Impurity analysis of methamidophos with chromatographic methods Original Research Article

An impurity analysis of methamidophos (O,S-dimethyl phosphoramidothioate) which is one of the frequently used pesticides for crop protections was carried out. Four impurity standards by synthesis and purifications to be used for quantitative impurity analysis were prepared. The chromatographic methods like gas chromatography (GC), liquid chromatography (LC), and gas chromatography–mass spectrometry (GC–MS) were used to analyze and compare the active ingredients and impurity contents. The active ingredient of methamidophos amounts to averages of 70.18% for technical materials and had averages of 52.61% for formulated materials, as determined by chromatographic analysis. There were seven impurities which were more than 0.1% of the sample weight. Hazardous impurities such as trimethyl phosphorothioate (TMP) and isotrimethyl phosphorothioate (i-TMP) were found to be 3.13% and 1.10% for technical methamidophos and 2.37% and 0.86% for formulated methamidophos, respectively. The analytical results of the active ingredient and impurity contents in this study were all consistent with impurity specifications from the Food and Agriculture Organization (FAO) of the United Nations. The residual components of technical methamidophos were around 12%, consisting of Al, Si, K, and P elements after ignition at 450°C for 4 h.