Atmospheric gas correction

The Atmospheric gas correction preprocessor is designed to remove H₂0 and CO₂ gas lines from spectra, based on a user-supplied reference spectrum.

The user defines a set of spectral ranges that will be corrected. The default ranges are 1330-2100 cm^-1 and 3410-3850 cm^-1 for H₂0 and 2190-2480 cm^-1 for CO₂. In each of these ranges (individually), the preprocessor either subtracts (or adds) as much of the reference spectrum as necessary to maximize the smoothness of the output (Correct), replaces the data with a smooth line (Bridge) or does nothing (No-op). Ranges to be corrected must not overlap.

For each range to be Corrected and for each spectrum, the amount of reference subtracted from (or added to) the spectrum is chosen such that the sum of squares of the first derivative (differences between consecutive points) is minimized.

If Use mean of references is unchecked and multiple reference spectra are used, the subtracted reference is a weighted sum of all the references. In practice, this may be a poor replacement for arbitrary linear mixes of the references; this should be investigated and developed further.

Optionally, the corrected ranges are smoothed with a Savitzky-Golay filter of user-defined Savitzky-Golay window size and polynomial order 3.

For each range to be Bridged, data are replaced with a spline that merges gradually with the data at its edges. The spline is derived from the level and slope in a window of Bridge base window size points, while the transition between data and spline follows a Tukey window with alpha=0.2.

Reference spectrum

To generate a suitable reference spectrum for your machine, measure the same sample at different levels of atmospheric gases (e.g. evacuating with clean air versus ambient air after breathing in the room), take the difference and pass it through a background correction such as ALS. (Suggested ASL parameters: smoothing constant 1000, weighting deviations 0.0001.)

Publication

https://doi.org/10.3390/mps3020034