Mass Spectrometry for the Detection of Chirality

One of the critical aspects of this approach is that two different experiments have to be performed between which the particular instrument conditions must be carefully kept constant in order not to affect the intensity ratios. This problem can be overcome by the enantiomer-labeled guest method [47]. It is based on the mass spectrometric examination of one enantiomer of the host with a pseudo-racemic mixture of the guest. In order to be able to detect both diastereomers separately, one enantiomer of the guest must be isotopically labeled, usually with deuterium. In the same experiment, both diastereotopic complexes are formed and their intensities can be compared directly. However, the stereochemical effect might additionally be superimposed by an unknown isotope effect. A way to separate stereochemical and isotope effects is to perform the same experiment with the second host enantiomer [4B]. In one experiment both stereochemical and isotope effects disfavor the same complex and thus work in the same direction. In the other experiment, they partly cancel each other. If both experiments have been performed, one can use the two experimental values for the intensity ratios of both diastero-meric complexes to deconvolute both effects [49]. 

The diastereoselectivities observed in the MS experiments for some systems differ from those found in solution. Furthermore, some cases have been found, where different ionization methods, e.g. FAB versus ESI [50], gave rise to completely different diastereoselectivities. These findings point to the fact that the ionization procedure might alter the ratios of the species present in solution. Therefore, an approach using true gas-phase experiments would be advantageous. Several complexes of chiral ammonium ions with the chiral crown ethers in Fig. 5.20 have 

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