Chem Notes, Analytical Chem

7/30/2019 Chem Notes, Analytical Chem

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CFK/PJB Analytical Chemistry: Page 1

Part I Chemical Engineering Section 2 (ex-ET)

ANALYTICAL CHEMISTRY

8 lectures, Lent Term 2012

Prof. Clemens Kaminski

Course outline

1. What is Analytical Chemistry?2. General features of molecular spectroscopy3. Ultraviolet/visible spectroscopy4. Infrared spectroscopy5. Microwave spectroscopy6. Nuclear magnetic resonance spectroscopy7. Methods of elemental analysis8. Mass spectrometry9.

Chromatography

Text books

These lecture notes contain all you need to know about analytical chemistry for examination

purposes. You can find out more (if you want to) from almost any textbook with Physical

Chemistry or Analytical Chemistry in the title.

Examples paper: One examples paper will be issued to test understanding and aid exam

preparation.


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1 What is Analytical Chemistry?

Analytical Chemistry is concerned with answering the questions:

What chemical species are present in a sample? How much of each chemical species is present?Analytical Chemistry is vital in the following areas:

Quality control in the process industrieso of starting materialso of intermediateso of products

confirmation ofpurity identification of impurities

Environmental analysiso Monitoring and control of pollutants in streams that are to be released

to the environment (in gas, liquid or solid form)

o Measurement of pollutants in the environment (air/river/ground) NOx, SOx, hydrocarbons in atmosphere Organic chemicals (polychlorinated biphenyls, detergents) Toxic heavy metals (lead, cadmium, mercury)

Clinical and biological studieso Measurement of nutrients, including trace metalso Measurement of naturally produced chemicals (cholesterol, sugar, urea)o Measurement of drug levels in body

Geological assayso Measurement of metal concentrations in ores and mineralso Measurement of oil/gas concentrations in rocks

Fundamental and applied research

o Chemical engineering: how much conversion (or separation) do we obtain underthese conditions?

o Organic molecule synthesis: what compound have we made?Analytical Chemistry is thus vital in the process industries and in research laboratories.

Qualitative analysis is the identification of elements, functional groups, orparticular compounds in a sample.

Quantitative analysis is the determination of the amount of a particularelement, species or compound in a sample.


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Analytical Chemists need to be good at careful accurate measurements, statistics and error

analysis:

samples of known concentration must often be prepared for calibration purposes samples must not become contaminated for environmental analysis, more than one measurement is often performed on more than

one sample to draw conclusions.

On a process plant, analytical chemistry is normally performed off-line:

a sample of product is removed and sent to the lab for testing might take hours or days for plant control purposes, we may need to infer composition indirectly

o e.g. from TandPmeasurements and a model of how conversion (or separation)varies with TandP

numerous off-line analytical techniques.However, an increasing number of analytical techniques can now be performed on-line:

sample the process stream in situ the plant can then be controlled using the direct composition measurement fewer techniques, and most will only work for certain reactions/products.Classical (old-fashioned) analytical chemistry is based on techniques such as:

Titration: volume of a standard reagent reacting with the sample is measuredo Acid-base titrations: e.g. monitor the colour of a solution containing a

pH-sensitive indicator as an acid (or base) is added.

o Complexation titrations: e.g. monitor the pH of a solution whilst reagent EDTA,ethylenediaminetetraacetic acid (HOOCCH2)2NCH2CH2N(CH2COOH)2, is added:

EDTA reacts in a 1:1 molar ratio with almost all metal cations (exceptalkali metals), enabling the metal cation concentration to be determined.

Gravimetry: measurements based on mass. Simple examples are:o

Mass lost on heating of a solid gives the amount of water of crystallisation.o Mass of precipitate formed during a reaction can be measured.

For instance, adding excess silver nitrate solution to determine theconcentration of chloride ions present.

Electrochemical methods:o pH measurement.o Ion-selective electrodes.

Modern analytical chemistry is largely based on instrumental techniques. In this lecture

course, we shall discuss:

Molecular spectroscopy techniques: first part of course


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Other analytical techniques: last two lectures


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2 General features of molecular spectroscopy

Quantum mechanics tells us that all energy levels are quantised. We can probe the separation between energy levels by spectroscopy:

o Test of quantum mechanics and our theories of bondingo Provides structural information

2.1 Absorption and Emission

The ground state of a molecule is the one of lowest energy. An excited state of a molecule is one of higher energy. Excitation refers to the process in which the molecule goes from a low to high energy

state: it requires the addition of energy by photon absorption.

Relaxation is the process by which a molecule falls from a high to low energy state: itinvolves the removal of energy by photon emission.

Whether the transition is permitted or not depends on:o The frequency of the photon: we require E= h o Selection rules: we require that the electromagnetic radiation interact

with the molecule, and that angular momentum is conserved as well as energy.

[Aside: photons have an intrinsic angular momentum] For example: an electron jumping between atomic orbitals has to obey:

n = anything ; l= 1 ; ml = 0, 1

This means an electron in the 1s orbital of a hydrogen atom could move to2p, 3p, or 4p orbitals by absorption of light of appropriate frequency, but

not to 2s, 3s, 4s, or 3d, 4d atomic orbitals.


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2.2 Schematic diagram of an absorption spectrometer

The monochromatorcauses light of just a single frequency of light to bedetected; it may be before or after the sample cell.

The monochromator is adjusted (e.g. by rotation of the prism) so that the frequency oflight reaching the detected is scanned.

This simple diagram is sufficient for this course. However, in practice better methodshave been developed than this basic set-up:

o Tunable diode lasers may be used: in this case, the light source is monochromaticbut its frequency can be tuned.

o Fourier transform infrared (FTIR) spectrometers use an interferometer technique.o NMR spectrometers use a very short pulse of radiation containing a distribution of

frequencies.

2.3 Factors affecting intensities of spectral lines

Transition Probabilityo This depends on the precise quantum mechanical wavefunctions of the initial and

final states (beyond the level of this course).

Some transitions may have zero probability in that case, they are said toviolate selection rules.

The Population of Stateso The initial population of an energy level obviously affects spectral intensities.o At thermal equilibrium, the relative populations of two energy levels may be

obtained from the Boltzmann factor:

kis Boltzmanns constant, 1.38066 x 1023 J/K

upper

Carrie

E


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When E>> kT, then only the lower level has significant population When E


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2.5 Regions of the electromagnetic spectrum

Depending on the wavelength used a variety of different structural information

may be obtained.


