Applied Physics

A portable modular gas sensor for measuring the 13 C/ 12 C isotopic ratio in CO 2 with a precision of 0.8%o(±1σ) was developed for volcanic gas emission studies. This sensor employed a difference frequency generation (DFG)-based spectroscopic source operating at 4.35 µm (∼ 2300 cm −1) in combination with a dual-chamber gas absorption cell. Direct absorption spectroscopy using this specially designed cell permitted rapid comparisons of isotopic ratios of a gas sample and a reference standard for appropriately selected CO 2 absorption lines. Special attention was given to minimizing undesirable precision degrading effects, in particular temperature and pressure fluctuations. 1 Introduction The ability to quantify gas concentrations with high sensitivity, excellent selectivity and rapid response by means of direct laser absorption spectroscopy has recently stimulated considerable interest in applying optical techniques for measuring isotopic ratios. Precision isotopic ratio measurements are important in a number of diverse areas, principally in the geochemical, environmental, biological and medical sciences. This paper describes the development of an optical instrument designed for field studies and capable of precise composition measurements of the ratio of the two stable carbon (13 C and 12 C) isotopes of CO 2. Such measurements can provide valuable information about CO 2 exchange processes in volcanic emissions and serve as potential predictors of increased volcanic activity, in particular of an impending volcanic eruption [1–3]. Changes in the isotopic ratio can be explained either in terms of variations in the volcanic sources, mixing with anthropogenic sources (e.g. agricultural or industrial) or could represent global and local changes. The 13 C/ 12 C isotopic ratio in carbon dioxide in volcanic systems and particularly in the Mid-Ocean Ridge Basalt (MORB) is usually between −4%o and −12%o, while it is usually close to 0% in crustal environment [4–6]. Continuous sampling and monitoring of volcanic gases indicate the presence of a steady source of carbon dioxide. The largest variation recorded for CO 2 was measured from the Bocca Grande fluids (Italy) in 1994, when −3.5%o was recorded instead of the previously measured steady −1.5%o value, indicating a possible MORB source below the crater [4]. Therefore, precise and fast measurements of the 13 C/ 12 C isotopic ratio in CO 2 are particularly important to study volcanic activities and to understand and model various environmental mechanisms in the Earth's atmosphere. Compact and robust gas sensor technology is a critical requirement for volcanic gas monitoring, as the …