to organic chemists are 1H, 13C, 19F and 31P, all of which have I = 1/2. Since the analysis of this spin state is fairly straightforward, our discussion of nmr will be limited to these and other I = 1/2 nuclei.

 

The following features lead to the nmr phenomenon:

1.       A spinning charge generates a magnetic field.
The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.

2.        In the presence of an external magnetic field (B0), two spin states exist, +1/2 and -1/2.
The magnetic moment of the lower energy +1/2 state is aligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field.

3.       The difference in energy between the two spin states is dependent on the external magnetic field strength, and is always very small. The following diagram illustrates that the two spin states have the same energy when the external field is zero, but diverge as the field increases. At a field equal to Bx a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field).

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/enrgdia1.gif

Figure 5.1.2.   Difference in energy between 2 spin-states

Strong magnetic fields are necessary for nmr spectroscopy. The international unit for magnetic flux is the tesla (T). The earth's magnetic field is not constant, but is approximately 10-4 T at ground level. Modern nmr spectrometers use powerful magnets having fields of 1 to 20 T. Even with these high fields, the energy difference between the two spin states is less than 0.1 cal/mole. To put this in perspective, recall that infrared transitions involve 1 to 10 kcal/mole and electronic transitions are nearly 100 time greater.

(1 kcal/mole = 4,184 kJ/mole)
For nmr purposes, this small energy difference (ΔE) is usually given as a frequency in units of MHz (106 Hz), ranging from 20 to 900 Mz, depending on the magnetic field strength and the specific nucleus being studied. Irradiation of a sample with radio frequency (rf) energy corresponding exactly to the spin state separation of a specific set of nuclei will cause excitation of those nuclei in the +1/2 state to the higher -1/2 spin state. Note that this electromagnetic radiation falls in the
radio and television broadcast spectrum. Nmr spectroscopy is therefore the energetically mildest probe used to examine the structure of molecules.
The nucleus of a hydrogen atom (the proton) has a magnetic moment μ = 2.7927, and has been studied more than any other nucleus. The following  diagram show the energy differences for the proton spin states (as frequencies).

 

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/enrgdia2.gif

Figure 5.1.3. Proton spin energy differences

 

4.        For spin 1/2 nuclei the energy difference between the two spin states at a given magnetic field strength will be proportional to their magnetic moments. For the four common nuclei noted above, the magnetic moments are: 1H μ = 2.7927, 19F μ = 2.6273, 31P μ = 1.1305 & 13C μ = 0.7022. These moments are in nuclear magnetons (µN), which are 5.05078•10-27 JT-1. The following diagram gives the approximate frequencies that correspond to the spin state energy separations for each of these nuclei in an external magnetic field of 2.35 T. The formula in the colored box shows the direct correlation of frequency (energy difference) with magnetic moment (h = Planck's constant = 6.626069•10-34 Js).

 

Figure 5.1.4.   Frequency to spin state energy seperations

5.2.          The proton NMR spectrometer

A proton nmr spectrometer must be tuned to a specific nucleus, in this case the proton. The actual procedure for obtaining the spectrum varies, but the simplest is referred to as the continuous wave (CW) method. A typical CW-spectrometer is shown in the following diagram. A solution of the sample in a uniform 5 mm glass tube is oriented between the poles of a powerful magnet, and is spun to average any magnetic field variations, as well as tube imperfections. Radio frequency radiation of appropriate energy is broadcast into the sample from an antenna coil (colored red). A receiver coil surrounds the sample tube, and emission of absorbed rf energy is monitored by dedicated electronic devices and a computer. An nmr spectrum is acquired by varying or sweeping the magnetic field over a small range while observing the rf signal from the sample. An equally effective technique is to vary the frequency of the rf radiation while holding the external field constant.

 

Figure 5.2.1. NMR schematic

 

Since protons all have the same magnetic moment, we might expect all hydrogen atoms to give resonance signals at the same field / frequency values. Fortunately for chemistry applications, this is not true. The following diagram shows a number of representative proton signals over the same magnetic field range. It is not possible, of course, to examine isolated protons in the spectrometer described above; but from independent measurement and calculation it has been determined that a naked proton would resonate at a lower field strength than the nuclei of covalently bonded hydrogens. With the exception of water, chloroform and sulfuric acid, which are examined as liquids, all the other compounds are measured as gases.

