Minimum refractometrically detectable concentrations of single atmospheric gases and simple mixtures with a Sagnac interferometer

Abstract

The minimum quantities of the nine most abundant, isolated, atmospheric gases that are detectable with a refractometer are calculated. An examination of the applicability of refractometric techniques for detecting and analyzing gaseous mixtures is discussed and a comparison made against other established techniques. Traditionally, most gas analysis performed with an interferometer is in determining the dispersion or refractivity of a known sample, presented here is the inverse approach, where refractivities are measured to determine the concentrations of particular species within a gas. The method, and experimental results for determining the minimum quantities of a particular species detectable in a mixture has been explored, as well as the complications, such as the indistinguishability of dynamic polarizabilities of different gases and the subsequent demands for accurate pressure and fringe measurements of using interferometric techniques. It is shown that the concentration of a single (isolated) gas, in units of number density, can be determined to within approximately 1-10 x 10(18) m(-3), and a mixture of the three most abundant gases, N2, O2 and Ar, to within 3.4 x 10(4) parts in 10(6) (ppm) when a minimum detectable fringe shift of lambda/100 is assumed.