Mid-infrared laser spectroscopic determination of isotope ratios of N2O at trace levels using wavelength modulation and balanced path length detection

We present a new mid-infrared laser spectrometer for high-precision measurements of isotopic ratios of molecules at ppm concentrations. Results are discussed for nitrous oxide (N2O), where a precision of 3‰ for a single measurement and a reproducibility of 6‰ have been achieved for a concentration of 825ppm. The room-temperature laser source employed is based on difference-frequency generation delivering a continuous-wave power up to 23μW at wavelengths between 4.3μm and 4.7μm and a line width of 1MHz. Two different measurement methods are compared; wavelength modulation with first-harmonic detection and direct absorption spectroscopy by recording the spectrum with a data-acquisition card. Two different detection schemes were employed; either all isotopomers were measured using the long path (36m) of the multipass cell or a balanced path length detection scheme was used, where the main isotope was measured with a beam along a shorter path (40cm) in the multipass cell. A single-pass reference cell was designed, offering two different path lengths for balanced path length detection. All combinations of measurement methods and detection schemes were tested regarding precision of a single measurement and long-term stability. The advantages and disadvantages of various measurement approaches are discusse