This invention relates to a detection system for detecting an analyte in a fluid medium. The detection system comprises a substrate that provides mechanical stability and is sized and shaped to intercept an infrared beam. A reactive material is coated on the substrate. When contacted with the analyte in the fluid medium, the reactive material reacts with the analyte and is altered. The detection system also comprises an infrared spectrometer producing the infrared beam that passes through the reactive material to a detector of the spectrometer. The alteration of the reactive material allows the spectrometer to identify and quantify the analyte. In one embodiment, the reactive material irreversibly reacts with the analyte. In another embodiment, the spectrometer is a non-ATR infrared spectrometer. In a further embodiment, the substrate is a disposable substrate.