Study of different parameters affecting the derivatization of acidic herbicides with trimethylsulfonium hydroxide to make them suitable for gas chromatography analysis

In this study, an orthogonal array design was applied to know the way different parameters affected the derivatization of some herbicides that are commonly applied in the soils. Herbicides formulated as esters have been reported to rapidly hydrolyse, in contact with soil, to their corresponding acids and phenols. What involves is that both forms need to be monitored. Acidic herbicides and phenols cannot be detected by gas chromatography (GC) due to their polarity and low volatility that cause peak asymmetry. Therefore, masking of these polar groups by eliminating the active hydrogen atom with derivatization to their corresponding esters/ethers is needed in order to yield products that possess enhanced volatility and improved GC properties. A lot of derivatization reagents have been proposed but trimethylsulfonium hydroxide (TMSH) was selected due to its easy and quantitative formation of methyl esters/ethers. It was observed that the addition of TMSH promoted not only esterification of acids/phenols but trans-esterification of the original non-hydrolyzed remaining esters to their corresponding methyl ones. As a result, methyl esters/ethers were the final product of both reactions. Different parameters were studied in the statistical design for both TMSH promoted reactions: type of solvent, pH, temperature and time of incubation. The amount of derivatization reagent was calculated to be high enough to ensure the complete derivatization of all compounds present in the sample. The reaction medium was shown as an important factor. The formation of some methyl esters/ethers decreased with increasing time and temperature because trans-esterification, being an equilibrium where the formation of smaller structures is promoted, was not enough shifted. However, the statistical analysis revealed that only the pH of the solution played an important role during the derivatization process. The presence of the anionic form of the acids appeared to be essential for derivatization, being diminished in strong acidic conditions. In addition, pre-heating was shown not to improve derivatization reaction, being easily carried on in the injector port of the GC system.