Hi,
My suspiction is that you work at too low pressure in the
inlet system. Try to increase the pressure to 30 Torr
and squize the capillaries to lower the ion currents.
At this pressure you will ensure the viscous flow condition,
at which no isotopic fractionation of the standard gas is
possible over infinitely long time.
Another reason of continuous variation in the standard isotope
ratio may be a leak in the standard side of the inlet system.
Hope this may help,
Stan Halas

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Professor Dr. Stanislaw Halas
Mass Spectrometry Laboratory
Maria Curie - Sklodowska University
20-031 Lublin, POLAND
tel: (48 81) 537 6275    fax: (48 81) 533 3669