Single particle inductively coupled plasma mass spectrometry: investigating nonlinear response observed in pulse counting mode and extending the linear dynamic range by compensating for dead time related count losses on a microsecond timescale†
In single particle inductively coupled plasma mass spectrometry (spICP-MS) the detection of small nanoparticles (NP) usually requires utilization of the pulse counting signal, which entails the risk of nonlinear response for larger NP. In this work, the suitability of a traditional dead time correction (DTC) method applied to both millisecond (msTR) and microsecond (μsTR) time-resolved spICP-MS is evaluated. A custom data acquisition system was used to record the pulse counting signal at a sustained rate of 2 × 105 samples per second. Model transients from microdroplets containing different concentrations of a thallium element standard and generated via a custom droplet introduction system were studied (Part A). Findings were compared to the analysis of eleven gold NP suspensions covering a size range of 10–100 nm, introduced via regular solution nebulization (Part B). Applying DTC to μsTR spICP-MS data allowed to increase the maximum number of counts tolerated per particle or droplet four- to fifteen-fold, resulting in a linear dynamic range (LDR) of 10–60 instead of 10–40 nm AuNP, or 9–4500 instead of 9–300 μg L−1 Tl. For Part B a cross-calibration between standard and attenuated sensitivity mode could be established, further extending the LDR (10–100 nm AuNP). Findings support the theory of dead time related count losses being the main reason for nonlinear response in pulse counting spICP-MS. However, results also indicate that DTC can lead to slightly distorted particle size distributions.