Single-Particle ICP-MS with Online Microdroplet Calibration: Toward Matrix Independent Nanoparticle Sizing
Though single particle inductively coupled plasma mass spectrometry (sp-ICP-MS) is readily used as an effective method for characterization of inorganic nanoparticles in various type of samples, some fundamental aspects, such as quantification accuracy of size and particle number concentration, still require more attention.
In this paper, Hendriks and Ramkorun-Schmidt et al. use a TOFWERK icpTOF mass spectrometer to characterize an online microdroplet-based particle size calibration approach with a new dual-inlet setup. The setup comprises a microdroplet generator to produce monodisperse single microdroplets of element calibration solution and a nebulizer/spray chamber to introduce a sample containing nanoparticles. Authors investigate the influence of steady-state matrix effects on the quantification accuracy of the method. This includes ionization suppression, acid effects, and space-charge effects caused by sample matrix. The icpTOF instrument was used in this study to enable simultaneous measurement of multiple elements in microdroplets and nanoparticles. For example, Au standard microdroplets could be introduced together with Au nanoparticles from the sample, because these two could be distinguished by doping standard droplets with a tracer element Cs.
The authors emphasize practical benefits of this online calibration method combined with icpTOF. This approach:
- Allows for absolute sensitivity (counts/g) determination of all elements present in calibration standard
- Enables quantification of multiple types of nanoparticles in a single measurement run
- Eliminates transport efficiency determination of the sample introduction system, as it is the case for the calibration method in standard sp-ICP-MS
- Does not require calibration bracketing because microdroplets account for instrumental drift
- Does not require matrix-matching of standards because standard droplets are introduce in parallel with matrix from the sample and experience the same plasma and transmission conditions as nanoparticles
“The use of ICP-TOFMS extends the benefits of our calibration system because this analyzer allows for the simultaneous measurement and quantification of diverse NP types, which will allow for high-throughput measurements that are necessary to understand NP populations in, for example, environmental samples”.