Thermospray Sample Introduction to Atomic Spectrometry

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Abstract
Thermospray aerosols are generated by forcing a liquid sample through a capillary tube that is heated to partially vaporize the solvent, resulting in a blast of vapor that converts the remaining liquid to droplets. The droplet size character of thermospray aerosols can be electrically varied by changing the temperature and degree of solvent vaporization of the liquid stream. The primary droplets produced by thermospray under optimal conditions are smaller on average then those produced by pneumatic nebulizers, particularly of the types used for inductively coupled plasmas (ICPs). Solvent vaporization is enhanced for smaller particles and higher temperatures, with both aspects leading to faster size reduction due to solvent evaporation than would occur with pneumatic sample introduction at room temperature. As smaller droplets are more efficiently transported through sample introduction systems, the use of thermospray aerosol generation provides higher analyte transport, higher sensitivity, and lower LODs than pneumatic sample introduction with most atomic spectrometric detectors. Factors that affect the extent of improvement are the operating temperature of the thermospray vaporizer, the temperature of the spray chamber, the presence or absence of a desolvation system, the diameter of the capillary, and the liquid sample flow rate. The absence of desolvation results in degradation of excitation conditions within ICPs, and in smaller improvements in analytical peformance with ICP atomic emission spectrometry (ICP-AES). Smaller capillary exit diameters provide better performance. Specific LOD improvements with thermospray sample introduction compared to pneumatic sample introduction vary, but typically are a factor of 15 to 25 times lower when desolvation is used with thermospray, and when both systems operate at comparable flow rates. Matrix effects are generally higher with thermospray sample introduction than with pneumatic sample introduction, but are comparable to those reported for ultrasonic nebulization. Thermospray systems have been shown to provide LODs an order of magnitude lower than that obtained with pneumatic sample introduction, even in the presence of high dissolved solids, such as 3000 μg/ml Ca. Thermospray capillaries as small as 25 to 50 μm can operate effectively at optimal conditions with high dissolved solids content samples, without problems of capillary clogging. Thermospray sample introduction has most often been applied to ICP-AES; but also has been studied with ICP-mass spectrometry, flame atomic absorption, and even graphite furnace atomic absorption. The principle applications of thermospray sample introduction to ICP-AES to date have been to environmental analyses, and for detection of discrete samples resulting from flow injection and liquid chromatography. For discrete sampling methods, the advantages are the low extra-column volumes of thermospray systems, which minimize dispersion, and improved sensitivity, which counteracts the effects of unaviodable dispersion, particularly during chromato-graphic separations.