3.2 Laser Cavity Spectroscopy

This method is more recent than mass spectrometry and was developed in the 1980’s (O’Keefe, 1988). The first form of this method was known as cavity ringdown spectroscopy (CRDS) and is still used, but more recent developments have broadened the method and its application. One of the more recent modifications is called off-axis integrated cavity output spectroscopy (OA-ICOS) and reduces the need for highly accurate mirror alignment (Los Gatos Research, 2020). As with mass spectrometry, CRDS is not only used for isotopic measurements, as it can be set up to determine the quantities of different gases in a sample, or when modified with a plasma source can analyze for heavy element concentrations (Wang, 2007).

CRDS works in roughly the following way (Figure 6). A cavity containing an inert gas (He, N, etc.) is injected with the gaseous phase of a sample. A short burst of laser light is emitted into the cavity, which contains at least 2 high reflectivity mirrors. The light bounces between the mirrors to give an effective path length of many kilometers, in which the light decays due to mirror inefficiency and absorption by the sample gas. By monitoring wavelengths which are both affected and not affected by the sample absorption, the instrument can compare the decay of light due only to reflection losses and that due to absorption by the sample as well. This means variations in laser intensity and power that occur between measurements are corrected for. Multiple measurements are taken of the time taken for the light to diminish (the ringdown time) and therefore the strength of the sample absorption can be calculated. This absorption factor can be converted into an isotopic composition, as the different isotopic species have different absorptivity.

Figure 6 – Schematic diagram of a laser cavity ringdown instrument.

As with mass spectrometry, laboratory standards are inserted every ten or so samples, to allow final corrections to be made to the CRDS results, and allow reporting to the international standards, SMOW or SLAP.