USING GAS CHROMATOGRAPHY FOR MEASURING ATMOSPHERIC METHANE CONCENTRATIONS IN THE FIELD


A major challenge facing environmental scientists is the generation of accurate meaningful data which shows how different gas compositions are changing within the atmosphere over short time frames. These measurements are being carried out on both global and highly localised levels in order to measure the long and short term impacts of greenhouse gases on the earth’s atmosphere. Methane currently makes up less than 5% of the total greenhouse gases globally which also include Carbon Dioxide, Nitrous Oxide and other fl uorinated gases, making it more diffi cult to detect [1].


There are six major sources of atmospheric methane: emission from anaerobic decomposition in natural wetlands; paddy rice fi elds; emission from livestock production systems (including intrinsic fermentation and animal waste); biomass burning (including forest fi res, charcoal combustion, and fi rewood burning); fossil methane emission during the exploration and transport of fossil fuels and anaerobic decomposition of organic waste in landfi lls [2].


This article will look at how gas chromatography (GC) can provide accurate data on methane levels in highly specifi c geographical areas. It will also look at the continuing role of the GC industry in measuring methane and the instruments which can be used to gather accurate data on emissions in the fi eld.


The growth of GC for environmental analysis


Much environmental analysis is performed by GC due to compound volatility. The technological advances and growth in the development of chromatographic instruments in terms of their size, versatility and power (including GC, for environmental analysis) have accelerated as the general knowledge of the impact of climate change has increased in the past decades. There are typically two types of GC columns, packed and capillary. Capillary column GC fi rst became commonly used in the late 1970s. The greatly increased separation effi ciency of capillary columns compared with packed columns has resulted in their widespread use for oil, petrochemicals and environmental analysis to the extent that packed columns are now rarely used [3]. This is due to the accuracy of results and ability to be used in the fi eld due to smaller, lighter instruments being developed. In parallel, with the development of more effi cient, robust and stable columns, improvements in electronics and data capture reduced the relative cost of chromatography when compared to mass spectrometry as an analytical technique [3].


When conducting environmental research in the fi eld a GC instrument gives positive confi rmation of the compounds. Researchers can use two columns for extra confi rmation that they are looking at the correct compound. When using GC for methane detection, a research team will traditionally run a sample on two columns of different polarity (e.g. polar and non-polar) to confi rm the identity of a particular compound. This is not necessarily something that should be done in the fi eld. Instead, it is more


useful to identify compounds by comparing both retention times and detector response factors with known standards, which is the basis of a number of environmental methods [4].


Using GC to measure methane levels in a specifi c area


Methane in air samples is measured using GC with FID. Typically, mass spectrometry is not suitable to carry out research in the fi eld due to practical limitations when compared to lab research. Using GC for environmental analysis in the fi eld relies on the retention of a specifi c compound on the GC column. The measurements are calibrated to the World Meteorological Organization standard scale, currently maintained by the National Oceanic and Atmospheric Administration (NOAA). Scale propagation errors are quantifi ed by repeated measurement of standards [5].


An effective method of collecting fi eld data by measuring atmospheric methane (CH4


) levels is to focus on a single point or


area where concentrations of methane are thought to be higher than the base levels. This can include an enclosed space, such as a chimney, or close to a valve or close to a landfi ll site. Exhaust systems can release methane through stacks; pneumatic valves and multiple discrete sources in both the petroleum and natural gas supply chains which emit methane as they operate [6].


One approach for measuring point sources is the use of ‘calibrated bags’, where the sample bag, when fully infl ated, contains a known volume of gas collected over a known time period. These bags can then be taken off site to a lab for analysis later. This gives the advantage of not requiring an on-site lab and the ability for researchers to collect samples over a period of time. Another approach is to install suitable analytical equipment on site and to perform constant real time analysis, which is more practical than transporting samples to a lab and provides fast results. These discreet areas of emission point testing allow for accurate reporting of methane levels, as emissions can be collected and analysed over a period of time.


An example of this in practice is a study aimed at measuring CH4 levels in East Anglia – around landfi ll waste sites, where four


specifi cally designed GC machines were installed in church towers which offered high points, protecting from the elements in a generally fl at terrain. This in turn minimised any local fl uxes which could have been recorded if the instrument was not in a protected space. It also ensured a wide data set was used to create an


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accurate picture of the methane levels across this area, reducing the risk of anomalous results.


For this study, methane mixing ratios were measured every 75 seconds over the period July 2012 to July 2015 at four different churches in the East Anglia area, known in this study as the ‘East Anglia measurement network’. The measurements were taken using the same model of 200 Series Ellutia GC-FID to ensure consistency of results in this experiment [7].


Before the experiment could take place, several bespoke 200 series instruments were developed to ensure the results provided would be as accurate as possible. The 200 series GC instrument was chosen due to its smaller footprint and more versatile confi guration when compared to other larger GC models. The developer, Ellutia, worked to create a confi guration of the 200 Series GC with a Flame Ionisation Detector (FID) attached, that would ensure methane-air separation could be achieved within the analysis cycle time required whilst giving results which were stable and reproducible.


The FID worked by detecting the ions formed by the combustion of a sample in a hydrogen/air fl ame. The introduction of air as a sample can affect the ratio of hydrogen to air within the fl ame and cause disturbances in the signal. In order to develop an instrument which would suit the needs of this experiment a number of challenges needed to be overcome. The length of the column was important to ensure good separation of the methane and the air present in the sample because of the disturbance to the FID the air can cause.


The sample size was important, if it was too small then the required sensitivity could not be achieved. If it was too large then the volume of air would potentially extinguish the FID fl ame. The choice of carrier gas was also important. Hydrogen was found to not give a stable enough baseline prior to the elution of methane. Nitrogen was found to greatly improve air disturbance, however, it also gave a negative response for methane, indicating the carrier gas had a higher concentration of methane than the sample. The quality of nitrogen could not be guaranteed unless a high purity nitrogen (6.0 Grade) was used. As such, helium was eventually chosen, as it offered minimal signal disturbance from the air and showed no other contamination of methane. Further adaptions were made to the instrument to allow for automated sample introduction and for remote ignition of the FID.


Using the adapted instrument, the air from outside the church was automatically analysed every 75 seconds. The GC instrument used


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