Microwave-assisted continuous leaching on-line with inductively coupled plasma mass spectrometry for exploration and environmental geochemistry

Focused-microwave heating was used in combination with simultaneous multielemental detection through ICP-TOFMS (inductively coupled plasma time-of-flight mass spectrometry) at a higher leaching flow rate than had been used with ICP-HRMS (ICP with high resolution mass spectrometry) to speed up the continuous leaching of soils. Compared to leaching at room temperature under otherwise identical conditions, heating the mini-column to 90 °C using focused-microwave energy significantly enhanced the release of several analytes from three mono-mineralic samples (pyrolusite, hematite and malachite), a 1:1:1 artificial mixture of the three, and a sandstone sample, by up to an order of magnitude. Furthermore, the resulting leaching profiles more closely resembled those obtained by ICP-HRMS at room temperature than those obtained at the same temperature by ICP-TOFMS. This indicates that the enhanced dissolution kinetics achieved through microwave heating compensated for the faster leaching flow rate. Thus, the whole fractionation study could be performed within about 16 min, which is substantially shorter than the several hours that are required for continuous leaching with ICP-HRMS. Although ICP-MS (with either TOF or quadrupole) is susceptible to spectroscopic interference, which could preclude the identification of some elements, it could nonetheless be used as a work horse for screening samples i.e. identify those that should be submitted to continuous leaching with ICP-HRMS for confirmation.