Detailed evaluation of conditions of photochemical vapor generation for sensitive determination of nickel in water samples by ICP-MS detection

Abstract

Photochemical vapor generation (PVG) of Ni was optimized using inductively coupled plasma mass spectrometry detection and a 19 W high-efficiency flow-through photoreactor operated in a flow injection mode (0.49 mL sample volume). The main aim was to achieve very high sensitivity and low limits of detection (LOD) for determination of Ni in fresh water samples, which necessitated to keep low Ni contamination and rather low dilution of samples by added formic acid as a photochemical agent. With respect to that a reaction medium composed of 30% (m/v) formic acid and a flow rate through the photoreactor of 1.5 mL min−1, which corresponds to an irradiation time of 29 s, were chosen as optimal. These PVG conditions were characterized by 42% overall PVG efficiency and by relative and absolute LOD of 1.9 ng L–1 and 0.9 pg, respectively. Almost double efficiency was achievable when pH was adjusted by an addition of 1.5 M ammonium formate, reflected in the proportional increase in sensitivity. However, no significant benefit to analytical performance was found at those conditions. Interferences from inorganic anions likely to be encountered during analytical application to real samples (i.e., NO3−, Cl−, SO42−) were investigated in detail as well as interferences from several transition metals and metalloids. The accuracy was successfully verified by an analysis of the standard reference material NIST 1643f (fresh water) and certified reference materials SLRS-6 (river water), ERM-CA713 (wastewater) and CTA-FFA-1 (fine fly ash), the last one after simple acid digestion. The applicability of this sensitive methodology was further demonstrated by a direct analysis of three water samples (river and well) that were collected and only acidified with formic acid to 30% (m/v).