Chromatography Forum

I have a 7890/5975C with an Entech Preconcentrator and we are using it to perform TO-15 analyses. I am currently having a problem getting the MS to pass a tune check. The m/z 176 is too high in relation to 174. I last cleaned the source two weeks ago. Since then, all I've analyzed on the instrument are standards, and the tune checks were passing with flying colors. Yesterday, the 176/174 ratio was too high, so I performed the BFB Dynamic Target Tune. This didn't help. The 174/95 and 176/174 ratios would time and again be too high. This morning I performed an autotune followed by the Target Tune and achieved the same results. I'm not really sure what to do at this point since the source should still be clean, even though when I would do the automatic tunes, the 502 peak would be a little noisy.

Here's a tune report from the latest tune.

Does your Air/Water Check typically run 3.7% Nitrogen and 1% Oxygen?

I'd go leak checking in the GC.

Are those values too high? What should they be?

174 to 176 rqtio usually impacted by peak with. The lens handle the ratios between the high-mass to mid-mass to low-mass ratios, but for neighboring ions it tends to be peak with issues. Try narrowing them a bit and see what happens.

Greg

Randy,

Right off the bat I see a couple of problems. One, your mass assignment for 502 is too far off. Should not be less than 501.9 to 502.1. Two, your abundance for 219 is too high, should probably be below 50%. Third, your peak width for 219 is too narrow in my opinion. Should be between 0.45 and 0.50. I would "fatten" wid219 a bit and try again.

Best regards.

How do I correct the problem with the 502 mass assignment?

And how do I fatten the 219 peak?

I've done GC/MS analysis in the past, but it's only been really as a button pusher, and someone else would come along and deal with the problems. It's only been in the last 4 to 5 months that I've gotten into MS analysis in an intensive manner, and I'm still in the steep part of the learning curve. I have a lot more to learn, but I do want to get better at troubleshooting and solving MS problems.

Randy,

Personally, I would start over again with a standard spectra autotune and add 100 volts to the EM. This would be the cleanest way to get to a tune that should work for BFB. I never had much luck with target tunes but I also am comfortable adjusting lenses on tunes to make them work for me. None of which answers your questions. Regarding the mass assignment for 502, go into your mass gain or mass offset and adjust them a little bit up or down to see which has the most impact on the 502 mass assignment. My guess would be the mass offset. Regarding the 219 peak width, I would start by adjusting the Wid219 field to see what that does.

All said, again, I would simply start from a standard spectra autotune.

Best regards.

I would also follow up on the leak check; nitrogen and oxygen in the ratio 4:1 is usually air and your vacuum seems a little low (i.e. too much gas). the turbo pump i use usually generates a vacuum 3 times lower pressure. that said I think your tune problems are more likely to do with your peak width and mass windows as said above

With the help of a coworker, I was able to get the MS to tune and pass the BFB tune check two days ago. This morning, the tune check failed again, this time with the 75/95 ratio being too high.

The MS vacuum seems to run around 7.5 x 10-6. I checked the interface nut for leaks; none there. When I did a profile scan of m/z 51 and checked the MS door with Dust Off, I got a peak with abundance around 3500. I don't know if this is considered a significant enough leak to try to remedy.

This system is very frustrating. The tuning problem is not the only one. The ISTD responses are too variable. The responses I got this morning are 40% of what they were when the instrument tuned two days ago. There is also acetone contamination that lingers even though the Entech passed a leak test last week. Too many things going wrong at once!

The tuning and variable response problems ended up being solved by replacement of the filaments.

Thanks to those who provided suggestions.

If you see this problem again you can adjust the 176/174 ratio by going
to edit dynamic lens ramping in the tune menu. You can add 0.1-0.2 v to the 131 value which would result in more 174, or you could subtract 0.1-0.2 v from 219 which would reduce the 176. Make only small changes save
the tune and then inject bfb again.