Improving detection capability for single particle inductively coupled plasma mass spectrometry with microdroplet sample introduction

Abstract

Single particle inductively coupled plasma mass spectrometry (sp-ICPMS) is an attractive technique for fast measurement of elemental composition, mass and particle number concentration (PNC) of metal containing nanoparticles (NPs). In order to investigate NPs <10 nm using ICPMS, low instrumental background and high detection efficiency are primary requirements. This study evaluated the performance of a sector-field ICPMS with standard and enhanced sensitivity (“Jet”) vacuum interfaces with different sample introduction setups: conventional pneumatic nebulization with (DSN) and without aerosol desolvation (PN) and microdroplet generation (MDG). Additionally, the influence of nitrogen gas as an addition to a dry aerosol was studied. In this study, transport efficiencies (TEs) and detection efficiencies (DEs) are determined for the different instrumental setups. Gold NP suspensions were analysed and evaluated for PNC and size. Applying counting statistics, the size limit of detection (LOD_size) of gold nanoparticles (Au NPs) was estimated to be 6.1 nm and 4.7 nm for PN and MDG with the standard interface, and 3.6 nm and 3.1 nm for DSN and MDG with the “Jet” interface and nitrogen addition, respectively. Additionally, DEs for various elements were determined. 11 isotopes (²⁷Al, ⁴⁷Ti, ⁶³Cu, ¹⁰⁷Ag, ¹¹¹Cd, ¹¹⁵In, ¹³³Cs, ¹⁴⁰Ce, ¹⁹³Ir, ¹⁹⁷Au, and ²³⁸U) were measured at a mass resolving power (MRP) of 300 while an MRP of 4000 was used for ⁵⁶Fe and ⁶⁶Zn. DEs obtained for the conventional nebulization system with a spray chamber (PN) were in the range of 10⁻⁴ to 10⁻² counts per atom (low resolution) and 10⁻⁶ to 10⁻⁵ counts per atom (medium resolution), while significant improvement in DE was obtained for the MDG setup with the “Jet” interface and nitrogen addition resulting in the range of 10⁻² to 10⁻¹ counts per atom (low resolution) and 10⁻⁴ to 10⁻³ counts per atom (medium resolution). The enhancement in DE was most pronounced for isotopes of lower m/z indicating reduced mass discrimination of the “Jet” interface with nitrogen gas added to the sample aerosol. The corresponding LOD_SIZE could thus be decreased by 10 or 2 times for example for Al- and Au-containing NPs, respectively. At the same time the use of an MDG for sample introduction allowed for 98.5% TE in the analyses of NP suspensions, while a TE of 10% (PN) or 23% (DSN) was obtained with pneumatic nebulizers.