| Description |
Some analytically important elements such as As, Sb, Bi, Se, Pb, Sn, Te or Ge can be efficiently converted from a liquid sample into their volatile hydrides, yielding a 100% analyte introduction efficiency into the spectrometric detector, and surpassing thus, by an order of magnitude, the performance of common pneumatic nebulizers (5-10%). Atomic absorption spectrometry (AAS) is still the daily workhorse in trace element analysis, offering affordable instrumentation, method robustness and operator friendliness. Externally heated quartz tube atomizers (QTA) are the most common hydride atomizers in hydride generation (HG) AAS. Recently, the dielectric barrier discharges (DBD), i.e. low power and low temperature plasmas sustained by alternating voltage at atmospheric pressure, have been proven as promising hydride atomizers at least for some of the elements listed above. Other types of atmospheric plasma discharges, namely a RF plasma jet called plasma pencil, or a discharge sustained inside a quartz tube between two bare electrodes, i.e. with no dielectric barrier, have been investigated as alternative hydride atomizers, especially to overcome difficulties with atomization of Pb, Sn or Ge hydrides in DBD. Experimental conditions were optimized individually for each analyte in all three plasma atomizer designs. Sensitivities and limits of detection were determined as basic figures of merit to assess the atomizer applicability to routine measurements. Each atomizer design was also characterized in terms of plasma temperature, absolute concentration and spatial distribution of H radicals as important species responsible for hydride atomization determined by two photon laser induced fluorescence (TALIF). For selected elements, the absolute concentration of free analyte atoms and their spatial distribution was measured by LIF allowing to quantify atomization efficiency. Detailed insights into hydride atomization mechanisms, which are strongly element-dependent, will be shown using Ge and Se as target analytes. The sensitivity trends observed in AAS measurements correlate perfectly with atomization efficiency determined by LIF. Also the tendency of individual analytes to deposit in the discharge area, quantified by leaching experiments, seems to agree with element dependent life-time of free atoms. The advantages of plasma diagnostics by LIF and TALIF, and the importance of these methods for further improving the hydride atomizers performance will be discussed. Potential coupling of the plasma sources investigated with other atomic spectrometric detectors such as atomic fluorescence or optical emission spectrometry will be outlined.
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