BSTFA + TMCS

Highlights

• Trimethylsilyl donor strength approximately equal to BSA
• Increased volatility of reaction byproducts mono(trimethylsilyl)trifluoroacetamide and trifluoroacetamide, over corresponding nonfluorinated compounds from BSA
• Increased volatility makes it possible to derivatize smaller molecules with which the TMS derivatives elute with the byproducts from BSA
• React with the same classes of compounds as BSA, producing the same derivatives

Applications

• Addition of 1% TMCS aids in derivatizing amides, many secondary amines and hindered hydroxyls that are not derivatized by BSTFA alone
• Excellent derivatization reagent for analyzing drugs of abuse (THC Metabolites, Morphine and PCP)
• Can substitute for BSA and BSA + 1 % TMCS in many derivatization techniques

BSTFA is an effective trimethylsilyl donor with donor strength approximately the same as its unfluorinated analog BSA, N,O-bis(trimethlysilyl)acetamide. It reacts with a wide range of polar compounds to replace labile hydrogens with a -Si(CH3)3 group. Therefore, it is used to prepare volatile and thermally stable derivatives for gas chromatography and mass spectrometry.

One of the particular advantages of BSTFA over many of the other silylating reagents is the volatility of its by-products, mono-(trimethylsilyl)trifluoro-acetamide and trifluoroacetamide. For example, in the gas chromatographic analysis of some of the lower boiling TMS-amino acids and TMS-Krebs cycle acids, the retention times of these derivatives cause them to be co-eluted with the by-products from most TMS derivatization reagents. Good chromatographic separations can be obtained with BSTFA, as the by-products from this reagent usually elute with the solvent front.

Amides, many secondary amines and hindered hydroxyls will not be derivatized completely by BSTFA alone; however, when a catalyst such as TMCS is added, many of these compounds can be derivatized satisfactorily. The mechanism for the catalytic effect of TMCS is not well understood; however, there is little doubt that the addition of the relatively weak silyl donor, TMCS, to BSTFA will enhance the donor strength of the stronger donor, BSTFA. The TMCS may participate through the formation of a reactive intermediate. Clearly, in those cases where amounts of TMCS up to 20% are used,7 the TMCS is not acting in a purely catalytic role.

The donor strengths of BSA and BSTFA are comparable and the reactivity enhancement from the addition of TMCS appears to be similarly comparable. Therefore, it is generally safe to assume that whenever a procedure calls for BSA + TMCS, BSTFA + TMCS can be substituted. This substitution is particularly appropriate when the peaks of interest have relatively low retention times and tend to be obscured by the derivatization reagent or the primary reaction products from the derivatization reagent. In some cases the combination of BSTFA and TMCS is a more powerful silyl donor than the comparable BSA and TMCS solution. In most cases the addition of 1% TMCS is sufficient to achieve the desired derivatization. If after using this reagent under forcing conditions (150°C for 12 hours) it appears that derivatization is not complete, additional TMCS may be added up to a final concentration of about 30%.

BSTFA + 1% TMCS or BSTFA + 10% TMCS can be used at full strength or diluted with a suitable solvent such as pyridine. In most applications it is advisable to use an excess of the silylating reagent, and at least a two to one molar ratio of BSTFA + TMCS per active hydrogen is recommended. Best results are obtained when the products of the silylation reaction are soluble in the final reaction mixture.

References