Methods to increase sample transport efficiency in single particle inductively coupled plasma mass spectrometry when analyzing nanoparticles

Abstract

Inductively coupled plasma mass spectrometry (ICPMS) can be used for the determination of the size and concentration of nanoparticles (NPs) suspended in solution in so-called single particle ICPMS (spICPMS) mode. Using a dilute suspension to introduce a single NP into the plasma at a time indeed allows for quantification of the mass of the NP (and thus its size if its density and shape are known) based on its signal intensity. Conventional sample introduction through continuous nebulization is typically used. However, heating of the sample introduction system can improve the transport efficiency by enhancing the formation of small droplets after nebulization, which enables more sample to reach the plasma. For the first time, the effect of infrared (IR) heating the sample introduction system on spICPMS was investigated. The number of detected NPs min⁻¹ increased from 540 at room temperature to 600 at 60 °C and 840 at 80 °C. Unfortunately, because IR heating was only done outside the torch box and condensation could occur on the way to the torch inside the torch box, no significant change in size detection limit resulted at 60 °C and degradation occurred at 80 °C. Another approach that increases the sample introduction efficiency is mono-segmented flow analysis (MSFA), where flow injection is done within an air bubble. For the first time, MSFA-spICPMS was tested. It improved transport efficiency for Au NPs to 840 NPs min⁻¹ without heating and to 900 NPs min⁻¹ with IR heating at 60 °C. Both approaches improved the precision of NP size measurements.