Improving the detection limits of near infrared spectroscopy in the determination of aromatic hydrocarbons in water employing a silicone sensing phase

This work describes the use of a silicone sensing phase for the detection of aromatic hydrocarbons in water employing near infrared spectroscopy and the exploitation of the salting-out effect as a means of improving the limits of detection of the method. Sensing phases of polydimethylsiloxane (PDMS) were prepared from Silastic T2 (Dow Corning). Determinations were carried out by immersing a disk in a flask containing an aqueous solution of aromatic hydrocarbon (benzene, toluene, ethylbenzene and the xylenes, commonly called BTEX) and left under constant stirring. Afterwards, the disk was removed from the contaminated water, dried and placed in a home-made holder for absorbance measurements from 800 to 2500 nm, employing an MB 160 Bomem spectrophotometer. By employing a 2.0-mm height disk, measurements performed in water provided detection limits (3sblank/sensitivity) of 1.1, 1.8, 1.1 and 1.1 mg L−1 for benzene, toluene, ethylbenzene and xylenes, respectively, while LOD values of 0.6, 0.7, 0.6 and 0.7 mg L−1 were obtained when sample solutions contained 2.0 mol L−1 sodium chloride. Detection limits of 0.080, 0.12, 0.14 and 0.27 mg L−1 were obtained with the use of a 5.0-mm height disk and 2.0 mol L−1 sodium chloride. Sodium chloride improves the limits of detection due to its salting out effect; however, headspace in the flask must be avoided during measurements, as the hydrocarbons are preferentially lost to the air. The precision of the proposed method was evaluated by constructing three analytical curves for toluene, providing average slopes and linear coefficients with relative standard deviations of 5.8 and 3.8%, respectively.