Derivatization chemical modification

Grafting reactions alter the physical and mechanical properties of the polymer used as a substrate. Grafting differs from normal chemical modification (e.g., functionalization of polymers) in the possibility of tailoring material properties to a specific end use. For example, cellulose derivatization improves various properties of the original cellulose, but these derivatives cannot compete with many of the petrochemically derived synthetic polymers. Thus, in order to provide a better market position for cellulose derivatives, there is little doubt that further chemical modification is required. Accordingly, grafting of vinyl monomers onto cellulose or cellulose derivatives may improve the intrinsic properties of these polymers. 

Many of the chemical derivatization methods employed in these strategies involve the use of an activation step that produces a reactive intermediary. The activated species then can be used to couple a molecule containing a nucleophile, such as a primary amine or a thiol group. The following sections describe the chemical modification methods suitable for derivatizing individual nucleic acids as well as oligonucleotide polymers. 

Peptide Derivatization. The sequence information obtained from a peptide can be significantly improved by chemical modification. These approaches are discussed in more detail in the following section. 

GC provides separations that are faster and better in terms of resolution than the older chromatographic methods (see Chapter 14). It can be used to analyze a variety of samples. However, many samples simply cannot be handled by GC without derivatization, because they are insufficiently volatile and cannot pass through the column or they are thermally unstable and decompose under conditions used for GC separations. It has been estimated that only 20% of known organic compounds can be satisfactorily separated by GC without prior chemical modification of the sample. 

Derivatization by modification of substrate chemical functionalities may be indicated for three reasons 74 ... 

There are three major ionization sources now used for steroid LC-MS electrospray ionization MS (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI). Under optimal conditions, all ionization methods have similar absolute sensitivities. ESI is preferred for polar and charged (steroid conjugates) molecules, but derivatization or chemical modification of less-polar molecules may be necessary for this technique to match the sensitivity of APPI or APCI. 

MFA by simple chemical modifications, such as N-l derivatization, oxidation, reduction etc., were not successful. 

Derivatizing the neutral sample molecules to form ionic species enhances their secondary ion yield. Derivatization of the neutral sample molecules can often be accomplished by simply adding acid or base to the sample solution, or through the chemical modification of specific functional groups of the molecules, e.g., quaternization reactions [20, 89]. 

In most of these applications, silica has the role of a support material. The popularity of silicon-based supports for multiple modification applications is well summarized by Mottola.1 Chemical modification requires seemingly paradoxal support properties (a) supports need to have a surface hydrophilic in nature but also to be insoluble in aqueous solutions and polar solvents (b) supports are required, in many instances, to be porous but retain mechanical stability and (c) they must be chemically stable but easily derivatized. Silica gel meets all of these requirements. 

The availability of different types of amino groups allows a variety of reactions typical for amines which are suitable for the derivatization of PEI. Reaction partners can be aldehydes, ketones, alkyl halides, isocyanates and thioisocyanates, epoxides, cyanamides, guanides, ureas, acids, and anhydrides. The synthesis and chemical modification reactions of PEI will be discussed in the next sections. 

Liu and Hop [19] have reviewed various LC-tandem MS strategies, with or without chemical modification or derivatization, which have been successfully applied in the identification and the rational, but tentative, structural determination of drug metabolites. These techniques are equally applicable to the analysis of known plant secondary metabolites or to the de novo structural identification of new compounds. 

Microderivatization or chemical modification of analytical samples can be useful in enhancing the ion response, and hence the LC-MS sensitivity, for any particular analyte. In the case of alkaloids, which are already capable of ready ionization, chemical modification can be more valuable in elucidating structures by providing tandem MS evidence for positions of substitution in the parent molecule. Such derivatization or chemical modification for HPLC-MS can include H/D exchange by using deuterated mobile phases, Jones oxidation of aliphatic hydroxyls, selective acetylation of hydroxyl and amine groups, and N-oxide reduction [19,20]. 

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