Figure 23 Apparatus for Generating Diazomethane for Esterification
The diazomethane is passed into the solution of the sample being derivatized until a yellow color persists. 0.5 to 30 mg of acid can be used which is dissolved in 2 ml of a 10% solution of methanol in diethyl ether. The nitrogen flow is about 4 ml/min and after the reaction is complete, the ether methanol mixture is removed by evaporation, leaving the ester derivative ready for analysis. The derivatization procedure does not work well with phenolic acids as the hydroxyl group is also methylated. Diazoethane can also be used employing a similar technique.
Both alcohols and acids react with silyl reagents to form trimethylsilyl ethers and trimethylsilyl esters respectively. These derivatives are volatile and allow the derivatives to be easily separated. The two reagents, N,N–bis(trimethyl–silyl)trifluoroacetamide (BSTFA) and bis(trimethyl-silyl)acetamide (BSA) react rapidly with organic acids and give high yields; the latter reagent is used if an electron capture detector is being employed. A few milligram of the acid is placed in a vial and about 50 ml of BSA or BSTFA added. Reaction is claimed to be complete on solution of the acid, but some recommend the mixture is heated for 5 to 10 min. at 60°C to ensure reaction is complete. The mixture can then be injected directly onto the gas chromatograph.
Tert-butyldimethylsilyl esters (TBDMS) are also used particularly for GC/MS. TBDMS esters are prepared by dissolving 5 mg of the acid in 100ml of dimethylformamide containing 20 mmol of imidazole and 10 mmol of TBDMS. The mixture is heated to 60°C for 15 minutes, an equal volume of 5% NaCl is added and the esters extracted with 1 ml of ether.
The halogenated silyl esters are also useful for producing silyl esters, for example, the chloromethyldimethyl silyl (CMDMS) and bromo-methyldimethylsilyl (BMDS) esters. A few milligram of the acid is dissolved in 600 ml of pyridine (a scavenger for released HCl) and 200 ml of di(chloro-methyl)tetra methyldisilazane and 100 ml of chloro-methyldimethylsilyl chloride is added. Reaction is allowed to proceed for 30 minutes at room temperature.
