Maintenance Protocol 7890 GC

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Abstract

Two Agilent 7890 Gas Chromatographs (GCs) with an FID (Flame Ionization Detector), ECD (Electron Capture Detector), and a Licor 820 – Infrared Gas Analyzer coupled to the GC (analog/digital signal transfer) for simultaneous N2O, CH4, and CO2 analyses. The combined lab capacity of the two instruments is 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 (see Figure 1 below) using Agilent Chemstation and Gerstel Maestro software, with results output to a .csv file. 

Sensitivity calculated by the coefficient of variation (CV) is always less than 10% for each trace gas measurement. Twenty atmospheric samples are collected in the field and measured by the equipment above. The resulting atmospheric concentrations are then statistically compared using coefficient of variation. Results most commonly range from 1-3% for each trace gas.

Protocol

Gas Cylinder System

  • The gas purity level used in the Robertson GHG Lab meets or exceeds all manufacturer specifications. Gas purity is also 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 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 if GC detector baselines rise dramatically, whichever occurs first.
  • Check linear velocities of each gas at the regulator. There should be no needle pulse on the gauges. Nitrogen is set to 80psi, P10 to 45psi, Air to 60psi, and Hydrogen to 80psi. Check once per month.

Safety

  • 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.

System Maintenance (by frequency, logged in instrumentation maintenance logbook)

  • Each Run:
    • 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.
    • It is very important to clean the sampled vial caps with a clean compressed air source (not from a can containing difluoroethane) prior to inserting the vials onto the autosampler tray. Dirt and residue from the field often plug the syringe and/or needle. This is a critical part of the analysis process, each run should be clean.
    • Always perform a CV test on the check standards from the previous run. If CVs are less than 10%, the system is functioning properly. If greater than 10%, check the needle and syringe for leaks, plugs, or breakage and check the integration parameters. Check standards will change slightly during a long run due to room temperature and pressure differences, a larger CV than below is acceptable in evaluating the GC and syringe performance. For directions on CV evaluations, see 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 with deionized water every two weeks. Dry with compressed air prior to returning the syringe. 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, than the autosampler is functioning properly.
    • Manually check the syringe plunger action every 2000 samples. If the plunger fit is loose, change the syringe immediately. It is hard to track the decline of syringe performance via chromatographs. This is very important to the data integrity of our samples.
  • Monthly or longer:
    • Run air CV evaluation monthly. CVs should be less than 3% for N2O and CH4, and less than 8% for CO2 analyses. If greater, check for leaks using a pressure drop test, GC methodology, and check the autosampler consumables. To evaluate, collect 12 vials of outdoor air (10ml overpressure) in vials and run them on the GC. Make sure to collect only fresh air by sampling into the wind. Sample away from the dairy or buildings (GLBRC field best). Run the samples with a full suite of standards.
    • 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 resolution is an indicator of bungee stretch or a clogged needle.
    • Change syringe and needle every 10,000 samples.

Sensitivity/Lowest Detectable Limit Test

  • To perform a sensitivity test, study within the range of expected values. In our case, this is 325 ppb N2O. Run a suite of 12-15 ambient outdoor air samples collected carefully in vials. Run with a suite of standards and 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.
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