Incoherent Broadband Cavity-enhanced Absorption Spectroscopy Research Articles

Cavity-enhanced absorption spectroscopy with a red LED source for NOx trace analysis

Incoherent broad-band cavity-enhanced absorption spectroscopy (IBB-CEAS) based on arc lamps has been around for a few years, but only two reports exist using light-emitting diodes (LEDs). We present a setup based on a 643-nm LED which is of interest for the simultaneous detection of NO3 and NO2. The latter is chosen for testing as it is stable and available in calibrated diluted samples. A detection limit in the ppbv range is obtained with 2-min averaging (5×10-9 /cm rms baseline noise level), comparable to the best performance of chemiluminescence devices used for pollution monitoring. At 1-s acquisition time, the detection limit is below 10 ppbv. Extrapolation to NO3 yields a detection limit of a few pptv for a few minutes averaging. We also test the retrieval of absolute sample absorption (and concentration) using the cavity mirror reflectivity obtained with a commercial spectrophotometer, and we conclude that a calibration based on a reference sample of known concentration is preferable for accurate concentration measurements with IBB-CEAS. Finally, we present a rigorous frequency-domain derivation of cavity transmission as a function of wavelength for a broad-band spectrally smooth source, which complements the time-domain derivation by Fiedler et al. This derivation exposes an issue with multiple transverse mode excitation inherent to this technique, which may result in slightly distorted spectral profiles.

Incoherent broad-band cavity-enhanced absorption spectroscopy of the marine boundary layer species I2, IO and OIO

The novel combination of incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS) and a discharge-flow tube for the study of three key atmospheric trace species, I(2), IO and OIO, is reported. Absorption measurements of I(2) and OIO at lambda=525-555 nm and IO at lambda=420-460 nm were made using a compact cavity-enhanced spectrometer employing a 150 W short-arc Xenon lamp. The use of a flow system allowed the monitoring of the chemically short-lived radical species IO and OIO to be conducted over timescales of several seconds. We report detection limits of approximately 26 pmol mol(-1) for I(2) (L=81 cm, acquisition time 60 s), approximately 45 pmol mol(-1) for OIO (L=42.5 cm, acquisition time 5 s) and approximately 210 pmol mol(-1) for IO (L=70 cm, acquisition time 60 s), demonstrating the usefulness of this approach for monitoring these important species in both laboratory studies and field campaigns.

Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy in the near-Ultraviolet: Application to HONO and NO2

The first application of incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) in the near-ultraviolet for the simultaneous detection of two key atmospheric trace species, HONO and NO2, is reported. For both compounds the absorption is measured between 360 and 380 nm with a compact cavity-enhanced spectrometer employing a high power light-emitting diode. Detection limits of approximately 4 ppbv for HONO and approximately 14 ppbv for NO2 are reported for a static gas cell setup using a 20 s acquisition time. Based on an acquisition time of 10 min and an optical cavity length of 4.5 m detection limits of approximately 0.13 ppbv and approximately 0.38 ppbv were found for HONO and NO2 in a 4 m3 atmospheric simulation chamber, demonstrating the usefulness of this approach for in situ monitoring of these important species in laboratory studies or field campaigns.

Influence of the cavity parameters on the output intensity in incoherent broadband cavity-enhanced absorption spectroscopy

The incoherent broadband cavity-enhanced absorption spectroscopy is a technique in measuring small absorptions over a broad wavelength range. The setup consists of a conventional absorption spectrometer using an incoherent lamp and a charge coupled device detector, as well as a linear optical cavity placed around the absorbing sample, which enhances the effective path length through the sample. In this work the consequences of cavity length, mirror curvature, reflectivity, different light injection geometries, and spot size of the light source on the output intensity are studied and the implications to the signal-to-noise ratio of the absorption measurement are discussed. The symmetric confocal resonator configuration is identified as a special case with optimum imaging characteristics but with higher requirements for mechanical stability. Larger spot sizes of the light source were found to be favorable in order to reduce the negative effects of aberrations on the intensity.

High Sensitivity in Situ Monitoring of NO3 in an Atmospheric Simulation Chamber Using Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy

We describe the application of incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) for the in situ detection of atmospheric trace gases and radicals (NO3, NO2, O3, H2O) in an atmospheric simulation chamber under realistic atmospheric conditions. The length of the optical cavity across the reaction chamber is 4.5 m, which is significantly longer than in previous studies that use high finesse optical cavities to achieve high absorption sensitivity. Using a straightforward spectrometer configuration, we show that detection limits corresponding to typical atmospheric concentrations can be achieved with a measurement time of seconds to a few minutes. In particular, with only moderate reflectivity mirrors, we report a measured sensitivity of 4 pptv to NO3 in a 1 min acquisition time. The high spatial and temporal resolution of the IBBCEAS method and its pptv sensitivity to NO3 makes it useful in laboratory studies of atmospheric processes as well as having obvious potential for field measurements.

Incoherent broad-band cavity-enhanced absorption spectroscopy of liquids

A new application of incoherent broad-band cavity enhanced absorption spectroscopy (IBBCEAS) to weak transitions in solution through a very straightforward modification of commercially available double-beam UV/VIS absorption spectrometers is reported. The improved sensitivity of the new approach is demonstrated on basis of the weak Franck–Condon inhibited absorption of the fifth C–H stretch overtone in liquid benzene. The theoretical limits of the enhancement of the signal-to-noise ratio of IBBCEAS in comparison with single pass absorption experiments are discussed for a set of given experimental cavity parameters. The optical loss properties of a typical transparent cuvette window in the cavity are also discussed.

Incoherent broad-band cavity-enhanced absorption spectroscopy of azulene in a supersonic jet

The application of incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS) to isolated jet-cooled gas-phase species is demonstrated on basis of the S 1 ← S 0 transition of azulene in a continuous supersonic expansion. By focusing white light from a high power Xe-lamp into an optically stable resonator, which is intercepted by the supersonic seeded jet, the absorption is measured over a broad spectral range between 17 300 and 20 255 cm −1 with a resolution of ≈4 cm −1. The resolution, which is due to the dispersion of transmitted light by a grating monochromator, is sufficient to monitor the highly congested vibronic structure >3000 cm −1 above the S 1,0 origin of azulene. The sensitivity required for this absorption experiment compares well to that of conventional pulsed cavity ring-down measurements.