Cavity Ring-Down Spectroscopy of Liquid Samples Using Standard Cuvettes at Normal Incidence

Cavity ring-down (CRD) spectroscopy has emerged as a sensitive analytical technique. In this method, a laser pulse is injected through one of two highly-reflective mirrors which form a stable optical cavity and the rate that the light leaves the cavity is monitored by a detector placed behind the second mirror. In this research a CRD spectrometer has been designed and constructed. The light exiting the cavity is collected via a fiber optic cable which is then directed toward a photo multiplier tube (PMT) detector. The signal is digitized and averaged by an oscilloscope and the data are transferred by an I 488 interface to a personal computer where the data are analyzed. Instrument command and data acquisition are controlled by a Visual Basic computer program. A short review of several attempts to measure liquid samples using CRD spectroscopy is presented; most discuss the necessity for the incorporation of Brewster’s angle at the liquid interface. This study integrates a 1 cm standard quartz cuvette at normal incidence. It was determined that there are significant losses from scattering and reflection; however, these losses were not so large as to negate the efficacy of the technique. The hypothesis tested here is that the light “lost” as reflections are collected by the cavity mirrors and redirected back into the cavity. Rhodamine 6G was used as the primary model absorber in these studies. Absorbance measurements were extracted from the measured ring-down times and a detection limit was obtained. Four cavity lengths were constructed to determine the effect on the scattering losses with varying cavity lengths. The calculated detection limit for the CRD spectrometer used in this study was found to be in the range of 4-5 nM. It was found that the detection limit of the CRD spectrometer was 36 times lower than that of the commercial instrument. Aligning the cavity mirrors at longer cavity lengths proved to be more difficult; however, there were no significant additional losses observed by incorporating longer cavities