Modern Analytical Techniques

1. Infra red Spectroscopy.

• State that absorption of infrared radiation causes covalent bonds to vibrate.

• Identify absorption peaks in an infrared spectrum.

• State that modern breathalysers measure ethanol levels by analysis using infrared spectroscopy.

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Infra red radiation and molecules

• When a molecule is exposed to electromagnetic radiation covalent bonds will absorb energy in the infrared region of the spectrum.

• Since each particular type of bond has its own natural vibrational frequency we can use these analytically.

• The amount of vibration depends on:• the bond strength• the bond length• the mass of each atom involved in the

bond.

• For IR radiation to be absorbed the vibrations must cause a change in the dipole moment of the molecule.

• Symmetrical bonds in e.g. O2 and N2 do not absorb radiation and are not IR active.

• Since most molecules have several different types of bond molecules will cause many absorptions.

• The same bond may have many different vibrational modes.

• http://en.wikipedia.org/wiki/IR_spectroscopy

Recording a Spectrum

• An infra red beam is split into 2 and passes through a sample and a reference cell containing solvent only.

• The solvent absorptions are thus removed from the sample and only the absorptions due to the test molecule are seen.

• N.B. Frequency is measured using WAVENUMBER which is actually 1/wavelength with the units cm-

1.• This makes the numbers in the spectrum easy to

handle.• Absorptions are diagnostic of bonds present.

Schematic diagram of an infrared spectrometer

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A typical infrared spectrum showing absorption peaks

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•Even though energy is absorbed and the spectrum consists of several troughs these are still described as peaks (since they represent increasing amounts of energy absorbed).

•Most organic compounds produce a peak at around 3000 cm-1 due to absorption by C-H bonds which is therefore NOT used diagnostically.

•However (see table p.168) O-H bonds and C=O bonds can be used diagnostically for alcohols, aldehydes, ketones and carboxylic acids.

•Very broad absorptions are due to the large amounts of energy absorbed by very polar bonds like O-H.

•When interpreting simple spectra:

•Concentrate on major absorptions only.

•Remember that absence of an absorption is diagnostic and popular in exam questions.

•Do not expect to make absolute identifications from IR spectra only.

• Identify, using C=O and O–H absorptions, an alcohol, an aldehyde or ketone and a carboxylic acid.

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Infrared spectrum of methanol

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Infrared spectrum of propanal

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Infrared spectrum of propanoic acid

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IR Analysis and Breath Testing

• The legal limit for blood alcohol in the UK is 80mg of ethanol per 100cm3 of blood.

• Initial breath testing using a breathalyser involves the use of potassium dichromate absorbed onto crystals.

• Exhaled breath passes through the crystals and ethanol is oxidised, causing the crystals to go green as the dichromate is reduced to Cr3+ ions.

• A positive breath test is a cue for an IR test back at the police station.

• A broad absorption at 3340 cm-1 and a sharp absorption at 2950 cm-1 can be used by the computer to calculate the % of ethanol present.

• Outline the early developments in mass spectrometry.

• Outline the use of mass spectrometry in the determination of relative isotopic masses and for identifying elements.

• Interpret mass spectra of elements in terms of isotopic abundances.

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Mass Spectrometry

Simple Mass Spectrometer

• Early developments in mass spectrometry listed on p. 170 in the text book are NOT required by the syllabus, but make interesting reading, nor are the workings of the machines.

• The key is that POSITIVE ions are created and accelerated then deflected (by a magnet) round a bend which serves to focus the beam of ions since heavy and light ones will hit the sides.

• To ensure that all ions are detected the magnetic field is varied.

• The ions are detected and recorded according to their mass to charge ratio (written m/z). For AS only ions with a single + charge are considered so m/z = Ar

• The use of mass spectrometers to determine relative isotopic masses are required.

Using the Mass Spectrum of Neon to work out RAM

• Neon shows 3 isotopes with m/z (mass number) 20,21 and 22.

• Peak heights show relative abundances as:• 90.5, 0.3 and 9.2 respectively.• To calculate the RAM remember:• RAM = (20 x 90.5)+(21 x 0.3)+(22x9.2) 100

• =1810+6.3+202.4• =20.19 (or 20.2 to 3sf)• Use the mass spectrum for magnesium on the

following slide to calculate the RAM for magnesium.

The mass spectrum of magnesium

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http://www.azcentral.com/community/westvalley/articles/2009/03/27/20090327gl-nwvvan0328.html

Syllabus requires that you know that mass spec is used as a method for identifying elements e.g. in the Mars space probe and in monitoring levels of environmental pollution such as lead.

Also mentioned: pesticides in the food chain.http://www.bbc.co.uk/radio4/science/smalldog.shtml

http://www.nytimes.com/1986/02/06/business/technology-detecting-food-contaminants.html?sec=health&spon=&pagewanted=all

• Use the molecular ion peak in an organic molecule’s mass spectrum to determine its molecular mass.

• Explain that a mass spectrum is essentially a molecule’s fingerprint that can be identified using a spectral database.

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Mass Spectrometry and Organic Molecules

• The value of mass spectroscopy with isotopes of elements is limited.

• However it is widely used as a routine analytical tool in organic chemistry because the beam of high energy electrons (or whatever the ionising source is) disrupts the covalent bonds within the molecule and fragments it.

• There is usually a fragment made which represents the MOLECULAR ION where a single electron has been knocked off the molecule leaving it positively charged with the same mass since the mass of an electron is negligible.

• Energy can cause the molecular ion to vibrate strongly enough to weaken the bonds and cause fragmentation.

Using Fragmentation Patterns to Confirm Molecular Structures

• The mass spectrum of ethanol has many peaks due to the fragmentation of the molecular ion.

• These can often be explained from a knowledge of the structure of the molecule under test or used diagnostically to identify unknown or new molecules.

• Examples include:• m/z 15 CH3

+ fragment• m/z 17 OH+ fragment• m/z 29 C2H5

+ fragment• m/z 31 CH2OH+ fragment• m/z 46 M+ C2H5OH+ the molecular ion.• Can you suggest fragments for peaks at m/z 45,

43, 27?

Mass spectrum of ethanol

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Using Fragmentation Patterns to Confirm Molecular Structures(2)

• Look at the mass spectra for Pentane and 2-Methyl pentane. Can you identify the key fragments which unambiguously identify which of thesespectra is which?

• Identify the M+ for A and B. Any ideas for possible identification?

Mass spectra of the isomers of C5H12, pentane and 2-methylbutane

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Spectra A and B

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• Suggest the identity of the major fragment ions in a given mass spectrum.

• Use molecular ion peaks and fragmentation peaks to identify structures.

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Mass spectrum for an unknown alkane (hexane).

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Equations explaining how some of the peaks arise in the mass spectrum for

hexane

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Using Fragmentation Patterns to Confirm Molecular Structures(3)

• Identify the fragments responsible for the peaks A,B and C for pent-1-ene as follows.

• Identify the molecular ion peak in the spectrum of the unknown alkane.

• It is a 2-methyl…… branched chain.• Draw structures for the fragment ions

represented by the peaks with m/z values of 43, 57 and 71.

Mass spectrum of pent-1-ene

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Mass spectrum of an alkane

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Combined Techniques

• It is worth remembering that no one single analytical technique is ever used alone.

• Mass Spectroscopy is invariably carried out in sequence with Gas Chromatography.

• IR and NMR are also used with the other 2.• The job of a modern analytical chemist is not a lot

different to the antics of the various CSI ‘scientists’ on the TV.

• Any new substance is routinely ‘fingerprinted’ and records of the various spectra stored on databases to enable swift analysis when required.