Analyze organophosphorus pesticides in the apple matrix by GC/MS/FPD using an Agilent J&W DB-35ms Ultra Inert GC Column

Solutions and Standards

The OP pesticide stock standard solution (100 μg/mL of 12 OP pesticides) was diluted in acetone to yield spiking solutions of one and 10 g/mL. A surrogate standard, triphenyl phosphate, was prepared at concentrations of one, 15, and 100 g/mL in toluene. The spiking solutions were used to prepare the calibration curves in the matrix blank extract by appropriate dilution.

Sample Preparation

An organic apple sample was purchased from a local grocery store. The apple was chopped into small cubes and frozen at minus 80 degrees C overnight. The samples were then comminuted thoroughly to achieve sample homogeneity. The sample extraction method used the QuEChERS method. Figure 1 illustrates the sample preparation procedure graphically in a flow chart.

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Figure 2: GC/MS-SIM and FPD chromatograms of a matrix matched OP pesticides standard analyzed on an Agilent J&W DB-35ms UI 20 m × 0.18 mm × 0.18 µm capillary GC column (Agilent p/n 121-3822UI). Chromatographic conditions are listed in Table 1. The effluent split ratio is MSD:FPD = 3:1.

Samples containing 15 (± 0.1) grams of apple were weighed into centrifuge tubes. Quality control samples were spiked with appropriate amounts of spiking solutions to yield QC samples with quantitative concentrations relative to the 3:1 split ratio of 150, 300, and 750 ng/mL levels for GC/MS-SIM determination, and 50, 100, and 250 ng/mL by flame photometric detection. Each sample received a 15-mL aliquot of ACN. Two ceramic bars (Agilent p/n 5982-9313) were added to each sample to aid in sample extraction. The samples were vortexed for one minute. An Agilent original QuEChERS extraction salt packet (Agilent p/n 5982-5555) containing 6 grams of MgSO4 and 1.5 grams sodium chloride was added to each centrifuge tube. The capped tubes were shaken on a Geno/Grinder at 1500 rpm for one minute. The samples were centrifuged at 4000 rpm for five minutes.

An 8 mL aliquot of the upper layer was transferred to an Agilent QuEChERS dispersive SPE 15-mL tube for general fruits and vegetables (Agilent p/n 5982-5058). The dSPE tube was vortexed for one minute and then centrifuged at 4000 rpm for three minutes to complete the sample extraction. The extract from the dSPE tube was transferred to a GC vial and analyzed by SIM GC/MS and GC/FPD using the chromatographic conditions listed in Table 1.

Extractions of water and acetonitrile aliquots were prepared in the same manner as the samples and served as reagent blanks.

Discussion of Results

The OP pesticides were resolved on an Agilent J&W DB-35ms UI 20 m × 0.18 mm × 0.18 μm analysis column in about 30 minutes. The 12-component pesticide matrix-matched standard shown in Figure 2 shows good peak shapes for the pesticides in the GC/MS-SIM and FPD chromatograms. OP pesticides, particularly those that are more polar, can be problematic, often yielding broad peak shapes or excessive tailing and making reliable quantitation at low levels difficult. The high level of inertness of the DB-35ms UI column results in better peak shape and decreased sample adsorption, allowing lower detection limits. Figure 3 depicts the excellent peak shape seen for four of the more polar OP pesticides—oxydemeton-methyl, methamidophos, mevinphos, and acephate—with the DB-35ms UI column.

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Figure 3: Enlarged section of GC/MS-SIM and FPD chromatograms of the more problematic polar pesticides analyzed on an Agilent J&W DB-35ms UI capillary column (Agilent p/n 121-3822UI). Chromatographic conditions are listed in Table 1. The effluent split ratio is MSD:FPD = 3:1.

The performance of the DB-35ms UI high efficiency column yielded excellent linearity and recovery over the calibration range of this study. The linearity of the column as defined by the r2 values of the pesticide standard curve was ≥0.992 for all the pesticides using both detectors. The individual OP pesticide analyte values are shown in Table 3.