A portable open-path FTIR instrument has been developed, that incorporates a Midac FTIR spectrometer, a modified White cell, and a battery pack. The design includes a removable cover for the White cell, which allows for ease in path length determination, calibration, and background collection, which are challenging using conventional open-path FTIR spectrometers. In the new instrument, a folded open-path of up to 120 meters is created using mirrors and two retroreflectors. Due to the long path lengths which can be achieved, chemicals can be detected at the part per billion level. In addition, the IR beam is contained within a 60 L cell. Therefore, when concentrations of a chemical are detected in a room, the location of the concentration can be pinpointed. The instrument has been evaluated in an exposure chamber using a range of known concentrations. The chemicals used were nitrous oxide, trichloroethylene, and methyl ethyl ketone. The limit of detection was calculated for each chemical, and found to be in the low part per billion level for nitrous oxide and trichloroethylene. For methyl ethyl ketone, the limit of detection was found to be approximately 0.1 parts per million. For nitrous oxide, performance parameters, including precision, accuracy, and bias were evaluated according to NIOSH guidelines, and the instrument performed well within the NIOSH recommendations, having bias better than +/-10% and accuracy better than +/-25%.