Quantitative sizing of microplastics up to 20 µm using ICP-TOFMS

Abstract

A fundamental study of four different sample introduction systems was carried out to evaluate the upper size limit of microplastics measured by inductively coupled plasma-time-of-flight-mass spectrometry (ICP-TOFMS). Three different, certified microplastic samples (PS, PMMA and PVC) within a size range of 3-20 µm in suspension were measured. In this study, no particles larger than 10 µm could be detected using pneumatic nebulization for sample introduction. However, we were able to extend the upper size limit to 20 µm by either using a falling-tube device or a vertical downwards-pointing ICP-TOFMS. Particle transport efficiencies could only be estimated and were within a range of 13% to 184%. The particle size was quantified by using dissolved citric acid (non-matrix matched) and agreed with reference values. The critical size values were 2.3 µm for PS, 2.4 µm for PMMA and 3.0 µm for PVC. Additionally, in the case of PVC, chlorine could also be detected and the critical size value was 3.9 µm based on the ³⁵Cl⁺ ion signal.

Fig. 1. Schematic sketch of the sample introduction systems used for the downward-pointing ICP-TOFMS (A) and the ICP-TOFMS with horizontal ICP alignment using different particle introduction systems like the cyclonic spray chamber (B1), falling-tube device (B2) and HECIS (C).

Fig. 2. Transient signals for a period of 2 second from the 5 µm PMMA (A and D), 10 µm PMMA (B and E) and 20 µm PMMA (C and F) measurements with the downward-pointing ICP-TOFMS (A–C) and the setup B2 (D–F). The transient signals were split-event and background-corrected. For each plot, 50 droplets were counted. The dispensing rate was 50 Hz. Asterisks indicate the signals for double droplet events, exceeding a threshold of 5 * σ +µ. The standard deviation σ and the mean value µ were obtained from the Gaussian fits shown in Fig. 3.

Fig. 3. Histogram of the droplet tracers (¹³³Cs⁺ and ²⁷Al⁺) from experiments shown in Fig. 2 of the 5 µm PMMA (A and D), 10 µm PMMA (B and E) and 20 µm PMMA (C and F) measurements. The histogram counts were normalized to the total number of droplets injected and fitted with a Gaussian function (red). The bin size was set to 10 counts.

Fig. 4. Histogram of the particle-derived carbon signals from the 5 µm (A and D), 10 µm (B and E) and 20 µm (C and F) PMMA collected from the experiment shown in Fig. 2A–F. The signal distribution was fitted with a Gaussian function. The ¹³C⁺ ion signal was collected only for the 20 µm particles due to exceeding the linear dynamic range of the ¹²C⁺ ion signal. The bin size was set to 20 counts.

Fig. 5. Transient signals during the injection of 10 µm (A) and 20 µm PS (B) particles into the downwards pointing ICP-TOFMS. The signal suppression of the droplet tracer for the 20 µm PS indicates a more pronounced matrix effect for the larger particles.

Fig. 6. Comparison of the measured and reference sizes for the different microplastics and sample introduction setups. (A) Downwards ICP, (B1) horizontal ICP using the pneumatic nebulizer and cyclonic spray chamber, (B2) falling-tube and the (C) HECIS. Numerical values are provided in Table S2. The plots were labeled the same as the four configurations in Fig. 1.

Fig. 7. Transient signals of the 4 µm (A), 6 µm (B) and 20 µm (C) PVC particles showing coincident particle-derived carbon and chloride ion signals.

Fig. 8. Histogram of the particle-derived ³⁵Cl⁺ ion signals for 4 µm (A), 6 µm (B) and 20 µm (C) PVC particles with a Gaussian fit function.

Fig. 9. The chlorine-derived particle ion signal was plotted against the particle volume, determined from the carbon particle mass, to show the linear correlation. The data points of the 4 and 6 µm PVC particles are shown in the smaller plot.