Solid Sampling Electrothermal Vapourization Coupled to Inductively Coupled Plasma Optical Emission Spectrometry for Geochemical Exploration

Abstract

The direct analysis of solid samples by electrothermal vapourization (ETV) coupled to inductively coupled plasma (ICP) optical emission spectrometry (OES) is typically reserved to niche applications. This thesis is focused on a new niche application: geochemical exploration. First, solid sampling (SS) ETV-ICP-OES was used to determine the distribution of elements in soil samples in the Flin Flon-Snow Lake terrain, Manitoba, Canada in order to locate the undercover ore deposit. Samples were first vaporized by ETV and the vapor was then introduced into the ICP. Under optimal conditions and with a four-step ETV temperature program, the surface distribution of some pathfinder elements in soils showed obvious anomalies delineating the undercover ore. External calibration with 0–4 mg of soil reference material, together with internal standardization with an argon emission line, yielded results for Zn and P in good agreement with those obtained, following aqua regia (AR) digestion, by ICP mass spectrometry (MS). The vertical distribution of target elements in soil samples from the same area was also determined, which revealed anomalously high concentrations toward the surface of the profile in the area above the undercover ore. Again, good agreement was obtained with results by AR-ICP-MS for those elements that could be determined by ICP-MS. Hence, SS-ETV-ICP-OES completely eliminates the need for extraction or digestion of samples prior to analysis, which significantly simplifies the analysis of soils. Finally, in order to increase the amount of solid analyzed, the outlet of SS-ETV-ICP-OES was connected to a switching valve (for venting of pyrolysis products), itself connected to the side arm of a sheathing device whose other inlet was connected to a nebulizer/spray chamber. The aerosol exiting the nebulizer and spray chamber was heated to 400 °C by applying infrared heating to the sheathing device and bottom of the torch while vaporized analytes produced by ETV were transferred to the ICP. This system allowed the amount of sample to be increased from 5 to 13 mg without extinguishing the plasma, resulting in a 2-10 fold improvement in sensitivity and detection limit versus conventional SS-ETV-ICP-OES, and as accurate but more precise analysis of soil .