5975 filament deterioration symptoms?

[James_Ball]

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.

[LALman]

...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.

With the standard filaments, It was always close or at the default maximum of 35 (as you say "up and up"). And, the peaks were barely rolling off when ramping repeller from 10 to 42. I let the Max Sensitivity Autotune set the Repeller. In this case Filament #1 is a standard filament and it came up with a repeller of 34.81 which just to the right the roll off at 36.

For the SisAlloy Filament #2, Autotune used the default Emission of 34.6 and EI Energy of 69.9; it chose a Repeller of 29.29 which is the center of the nice guassian peaks when ramping repeller from 10 to 42.

[tlahren]

Hi, I realize this is a 6 year old post but I just wanted to correct this for other people who stumble upon it.

The second filament is definitely no marketing gimmick -- it is actually critical to the function of the EI source. When you are using filament 1 the electrons emitted are reflected back by the 2nd filament. The electron can make several reflections back and forth in the ion volume before making a collision.

Thank you for the clarification. I guess I can understand the necessity for the reflection using the bottom filament or as I assumed previously a "collector plate" as found on instruments with only one filament. I was just told by an old Agilent Sales guy at one time that they decided to put two filaments in the final designs for the single quads because they could use it as a Marketing advantage by saying you can simply switch filaments without venting (I cannot personally verify this info). I was told that they never see the same performance out of filament #2. You're right, though, I was grossly inaccurate in referring to it as a "Gimmick." It is indeed more than that on these systems.

[aldehyde]

Hi, I realize this is a 6 year old post but I just wanted to correct this for other people who stumble upon it.

The second filament is definitely no marketing gimmick -- it is actually critical to the function of the EI source. When you are using filament 1 the electrons emitted are reflected back by the 2nd filament. The electron can make several reflections back and forth in the ion volume before making a collision.

Thank you for the clarification. I guess I can understand the necessity for the reflection using the bottom filament or as I assumed previously a "collector plate" as found on instruments with only one filament. I was just told by an old Agilent Sales guy at one time that they decided to put two filaments in the final designs for the single quads because they could use it as a Marketing advantage by saying you can simply switch filaments without venting (I cannot personally verify this info). I was told that they never see the same performance out of filament #2. You're right, though, I was grossly inaccurate in referring to it as a "Gimmick." It is indeed more than that on these systems.

I'm sure there is some degree of truth to the marketing bit.. When doing testing / troubleshooting / validation work I always tune both filaments. Generally the tune gain factor should be very similar on both filaments; tune gain factor is the detector amplification needed to get to 500k counts of PFTBA base peak. As long as the source and vacuum are OK, this value should be about 0.5-3.

If filament one breaks, it could definitely cause filament 2 to perform poorly. But if both filaments are functional they should work equally as well.