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: Cu₁S_x⁻ (x = 2, 3, 4…), Cu₂S_y^−[rad] (y = 3, 4, 5…), and Cu₃S_z⁻ (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 ⁶³Cu₂⁶⁵Cu³²S₃⁻ and ⁶³Cu³²S₇⁻ 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.