OUR HP 6890 GC/FID HAS JUST WITHIN THE PAST FEW DAYS BEEN EXPERIENCING EXTREME FLUCTUATION IN THE AREA COUNTS OF OUR INTERNAL STANDARDS. THESE INTERNAL STANDARDS FALL USUALLY BETWEEN 125,000 AND 160,000. THEY ARE NOW AROUND 500,000 AND THEN CAN DROP INTO THE 250,000 RANGE AND UP AGAIN. WE ROUTINELY RUN ORGANIC AIR SAMPLES FOR INDUSTRIAL HYGIENE MONITORING WITH CARBON DISULFIDE AS OUR DESORPTION SOLUTION. WE HAVE RECENTLY BEEN HAVING TROUBLE WITH OUR GAS COMPANY AND NOTICE OBVIOUS CHANGES IN BASELINE WHEN THE AIR IS NOT ULTRA PURE. I HAVE OFTEN GOT MIXED ANSWERS WHEN ASKING IF THE HYDROGEN (OUR CARRIER GAS) IS NOT ULTRA PURE IF THIS COULD SOMEHOW AFFECT OUR AREA COUNTS.....ALSO IF THE SCRUBBERS (AIR PURIFIERS) ARE BAD WILL THIS AFFECT THEM AS WELL??? I HAVE THE URGE TO SAY THE H2 IS THE CULPRIT BUT WANT TO INVESTIGATE ALL AVENUES. THE JETS WERE REPLACED THIS WEEKEND AND THE INITIAL SHOOTS WERE FINE. JUST SEEMS AS THOUGH THIS PROBLEM CAME OUT OF NOWHERE. I GUESS WHAT I NEED IS SOME BROAD ADVICE SINCE THIS IS SORT OF A BROAD QUESTION. MY SUPERVISOR AND I HAVE ALOT OF EXPERIENCE IN THIS DEPARTMENT AND HAVE TROUBLE SHOOTED EXTENSIVELY TO NO AVAIL.......SUGGESTIONS???
By chris on Thursday, March 22, 2001 - 10:17 am:
could be your gas. Also, look for leaks. These can sometimes cause these kinds of problems. What does your baseline looklike?
By Ted on Tuesday, March 27, 2001 - 04:19 am:
I agree with Chris that leaks are definitely a possibility. Other possibilities: valve malfucntions (splitless injection purge valve? gas valve?), syringe/plunger malfunctions. It is hard to imagine that carrier gas purity would cause "extreme fluctuations in area counts" but perhaps a lot of water in the carrier gas could be trouble. In my experience, such large area fluctuations are usually caused by some sort of injection problem; the amount of sample/standard getting on to the column is changing. Good luck.
By Anonymous on Tuesday, March 27, 2001 - 06:14 am:
Variations in the air to hydrogen ratio or detector make up gas flow could also cause changes in response. Are you sure your detector gas flow rates are stable? What flow rates (hydrogen, air, carrier, make up gas) are you using? Does your instrument have electronic flow control for the detector gasses?
By Ron on Wednesday, March 28, 2001 - 10:56 am:
Let's go back to the basics, and check the last thing done to the system before it started performing poorly. As near as I can tell, that was replacing the jets. There are different jets available for the 6890 GC, so verify the correct jet is installed. Also, check for leaks at the detector, since it was apart and may not have been sealed completely when reassembled. Detector leaks, while not as common as injection port leaks, can cause the same type of problems.
By Anonymous on Monday, April 2, 2001 - 08:24 am:
We do have electronic flow on the 6890 not on the 5890 and the baseline is fantastic on both instruments. Problem is, as I look back, I did gas changes and some maintenance which included changing the jets (I double checked to make sure they are the correct ones),changing the septum and liner, and allowed both instruments to cook overnight. Is it possible for the retention times to remain stable if there is a leak?
By Ron on Monday, April 2, 2001 - 10:37 am:
The retention times often are unstable or change to a later time if there is an inlet leak. The retention times will remain more stable with a detector leak, as column flow is not affected.
By Anonymous on Tuesday, April 3, 2001 - 08:02 am:
Do you measure the gas flows on the 6890 or do you just set them at the instrument? If there is a problem with a flow module, you might be able to detect fluctuations in a flow rate if you actually measure them. Do both instruments use the same gas source or are they different? This may not apply in your case but I read where if the gas cylinder valves or regulator valves are only slightly opened, there can be changes or fluctuations in the gas supply. The article I read suggested fully opening the valves then closing them slightly (less than 1/4 turn). Another thing you might try is checking your supply lines and connections for leaks. If you do not have one of the electronic leak detectors, you could pressure check the supplies.
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