Cavity Ring-down Spectroscopy for Detection in Liquid Chromatography: Extension to Tunable Sources and Ultraviolet Wavelengths

Abstract

In earlier studies, it was demonstrated that the sensitivity of absorbance detection in liquid chromatography (LC) can be improved significantly by using cavity ring-down spectroscopy (CRDS). Thus far, CRDS experiments have been performed using visible laser light at fixed standard wavelengths, such as 532 nm. However, since by far most compounds of analytical interest absorb in the ultraviolet (UV), it is of utmost importance to develop UV-CRDS. In this study, as a first step towards the deep-UV region, LC separations with CRDS detection (using a previously described liquid-only cavity flow cell) at 457 and 355 nm are reported for standard mixtures of dyes and nitro-polyaromatic hydrocarbons (nitro-PAHs), respectively. For the measurements in the blue range a home-built optical parametric oscillator (OPO) system, tunable between 425 and 478 nm, was used, achieving a baseline noise of 2.7 3 10 À6 A.U. at 457 nm, improving upon the sensitivity of conventional absorbance detection (typically around 10 À4 A.U.). An enhancement of the sensitivity can be seen at 355 nm as well, but the improvement of the baseline noise (1.3 3 10 À5 A.U.) is much less pronounced. The sensitivity at 355 nm is limited by the quality of the UV-CRDS mirrors that are currently available: whereas the ring-down times as obtained at 457 nm are around 70-80 ns for the eluent, they are only 20-25 ns at 355 nm. Critical laser characteristics for LC-CRDS measurements, such as pulse length and mode structure, are given and prospects for going to shorter wavelengths are discussed.

Liquid-Phase Broadband Cavity-Enhanced Absorption Spectroscopy Measurements in a 2 mm Cuvette

“…A comparison between this study and previous liquid-phase cavity studies as a function of technique, the mirror reflectivity, base path length, the wavelength of measurement, the lowest value of a min , the minimum LOD for an analyte, and the molar extinction coefficient e for that analyte. 1.06 3 10 5 Xu et al 19 CRDS 0.9997 1 607 1 3 10 À5 ÁÁÁ ÁÁÁ Snyder et al 20 CRDS 0.9993 0.03 470 2.5 3 10 À4 9.2 3 10 À8 9 3 10 3 Hallock et al 22 CRDS 0.9998 21 620-670 1 3 10 À6 ;1 3 10 À11 1 3 10 5 Hallock et al 23 CRDS 0.9998 23 655 3.3 3 10 À7 ÁÁÁ ÁÁÁ Bechtel et al 24 CRDS ÁÁÁ 0.03 488 2.6 3 10 À6 1 3 10 À7 ÁÁÁ Bahnev et al 25 CRDS 0.9998 0.2 532 1.6 3 10 À4 3.7 3 10 À9 5.45 3 10 4 van der Sneppen et al 26 CRDS 0.99 996 0.2 532 1.0 3 10 À5 1.5 3 10 À8 1.4 3 10 4 van der Sneppen et al 27 CRDS 0.99 993 0.2 457 1.0 3 10 À5 1.2 3 10 À8 3.6 3 10 4 van der Sneppen et al 27 CRDS 0.9995 0.2 355 5.0 3 10 À5 7.5 3 10 À8 1.02 3 10 4 Fiedler et al 28 BBCEAS 0.99 1 607 2 3 10 À5 ÁÁÁ ÁÁÁ Fiedler et al 28 analyte in question. These are compared with corresponding data, where available, from the small number of previous liquid-phase cavity studies.…”

“…22,23 A few studies have applied liquidphase CRDS to high-performance liquid chromatography (HPLC) systems. [24][25][26][27] Currently, only two studies using CEAS for liquid-phase measurements have been reported. Fielder et al 28 made cuvette based measurements at normal angles of incidence using BBCEAS in a modified conventional double beam ultraviolet (UV)-visible spectrometer.…”

“…One drawback of liquid-phase studies is that the losses due to scattering and or absorption by the solvent and the cuvette windows can be significant and can reduce the number of passes compared to gas-phase measurements. However, through the use of high reflectivity mirrors, careful choice of solvents, and reduction of scatter from cuvette surfaces, greater than 1000 passes have been demonstrated for liquid-phase CRDS studies [22][23][24][25][26][27] and also recently for CEAS measurements. 29 At first glance, BBCEAS would appear to be a less than ideal method for the study of liquids.…”

Liquid-phase broadband cavity enhanced absorption spectroscopy (BBCEAS) studies in a 20 cm cell

“…The measurement of the absorption coefficient of an analyte does, however, require the reflectivity of the mirrors to be known, or more typically a separate calibration to be performed. The experimental difficulty in using CRDS increases when experimental schemes employ short cavity lengths such as those used by van der Sneppen et al 12,13 as these lead to shorter ring down times, which are worsened by the inherent high losses present in liquid-phase studies. These limit the range of analyte concentrations which can be studied before the ring down time becomes too short to measure.…”

Liquid-Phase and Evanescent-Wave Cavity Ring-Down Spectroscopy in Analytical Chemistry

“…Recently, liquid-phase CRDS has received much attention, and its potential in analytical chemistry has been demonstrated. It has been shown that compared to conventional absorbance detection, the baseline noise and limits of detection (LODs) in liquid chromatography (LC) can be significantly improved (1)(2)(3)(4)(5)(6)(7).…”