Rapid analysis of the nitrification inhibitor 3,4-dimethylpyrazole phosphate in soil using LC-MS/MS

摘要

Nitrate from the biological nitrification of ammonium fertilisers causes environmental damage via groundwater contamination and nitrous oxide emission. To limit nitrate formation, nitrification inhibitors (NIs) are used in conjunction with ammonium-based fertilisers in agricultural land management. The NI 3,4-dimethyl-1H-pyrazole phosphate (DMPP), with an active constituent 3,4-dimethyl-1H-pyrazole (3,4-DMP), is commercially available and its effectiveness and behaviour in soils have been studied. However, only one method for the analysis of 3,4-DMP in soil has been reported and relies on extensive sample preparation to remove matrix interferences prior to HPLC analysis. A new method was developed to allow monitoring of 3,4-DMP residues in soil after appliaction, which utilises the greater selectivity and sensitivity of LC-MS/MS. A 3,4-DMP limit of quantitation of 0.5ng/g was achieved, which is 10 times more sensitive than the published method, and was achieved using 10,000 times less 3,4-DMP injected on-column, with an injection volume 100 times smaller. Four internal standards were evaluated to improve the accuracy of the extraction method. The isotope-substituted structural isomer 3,5-dimethyl pyrazole-N-15(2) provided the best and most consistent recoveries over the 300-fold concentration range tested. The new method was employed to investigate the persistence and mobility of 3,4-DMP in an agricultural soil. 3,4-DMP had a half-life of 5days in the top 0.5cm of soil at normal and double recommended application rates, while half-lives in the 2.5cm soil profile were 28 and 21days, respectively. 3,4-DMP mobility in the clay loam soil tested was low, with only 15-25% of applied 3,4-DMP detected below the top 0.5cm, suggesting the loss of 3,4-DMP was either due to volatilisation or degradation, rather than leaching into the soil profile.