Comparison of direct and indirect measures of transport efficiency in single particle inductively coupled plasma mass spectrometry

Abstract

Accurate calibration of the fraction of introduced sample that is transported to the plasma, termed “transport efficiency” (TE), is required for particle sizing and number concentration determination by single particle inductively coupled plasma mass spectrometry (spICP-MS). In this study, we systematically compare three methods for the measurement of TE: the particle frequency (TEF), particle size (TES), and dynamic mass flow (DMF) methods. The TEF and TES methods provide a direct measure of TE but require a single nanoparticle (NP) reference material accurately value-assigned for particle size and number concentration of which few are available. The DMF method provides an indirect measure of TE and only requires measurement of the mass difference between the amount of sample solution introduced to the instrument and the amount of effluent exiting the spray chamber. Because the DMF method relies solely on mass measurements, it can provide an SI traceable measure of TE. However, the DMF method assumes that the mass difference represents the fraction of sample transported to the plasma. The veracity of the three approaches to account for TE is assessed using three different spray chamber types, i.e., Scott-type double pass, conical impact bead, and baffled cyclonic spray chambers operated at cooled (2 °C to 10 °C) and ambient temperature (19 °C to 21 °C) conditions equipped with different nebulizers on different ICP-MS platforms. When operating the spray chamber at ambient temperature, the DMF method yielded systematically higher measures of TE than the TEF and TES methods regardless of nebulizer type, spray chamber type, or ICP-MS platform. While better agreement between the three measures of TE was achieved when operating the spray chambers at 2 °C, DMF repeatability was poor. The deviation of particle number concentration, expressed as the median percent difference from the known value for various in-house value-assigned AuNP suspensions, ranged from – 18% to +8% (TEF) and from – 1% to – 70% (DMF) whereas the deviation for particle size ranged from – 3% to +1% (TES) and from – 4% to +44% (DMF) across all conditions. The large negative bias for counting and positive bias for sizing by the DMF method indicates an overestimation of TE which was traced to low recovery of the mass of effluent exiting the spray chamber. We found that the indirect measure of TE (DMF method) yielded acceptable results (within ±10% of known values for particle size and ± 20% of known values for number concentration) under select conditions (cyclonic spray chamber at 2 °C and Meinhard nebulizer), but the biases inherent to the DMF method are not fully understood for all studied conditions. Only the direct measures of TE (TEF and TES) yielded acceptable results across all use conditions.