Lewicki and Oldenburg [8] show by numerical investigations that monitoring of CO2 in the subsurface has greater potential to detect and quantify gas dynamics in heterogeneous ground than above-ground techniques. But up to now the development of a suitable measurement system for in situ gas monitoring remains to be a challenge, for both scientific and technical reasons. With respect to the heterogeneity of natural systems membrane based monitoring techniques particularly those based on polymers, gain increasing importance for environmental gas measurement.Typically, membranes are used as a gas-permeable phase boundary. Based on this approach a gas saturometer was introduced already in 1975 to measure the equilibrium gas pressure for a given dissolved gas in a liquid.

This technique is still available as Total Dissolved Gas sensor [9]. Numerous different applications combining standard analytical techniques and phase separation were developed e.g., [10-13]. Due to its low interaction such combinations of standard analytics with phase separating tubes have a significant importance for the in situ measurements.The measurement behind a phase separating membrane requires the equilibrium for all permeating substances and therefore, a high permeability of the membrane would be preferable. On the other hand, low gas permeability is required to conserve the equilibrated gas constitution inside the membrane tube during its transport to the analytical device. This problem of optimization restricts the temporal resolution of the readings and the spatial extend of such a measurement system.

To overcome this limitation, we developed a flux-based measurement method [14] operating near the dynamic equilibrium, which is reached fast in contrast to thermodynamic equilibrium. The gas selectivity of membranes is used as sensory principle and no transport of some gas Carfilzomib sample towards an analytical device is required. The robust method is applicable for quantification of the constitution of a multi-component gas [15] e.g., in soils, aquifers or bodies of water.One objective of this paper is to demonstrate theoretically the equivalence of a continuous (volume-based) application of the sensor with the discontinuous (pressure based) method. In many practical cases one is interested in the concentration of only one gas component within a given gaseous or liquid phase.

Therefore, we present a new concept for a single component analysis, which is a special case of the multi-component theory and which is the main objective of our paper. For this special case the constructive effort can be reduced and the sensor handling becomes relatively simple. We demonstrate the application of the single component analysis for monitoring of O2 and CO2 in a water-unsaturated soil.2.?Theory2.1.