The infrared sensing method is a powerful tool in the detection of organic species in aqueous solutions because its detection speed can be quite fast and it can also provide abundant chemical information about the examined species. Among the detection of organic species in aqueous solutions, attenuated total reflection (ATR) 1 is most suitable to handle aqueous solutions, because the evanescent wave penetrates into the adjoining medium a short distance and, typically, the depth of penetration (dp) is in the range of a few micrometers. Many successful applications of the ATR-IR method can be found in the literature using bare ATR sensing fibers. 2-6 Although the short dp can limit the absorption of a strong absorber, water, the signals of organic species were also limited. To increase the sensitivity, the principle of solid-phase micro-extraction 7-10 was applied to the ATR-IR sensing method. [11][12][13][14][15][16][17][18] The sensitivity of the SPME/ATR-IR method was largely enhanced because SPME films exclude water molecules from the evanescent wave and attract organic species into a region near the sensing crystal to enhance the signal. Although this type of method provides a direct way to detect organic compounds in aqueous solution, a tedious optical arrangement and an expansive ATR crystal are required.To eliminate these weaknesses, transmission absorption (TA) infrared spectroscopy has also been applied to detect small amounts of organic compounds in aqueous solutions by combining the principle of SPME to that of the transmission mode. Successful applications can be found in the literature. [19][20][21] For example, poly(dimethylsiloxane), 19 parafilm, 20 and perfluoroalkoxy Teflon 21 have been used as the SPME phase to extract organic compounds in aqueous solutions, and be sequentially detected by the TA-IR method. Although this method can eliminate the use of expensive IR windows, problems arise in preparing the standing films, which should exhibit low compactness, high IR radiation transmission, and a small variation after soaking in aqueous solutions. These problems restrict the application of the TA-IR method to detect organic compounds in aqueous solutions, especially in the detection of more polar compounds.To eliminate both problems associated with the ATR and TA methods, a reflection-absorption mode has also been proposed to detect organic compounds in aqueous solutions. 22,23 In this method, hollow waveguide samplers are prepared by coating hydrophobic film on the inner surface of hollow IR waveguides. After the aqueous solution passes through this coated waveguide, the organic compounds can be trapped into the SPME layer. The absorbed organic compounds were further sensed by infrared radiation based on the principle of multireflection-absorption. However, the complex production procedures in the formation of a hollow waveguides sampler restricted the spreading of this technique. To further simplify a reflection absorption type of method, a single-reflection method combined with the principle o...

Infrared (IR) optical sensors provide fast and nondestructive methods for the detection of organic compounds in aqueous solutions. Among the existing IR methods, attenuated total reflection (ATR) combined with the solid-phase micro-extraction (SPME) principle is the most frequently used method in the detection of organic compounds in aqueous solutions. In an effort to simplify this method for aqueous solution analysis, the SPME/transmission absorption infrared (TA-IR) method was developed in this work, and its performance was compared with that of the ATR-IR method. Results indicated that the SPME/TA-IR method provided better linearity (0 to 200 ppm concentration range) and sensitivity than the SPME/ATR-IR method in detection of chloroanilines. The lower linearity in the SPME/ATR-IR method was most likely caused by the change in the refractive index of the SPME phase after the absorption of different amounts of analytes. Because of the high-energy throughput in the SPME/TA-IR method, which largely reduced the noise level in the spectra, this method can provide slightly lower detection limits than the SPME/ATR-IR method. Another advantage of this new method is that the influence on IR signals by the SPME phase thickness phase can be partially compensated.