Method development for the determination of phosphine residues in foods

Phosphine (PH₃) is a fumigant used for pest control of stored products and foods. In Taiwan, PH₃ has the maximum residue level (MRL) in 14 foods, including dried fruits, vegetables, spices, nuts, crops, roots, and tuber vegetables. In this study, gas chromatography/mass spectrometry coupled with a headspace sampler (HS-GC/MS) was used to determine the PH₃ content in foods. The stability of the PH₃ standard was evaluated either directly using the gas standard or indirectly using zinc phosphide (Zn₃P₂) with an acid. The optimal conditions for the headspace agitator were determined to be an incubation temperature of 65 °C, rotation speed of 750 rpm, and shaking time of 20 min. The PH₃ residue in the sample was sufficiently released when 15 mL of 5% sulfuric acid was added. The PH₃ gas standard was applicable for quantification because the correlation coefficients of the standard and matrix-matched calibration curves were greater than 0.99, and the coefficient of variation of the repeatability test was less than 20%. The signal-to-noise ratio was greater than 3 when PH₃ was fortified into the testing matrix at 5 ng/g; therefore, the limit of quantification of PH₃ was determined to be 0.005 ppm. Because of the significant matrix effects and difficulty in obtaining representative matrices, the developed method referred to the EU Reference Laboratory-single residue method to estimate the PH₃ content in the sample using a standard curve and the PH₃ content was accurately quantified using the standard addition method. The proposed method was used to analyze 44 real samples obtained from markets and food factories, including various food commodities. Among these samples, three showed detectable PH₃ residues, with concentrations ranging from 0.006 to 0.066 ppm, which complies with the MRLs in Taiwan. This study successfully utilized HS-GC/MS with a gaseous PH₃ reference standard to develop a facile sample preparation method for detecting PH₃ in foods.