GC-FID Agilent 6850

[carol]

Hi all,

I have a college who is working with a GC-FID and she is doing fatty acid analysis. The lab bought six month ago a new instrument an Agilent 6850 and a method for the analysis of fatty acid conducted on an old instrument was transferred to the new instrument. However, we have a huge problem . Rt of the fatty acids change every day. Different technicians form Agilent came to the lab to try to resolve the problem and no solution has been found .
The technicians keep saying that maintenance of the instrument need to be conducted every day like cut the column; change the liner and things like this. My college got fed up with the situation. So now we are conducting the fatty acid analysis and every two days we have to update the Rt of each analytes.
I would like to know if somebody had similar problem with this instrument o have any idea of how to resolve this. We have checked all chromatograms, and on the old instrument Rt have never changed. ....With my experience on GC I know that Rt of analytes do not change, however, I would like to have other user opinion as we would like to solve this problem.

Thanks a lot in advance for your help,
Carolina

[GasMan]

We need to know more details. What column is being used and what are your conditions? You say that the retention times are changing. Are they increasing, decreasing or randomly changing. If you are running close to the maximum temperature of the column, you can expect column bleed, which means that you are loosing liquid phase from your column and there will be a change in retention times. I have seen this in biodiesel analysis. If you are using a data system, it should be possible to set 'Update Retention times' when you do a calibration. This will update automatically the retention times in your calibration table.

Gasman

[Peter Apps]

Hi Carolina

Does the daily maintenance solve the problem ? What was the old instrument, and what has changed in terms of columns, inlet liners, gas flows etc between the new one and the old ?.

Regretably, analytical chemistry often involves tedious stuff like making sure that instruments are operating correctly. If you can't stand the heat .......

Peter

[Consumer Products Guy]

Hi all,

I have a college who is working with a GC-FID and she is doing fatty acid analysis. The lab bought six month ago a new instrument an Agilent 6850 and a method for the analysis of fatty acid conducted on an old instrument was transferred to the new instrument.

OK, the method was "transferred" to the new 6890 GC, understand that. I want to share that I've done fatty acid analyses for over 30 years, and industry standard is to first convert to fatty acid methyl esters, because they're FAME are tons easier to obtain better and reproducible chromatography. Underivatized fatty acids are way tougher, tailing, etc....

[carol]

Hi again,

We haven't change any characteristic of the method when we when from the old instrument to the new instrument. What we have changed are the flow to achieve the old Rt and the gradient.

The flow is 1,8 ml/min and it works in a gradient of temperature form 60 ºC to 240 ºC.

The fatty acids are derivatized.
Rt degrees every 5 samples 0,02 min and therefore after 50 min the Rt have moved 1 min approx.

We think that it is the instrument which is not working properly however we don't know how to prove this as it is completely new.

Thanks a lot for your comments and suggestions, really appreciated.

Carol

[Consumer Products Guy]

Set up a long sequence (like over the weekend) and inject a sample or standard like every two hours, document if the retention time or chromatograhy changes.

(1) Was the column in the 6850 new? Do you have a new column to try?

(2) What septa are you using?

(3) How often do you change the split liner (or other liner), and which type do you use?

(4) Monday post early and late chromatograms.

[CE Instruments]

What we have changed are the flow to achieve the old Rt and the gradient.

Just a linear change ? Was the old GC EPC ? Assume you are not pushing the limits of the new GC compared to the old (using step gradient or close to one ?) Did you run retention time locking with either ? Was the old GC an Agilent ?

I would first of all find out if it is sample/method related. Run a simple test standard on your standard GC and method. Hopefully the peaks will separate enough and you will be able to see if the retention time of these changes. If the test standard runs to specification the problem is sample related if not the GC may have a fault with it's EPC

[Consumer Products Guy]

What we have changed are the flow to achieve the old Rt and the gradient.

I think the term is temperature program rather than the HPLC term "gradient". Anyway, while a run is occuring, with the temperature ramping up, feel the back left side (as you face it) of the GC and see if you feel warm air escaping. The cooling flaps should only be open during cool down after a run to let hot air out, so ensure that those are closed during a run.

[Peter Apps]

Hi Carolina

If the column is the same and the temperature programme is the same you would not need to change the flow to get the same retention times. Have you checked for leaks (with a leak seeker not with on-board diagnostics).

The way to get useful help quickly on this forum is to post the details of what you are doing, and respond to the questions that are asked.

Peter

[aldehyde]

More method information please. Inlet temperature? Oven temperature program? What type of column and what are the column dimensions?

As others have said fatty acids are very reactive and derivitization is necessary to reduce these problems. Even when converted to FAME they can be very tricky. Once we have more information we can give you some recommendations.

[Glenn]

Presumably you're doing FAMEs and not fatty acids and you will have a solution in heptane or iso-octane. Given the standard method, your samples should be so clean that you hardly need any maintenance, you certainly don't need to be cutting anything off your column. I used the same 30m column for over 6 years with no loss of resolution and no change in RRTs whilst running upto 100 samples a day (anyone telling you to routinely cut bits off your column is a sales rep trying to make more money out of you).

[aldehyde]

anyone telling you to routinely cut bits off your column is a sales rep trying to make more money out of you.

Yeah.. thats not application dependent at all..