I run breath CO2 using cf-IRMS on an VG Optima.
No problem (almost) for eight years.
Now I get very frustrating results. This morning for example I got
completely normal delta values (13C vs PDB)of -29.20 with a precision of
<0.02 per mil (n=6) Very good and all normal.
Suddenly the same samples get -33 per mil for a while going back to -29.2 or
so after that.
Earlier this year I have seen -80 per mil for a while (with poor precision)
and then -40 or -29.
Nothing strange at all with the chromatograms.
I have noticed though a high filament current of 4mA (normal 2.4 mA) and a
magnet scan reveals a number of large peaks indicating possibly leakage
But still, why these big jumps?
Stability tests always give very good precision.
Linearity seems to be decent as well.
Any ideas appreciated.