The answer is blowing in the wind

The concentration of greenhouse gases in the atmosphere has been steadily increasing – but how can they be accurately measured? Chunxiao Wang thinks gas chromatography is the answer

GREENHOUSE gases are atmospheric gases that absorb and emit radiation within the thermal infrared range – this process is the fundamental cause of the greenhouse effect. The main greenhouse gases in the Earth's atmosphere are water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone. Greenhouse gases greatly affect the temperature of the Earth; the contribution to the greenhouse effect by a gas is affected by both the characteristics of the gas and its abundance. For example, on a molecule-for-molecule basis methane is about eight times stronger as a greenhouse gas than carbon dioxide, but it is present in much smaller concentrations so that its total contribution is smaller.

Carbon dioxide was the first greenhouse gas shown to be increasing in atmospheric concentration with the first conclusive measurements being made in the latter half of the 20th century. Prior to the industrial revolution, concentrations were fairly stable at 280 parts per million (ppm). Today, they are around 370ppm, an increase of well over 30%. Direct measurement of atmospheric methane has been possible since the late 1970s and its concentration rose from 1.52 parts per million by volume (ppmv) in 1978 by around 1% per year to 1990, since when there has been little sustained increase. The current atmospheric concentration is approximately 1.77ppmv, and there is no scientific consensus on why methane levels have remained constant since 1990.

Concentrations of nitrous oxide also began to rise at the beginning of the industrial revolution and are understood to be produced by microbial processes in soil and water, including those reactions which occur in fertiliser containing nitrogen. These fertilisers have been used increasingly over the last century. Global concentration for N2O in 1998 was 314 parts per billion (ppb), and in addition to agricultural sources for the gas, some industrial processes (fossil fuel-fired power plants, nylon production, and nitric acid production and vehicle emissions) also contribute to its atmospheric load.

Continuous measurement of these gases provides meaningful information to track greenhouse gas emission trends and help in the fight against climate change. From 1st January 2010, the U.S. Environmental Protection Agency requires large emitters of heat-trapping emissions to begin collecting greenhouse gas data under a new reporting system1. Two different configurations in gas chromatography have been developed to support the analysis of the greenhouse gases CH4, N2O and CO2.2

For the first system the gas chromatographer (Agilent 7890A GC) has three valves and two detectors: a flame ionisation detector (FID) and micro-electron capture ionisation (ECD), using 1/8-inch stainless steel packed columns (HayeSep Q 80/100). This system can analyse CO2, CH4, N2O, and SF6 in air samples. The detection of organic compounds is most effectively done with flame ionisation using a methaniser. Hence the methaniser/FID combination is used to measure low levels of CH4 and CO2. The micro-ECD – a device for detecting atoms and molecules in a gas through the attachment of electrons via electron capture ionisation – is 10-1000 times more sensitive than a FID and is used to detect N2O.

Samples are injected into a short column which separates the components including air (O2), CO2 and CH4 from water. All analytes after N2O are backflushed and the air (O2) is vented away from the methaniser and micro-ECD. CO2 is converted to CH4 through the methaniser and measured by FID and N2O is introduced to the micro-ECD for measurement.

A quantitative precision study with 21 consecutive analyses was performed with results tabulated in Table 1. Excellent peak area repeatability for the analysis of CH4, CO2, and N2O standards was observed with this configuration.

To improve the sensitivity of micro-ECD, Ar-5% CH4 is recommended as the make-up gas, which can lower the detection of N2O to approximately 32ppb with the good signal-to-noise (S/N) ratio. The injected standard is prepared by dynamic blending with a 100-times dilution.

The same configured system was used to analyse real samples. In this experiment, laboratory air is analysed with Method 1. The chromatogram is shown in Figure 1. The measured concentrations of N2O, CH4, and CO2 are 473ppb, 2.7ppm, and 380ppm respectively.

