A method comprising: determining a first measured concentration of a reactive gas in a first sample of a gas mixture, the gas mixture comprising the reactive gas at a reactive gas concentration and a first background composition that contributes spectral interference that hampers direct spectroscopic measurement of the reactive gas concentration in the gas mixture, the determining of the first measured concentration comprising generating a first differential absorption spectrum by subtracting a first background absorption data set from a first sample absorption data set collected for the first sample and converting the first differential absorption spectrum to a first measured concentration of the reactive gas in the gas mixture using calibration data, the first background absorption data set comprising data characteristic of absorption characteristics of the first background composition; selecting whether to use the first background absorption data set or a second background absorption data set to determine a second measured concentration of the reactive gas in a second sample of the gas mixture based on whether a correlation of the first background absorption data set to the first background composition is within a pre-defined tolerance; and determining the second measured concentration by generating a second differential absorption spectrum by subtracting, in dependence on the selecting, either the first background absorption data set or the second background absorption data set from a second sample absorption data set for a second sample of the gas mixture and converting the second differential absorption spectrum to the second measured concentration of the reactive gas in the gas mixture using the calibration data. determining a first measured concentration of a reactive gas in a first sample of a gas mixture, the gas mixture comprising the reactive gas at a reactive gas concentration and a first background composition that contributes spectral interference that hampers direct spectroscopic measurement of the reactive gas concentration in the gas mixture, the determining of the first measured concentration comprising generating a first differential absorption spectrum by subtracting a first background absorption data set from a first sample absorption data set collected for the first sample and converting the first differential absorption spectrum to a first measured concentration of the reactive gas in the gas mixture using calibration data, the first background absorption data set comprising data characteristic of absorption characteristics of the first background composition; selecting whether to use the first background absorption data set or a second background absorption data set to determine a second measured concentration of the reactive gas in a second sample of the gas mixture based on whether a correlation of the first background absorption data set to the first background composition is within a pre-defined tolerance; and determining the second measured concentration by generating a second differential absorption spectrum by subtracting, in dependence on the selecting, either the first background absorption data set or the second background absorption data set from a second sample absorption data set for a second sample of the gas mixture and converting the second differential absorption spectrum to the second measured concentration of the reactive gas in the gas mixture using the calibration data.