Direct quantification of aromatic hydrocarbons in geochemical fluids with a mid-infrared attenuated total reflection sensor

An improved analytical method for directly and rapidly quantifying various monocyclic and polycyclic aromatic molecules in oil-water mixtures has been developed. This study reports on the application of a sensor based on attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) for determining the concentration of a number of volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) in water. The mid-infrared (MIR) sensor consists of a zinc selenide (ZnSe) waveguide with the surface modified by a thin poly(isobutylene) (PIB) coating. The sensitivity was investigated at different polymer film thicknesses and molecular weights. The analytical performance of the sensor was validated in the laboratory against a standard analytical technique for analyzing petroleum based samples. We have shown that the sensor may accurately quantify the benzene, toluene, ethylbenzene, xylenes and naphthalene (BTEXN) concentration in an oil-water mixture. The ATR-FTIR method overcomes the limitations associated with sampling/sample preparation and has the advantage that it may easily discriminate between meta- and para-xylene, which is difficult with conventional analytical techniques (i.e., GC). In addition, this technology may potentially be deployed in the field for geochemical mapping and to monitor in situ the concentration profile of a number of hydrocarbons in geological formations (e.g., petroleum systems). To demonstrate this, a miniaturized prototype of an ATR-IR sensor system taking advantage of planar-tapered silver halide fibers has been developed and tested.