Instrument Support Level 2
The gas chromatograph is an Agilent 7890GC. The auto-sampler is a Markes CIA-Advantage-xr. The water removal system is a Markes Kori-xr and the preconcentrator is a Markes unity-xr
ncas-dc-gc-fid-1 (lab), ncas-dc-gc-fid-2 (field)
300 cm x 70 cm x 70 cm. 120kg
See page for details
Dual Channel Gas Chromatograph with Flame Ionisation Detector (DC-GC-FID)
The Dual Channel Gas Chromatograph with Flame Ionisation Detectors is capable of reporting mixing ratios of a wide range of volatile organic compounds in the atmosphere.
Samples are selected, dried and preconcentrated before being desorbed within a stream of helium (or hydrogen) and injected into the GC oven for analysis. The eluent is split in approximately equal portions between a Na2SO4 deactivated aluminium oxide (Al2O3) porous layer open tubular (PLOT) column (50 m, 0.32 mm id, Varian Netherlands) for analysis of NMHCs and a stabilwax column (60 m, 0.32 mm id, Restek) for analysis of the more polar VOCs including selected oxygenated species. Analytes elute from the GC columns into two flame ionisation detectors for detection. The GC oven has been programmed for optimal separation of all compounds of interest. NOTE: we are continually striving to improve the performance of our methods and so there may be small variations in the instrument set-up, depending on the needs of the project.
There are two units available:
• unit 1 – Lab use (to receive whole air samples including aircraft samples)
• unit 2 – ground use only (deployable for field detachments)
The dual-channel gas chromatography system with flame ionisation detection (DC-GC-FID) is used to measure atmospheric concentrations of a range of volatile organic compounds (VOC) for both in situ ground-based deployments and for analysis of whole air samples from canisters collected from aircraft, glass tubes, and Tedlar bags. In each case the instrument is operated in much the same way, with the only difference being in the delivery of the sample into the system.
Many of the compounds of interest are present in the atmosphere at levels too low for quantitative detection, therefore, large volume sampling involving a concentration stage using an adsorbent trap is required before the samples are analysed.
Gas chromatography is a separation method that relies on differences in partitioning behaviour between a flowing mobile phase (carrier gas) and a stationary phase (column coating) to separate the components in a mixture.
Flame ionisation detection
The flame ionisation detector (FID) consists of a hydrogen-air flame within which organic compounds contained in the column eluent burn to produce ions which travel to the collector electrode held at a negative potential. The current produced is measured and is proportional to the number of ions present in the flame which in turn is proportional to the concentration of each species.
Example of the chromatograms produced by the instrument
For a given set of conditions, the retention time (the time it takes for a particular compound to reach the end of the column) is fixed for each compound. Using calibration mixtures of known composition and concentration allows us to positively identify each compound/peak and also calibrate the instrument.
Given the large number of compounds that the instrument is capable of measuring, a number of different calibration methods are required. We work closely with European and global partners in an effort to devise best practices in the calibration of this instrument. For more information on these, please contact the Instrument Scientist (IS). The most common calibration methods are detailed below.
Non-methane hydrocarbons (NMHCs)
Calibration of NMHCs is performed by reference to an NPL ozone precursor mix. This calibration scale has been adopted by the GAW-VOC network and hence the measurements of NMHCs made by this instrument are directly comparable to those made by all of the GAW global observatories. Calibrations are performed each month or more frequently if field deployment allows.
Oxygenated volatile organic compounds (OVOCs)
Calibration of oxygenated volatile organic compounds (OVOCs) is performed by reference to an NPL gas mixture. In order to achieve good stability, the concentrations in the cylinder are high (ppm level) and so is diluted (to reach typical ambient concentrations) using a dynamic dilution system featuring two mass flow controllers.
The linearity of the flame ionisation detector (FID) allows the calibration factors for other VOCs to be calculated based on their structure. This enables certain compounds, not contained in the certified gas mixtures, to be reported with a high level of confidence. For more details on this please contact the Instrument Scientist.
