RationaleThe deep geological repository is considered the international reference for radioactive waste management. All gas exchanges must be understood in the context of the feasibility of such a repository. The technological challenge is to continuously monitor a wide range of gaseous molecules at low concentrations in confined spaces.MethodsA gas monitoring station, composed of two complementary analyzers, was developed: an electron impact quadrupole mass spectrometer (HPR‐20 R&D Hiden Analytical) and an infrared laser spectroscope (Picarro). The spectrometer was calibrated using simple mixtures (i.e., C2H6 in N2) and multiple mixtures (i.e., H2, He, CO2, CH4, and O2 in N2) at different concentrations to correct interferences. A matrix calculation is proposed to calculate the relative concentrations.ResultsThe method developed allows the measurement of gaseous species: light hydrocarbons, noble gases, sulfides, greenhouse gases, oxygen, hydrogen, and nitrogen in the same mixture. For each gas, the SDs and the limits of detection and quantification were calculated. The method was validated by comparing the concentrations of the measured gas species with the reference values of two standard gas cylinders.ConclusionsCalibration of a complex gas mixture remains a challenge because fragmentation of molecules, especially hydrocarbons, reduces the sensitivity of the method. The method developed is suitable for continuous gas monitoring in a confined environment and can be implemented to perform experiments in underground structures: galleries, microtunnels (cells), and boreholes.
Within the framework of radioactive waste management, the evolution of
the gaseous composition inside the underground repository structures
must be understood. Indeed, chemical and microbial reactions and
exchanges occur between the rock, the structures and the air. These
processes are studied in the Andra’s Meuse/Haute-Marne Underground
Research Laboratory’s (URL). Recently, a gas monitoring station “Flair
soil™” has been designed to monitor the gaseous composition inside this
URL. This station is composed by two analyzers: a quadrupole mass
spectrometer (QMS) which allows to follow the evolution of several gases
and an infrared laser spectroscope (Picarro) providing simultaneous
measurements of CH4, CO2 and CO. Thus, a multivariate calibration method
for the quantitative detection of interfering and non-interfering gases
in a nitrogen matrix has been developed for the QMS. The MS was
calibrated from pure gases in a nitrogen matrix, with known
concentrations and ion currents obtained from the measurement of each
species of gas. This method uses matrix calculations to calculate the
relative concentrations of an unknown gas mixture from the ion currents
measured directly in the MS. The test gases T1 and T2 were used to
assess the accuracy of the method. Daily ion currents are corrected from
theoretical ion currents obtained from calibration coefficients and test
gas concentrations. Some gases are less well quantified due to their low
concentration in the test gases and interferences on the measured
masses. One of the motivations of this study lay in developing an
advanced measurement tool allowing to be low in sensitivity, and thus to
improve the detection and quantification of gases at low concentrations.
The objective of this paper is to propose an analytical method to
measure the gaseous composition with several molecules. This method was
able to detect hydrocarbons, noble gases, sulfides, greenhouse gases,
oxygen, hydrogen, nitrogen in the same mixture.
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