MS calibration

The mass-spectrometer of COSAC delivers data in the following way:  Every
ion hitting the detector produces a digital pulse.  These pulses are 
counted and stored in different bins according to their arrival time at 
the detector measured from the starting signal.  This procedure is repeated
 until the spectrum acquisition is considered sufficient, i.e. after a 
pre-defined time.  Hence the signal itself is digital in nature and not the
 result of an analogue-to-digital conversion.  The natural format for the 
intensity information is integer.

For the quantitative intensity calibration one needs: a) the ionisation 
current, b) the detector voltage, and c) the integration time.  All these
 values are contained in the header.  Since at first approximation parameters
 a) and b) are kept constant, mainly the integration time is important.

For the mass scale calibration one needs to know that the ions start at the
 same time with the same energy per unit charge and are separated in time due
 to their different mass to charge ratio.  The ion mass is usually quoted in
 atomic mass units (amu) which is defined by 1/12 of the mass of the carbon
 12 isotope.  The relation between mass/unit charge and flight time is:

m=(binNo*1.17E-3 - 0.4)^2

The above equation is sufficiently precise for unique identification of 
integer amu/q numbers and therefore the interpretation of the spectra.  For
 a more detailed analysis the precision of the fit may be fine tuned furthe
r by the end user.

GC calibration

The COSAC gas-chromatograph produces a signal which is proportional to the
 thermal conductivity of the measured gas.  The intensity of the signal is 
commonly not given in physical units for two reasons.  The first is that it
 may vary for different sample constituents; the second is that it is not
 constant over the measurement time since the peaks tend to be wider for
 longer times.  A more meaningful information is the area under a peak, 
which, on the other hand, can only be properly calibrated if the component
 causing the peak is known.  Example:  The catalogue of the company RESTEK
 which offers all sorts of chromatography components contains several 
hundreds of reference chromatograms.  None of them carries a labelled y-axis.

When quantitative chromatography is performed on earth this is usually 
achieved by simultaneous injection of a well characterised standard.  Such
a procedure is not possible with COSAC.