Analytical Aspects

The technique of gas chromatography linked to mass spectrometry is an important tool in the study of volatile insect substances and also for the study of their biosynthesis. The subject of linked chromatographic-mass spectrometric methods, and the newer techniques that can be achieved with mass spectrometry are beyond the subject of this book. We can expect to see much wider use of high resolution mass spectrometry in the study of enzymes and biosynthetic pathways.

The position of double bonds in alkenes can be learned by reacting them with dimethyl disulphide (Figure 5.12), the products have characteristic strong fragment ions in their mass spectra, corresponding to cleavage at the S-C-C-S bond. Another way is through chemical ionization mass spectrometry, using special reagents such as acetonitrile.

mass i„ spectrometer r-ch—ch-r' + ch3ssch3 —2—- r-ch-ch-r' -- r~ch + ch-r'

ch3s sch3 ch3s+ +sch3

Figure 5.12 The use of dimethyl disulphide to determine double bond positions by gas chromatography-mass spectrometry

The position of methyl branches in mono-methylalkanes can also be deduced from mass spectra. As one progresses to more methyl groups per molecule, interpretation becomes more difficult. For a detailed consideration of methylalkane identification see Carlson, Bernier and Sutton, Journal of Chemical Ecology, 1998, 24, 1845.

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