μ-dDIHEN: a new micro-flow liquid sample introduction system for direct injection nebulization in ICP-MS

Abstract

The μ-dDIHEN allows direct nebulization of small samples into the plasma at a flow rate down to 5 μL min⁻¹. It is composed of a demountable direct injection high efficiency nebulizer (d-DIHEN), a flow injection analysis (FIA) valve and a gas displacement pump (GDP) connected to a μ-mass-flow meter delivering very stable liquid flow. The system was successfully tested with sample loops of 10 and 50 μL and for liquid flow rates between 5 and 50 μL min⁻¹ on two different inductively coupled plasma mass spectrometry (ICP-MS) instruments, a sector-field-ICP-MS (SF-ICP-MS) and a multi-collector-ICP-MS (MC-ICP-MS), and for three different applications: boron isotopic ratio measurement of geological samples, multi-element analyses of natural water samples and gold nanoparticle characterization by single particle ICPMS (spICP-MS). Signal sensitivity was increasing with the liquid uptake rate of the μ-dDIHEN (and the GDP gas flow rate), but stopped increasing above 30 μL min⁻¹, due to bad spray quality and poor ionization under extra-wet conditions. We successfully measured boron isotopic ratios (δ¹¹B) for reference material solutions with B concentrations between 10 and 200 μg L⁻¹; the reproducibility of the measurements depending on both the B concentration of the sample and the uptake rate: ¹⁰B signals below 20–30 mV (with 10¹¹ ohms amplifiers) gave 2SD reproducibility ≥0.5‰ (n = 5). The best measurement conditions were not necessarily with the lowest uptake rate but rather at 25 μL min⁻¹, allowing for higher signals. We also tested transient versus continuous measurement modes for a series of natural samples (with 10 and 50 μL sample loops). The two modes gave fully comparable results but the transient mode allows triplicate δ¹¹B measurements and is recommended for very small samples (≤100 μL and B content of a few ng). For multi-element analyses by SF-ICP-MS, the limits of quantification (LOQs) of the majority of the trace and ultra-trace elements were lower (up to 15 times) with the μ-dDIHEN than with a concentric nebulizer and spray chamber, due to the much lower sample volume needed. Lastly, the μ-dDIHEN is highly promising for nanoparticle (NP) characterization by spICPMS. The first measurements for gold NPs of 40 nm certified size led to a transport efficiency of 85%.