- Deployment of the instrument typically requires an NCAS Instrument Scientist operator, in which case the consumables required will have already been arranged by them for the deployment.
- Typical consumables required for instrument deployment include:
- 1 x cylinder of CP Grade Helium (carrier gas).
- 1 x cylinder of Nitrogen N6.0 grade (blank gas).
- 1 x cylinder of hydrogen research grade (detector gas).
- Fittings and ferrules for GC column installation.
- Fittings, ferrules and tubing for connecting to a sampling manifold
- Instrument Insurance:
- This system must be insured by the user for £90K and covers loss, theft or damage to the instrument: damage being that over and above general wear and tear. The system has been designed to be rugged and autonomous. Even so, the end-user must respect the fact that the system is a precision instrument that must be treated with great care. The system is insured whilst it is on the research aircraft.
- The user is responsible for the instrument from the time it leaves to the time it is returned and signed off as in an acceptable operating condition by the Instrument Scientist (IS): this will be done as soon as is possible on its return.
- Public Liability Insurance
- AMOF is not liable for any damage or injury arising from the deployment or operation of this instrument when unattended by the IS.
- Shipping Expenses
- The user is liable for all costs arising from the shipping of the instrument both to and from deployment.
- IS T&S
- The user is responsible for covering the travel and subsistence expenses of the IS during mobilisation and demobilisation.
The system when packed ready for shipping consists of multiple flight cases (typically four) for each of the components and related equipment, tubing and fittings.
Shipping dimensions: Approximately 4 boxes of around 100 cm x 70 cm x 70 cm
Shipping weight: 200 kg
The equipment requires approximately 3 m of bench space, 3kW of power and needs t be maintained at a temperature between 18 and 25 °C (higher temperatures can be tolerated, but may require changes to the analytical method.
If the system is used to analyse Whole Air Samples (WAS) then adequate room in front of the instrument is required for the WAS samples/cases (up to 2 m of space).
NOTE: In the case of WAS analyses for FAAM detachments there may also be the requirement for storage and preparation (evacuation) of up to three SETS of WAS cases. This is a significant space – approximately 4 m x 6 m.
- Set-up of the instrument requires a minimum of two persons. Individual components (boxed) weigh up to 70 Kg. Power cables and gas lines may represent a trip hazard.
- Individual components of the system are commercially available instruments built to a high standard. No attempt should be made to remove the outer covers of the instruments.
- There is no requirement for the system to be attended during operation from a safety standpoint. However, regular checks (daily) should be made to ensure the instrument is working normally and there is normal consumption of gases etc.
When unpacked the instrument has the following physical specification
- Footprint: 300 cm (L) x 70 cm (W) x 70 cm (H)
- Weight (not including shipping case): 120 kg
- Power: 3 kW at 230 V
- Operation temperature: 18°C to 25°C
The Dual Channel Gas Chromatograph with Flame Ionisation Detectors is capable of reporting mixing ratios of a wide range of volatile organic compounds in the atmosphere. Below is a complete list of the most commonly reported compounds able to be detected in the atmosphere. The instrument is capable of detecting other compounds, so please contact the IS for more details.
Table: A list of the most commonly reported compounds in ambient air samples
The above list is not exhaustive, please contact the instrument scientist if you would like further information. We may also be able to give advice on other instruments available for any VOCs not covered by this instrument (for example heavier weight or halogenated compounds).
- The instrument produces a range of out files and all are text format.
- The user can download (but not delete) this data from the instrument but it should be noted that this data will not have been quality controlled.
- Data is provided in NetCDF files following the AMOF data standard
- Files contain no more than 24hr of data.
- Instrument name is
- ncas-dc-gc-fid-1 (lab)
- ncas-dc-gc-fid-2 (field)
- The data product(s) associated with this instrument:
- Example data file