Anal. Chem. 2001, 73, 4688-4693
Microfluidic Device for Airborne BTEX Detection Yuko Ueno, Tsutomu Horiuchi, Takashi Morimoto, and Osamu Niwa
NTT Lifestyle and Environmental Technology Laboratories, 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan
We fabricated a microfluidic device for the optical detection of airborne benzene, toluene, ethylbenzene and xylenes (BTEX). The device consists of concentration and detection cells formed of 3 cm × 1 cm Pyrex plates. The concentration cell is composed of an adsorbent to concentrate the BTEX gases and a thin-film heater to desorb the concentrated gases from the adsorbent thermally. The collected gases are introduced into the detection cell, which is connected to optical fibers, to measure their absorption spectra. We optimized the device’s operating conditions by studying the thermal characteristics of the concentration cell and the time profile of the gas concentration flowing in the detection cell. We used the device under optimized operating conditions to detect toluene gas as a typical example BTEX. The gas concentration amplification rate was ∼2 orders of magnitude, and we successfully measured parts-per-million levels of toluene gas with this device. Airborne benzene, toluene, ethylbenzene, and xylenes (BTEX) are volatile organic compounds (VOCs) of great social and environmental significance. Because they are toxic even at partsper-billion concentrations, it is important to know their concentration in the air. Benzene is classified as a human carcinogen and is a risk factor for leukemia and lymphomas.1 The regulated standard concentration of benzene is 3 µg/m3 (1.0 ppb) in Japan. The guidelines for the upper indoor concentration limits of toluene, ethylbenzene, and xylenes are 260 µg/m3 (0.07 ppm), 3800 µg/m3 (0.88 ppm), and 870 µg/m3 (0.20 ppm), respectively. The most widely used method of detecting VOCs is gas chromatography (GC)/mass spectrometry (MS).2 A thermal desorption and cold trap (TCT) injection system is usually combined with GC to provide higher resolution.3 The TO-14 method developed by the United States Environmental Protection Agency (EPA) also employs the GC/MS method for VOC detection,4 because this method has several advantages, namely a parts-per-trillion detection limit, high selectivity, and high (1) See, for example: Carcinogenic Effects of Benzene: An Update, 1998; EPA/ 60/P-97/001F; U. S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 1998. (2) See, for example: Bruner, F. Gas Chromatographic Environmental Analysis: Principles, Techniques, Instrumentation; John Wiley & Sons: New York, 1993. (3) Suzuki, S. Anal. Sci. 1995, 11, 953-60. Pankow, J. F.; Luo, W.; Isabelle, L. M.; Bender, D. A.; Baker, R. J. Anal. Chem. 1998, 70, 5213-5221. (4) Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air, 2nd ed.: 1999; EPA/625/R-96/010b, Method TO-14A; U. S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 1999.
4688 Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
accuracy. However, in terms of field monitoring, it has a crucial disadvantage in that the size and weight of a GC/MS instrument can only be reduced to a certain degree without degrading selectivity and sensitivity, although several techniques related to portable GC or GC/MS instruments have been investigated.5 Other kinds of portable VOC sensors have also been studied. Examples include quartz crystal microbalances,6 surface acousticwave arrays,7 and metal oxide sensor arrays;8 however, they are poor at identifying chemically and structurally similar BTEX because the BTEX responses to these sensors are very similar. The aim of this work is to develop a microfluidic device designed to detect and identify atmospheric levels of BTEX. We used an optical method, absorption spectroscopy, for BTEX detection. Our microfluidic device combined with an optical detection system has a number of merits. (1) The use of a microfluidic system makes it possible to reduce the sample amount. Our device requires ∼10 mL of sample gas and