 

 

Figure 5.2.2. Chemical shift

 

Chemical shift (due to different electron density = less or more shielded):

A spinning charge generates a magnetic field that results in a magnetic moment proportional to the spin. In the presence of an external magnetic field, two spin states exist (for a spin 1/2 nucleus): one spin up and one spin down, where one aligns with the magnetic field and the other opposes it. The difference in energy (ΔE) between the two spin states increases as the strength of the field increases, but this difference is usually very small, leading to the requirement for strong NMR magnets (1-20 T for modern NMR instruments). Irradiation of the sample with energy corresponding to the exact spin state separation of a specific set of nuclei will cause excitation of those set of nuclei in the lower energy state to the higher energy state.

For spin 1/2 nuclei, the energy difference between the two spin states at a given magnetic field strength are proportional to their magnetic moments. However, even if all protons have the same magnetic moments, they do not give resonant signals at the same field/frequency values. This is because this is dependent on the electrons surrounding the proton in covalent compounds. Upon application of an external magnetic field, these electrons move in response to the field and generate local magnetic fields that oppose the much stronger applied field. This local field thus "shields" the proton from the applied magnetic field, which must therefore be increased in order to achieve resonance (absorption of rf energy). Such increments are very small, usually in parts per million (ppm). The difference between 2.3487T and 2.3488T is therefore about 42ppm. However a frequency scale is commonly used to designate the NMR signals, even though the spectrometer may operate by sweeping the magnetic field, and thus the 42 ppm is 4200 Hz for a 100 MHz reference frequency (rf).

The chemical shift provides information about the structure of the molecule.

 

J-Coupling (due to possible spin combinations of adjacent hydrogens):

Some of the most useful information for structure determination in a one-dimensional NMR spectrum comes from J-coupling or scalar coupling (a special case of spin-spin coupling) between NMR active nuclei. This coupling arises from the interaction of different spin states through the chemical bonds of a molecule and results in the splitting of NMR signals. These splitting patterns can be complex or simple and, likewise, can be straightforwardly interpretable or deceptive. This coupling provides detailed insight into the connectivity of atoms in a molecule. The number of peaks is equal to the number of adjacent hydrogens + 1.

 

File:1H NMR Ethanol Coupling shown.GIF

Figure 5.2.3.   Example of a NMR: Ethanol

Coupling combined with the chemical shift (and the integration for protons) tells us not only about the chemical environment of the nuclei, but also the number of neighboring NMR active nuclei within the molecule. In more complex spectra with multiple peaks at similar chemical shifts or in spectra of nuclei other than hydrogen, coupling is often the only way to distinguish different nuclei.

 

5.3.          The carbon NMR spectrometer

The power and usefulness of 1H nmr spectroscopy as a tool for structural analysis should be evident from the past discussion. Unfortunately, when significant portions of a molecule lack C-H bonds, no information is forthcoming. Examples include polychlorinated compounds such as chlordane, polycarbonyl compounds such as croconic acid, and compounds incorporating triple bonds (structures below, orange colored carbons).

Figure 5.3.1. C-NMR examples

 

Even when numerous C-H groups are present, an unambiguous interpretation of a proton nmr spectrum may not be possible.

These difficulties would be largely resolved if the carbon atoms of a molecule could be probed by nmr in the same fashion as the hydrogen atoms. Since the major isotope of carbon (12C) has no spin, this option seems unrealistic. Fortunately, 1.1% of elemental carbon is the 13C isotope, which has a spin I = 1/2, so in principle it should be possible to conduct a carbon nmr experiment.

 

5.4.          Electron spin resonance

Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a technique for studying materials with unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of the spins of atomic nuclei. Because most stable molecules have all their electrons paired, the EPR technique is less widely used than NMR. However, this limitation also means that EPR offers great specificity, since ordinary chemical solvents and matrices do not give rise to EPR spectra.