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3 Ultraviolet/visible (UV/vis) spectroscopy

Absorption in the UV/visible region is associated with transitions between electronicenergy levels.

o Colours of compounds/solutions arise in this way. The transition of interest is normally that between the highest occupied

molecular orbital (HOMO) and the lowest unoccupied molecular orbital

(LUMO).

o Other transitions involve greater energy separations, and so are further away fromthe visible region; band overlap for transitions at higher energies tends to result in

uninformative spectra.

o Wavelengths below ~200 nm cannot easily be studied for instrumental reasons -the sample cell window absorbs radiation at these wavelengths.

The UV spectrum is normally plotted as the absorbance against wavelength; peakpositions are identified by quotingmax and .

Typical UV spectrum:

molecular orbitals

HOMO

LUMO

Efor transition of interest

Energy


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(fromhttp://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm#uv3)

Aside: due to simultaneous vibrational and rotational transitions (see Sections 3 and 4),UV spectra normally consist of fairly broad peaks.

The absorbing groups in a molecule are called chromophores. Two isolatedchromophores in a molecule give roughly independent absorptions for each one:

o e.g. CH3CH2CNS: max= 245 nm and = 800SNCCH2CH2CH2CNS: max= 247 nm and = 2000

In organic chemistry, -conjugated systems (when multiple bonds are separatedby a single bond) tend to give particularly informative spectra.

o Overlap of adjacent orbitals results in a decrease in the energy gap between theoccupied orbital and the unoccupied * antibonding orbital.

o This results in an increase in absorption wavelength (even into the visible regionfor greatly conjugated systems e.g. organic dyes), and normally an

increase in the intensity as well.

General rule: increased conjugation increases max and .o Aromatic systems exhibit conjugation, but tend to give complex spectra,

frequently with more than one absorption band.

Conjugation with lone pairs (n- conjugation) can also result in spectral transitions,though these are much weaker than those originating from overlap of orbitals.


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Example UV spectrum results:


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3.1 Uses of UV/vis spectroscopy

Sample is usually liquid. Can follow changes in colour/composition quite rapidly (timescale may be down to ~1 s). Can measure concentration of any coloured compound, or any compound that absorbs in

the UV region.

Beer-Lambert law is useful, though for accurate work absorbance will bemeasured on solutions of known concentration for calibration purposes.

Reasonably straightforward to do the measurement on-line:o Can study the process fluid through a glass window, or using a fibre optic cable.o In practice, a technique called attenuated total reflectance is likely to be used if the

sample absorbs strongly.

This technique is limited:o It doesnt work if the sample doesnt absorb in the UV/vis region!o Its not good if the sample contains several species that absorb in the UV/vis

region: the absorption bands are broad and so overlap too much.


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4 Infrared spectroscopy

Absorption in the infrared region is associated with transitions between vibrational energylevels.

4.1 Diatomic molecules: ideal case

Let us consider a diatomic molecule first (such as HCl) as it contains just a single bond. Imagine the bond behaves like a perfect spring with a force constant k

This is often called the simple harmonic oscillator (SHO)approximation.

We can solve the Schrdinger equation exactly in this case to derive the energy levels ofthe molecule

The vibrational energy levels are characterised by a quantum number v.o E= h 0 (v + ) v = 0, 1, 2, o [Note that this corresponds to the vibration frequency of the

spring]

is the reduced mass, given by

At room temperature,E >> kT, implying that only the v = 0 quantum level issignificantly populated.

m1 m2

spring constant = k

vibrational energy levelsEnergy

v = 0

v = 1

v = 2

v = 3

v = 4

E

E= 1/2h 0

E= 3/2h 0

E= 5/2h 0

E= 7/2h 0

E= 9/2h 0


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The first selection rule is thatv = 1o This means that for a diatomic molecule we expect to see a single absorption peak

corresponding to E= h 0

o The peak tells us 0, the bond vibration frequency, from which we get informationabout kand/or.

The second selection rule is that the bond has to have a permanent dipolemoment:

o Variation of the molecules dipole moment upon vibration is needed to interactwith the oscillating electric vector of the electromagnetic radiation.

o This means that diatomic molecules such as O2 and N2 wont show any absorptionin the infrared region (and so arent greenhouse gases); a molecule such as HCl

will show absorption in the infrared region.

4.2 Diatomic molecules: real case

Real bonds do not behave as ideal springs; they behave as anharmonic oscillators. Potential energy diagram:

The simple harmonic oscillator SHO model discussed earlier provides a goodapproximation to the real case at the lowest energy level when ris always close to

requilibrium.


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For real molecules, the bond will break at high energies before v .o In practice this means that the vibrational energy levels get closer together as

quantum number v goes up.

Selection rules for real diatomic molecules:o v = 1 (as for SHO), but v = 2, 3, are now weakly allowed as well

(transition probability is small but is now greater than zero).

o Molecule still needs to have a dipole moment for interaction with photon to occur. Its still the case that only the v = 0 level is significantly populated at room temperature. Spectrum will thus show an absorption band at 0, plus a weak band at ~20 and possibly

~30

Note that the ground state of the molecule (i.e. the state of lowest energy) doesnt haveE= 0. Two consequences of this are:

o Even at a temperature of absolute zero, bonds will still have a non-zero vibrationalenergy that depends on kand:

The molecule is said to possess zero-point energy.

Atoms still move by vibrations at absolute zero; whilst seen here from themaths, its a consequence of the Heisenberg uncertainty principle.

o Consider bonds involving different isotopes, e.g. compareOH and OD bonds:

They involve the same number of electrons, and so have identical forceconstants and bond lengths.

However, they have different zero-point energies because of different. They will have different bond dissociation energies, as this is

the energy required to take the bond from its zero-point energy up to an

energy corresponding to the atoms being widely separated.

vibrational energy levelsEnergy

v = 0

v = 1

v = 2

v = 3

v = 4

E

E=1

/2h 0

E= ~ 3/2h 0

E= ~ 5/2h 0

E= ~ 7/2h 0


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4.3 Polyatomic molecules

Polyatomic molecules have more than one vibrational mode.o The location of allNatoms in a molecule needs 3Nparameters to specify it. These

are normally specified by:

Translational position: specify the centre of gravity of the molecule using3 parameters.

