Quantum cascade lasers render the mid-IR spectral range available for laser gas monitors. For PFC's, continuous emission monitors are now commercially available. Since these instruments can only make use of a narrow band, spectral interference from other gas species is not easily resolved. In this paper a commercial QCL CF4 analyser is validated with respect to sensitivity and selectivity. In the laboratory, the laser was mounted on a 0.3 meter measurement cell. A gas mixer provided humidified gas mixtures of CF4 and methane. The results indicated that while there were no interference from water in the range up to 8000 ppm mol, a small interference from methane was observed. This was quantified to be approximately 3 ppb mol CF4 per ppm mol of methane. The sensitivty of the instrument was found to be 15 ppb mol CF4 for one meter light path.