Method of calibrating a wavelength-modulation spectroscopy apparatus using a first, second and third gas to determine temperature and pressure values to calculate concentrations of analytes in a gas

A method of calibrating a wavelength-modulation spectroscopy apparatus configured to measure a concentration of an analyte in a sample gas, said method comprising the steps of:

• determining an intercept coefficient reflecting contribution from the noise floor, said determination based on a plurality of harmonic spectra of known concentrations of said analyte at a reference pressure and a reference temperature of a first sample gas;
• determining a temperature-dependent constant reflecting spectral line intensity corresponding to a known temperature of a second sample gas, said determination based on said intercept coefficient and at least one harmonic spectrum of known concentration of said analyte at a known pressure and said known temperature of said second sample gas;
• determining an effective pressure-broadening coefficient of a third sample gas, said determination based on said intercept coefficient, said temperature-dependent constant, a pressure of said third sample gas, a temperature of said third sample gas, and at least one harmonic spectrum of said analyte in said third sample gas;
• and
• determining a concentration of said analyte in said third sample gas, said determination based on said intercept coefficient, said temperature-dependent constant, said effective pressure-broadening coefficient, the pressure of said third sample gas, the temperature of said third sample gas, and at least one harmonic spectrum of said analyte in said third sample gas.