In inductively coupled plasma mass spectrometry (ICP-MS), short transient signals originating from individual nanoparticles are typically recorded in a time-resolved measurement with reduced dwell times in the millisecond time regime. This approach was termed single-particle ICP-MS in the past and used for particle counting and sizing but is not without limitations. In this work, a home-built data acquisition unit (DAQ) specifically tailored to the needs of single-particle ICP-MS applications was developed to study and alleviate some of these limitations. For best comparison, data were acquired simultaneously with both techniques. Each experiment was carried out as a conventional time-resolved measurement, while the DAQ directly probed the instrument's detection circuitry. Our DAQ features dwell times as low as 5 ms during continuous data acquisition and can be operated for virtually unlimited measurement time. Using a time resolution much higher than the typical duration of a particle-related ion cloud, the probability of measurement artifacts due to particle coincidence could be significantly reduced and the occurrence of split-particle events in fact was almost eliminated. Moreover, a duty cycle of 100% of the counting electronics improves the method's accuracy compared to the acquisition system of currently available ICP-Q-MS instruments.Fully time-resolved temporal profiles of transient signals originating from single gold nanoparticles as small as 10 nm are presented. The advantages and disadvantages of millisecond versus microsecond dwell times are critically discussed including measurement artifacts due to particle coincidence, split-particle events, and particle number concentration.
Capillary electrophoresis (CE) coupled to single particle inductively coupled plasma mass-spectrometry (SP-ICP-MS) was used for the first time with a prototype data acquisition (μsDAQ) system that features 5 μs time resolution (100% duty cycle) to separate and quantify mixtures of silver nanoparticles (Ag NPs). Additionally, an online preconcentration technique, reversed electrode polarity stacking mode (REPSM), was applied for Ag NPs analysis with CE-SP-ICP-MS for the first time. After optimization, best results were achieved using a injection time of 110 s and a constant pressure of 50 mbar in hydrodynamic injection mode. It was possible to detect 14.3 ± 1.5× more 20 nm sized, 21.0 ± 4.2× more 40 nm sized, and 27.7 ± 4.9× more 60 nm sized Ag NPs compared to the standard injection time of only 3 s. The effect of applied voltage on the NPs separation was studied, and a CE separation at 20 kV was found to be optimal for the present setup. The capability of CE-SP-ICP-MS for quantification of particle number concentration was investigated, and detection limits in the submicrogram-per-liter range were achieved. The possibility to separate 20, 40, and 60 nm sized Ag NPs simultaneously present in a mixture was demonstrated over a broad concentration range.
Sampling of the pulse-counting signal with μs time-resolution provided a functional compensation for dead-time related count losses in spICP-MS, ultimately improving the linear dynamic range by one order of magnitude towards higher count rates.
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