Analytical Techniques
Analytical techniques covers how mass spectrometry, infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and chromatography are used to identify and determine the structure of organic and inorganic compounds. It draws on isotopes from atomic structure and functional groups from organic chemistry, and is often examined as a synoptic topic in A Level Chemistry.
Before you start
Make sure you're comfortable with these topics first:
Method
- In mass spectrometry, use the m/z value of the molecular ion peak (the peak at the highest m/z, formed when a molecule loses one electron without fragmenting) to find the relative molecular mass, and interpret smaller fragment peaks by subtracting their m/z from the molecular ion to identify the fragment lost.
- Recognise a compound containing chlorine or bromine from a mass spectrum by the characteristic pair of molecular ion peaks 2 mass units apart (from the two chlorine isotopes, 35Cl and 37Cl, in an approximate 3:1 ratio, or the two bromine isotopes in an approximate 1:1 ratio).
- In infrared spectroscopy, compare the absorptions present in the spectrum against those known to be absent (using a table of characteristic wavenumber ranges) to identify which functional groups, such as O-H, N-H or C=O, are present or absent in a molecule.
- In 13C NMR, count the number of distinct carbon environments in a molecule (carbons identical by symmetry count as one environment) to predict the number of peaks, then use typical chemical shift ranges to assign each peak to a type of carbon.
- In 1H NMR, count the number of distinct proton environments to predict the number of peaks, use relative peak areas (integration) to find the ratio of protons in each environment, and apply the n+1 rule to a peak's splitting pattern to find the number of protons on adjacent carbon atoms.
- In thin-layer chromatography, calculate Rf = distance travelled by the spot / distance travelled by the solvent front, and compare this value with reference standards run under identical conditions (ideally on the same plate) to identify an unknown component.
Worked example
Three isomers of molecular formula C4H10O are butan-2-ol (CH3CH(OH)CH2CH3), 2-methylpropan-1-ol ((CH3)2CHCH2OH) and 2-methylpropan-2-ol ((CH3)3COH). A sample is analysed by 13C NMR and gives exactly two peaks. Identify which isomer this is, and justify your answer by describing its carbon environments.
- Count the distinct carbon environments in butan-2-ol: all four carbons (CH3, CH(OH), CH2, CH3) are in different environments, giving 4 peaks.
- Count the distinct carbon environments in 2-methylpropan-1-ol: the two CH3 groups are equivalent by symmetry (1 environment), plus the CH carbon and the CH2OH carbon (2 more environments), giving 3 peaks.
- Count the distinct carbon environments in 2-methylpropan-2-ol: the three CH3 groups are all equivalent by symmetry (1 environment), plus the central carbon bonded to OH (1 environment), giving 2 peaks.
- Only 2-methylpropan-2-ol gives exactly 2 peaks, matching the spectrum described.
- Final answer: the sample is 2-methylpropan-2-ol, (CH3)3COH
Practice questions
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Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
A sample of 1-bromopropane, C3H7Br, is analysed by mass spectrometry. Use Ar: C = 12.0, H = 1.0, Br = 79.0, to calculate the relative molecular mass, Mr, of 1-bromopropane.
The infrared spectra of propanoic acid, CH3CH2COOH, and propanal, CH3CH2CHO, both show a strong absorption at about 1720 cm^-1. The spectrum of propanoic acid also shows a broad absorption between 2500 and 3300 cm^-1 that is absent from the spectrum of propanal. Identify the bond responsible for the absorption at about 1720 cm^-1, and explain how the broad absorption between 2500 and 3300 cm^-1 allows the two compounds to be distinguished.
A student separates a mixture of three food dyes, D, E and F, using thin-layer chromatography. The solvent front travels 7.60 cm from the baseline. The spot corresponding to dye E travels 2.66 cm. Calculate the Rf value for dye E. Reference standards run under the same conditions give Rf values: D = 0.25, E = 0.35, F = 0.60. State which dye matches the value found, and explain why the reference standards must be run under identical conditions to the unknown sample for a valid comparison. Suggest one precaution that should be taken when spotting the dye mixture onto the TLC plate to obtain reliable results.
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