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Inorganic arsenic is a primary monitoring indicator of soil heavy metal pollution.Recently,diphenylarsinic acid(DPAA)has received increasing attention as novel environmental pollutants derived from chemical warfare agents.DPAA can be naturally transformed to phenylarsinic acid(PAA)and inorganic arsenic under aerobic or anaerobic conditions.Our recent studies also demonstrated that chemical oxidation(e.g.Fenton oxidation)or enhanced microbial degradation(DPAA degrading bacteria/sulfate reducing bacteria)process also transformed DPAA into phenylarsinic acid(PAA),even inorganic arsenic,which offers the possibility of further phytoremediation in former warfare agent deposits.Therefore,we established high performance liquid chromatography-tandem mass spectrometry(HPLC-MS/MS)method to simultaneously determine inorganic arsenic and organic phenylarsinic acid in soil.Common problems associated with HPLC-MS/MS lies on matrix effects,so different pre-treatment procedures including sample dilution,smaller injection volume and applying various extraction solvents(disodium hydrogen phosphate,sodium hydroxide in methanol,citric acid-sodium citrate buffer,phosphate)were carefully optimized to achieve higher recovery efficiency and reduced matrix effects for subsequent HPLC-MS/MS analysis.Under the optimized conditions,limits of detection were 0.01,0.1 μg L-1 for DPAA and PAA,25,80 μg L-1 for arsenite and aesenate,respectively.Validation data show alkaline extraction solvents exhibit higher recovery efficiency,while acid ones with reducing matrix effects.Overall,disodium hydrogen phosphate achieved high recovery efficiency for DPAA(82%-111%)and PAA(75%-109%)at both low(20 mg kg-1)and high(50 mg kg-1)concentration level regardless of soil types.Matrix effects(85%≤ME≤115%)and coefficients of variations(CVs≤3%)for DPAA and PAA in different soils were within the permissible range,demonstrating HPLC-MS/MS is suitable to detecting inorganic arsenic and phenylarsinic acid in soil.This provides one analytical method to investigate environmental behavior and pollution remediation of soil inorganic and organic arsenic.