A single-beam device for photoacoustic measurement with a single wavelength laser is described and its use in the determination of a trace sample is demonstrated. Using a 8mW He-Ne laser, the determination of trace amounts of Co in PAN-Co（Ⅲ） complex has been completed with a detection limit less than 1 ng Co. The dependence of the photoacoustic signal size on some parameters such as the modulation frequency and the backing materials have been examined. In addition, both a photoacoustic cell consisting of a double-microphone and a method in which collinear beams from He-Ne and He-Cd lasers are focused onto the sample are used to significantly improve detection sensitivity by overlapping the photoacoustic signals.