Using citric acid stabilizing reagent to improve selective hydride generation-ICP-MS method for determination of Sb species in drinking water

Abstract

An improved selective hydride generation-inductively coupled plasma-mass spectrometry (SHG-ICP-MS) method using citric acid as the stabilizing reagent was established and evaluated for the speciation of inorganic Sb in drinking water. The use of citric acid as the stabilizing reagent improved the sensitivity of the Sb species over 2-fold, when compared to the conventional pH-dependent SHG strategy. The SHG reaction in the presence of citric acid was studied using high-resolution quadrupole-Orbitrap mass spectrometry (HR-MS). Almost 100% of the Sb(V) and Sb(III) reacted with citric acid to produce [Sb(OH)₃(C₆O₇H₅)]⁻ and [Sb(C₆O₇H₆)₂]⁻, respectively. The resultant Sb(V)-complex ([Sb(OH)₃(C₆O₇H₅)]⁻) cannot be reduced by NaBH₄, while the resultant Sb(III)-complex ([Sb(C₆O₇H₆)₂]⁻) originating from Sb(III) can be reduced thoroughly by NaBH₄. The ICP-MS signal intensity for Sb was initially low due to its high first ionization energy of 8.61 eV; however, an increase in sensitivity was observed with an addition of 2.0 mL min⁻¹ CH₄ to the ICP. Three elements (Ge, Sn, and Te) were evaluated as potential internal standards (ISs) to improve the stability of the HG signal. After careful evaluation, ¹¹⁸Sn was selected as the IS, and the relative standard deviation (RSD) for the of Sb(III) signal (1.0 μg L⁻¹) was improved from 5 to 2%. Under optimized conditions, excellent detection limits (4.0 ng L⁻¹ for Sb(III) and 6.0 ng L⁻¹ for total Sb) and satisfactory recoveries for Sb(III) (from 93.1 to 108%) and total Sb (from 98.5 to 103%) were obtained. The proposed method was employed for analysis of three types of drinking water: bottled water, tap water, and well water. The average Sb(III) and total Sb contents were 0.032 and 0.320 μg L⁻¹, respectively, in bottled water (N = 10), 0.197 and 1.05 μg L⁻¹, respectively, in tap water (N = 10), and 0.211 and 1.93 μg L⁻¹, respectively, in well water (N = 40). Thus, this high throughput method has great potential for the determination of inorganic Sb species at ultra-trace levels in drinking water.