Infrared Reflection-Absorption Method for the Detection of Aromatic Compounds in Aqueous Solutions with Limited Sample Volumes

Abstract

An infrared reflection-absorption (IR/RA) method was developed to detect aromatic organic compounds in aqueous solutions where the required sample volume can be as low as 50 µL. Two aluminum plates were used to form the sampling cell for the detection of small amount of aqueous samples. One plate was used as an IR reflection substrate and a second plate, in which several holes were drilled, was placed tightly on the top of the reflection plate to form cavities for sampling. The cavities were further coated with hydrophobic film. After the hydrophobic film dried, a certain amount of aqueous sample was injected to the cavity. Analytes in the aqueous solution were attracted into the hydrophobic film through the solid phase micro-extraction principle. After residual water was removed from the cavity, organic compounds absorbed by the hydrophobic film could be sensed using IR radiation based on the reflection-absorption mode. To investigate the applicability of this type of sensing method for small-volume detection, factors such as the volume of the aqueous solution, the sample concentration, size of the cavity and the sensitivity of this method were investigated. An examination of the linear relationship between the signals and the analyte concentrations showed regression coefficients that were generally in the range of 0.992 to 0.999 for the examined analytes in the concentration range of 10 to 100 ppm. Under the condition that the sample volume was 100 µL and based on three-times the spectra noise level, the calculated detection limits for this method were found at around 1 ppm for the examined analytes.