The analysis of greenhouse gas (GHG) emissions has been a standard and consistent gas chromatograph (GC) application for many years. Recently though, this application has experienced a resurgence with the global urgency to address causes of global warming. As consumers, we are constantly hearing marketing campaigns of companies striving for “zero carbon emissions” in the upcoming years. Also in the media, we hear phrases like “carbon neutrality” and countries like the United States are passing laws like the Energy Act of 2020 to focus on green energy and curb GHG emissions. The underlying connection between all of these policies is the need to quantify GHG emissions. It is no surprise then, that a tried-and-true GC application seems to have come back to the forefront of the chromatography application arena.
Why is this application important?
Before we discuss the primary instrument used, let us review the general information on greenhouse gas emissions. Gases that make up GHGs collect in the atmosphere where infrared light from sunlight is reflected throughout. The collected GHGs absorb this energy and reflect it back toward the Earth in what is known as the greenhouse effect. While researching the trapped gases in the atmosphere is not new and has been ongoing since the mid-1800s, there has been a steady increase in interest for this application over the last 20 years as we learn more about the negative affects this has on our planet. Understanding where these compounds are coming from is key. Sadly, the gases of interest have many different sources including agriculture and electricity generation. While many of the sources are within human control, not all can be changed.1
What are the major compounds of interest?
According to the US EPA, the main gases of interest are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases like chlorofluorocarbons (CFCs).2 For the application discussed here, we will be focusing on the big three of CO2, CH4, and N2O.
All these gases can easily be quantified on a GC using specialized detectors. In the simplest system discussed here, two detectors are used; however, additional detectors may be necessary if there are other compounds of interest. An electron capture detector (ECD) is highly sensitive to N2O in the sample. A flame ionization detector (FID) is used for methane detection. A methanizer (typically using a nickel-based catalyst) is used to reduce CO2 in the sample to CH4 for detection on the FID.
The sample is introduced into the system either by gas valve injection or gas syringe injection. In the case of a gas valve injection, a six-port gas sample valve with a fixed loop injects the sample into the system. Gas syringe injections can be completed either manually or using an autosampler with the ability to inject headspace. In both cases, the sample is sent through a split/splitless inlet which allows for additional control of the injection volume. Sample is carried out of the inlet into analytical columns for separation and to the detectors for quantification. Depending on column and detector selection, additional valves and columns may be incorporated into the flow path for additional analyte speciation or to vent possible contaminants.
From an analysis perspective, the systems can have a turnaround of under five minutes to upwards of 15 minutes depending on additional compounds of interest as well as the columns being used. These systems can be configured with either packed or capillary columns and the column inner diameter will significantly affect chromatographic parameters such as loading capacity, peak resolution, carrier gas consumption, and total run time. From a quantitation perspective, the FID and ECD can detect below 1 ppm on most systems (and even upper ppb levels on some).
From a maintenance perspective, these systems are not much more complicated to upkeep compared to a basic inlet to detector GC system. Keeping the carrier and detector gases clean by using filters is critical for longevity and performance on any GC. Standard preventative maintenance including replacing inlet consumables, FID consumables, carrier gas filters, and cleaning the ECD will keep these systems running well for many years. One point to note about systems using valves for column selection or contaminant venting is that they are precisely timed to keep analytes moving in specific directions. Improper valve timing could cause unnecessary wear and tear on detectors and columns. Be mindful that as columns age or are replaced, the system may need minor adjustments to continue operating at peak performance. Users should always consult the manufacturers on best practices when upkeeping the system.
Innovations over the years
While this application is not necessarily new, we are seeing new developments in the configuration of the system to make it simpler and faster. One large change that has occurred in recent years is the conversion of the traditional system from packed to capillary columns. In many cases, GC manufacturers are no longer optimizing instruments to run older packed column technologies. Also, in applications such as this, throughput and run time have become critical performance metrics. Moving to capillary columns with their higher resolution and faster analysis times have allowed this system to increase throughput.
Another change that has come over the years is the sample introduction to the system. For many years, simple gas injection was made either via a valve or a manual injection with a gas syringe. As customers are wanting to run more samples and perform less transfers of collected gas to other vessels, many have started to favor autosampler systems as their form of sample introduction. The most notable style autosampler would be a multifunction unit like that from CTC Analytics. Robotic, multifunction autosamplers offer the best out of the box options as well as flexibility to use non-traditional sample containers such as pre-evacuated tubes and even mason jars.
One of the most recent advancements is the conversion from a traditional nickel catalyst methanizer to an in-jet style non-nickel catalyst from Activated Research Company (ARC).3 In traditional methanizers, the nickel catalyst used within the system varies in amount but is environmentally toxic and susceptible to poisoning from oxygen. This is a significant challenge in GHG applications where the sample matrix will consistently be about 20% oxygen, rendering the methanizer as essentially an environmentally toxic consumable. With an in-jet style methanizer using non-traditional catalyst materials, the consumable is simply the nozzle of the FID but the catalyst material in now incorporated into the jet. The in-jet non-nickel methanizer is more robust, has better overall range of linearity, and is robust against oxygen poisoning. This advent is also great because unlike traditional methanizers, it also takes up no additional space and requires no additional hydrogen fuel lines since it is integrated into the FID.
The analysis of GHG has been a consistent application for the GC over the years and is important area of research for the sustainability of mankind. With recent challenges like the helium shortage, many new systems have already converted this traditionally helium application over to alternative carrier gases like nitrogen and argon. While the application may seem like it has reached maximum optimization, continued innovations in gas chromatography hardware offer significant improvements in this analysis. Analytical intelligence is being explored to help leverage sample preparation and optimizing systems, methods, and data analysis with minimal user input. Advancements like these and others will keep this application at the forefront of gas chromatography for years to come.
References
- GHG Inventory Data Explorer: https://cfpub.epa.gov/ghgdata/inventoryexplorer/
- US EPA website: https://www.epa.gov/ghgemissions/overview-greenhouse-gases#:~:text=GHG%20emissions%20are%20often%20measured,CO2%2C%20per%20unit%20mass.
- ARC website: https://www.activatedresearch.com/jetanizer-methanizer-astm-d6158-astm-d2505-uop603-18/
About the Author: Nicole Lock is the GC Product Manager at Shimadzu Scientific Instruments and a member of the Labcompare Editorial Advisory Board. Nicole graduated from Seton Hill University with a Bachelor of Science in Chemistry. She started her career as a service engineer in a small third-party service organization repairing multiple brands’ GC and GCMS units. This skill set led Nicole to Shimadzu where she has spent the last 15 years working in both the service and marketing department representing the GC and GCMS product lines. Her passion is helping new customers find answers to their unknown problems and current customers make the most of the systems they use every day.