Người ta có thể phân biệt R-so với S-enantiomers GC-MS nhưng chỉ bằng một nỗ lực bổ sung đáng kể. Một số phương pháp được sử dụng bao gồm cả việc sử dụng của một cột chiral. Nói cách khác, sắc ký cột có thể được lựa chọn đặc biệt hoặc đặc biệt sửa đổi để nó nhạy cảm với enantiomers. | One can distinguish R- vs. S-enantiomers by GC-MS but only by a significant additional effort. Several methods are employed including the use of a chiral column. In other words the chromatographic column can be specially selected or specially modified so that it is sensitive to enantiomers. In one type of column for example L-valine tert-butylamide is covalently bonded to the polysiloxane backbone of a regular column. The modified packing forms different diastereoisomers as hydrogen bonds to the different enantiomers passing through the column. R- and S- forms have different retention times. The most common method for GC-MS differentiation of R- and S-methamphet-amine is by derivatizing the methamphetamine with a chiral derivatizing agent. For this purpose N-trifluoroacetyl-L-prolyl chloride is frequently used. The derivatizing agent contains a chiral carbon and the drug to be detected also contains a chiral carbon. Therefore the derivatized methamphetamine contains two chiral carbons. Substances with more than one chiral carbon may have diastereoisomers . structures that are not mirror images not enantiomers but they are stereoisomers of each other. These molecules are distinguishable from each other because some intramolecular forces are slightly different. Diastereoisomers often have different retention times on chromatographic columns and this enables us to distinguish between them. The derivatization reaction of methamphetamine with N-TFA-L-prolyl chloride is shown in Figure . OTHER MODES OF MASS SPECTROMETRY The most common modality in which mass spectrometry is used is in the electron impact mode. In this form high energy electrons strike molecules as they emerge from the gas chromatograph. This occurs in a very high vacuum. As described earlier mass spectra are produced that are usually quite specific for a particular compound. Electron impact mass spectrometry EI-MS is a superb technology that is satisfactory for the large majority of toxicological
