Platinum Nanoparticle Extraction, Quantification, and Characterization in Sediments by Single-Particle Inductively Coupled Plasma Time-of-Flight Mass Spectrometry

Single-particle analyses were performed using an ICP-TOF-MS instrument (icpTOF 2R, TOFWERK AG, Thun, Switzerland). A detailed description of the instrument can be found elsewhere [22,39,40]. Operational parameters can be found in Table S3. In this study, the samples were introduced through a desolvation membrane system (Table S4), consisting of a concentric pneumatic nebulizer combined with a membrane desolvation unit (Apex Omega, ESI, Omaha, NE, USA). By producing a dry aerosol, solvent-generated interferences, such as oxide and hydroxide interferences, were reduced, and the signal intensity was maximized. Each Pt-spiked sediment dispersion was measured for 1 min, and the pristine sediment dispersions for 1, 2, and 5 min (Table S2).

For instrument calibration, dissolved calibration standards were prepared from single-element solutions (Inorganic Ventures, Christiansburg, VA, USA). Elements with no known interferences were grouped into multi-element standard batches. The first batch [Au, Ir, Pd, Pt, Rh, Ru, Te] was prepared in 1 wt% HCl (ROTIPURAN Ultra, ROTH, Karlsruhe, Germany). The other three batches [Ba, Cd, Ce, Co, Cr, Pr, V, Y, Zr], [As, Bi, Cu, La, Mo, Ni, Pb, Sb, Sn, Zn], and [Al, Fe, Mg, Mn, Na, Si, Ti], were prepared in 2 wt% HNO₃ (ROTIPURAN Ultra, ROTH, Karlsruhe, Germany). The transport efficiency was determined with the particle size method as in Pace et al. [41], using 100 nm Au NPs (BBI Solutions, Crumlin, UK) in UPW and dissolved Au standards in 1 wt% HCl. The autotuning module of the control software TOFpilot (V2.8, TOFWERK AG, Thun, Switzerland) was used to tune and optimize the instrument for the best sensitivity and resolution as well as the lowest oxide (CeO⁺/Ce⁺) and doubly charged (Ba⁺⁺/Ba⁺) levels (Table S3) using a solution of 1 μg·L⁻¹ Ba, Bi, Ce, Co, In, Li, and U (Thermo iCAP Q, Thermo Scientific, Waltham, MA, USA). The sensitivity and resolution of ⁵⁹Co, ¹¹⁵In, and ²³⁸U were monitored before, during, and after each measurement to account for any drift in instrument sensitivity. The sensitivities and resolutions were stable throughout the measurement time and within the average values commonly achieved with the ICP-TOF-MS (Figure S1).

Single-particle analyses were performed with the single-particle workflow featured in the TOFpilot software, which provided experimental set-up and data processing and quantification. The software determined mass calibration curves using liquid standards, transport efficiency using the particle size method, limits of detection, particle number concentrations, and mass distributions all according to Pace et al. [41]. The collected data were processed after the measurements using the TOFpilot liquid reprocessing module (TOFWERK AG, Thun, Switzerland). For every single isotope, particle signals were separated from the background by performing iterative signal/background separation. A window of 100 data points was used, and the threshold was determined according to Equation (3) [42]

The average (Avg) and standard deviation (SD) were calculated for each window. Signals above the threshold were selected and extracted as particle signals, and the iterations continued until no more peaks were detected. The ICP-TOF-MS operational parameters for each measurement can be found in Table S3. The limits of detection (LOD) of all monitored isotopes were determined using acidified calibration standards with the 3 sigma formula [43] (Table S5). The LOD of Pt was 4.55 ng·L⁻¹, corresponding to 0.014 fg and 11 nm, according to Pace et al. [41]. Considering the dilution factors, the LOD corresponded to a method limit of detection of 1.77 × 10⁻³ ng·g⁻¹ for Pt. The particle number concentration limit of detection (LOD_NP) was determined as LOD_NP = 3/(Transport efficiency × Flow rate × Total acquisition time) [33,44] and corresponded to 4.88 × 10⁴ L⁻¹. Current ICP-MS instruments have LOD_NP values in the range of 10⁶ L⁻¹ [33,44,45].