The Medusa-GCMS (gas chromatograph mass spectrometer) is a complex instrument that can measure a wide range of trace gas species.The instrumentation has a unique pre-concentration system coupled with a GCMS which enables the measurement of a wide range of very high volatility trace gases at low parts per trillion (ppt) levels. It was developed at Scripps Institute of Oceanography, with input from the University of Bristol's Atmospheric Chemistry research group to measure the full range of Montreal (ozone depleting) and Kyoto Protocol (global warming) gases and has been adopted by the entire AGAGE network.
Central to the Medusa-GCMS design is the 'Cryotiger' which cools adsorbent traps to -165oC. The use of two traps allows more abundant gases (N2, O2, H2O, CO2 and CH4) to be removed prior to further analysis, avoiding interference with the lower concentration trace gas species. Trace species are isolated by desorption from the first trap onto a smaller second refocusing trap. The two litre air sample is then injected onto the GC column by flash heating of the second trap to release analytes. The air sample must be concentrated so that levels of trace species are high enough to be measured by the mass spectrometer.
On exiting the GC columns the analytes are detected via a mass spectrometer, which enables accurate identification of the complex mixtures within the air samples once samples have been separated during chromatography. A sample analysis time of one hour enables a high measurement frequency of 12 air measurements per day.
Calibration of the Medusa-GCMS is done using the AGAGE method. This is controlled by a central calibration centre at Scripps Institute of Oceanography (SIO). Tanks of air (J-tanks) are filled at Trinidad Head and calibrated at SIO against the gold standards here to assign concentration numbers to the range of species measured. These J-tanks are then sent out to the stations and used to calibrate measurements of Medusa-GCMS species. A similar method (centralised at SIO) is used for N2O, SF6, H2 and CO calibration.
J-tank standards are sent back to SIO once almost empty and re-measured to ensure no drift, loss or growth of any species has occurred within the tank whilst on site. In this way any drift can be monitored and corrected for. J-tanks are measured weekly against H-tanks (air filled at Mace Head). These H-tanks are measured in every second run to ensure accurate calibration of all species. This method also means that the J-tanks last for up to 1 year while the H-tanks last for around 13 weeks.