Gas Chromatography - Quantitative Analysis > Derivatization > Page 58

 

Figure 37 Apparatus for Generating Diazomethane for Esterification

Nitrogen is bubbled through ether so that the gas is saturated with ether and then passed through a vessel containing a solution of N–methyl–N–Nitroso–p toluene sulfonamide. When potassium hydroxide, is added through a dropping funnel, diazomethane is generated. The nitrogen containing the diazomethane is passed into a solution of the sample until a yellow color persists. The sample solution consists of 0.5 to 30 mg of acid dissolved in 2 ml of a 10% solution of methanol in diethyl ether. About 4 ml/ min. of nitrogen is used and after the reaction is complete, the solvent is removed by evaporation. Due to the methylation of the hydroxyl groups the procedure does not work well with phenolic acids. Diazoethane can also be used employing a similar technique.

Silyl reagents will react with both alcohols an acids to form trimethylsilyl ethers and trimethylsilyl esters respectively. These derivatives are volatile and for the most part, are easily separated. The two most popular reagents are N,N–bis(trimethyl–silyl)trifluoro-acetamide (BSTFA) and bis(trimethylsilyl)–acetamide (BSA). Each react rapidly with organic acids to give high yields; the latter reagent is often used when an electron capture detector is employed. A few milligram of the acid is placed in a vial and about 50 ml of BSA or BSTFA added. Reaction can be expected to be complete directly on solution, but the mixture can be heated for 5 to 10 min. at 60˚C to ensure that reaction is really complete. The reaction mixture can be injected directly into the gas chromatograph.