We have a VG 602C mass spectrometer configured for CO2.
All electronics beyond the preamplifiers were upgraded
before the problems described here appeared.
Until about 4 years ago, peak shapes were excellent;
it was possible to obtain a peak-width of 10 volts for mass 45.
In the last 4 years, the best possible 45 peak has been
narrower, but usable . . . until now.
At present, we cannot find a magnet position/alignment
that gives a usable peak.
We have accuracy, but poor precision.
We now have a peak shape that is almost the classic Gaussian
bell shape, with long tails so that the peak tails overlap.

We have checked the electronics thoroughly, and noise is
insignificant (less than 40 mv rms) in the high voltage
power supply and on the half plates.
The filament current, the filament bias and the filament
emission all appear to be relatively stable.
The source vacuum appears to be adequate.
We checked the magnetic field across the poles on the
deflection magnet and the field appears uniform
within a few percent.

Has anyone experienced such a problem,
and come up with a diagnosis?
Could deterioration of the magnet or misalignment
in the source cause what we are observing?

Any suggestions and advice (other than buying a new
Mass spec) are welcome.

Austin Long