Kenji Uchiyama 1 Akihide Hibara 2 Kiichi Sato 2 Hideaki Hisamoto 2 Manabu Tokeshi 1 Takehiko Kitamori 1, 2 1 Integrated Chemistry Project, Kanagawa Academy of Science and Technology, Kawasaki, Japan 2 Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Tokyo, Japan An interface chip connection between capillary electrophoresis and thermal lens microscope A thermal lens microscope (TLM) detection of capillary electrophoresis (CE) utilizing microchip technology was developed. Fused-silica capillaries with an inner diameter of 50 mm were directly connected to a microchannel in a microchip. The detection limit by TLM was estimated as 2.8610 27 absorbance by measuring pure water. The detection limit of derivatized amino acids determined by CE-TLM was estimated as 2.4610 28 M, which was 100 times lower than that of conventional absorbance detection. Keywords: Capillary electrophoresis / Dead volume-free connection / Microchip / Microfabrica- tion / Thermal lens microscope EL 5209 1 Introduction Capillary electrophoresis (CE) is one of the most widely used techniques for high-resolution separations of chem- icals and biochemicals [1]. A wide variety of detection techniques have now been coupled to CE with varying degrees of success [2]. Although an ultraviolet-visible (UV-Vis) absorbance technique is most frequently used for detection in CE because of its wide applicability, it is not highly sensitive [1]. A laser-induced fluorescence (LIF) technique is a very sensitive detection method which can be used with CE, and is capable of single molecule detection [3], but often requires chemical derivatization. Other detection methods have been used with CE for var- ious purposes, i.e., potentiometric [4], conductivity [5], amperometric [6], refractive index [7], Raman [8], NMR [9], radioisotope [10], and laser-induced capillary vibration [11]. The requirements placed on the detection method used with CE are wide applicability and high sensitivity. On the other hand, microfabricated devices known as micrototal analysis systems (m-TAS) or lab-on-a-chip have become of major interest recently [12]. For detection on microfabricated devices, a variety of methods have been proposed in the same manner as CE [13]. We have developed a thermal lens microscope (TLM) based on photothermal effect in order to detect nonfluorescent molecules in a liquid [14–16] and on a surface [17]. Photo- thermal spectroscopy offers highly sensitive methods to monitor the heat distribution (refractive index distribution) resulting from absorption of optical irradiation [18]. Be- cause the most common relaxation process following absorption of light causes photothermal energy-conver- sion phenomena, the applicability of the TLM is very wide and we have successfully demonstrated this suit- ability in microfabricated devices [19–24]. Microfabrication techniques have allowed complex pat- terns and designs of microchannels to be fabricated easily on glass and plastic substrates [25, 26]. Since it is possible to fabricate several microchannels on a sub- strate, on-chip postcolumn reactions could be integrated on a microchip [27]. Although microchip electrophoresis has great potential, folded long microchannels are required for better separation. Band broadening which affects the separation conditions and is due to the curva- ture of the folded microchannel should be minimized by controlling the channel width and curvature radius [28]. In addition, highly sensitive detection of microchip elec- trophoresis has only been realized using a laser-induced fluorescent (LIF) method. Due to the short optical length of the microchip, highly sensitive detection is difficult. Some groups have tried to increase the optical length in the microchannel by utilizing some part of the micro- channel as an optical cell, where the length corresponds to the slit width of the detector. Therefore, optimization of the optical length and the slit width should be considered in this approach in order to provide high sensitivity of separation. Dovichi and co-workers [29, 30] developed a thermoopti- cal absorbance detection system for CE. Although the system had very good sensitivity and applicability, optical alignment of the setup seemed very difficult and it may Correspondence: Professor Takehiko Kitamori, Department of Applied Chemistry, Graduate School of Engineering, The Univer- sity of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan E-mail: kitamori@icl.t.u-tokyo.ac.jp Fax: +81-3-5841-6039 Abbreviations: DABSYL, 4-dimethylaminoazobenzene-4’-sul- fonyl; TLM, thermal lens microscope Electrophoresis 2003, 24, 179–184 179  2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0173-0835/03/01-0201–179 $17.501.50/0 Miniaturization