 

http://upload.wikimedia.org/wikipedia/commons/a/a9/EPR_splitting.jpg

Figure 5.4.1.    Energy in function of the magnetic field

 

An unpaired electron can move between the two energy levels by either absorbing or emitting electromagnetic radiation of energy  \varepsilon = h \nu  such that the resonance condition,  \varepsilon = \Delta E , is obeyed. Substituting in  \varepsilon = h \nu  and  \Delta E = g_\mathrm{e} \mu_\mathrm{B} B_\mathrm{0}  leads to the fundamental equation of EPR spectroscopy:

 

 h \nu = g_\mathrm{e} \mu_\mathrm{B} B_\mathrm{0}

 

Experimentally, this equation permits a large combination of frequency and magnetic field values, but the great majority of EPR measurements are made with microwaves in the 9000–10000 MHz (9–10 GHz) region, with fields corresponding to about 3500 G (0.35 T). See below for other field-frequency combinations.

In principle, EPR spectra can be generated by either varying the photon frequency incident on a sample while holding the magnetic field constant or doing the reverse. In practice, it is usually the frequency that is kept fixed. A collection of paramagnetic centers, such as free radicals, is exposed to microwaves at a fixed frequency. By increasing an external magnetic field, the gap between the   m_\mathrm{s} = + \tfrac{1}{2}    and  m_\mathrm{s} = - \tfrac{1}{2}     energy states is widened until it matches the energy of the microwaves, as represented by the double-arrow in the diagram above. At this point the unpaired electrons can move between their two spin states. Since there typically are more electrons in the lower state, due to the Maxwell-Boltzmann distribution (see below), there is a net absorption of energy, and it is this absorption that is monitored and converted into a spectrum.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/imgmol/esr1.gif

Figure 5.4.2.  Electron spin resonance detection

http://img.directindustry.com/images_di/photo-g/electron-paramagnetic-resonance-spectrometer-epr-20754-2802025.jpg

Figure 5.4.3.    EPR Spectrometer

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Figure 5.4.4.   EPR of Cupper (II) compounds

6.       Raman spectroscopy

Raman spectroscopy ( named after Sir C. V. Raman) is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.  It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy yields similar, but complementary, information.

Typically, a sample is illuminated with a laser beam. Light from the illuminated spot is collected with a lens and sent through a monochromator. Wavelengths close to the laser line due to elastic Rayleigh scattering are filtered out while the rest of the collected light is dispersed onto a detector.

Spontaneous Raman scattering is typically very weak, and as a result the main difficulty of Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh scattered laser light. Historically, Raman spectrometers used holographic gratings and multiple dispersion stages to achieve a high degree of laser rejection. In the past, photomultipliers were the detectors of choice for dispersive Raman setups, which resulted in long acquisition times. However, modern instrumentation almost universally employs notch or edge filters for laser rejection and spectrographs (either axial transmissive (AT), Czerny-Turner (CT) monochromator, or FT (Fourier transform spectroscopy based), and CCD detectors.

There are a number of advanced types of Raman spectroscopy, including surface-enhanced Raman, resonance Raman, tip-enhanced Raman, polarised Raman, stimulated Raman (analogous to stimulated emission), transmission Raman, spatially offset Raman, and hyper Raman.

The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule. For the spontaneous Raman effect, which is a form of light scattering, a photon excites the molecule from the ground state to a virtual energy state. When the molecule relaxes it emits a photon and it returns to a different rotational or vibrational state. The difference in energy between the original state and this new state leads to a shift in the emitted photon's frequency away from the excitation wavelength. The Raman effect, which is a light scattering phenomenon, should not be confused with absorption (as with fluorescence) where the molecule is excited to a discrete (not virtual) energy level.

If the final vibrational state of the molecule is more energetic than the initial state, then the emitted photon will be shifted to a lower frequency in order for the total energy of the system to remain balanced. This shift in frequency is designated as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the emitted photon will be shifted to a higher frequency, and this is designated as an Anti-Stokes shift. Raman scattering is an example of inelastic scattering because of the energy transfer between the photons and the molecules during their interaction.

A change in the molecular polarization potential — or amount of deformation of the electron cloud — with respect to the vibrational coordinate is required for a molecule to exhibit a Raman effect. The amount of the polarizability change will determine the Raman scattering intensity. The pattern of shifted frequencies is determined by the rotational and vibrational states of the sample. This dependence on the polarizability differs from Infrared spectroscopy where the interaction between the molecule and light is determined by the dipole moment; this contrasting feature allows to analyze transitions that might not be IR active via Raman spectroscopy, as exemplified by the rule of mutual exclusion in centrosymmetric molecules.

