Agilent 7890 Gas Chromatography - Analysis of Greenhouse Gases
Greenhouse gases (N2O, CH4, and CO2) are analyzed simultaneously from a single sample using an Agilent 7890 Gas Chromatograph (GC) equipped with a flame ionization detector (FID), a 63Ni electron capture detector (ECD), and a Licor 820 – Infrared Gas Analyzer coupled to the GC via an analog/digital signal transfer. The Robertson GHG Lab has two of these instrumental setups for a combined analytical capacity of approximately 50,000 samples (three gases per sample) per sampling season (April through November). Each GC is coupled to a Gerstel MPS2XL autosampler with a custom sampling tray using Agilent Chemstation and Gerstel Maestro software, which outputs results to a .csv file.
Instrument sensitivity is calculated by the coefficient of variation (CV) and is acceptable when less than 10% for each trace gas analyzed. The CVs of 12-15 atmospheric samples collected in the field and measured with this instrument commonly range between 1-3% for each of the trace gases.
CH4 and N2O are determined by GC after separation on a Porapak Q column (1.8 m, 80/100 mesh) at 80° C. CH4 is analyzed with the FID (300 degrees C) using nitrogen as a carrier gas. N2O is analyzed with the ECD (350° C) using argon/methane (90/10, P10) as a carrier gas. CO2 is analyzed using a Licor-820 infrared gas analyzer with nitrogen carrier gas.
Carrier Gas Purity
- The carrier and standard gas purity in the Robertson GHG Lab meet or exceed all Agilent specifications. Gas purity is certified down to 10% of the starting pressure. For most cylinders, this means the cylinder should be replaced at no lower than 200 psi remaining.
- Traps are also employed as an additional step to ensure gas purity. Traps on UHP or Ultra Zero grade gas purities last for dozens of cylinders in our application. It is recommended that traps be changed every two years or earlier if the GC detector baseline rises dramatically.
- Linear velocities of each gas at the regulator are checked monthly. There should be no needle pulse on the gauges. The regulator on the nitrogen tank is set to 80psi, P10 to 45psi, air to 60psi, and hydrogen to 80psi.
Analysis of Samples
- Ensure that applicable steps in the System Maintenance schedule below have been performed prior to initiating an analytical run of samples.
- Prepare analytical run to include four sets of calibration standards. Two sets on the front end and two sets at the end of each sample sequence. Check standards should be placed at a minimum frequency of every twenty samples. Blanks are run with the calibration standards.
- Calibration and check standards are made from a certified primary standard manufactured by Airgas.
- Clean the caps of sample vials with a clean compressed air source (not from a can containing difluoroethane) prior to loading the vials into the autosampler tray. Dirt and residue from the field often plug the syringe and/or needle. This is a CRITICAL step in the analysis of field samples.
System Maintenance (logged in instrument maintenance logbook)
- Bake oven when not in use or for one hour between sequence runs. Set oven temperature to 120° C during bake out. The baseline trace should rise, bounce for some time, then slowly drop to a stable setting. When a new sequence is run, the bakeout temperature will automatically default back to the sequence method setting. There is no need to change the setting on the GC keyboard.
- Change the injector septa every 600-800 samples. Septa should be changed at all three injection ports. Tighten the septa nuts finger tight ONLY.
- Perform a CV test on the check standards from the previous run. If CVs are less than 10%, the system is functioning properly. Check standards values may drift slightly during a long run due to room temperature and pressure differences;
- If the CV test on check standards is greater than 10%, check the needle and syringe for leaks, plugs, or breakage and check the integration parameters.
- For directions on CV evaluations, contact Kevin Kahmark. It is possible to run the samples two or three times if a problem occurs during analysis.
Weekly to every two weeks:
- Check column fittings in oven weekly.
- Clean internal parts of the syringe by rinsing with deionized water every two weeks. Dry with compressed air before reassembling.
- Manually check the syringe plunger action every 2000 samples. If the plunger fit is loose, change the syringe immediately. It is difficult to track the decline of syringe performance via chromatographs. This is very important to the data integrity of our samples.
- Run two or three blanks and then a suite of standards to assess performance. If the N2O and CH4 CV’s are 10% or less, then the syringe/autosampler is functioning properly.
Monthly or longer:
- Run an air CV evaluation monthly. Collect 12 vials of fresh outdoor air (at 10ml overpressure) by sampling into the wind and away from the dairy or buildings (GLBRC field best). Run the samples with a full suite of standards. CVs should be less than 3% for N2O and CH4, and less than 5% for CO2 analyses. If greater, check for leaks using a pressure drop test, GC methodology, and check the autosampler consumables.
- Gently check the retention column in the oven monthly for nicks, cuts, or loss of column integrity. Use proper white linen gloves.
- Change bungees on the autosampler every 10,000 samples or sooner if the bungees look stretched. Loss of sensitivity is an indicator of bungee stretch or a clogged needle.
- Change syringe and needle every 10,000 samples.
Sensitivity/Lowest Detectable Limit Test
A sensitivity test is performed on ambient air samples. Collect 12-15 fresh outdoor air samples and run with a suite of standards. Calculate the CV of each GHG. The percent CV is the percent of the normal ambient air concentration of each gas. For example, a 1% CV of N2O would be 328ppb * .01 = 3.28ppb sensitivity or lowest detectable limit. The instrument should detect changes in concentration down to this level for N2O in this scenario.
- Check all cylinder safety brackets monthly.
- Do not leave hydrogen and air gases on simultaneously when checking flow from the FID. This could cause an explosion.
- Move cylinders only with caps in place.
- Keep hands away from the injection needle during operation.
Date modified: Monday, Apr 01 2019