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- Posts: 3475
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- Location: Western Kentucky
LALman wrote:James_Ball wrote:
One thing I have read on the SIS yttria filaments is that you need to lower the emission current a little because they produce more electrons at a given current. Not sure if that would cause the falloff with the first attempt or not.
I've not seen that. Their own SISweb Note 92: Yttria Coated Mass Spectrometer Filaments says to leave the Agilent default EIEnergy at 69.9 and that current and voltage will be correspondingly lower because it happens at about 500C lower temperatures while still producing 70eV. See figure #4.
I did have a source thermocouple fault on the second pump down (temperature was all over the place (target: 230C: 225,226,227,228,350, 450, 225, 228, 229, 280, etc...) I had the same fault during my May 2017 source cleaning and then it stabilized when I checked the mount. The instrument ran without problem right up till I burnt the filament. So I had bought a new thermocouple back in May and had it available and put it in right away when the fault happened again. I had wondered if temperature drift might be responsible but hard to see how it could vary enough without tripping a fault and I think I would have noticed changes while calibrating because I keep the source temperature visible on the Top App.
EI Energy will always be 69.9 but the Emission Current can be lowered because the filaments are more efficient at producing electrons. I think you have to manually lower the Emission Current as the autotune setting don't normally adjust that value. Best way to adjust the Emission Current is to run Repeller profiles and if the profile is not a nice bell curve shape, lower the current until it gives a better profile. If the profile from 219 and 69 just go up and up until the repeller maxes out, then the current is too high. I have had to drop emission current down as low as 20uA even on regular filaments, I believe default is 35uA.