| Novel approaches to thermal vaporization sample introduction for inductively coupled plasma (ICP) spectrometry using techniques with demountable sample supports are presented and compared. Developments and applications are presented for two sample introduction arrangements that use interchangeable sample probes in contact-free heating environments: direct sample insertion (DSI), and induction heating-electrothermal vaporization (IH-ETV). For the well-established technique of DSI, studies focused on the development and application of a pyrolytically coated graphite sample support, a feature common to conventional ETV systems. A process for coating sample probes in the ICP discharge was developed, and improvements were seen in both the reproducibility of the volatilization event and the appearance of the transient emission signals with ICP-optical emission spectrometry (OES) when a coated probe was used over an uncoated one. The pyrolytically coated sample probe was successfully used for the direct determination of several metals in wood pulps by ICP-OES, and was found not only to improve the appearance of the temporal signals, but also demonstrated a heightened resistance to chemical attack. For the prototypical IH-ETV system, a general study was first conducted to establish performance attributes and benchmarks such as heating characteristics, transport efficiency, and ICP-OES detection limits using several mixed carrier gases. The IH-ETV arrangement was found to be capable of rapid heating rates and precise, reproducible temperature control suitable for a thermal vaporization sample introduction technique. Of all the gas mixtures studied, the incorporation of 15% (v/v) SF6 into the Ar carrier flow resulted in the best overall conditions for the vaporization, transport and detection of analytes by ICP-OES. These conditions were successfully used for the determination of various metals in soil and sediment samples by ICP-OES using IH-ETV sample introduction. Additionally, a novel arrangement that uses gas sampling bags for trapping and dissoluting analyte was developed and validated for the purposes of determining IH-ETV transport efficiency. This closed-system trapping method, which can be used with any ETV system, generally performed as well as the reference method and was simpler to carry out. Based on the examination and comparison of the above experiences with DSI and IH-ETV, the utility of demountable sample support arrangements for thermal sample introduction are evaluated and recommendations for future instrumentation and methodologies are made. |
