Determination of carbon dioxide by gas chromatography using an electron capture detector for the analysis of greenhouse gases: A comparison and validation with the standard method

The goal of this study is to develop a novel analytical method for the simultaneous determination of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O)—the main greenhouse gases—from environmental samples using gas chromatography (GC) equipped with an electron capture detector (ECD). The newly developed method was compared and validated against the standard GC method using a thermal conductivity detector (TCD) for CO₂ analysis.

The study evaluated key performance parameters including selectivity, linearity, precision, accuracy, and detection limits. Despite a higher quantitation limit, the ECD method showed comparable accuracy and precision to the TCD method and proved adequate for most environmental applications. Once validated, the method enabled the simultaneous analysis of CO₂, CH₄, and N₂O using a simplified GC setup, reducing equipment needs and operational costs while maintaining analytical reliability.

The original article

Determination of carbon dioxide by gas chromatography using an electron capture detector for the analysis of greenhouse gases: A comparison and validation with the standard method

Joan Noguerol Arias, August Bonmatí, Francesc X. Prenafeta-Boldú, Míriam Cerrillo

Journal of Chromatography A, Volume 1745, 29 March 2025, 465750

https://doi.org/10.1016/j.chroma.2025.465750

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) are considered the main greenhouse gases (GHG) in the Earth's atmosphere. These gases are relatively transparent to the solar radiation but absorb and emit radiant energy within the thermal infrared range emitted by Earth, thus causing global warming. The global average surface temperature has increased by 1.10 °C by 2011–2020 compared to 1850–1900 [1]. Atmospheric concentrations of CO₂, CH₄ and N₂O are increasing at a rate of approximately 0.4, 0.6 and 0.25 % per year, respectively [1]. Anthropogenic activities like energy supply, industry, transport, and agriculture contribute to the increase of GHG concentration in the atmosphere, resulting in a higher global warming potential [[1], [2], [3]]. Since these increases contribute to changes in the Earth's climate, there is a growing interest in quantifying the sources and sinks of GHG, and in monitoring the emission of these gases, as a fundamental tool for tackling climate change.

Several analytical methods are available to quantify the CO₂ concentration and other GHG including, among others, gas chromatography (GC), open path Fourier-transform infrared spectroscopy (FTIR) or photoacoustic spectroscopy (PAS) [4]. Analysis of CO₂ in environmental samples containing GHG is usually performed by GC equipped with a TCD detector [5,6] or by using a nickel catalyst accessory (a methanizer appliance that converts CO₂ into CH₄) coupled to an FID detector [7,8]. The concentration of CH₄ can be analyzed with accuracy and enough sensitivity by GC-FID. The measurement of N₂O using GC combined with an ECD detector, provides both an excellent selectivity and high sensitivity for this compound [9]. Nowadays, there is a growing interest in the development of analytical methods that allow the simultaneous determination of GHG, by the direct analysis of a single gaseous sample (e.g. ambient air, environmental emissions) to increase the throughput of analytical determinations. In this sense, special attention has been given to GC instruments that integrate different valves and detectors that allow the simultaneous determination of CO₂, CH₄ and N₂O [[6], [7], [8],[10], [11], [12]]. It is well known that ECD detectors can be used for the analysis of several permanent gases such as H₂, O₂, CO₂, N₂, NO and N₂O, with a different sensitivity depending on the analyte [[13], [14], [15]]. Simmonds demonstrated that it is possible to analyze simultaneously N₂O and CO₂ using an ECD detector with nitrogen doped with 100 ppm of O₂ as the carrier gas [14]. Nevertheless, this type of detector has not been commonly used in the laboratory for routine analysis of GHG, except for N₂O.

Method validation is the process of proving that any given analytical procedure is acceptable for its intended purpose [16,17]. Therefore, there is a pressing need for the standardization and validation of the analytical methods, to improve the quality, reliability and consistency of analytical results [18]. Thus, method validation is an important requirement in chemical analyses for testing the suitability of methods as well as the capacity and performance of the analyst and laboratory. Several guidelines concerning validation can be found in the literature. Some of them are reported by international organizations, working groups or committees [19,20]. The main performance parameters to consider for the validation of an analytical method are selectivity/specificity, linearity/working range, precision, trueness, limit of detection (LOD) and quantitation (LOQ), and robustness.

