Effect of sampler orifice diameter on dry plasma inductively coupled plasma mass spectrometry (ICP-MS) backgrounds, sensitivities, and limits of detection using laser ablation sample introduction

High backgrounds from polyatomic ions, formed from the argon plasma gas, impurities in the gas supply and atmospheric air entrainment of especially H, C, N, O, and Ar, are a limitation to Inductively Coupled Plasma-Mass Spectrometric (ICP-MS) analysis, especially for analytes with masses below 80 Dalton. When a 'dry' plasma is utilized, instead of the more common 'wet' plasma created when solution nebulization sample introduction is used, the relative contribution of air entrainment becomes more important. In this study, the sample introduction system used was an in-house constructed laser ablation microprobe designed for small volume sampling of geological materials. An enhanced sensitivity VG PQII + 'S' ICP-MS was applied with different sample cone orifice diameters. Background, signal to background ratio, and detection limits are reported for ablated NIST glass reference material (SRM 612). By a reduction of the sample cone orifice diameter from 0.7 mm to 0.5 mm, the background was lowered by factor of ca. 100 in the low mass range and by a factor of ca. 10 in the higher mass range, while similar sensitivities were maintained. The reduced background improved the limits of detection from ca. 1 mg/μg/g to <0.5 μg/g in 10 μm diameter pits and from ca. 100 ng/g to < 10 ng/g in larger, 30 μm pits.