Direct determination of arsenic in sea-water by reaction cell inductively coupled plasma mass spectrometry

Abstract

A new procedure for the direct determination of total arsenic in sea-water has been developed. In order to remove the polyatomic interferences arising from the direct analysis of 15-fold diluted sea-water samples, the dynamic reaction cell (DRC) technique was applied. Different analytical strategies, such as reactive removal of the interfering species by ammonia, methane or hydrogen, as well as arsenic oxidation by oxygen or nitrogen protoxide, followed by detection of the AsO⁺ ion were explored. The use of DRC pressurized with ammonia, methane or hydrogen was not able to reduce the spectral interfering effects at m/z = 75. On the other hand, satisfactory results were obtained by using oxygen at 0.8 cm³ min⁻¹ or nitrogen protoxide at 0.5 cm³ min⁻¹ and detecting the analyte at m/z = 91. The relevant parameters, such as the reactive gas flow rate, the Mathieu stability parameters a and q of the DRC, and the rod offset voltages of both cell and mass analyzer quadrupole, were optimized for maximum ion transmission and selectivity. Following the optimization of the DRC conditions, a new analytical procedure for the direct determination of total arsenic concentration in sea-water was hence developed. The main analytical performance characteristics of the optimized method were evaluated, with special attention to the determination of the “procedural” and “field” blanks. A detection limit as low as 40 ng L⁻¹ and instrumental precision of about 2% were achieved. The accuracy was verified by the analysis of certified reference materials (CRM) of sea-water (NASS-4, CASS-4) and estuarine water (SLEW-3). The analytical precision for replicated analyses of the CRMs ranged from 2.3% to 5.0% (n = 5). Finally, the developed procedure was applied to a number of sea-water samples from the Ross Sea (Southern Ocean, Antarctica). Arsenic concentration ranged from 0.88 to 1.03 μg L⁻¹ (n = 12), which are typical values for the uncontaminated offshore waters.