A deep-UV light-emitting diode-based absorption detector for benzene, toluene, ethylbenzene, and the xylene compounds

The BTEX detector employs the emission band of an ultraviolet light-emitting diode at 260 nm as monochromatic light source and photodiodes for the deep UV-range as reference and signal detectors. Optical fibres are used for coupling the light to the absorption cell of 40 cm length as well as to the reference photodiode. The use of an integrated circuit log-ratio amplifier allows the direct determination of absorbance values according to Lambert-Beer's law. Linear calibration curves over two orders of magnitude, between about 1 ppm and 100 ppm, were obtained for benzene, toluene, ethylbenzene, o-xylene, m-xylene and p-xylene. There is some variation of the sensitivity between the six species, which correlate with differences in the absorption spectra, with toluene, ethylbenzene, o-xylene and m-xylene showing close values between 150 μAU/ppm and 185 μAU/ppm, while benzene with 62 μAU/ppm has a somewhat lower and p-xylene with 235 μAU/ppm a somewhat higher sensitivity. The limits of detection were determined as approximately 1 ppm.