Solid Phase Extraction (SPE) of Pesticide
Residues with CarbonX for QuEChERS
Graphitized carbon black (GCB) is a sorbent that is widely used in solid phase extraction (SPE) to extract polar and non-polar pesticides/compounds from water samples. GCB struggles with planar pesticides such as Chlrothalonil in this type of application. CarbonX for QuEChERS on the other hand demonstrates excellent recoveries of water soluble pesticides including planar pesticides such as Chlorothalonil.
2. SUMMARY OF PROCEDURE
Extract pesticides including one planar pesticide in water (pH 2) with 1 g CarbonX for QuEChERS in 6 mL cartridges; elute with ethyl acetate and dichloromethane; dry extract with sodium sulfate and concentrate to 1 mL; and analyze by GC/MS under full scan mode.
3. STANDARDS AND REAGENTS
1000 ppm semi-volatile pesticide mixture and 5000 ppm triphenyl phosphate (TPP) were purchased from Cerilliant (Round Rock, TX). Chlorothalonil, dichlofluanid, dicofol, tolyfluanide, captan, and folpet were purchased from Chem Service (West Chester,PA). GC grade ethyl acetate (EA), dichloromethane (DCM), and HPLC grade methanol (MeOH) were purchased from Spectrum (New Brunswick,NJ). 6 N hydrochloric acid (HCl) was purchased from Ricca Chemical Company (Arlington,TX).
4. PREPARATION OF SOLUTIONS
50 ppm standard mixture was made by mixing 200 µL of the 1000 ppm semi-volatile pesticides mixture, 200 µL each of the 1000 ppm chlorothalonil, dichlofluanid, dicofol, tolyfluanide, captan, and folpet with 2.6 mL MeOH. 50 ppm TPP used as internal standard (IS) was made by mixing 40 µL of the 5000 ppm TPP solution with 3.96 mL of EA.
GC/MS: Agilent 6890N GC coupled with 5975C MSD, equipped with 7683 auto sampler.
6.1 SPE cartridges:
A. CarbonX for QuEChERS: Pack 1 g of CarbonX for QuEChERS in 6 mL cartridge.
1. Spike 100 µL of the 50 ppm pesticide mixture into 100 mL of water, adjust pH to less than 2 with 6 N HCl;
2. Condition the cartridges with 10 mL of MeOH and 10 mL of DI water;
3. Load samples at a fast dropwise fashion (about 10 mL/min);
4. Wash the cartridges with 10 mL of DI water;
5. Dry the cartridges under full vacuum for 10 min;
6. Elute the cartridges with 6 mL of EA; and 6 mL of DCM;
7. Dry the extracts with sodium sulfate held in blank cartridges; rinse test tubes and salt with DCM;
8. Concentrate to 0.9 mL under a gentle stream of nitrogen at 35 ºC;
9. Add 100 µL of the 50 ppm TPP (IS), inject 1 µL to GC/MS for analysis.
7. INSTRUMENTAL METHOD
Injector: 1 µL splitless injection at 250 ºC, 30 mL/min split vent at 1 min
Liner: 4 mm splitless gooseneck with deactivated glass wool, 4mmID*6.5mmOD*78.5mm
GC capillary column: Restek Rxi-5sil MS 30m*0.25mm*0.25µm integrated with 10m guard column
Oven temperature program: Initial temperature of 55 ºC, hold for 1 min; ramp at 10 ºC/min to 200 ºC; ramp at 7 ºC/min to 320 ºC and hold for 0.36 min. Start data acquisition at 7 min.
Carrier gas: Ultra high purity Helium at a constant flow of 1.2 mL/min.
MSD: Transfer line: 280 ºC;
MS Source (ESI+): 250 ºC;
MS Quad: 150 ºC
Full Scan: 45-500 amu
Figure 1. Recoveries of pesticides using CarbonX for QuEChERS cartridges. All recoveries are higher than 80% including planar pesticide Chlorothalonil.
All recoveries are at or above 80%, even that of Chlorothalonil, a planar pesticide that commonly used carbons such as GCB have difficulty recovering.