Sensitivity of blended baseline fitting method for direct absorption spectroscopy

Measurement of multiple species at moderate-to-high pressures in a single wavenumber scan using a tunable diode laser can result in blended features in the spectrum. A blended-feature baseline fitting method can be used to simultaneously determine the concentration of two species present in the laser scan range. The baseline fitting method was validated with previous experimental measurements at 1 atm, where a laser scanned the absorption lines of both H₂ O and CO₂ between 3683.5 cm⁻¹ and 3686.5 cm⁻¹. This study explores the extension and application of the fitting method to higher pressures using synthetically-created datasets. The peak wavenumber ranges used in the blended-feature baseline fitting method is determined to be generally constant for varying pressures. A metric from the creation of error surfaces is used to evaluate the sensitivity of the fitting of species relative to each other, and is used to evaluate the method at increasing pressures. The result of increasing the weighting of the error calculation of H₂ O features over CO₂ features results in increased errors in the simultaneous fitting of CO₂ features. The fitting method is also found to be insensitive to increased levels of random noise in the raw dataset for all pressures.