Gas Chromatography - Quantitative Analysis > Derivatization > Page 55

 

The water sample was pumped through the extraction tube at a rate of 100 ml/min. The solutes removed were displaced from the extraction tube with 10 ml of methanol followed by 10 ml of (MTBE) and dried over anhydrous sodium sulfate. It is seen that all the chlorinated pesticides were extracted and concentrations down to 1 ppb could be easily identified.

Derivatization

GC samples are usually derivatized to render highly polar materials sufficiently volatile so that they can be eluted at reasonable temperatures without thermal decomposition or molecular re-arrangement. Examples of such materials that need to be derivatized are the organic acids, amides, poly hydroxy compounds, amino acids etc. In order to render such materials more volatile, they are either esterified, silanated or acetylated using one of a number of different methods of derivatization.

Acids can be esterified by treating them with an appropriate alcohol using an inorganic acid to catalyze the reaction. Hydrochloric acid was popular for this purpose because it's strength was adequate and any excess could be easily removed. Sulfuric acid is not very suitable as it can cause charring and any excess is difficult to remove. Other catalysts that have been found effective are trifluoroacetic acid, dichloroacetic acid, benzene sulphonic acid, p-toluene sulphonic acids and suphuryl and thionyl chlorides. A volatile acid is recommended such as hydrochloric acid or thionyl chloride. However, the derivative must be must be sufficiently involatile not to allow loss when removing the excess alcohol and where appropriate the catalyst itself. A general method would be to treat one or two milligrams of the acid contained in a small with 125 ml of either methanol or ethanol that contains 3M hydrochloric acid and heat at 65˚C for about 35 minutes. A stream of nitrogen is bubbled through the reaction mixture to remove the alcohol. It is clear that the derivative must be sufficiently involatile, (i.e., has an adequately low vapor pressure) to prevent any loss during the removal of the alcohol.