Rotational motion: requires 2 parameters for a linear molecule; 3parameters for a non-linear molecule.

Vibrational modes: there will thus be 3N5 of these for a linear molecule,and 3N6 of these for a non-linear molecule.

The vibrational modes in polyatomic molecules may involve movement of all the atoms,rather than just a single bond vibration.

Each vibrational mode will have its own frequency. For example, the three vibrationalmodes of the water molecule are:

Symmetic stretch Asymmetric stretch Bending mode

1 = 3655 cm1

2 = 1595 cm1

3 = 3755 cm1

Each vibrational mode can be considered individually. We therefore expect absorptions at frequencies corresponding to 1, 2, 3, providing the

dipole moment of the molecule is changing during the vibration (as was the case for

diatomic molecules).o All three vibrational modes of H2O are IR-activeo The symmetric stretch of CO2 is IR-inactive as it doesnt change the

dipole moment); the other vibrational modes of CO2 are IR-active

o Note that H2O and CO2 are greenhouse gases because they absorb in the infraredregion.

Overtone and combination bands (e.g. 21, 1+3) are very weaklyallowed (as was the case for real diatomic molecules).

O

H

H

O

H

H

O

H

H


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4.4 Uses of IR spectroscopy

The frequencies of some vibrational modes are almostindependent of thestructure of the compound in which they are located.

o For instance, OH stretching vibrations occur at 3200-3600 cm1 Detecting an absorption band in this frequency range is evidence that the

sample contains OH functional groups

[Aside: its quite a broad range in this case because OH bond strengthsare affected by the extent of hydrogen bonding to them]

We can thus use IR spectroscopy to identify structural groups present in the sample:

From our discussion above, we note that:o Bonds involving lighter atoms absorb at higher frequency

than bonds involving heavier atoms: CH > CC OH > OD

o Stronger bonds absorb at higher frequencies: CC 2150 cm1 C=C 1650 cm1

o Bonds with large dipole moments give strong absorptions; those without giveweak (or no) absorption:

C=O strong C=C often weak Organic chemists used to be very good at knowing precise vibrational frequencies of

different functional groups and how there were affected by substituents.

o For instance, theyd know 1710 cm1 was likely to be a C=O group in a ketone,while recognising 1730 cm1 was more likely to be a C=O group of an aldehdye.

However, structural identification is now almost always done by comparison of thespectrum obtained with one in a database: a fingerprint method ofidentification.


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Example IR spectrum:

(fromhttp://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm#ir1)

The sample for IR spectroscopy may be solid/liquid/gas.

For lab measurements, the sample is held in a special cell:o the windows shouldnt absorb above ~500 cm1 (e.g. cant use glass; KBr is quite

common)

o the path length is usually short (because absorbances tend to be strong). Beer-Lambert law can be used to estimate concentration; this is difficult in practice to do

accurately due to scattered radiation affecting the baseline.

Limitation: need to identify a band from the component of interest that isnt overlappingwith bands from any other components that may be present; usually okay for simple

mixtures.

Rough cost estimate of basic spectrometer: 12k Timescale: Typically 10 s for a modern instrument, but it depends on sensitivity and

resolution required.

On-line measurement of process fluids is not straightforward:o Most of the materials used for cell windows arent resistant to chemicals (e.g. KBr

dissolves in water).

o Normal fibre optic cables absorb in the infrared region so we cant use them.o Special materials for fibre optic cables that dont absorb above, say, 800 cm1 are

being developed; thus far they tend to be expensive and react with acid/alkali.


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Alternative on-line measurement technique: investigate the near-infrared (NIR) regioninstead (wavenumbers between 4500 and 12,000 cm1):

o Fibre optic cables do exist that dont absorb in this region, so remote sensing ispossible.

o Well only see weak overtone and combination bands (e.g. 21 and 1 + 2) ratherthan fundamental vibrations; spectra are far harder to interpret.

There tend to be lots of weak overlapping bands in this region, Calibrations involve running pure components and developing a

mathematical model of the behaviour for mixtures very time consuming

process

o The NIR method is now used forcontinuous monitoring of somebulk chemicals in industry (e.g. gasoline; polymer melts), and is just beginning to

be used in food and pharmaceutical industry.

Example: NIR spectra of C-H stretching overtone region for water-ethanol mixtures.

(www.axsun.com)

New techniques are being devised for on-line process analysis one called EncodedPhotometric Infrared (EPIR) spectroscopy has great potential, but its too early to say

how useful it will be.


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4.5 Raman vibrational spectroscopy

Raman spectroscopy is another way of doing vibrational spectroscopy. Let us consider what happens when a laser emitting fixed frequency light is fired at a

sample.

Most photons scattered by a sample will be at the same frequency as the incident light:o Photons interact with the sample.o During interaction, the molecule is raised from the v = 0 vibrational level to a so-

called virtual state.

o Usually the molecule then relaxes back down to the v = 0 level.o The scattered photon thus has the same frequency as before this is termed

elastic scattering or Rayleigh scattering.

However, a very small number of photons (~1 in 107) will be scattered at a different(usually lower) frequency than the incident light:

o This effect is called the Raman effect.o During the photon interaction with the sample, the molecule in the virtual state

may relax back to the v =1 vibrational state.

o In this case, photons will have a scattered frequency ofvlaser0 where 0 is the vibration frequency.

o Hence we can measure the vibrational frequency 0.o Other transitions may also occur (e.g. from initial v = 1 level to level v = 0 or

v = 2).

The selection rule for Raman spectroscopy is different to that for infraredvibrational spectroscopy:

o Raman bands require there to be a change in polarizability of the molecule uponvibration; theres no need for there to be a changing dipole moment.

o As a result, bands that are inactive in IR spectroscopy are normally active inRaman spectroscopy.

oSimilarly bands that are weak in IR spectroscopy (because they dont changedipole moment much) are usually strong in Raman spectroscopy.


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Example: IR and Raman spectrum of L-cystine illustrating the different selection rules(www.jascofrance.fr):

Another advantage: light at the laser frequency can be focussed on to a small area ofsample easily Raman microscopy can record a vibrational spectrum on just a

small part of the sample (down to ~1 m). Main limitations:

o if the laser also promotes electrons into a higher energy level, then light will beemitted as the electron relaxes back down hence there may beinterference from sample fluorescence

o interference from background radiationo quantifying signal the Beer-Lambert law doesnt applyo its not good for complex mixtures

Rough cost estimate of basic spectrometer: 15k ?

