Assessing the impact of common sample preparation strategies for single particle ICP-MS regarding recovery and size distribution of natural single particles

Abstract

Particles on the nano- and micro-scale are produced in a wide range of natural and anthropogenic processes and play a significant role in the biogeochemical cycling of major and trace elements in the environment. Single particle inductively coupled plasma – mass spectrometry (SP ICP-MS) is quickly becoming one of the premier techniques for analysis of nano- or micro-entities. However, SP ICP-MS analysis requires dilute aqueous solutions, free from large particles that could cause blockages. For environmentally relevant samples, like soil extracts, this typically calls for sample clean up. Sample preparation strategies like syringe filtration or ultra-centrifugation are regularly applied to handle complex matrices. The aim of this article is to examine the influence of common preparative strategies on the analysis of both naturally formed and synthetic nanoparticles in complex matrices. To achieve this, water extracts of mineral and sediment standards were spiked with Au nanoparticles and a variety of chemical and physical approaches were investigated to identify which strategies provide the best route to accurately quantifying particle numbers, masses and sizes. In a vast majority of cases, at least 90% of the detectable particles were lost for both particle types whenever filtration or centrifugation was applied. The addition of surfactants like Triton X-100 proved to promote relative particle recoveries of up to 30% for spiked Au particles but the extracted Fe-containing particles continued to have losses of up to 99%. Therefore, common sample preparation strategies are directly impeding the possibilities for quantitative particle analysis by SP ICP-MS. Furthermore, commonly used nanoparticles like Au do not necessarily reflect the reality of nano- and microparticles found in the environment. It is apparent that for SP ICP-MS to become a useful, quantitative method for environmental analysis there must be a high degree of care taken in the collection and preparation stages of analysis.