Single-particle ICP-TOFMS with online microdroplet calibration for the simultaneous quantification of diverse nanoparticles in complex matrices†
Inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) is unique in its ability to provide multidimensional information about nanoparticles (NPs) including multi-element composition, size/mass distribution, and number concentration. We present an online and matrix-matched calibration method for the multiplexed analysis of NPs using ICP-TOFMS. In our system, NP mass is determined based on absolute sensitivities measured with microdroplet standards. Because we introduce microdroplets along with NP-containing samples, they provide matrix-matched calibration of element mass. For accurate determination of particle number concentration (PNC), we spike a plasma-uptake standard element, e.g. Cs, into NP containing samples and then—based on absolute sensitivity from microdroplet signals—can directly determine the sample uptake rate into the plasma. Our online microdroplet method requires no external NP standards and a detailed explanation of the approach is provided. As a proof-of-principle, we applied this approach for the quantification of well-characterized engineered NPs (Ag, Pt, and Au NPs) in different matrices, including phosphate-buffered saline, triton-x surfactant, and effluent from a waste water treatment plant (WWTP). Results demonstrate accurate multiplexed quantification of spiked NPs in all matrices in terms of both element mass and PNC, which suggests the utility of the approach for quantification of NPs in challenging or not well-defined environmental matrices. For the WWTP effluent sample, endogenous NPs and spiked NPs are quantified in a single run.