Many thanks to everybody for the many answers!!
The comments already helped a lot to overcome my confusion and i am now able to achieve a H3 factor of around 7 with good reproducibility, but what is far more interesting than that is the way how i can do it (which is in contradiction to what i read in the archives, but in accordance with your recent comments...!)
Please, let me explain:
Already before my first posting, i read this in the archives (the author is Willi A. Brand):
He wrote in 2005: "The awkward mass 2 peak arises from He2+ ions which can become prominent when the electron beam energy is set to values of 90V or larger. The mass separation of the Delta machines is not large but large enough to show a composed peak. Remedy: Adjust the electron energy to around 70 eV. This will suppress the He2+ to less than 200 mV (I think).
and earlier in 2002:
"Dear Paul (and others interested),
you are observing the doubly charged helium ion which is formed in the ionization step. The size of the signal depends o the amount of helium in the ion source (of course) and on the electron energy. You can easily increase the signal to 1 nanoamp or more by turning up the electron acceleration, and you can discriminate against He++ formation by setting the value to less than 70 eV. The second ionization potential of He is 54.4 eV.
(the subjects were called: "water SRM for H and O" and "HD tuning on Delta Plus XL",respectively, for those who want to refer to the archives)
...now, what i observe for the behaviour of the H3 factor at our mass spec is that it is decreasing with increasing electron voltage. At 70eV, the H3 factor is around 9 (but only if i keep extraction voltage low, too (around 30)). From there on, the H3 factor decreases to 6-7 at an electron voltage of 100. This is in accordance with your postings, i think. My question is now: What if a very low H3 factor at high electron voltages is the result of interference of He2+ with the mass 2 beam?? This would of course minimize the mass 3/mass 2 ratio and decrease the H3 factor, but would produce false measurements, or not??
...I hope i am not making a fool of me with this question as i do not know whether i am mixing things up or overlooking something...is there a point in what i wrote or did i get something wrong? I am confused about this.
Thank you so much!