The system can easily include the analysis of SF6 by delaying the backflush time to allow SF6 to elute into the precolumn. Figure 2 shows the chromatogram of SF6 at approximately 0.5ppb with a 1-mL sample size. The 0.5ppb SF6 standard is prepared by dynamic blending with 200 times dilution of the standard (original standard of SF6 is 100ppb).

For the second system the gas chromatographer is configured with two separate channels with 1/8-inch stainless steel packed columns. It uses three detectors (FID, micro-ECD and thermal conductivity detector (TCD)) for the analysis of CO2, CH4, N2O, and SF6 in air samples. CO2 can be analysed at wide concentration levels: high levels of CO2 can be analysed by TCD and low concentrations can be analysed by a methaniser with an FID. A dynamic blending system is used to prepare the low level calibration standards using N2 as a diluent.

The first channel employs two valves with TCD and FID. The TCD and methaniser-FID are connected in series to measure CH4 and CO2. This channel provides the flexibility for CO2 in varying levels. Low level CO2 can be converted to CH4 through the methaniser and measured by FID. The system is flexible depending on the requirements. The TCD can be used for high concentrations of CO2. If only higher levels of CO2 (higher than 50ppm) analysis are required, the methaniser can be removed. This channel can be expanded to include O2 and N2 analysis by adding an additional Molsive column.

Another micro-ECD channel with two valves is dedicated to measuring N2O and SF6. The precolumns direct heavier components (mainly water) to be backflushed whilst O2 is vented.

Results obtained for greenhouse gases (N2O, CH4, CO2 and SF6) by Method 2 are equivalent to those obtained by Method 1. In addition, with this setup, high levels of CO2 can now be measured by the third detector, TCD. The dynamic blending system is also used for Method 2 to prepare the low level standards. Table 2 shows very good repeatability of peak areas for the analysis of the greenhouse gas standards.
Real sample (laboratory air) is analysed with Method 2. The chromatogram is shown in Figure 3. The concentrations of N2O, CH4 and CO2 measured are 441ppb, 2.2ppm and 398ppm respectively.

Two methods have been developed to meet the different requirements for simultaneous analyses of greenhouse gases including CH4, CO2, and N2O in air samples. Method 1 has a simpler valve configuration and with minor modifications, accommodates autosampling by a headspace sampler. Method 2 has two separate channels with three detectors and can achieve even faster results. The separate channels increase flexibility to make the valve switching time less critical and the method easier to set up. The use of the third TCD allows measurement of a wide concentration range of CO2 (0.2ppm to 20%).

Although results obtained on both analysers are the same for greenhouse gases (N2O, CH4, CO2 and SF6), Method 2 enables detection of higher concentrations of CO2 ¬¬– for example in areas of higher pollution such as power plants and factories. These methods can also be used for other samples such as soil gases analysis or plant breathing studies where the analytes of interest contain gases such as CH4, N2O and CO2.3
Businesses worldwide face pressure to reduce the impact their activities have upon the environment, and in particular the volume of greenhouse gases they produce. With new requirements emerging from regulatory bodies, these methods will help businesses measure and report on their emissions and develop environmental strategies.

References
1. Environmental Protection Agency (EPA), “40 CFR Parts 86, 87, 89 et al. Mandatory Reporting of Greenhouse Gases; Final Rule”.
2. C. Wang, “Simultaneous Analysis of Greenhouse Gases by Gas Chromatography” Agilent Technologies Publication 5990-5129EN.
3. Teri Kanerva, Kristiina Regina, Kaisa Ramo, Katinka Ojanpera, Sirkku Manninen, “Fluxes of N2O, CH4 and CO2 in a meadow ecosystem exposed to elevated ozone and carbon dioxide for three years”, Environmental Pollution 145 (2007) 818-828.

Table 1.Quantitative Precision for Analysis of Greenhouse Gas Standards (n=21, Excluding the First Run)

Table 2.Quantitative Precision for Analysis of Greenhouse Gas Standards (n=20, Excluding the First Run)