Carrier gas flow optimization for an end-on type of electrothermal vaporizer used for sample introduction in plasma source spectrometry

Abstract

In case of end-on type of graphite tube electrothermal vaporizers (ETVs) the vaporization product is streamed towards the plasma through one of the ends of the graphite tube, which is represented in the present work by the commercial unit of Perkin–Elmer HGA-600MS. The properties of this ETV unit are compared under two sets of gas-flow conditions: (A) using a high flow rate of carrier gas within the graphite tube and a low flow rate of make-up gas that is introduced into the interface close to the outlet end of the graphite tube, and (B) using a decreased carrier gas flow and a relatively high flow rate of make-up gas, while the total transport gas flow rate was kept constant. The ETV unit was coupled to a multi-channel optical emission spectrometer and time integrated line intensities for 26 elements were measured under (A) and (B) conditions and also with the use of pneumatic nebulization of the same multi-element standard solution. A relatively low heating rate was applied to the ETV to minimize plasma loading and also for a better estimation of the appearance temperatures (volatility) of the elements. The intensity ratios with low and high flow rate of the carrier gas (B/A) were between 1.2 and 0.8 for 20 elements. Significantly higher ratios (B/A) were found for volatile elements (Cd and Zn), while the ratios were significantly lower for heavy volatile elements (V, Ti, B). This means that apart from the latter group of elements, the use of a relatively low flow rate of the carrier gas does not result in an essential decrease of analytical sensitivity. This finding can be utilized, e.g. to the direct analysis of finely powdered samples which is handicapped under high carrier gas flow conditions, due to possible sample loss. Comparing the sensitivities with ETV(A) conditions and pneumatic nebulization sample introduction methods there was an unfavorable tendency for the ETV(A) method with decreasing volatility of the analytes. This observation suggests an increased vapor condensation of low volatile elements close to the graphite tube end that is characteristic for this type of ETV units.