The Medusa gas chromatograph-mass spectrometer (Medusa-GCMS) can measure a wide range of trace gas species through the use of pre-concentration techniques.
AGAGE calibration scales are used for all species measured in the network with the exception of CO2 and CH4. A standard calibration tank is measured alternately to each air measurement.
Calibration of the Medusa is controlled by a central calibration centre at Scripps Institute of Oceanography (SIO). Tanks of air (J-tanks) are filled here and calibrated 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 species. 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 the 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 rather whereas the H-tanks last for approximately 13 weeks.
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