Raman spectroscopy is beginning to be used for on-line process analysis:o the laser can be in the near-infrared region meaning that it can travel through glass

windows, or be transported by fibre optic cables; the latter makes possible remote

on-line sensing of the process fluid.


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Example: in-situ Raman spectra of the polymerisation of styrene (C6H5CH=CH2) in abatch reactor as a function of time (in minutes).

(http://www.surrey.ac.uk/PRC/Facilities/raman.htm)


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5 Microwave Spectroscopy

This technique is not really useful in analytical chemistry, but is covered briefly here forcompleteness and because it can impact vibrational spectra.

Absorption in the microwave region is associated with transitions between rotationalenergy levels.

o This is how microwave ovens work.5.1 Pure rotational spectroscopy

We shall only consider linear molecules in this section (e.g. diatomic molecules, or CO 2). In the same way that we write down and solve the Schrdinger equation for vibrations we

can also do so for pure rotational motion.

For rigid linear molecules this gives energy levels characterised by the quantum numbersJandMJ

o E = B J (J+1) J= 0, 1, 2, 3, o MJ=J,J1,J2, , J [i.e. there are 2J+1 values ofMJ]o whereIis the moment of inertia

Note thatI=r2 for a diatomic molecule, where is the reduced mass

Selection rules:o J= 1 [to conserve angular momentum]o Molecule needs a permanent dipole moment (to interact with EMR)

Population of levels depends on the degeneracy (number of levels having the sameenergy) and the Boltzmann factor:

rotational energy levelsEnergy

J = 0J = 1

J = 2

J = 3

J = 4

E= 0 1 levelE= 2B 3 levels

E= 6B 5 levels

E= 12B 7 levels

J = 5 E= 30B 11 levels

E= 20B 9 levels


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o Several of the lowest energy levels are occupied at room temperature

o Indeed, differentiation of the above equation forN(J) with respect toJshows thatthe most populatedJlevel corresponds to:

Hence we will observe a series of spectral lines if we do microwave spectroscopy, atenergies corresponding to:

o E (J J+1) = 2B(J+1)o Measurement enables parameterB to be determinedo Extremely accurate method of measuring moment of inertias (and thus bond

lengths) of gaseous molecules.

Because the upper rotational levels are occupied, we can measure microwave emissionspectra from remote objects:

o Method for identifying molecules in planetary atmosphereso Method for estimated temperature of remote objects, as the intensity distribution

of the lines in the spectrum depends on temperature.

Fornon-rigid molecules (i.e. real ones!):o Analysis is similar to above, but it is found that bond lengths increase very

slightly the faster the molecule rotate, meaning the apparent value ofB decreases

slightly asJgoes up.

5.2 Infrared spectroscopy revisited: vibrational-rotational spectroscopy

Vibrations and rotations can be treated as being independent of each other as they occuron different timescales.

For a diatomic molecule, the selection rule is:

v = 0, 1 (for SHO) and J= 1 and molecule needs a permanent dipole moment

Thus pure rotational spectra may be observed, but pure vibrational spectraare forbidden despite our discussion in Section 4!

o Each transition from vibrational level v = 0 to v = 1 has to be accompanied bya rotational change J= 1 in order to conserve angular momentum.


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o For instance, from the v = 0J= 3 level we get:

This means infrared transitions of diatomic molecules actually obey:o E= h0 2B(J+1) J= 0,1,2,3, (up to the last thermally populatedJ

level)

o We thus expect to get a series of peaks on either side of0 For gas-phase samples, this so-called rotational fine structure is often

observed when recording infrared spectra.

Energy

J = 0J = 1

J = 2

J = 3

J = 4

J = 5

J = 1

J = 2

J = 3

J = 4

J = 5

J = 0

v = 1

vibrational

level

v = 0

vibrationallevel


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Example 1: here is the background spectrum of air in an IR spectrometer. Note:o The baseline is not flato Lots of rotational fine structure on the water vibrational modeso Some evidence of fine structure on the CO2 asymmetric stretch

Example 2: IR spectrum of gases in a plume from a volcanic eruption: remote measurement

is possible using the sun as the source of infrared light. [Spectrum from Love et al., OSA

topical conference, 1991]

For liquid samples, intermolecular collisions usually mean that the linewidths aretoo broad to see rotational fine structure in infrared spectra.


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5.3 Energy diagram for a diatomic molecule

r


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6 Nuclear magnetic resonance (NMR) spectroscopy

This is usually the best method for the structural identification of organicmolecules.

The sample normally needs to be a liquid.o Where possible, dissolve solids to make a solution before doing NMR

spectroscopy.

o Far more limited information can be obtained on solids (or gases).6.1 Simplified background theory

In the same way that an electron has angular momentum (described by spin), nucleimay also possess angular momentum and so be described as having spin.

o The nuclear spin quantum number,I, gives the total angular momentum. The value ofIfor a particular nucleus depends on the detailed arrangement of

protons and neutrons within the nucleus.

o 1H hasI= 1/2 2H (0.015% natural abundance) hasI= 1o 12C hasI= 0 13C (1.1% natural abundance) hasI= 1/2o 16O hasI= 0 17O (0.04% natural abundance) hasI= 5/2

The z-component of nuclear angular momentum is given by the quantum numbermI,which can take valuesI,I1, ..., I.

Ordinarily the mI quantum levels all have the same energy (i.e. are degenerate). However they will split into 2I+1 different energies if a large magnetic fieldB0 is applied. Transitions between these energy levels is termed nuclear magnetic resonance (NMR)

spectroscopy. The selection rule is mI=1.


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We shall concentrate only on the case of nuclei havingI= , the most important of whichare 1H and 13C.

o Nuclei withI= 0 will not show NMR spectra.o Nuclei withI> tend to give broad uninformative spectral lines.

