Specific derivatizing methods

Specific Derivatization Methods Reactions to Alter Structure... 

The development of new fiber coatings in the near future should further improve the specificity of SPME and overcome some of the observed matrix effects. Quantification by stable isotope dilution gas chromatography/mass spectrometry (GC/MS) may assist in improving analytical performance. Along with the possible application of micro LC and capillary LC columns to in-tube SPME, the development of novel derivatization methods and the potential for the analysis of fumigant pesticides, SPME appears to be a technique with a future in the analysis of pesticide residues in food. 

As mentioned earlier, the response of each protein will vary. This is especially apparent with colorimetric assays or derivatization methods requiring a chemical reaction. These protein-to-protein reactivity differences mean that a protein assay suitable for one protein may not be suitable for another. Even for a given protein and a specific protein determination method, results may still vary based on limitations of the assay. Methods requiring extensive sample preparation including protein concentration, buffer exchange, and time-sensitive reactions are liable to be less reproducible than direct measurement techniques, which have fewer variable parameters. The application will determine the suitability of the method. 

Niacin can be detected by UV, ED, or FLD. UV is a widespread technique but it needs a longer preparation step and it does not reach high sensitivity. The FLD is more sensitive but it needs a pre or postcolumn derivatization to make niacin fluorescent. Krishnan et al. [599] describe a postcol-umn derivatization using cyanogens bromide and p-aminophenol, but this method involves toxic reagents. Mawatari et al. [600], instead, propose a fast, highly specific derivatization procedure, which involves UV irradiation at 300 nm in the presence of hydrogen peroxide and copper(II) ions. 

In short, the spectroscopic methods appear to be reliable and specific for HCHO. The derivatization methods are generally in reasonable overall agreement with the spectroscopic methods where intercomparisons have been carried out, but there can be very large discrepancies in individual measurements. Part of the reason for these discrepancies may be related to the fact that some of the spectroscopic methods average over long distances whereas the derivatization methods sample at a point. On the other hand, the latter methods involve collecting the sample over a period of time, usually several hours, whereas the spectroscopic methods are real-time measurements. Finally, variations in collection efficiencies and possible interferences must be taken into account for the derivatization methods. 

This book provides a practical guide to various aspects of lipid analysis, covering topics from sample preparation (extraction, fractionation, and deri-vatization) to CC analysis. Various derivatization methods are discussed and specific procedures are given for each of them. The book provides a comprehensive overview of GC technology including instrumentation (i.e., column, oven, carrier gas, injector, and detector) and data collection. 

M Lehr, W Schmid. Simple and specific HPLC method for determination, after derivatization to N,N-dichlorocyclohexylamine, of cyclamate in beverages containing fruit juice. Z Lebensm Unters Forsch 192(4) 335-338, 1991. 

Introduction to Derivatization For many years derivatization by alkylation -especially as ethylation, but also as propylation - has been applied to transform Hg species into volatile Hg species before measurement with hyphenated techniques [2, 50, 52]. Sodium tetraphenylborate (NaBPh4) was also used for derivatization prior to measurement with GC-MIP-AED [53], Studies of possible species transformation, for example, during the analytical procedure, have been carried out with isotope-specific determination methods. The results showed that a direct ethylation of Me-Hg in an atmospheric precipitation sample by NaBEt4 produced no significant amount of artifactual Me-Hg [54]. Others investigated the species transformation processes in synthetic solutions to simulate environmental matrices. From the experiments it could be concluded that the species conversion, for example, of Me-Hg into zerovalent Hg, depends on the concentration levels of the halide [2]. Furthermore, the procedural order is of great importance, for example, ethylation should be done after addition of the organic phase to avoid species transformation [55]. 

Compared to LC/MS, GC/MS methods are applied to a much smaller extent, mainly because many pharmaceutical compounds cannot be eluted from a GC column without prior derivatization steps. However, GC/MS is still the method of choice for the analysis of small molecules that can be evaporated without decomposition due to its unmatched chromatographic resolution and highly reproducible, compound-characteristic EI-MS spectra [40]. Whereas compound-specific derivatization is time-consuming, the GC/MS analysis itself can be very fast, as a review by Leclercq and Cramers demonstrates [41 ]. 

ISO-DALT uses protein-specific detection methods such as autoradiography and dye-binding, e.g., with Coomassie Brilliant Blue. Iso-tachophoresis and HPLC both use UV-absorption for the detection of proteins, the equipment for the former being usually at a fixed wavelength and for the latter a variable wavelength. These detection methods are sensitive but not specific. With HPLC, there is the possibility of pre-or post-column derivatization, e.g., with fluorescamine, which may improve both sensitivity and specificity. 

In contrast to liquid chromatography which is the technique of choice when considering high molecular mass constituents, gas chromatography (GC) is well-suited for the fractionation of low molecular mass species which are volatile, thermally stable and preferably neutral. Some species that do not already possess the above properties can be converted into forms that are amenable to GC. Various derivatization procedures that can be used to perform such conversions have been described by Poole and Schuette (1984). However, it Is worth mentioning that hydride formation and alkylation are the two most commonly used derivatization methods that have found application in the study of the speciation of various elements like As, Bi, Ge, Hg, Pb, Sb, Se, Sn, Te and Tl. Two points have to be considered when derivatization is performed first, the specificity of the chemical conversion second, the percentage yield of the reaction. A specific reaction is desirable in order to avoid the introduction of artefacts. Moreover, if a quantitative estimate of the amount of the original species is required, then it is essential that the extent of the conversion is known. 

Table 1 Specificity and sensitivity of precolumn derivatization methods for sugars...

One of the main, but not sole, purposes of derivatization is the transformation of non-volatile compounds into volatile derivatives. Each chromatographic method [GC, GC/ mass spectrometry (MS), high-performance liquid chromatography (HPLC), capillary electrophoresis (CE), etc.] being supplemented by derivatization of analytes permits us to solve some specific problems. The principal among them are summarized briefly in Table 1 more detailed comments follow. Some of the derivatization methods mentioned can also be used in mass spectrometry, which includes no preliminary chromatographic separation of analytes, but there are special derivatization techniques... 

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