ABSTRACT
Single particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS) is a powerful analytical technique for quantifying elements in nanoparticles and microparticles; however, like most ICP-MS-based measurements, matrix effects can be a major challenge for accurate quantification in spICP-MS. Here, we report the use of online microdroplet calibration to overcome extreme matrix effects observed for the analysis of nanoparticles and microparticles in seawater. With online microdroplet calibration, particle-containing samples are introduced into the ICP along with monodisperse microdroplets containing known element mass amounts. The microdroplet standards, which experience the same plasma conditions as the analyte particles, are used to measure matrix-matched absolute element sensitivities. With online microdroplet calibration, one multielemental standard can be used to determine the element mass amounts in diverse types of analyte particles independent of the sample matrix. We evaluate the matrix tolerance of spICP-TOFMS with online microdroplet calibration through the analysis of metal nanoparticles, polystyrene microplastic beads doped with rare-earth elements, and metal-oxide submicrometer particles in artificial seawater. Our results demonstrate mass recoveries of 98-90% for the analysis of individual gold NPs in ultrapure water to 99% seawater. In the analysis of food-grade TiO2 submicron particles, accurate Ti-mass per particle is determined with matrix-caused signal attenuation up to 80% in a pure seawater matrix. We also demonstrate accurate diameter determinations of individual 3.4 µm polystyrene beads at concentrations of up to 80% simulated seawater. Furthermore, simultaneous and accurate quantification of rare-earth elements in the polystyrene beads is achieved.
ABSTRACT
This study describes an interlaboratory comparison (ILC) among nine (9) laboratories to evaluate and validate the standard operation procedure (SOP) for single-particle (sp) ICP-TOFMS developed within the context of the Horizon 2020 project ACEnano. The ILC was based on the characterization of two different Pt nanoparticle (NP) suspensions in terms of particle mass, particle number concentration, and isotopic composition. The two Pt NP suspensions were measured using icpTOF instruments (TOFWERK AG, Switzerland). Two Pt NP samples were characterized and mass equivalent spherical sizes (MESSs) of 40.4 ± 7 nm and 58.8 ± 8 nm were obtained, respectively. MESSs showed <16% relative standard deviation (RSD) among all participating labs and <4% RSD after exclusion of the two outliers. A good agreement was achieved between the different participating laboratories regarding particle mass, but the particle number concentration results were more scattered, with <53% RSD among all laboratories, which is consistent with results from previous ILC studies conducted using ICP-MS instrumentation equipped with a sequential mass spectrometer. Additionally, the capabilities of sp-ICP-TOFMS to determine masses on a particle basis are discussed with respect to the potential for particle density determination. Finally, because quasi-simultaneous multi-isotope and multi-element determinations are a strength of ICP-TOFMS instrumentation, the precision and trueness of isotope ratio determinations were assessed. The average of 1000 measured particles yielded a precision of below ±1% for intensity ratios of the most abundant Pt isotopes, i.e.194Pt and 195Pt, while the accuracy of isotope ratios with the lower abundant isotopes was limited by counting statistics.
ABSTRACT
Single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS) is a high-throughput method for the multiplexed analysis of nanoparticles (NPs). However, matrix interferences are a significant challenge that must be overcome for accurate NP sizing and number concentration determination. In online microdroplet calibration, multi-element droplet standards are mixed with nebulized aerosols from a NP-containing sample in a dual-sample introduction system before entering the ICP. Microdroplet standards are used to determine absolute sensitivities and calibrate the mass amount(s) of analyte element(s) in particle events. A plasma-uptake standard is added to NP-containing samples to account for variations in the nebulization and transport efficiency of aerosols into the plasma, which are used in the determination of particle number concentrations (PNCs). Since the microdroplets experience the same plasma conditions as analyte NPs, this creates a matrix-matched calibration. We studied the efficacy of online microdroplet calibration for the quantification of NPs in three organic matrices: ethanol, acetone, and acetonitrile. Organic solvents can cause a pronounced increase in sample uptake and cause plasma-related signal attenuation for nanoparticle measurements. Online microdroplet calibration corrects for both effects without the use of nanoparticle standards. Our results demonstrate accurate NP sizing and PNC determinations in organic matrices up to 98% (v/v), even when these matrices caused signals to be attenuated up to 35-times and nebulizer transport efficiencies to be up to four-times higher than that of a pure water matrix.
