Leco Pegasus III high background in TIC?

[walshbig]

I am getting very high background with a recently installed Leco Pegasus III after years of being out of service. Even when nothing is injected I am getting almost flat line at about 200000 count in TIC with wide mass 10-300 for example. When I close mass range (10-50 for example) it comes down to about 60000. Is this normal for a stored instrument for a long time with air inside or I am missing something here?

[Don_Hilton]

I assume that you ran the acquisition offset adjust in the tune sequence?

The TOF does not have an acquisition threshhold , so unlike some other systems it counts all the noise. Zero through an operational amplifier is actually what ever you want it to be. The Pegasus will set "zero" to a high enough signal level that there should not be noise with a negative measurement.

If you are concerned that there is signal from the column or leakage of air. look at the spectrum as it is acquired during the run. Column bleed will show up as will air. (And don't bother collecting below m/z 28 -- and down that low only if you need the smaller masses. There is not much to see down there - and a lot of background from air coming thorugh the split vent if you column head pressure is close to atmospheric pressure.) You should have a good handle on the air background, having run the leak check as part of the tune sequence before trying to make an injection.

The integration software finds the baseline and determines zero. So once you start finding peaks with the integration routine, the signal level for the baseline in the TIC will drop dramatically - and things like a contant signal for m/z 28 or 40 through the run largely vanish.

I don't remember what my Pegaus gives for background counts on the TIC, so I can not give you an idea of what you should get, but the Pegasus does give higher background counts during acquisition than other instruments. But then again the detectors, amplifiers, and numeric scale for results differ as well.

[walshbig]

"acquisition offset adjust in the tune sequence" ? I Haven't seen this anywhere in Chromatof. Where is this "acquisition offset adjust"?

Is there anyway of decreasing background in TIC without decreasing signal level? It is really annoying to see such a large black area at the right hand side when I browse through TIC. I know if I could get to 1E-8 or lower vacuum I could get 10 times lower background but achieving such a low pressures seems impossible. Best I am getting is 1E-7.

A new problem I am having since a few days ago. Problem is instrument doing "acquisition system adjust" with every measurement automatically by itself at the beginning. It just does about 50 quick collection and goes into "waiting for transfer line" right away. This is annoying to see instrument goes to "acquisition system adjust" with every acquisition task unless it is normal. In the log file I see offset adjust is recorded after this. I think something should be wrong but I don't know what. I just noticed once "Z deflection" was faulty when I was inspecting Diagnostic but later it went away.

[Don_Hilton]

about 1 E - 7 is where you want to be with the vacuum. Even if you get lower than this, electronic noise will be part of the signal.

I'm not sure why I put offset in the middle of "Acquisition System Adjust" or "Acquisition Adjust" (without an instrument in front of me, I can't open the software to check.") - other than I was thinking about the offset adjust step in the procedure. It is listed as one of the types of acuisitions avaialble in the run queue - as an alternative to "Sample."

The acquisition adjust step that you have at the start of the run, you want. This is an adjustemtent of the offset in the operational amplifier to account for any changes in room temperature before you start the run.

At one time some of the folks in LECO service were talking about making some videos to show how to get started with using the instrument. I have no idea if this was ever done. Call LECO service and see what kind of help they can get for you. You seem to be at a point where someone pointing out the tricks to efficient use of the instrument would help. If videos do not exist, perhaps there is a way that you could visit with someone from LECO and get some quick training. There is a three day class taught by LECO - and the materials have improved since my day. The Pegasus does some things differently from other instruments (actually all instruments do things differently from each other) so it is helpful to hear the point of view of the peole who built the instrument. I have learned other instruments by hacking my way through getting them to work and by having information passed on to me (and sometimes remembered correctly) by other users. No matter the instrument, you get more if you go throug the program the vendor has created to be sure you don't miss critical features.

[walshbig]

Thank you. Very helpful as usual. I will try to attend one of those 3 day courses but still hearing others opinion is sometimes the best way of learning since people has practical experiences.