Summary

1. Determination of CH4, CO2 and N2O in air samples and soil atmosphere by GC-MS Dag Ekeberg1, Gunnar Ogner2, Monica Fongen3 1)The Norwegian University of Life Sciences, IKBM, P.O. Box 5003, 1432 Ås, Norway 2)Drottveien 4, N-1432 Ås, Norway 3)Norwegian Forest Research Institute, Høgskoleveien 12, N-1432 Ås, Norway dag.ekeberg@nlh.no • If it was necessary to repeat an analysis, one extra 400 mL sample could be drawn from the sample vial without significant decrease in the concentration determined. Ten following injections from the same vial filled with a calibrated air sample gave a continuous decrease in the CO2 concentration analysed due to reduced pressure in the vial. • The long-term instrument stability was tested by analysing 180 samples of laboratory air. The air was sampled at 3.3 min intervals just prior to analysis during a period of approximately 10 h. • The CO2 concentration decreased from 312 to 288 µLL-1, the mean concentration for the 10 first samples was 312±4 and the 10 last 286±2 µLL-1 for CO2. When recalibrations were performed every 1 h the corresponding values for the first and last set of 10 samples were identical (315±4 µL L-1 for CO2). The results for CH4 and N2O gave a corresponding stability. Additional information on detector stability was obtained from m/z 22, Ne+, the first peak at RT=1.02 min in the m/z 22 graph of Fig. 2. The first and last set of 10 samples had mean concentrations (±2s) of 17.7±0.6 and 17.9±0.4 µL L-1 for Ne (relative to a literature value of 18.2 µL L-1 for Ne) . • GC-MS analyses of gases in the headspace of bottles with incubated soil showed no differences in production of trace gases between soils with different nitrogen input history under aerobic conditions. Under anaerobic conditions, however, N2O production was markedly higher in the soil having received additional nitrogen for 10 years. The addition of C2H2 had no impact on N2O production, whereas it increased CO2 production by approximately 40 % and CH4 production about 200 times. Apparently, C2H2 served as a readily available substrate for microbial methanogenesis. • These and other results show clearly that this method is satisfactory for environmental gases analysed. Summary A method for determination of the climate gases CH4, CO2 and N2O in air samples and soil atmosphere was developed using GC-MS. The method uses gas chromatography with a mass spectrometric detector in the single ion mode. The gases were determined with high sensitivity and high sample throughput (18 samples/hour). The LOD (3s) for the gases were 0.10 µLL-1 for CH4, 20 µL L-1 for CO2 and 0.02 µLL-1 for N2O. The linear range (R2=0.999) was up to 500 µLL-1 for CH4, 4000 µLL-1 for CO2 and 80 µLL-1 for N2O. The samples were collected in 10 mL vials and a 5 mL aliquote was injected on column. The method was tested against certified gas references, the analytical data gave an accuracy within ± 5 % and a precision of ± 3%. The presence of ≤ 10 % by volume of C2H2 (often used experimentally to prevent N2 formation from N2O) did not interfere with detection for the targeted trace gases. • Aim of investigation • Due to concern about climate change, there are increased demands for analysis of climate gases. CO2, CH4, and N2O are important for the climate and for processes in plants and soils. These gases, in air samples collected from various matrixes, have earlier been analysed mostly by complex gas chromatographic systems using multiple detectors with flow switching, with conversion of CO2 to CH4 prior to detection, , or by photoacoustic infrared spectrometer technique. The purpose of this work was to simplify the analysis of climate gases in air and soil atmosphere by using straightforward gas chromatography with component separation on one single column, with a specific mass spectroscopic detection. The method for determination of the climate gases should be automatic, have high sensitivity and high sample throughput. Fig. 2 The SIM chromatograms of outdoor air as recorded by m/z 15, m/z 22 and m/z 44. The peaks at RT = 1.28 min, RT = 1.81 min and RT = 2.21 min represent CH4 (1.75 μL L-1), CO2 (400 μL L-1) and N2O (0.33 μL L-1), respectively. The chromatographic details are described in the text. • Results and Discussion • Analysis in scan mode (from m/z 12 to m/z 50) is not sufficient for determination of CH4 and N2O in the concentrations usually found in the atmosphere. We therefore used selected ion monitoring (SIM) mode to achieve sufficient sensitivity. For the qualitative analyses of methane, CH3+ ions at m/z 15 were used. This ion is present in 89 % of the base ion The base peak in the mass spectrum of methane that corresponds to CH4+ could not be used due to interference from O2 in air (formation of O+). The base ion of N2O is m/z 44. This ion gives satisfactory results when the concentration of CO2 was below 4000 µLL-1. At higher concentrations of CO2 the tailing of its peak interfered with N2O analysis. CO2 was analysed at m/z 22, (CO22+), an ion present in 1.4 % of the base ion. The base ion for CO2 is found at m/z 44, but this had too high intensity to give good determinations at concentrations above approximately 3000 µLL-1 for CO2. In conclusion, the presented GC-MS method for analysis of climate gases has high sample throughput, high sensitivity and performs with sufficient stability to give reliable analytical data.