File:Raman energy levels.svg

Figure 6.1.   Energy level diagram showing the states involved in Raman signal. The line thickness is roughly proportional to the signal strength from the different transitions.

http://newton.ex.ac.uk/research/biomedical-old/optics/images/Raman.jpg

Figure 6.2.   Measuring principle

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Figure 6.3.    Raman spectrum of Sulfur

 

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Figure 6.4.     Difference in Raman spectrum between Acetone and Ethanol

 

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Figure 6.5.    A Raman spectrometer

7.       Diffraction

7.1.          X-ray

When X-ray radiation passes through matter, the radiation interacts with the electrons in the atoms, resulting in scattering of the radiation. If the atoms are organized in planes (i.e., the matter is crystalline) and the distances between the atoms are of the same magnitude as the wavelength of the X-rays, constructive and destructive interference will occur. This results in diffraction where X-rays are emitted at characteristic angles based on the spaces between the atoms organized in crystalline structures called planes.

 

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Figure 7.1.1. Principle of x-ray diffraction

Bragg's Law can easily be derived by considering the conditions necessary to make the phases of the beams coincide when the incident angle equals the reflecting angle. The rays of the incident beam are always in phase and parallel up to the point at which the top beam strikes the top layer at atom z (Fig. 7.1.2). The second beam continues to the next layer where it is scattered by atom B. The second beam must travel the extra distance AB + BC if the two beams are to continue traveling adjacent and parallel. This extra distance must be an integral (n) multiple of the wavelength  for the phases of the two beams to be the same:   

 nl = AB +BC .

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Figure 7.1.2. X-ray diffraction calculation

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Figure 7.1.3.    x-ray diffractogram

 

Various X-ray diffraction instruments

Figure  7.1.4.    Equipment for x-ray diffraction

7.2.          Electron diffraction

Electron diffraction refers to the wave nature of electrons. However, from a technical or practical point of view, it may be regarded as a technique used to study matter by firing electrons at a sample and observing the resulting interference pattern. This phenomenon is commonly known as the wave-particle duality, which states that the behavior of a particle of matter (in this case the incident electron) can be described by a wave. For this reason, an electron can be regarded as a wave much like sound or water waves. This technique is similar to X-ray and neutron diffraction.

Electron diffraction is most frequently used in solid state physics and chemistry to study the crystal structure of solids. Experiments are usually performed in a transmission electron microscope (TEM), or a scanning electron microscope (SEM) as electron backscatter diffraction. In these instruments, electrons are accelerated by an electrostatic potential in order to gain the desired energy and determine their wavelength before they interact with the sample to be studied.

The periodic structure of a crystalline solid acts as a diffraction grating, scattering the electrons in a predictable manner. Working back from the observed diffraction pattern, it may be possible to deduce the structure of the crystal producing the diffraction pattern. However, the technique is limited by the phase problem.

Apart from the study of crystals i.e. electron crystallography, electron diffraction is also a useful technique to study the short range order of amorphous solids, and the geometry of gaseous molecules.

 

Limitations:

Electron diffraction in TEM is subject to several important limitations. First, the sample to be studied must be electron transparent, meaning the sample thickness must be of the order of 100 nm or less. Careful and time consuming sample preparation may therefore be needed. Furthermore, many samples are vulnerable to radiation damage caused by the incident electrons.

The study of magnetic materials is complicated by the fact that electrons are deflected in magnetic fields by the Lorentz force. Although this phenomenon may be exploited to study the magnetic domains of materials by Lorentz force microscopy, it may make crystal structure determination virtually impossible.

Furthermore, electron diffraction is often regarded as a qualitative technique suitable for symmetry determination, but too inaccurate for determination of lattice parameters and atomic positions. But there are also several examples where unknown crystal structures (both inorganic, organic and biological) have been solved by electron crystallography. Lattice parameters of high accuracy can in fact be obtained from electron diffraction, relative errors less than 0.1% have been demonstrated. However, the right experimental conditions may be difficult to obtain, and these procedures are often viewed as too time consuming and the data too difficult to interpret. X-ray or neutron diffraction are therefore often the preferred methods for determining lattice parameters and atomic positions.

