Abstract
Monitoring nitric oxide at the trace level is required in a large range of applications. We report on a trace gas analyzer optimized for nitric oxide measurements by Optical Feedback Cavity Enhanced Absorption Spectroscopy with an interband cascade laser at 5.3 µm. The short response time of the instrument allows for reaching the level of 50 ppt in only 180 ms. Its stability enables averaging up to 12 min to reach a detection limit of 0.9 ppt. Absolute concentration calibration requires to account for the optical saturation effect that results from the intense absorption line intensity addressed here, in the mid infrared region, in contrast to instruments that are operating in the near infrared region.
Highlights
Nitric oxide (NO) is a molecule of high interest at the trace level in a large range of applications.It is the most abundant of the nitrogen oxides (NOx ) that are involved in the atmospheric ozone cycle [1]
The baseline is included in the fit as a second order polynomial
We will show in the following, that due to optical saturation effect occurring in the mid-infrared region (MIR), the normalization procedure for NO concentration has to be adapted
Summary
Nitric oxide (NO) is a molecule of high interest at the trace level in a large range of applications. To obtain a sub-ppb sensitivity by direct measurement, strong absorption lines in the mid-infrared region (MIR) need to be addressed In this spectral region, lasers, detectors, and optical components are not as well developed as in the near infrared region (NIR). A research-grade chemiluminescence instrument was developed more than 15 years ago [18], and it is still at the state of the art for NO detection, featuring a sensitivity of 5 ppt within 1 min averaging time (with 1 Hz acquisition frequency). The newly developed MIR NO analyzer has a sensitivity of 6 × 10−10 /cm (in absorption units) for a single absorption spectrum This is nearly equivalent to the best OF-CEAS instruments optimized over more than a decade in the NIR (smallest detectable absorption coefficient of ~5 × 10−10 /cm [26]). We believe that this work may open the path for the industrialization of OF-CEAS NO trace analyzers with a detection limit in the ppt range
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