LA-ICP-MS analysis of fluid inclusions: contamination effects challenging micro-analysis of elements close to their detection limit
This paper presents a practical guide to an optimized analytical procedure for the reliable quantification of trace element concentrations in fluid inclusions hosted by natural minerals, using laser ablation inductively coupled plasma mass spectrometry (ICP-MS). With the improved sensitivities of modern ICP-MS instrumentation, more stringent guidelines for cleaning and contaminant reduction become decisive in extending the limits of trace element analysis, particularly for dilute fluids and smaller inclusions investigated by highly sensitive sector field ICP-MS. We have identified three sources of contamination that limit the recording and quantification of low-count signals, namely: (1) the carrier gas quality that affects the continuous instrumental background. (2) The material and cleanliness of the ablation cell, connection tubing material, and sampler and skimmer cone, all of which may cause a falsely elevated signal during ablation. (3) Contamination of the sample surface during its preparation and from the deposition of previously ablated material, including aerosols produced during extended ablation of an ICP-MS tuning material. Contamination from these sources can overlap with the host mineral and with the fluid inclusion signal. Based on extensive test experiments, we propose a workflow that allows minimization of these contamination contributions and optimizes representative sampling and quantification of fluid inclusion data. For example, Au and S in low-density vapor-dominated inclusions may be quantified down to a few ng g−1 and a few hundred μg g−1, respectively.