by
LALman » Wed Oct 24, 2018 5:06 pm
I replaced my filiments with the yttria coated yttrium-rhenium alloy when I repaired/replaced my diffusion pump thermal sensor, and have let it sit unused under vacuum for nearly a year. I'm getting poor tunes and ramping lenses are very ragged. I thought it might be a leak. But with leaks eliminated, I'm venting and giving the source a good cleaning and trying again. I used to clean my parts and then dry them in an oven before reassembly. But this is apparently wrong. I recently found this Agilent document:
How to clean an Agilent GCMS Ion Source. The lense parts have to be assembled after three different solvent rinses and then dried by the vacuum and not heated untill under hard vacuum. It also has some very particular directions about which parts of the lenses are critical to clean and which parts must touch properly. I've been itching to try it this way and see if I get improved performance.
By the way, I took your advise about yttria coated yttrium-rhenium filaments and turned my emission current on my 5973 MS down to 19.8mA. The linearity has improved tremendously! Prior, it was saturating at about 200ppb. Now with the emission current down to 19.8, I an getting a near perfect set of concentrations from a 400 ppb 8260 standard.
With a rhenium filament, a really huge (4000 ppb) peak does not affect the next peak in the chromatogram. But, even with this 19.8mA setting, A 550 ppb 1,2,4-trimethylbenzene peak will cause the following 1-4-dichlorobenzene-d4 peak to drop by 75%. So 400 ppb OK but not 550 ppb. Perhaps I should try 15mA.
I am puzzled how I am going to coax the same fix out of this 5971 MS.