Cavity enhanced spectroscopy (CES) methodology provides a much higher degree of sensitivity than that available from conventional absorption spectrometers. The aim of this chapter is to present the fundamentals of the method, and the various modifications and extensions that have been developed. In order to set the stage, the limitations of traditional absorption spectrometers are first discussed, followed by a description of cavity ring-down spectroscopy (CRDS), the most popular CES embodiment. A few other well-known CES approaches are also described in detail.

A novel instrument, based on cavity-ringdown spectroscopy (CRDS), has been developed for trace gas detection. The new instrument utilizes a widely tunable optical parametric oscillator (OPO), which incorporates a zinc–germanium–phosphide (ZGP) crystal that is pumped at 2.8 μm by a 25-Hz Er,Cr:YSGG laser. The resultant mid-IR beam profile is nearly Gaussian, with energies exceeding 200 μJ/pulse between 6 and 8 μm, corresponding to a quantum conversion efficiency of approximately 35%.

The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotopeanalysis of water is increasing. While IRIS has many advantages over traditional isotope ratio massspectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Ofparticular concern is the potential for contaminants in the water sample to interfere with thespectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soilsamples may often contain organic contaminants.

The quantification of greenhouse gas emissions requireshigh precision measurements made with high spatial resolution.Here we describe measurements of carbon dioxide (CO2)and methane(CH4) conducted using Purdue University’s AirborneLaboratory for Atmospheric Research (ALAR), aimed at thequantification of the “footprints” for these greenhouse gasesfor Indianapolis, IN. A cavity ring-down spectrometer measuredatmospheric concentrations, and flask samples were obtainedat various points for comparison.