Hey,

we are using a Finigan MAT 252 mass spectrometer to measure the 13C
isotopy on CO2. When I started to reproduce the results obtained by the
intern isodat software, I had a look to the raw data. Thus, to the
mass44, mass45 and mass46 signals in time. I recognized, that the time
intervals are not equidistant, although they are set to 0.25s. It seemed
to be dependent on the actual signal. If the signal is high, like when
measuring a peak, 0.25s is the normal time interval between two points.
If only background values are measured, the time interval often changes
to 0.26s. 
So, I would like to know, how the time column might be produced. Is it
an intern effect of the farraday cups and how often they can be read in
a certain accuracy, or does the computer-speed maybe influence the read
out time? I just want to figure out in which way I can trust this time
column and how the raw data are basically produced at all inside the
mass spectrometer.

Any help would be useful.

Thanks in advance.

respect,

Robert