The separation of the two energy levels for a spinI= nucleus is given by:E= (h/2) B0 (1)

o h = Plancks constanto = magnetogyric ratio of the nucleus (ratio of magnetic moment to angular

momentum):

For1H: = 26.752 x 107 rad T1 s1 For13C: = 6.7283 x 107 rad T1 s1

Example:

anI=1

/2 nucleus

No magnetic

field

Sample in strong

magnetic fieldB0

Energy

E= (h/2) B0 (1)

mI = 1/2

mI =1/2

mI = 3/2

Energy

IncreasingB0

Example:

anI= 3/2 nucleus

4 degenerate levels

whenB0 = 0

mI =3/2

E= h

mI = 1/2

mI =1/2

E= h

E= h


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o B0 is the strength of the applied magnetic field (SI unit = Tesla)o is a very small shielding term which will depend on the precise

chemical environment of the nucleus under investigation.

The separation between the energy levels is far smaller than the other techniquesdiscussed so far.

o We need to use largeB0 values to get a measureable population differencebetween levels; it turns out NMR signal intensity is proportional toB0

2.

o Modern NMR spectrometers in research laboratories employ superconductingmagnets:

Typical field strength is 4.7-9.4 Tesla, corresponding to a 1Hresonance frequencies of 200-400 MHz.

Highest available field strength is 21.1 Tesla, corresponding to a 1Hresonance frequency of 900 MHz.


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The exact separation of energy levels for a nucleus in a molecule depends slightly on itschemical environment because of the shielding term:

o Electrons immediately surrounding the nucleus will tend to circulate in aparticular direction in the applied magnetic field.

o This circulation induces a very small magnetic field B at the nucleus whichopposes the large applied magnetic field.

o The result is that the nucleus actually experiencing a magnetic fieldB0 (1)

There are also some other effects that cause chemical environment to give a shielding term that we dont have time to discuss (e.g. hydrogens attached to

aromatic rings have less shielding than might be expected).

Hence nuclei in different chemical environments in the molecule will have peaks atdifferent frequencies in the NMR spectrum.

o Those with fewer electrons around them will have less shielding. The peaks in NMR spectra are usually quoted using the chemical shift

scale in parts per million (ppm).o This is effectively a dimensionless frequency scale relative to a standard reference

substance:

o The reference sample for both 1H and 13C NMR spectra is a compound known asTMS, tetramethylsilane, Si(CH3)4,

o The chemical shift scale is used because it is independent ofB0value this is helpful becauseB0 is rarely known exactly as it changes slightly day by dayfor a superconducting magnet.

e

B0

B


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6.21H NMR spectroscopy

Peaks in 1H NMRspectra are quantitative i.e. their area is proportional tothe number of hydrogens in that chemical environment in the molecule.

Example: low resolution NMR spectrum of ethyl acetate (CH3COOCH2CH3)

High-resolution 1H NMR spectra show an important additional effect called spin-spin coupling, or alternatively J-coupling.

o The quantum state of a 1H spin is slightly affected by the spin states of hydrogennuclei that arent chemically equivalent to it, provided that they are within 3 bonds

of it.

Example 1:

o Consider nucleus HA: Spin HB may be in the same or the opposite direction to it. Hence HA sites in a sample have two slightly different energy states.

o NMR signal from HA will therefore be a 1:1 doublet due to coupling to HB.o Similarly, the NMR signal from HB will be a 1:1 doublet due to coupling to HA.

chemical shift,

frequency,

shielding,

J/Hz J/Hz

/ppm HA HB


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Example 2:

o Nucleus HA can now couple to two chemically equivalent HB nuclei: Both HB spins may be in same direction as HA (probability ). Both HB spins may be in opposite direction as HA (probability ). One HB may be in same direction as HA, and one opposite (probability ).

o NMR signal from HA will therefore be a 1:2:1 triplet due to coupling to HB.o Note that one HB nucleus does not couple to the other HB nucleus because they are

in chemically equivalent environments.

o The NMR signal from HB will therefore be a 1:1 doublet due to coupling to HA

Example 3:

o Nucleus HA now can couple to three chemically equivalent HB nuclei.o The resulting pattern for HA will be a 1:3:3:1 quartet (reflecting the probabilities

of the different possible spin states), while the signal from HB will be a 1:1

doublet due to coupling to HA.

J

J/Hz

/ppm HA HB

J J

J

/ppm HA HB

J

J/Hz


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Expert NMR spectroscopists are able to predictJvalues for different functional groupsand molecular conformations.

o Values are normally in the range 1-12 Hz for a 3 bond coupling. Example: high-resolution spectrum of ethyl acetate at a medium magnetic field ( 1H

frequency 100 MHz)

Example: high-resolution spectrum of ethyl acetate at a high magnetic field(1H frequency 500 MHz)

Timescale for1H NMR experiment is ~1 minute, but setting up may take more timedepending on the experiment being performed.

Each chemically distinct environment gives rise to a chemical shift (fixed in ppm), whichmay then split up due to J-coupling (fixed in Hz).

Peak areas give relative amounts of each H environment.

500 Hz

7 Hz

100 Hz

7 Hz


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Functional groups can be identified (via chemical shifts and intensities). Linkages between functional groups can be identified (via J-coupling). NMR experts are good at identifying molecules from chemical shift and J-coupling

patterns.

Nowadays, identification of unknown molecules is often by comparison with:o Established databases of NMR spectrao The results of computer programs that predict NMR spectra.

For very large molecules (even proteins!), there are huge numbers of spectral lines:o NMR techniques have been developed that allow assignment of these, e.g. 2-

dimensional and 3-dimensional spectra reduce overlap between peaks.

Example 1H NMR spectrum: vitamin K1 (C31H46O2) (1H frequency 400 MHz)

(from http://riodb01.ibase.aist.go.jp/sdbs/)


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6.313

C NMR spectroscopy

The first use of13C NMR spectroscopy is to give the number of chemically inequivalentcarbon atoms in the sample.

Note that symmetry considerations may mean that this is less than thenumber of carbon atoms in the molecule.

3 carbons 5 carbons 4 carbons

(2:2:4) (2:2:2:1:1) (2:2:2:2)

The actual 13C chemical shifts provide structural information. For example:o C=O carbonyl 160-220 ppmo C=C alkene/aromatic 100-150 ppmo Saturated C alkyl 10-50 ppm

As chemical shifts are affected by the shielding given by the surrounding electrons,substituents have an inductive effect. For example:

o COH 50-80 ppm (compared to 10-50 ppm without OH group) Identification of structural groups is therefore possible:

o By comparison with chemical shift values in reference bookso By comparison with databases of13C NMR datao By comparison with results of NMR prediction programs

J-Coupling to other nuclear spins is possible:o Probability of13C of interest being bonded to another13C nucleus is small (as the

natural abundance of13C is only 1.1%)

o Coupling to 13C of interest to any 1H bonded to it will happen this will result in asplitting up of each 13C peak according to how many hydrogens are directly

bonded to the carbon.

o J-coupling values in this case are ~120 Hz.o 13C multiplets due to coupling to 1H are:


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13C NMR spectra take a long time to acquire if good signal-to-noise is desired (timescaleis hours per spectrum).

