Analysis of dimethyl hydrogen phosphite and its stability under simulated physiological conditions

In a study by the National Toxicology Program, dimethyl hydrogen phosphite (DMHP) was shown to cause dose-related neoplastic lesions in the lungs and forestomachs of Fischer 344 rats. This investigation was carried out to study the stability of this carcinogen under conditions similar to those that the chemical may encounter in the physiological environment. To carry out these studies, capillary gas chromatography was utilized to analyze DMHP. The method was linear over a range of 10 to at least 1000 ng. High-performance liquid chromatography (HPLC) also was used to analyze DMHP and its degradation products. In aqueous solutions, DMHP was stable for a period of time before degradation began. Once the process began, degradation continued until 10-20% of the original concentration was reached and further decomposition was minimal. At 10% concentration in 0.1M phosphate buffer, pH 7.4, DMHP was stable for 3.6 h at 37 degrees C. This stability period increased at lower temperature, at lower DMHP concentration, and in slightly more alkaline buffer (pH 8). After the stability period, DMHP disappeared from the solution and this disappearance followed a first order kinetics with a rate dependent upon temperature, concentration of DHMP, and the pH of the solution. The half-time of disappearance of DMHP under the conditions mentioned above was 2.4 h, which increased at lower temperature, at lower DMHP concentration, and in slightly more alkaline buffer (pH 8). The decomposition products of DMHP were identified by HPLC and proton nuclear magnetic resonance (1H-NMR) spectroscopy as methanol, monomethyl hydrogen phosphite, and orthophosphorous acid.