Quick questions on Mass spectrometry and IR spectroscopy (QCE Chemistry Unit 4)

Principle. A sample is vaporised, ionised by an electron beam, accelerated through an electric field, and deflected in a magnetic field. The deflection radius depends on mass-to-charge ratio (m/z); the detector records intensity vs m/z.

Principle. Each chemical bond vibrates (stretches and bends) at a characteristic frequency. When IR radiation of that frequency passes through a sample, the bond absorbs energy and the transmission drops. The transmission vs wavenumber spectrum has dips (absorptions) at the resonant frequencies, identifying which bonds are present.

A typical IA3 / EA stimulus gives you both spectra and asks you to propose a structure. The systematic approach:

MS strengths. Determines Mr exactly. Fragmentation pattern identifies many functional groups indirectly.

Misidentifying M+. Some compounds fragment so completely that M+ is very weak or absent. Look for the highest reasonable m/z; if there are isotope peaks (M+1, M+2), the M+ is the lowest of the cluster.

A sample is vaporised, ionised by an electron beam, accelerated through an electric field, and deflected in a magnetic field. The deflection radius depends on mass-to-charge ratio (m/z); the detector records intensity vs m/z.

The first ionisation event removes one electron from the parent molecule, giving the molecular cation M+. Its m/z equals the molecular mass (Mr) of the original molecule. The M+ peak is usually the highest m/z peak in the spectrum (occasionally with weak M+1 isotope peaks from 13C).

The molecular ion has high internal energy and fragments before reaching the detector, producing smaller cations and radical species. The radicals are not detected; the cations are. Common fragment losses for organic molecules:

The Mr from M+ narrows the possibilities. For example, Mr = 60 is most commonly C2H4O2 (carboxylic acid or ester) or C3H8O (alcohol or ether), depending on which IR absorptions are present.

Below 1500 cm^-1, IR spectra have many small peaks that are characteristic of the whole molecule. QCAA does not expect you to interpret fingerprint peaks individually, only to use them to confirm an identification by matching to a reference spectrum.

Determines Mr exactly. Fragmentation pattern identifies many functional groups indirectly.

Cannot distinguish isomers with identical fragmentation (rare for simple compounds, more common for larger ones). Requires sample volatilisation; not all compounds are stable to ionisation.

Fast, non-destructive, identifies functional groups directly. Routine in industry and IA2 / IA3 contexts.

Does not give Mr. Cannot count carbons or hydrogens. Cannot distinguish chain isomers (propan-1-ol vs 2-methylpropan-2-ol both show O-H and C-O). Pair with MS and NMR for unambiguous identification.

Some compounds fragment so completely that M+ is very weak or absent. Look for the highest reasonable m/z; if there are isotope peaks (M+1, M+2), the M+ is the lowest of the cluster.