Unable to establish baseline

[ProChem]

Hello,

This is my first post, and I should disclose this upfront. I'm an IT person not a chemist. That said, I am posting on behalf of the company I work for with an issue we've been having with a HP 5890 SeriesII GC.

The GC machine will not return a correct readout. I'll run a burnoff macro and it will show both on screen and on the GC 4.0 - 4.1. once I hit stop on the GC and it begins generating a report, the report says 1,300.

We've also run a "loop test" I believe it was called. The GC would generate it's own signal, generating simulated peaks. The first peak on the GC LCD screen and the computers real-time readout was 127. Yet when the run ends and it spits out the report 127 becomes 30,000 or higher.

We've talked to several technicians and they all don't know what's happening. Thus I went looking for a forum to be able to ask more than one person at a time. Anybody have any ideas as to what would cause this issue? We've changed out several components ( Main Board, PROM chip, Power supply, Controller card). Yet still having this issue.

If I'm not supplying enough information, please let me know and I will have the manager answer them, since he knows much more about this then I do. Thanks in advance.

Jeremy

[GasMan]

Prochem,

The signal that you see on the 5890 screen is the value of the signal at that moment in time. The numbers that you see in the report are this signal collected over time. In chromatography we are interested in the area of a peak. This area is collected by taking and storing the signal maybe 20 times a second. Algorithms are then used to find the area of the peak.This is how you will get the large numbers.

Check your report generating software. It may be possible to have the report in units of height rather than units of area ( which is the norm). If you have a report in heght, I am sure that the numbers will be closer to what you are seeing on the screen. I do not think that you have a problem with your 5890.

Gasman

[aldehyde]

How are you collecting the signal? A/D converter/integrator or a computer? If computer, what data system? It is possible that you have the signal range/attenuation incorrectly set and it is scaling the signal way high.

Does the baseline as reported from the GC front panel stay low? Is the signal always huge on whatever readout you're looking at?

http://www.chem.agilent.com/Library/Sup ... a16126.pdf

[ProChem]

The System is hooked up to a computer via Hpib and has chemstation doing the integration. Although I started with the same conclusion that it must be the software or communication port I was let down when I reloaded the software and even down graded it to version a10. Then to be sure it was not a computer problem I brought in another computer and booted it on that computer. And to really be sure it was not a windows problem I did a fresh install of the windows XP and did not let it update to make sure it was not something from windows.

As for the settings on the GC the attenuation and range are at 0. And the GC front panel and real time output in chemstation are correlating. It is just in the integration of the report.

To further reduce the amount of variables. I took out the FID and TCD detection boards to run just a straight Test Plot and it still skews the results. The front panel and the chemstation both say the first peek should be 127 counts and the report states more than 30,000.

Also we put in a new Hpib communication boards and cable to try to solve this.

[GasMan]

Prochem,

You have to understand that you are looking at two different numbers. The number that you see in the 5890 display will be pA for the FID. The number that you see in the ChemStation report is NOT in pA but is an area measurement. It is the product of the signal multiplied by time.

I do not know if the following analogy will help. If you are driving your car on the highway at 55 mph and you drive for 3 hours, the odometer will show 165 ( if you set it to zero at the start). It does not show 55. The odometer is showing the result of the speed (signal) multiplied by time ( the area in your report).

I suggest that you try to find some basic literature on peak integration which may be able to explain it better than I can.

As I mentioned in my first post, you can try to get a report based on height rather than area. In ChemStation, go into the Data Aanalysis screen and load your signal file. In the menu bar click on 'Report' and then click on 'Specify report'. In the resulting window you will see at the top an entry for 'Based On:' . Set this to height, and re run your report. You should now see numbers that you are expecting.

Gasman

[ProChem]

I understand the percent area and what it mean. That is not what I am trying to figure out. It is that if my display on my gc and my display in chemstation are reading 4.3 counts as a start base line, and my chromatography report has shown the same starting base line, why would my gc and chemstation still show the same but my report suddenly show my start baseline at 12,000+ counts.

So if I run my gc with nothing running through it and is oscillates at say 4.3 to 4.4 counts my chromatography report should reflect the same thing. And has for over 2 years as we use this as a QC part of our process. And now it looks like static. But is not as reported by doing the Test Plot program. It is just way off scale but no scaling is set anywhere that I can find in any program parameter or GC setting.

[aldehyde]

Could you possibly post a screenshot of the report?

[ProChem]

aldehyde,

The last few post were from the manager, him and I decided to video the problem and post a video for people to see. Unfortunately the video equipment is at home, but thankfully the work day has drawn to a close. We'll video and post a link the the video tomorrow.

Thanks for your help

ProChem

[aldehyde]

No problem . A screenshot would probably be easier, hopefully we'll be able to see the details.

[AICMM]

Prochem,

I think the issue you are facing is one I have scratched my head over for years with Chemstation. The older versions of Chemstation report signal in a manner that it basically disconnected from PA on the GC. The newest versions of Chemstation tracks pA from the GC with pA on the screen and in the data but the older systems are not 1:1 in the data analysis of chromatograms. The best thing I can tell you is to track a peak, look at it's max in pA then look at its height in data analysis and generate a conversion factor for the data analysis window.

Best regards.

[ProChem]

aldehyde,

Since I never posted the link to the video I figured I'd should at least inform as to why. The manager and I think we've come to a conclusion to our issue.

We were in contact with a technical specialist that has serviced GC machines for the last 25 years and he had us look at a sample readout. It turns out, at least to our understanding now, the machine is working just fine. Here's what I mean, and forgive me for it being confusing.

For the longest time it worked the way we knew it worked. We'll call this "way A". Then at some point with the upgrading of software and hardware it started working "way B". Now, we thought that "way B" was the broken way, and we were desperately trying to get it back to working "way A". After looking at the technicians sample and comparing it to one of our readouts, "way B" matches his. So... a complete mindtrip to us, we now think "way A" was the broken way and we desperately trying to break it to make it work how we thought it should. Still going to run a few more test, and will definitely post here if we run into more trouble. I thank you for all your help.



AICMM,

I will have the manager read your post Tues when I get back to the plant. Some of what your saying is currently over my head a little. I thank you though for the post.


Respectfully,

ProChem

[JTM]

"way A" ... "way B"

i laughed when i saw this. pretty funny.