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

Single particle inductively coupled plasma mass spectrometry (spICP-MS) offers a new tool to explore for con-cealed 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 mu M 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. Addi-tional 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.