Photoacoustic spectroscopy is a sensitive, on-line and non- invasive tool to monitor concentrations of trace gases in ambient air. With the appropriate high power lasers in the mid-IR wavelength region gas mixtures can be analyzed, at and below the part per billion level. Within the development of novel IR laser sources, a continuous wave optical parametric oscillator based on periodically poled lithium niobate in combination with photoacoustic detection has been applied to detect traces of several hydrocarbons in nitrogen. At an idler wavelength of around 3.3 micrometers , the cw OPO produced approximately 300 mW of single mode radiation. Preliminary results show detection limits on methane, ethane, butane and pentane of around 1 ppb. This trace gas detector will be used within medical applications. E.g., the trace gas composition of exhaled air is able to give information about a wide variety of processes in human body. In addition, such analysis has the potential to monitor processes non-invasive, on-line and fast for diagnostic purposes related to acute or chronic diseases.