Chemical derivatization techniques

There are ill-defined limits on EI/CI usage, based mostly on these issues of volatility and thermal stability. Sometimes these limits can be extended by preparation of a suitable chemical derivative. For example, polar carboxylic acids generally give either no or only a poor yield of molecular ions, but their conversion into methyl esters affords less polar, more volatile materials that can be examined easily by EL In the absence of an alternative method of ionization, EI/CI can still be used with clever manipulation of chemical derivatization techniques. 

Other spectroscopic methods such as infrared (ir), and nuclear magnetic resonance (nmr), circular dichroism (cd), and mass spectrometry (ms) are invaluable tools for identification and stmcture elucidation. Nmr spectroscopy allows for geometric assignment of the carbon—carbon double bonds, as well as relative stereochemistry of ring substituents. These spectroscopic methods coupled with traditional chemical derivatization techniques provide the framework by which new carotenoids are identified and characterized (16,17). 

Figure 1. Variation in number of publications dealing with chemical derivatization techniques for pesticides from 1962-1978 ((-)-) total (including fungicides) (A) herbicides (O OC s (including Mirex, PCB s, etc.) (Q) DP s)...

Over the years, several techniques have been developed to elucidate the structure of a new lantibiotic. The extensive post-translational modifications limit Edman degradation to a stretch of amino acids from the N-terminus to the first modification. Various chemical derivatization techniques have been used to allow continuation of the sequence and to reveal the position of the posttranslationaUy modified residues. Originally, these techniques relied on treatment with ethanethiol under highly basic conditions that result in elimination reactions of the thioethers... 

Chemical Derivatization Techniques for Confirmation of Organochlorine Residue Identity... 

The above brief review illustrates that chemical derivatization techniques have been used extensively for the confirmation of identity of organochlorine residues. In most instances, the lower limits of detectability of the derivatives are substantiaUy lower than the established tolerance values for the parent compounds. Taken in conjunction with the many other modes of derivatization—e.g., during or after gas chromatography (59)— the analyst has a vast array of modification procedures at hand to aid in residue identification. They can be employed for residues in soil, biological, fat, and nonfat extracts and can be successfuUy extended, especially the more specific tests, to the identification of crosscontaminants in pesticide formulations and also fertilizer mixtures. So far, these latter two cases have only been a fringe area of application (60,61). 

The chemical derivatization technique in combination with LC-MS/MS has also been used as an effective tool to facilitate the identification of metabolites, especially for unusual or unstable metabolites [277-279]. The MS detection sensitivity for poorly ionizable compounds can be increased by introduction of an easily ioniz-able functional group. Derivatization can reduce the polarity of very polar small molecular weight metabolites that elute very early together with many endogenous materials in the HPLC columns. As a result, the derivatized metabolite can be... 

Some of the variety of techniques described In the literature have resulted in the commercialization of modules independent of the chromatograph and providing it with different degrees of automation. Such modules are based on extraction (both liquid-liquid and solid-liquid), sorption (adsorption, ion exchange), vaporization, filtration (simple or through molecular sieves) or dialysis processes, or on chemical derivatization techniques. Some of these preliminary operations are better suited to HPLC, others to GC and the remainder equally to both. Only those involving the reduction of human Intervention to some extent are described here. This is a wide topic, so a comprehensive treatment is beyond the scope of this book. Below are described some representative examples of both HPLC and GC. Many of the systems described are based on the continuous separation systems dealt with in Chapter 4, devoted to the automation of sample treatment. The foundation of continuous and segmented flow analysers plays a major role in this context. 

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