342 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA trends in analytical chemistry, vol. 16, no. 6, 1997 [ 43 ] Y. Guo, L.A. Colon, J. Microcol. Sep. 7 ( 1995) 485. [44] J.T. Wu, P. Huang, M.X. Li, M.G. Qian, D.M. Lubman, Anal. Chem. 69 (1997) 320. [ 45 ] J.H. Wahl, D.R. Goodlett, H.R. Udseth, R.D. Smith, Anal. Chem. 64 (1992) 3194. [ 461 J.H. Wahl, D.C. Gale, R.D. Smith, J. Chromatogr. A 659 (1994) 217. The authors are at the Laboratory for Analytical Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 zyxwvutsrqponmlkjihgfedcbaZYX WV, The Netherlands. Determination of inorganic species by thermal lens spectrometry Y. Martin-Biosca, M.C. Garcia-Alvarez-Coque, G. Ramis-Ramos* Burjassot, Valencia, Spain The application of thermal lens spectrometry (TLS) to the determination of inorganic spe- cies is reviewed. The requirements of a chro- mogenic reaction to be advantageously adapted to TLS detection, and the necessary conditions for reaching very low limits of detection are discussed. Methods for the determination of metals and P, Si, S and N compounds in several samples are exam- ined. Flow-injection and extraction methods, associated photoinduced reactions and sur- face phenomena, speciation and equilibrium studies are discussed. Gas phase NO2 meth- ods are also commented upon. Trends are given. 1. Introduction Thermal lens spectrometry (TLS) is a very sen- sitive absorptiometric technique that is used to measure extremely low absorbances [ 14 1. The principle of TLS is the optical measurement of the refractive index gradient which is generated by absorption of a laser beam, followed by degra- dation of the electronic energy to thermal energy. The fundamentals including the models that describe the relationship between the signal, the chromophore concentration, and other experimen- tal conditions, the instrumental approaches and applications with the metrological characteristics *Corresponding author. 01659936/97/$17.00 WSO165-9936(97)00041-l of the corresponding procedures, have been reviewed [ 5-12 1. In a previous work, the determi- nation of organic species by TLS, including bio- chemical applications, was examined [ 13 1. The TLS determination of inorganic species is consid- ered here. The pump-probe double beam design is followed in most TLS experiments. Two lasers, rather than a single laser head with a split beam, are imple- mented to provide the pump and probe beams. The pump beam is used to create the thermal gra- dient and is absorbed by the analyte. Since the sen- sitivity is directly related to the power density, moderate to high pump laser powers, and tight focusing of the pump beam at the sample location, are used. The gradient is measured by the deflection or the defocusing of the probe beam, which pref- erably should not be absorbed by the sample. In addition, the beams in the sample region can be geometrically arranged following either the coaxial or the crossed-beam configurations. The latter has a reduced measurement volume, limited to the region where the two beams cross, which is useful when spatial resolution is required, e.g., for detection in capillaries and small cells. The pump beam can be used simultaneously to generate the thermal gra- dient and to excite the laser-induced fluorescence of the sample. This approach has been applied to the detection of rare-earth ions after HPLC separation zyxwvu [141. To estimate how much the sensitivity is enhanced with respect to conventional spectropho- tometry, the enhancement factor, E, is used: E--lI A (1) where (AZ/Z) is the TLS signal and A is the absorb- ance. This factor is sometimes reported as related to 0 1997 Elsevier Science B.V. All rights reserved.