Page 204 - Tandem Techniques

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It is seen that a good separation is obtained from the individual impurities and so the actual cholesterol
content can be easily assayed and with the accompanying mass spectra the identification of the
individual impurities could be easily ascertained. The two major impurities were desmosterol (cholesta-
5,24-dien-3ß-ol) and lathosterol (5a-cholest-7-en-3ß-ol). Other impurities that were also found present
in certain batches were 5a-cholestan-3ß-ol, cholesta-3,5-dien-7-one and cholest-4-en-4-one. A number
of volatile lipids were also identified.

Some General Applications of GC/MS tandem Systems

Dugay et al. [14] employed a GC/MS tandem system to investigate the effect of hydrogen
rearrangement on the determination of [15N]leucine enrichment, when measuring the ratio of 15/14 N-
labeled leucine. Eleven different ester derivatives were examined, and it appeared that as result of the
type of alcohol that was used in the derivatization process, hydrogen rearrangement could occur. The
labeling ratio was found to increase with the length of the alkyl chain of the ester as well as with the
number of hydrogen atoms at the ß-site and, to a lesser extent, at the y-site on this chain.

The leucine derivatives were made according to the method of Mackenzie and Tenaschuk [9,10]. The
GC/MS system comprised the Fisons model MD 800 mass spectrometer used in conjunction with the
8000 series gas chromatograph. The capillary column employed was the DB-5, which was 30 m long
and 0.25 mm I.D. The mass spectra of the 1-propyl N (heptafluorobutyryl) and the 2-propyl N-
(heptafluorobutyryl) esters of leucine were easily identified and the same major ion fragments at m/z,
240, 241, 282 and 283, which also appear in the mass spectra of other amino acid derivatives were also
disclosed. Irrespective of whether hydrogen rearrangement takes place or not, it was shown that the N
15
abundance can be obtained from either the mass fragments 240 and 241 or 282 and 283, the N
15
15 14
fragments being shifted to 1 unit mass greater. For example, the / N-labeled leucine ratio can be
taken as proportional to the ratio of the 283 peak to the 282 peak. It is seen that GC/MS tandem systems
are useful for solving a range of analytical problems, and the