Monodisperse microdroplets: a tool that advances single-particle ICP-MS measurements

Abstract

In single-particle inductively coupled plasma mass spectrometry (sp-ICP-MS), individual nanoparticles are detected by measuring ICP-MS signals with high temporal resolution. At high time resolution, particle-based signals—which are around 200–500 μs in duration—make up a larger fraction of the signal measured. If the dissolved background is low enough and the mass of element(s) of interest in the particles is high enough, then nanoparticle (NP) signals are recognized as signal spikes on the time trace. With modern instrumentation, sp-ICP-MS can be used to quantify NPs with element mass down to single-digit attogram levels. Monodisperse microdroplets composed of elemental solutions are vehicles for the transport of discrete amounts of elemental species into the ICP, and signals produced from microdroplets closely match those produced by NPs. Temporal durations and element sensitivities obtained from microdroplets and NPs are so similar that microdroplets may be used as NP proxies. Unlike NPs, microdroplets offer a flexible platform for user-designed sp-ICP-MS measurements because—with microdroplets—we can precisely control the absolute mass of elements injected into the plasma in each droplet. Controlled introduction of analyte mass into the ICP enables us to use microdroplets to generate sensitivity calibration factors and also to use microdroplets as NP proxies to study sp-ICP-MS measurement accuracy. Here, were report several instances of how measuring microdroplet-based element signals with ICP-TOFMS has allowed us to expand the versatility, as well as the general understanding, of sp-ICP-TOFMS measurements. First, we summarize how microdroplets can be used for online calibration of analyte NP element mass and particle-number concentration. Second, we describe how the measurement of microdroplets with tailored analyte masses helps us to refine, test, and validate sp-ICP-TOFMS data analysis strategies. Specifically, we use microdroplets to study the accuracy and robustness of split-event correction and signal-thresholding approaches for NP detection. Together, these experiments describe how the use of monodisperse microdroplets allows us to design better sp-ICP-MS experiments.