Choice of derivatives

In these procedures, the choice of derivatives and enzyme is important. Sometimes it is possible to get a D-amino acid which remains in the microbial culture supplemented with DL-amino acids (54). 

The choice of derivative is dictated by the exact problem under study. Frequently, it may be necessary to prepare both types of derivative in order to obtain the maximum structural information. The following factors need to be taken into account when choosing the most appropriate derivative. 

The situation often occurs where more than one derivative can be used to resolve a particular compound. For example, racemic ephedrine has been successfully resolved on type I CSPs as an oxazolidine (25), a naphthylurea (26), and a naphthylcarbamate (27). With such a wide range of possibilities, the choice of derivative can be based on either the availability of a particular type I CSP or the availability of various derivatizing agents. 

For GC analysis, several of these advantages may be combined by preparing, for example, heptafluorobutyrate esters of phenols or trichloroethyl esters of carboxylic acids. Although trimethylsilylation is widely used and gives derivatives with structurally valuable mass spedra, the trimethylsilyl ethers and particularly esters are often relatively unstable. The choice of derivatives should therefore take into consideration their suitability for the cleanup procedures that may subsequently be employed. 

Little work has, however, been done on the preparation of insolubilized antibiotics, but, in view of the success of initial trials, the field may be expected to develop quite rapidly. Many of the polysaccharide derivatives discussed in the present article may have antibiotic characteristics, although few have been tested for such. Furthermore, many of the polysaccharide derivatives used for insolubilization of other molecules (see Sections VIII-X, pp. 361-387) are suitable for insolubilization of antibiotics, the choice of derivative depending upon the functional groups available in the particular antibiotic substance under consideration. 

We believe that the choice of derivatives is representative for the materials intended for study here. Note the elimination of temperature memory otherwise the local equilibrium might be invalid (cf. Sects. 2.2,4.5). 

Alternatively, the scanned mass spectrum may be compared with a library of reference spectra, and many spectra of derivatives have been published for this purpose. This comparison step may be done by computer or manually using a variety of suitable reference publications. The comparison, when supplemented by the GC retention index, demonstrates the true power of GC-MS for the unambiguous identification of mixture components at trace levels. The choice of derivative may be influenced by that used in specialist libraries of reference spectra, e.g. methyl esters of organic acids. 

Perfluoroacyl derivatives such as trifluoroacetyl (TF.A), pentafluoropropionyl (PFP) and heptafluorobutyryl (HFB) are very commonly employed in GC-MS. Much of their popularity arose from their ease of preparation and their useful employment in GC with ECD. Workers then employed them directly in the GC-MS system and in many cases excellent results were obtained—high mass increments which can be conveniently adjusted by choice of derivative (see Table 3) and high abundance fragment ions. All this is in addition to their advantages for negative-ion chemical ionization mass spectrometry, where their electron capturing properties can enable highly sensitive analyses to be carried out. 

There is clearly considerable scof>e for the choice of derivative to achieve optimization of the assay prcvredure for an analyte or analyte group. Indeed, there is an excitement to be had from preparation of a new derivative and first sight of the mass spectral fragmentation pattern, particularly if it has the desired features ... 

As will be abundantly clear from the rest of this handbook, derivatization for chromatography is still most usually done before the chromatographic separation step, because the choice of derivative includes as one of its aims conversion to compounds with improved chromatographic properties. However, this chapter describes many situations where post-chromatographic derivatization has been found necessary or desirable, not only as an alternative to pre-chromatographic derivatization, but also as a complementary additional procedure. 

GC-MS was used to tentatively identify CLA in total FAME from erythrocyte membrane lipids from diabetic patients (78). Two peaks with mass spectra similar to that of linoleic acid methyl ester, but eluting later, were detected however, the choice of derivative and a nonpolar GC column, which poorly resolved the CLA isomers, was a limitation on the quahty of information. 

THE CHOICE OF DERIVATIVES FOR SOME OF TflE COM IONER CLASSES OF COMPOUNDS... 

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