Interference filter tilting to detect a polycyclic aromatic hydrocarbon at the second harmonic of wavelength modulation frequency.

We present a practical implementation of the wavelength-modulation spectroscopy technique with second-harmonic detection for selective detection and concentration measurements of polycyclic aromatic hydrocarbons (PAHs)-common fossil fuel pollutants. The method is based on excitation light wavelength modulation around the maximum of a narrow peak in the absorption spectrum of a PAH and lock-in detection of fluorescence at the second harmonic of the excitation wavelength modulation frequency. A violet LED is used as the excitation source and wavelength modulation is performed via narrowband interference filter tilting. Feasibility is demonstrated with a carcinogenic PAH, benzo(k)fluoranthene. Model computations are presented to estimate the influence of the background signal on the sensitivity of the method.