Improving the Sensitivity of NIR Spectroscopy with an Enrichment Technique: Determining a Trace Analyte of Ethyl Carbamate

Abstract

Near-infrared (NIR) spectroscopy has proven to be a powerful technique for the quantitative analysis of chemical samples. Normally, NIR absorption bands are far weaker than IR absorption bands; for example, a molar absorption coefficient of water in the NIR region is about 1/1000 of that in the IR region. Therefore, an NIR analysis of samples often shows a lower sensitivity, which encounters difficulty in the determination of low-concentration analytes. Only a few reports were found to be related to the use of NIR to determine low-concentration analytes.Quite recently, Blanco et al. 1 reported their work on the determination of low-concentration analytes with NIR spectroscopy directly. Chemometric methods, orthogonal signal correction (OSC) and net analyte signal (NAS), were used to pretreat the NIR spectra in order to reduce noise or to suppress extraneous contributions and improve the results when the amounts of the analyte were of low level. The samples that they used were mixtures of esters of 2-ethylhexanol with middle-and long chain fatty acid. The concentrations' range of 2-ethylhexanol was from 20 to 1600 ppm. With the method they developed, the detection limit was measured and computed; the value was around 100 ppm.Lima et al. 2 prepared a sensing phase of polydimethylsiloxane (PDMS) and enriched aromatic hydrocarbons on its surface to increase the concentration of analytes, which made it feasible for NIR spectroscopy to determine aromatic hydrocarbons of trace-level concentration in water.The water samples, separately with known amounts of benzene, toluene, ethylbenzene, and m-xylene, were used for evaluating the PDMS sensing phase; measurements were made in a closed reactor with constant stirring. Equilibrium states spent 90, 180, 360, and 405 min for benzene, toluene, ethylbenzene, and m-xylene, respectively. The PDMS sensing phase showed a reversible response, presenting linear response ranges of up to 360, 290, 100, and 80 mg L -1 , with detection limits of 8.0, 7.0, 2.6, and 3.0 mg L -1 for benzene, toluene, ethylbenzene, and mxylene, respectively. Later, they 3 employed a salting-out effect to improve the limit of detection of this method. Measurements performed in water provided detection limits (3sblank/sensitivity) of 0.080, 0.12, 0.14 and 0.27 mg L -1 for benzene, toluene, ethylbenzene and m-xylene, respectively, under the optimum condition of a 5.0 mm-height disk and 2.0 mg L -1 sodium chloride. Their work 2,3 showed a wonderful perspective of lowconcentration analytes determination by NIR spectroscopy coupled with an enrichment technique.In the quantitative analysis of micro and trace quantities, even ultratrace samples, extraction and enrichment in sample preparation are often used. With extraction and enrichment, the concentrations of analytes may increase by tens to hundreds of times. Therefore, it is an expectable method to improve the power of the NIR spectroscopy technique in low-concentration analysis. Although Pimentel and his coworkers 2,3 have shown some wonderful results in this ...

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