Detection and quantification of trace organic contaminants in water using the FT-IR-attenuated total reflectance technique.

The Fourier transform infrared-attenuated total reflectance (FT-IR-ATR) technique has been used to detect and quantify the following volatile organic compounds (VOCs) in water: 1,1,1,2-tetrachloroethane, 1,1,2,2-tetrachloroethane, styrene, and tetrachloroethylene, among which the first three compounds were investigated at parts per million levels for the first time. Enhancement of the detection was made by (1) coating the ATR crystal with a hydrophobic polymer membrane, (2) optimizing the flow rate of the sample solution, (3) varying polymer membrane thickness, and (4) increasing the number of reflection bounces within the ATR crystal. Our flow rate optimization confirmed a previous finding that turbulent flow is more favorable than laminar flow in detecting the VOCs in water. However, decreases of ATR signal intensity were observed at very high turbulency due to analytes flowing too quickly through and exiting the ATR cell to be adsorbed onto the polymer membrane. The optimal membrane thickness was found to be associated with the maximum overlap between the IR evanescent wave penetration depth and the analyte diffusion depth. Consequently, there is no universal optimal flow rate and optimal polymer membrane thickness for detection of all VOCs. Doubling the number of IR reflection bounces within the ATR crystal enhanced both detection and sensitivity by about a factor of 2. Finally, it was observed that the detection limit concentrations decrease with the water solubility of the VOCs.