Cavity-enhanced absorption spectroscopy with a red LED source for NOx trace analysis |
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Authors: | M Triki P Cermak G Méjean D Romanini |
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Institution: | (1) Laboratoire de Spectrométrie Physique, CNRS UMR 5588, Université J. Fourier de Grenoble, 38402 Saint Martin d’Hères, France |
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Abstract: | 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.
PACS 07.88.+y; 07.60.Rd; 42.60.Da |
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Keywords: | |
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