Sensitive spectroscopic methods are required to probe chemical kinetics because, in general, the reactive radical species of interest are present only at low concentration. Fluorescence methods are exquisitely sensitive; however many important radicals, for example HO2, do not fluoresce. Moreover, determining absolute concentrations from fluorescence measurements normally requires extensive calibration. For these reasons, sensitive absorption techniques have been used in the Laser Chemistry Laboratory (PI: Craig A. Taatjes). As shown in the figure, a Herriott-type multipass cell is used to send a probe beam many times through a long flow reactor. The multipass cell is combined with either simple differential absorption or two-tone frequency modulation absorption spectroscopy to further enhance the sensitivity. The overlap between the probe laser beams and the large-diameter photolysis beam, which triggers the chemical reactions, is confined to the middle region of the reactor. This probe region is specifically designed to be precisely temperature controllable from room temperature to over 800 Kelvin. Multiple probe laser beams at different wavelengths are coupled into the cell to monitor several species simultaneously. For example, this setup has recently been applied to measure absolute concentrations of both HO2 (in the near infrared) and OH (in the mid infrared) formed in laser-initiated biofuel oxidation reactions. These two radicals are the most important radical products in the initial steps leading to autoignition, and knowing their production rates is key to understanding these important chemical steps.