| A new liquid-phase absorption detector is developed that demonstrates a minimum detectable absorbance of 10-8 absorbance units (AU). This value is more than 40 times lower than commercially available UV-Vis detectors. This new detector is a result of the extension of cavity ring-down spectroscopy (CRDS), a primarily gas-phase technique, to the liquid phase.; Liquid-phase CRDS is accomplished by incorporating a specially designed flow cell into the ring-down cavity. The flow cell minimizes optical losses by allowing p-polarized light to refract through the cell interfaces at Brewster's angle. This flow cell has been coupled to the output of an HPLC separation, enabling the detection of analytes by CRDS.; This technique was initially demonstrated by the separation and detection of a series of anthraquinones using a pulsed laser source at 470 nm. Ring-down time constants with the Brewster's angle flow cell, having an interior optical pathlength of 0.3 mm, were up to 2.5 mus in a 1-m cavity. The baseline noise level (rms) of this system was 3.2 x 10-6 AU, rivaling the best commercial UV-Vis detector, which exhibits a baseline noise of 3.0 x 10-6 AU. The CRDS detector performance, while notable, was limited in this case because of the nature of the light source: excitation of multiple cavity modes resulted in a 1% shot-to-shot variation in the ring-down time constant.; The detection limit of the liquid-phase CRDS detector was improved through the use of a single-mode continuous-wave (cw) laser source at 488 nm. Its narrow linewidth enabled excitation of a single cavity mode, resulting in shot-to-shot variations in the ring-down time constant as low as 0.04%. Furthermore, through improved cell characterization, ring-down time constants with the same flow cell and cavity length were nearly 6 mus. The baseline noise (rms) for this system during an HPLC separation of the same analytes was 6.7 x 10-8 AU. These results clearly illustrate the value of liquid-phase CRDS and its use as an absorption detector for HPLC measurements. |
