Single particle inductively coupled plasma mass spectrometry: A new method to detect geochemical anomalies in stream sediments

Abstract

Single particle inductively coupled plasma mass spectrometry (spICP-MS) offers a new tool to explore for concealed mineral deposits. Through detection of metal nanoparticles (NPs) present in stream sediments, subtle geochemical anomalies may be identified farther from their source. Geochemical exploration with spICP-MS lowers the detection limit substantially from conventional ICP-MS for insoluble elements of interest. Analyzing only 197Au signal generated by individual NPs rather than an integrated signal, which can have a significant contribution from detector noise, resulted in a 53-fold reduction in detection limit compared to a digestion/quadrupole ICP-MS determination (21 ng∗L−1 to 0.39 ng∗L−1). Method development included analysis of dilute suspensions of manufactured 50 nm Au particles and three stream sediment samples (5–15 g sonicated in 20 mL 25 μM 11-mercaptoundeconoic acid). Au and Ag NPs were identified in the two sites downstream from a known area of mineralization, whereas none were detected in a background stream sediment site. Average particle numbers and masses of Au and Ag were highest in the sample closest to known mineralization. Additional results indicated that use of surfactant is an important contribution to the procedure. This pilot study of three sites provides a proof of concept in the analysis of spICP-MS for determination of Au and Ag in stream sediment samples, which could augment or improve current cyanide/acid dissolution/ICPMS methods for the ultra-trace concentration range.