Thiabendazole – Planar Pesticide Loss to Graphitized Carbon Black during Dispersive Solid Phase Extraction Cleanup for QuEChERS

 Anyone who has done QuEChERS for pesticide residue sample preparation knows that a critical part of the approach is the novel dispersive solid phase extraction (dSPE) cleanup, whereby the extract is simply shaken with loose sorbent material to remove matrix interferences and then centrifuged.  An aliquot of the cleaned extract is then removed for analysis by LC and/or GC.  Sorbents can include primary secondary amine (PSA, which removes fatty acids and sugars), C18 (which removes lipids, some sterols, and some pigments), and graphitized carbon black (GCB, which removes chlorophyll and other pigments).  GCB is especially useful for chlorophyll removal, since chlorophyll is a non-volatile material that, when injected into a gas chromatograph, quickly degrades GC inlet and column performance.  Chlorophyll can also have a negative impact on LC-MS/MS analysis by dirtying the source prematurely, so recently when analyzing some QuEChERS marijuana extracts for pesticides, we employed GCB dSPE.  We used 3 dSPE formulations that contained 2.5, 7.5, and 50 mg GCB, along with MgSO4 and PSA, and visually noted removal of chlorophyll (in the figure inset photo, from left to right are: untreated QuEChERS marijuana extract, extracts treated with 2.5, 7.5, and 50 mg GCB).  Importantly, some planar pesticides are subject to loss on GCB, but a rule of thumb is that if there is color left in the extract, the planar pesticides won’t be lost.  We haven’t always found that to be the case, as noted in this experiment, where even with color left in the “7.5 extract”, we saw loss of Thiabendazole (about 50% versus the 2.5 extract).  By the time we got to 50 mg of GCB, we lost almost all Thiabendazole.  I guess you could say we now thumb our nose at that particular rule…

Loss of Thiabendazole to GCB when using dispersive solid phase extraction cleanup of a QuEChERS extract of marijuana. Inset photo (from left to right): untreated extract, 2.5 mg GCB treated extract, 7.5 mg GCB treated extract, 50 mg GCB treated extract.

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