LOW-COST COMPACT LASER POINTER-BASED TLS SYSTEM FOR Cr(VI) DETECTION

E. LESCANO; N.L. BARREIRO; V.B. SLEZAK

Congreso; 18th International Conference on Photoacoustic and Photothermal Phenomena; 2015

The objective of this work is developing a low-cost, compact and portable Thermal Lens Spectrometry (TLS) device for in situ Cr(VI) detection using the widely known colorimetric reaction with DPC. Up to now, its use is limited to laboratory work since the alignment is very sensible to instabilities and laser sources are usually big (Ar-Ion laser, Krypton laser, Dye laser, among others). In this work, we use a small inexpensive green laser pointer controlled by a simple modulation circuit to excite a Cr(VI)-DPC samples and a red laser diode as probe, both focused in the cuvette by a single lens with short focus to obtain compactness and stability. An optic fiber is used as pinhole, a photodiode at its other end detects variations of the probe beam generated by the thermal lens in the sample and a lock-in amplifier is used for synchronous detection. To improve and optimize the system we studied different configurations where the optical power of the focusing lens and the diameter of the fiber are changed. The chromatism of the lenses defines the distance between the focus of excitation and probe beam. Based on the different lens materials (different refraction index) and radius of curvature we calculated this distance, which, as the theory shows, is optimum when the distance between both focus is the product between three root and the Rayleigh distance of the excitation beam. This work shows that different limits of detection are achieved for four different configurations, based on three different lenses and fibre radius. In all cases the results indicate that the device is able to evaluate if a sample has a concentration of Cr(VI) which is safe for humans (0.05 mg/l).