However, the main limitation of electron diffraction in TEM remains the comparatively high level of user interaction needed. Whereas both the execution of powder X-ray (and neutron) diffraction experiments and the data analysis are highly automated and routinely performed, electron diffraction requires a much higher level of user input.

 

Ultrafast Electron Diffraction:

Ultrafast Electron Diffraction (UED) with atomic-scale, combined spatial and temporal resolution is a powerful tool for determination of structural dynamics. In order to spatially resolve a molecular structure, the wavelength required is sub-Ĺ, which can be easily obtained using accelerated electrons: λde Broglie = 0.067 Ĺ at 30 keV. The atoms in a specimen act as scattering centers for incident electrons, and each atom becomes a coherent source of an outgoing spherical wave. The electron diffraction experiment thus becomes a conceptual analog of the well-known multi-slit diffraction experiment, but at the molecular length scale. That is why electron diffraction is often used to elucidate the three-dimensional architecture of isolated molecules, amorphous materials, and (thin) crystals.

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Figure 7.2.1.

Because of the very large cross-section for scattering of electrons, as compared to that of X-ray light, it is possible to archive the ultrafast time resolution when studying molecular systems in the gas phase (a nontrivial task because of the lack of long-range order and low molecular density). The UED approach developed at Caltech has been successfully used to study chemical reactions, excited-sate structure dynamics, and nonequilibrium conformational changes on their native ultrafast time scales (see abstracts below). With UED, dark transient structures unobservable by spectroscopic methods can be determined and visualized. Ongoing UED research involves studies of complex chemical structures and biological chromophores. The construction of a fourth-generation instrument capable of investigating biomolecules in the gas phase is a significant part in this effort. The goal is to reveal, in the absence of a perturbing solvent, the intrinsic structural dynamics of the species, in the hope of identifying degrees of freedom critical to the understanding of their biological functionality.

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Figure 7.2.2.    Diffraction patterns : left = x-ray , right = electron

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Figure 7.2.3.    Electron diffraction instrument

8.       Mass spectrometry

8.1.          Mass spectrometry

Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio of charged particles. It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and other chemical compounds. MS works by ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios. In a typical MS procedure:

1.    A sample is loaded onto the mass spectrometer, and undergoes vaporization

2.    The components of the sample are ionized by one of a variety of methods (e.g., by impacting them with an electron beam), which results in the formation of charged particles (ions)

3.    The ions are separated according to their mass-to-charge ratio in an analyzer by electromagnetic fields

4.    The ions are detected, usually by a quantitative method

5.    The ion signal is processed into mass spectra

MS instruments consist of three modules:

·       An ion source, which can convert gas phase sample molecules into ions (or, in the case of electrospray ionization, move ions that exist in solution into the gas phase)

·       A mass analyzer, which sorts the ions by their masses by applying electromagnetic fields

·       A detector, which measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present

The technique has both qualitative and quantitative uses. These include identifying unknown compounds, determining the isotopic composition of elements in a molecule, and determining the structure of a compound by observing its fragmentation. Other uses include quantifying the amount of a compound in a sample or studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in a vacuum). MS is now in very common use in analytical laboratories that study physical, chemical, or biological properties of a great variety of compounds.

 

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Figure  8.1.1.   Principle of mass spectrometry

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Figure 8.1.2.    Mass spectrogram of Methane (CH4)

18

   < 0.5

   M+2

   Molecular ion

17

   1.1

   M+1

   Molecular ion

16

  100.0

   M

   Molecular ion; base peak

15

   85.0

   M - H

   Fragment

14

   9.2

   M - 2H

   Fragment

13

   3.0

   M - 3H

   Fragment

12

   1.0

   M - 4H

   Fragment

 

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Figure  8.1.3.     Mass spectrometer

8.2.          Gas chromatography – mass spectrometry

Gas chromatography–mass spectrometry (GC-MS) is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation, and identification of unknown samples. GC-MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification.

GC-MS has been widely heralded as a "gold standard" for forensic substance identification because it is used to perform a specific test. A specific test positively identifies the actual presence of a particular substance in a given sample. A non-specific test merely indicates that a substance falls into a category of substances. Although a non-specific test could statistically suggest the identity of the substance, this could lead to false positive identification.

 

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Figure 8.2.1.     GC-MS Schematic

 

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Figure 8.2.2.    a GC-MS