Model 7700x icp ms

For the determination of total Au concentration and single particle analysis, a model 7700x ICP-MS from Agilent Technologies (Santa Clara, CA, USA) was used. The instrument was equipped with a concentric (Micromist) nebulizer and a refrigerated Scott chamber (2 °C). Before analysis, the ICP-MS was tuned using a multi-element standard solution from Agilent (1 μg L⁻¹ each of ⁷Li, ⁸⁹Y, ¹⁴⁰Ce and ²⁰⁵Tl in 2 % v/v HNO₃) to ensure maximum sensitivity and minimum oxide (¹⁵⁶CeO/¹⁴⁰Ce) and doubly charged (⁷⁰Ce⁺⁺/¹⁴⁰Ce⁺) level (< 2 %). Data were collected at m/z 197 for Au and m/z 195 for Pt. Dissolved Au calibration standards were prepared over a mass fraction range from (0.05 to 5) μg kg⁻¹ either in diluted aqua regia (1.0 % v/v 3 parts HCl/1 part HNO₃, AR) or in DI water. Platinum was used as an internal standard for total analyses (spectrum analysis mode). For the spICP-MS analyses, the dwell time was set to 10 ms with a measurement time of 300 s for each sample (time resolved mode). The transport efficiency was determined daily by the use of standard dissolved Au solutions and AuNP reference materials (RM 8012 and RM 8013) according to a previously described method;^{34 (link)} the value averaged ≈ 6 % during the course of this study. Between each sample analysis, the system was washed with DI water, 2 % v/v HNO₃ and/or 1% AR, as necessary to avoid contamination.