Evaluation of headspace Solid Phase Micro-extraction method for analysis of phosphine residues in wheat

This new method utilizes headspace-solid-phase micro extraction (HS-SPME) for pre-concentration of PH3. Phosphine was determined with gas chromatography/pulsed flame photometric detector (PFPD). Spiked samples were used for calculation of phosphine residue in grain. Four types of fibres (100μm- PDMS, 85μm-CAR/PDMS, 75μm-CAR/PDMS and 65μm-PDMS/DVB) were tested. The bipolar fibres (CAR/PDMS and PDMS/DVB) can extract PH3, but the non-polar fibre (PDMS) did not. Larger size fibres extracted PH3 more efficiently than the smaller size fibres (e.g., 85 μm > 75 μm > 65 μm). The 85μm CAR/PDMS fibre was used to optimize the different parameters that affect the SPME extraction efficiency of PH3. In the validation study, 50 grams of wheat in a 250 mL glass flask and capped with an open-top screw cap and PTFE/Silicon septa were spiked at 0.02 ng PH3/g of wheat. The flask was then heated to 45°C in an oil bath for 45 min, after which time the 85 μm CAR/PDMS fibre was exposed for 20 min and then exposed in the heated injection port of a GC/PFPD and desorbed for 2 min. Under conditions of the validation study, the limit of detection (LOD) or level of quantification (LOQ) was in the range of 0.005–0.01 ng PH3/g of wheat.