Gas Chromatography - Quantitative Analysis > Acylation Reactions > Page 59
For GC/MS analyses Tert-butyldimethylsilyl esters (TBDMS) are recommended. The TBDMS esters are prepared by dissolving about 5 mg of the acid in 100 ml of dimethylformamide containing 20 mmol of imidazole and 10 mmol of TBDMS. The mixture is then heated to 60˚C for about 15 minutes, an equal volume of 5% NaCl is added and the esters extracted with 1 ml of ether.
Acylation is also a popular reaction for the production of volatile derivatives of highly polar and involatile organic materials. The technique, however, has a number of other advantages. In addition to improving volatility, acylation reduces the polarity of the substance and thus can improve the peak shape and, reduce peak tailing. As a consequence amide esters are usually well separated with symmetrical peaks. By inserting protecting groups into the molecule, acylation also improves the stability of those compounds that are thermally labile. Acylation can render extremely polar materials such as sugars amenable to separation by GC and, consequently, are a useful alternative to the silyl reagents. Acylation is frequently used to derivatize amines, amides, alcohols, thiols, phenols, enols, and glycols etc..
A typical example of anacylation is the reaction between acetic anhydride and an alcohol
R–CO–CO-R + R'–O–H = R–CO–O–R' + R-CO-O-H
About 5 mg the acid is dissolved in 5 ml of chloroform together with 0.5 ml of acetic anhydride and 1 ml of acetic acid and is heated for 2-16 hours at 50˚C. The excess reagents are removed under vacuum and the residue dissolved in chloroform and injected directly onto the column. Sodium acetate can be used as an alternative to acetic acid in which case, 0.3 ml of acetic anhydride is added to 12 mg of sodium acetate. The reaction is carried out at 100˚C for about an hour, excess reagent is removed by evaporation and the residue taken up in a suitable solvent for analysis. An excellent handbook describing a wide range of procedures used to produce derivatives for chromatographic analysis has been compiled by Blau and Halket (12).