Its therefore common to record the spectrum using a technique called continuousproton decoupling:

o Almost all 13C NMR spectra are recorded this wayo This eliminates the J-coupling to H nuclei, and so concentrates multiplet signal

intensities into a single peak

o The sample is irradiated with a broad bandwidth pulse whilst the 13C NMRspectrum is recorded such that all protons rapidly cycle between their spin up and

spin down states, so that they become decoupled from 13C.

o Coupling to protons other than that of H can still take place.o The result is a significant saving in the time it takes to record the spectrum.

However, doing the experiment in this way means that the 13C NMRspectra dont have absolutely reliable relative intensities.

Example 13C NMR spectrum (proton decoupled): riboflavin (vitamin B2, C17H20N4O6)

(from http://riodb01.ibase.aist.go.jp/sdbs/)


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6.4 Industrial on-line NMR spectroscopy

The NMR techniques described above are those used in research and specialist analyticallaboratories.

They cant be implemented on a chemical plant: theyre too expensive and the hardwareis not robust enough.

Theres also a problem measuring NMR spectra of flowing samples the nuclei underinvestigation need to be in the magnet for a certain length of time before they can be

measured by NMR.

o Slow flow only, or need to stop the flow for the measurement. On a chemical plant, we can use permanent magnets of far lower magnetic field strength

(corresponding to a 1H frequency of, say, 20 MHz)

o Cant do high-resolution spectroscopy but can sometimes separate low-resolutionsignals: for instance, we can get a measurement of the ratio of aromatic to

aliphatic 1H environments in gasoline.

o 1H NMR signal intensity can be used to estimate the water content in solids.o 1H NMR relaxation times (how long spins take to move from the upper energy

level to the lower energy level) of liquids in porous solids gives some information

on the pore sizes present; this is used in oil-well logging.

Magnetic resonance imaging (MRI) is also being developed for use in a process plantsetting.


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7 Methods of elemental analysis

In this section, three principal methods ofelemental analysis are described. They arent the only methods for doing elemental analysis:

o For example, titration, gravimetry, and electrochemical measurements can also beused.

7.1 CHN Analysis

Organic compounds are normally analysed by flash combustion of a smallsample (typically 1-2 mg) in oxygen unlike spectroscopic techniques, this is destructive

test.

An exact amount of sample is weighed inside a small tin capsule. The capsule is introduced into the analysers furnace which is at 950C. The tin capsule combusts, elevating the temperature to >1800C. At this temperature, the sample is vaporised and forms CO2, H2O and a mixture of N2,

NO, NO2.

CxHyNzO + O2xCO2 +y/2 H2O +zNO2 + O2

The product gases are then analysed:o Instruments differ on whether they try to remove SOx, halogens, phosphorus etc.

before analysing the product gases, or whether they try to measure them.o The product gases are normally reduced (using copper at high temperature)

to remove O2 and convert NOx gases to N2 before analysis.

o Some instruments separate the product gases: This can be by using chromatography (see Section 9), or by having a water

and a CO2 trap (using Mg(ClO4)2 and NaOH respectively).

A thermal conductivity detector is normally used to measure the gasconcentrations.

oA few instruments use infrared spectroscopy of the mixture as the detectiontechnique.

From the product gas concentration, the original mass fractions of C, H, and N in thesample are calculated.

With modern CHN analysers:o Little attention is required by operator, other than a daily calibration of instrument.o Analysis is complete in 15 minutes.

Oxygen contents of the sample can only be obtained indirectly using this method (e.g. bycomparing the sample mass used, and that calculated for C+H+N components).


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o For a direct measurement of oxygen, pyrolysis (=heating in absence of air) in thepresence of platinum-carbon can be used: this gives CO, which is then converted

to CO2 and detected quantitatively.

7.2 Atomic spectroscopy

Atomic absorption spectroscopy (AAS) is a very sensitive methodfor detecting the presence and concentration of about 70 elements.

A sample of solution is vaporized to its constituent atoms in a hot flame. A hollow cathode lamp containing the element under investigation emits lights at specific

frequencies for that element.

The photons will be absorbed by the sample if the frequencies correspond to an allowedtransition of atoms in the flame.

o Recall selection rule for transitions of electrons between atomic orbitals is:n = anything ; l= 1 ; ml = 0, 1

The absorbance obeys the Beer-Lambert law to a good approximation, and soconcentrations can be determined quantitatively.

In practice the spectrometer will be calibrated on solutions of known concentrationbeforehand to achieve high accuracy.

Advantages of AAS:o Sensitivity to a wide range of elements (typically down to 1 ppm)o High accuracy if care is taken over sample preparation and calibration.

Disadvantages of AAS:o Some solid samples are difficult to get into solution form.o Need a hollow cathode lamp for sharp monochromatic lines for each element.o Different atoms require different flame temperatures to achieve reliable results

(e.g. air/acetylene 2250C; NO/acetylene 2955C).

Light at

(chosen for specific

element of interest) Measure absorptionF

L

A

M

E

Atomic orbitals

h

Solution of sample


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o Other factors influencing absorption may need to be taken into account: excited states of the atom under investigation (e.g. ionisation of Na/K) potential interferences from other elements present.

Atomic emission spectroscopy (AES) is a similar process:o Again a solution of sample is introduced into a hot flame.o This time the intensity of a particular frequency emitted is measured.o The advantage here is that there is no need for individual lamps for each element.o The disadvantage is that it is far less sensitive than AAS.

Inductively Coupled Plasma (ICP) spectrometers use a plasma(temperature >7000 K) rather than a flame:

o Under these conditions, atomic emission spectra (ICP-AES) can be measured withsimilar sensitivity to AAS, while removing many of the chemical interferences

present in a flame.

o Mass spectrometry (ICP-MS) may be used to measure mass of atoms/ions presentin the plasma directly using a quadrupole mass spectrometer [see Section 8].


