Multiplexed carbon braid ETV and tandem ETV-nebulizer sample introduction for ICPMS
This research focuses on electrothermal vaporization (ETV) as a sample introduction source for inductively coupled plasma mass spectrometry (ICPMS). ETV creates a dry plasma that causes problems when used at the high applied powers (e.g., 1.2 kW) typically employed for nebulizer-based sample introduction for certain ICPMS instruments. A secondary discharge forms in the sampling region of the spectrometer, but this effect was removed by reducing the applied power (e.g., 0.7 kW). A novel, steady state, dry aerosol introduction system was developed to permit optimization of the ICPMS settings. The device used solid NbF₅, SnBr₄ and a W filament plated with Pb to generate dry aerosols which produced ²⁸Si⁺, ⁷⁹Br⁺, ¹²⁰Sn⁺, ¹⁸⁴W⁺, and ²⁰⁸Pb⁺ that were used to optimize the ICPMS. When compared to an ICPMS optimized using a nebulizer, the dry optimized plasma produced an average enhancement of 4.5(±0.4) for 26 elements when using ETV sample introduction. The ETV produces a short (0.5-2 s) transient pulse once every 2-3 min, resulting in a sample throughput of 20-30 samples/h. To increase this throughput, a low power, low background multiplexed ETV device using carbon braids as vaporizers was developed. Oxygen ashing was demonstrated with recoveries of 96±17% for a suite of elements. Refractory elements (e.g., V and U) showed precision of greater than 25% while medium to high volatility elements were in the 10-20% range. The lifetime of the braids was limited to ca. 30 vaporizations when heated to 2,800°C. A device for coupling an ETV in parallel with a nebulizer was also designed and characterized. The device was designed to minimize the impact of the ETV's presence on nebulizer-based ICPMS performance. The ETV could be easily switched on line to provide complimentary information that may be unavailable with nebulizer sample introduction because of isobaric interference problems. For example, in a 1% HCl matrix, the detection limits for ⁵¹V⁺ (⁵¹ClO⁺ interference), ⁷⁵As⁺ (⁷⁵ArO⁺ interference) and ⁷⁸Se⁺ were found to be 0.008 ppb, 0.088 ppb, 0.063 ppb, respectively. By contrast, the nebulizer detection limits in 1% HCl for ⁵¹V⁺, ⁷⁵As and ⁷⁸Se were found to be 0.593 ppb 1.488 ppb and 1.158 ppb, respectively.