## 3 Results and Discussion
### Combined Method and Performance  
For the combined method, it was imperative to ensure proper dissolution of the derivative in the ethanol SPME extract. Sonication was employed in an attempt to ensure full dissolution, and thus, good repeatability. The full method was also tested for two phenoxy herbicides: 2-methyl-4-chlorophenoxyoxyacetic acid (MCPA) and 2,4-dichlorophenoxyoxyacetic acid (2,4-D). It was possible to extract these compounds along with glyphosate due to the fact that at the extraction pH (around 12) the carboxylate is dominant, with almost no neutral carboxylic acid molecules present (calculated from the phenoxyacetic acid pKa). However, the repeatability was poor at every concentration (20-100 \(\mathrm{\SIUnitSymbolMicro g}\)/kg), with coefficients of determination of 0.87 for MCPA and 0.88 for 2,4-D. This was likely caused by the extraction itself, or by the fact that a certain amount of the analytes might have been dissolved in the dichloromethane phase. Naturally, the isotopically labeled glyphosate was very poor at correcting extraction variability, but it may be possible to analyze these compounds with this methodology by using another internal standard which is chemically similar to them, although adding more isotopically labeled compounds to a method will significantly increase its cost per sample. For AMPA, there was also significant lack of repeatability (R\({}^{2}\) = 0.91), but this was probably also due to the poor chromatographic performance, since the volatile AMPA derivative is not compatible with the programmed temperature volatilization injection technique used. Table 2 presents the method performance parameters. The extraction recovery for glyphosate was 92%, with a relative standard deviation of 12% (n = 3). Although for some compounds the internal standard is essential, for others (such as tebuconazole and tefluthrin) it did not increase repeatability. Glyphosate had a lower limit of detection, likely because of the greater concentration factor. Even though SPME is known for greatly concentrating samples, that effect is not present in this method, because instead of direct desorption onto the GC inlet (which transfers the whole extracted analyte mass onto the GC-MS), it employs solvent desorption onto 120 \(\mathrm{\SIUnitSymbolMicro L}\) of ethanol, of which only 5 \(\mathrm{\SIUnitSymbolMicro L}\) is injected. Nevertheless, in the future an increase in soil sample mass (e.g., to 4 g) might be an interesting avenue for exploration; it could lower limits of detection, although it could also increase RSDs. Another weakness of the method is that it does not take advantage of the inherent clean-up performed by SPME, since the SPME extract is then mixed with the glyphosate extract, whose only clean-up was the addition of dichloromethane. This results in a final sample with much more matrix interferets than the SPME extract alone, although these were not noticeable in the MRM chromatogram. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Analy** & **LoD** & **LoQ** & **R\({}^{2}\)** & \%RSD** & \%RSD with IS \\ \hline Aldrin & 1.0 & 20 & 0.989 & 28 & 13 \\ Suprofezin & 1.0 & 20 & 0.983 & 36 & 10 \\ Chlorpyrifos & 0.1 & 1.0 & 0.988 & 29 & 6.9 \\ Lindea & 1.0 & 20 & 0.981 & 21 & 22 \\ Malathion & 0.1 & 1.0 & 0.982 & 19 & 17 \\ Metazachlor & 1.0 & 20 & 0.954 & 17 & 18 \\ Metolachlor & 0.1 & 1.0 & 0.992 & 21 & 13 \\ Penconazole & 0.1 & 1.0 & 0.994 & 26 & 3.0 \\ Pentachloroenisole & 1.0 & 20 & 0.990 & 18 & 15 \\ Prosulfocarb & 1.0 & 20 & 0.979 & 27 & 2.5 \\ Tebuconazole & 1.0 & 20 & 0.965 & 35 & 37 \\ Tefluthrin & 0.1 & 1.0 & 0.986 & 35 & 37 \\ Tetraconazole & 1.0 & 20 & 0.990 & 26 & 5.6 \\ Glyphosate & 0.01 & 1.0 & 0.991 & 26 & 5.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Method performance parameters. Limit of detection (LoD) was the lowest calibration concentration with a signal-to-noise ratio greater than 3, and limit of quantitation (LoQ) was that which was greater than 10. Values for LoD and LoQ in \(\mathrm{\SIUnitSymbolMicro g}\)/kg.