Choosing the right dispersive SPE for GC-MS/MS analysis of celery

I’ve recently started experimenting with QuEChERS extractions for pesticide analysis. The available options are overwhelming, especially when it comes to dispersive solid phase extraction (dSPE) for the cleanup. For starters, I’ve been looking at the smaller volume dSPE (2 mL, summarized in Table 1), because I didn’t want to waste the raw material, solvents and salts. Using 15 mL dSPE we’d be using 6-8 mL of extract, while 2 mL dSPE uses only 1 mL, which allows for more replicates per extraction.

Table 1: Summary of available dSPE

Table 1 includes cleanup materials covering a variety of matrices, however, the choice of cleanup should be ultimately based on analyte performance with the particular matrix and extraction techniques. The sample types listed above are good starting points for dSPE selection based on your matrix. I started with celery, which falls into the general fruits and vegetables category and is notorious for testing positive for residual pesticides, according to the Environmental Working Group. The dSPE recommended is either 50 mg or 25 mg PSA (#26124 and 26215, respectively), however, my colleague Landon, has had good experience with 25 mg PSA and 7.5 mg GCB (#26218, used for highly pigmented commodities). Because of this mismatch between the suggested dSPE and the one used previously, I’ve decided to try all the dSPE with exception of dSPE that targets foodstuff with fats and waxes. In order to streamline the analysis, I choose to use the QuEChERS performance mix (Cat #31152 or sold individually as #31153, #31154 and #31155), which covers a wide range of pesticides without overwhelming data processing.

Figure 1: Comparison of mean responses of pesticides using individual dSPE

Figure 1 shows a comparison of all of the chosen SPE. The dSPE meant for more pigmented samples (#26123 and 26219, both with 50 mg graphitized carbon black – GCB) have very low recoveries for planar pesticides, such as thiabendazole or chlorothalonil, compared to other analytes. Figure 2 shows the same comparison but limited to pesticides commonly found in celery.

Figure 2: Comparison of mean responses of selected pesticides using individual dSPE

Cutting down the extra pesticides helps with chart’s legibility, however, it doesn’t help with selection apart from excluding the dSPE with high CGB content. So I decided to use Cat #26218 (the one used by my colleagues previously) as a baseline to normalize the rest of the results. Table 2 shows a comparison of the recoveries normalized with recoveries when #26218 was used.

Table 2: Comparison of recoveries normalized by #26218 recoveries

Figure 3: Comparison of mean responses of pesticides using dSPE #26215 and #26218

From this analysis, the dSPE #26215 (150 mg MgSO4, 25 mg PSA) has the highest average and the highest minimum recovery (Table 2, Figure 3), which makes it the best choice for the chosen set of pesticides.

Last (but not least!) this approach can be used for optimization of QuEChERS salts, too (Figure 4)! We achieved the best extraction using the AOAC 2007.01 salts (6 g MgSO4, 1.5 g NaOAc).

Figure 4: Comparison of mean responses of selected pesticides using individual QuEChERS salts

 

Next time I will look into which dSPE is the best for spinach!

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