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7.3 X-ray fluorescence (XRF)

This is a method of elemental analysis for solid samples. Its useful for those that donteasily dissolve, e.g. some minerals and ores.

X-rays are fired at the sample, and these knock out a core electron from an inner quantumlevel (e.g. from the 1s, 2s or 2p atomic orbital).

An electron then falls from an upper energy level to fill the core-level vacancy; this isaccompanied by the emission of a photon of frequency h.

The wavelengths of emitted photons enable the elements present in the sample to bedetermined, while the intensities give the amount of each element present (after careful

calibration).

This method is only appropriate for elements with atomic numbers greaterthan ~20. Gram quantities of a solid sample are required.

1s

2s

2p

3s

3p3d

4s4p

h

Hole created

by X-rays


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8 Mass spectrometry (MS)

Mass spectrometry is a widely used technique in analytical chemistryo It can measure the molecular weight of components present in a sampleo It can identify particular chemicalso After calibration, it can be used quantitatively.

All mass spectrometers involve the following steps:o Production of ions in the gas phaseo Separation of the ions according to their mass-to-charge (m/z) ratio.o Detection of ions

There are several different ways of performing each step.

8.1 Production of ions: five methods

Electron Ionisation (EI) is the most common method of ionisation for small organicmolecules:

o The sample must first be vaporized (by heat or a spark) if it isnt already in gasphase.

Some sample decomposition may occur for thermally unstable samplesduring this step.

o The sample is then bombarded with electrons that knock an electron out:M + e M

+ + 2e

o The parent ion is always produced in a vibrationally excited state and so mightfragment (often in a fairly predictable manner) to smaller ions before it reaches

the detector.

o EI is a harsh ionisation method: fragmentation may be so extensive that the parention is absent from the spectrum.

oThe fragmentation pattern provides a fingerprint method of sampleidentification by comparison of results with MS databases.

o Example: EI MS of vinyl chloride

[Figure from //www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/]


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Chemical Ionisation (CI) is a common alternative ionisation approach forsmall organic molecules:

o Again, the sample is first be vaporized (by heat or a spark)o A small amount of methane is present in the ionisation chamber, which is ionised

by electron impact.

o This then reacts to form species such as CH5+ that can donate H+ to the sample ofinterest:

CH4 + e CH4

+ + 2e

CH4+ + CH4 CH5

+ + CH3

M + CH5+ MH+ + CH4

o Ionisation of M to give MH+ by this method is thus by proton transfer.o This is a softer method of ionisation than EI, so less fragmentation occurs.o The parent ion [MH]+ is likely to be the most prominent peak; note that it will be

at a molecular weight one greater than that of molecule M.

o If the methane concentration is too high, then sometimes the species [MC2H9]+ isdetected as well.

Fast-Atom-Bombardment (FAB) may be used to ionise medium-sizedorganic molecules:

o High-energy atoms hitting the sample can vaporise and ionise it in the same step Since no heating is required, this method can be used to analyse samples

that arent thermally stable.

o The parent ion [MH]+ is normally detected together with some usefulfragmentation.

o The method works for quite large molecules (up to about 5000 Da). For very large macromolecules such as proteins, the above methods dont work well:

o multiple fragmentation makes interpretation difficult/impossible.o

its difficult to separate species with high m/zratios (e.g. above 5000 Da).

Electrospray ionisation (ESI) has now become common for MS ofproteins:

o A solution of the sample passes through a metal capillary that has a high appliedvoltage, and is then sprayed out to produce droplets (10 m) with a very high

charge.

o The droplets shrink as the solvent evaporates, so Coulombic repulsion between thecharges increases until it causes each droplet to break up into smaller droplets.


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o Continuing the shrinking/breaking up process eventually leads to the molecule ofinterest being a lone ion it may be singly or multiply charged.

o This is a very soft method of ionisation, and so negligible fragmentation occurs.o For proteins, the method produces a range of ions, e.g. from [MH10]10+ to

[MH20]20+

o Example: ESI MS of myoglobin (mass 16955 Da).

[Figure from http://www.chm.bris.ac.uk/ms/theory/]

Matrix-Assisted Laser Desorption Ionisation (MALDI) is another ionisation methodused for very large molecules:

o The sample is dispersed in a solid matrix to form a solid solution.o A laser is then used to disintegrate the solid solution the matrix material is

chosen so that it absorbs the laser wavelength.

o Clusters ejected from the surface break up to give the sample molecule in [MH]+or [MNa]+ form; some are multiply charged ions.

o There is little fragmentation using this technique.


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8.2 Analysis and detection of ions: three methods

Once the ions have been formed, we need to separate them according to m/zratio. This part of the spectrometer will need to be at a good vacuum because we dont want the

ions to hit other molecules before reaching the detector.

The analysis method used will depend on mass resolution required, mass range, scan rate,and detection limit required.

Magnetic-sector instruments: this is the traditional method, but is nowbecoming rarer.

o These accelerate ions through a voltage Vand then deflect them by a magneticfield (B) through a radius of curvature r.

o Only ions of correct mass-to-charge ratio (m/z) will reach the detector:

o Scanning either the magnetic field or the accelerating voltage produces aspectrum.

[from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htm#ms1]

Time-of-flight (TOF) method:o The ions are accelerated through a voltage (V) and the time taken for them to

travel a specific distance to reach the detector is measured.

o The kinetic energy that each ion has is given by:

o Hence ions of low m/zwill have large v and reach the detector first.o TOF instruments are usually very sensitive, and can scan a large mass range.


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Quadrupole detectors:o The ions pass through an area with four hyperbolic magnetic poles created by a

radiofrequency field.

o Only certain ions can take a stable path through the field and be detected.o Scanning the rf field (by increasing voltage) allows a mass spectrum to be quickly

and easily recorded, but resolution is more limited than the other methods.

8.3 Uses of MS

Low-resolution mass spectrometry gives integer masses for peaks:o Useful for structural identification

High-resolution mass spectrometry can be very accurate:o It can distinguish between CO, N2 and C2H4; these have exact molecular weights

of 27.9949, 28.0062 and 28.0313 mass units respectively.

o Even more useful forstructural identification. For small organic molecules, the fragmentation pattern in EI MS often provides structural

information:

o For instance a peak at M+16 may correspond to loss of NH2 suggesting an acidamide to be present.

o Fragmentation can permit identification of compounds by comparison withdatabase of known mass spectra.

