Adelina Smirnova 1, 2 Mikhail A. Proskurnin 3 Svetlana N. Bendrysheva 3 Dmitry A. Nedosekin 3 Akihide Hibara 1 Takehiko Kitamori 1, 2 1 The University of Tokyo, Tokyo, Japan 2 Japanese Science and Technology Agency (JST), Saitama, Japan 3 Analytical Chemistry Division, M. V. Lomonosov Moscow State University, Moscow, Russia Received December 15, 2007 Revised February 15, 2008 Accepted February 19, 2008 Research Article Thermooptical detection in microchips: From macro- to micro-scale with enhanced analytical parameters In this paper, we compared the methods of photothermal spectroscopy used in different spatial scales, namely thermal-lens spectrometry (TLS) and thermal-lens microscopy (TLM) to enhance the performance parameters in analytical procedures. All of the experimental results were confirmed by theoretical calculation. It was proven that the design for both TLM and TLS, despite a different scale for the effect, is governed by the same signal-gen- erating and probing conditions (probe beam diameter at the sample should be equal to the diameter of the blooming thermal lens), and almost does not depend on the nature of the solvent. Theoretical and experimental instrumental error curves for thermal lensing were coincident. TLM obeys the same law of instrumental error as TLS and shows better repeat- ability for the same levels of thermal-lens signals or absorbances. TLS is more advantageous for studying low concentrations in bulk, while TLM shows much lower absolute LODs due to better repeatability for low amounts. The behavior of the thermal-lens signal with differ- ent flow rates was studied and optimum conditions, with the minimum contribution to total error, were found. These conditions are reproducible, are in agreement with the exist- ing theory of the thermal response in thermal lensing, and do not significantly affect the design of the optimum scheme for setups. TLM showed low LODs in solvent extraction (down to 10 –8 M) and electrokinetic separation (10 –7 M), which were shown to be governed by discussed instrumental regularities, instead of by microchemistry. Keywords: Detection in microchips / Optical-scheme optimization / Photothermal spectros- copy / Thermal-lens microscopy / Thermal-lens spectrometry DOI 10.1002/elps.200700914 Electrophoresis 2008, 29, 2741–2753 2741 1 Introduction Thermal-lens spectrometry (TLS) has rapidly increased in demand as a nondestructive technique for molecular absorption spectroscopy. It has already evolved into a highly sensitive and versatile analytical technique [1, 2]. A primary feature of TLS is that its overall sensitivity (subnanogram LODs down to 10 –7 absorbance units) is achieved not only through instrumentation, but also because of better condi- tions for photometric reactions at trace levels [3]. Other key features of TLS, locality and high spatial resolution, have led to the vigorous use of thermal-lens microscopy (TLM) as a novel detection method [4–7]. During the last decade, the development and application of micro-total analytical sys- tems (mTAS) has become the most progressive area of scien- tific research in analytical chemistry, biochemistry, and medicine [8–10]. TLM perfectly fits microfluidic analysis (reactions and separations on microchips occurring at high rates and in nanoliter-scale volumes) [5]. In TLM–mTAS, fine- tuning of experimental conditions from the viewpoint of integrated chemistry on in a micro-scale, accounting for micro-detection systems, is crucial for achievement of the full potential of this combination. In our previous reports devoted to the instrumentation of thermal lensing, we showed that the combined use of the data on accuracy and precision of thermal lensing [11] and specific features of the reactions at the nanogram level enhances the sensitivity and selectivity of procedures. We also found that crossed-beam TLS (CB-TLS), the most accepted photothermal-detection technique in capillary elec- trophoresis [12], and TLM [7, 13] have differences in appara- Correspondence: Professor Takehiko Kitamori, Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan E-mail: kitamori@icl.t.u-tokyo.ac.jp Fax: 181-3-5841-6039 Abbreviations: CB-TLS, crossed-beam thermal-lens spectrome- try; Co-nn 3 , Co(III) Tris-(2-nitroso-1-naphtholate); Fe-Phen, Fe(II) Tris–(1,10-phenanthrolinate); TLM, thermal-lens microscopy; TLS, thermal-lens spectrometry  2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com