Soil monitoring of arsenic by methanol addition DRCICP-MS after boiling aqua regia extraction

Abstract

Trace levels of arsenic (As) in soil samples can be determined by conventional ICP-QMS with closed vessel digestion, but always suffer from interference of ions originating from the complicated matrix, mainly including ⁴⁰Ca³⁵Cl⁺, ⁴⁰Ar³⁵Cl⁺, ³⁹K³⁶Ar⁺, ¹⁵⁰Nd²⁺, ¹⁵⁰Eu²⁺ and ¹⁵⁰Sm²⁺etc. In this study, ⁷⁵As⁺, the mass for general detection, was effectively changed to ⁷⁵As¹⁶O⁺ which could be detected at m/z 91 by reaction with oxygen in a Dynamic Reaction Cell (DRC). For the new interference of ⁹¹Zr⁺ on ⁷⁵As¹⁶O⁺, boiling aqua regia extraction was used to remove most of the zirconium in the sample preparation procedure, and the residual ⁹¹Zr⁺ was oxidized to ⁹¹Zr¹⁶O⁺ by oxygen in the DRC. In addition, the signal intensity of ⁷⁵As¹⁶O was improved 4-fold by addition of 4% methanol into the sample solution. The signal to background ratio (SBR) of As was obviously improved from 0.18 to 205 (the certified arsenic concentration is 1 μg L⁻¹), and a limit of quantification (LOQ, 10σ) of 0.1 ng g⁻¹ was obtained. The proposed method was successfully applied to analyze 20 soil samples collected from pig farms in livestock farming processes, and shows its great potential for trace As determination in environmental and geological samples.