## 3 Results and Discussion
### Glyphosate Extraction  
The extraction of glyphosate from the soil matrix had to be carried out via some modifications to the extraction solvent for immersion SPME. After the three hours of extraction, the SPME fiber was removed from the soil slurry and retroextracted onto ethanol. Concurrently, the glyphosate extraction was performed. There were two possibilities explored: addition of potassium hydroxide (KOH) [3] or ammonia solution [17]. The ammonia was chosen because it could be almost entirely removed by drying the extract. KOH, on the other hand, could not be removed from the extract, and although it has a relatively low solubility in ethanol (as does KCl, which is formed after neutralization with hydrochloric acid), it is still sufficient to cause accumulation in the GC liner and eventual clogging and chemisorption. Thus, when an extraction with KOH was assayed (0.6 molar, added dry to the extraction slurry), an extra step of extraction after the derivatization was required. Furthermore, whether neutralization with hydrochloric acid was performed or not, the large amount of salts interfered with the derivatization step, especially at low concentrations of glyphosate. The sample-to-extraction solvent ratio was slightly modified from a previous work [17], except only one extraction was performed instead of two. Using a larger sample size (e.g., 3 or 4 g) while retaining a similar extraction solvent volume could have been employed in an attempt to improve detection and quantification limits for the multiresidue pesticides extracted with DI-SPME, but would have been operationally more challenging (especially in terms of shaking), and might have compromised repeatability and extractability in terms of the soil-solvent equilibrium, even though it would ultimately result in a larger analyte mass being extracted onto the SPME fiber. Also, this could compromise the glyphosate recovery, unless a larger extraction solvent volume was added after DI-SPME. After the extraction, a small amount of dichloromethane had to be added to the extract because it was found that it contained non-negligible amounts of the other analytes, and their presence in the aqueous solvent could compromise repeatability. The simple centrifugation of the extract both removed suspended soil particles, thus eliminating the need for filtration, and settled the dichloromethane layer. However, for finer soils (notably clay), it may be required to centrifuge at higher speeds for longer. Neutralization of the extract with hydrochloric acid before centrifugation (in order to precipitate some soil matrix components) was not feasible because of the formation of non-volatile salts which could not be removed in the drying step. For derivatization of the glyphosate extract, two methods were tested: silylation with N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide (MTBSTFA) co-dissolved with acetonitrile [18; 19], and simultaneous acylation and esterification with trifluoroacetic anhydride (TFAA) and trifluoroethanol (TFE) [17; 20]. The silylation with MTBSTFA suffers from several drawbacks, including high reactivity of the reagent with water (more problematic than for TFAA), high cost per sample, and poor reaction yields for trace amounts of glyphosate [21], possibly caused by low solubility of the underivatized compound and adsorption to the glass vial. Furthermore, neither MTBSTFA nor the other reaction products are sufficiently volatile to allow drying of the mixture after derivatization, which compromises the subsequent combination with the SPME extract. Thus, derivatization with TFAA + TFE was chosen. One of the drawbacks of this approach was that the glyphosate derivative was much more volatile than the other pesticides analyzed. This was even more pronounced for glyphosate's degradation product aminomethylphosphonic acid (AMPA), which was also tested in this method. When a starting GC oven temperature of 50 \({}^{\circ}\)C was used, AMPA's derivative showed poor peak shapes with significant variability between injections of the same extract.