Chalcophile chemistry for enhanced detection of copper in its compounds and minerals

Samples of elemental copper, upon laser irradiation under negative-ion LDI-MS conditions, do not generate noticeable signals for copper-bearing gaseous ions. In contrast, a several thousand-fold enhancement of the generated ion current was observed when freshly made mixtures of copper and sulfur powders were laser-ablated. Time-of-flight mass spectra recorded showed a range of peaks with m/z ratios extending to over m/z 8000, indicating the formation in situ of an array of Cu-S ion clusters by a chemical reaction that takes place between the two elements upon laser irradiation. Evidently, sulfur acts as a potent reactive matrix and generates at least three distinguishable series of ion clusters: Cu1Sx- (x = 2, 3, 4 ...), Cu2Sy-center dot (y = 3, 4, 5 ...), and Cu3Sz- (z = 3, 4, 5 ...). Laser ablation of CuS alone did not produce signals beyond m/z 600. Some of the high-mass ions formed in this manner were isobaric: consequently, composite peaks were observed under low-resolution MS conditions. Under high-resolution conditions, however, we were able to separate and determine the composition of some isobaric mixtures. For example, the peak recorded at m/z 287 was resolved to two peaks that represented primarily the (Cu2CuS3-)-Cu-63-Cu-65-S-32 and (CuS7-)-Cu-63-S-32 ions. Intriguingly, the spectra recorded from many copper-containing minerals mixed with sulfur were remarkably similar to those acquired from copper-and-sulfur mixtures, which demonstrated that elemental sulfur has the ability to extract chemically bound copper even from its compounds, and form gas-phase Cu-S clusters of varying composition. (C) 2019 Elsevier Ltd. All rights reserved.