Silylating Agents

Silylation is the most widely used method for metabolic profiling, especially for urinary organic acids. On the other hand its use for amino acids has a number of disadvantages as already mentioned. For some applications, silylating agents are too powerful in that they are able to react with compounds which will not elute from the column. The most popular reagents are listed in Table 3 and many others are described in detail in the Fluka handbook edited by van Look (see Further Reading). For metabolic profiling the TMS donors are used much more frequently than the TBDMS donors although the latter are more convenient for treating amino groups in those applications where greater hydrolytic stability is required. Reactivity of functional groups towards silylation is as follows: alcohols > phenols >acids >amines > amides. Disadvantages include the need to operate under

Table 3 Reagents for silylation: (I) most common TMS donors; (II) a TBDMS donor; and (III, IV) reagents for special application

(I) Trimethylchlorosilane (TMCS) \

First silylation reagent prepared (1944) Rarely used in analytical applications alone, rather in mixtures with HMDS and pyridine

Hexamethyldisilazane (HMDS)

One of the earliest (1957)

Favorable solvating properties for many compounds Not a strong silyl donor but more selective

N,0-Bis(TMS)acetamide (BSA)

First reported in 1963 as a more potent TMS donor;

usually in combination with TMCS (1-20%)

N, 0-Bis(TMS)trifluoroacetamide (BSTFA) /

Most widely used for silylation in general (prepared 1968) Very versatile, reacting with all the common protic sites in organic material present; more volatile by-products!

TMSimidazole (TMSIM)

Prepared in 1965; considered to be the strongest reagent available for silylation of hydroxyl groups

N-Methyl-N-TMS-trifluoroacetamide (MSTFA) 0

Introduced in 1969, has become one of the most important reagents; can be used for all protic groups and its by-products are more volatile than BSTFA

(II) N-Methyl-N-t-butyldimethylsilyl-trifluoroacetamide (MTBSTFA)

Alternative silyl derivatives for enhanced hydrolytic stability (1975/1980) increasingly popular, suitable even for amino acids

(III) Pentafluorophenyldimethylsilyl (flophemesyl) reagents halogenated reagents for EC detection

Si-NH2

F F I (1975), particularly suitable for analysis of sterols; silylation power: flophemesylamine > flophemesylchloride

(IV) N-Methyoxy-N,0-bistrimethylsilyl carbamate (BSMOC)

for simultaneous oximation & silylation at room temperature (reported 1986) as follows:

Si(CH3)3

anhydrous conditions, to heat the sample and to inject a reactive mixture onto the column. This means that the compounds of interest have to be isolated, the extraction medium evaporated and the column replaced more frequently. For further information on profiling see the review by Sweetman.

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