New method for isotopic ratio measurements of atmospheric carbon dioxide using a 4.3 μm pulsed quantum cascade laser

We present a new approach to the measurement of stable isotopic ratios of carbon dioxide using a near-room-temperature pulsed quantum cascade laser and a spectral ratio method based upon dual multiple pass absorption cells. The spectral ratio method improves precision and accuracy by reducing sensitivity to variations in the laser tuning rate, power and line width. The laser is scanned across three spectral lines (near 2310 cm(-1)) quantifying three CO2 isotopologues: (CO2)-C-12-O-16, (CO2)-C-13-O-16 and (COO)-C-12-O-16-O-18. Isotopic ratios are determined simultaneously with a precision of 0.2 delta for each ratio with a one-second measurement. Signal averaging for 400 s improves the precision to better than 0.03 delta for both isotopic ratios (R-13 and R-18). Long-term accuracy of 0.2 to 0.3 delta is demonstrated with replicate measurements of the same sample over a one-month period. The fast time response of this instrument is suitable for eddy flux measurements.