The aim of this study is to develop an analytical method for the determination of CO₂ using a GC equipped with an ECD detector, and the comparison and validation of the new analytical method versus the standard method by GC-TCD. By using an adequate set up of valves, and of an FID and an ECD detectors, it is possible to determine simultaneously CO₂, CH₄ and N₂O in a single GC instrument, avoiding the necessity of an additional TCD detector or a methanizer, simplifying the equipment and reducing its investment and operational costs. Unlike other instruments such as PAS, FTIR, and Open-Path Tunable-Diode-Laser Spectroscopy (OP-TDLS), GC does not provide continuous monitoring of GHGs. However, its ability to simultaneously analyze multiple gases makes it particularly advantageous in specific scenarios. For instance, GC is a preferable option when monitoring emissions from numerous samples collected at various locations separated by significant distances, especially in situations when electric power is unavailable. Furthermore, GC simplifies the sampling procedure by requiring only gas syringes and evacuated vials, eliminating the need for bulky, battery-dependent field equipment. This practicality makes GC an effective solution for emissions monitoring in remote or resource-constrained environments.

2. Materials and methods

Apparatus. CO2 was analyzed by a GC CP-3800 Varian (USA) equipped with an on-column injector model 1041 and TCD detector, using a packed column Hayesep-Q 80–100 mesh 2 m x 1/8″ x 2.0 mm SS Varian (USA). CH4, CO2 and N2O were analyzed simultaneously by a GC 7820A Agilent (USA) system equipped with a single channel and 2 valves of ten-port gas sampling with back-flush to vent, and 6-port to change between the FID and micro-ECD detectors, using 2 packed columns Hayesep-Q 80–100 mesh 2 m x 1/8″ x 2.0 mm Ultimetal Agilent (USA).

4. Results and discusion

The simultaneous determination of CO₂, CH₄ and N₂O in GHG samples by using a GC equipped with an ECD and FID detectors connected in series with a system of valves (GC-ECD/FID) was carried out using the configuration shown schematically in Fig. 1:

Journal of Chromatography A, Volume 1745, 29 March 2025, 465750: Fig. 1. Diagram of the configuration of the GC equipped with ECD and FID detectors connected in series with a valve system (GC-ECD/FID). PCM (Pneumatic Control Module).

The optimized chromatographic conditions used in each instrument, GC-TCD or GC-FID/ECD are shown in Table 1. Under these conditions, suitable separation of different compound peaks was achieved so that an adequate selectivity was accomplished (GC-TCD retention times (t_r): 0.33 and 0.75 min for air and CO₂; GC-ECD/FID t_r: 2.44, 3.94 and 4.83 min for CH₄, CO₂ and N₂O, respectively). See Fig. 2, Fig. 3.

Journal of Chromatography A, Volume 1745, 29 March 2025, 465750: Fig. 2. Signal of CH4 in GHG samples obtained with the FID detector.

Journal of Chromatography A, Volume 1745, 29 March 2025, 465750: Fig. 3. Signal of CO2 and N2O in GHG samples obtained with the ECD detector.

Fig. 2 also shows the signal of the FID detector at different CH₄ concentrations (ranging from 2.5 to 25.3 µmol mol⁻¹) in GHG samples obtained under various conditions and agricultural environments. Similarly, Fig. 3 illustrates the response of the ECD detector at different CO₂ and N₂O concentrations in GHG samples (CO₂ ranging from 410 to 3250 µmol mol⁻¹).

5. Conclusions

Based on the results of this study for the measurement of CO₂ in samples from environmental GHG emitting sources by using a ECD detector, the following conclusions are drawn:

• It has been possible to develop a new chromatographic method for the measurement of CO₂ in complex GHG-containing samples, using an ECD detector with precision and accuracy.
• The main performance parameters of the GC-ECD method have been validated: selectivity/specificity, linearity/working range, precision, trueness, LOD and LOQ; and compared with the standard GC-TCD method. The precision (3.1–3.4 %), and accuracy (101–106 %) of both chromatographic methods are similar.
• The working range of CO₂ have been established from 300 to 4000 µmol mol^-1. This range can be extended to 16,000 µmol mol^-1 using a volume of sample of 250 µL instead of 2000 µL, increasing considerably the range of applicability of the new analytical method.
• The results of LOD and LOQ, obtained for CO₂ analysis were 99 and 300 µmol mol^-1 for ECD detector. The sensitivity of the ECD detector is sufficient for the monitoring of CO₂ emissions in GHG containing samples from most settings.
• The simultaneous determination of CO₂, CH₄ and N₂O in GHG samples by using a GC equipped with an ECD and FID detectors connected in series with a system of valves (GC-ECD/FID) has been demonstrated. This approach is a good option when a laboratory is considering the purchase of a gas chromatograph to analyze GHG, being more simple and less expensive than other alternatives, such as the use of instruments equipped with 3 detectors connected in series, or those incorporating a methanizer.
• The simultaneous determination of gases with this new chromatographic method offers an effective solution for emissions monitoring, particularly in remote or resource-constrained environments where numerous samples must be collected from various locations separated by significant distances. It also simplifies the sampling procedure, requiring only gas syringes and evacuated vials