Leco Pegasus III (Chromatof) goes to optimization by itself?

[walshbig]

I prefer to do optimization manually but each time after about 48 hours when I am starting an acquisition task Chromatof goes to optimization first by itself. I haven't scheduled any automatic tuning. How to disable this?

[Don_Hilton]

Look at the QC methods. I would reccomend making a QC method ans specify what you want for the various optimization steps and use that with all samples you run. The QC method will only run based on the time specified in the method and the record of the last execution of the specified step in the log. Leaving the QC method blank with a sample run specifies the system default QC method.

I would suggest leaving the leak and tune checks to automatically run at 24 hours. This will give you a warngin if something has changed.

When you set the time specification to "24" hours, this means: "Dear Pegaus, when you run a line in the sample table with this QC method specified chck the log and the clock and if the QC step has not been run in the last 24 (number of hours I specified) then run it - otherwise, just run the sample."

If you are running the latest version of ChromaTOF, some news letters (LECO Retention Times and Chronicle) are included in the help file materials. There is a decription of using QC methods (Under ChromaTOF 3.x) and other helpful things. I don't believe they have been updated for ChromaTOF 4.x - some things have moved around a bit and some functions have been added.

[walshbig]

One problem I am having during optimization is a couple of "mass calibration failed" message sometimes before optimization finishes. It finally finishes optimization successfully but sometimes in the first go and sometimes after a few mass calibration faile. Other steps before mass calibration goes fine. Is this normal or I should expect a problem in the near future?

[Don_Hilton]

I believe that I have heard more than one explanation for this - but from what I can understand, if the m/z 69 signal is too intense, it will be a bit flat on top and ChromaTOF will have a bit of difficulty in locating the center of the mass peak. I've not really worried about it. You look at the mass error and, it is not going to make any real difference. If you are running qualitiative scans, you'll never notice it. And, I don't expect an noticible effect on quantitative runs - but good practice is to run a check standard or two after a tune anyhow (no matter what instrument you are working on), so I'll let the instrument answer the question with data.

[walshbig]

I tried to check detection limit today and used HCB solution shipped with the instrument which haven't been used for years. I didn't see any 284 peak no matter what I did. Obviously the solution is expired. Is this normal not to show any peak or something was wrong with my acquisition parameters?

[Don_Hilton]

If you used a freshly opened vial, it should be good enough to see something. This is normally at a pretty high conentration relative to what you should see for making specification. So shoot ~ 2 ng on column and look for the peak - then go lower (I used to do this as a class exercise with students learning to use the Pegasus. I got it from the fellow who taught the class before me - and I assume LECO still uses the exercise...)

The instrument should have a set of conditions that were used when the instrument was installed. Be sure you use a DB-5 column of the same dimensions. You will probably have to increase the detector voltage by about 200 volts from that obtained by tuining. You may need to go up to about -1800 v. A rule of thumb with the detector on the Pegasus: The signal approximately doubles for each 50 v increase in detector voltage. Noise does not necesarily follow that trend.

Operate the detector at as low a voltage is practical to get the desired LOD for your method. There are three things that age a detector: voltage, ions striking the detector, and moisture.

To avoid these - use the voltage you need to obtain signal. No point in cooking a detector just because you can. But, keep in mind that the detector is a consumable - use it! but use it well.

ions striking the detector: Acquire the mass range you need. (This will also save you time on processing data files.) If you do not need the low masses down below 45 or 50, don't collect them. The instrument has some smarts and will deflect some of the low mass ions if you are not collecting them. And, this keeps background air and smaller ions from solvent from hitting the detector. Also, keep the filament turned off untill the solvent peak has passed - it saves on the life of the filament and keeps from sending all those solvent ions down the flight path to the detector.

And moisture. Don't leave your system vented for extended periods of time. When you work on the instrument, vent it get the job done and get it back under vacuum. If you need to vent the instrument for an extended period of time, flood the instrument with dry nitrogen. This is not a new question, but call LECO service to see what they have for current advise on this.