Use of the optogalvanic effect (OGE) for isotope ratio spectrometry of ¹³CO₂ and ¹⁴CO₂

The use of isotopic carbon dioxide lasers for determination of carbon (and oxygen) isotope ratios was first demonstrated in 1994. Since then a commercial device called LARA™, has been manufactured and used for Helicobacter pylori breath tests using ¹³C-labelled urea. The major advantages of the optogalvanic effect compared with other infrared absorption isotope ratio measurement techniques are its lack of optical background and its high sensitivity resulting from a signal gain proportional to laser power. Continuous normalisation using two cells, a standard and sample, lead to high accuracy as well as precision. Recent advances in continuous flow measurement of ¹³C/¹²C ratios of CO₂ in air and extensions of the technique to ¹⁴C, which can be analysed as a stable isotope, are described.