Abstract
To measure small absorbances in tunable diode laser infrared spectrometry, second-harmonic detection is often used. The spectrum obtained is often used. The spectrum obtained is the sum of the desired molecular line profile, a contribution of the laser emission profile and various types of noise. It is shown that the undesired contributions can be reduced significantly by subtracting a least-squares fitted low-degree polynomial from the spectrum, followed by applying a digital low-pass filter to the remainder. A theory is developed for estimating the systematic error introduced by this procedure. The method is applied in a calibration experiment for nitrogen dioxide in air at low part-per-billion by volume levels.
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