The different isotopes of the elements are very important in MS:o For instance, molecules containing a single chlorine atom will give separate MS

peaks for the 35Cl and 37Cl isotopes (in a relative ratio of ~3:1)

o The natural abundance of13C is 1.1%; MS peaks due to ions with one (or more)13C isotope in them will be observed, particularly for medium and large

molecules. MS is used sometimes on-line in the process industries:

o Quadrupole detectors arent too expensive and are fairly robusto The most common use is for analysis of gas-phase species; only difficulty is

reducing pressure from process operating conditions to good vacuum inside

instrument.


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9 Chromatography

All the techniques discussed so far are limited in one important respect:o They can only easily analyse pure compounds, or simple mixtures at best.

What do we do if the sample is a complicated mixture? Chromatography is the key separation technique used by analytical chemists when

the sample is a mixture.

After calibration, chromatography can be used for structural identification andquantitative measurement as well as simply being a separation technique.

Chromatography is also a chemical engineering unit operation for purification of high-value chemicals, particularly in the pharmaceutical and biotechnology industries.

Chromatography is based on the physical separation of individual chemical componentsin a sample:

o The sample is present in a mobile or carrier phase: may be gas, liquid, or evensupercritical fluid.

o The sample is separated into components due to differences in affinity for astationary phase.

9.1 Gas chromatography (GC)

For GC, the carrier phase is an inert gas (e.g. He, Ar, N 2). The sample needs to be vaporised if its not already a gas, and injected as a pulse into the

carrier stream.

The stationary phase is usually a column containing the stationary phase on a fused silicasupport:

o The column is usually very narrow (say 2 mm diameter) and may be 1-10 m long;physically it will look like a coiled loop.

oThere are many types of silica and modified silica so different separations can beachieved.

Separation is based on the components having different retention times on the column:o affected by boiling points of the substances to be separated.o affected by selective adsorption of a component onto the stationary phase.

The column is located in an oven, the temperature of which can be controlled. For good separations in a reasonable length of time, its common for the temperature of

the oven to be increased over the course of the experiment.


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GC experimental configuration:

[from http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm]

There are a range of detectors available. Well mention the two most common.

Flame ionisation detectors (FID):o These are widely used for analysis of organic compounds.o Gas at the column exit is mixed with hydrogen and air and burnt. Any organic

compounds present produce ions and electrons in the flame making it capable of

conducting electricity. The FID measures the current response to an electric

potential at the burner tip.

o The FID response has high sensitivity, a large linear response range, and lownoise. It is also robust and easy to use, but it destroys the sample.

o For hydrocarbons, the FID peak areas are proportional to the number of carbonatoms present in that component of the sample.

Thermal conductivity detectors (TCD):o These compare the thermal conductivity of the gas at the column exit with a

reference flow of carrier gas (usually He). Any change is due to the presence of

sample compounds.

o Disadvantage: TCDs are slightly less sensitive than FIDs and have slightly lowerresolution (as they have a larger dead volume).

o Advantage: TCDs can be used to detect any compound (i.e. not justhydrocarbons), and the sample isnt destroyed.

o Because the thermal conductivity of organic compounds tend to be similar to eachother, TCD peak areas for hydrocarbons are roughly proportional to the

concentration of that component.


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Example: GC of a mixture of air + 9 hydrocarbons:(a) column at 45C separate components 1-5; dont get components 6-9.

(b) column at 145C doesnt separate 1-4; does separate components 5-8.

(c) column heated a linear rate from 30-200C separates components 1-9. Theres also less band spreading

as a function of time.

From http://www.uft.uni-bremen.de/chemie/pdf/GC_Intro_Christian_Jungnickel.pdf

45C

145C

time,min

time,min


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9.2 Liquid chromatography (LC)

Note modern instruments often use the acronym HPLC (for high-performance orhigh-pressure LC).

In this case the sample is in the liquid phase (by dissolving into solution if its not alreadya liquid).

The stationary phase is a solid packed into a column; it can be a liquid-coated solid. Different detector systems are used (e.g. UV, fluorescence, refractometry). A variety of different separation mechanisms are used. Liquid-Solid separations are based on the intermolecular interactions between sample

molecules and the solid phase.

o For instance, these may be polar interactions or hydrogen bondinginteractions.

o The normal case is that the solid has hydroxyl groups at the surface and so hasan affinity for polar groups:

Less polar molecules will pass through the column faster than polarmolecules if the surface of the solid likes polar species.

o In reverse phase chromatography, the stationary phase is made hydrophobic(e.g. silica with n-alkyl chains covalently bound to its surface):

In this case, hydrophobic compounds will have longer retention times thanhydrophilic ones.

o The retention times are critically affected by the polarity of the solvent (carrierphase).

Tramp:

30-200C


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Mixtures that dont separate when using one solvent as carrier phase mayseparate easily using a solvent of different polarity.

o This technique is widely used in synthetic organic chemistry labs as a bench-scalepurification technique.

Liquid-Liquid separations are based on the partition of the sample between two liquidphases.

Size-Exclusion chromatography is based on the molecular size of the compoundspresent.

o The stationary phase consists of solid beads containing small pores.o Large compounds cant enter inside the beads and thus will elute first.o Smaller compounds enter the beads and will have longer retention times.

Ion-exchange chromatography operates on the basis of selective exchange of ionsin the sample with those in the stationary phase.

Carrier solvent

Inject

sample

Packed

column

Carrier solvent Carrier solvent Carrier solvent

Collect

product 1

12

3

1

2

3

1

2

3


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o The column consists of a polymer matrix bearing certain ionic functional groups: e.g. MSO3H+ for the case of cation exchange (anion exchange columns

also exist).

o Molecules capable of ion-exchange will be retained at these sites : e.g. if they contain cations or acidic hydrogens in this example.

o Molecules retained on the column can be subsequently collected by changing theproperties of the mobile phase:

e.g. by changing the pH so that the carrier liquid will displace samplemolecules attached to the column.

Affinity chromatography uses immobilized biochemicals that have aspecific affinity to the compound of interest.


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9.4 Hyphenated techniques

These are simply a combination of the techniques that weve discussed so far. Examples include:

o GC MSo LC MSo LC NMR

With these techniques, separation and sample identification of complex mixtures can beperformed in a single piece of equipment.

This represents the state of the art in modern analytical chemistry.

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