Stable Isotopes by FTIR

Chemical and biological processes in nature, such as respiration, photosynthesis and atmospheric chemical reactions, often discriminate between different isotopes in the chemical species concerned. For example photosynthesis discriminates against the heavy ¹³C isotope, and plant matter and respired CO₂ is therefore depleted in ¹³C relative to the atmosphere. Careful analysis of the isotopic composition of atmospheric trace gases can thus provide very valuable information on the sources and sinks of the gases concerned because each natural process leaves its isotopic "signature" in the gases it produces.

High resolution FTIR spectroscopy can also discern the different isotopic components of trace gases and in certain cases avoid these limitations. The molecular spectroscopic properties of e.g. ¹⁵N¹⁴N¹⁶O are different from ¹⁴N¹⁵N¹⁶O and are thus two different species to the spectroscopist. In very recent research, we have begun to investigate the use of high resolution FTIR spectroscopy for isotopic analyses of gases which can either not be easily measured or not at all be measured by mass spectrometry. These include measurements of isotope ratios such as ¹⁷O/¹⁶O and ²H/¹H which are masked by ¹³C/¹²C in mass spectrometric measurements. Our current focus is on ¹⁵N₂O and CH₃D. In other studies we have used low resolution FTIR spectroscopy to determine ¹³C/¹²C ratios in air samples as a considerably cheaper and more convenient alternative to isotope ratio mass spectrometry.