GC hp6890 peak response differences

[ydna6969]

Hi,

Anybody have a clue what is going on .

2 identical (from the outside at least) GC HP6890's - peak area responses between systems differ by 25% for various methods
Same column ,liner , carrier gas, detector parameters etc.
Even engineer confirmed both systems are fine.(robustness of methods questioned but I am not convinced)

We use peak area ratios to quantify so these are the same on both systems (due to 25% loss in int std too)
Concerned for Low level methods /volatile solvents methods we only have one GC ,,
any ideas much appreciated

[tangaloomaflyer]

Does it matter? If your instrument is calibrated then absolute response shouldn't be a problem as long as you can meet your detection limit

[ydna6969]

Hi ,

That's just the problem , detection limit will be fine on one system (lets say for a residual solvent method) but not on another using the same setup ,column, liner etc.
I am not sure why this is the case or how to explain this difference in response and sensitivity on 2 identical systems

[Peter Apps]

If a 25% difference in peak area makes or breaks a method's detection limit, then it is not very robust.

What are the noise levels on the two instruments ?.

What are the repeatabilities of 5 replicate injections on the two systems ?

Nobody can make sensible suggestions about the cause of the difference unless you provide details of the set-up, and what you are analysing.

Peter

[ydna6969]

Hi

The noise levels are very similar between the 2 HP6890's
Replicate injections between the 2 HP6890 are < 1.0 (0.4 and 0.6% RSD)

The 25 % loss has been noticed on a number of different methods (by robust I assume you mean the methods)
All ICH validation criteria was met on both systems accept at low level so LOQ/LOD failed as we only have one gc system

[Peter Apps]

I am slightly confused - you have a comparison between two systems, but you only have one system. Does this mean that you are comparing the one system that you have with a system in another laboratory ?

I repeat: without details of what you are doing we cannot help you. You do not even tell us what detector you are using. FIDs are much more robust than ECDs or NPDs for example.

Peter

[GasMan]

As Peter mentions, more information is needed. What columns and what detectors. Assuming that you are using FID detectors, are you aware that the 6890 has two different FIDs available. One is set up for capiilary columns only and the other can be used for capillary or packed columns with the appropriate adapter. What is not realised is that they have differet jets installed, with different internal diameters and this would certainly give the variations that you are seeing.

Gasman

[ydna6969]

Hi ,
Thanks for replies ,

We have 2 hp6890's in our lab,, both identical but only one is suitable for low level analysis.
Detector is FID... columns are capillary and doesn't matter about dimensions .
This loss in response is on a number of columns of different dimensions and methods

[Peter Apps]

To quote one of the experts on this forum ....

"The more you don't tell us, the more we can't help you".

Peter

[ydna6969]

Hi ,

If I quoted all GC method parameters that have been used on the HP6890 I would be here all day
Does it matter about parameters when the other HP6890 is spot on to a 3rd instrument in another lab?

I suspect it is something to do with the detector or set up on Chemstation

[Peter Apps]

Hi ,

I suspect it is something to do with the detector or set up on Chemstation

You might well be right - but unless you think that we are telepathic you cannot really expect us to offer advice unless you tell us how the detector is set up, what troubleshooting you have done and what the results were, and how Chemstation is set up.

If I were to list all the possible causes of a bias in signal between two instruments I would be here all day.

Peter

[ydna6969]

Understood peter apps,

But a list would be nice !

Various detector set ups are used - both identical on each system.

As I mentioned engineer was brought in to investigate ,,, his comment :

The HP1 GC system was giving smaller peak areas for the standard injection than were being measured when using the HP2 GC system. I have
checked over the operation of the GC inlet, split vents, purge vents, FID detector and autosampler but I have been unable to find any problems
on or differences between either GC. I then ran our standard GC test mix using our standard method in both split and splitless modes on both
GC systems and found no differences between the results obtained on each GC.

Then concluded methods were not robust but Im not convinced ,, and 1600 quid down !

[sepscientologist]

So what is 25% smaller? Peak width or peak height or both?

[ydna6969]

Hi ,

peak area and height are about 20-25 % less on one system than the other
Exactly the same set up on both systems as mentioned earlier ,,unless im missing something (and technician did too !)

[Peter Apps]

"I then ran our standard GC test mix using our standard method in both split and splitless modes on both
GC systems and found no differences between the results obtained on each GC."

So sometimes the results are 25% different and sometimes they are not ? And yet earlier you said that all results were 25% different. Which is it ?

And you still do not provide any of the details that we need to diagnose the problem. Repeating over and over that the two systems are the same is not getting us anywhere. Let's start with the conditions that did not generate the 25 % difference (your test mix runs) compared with one of the methods that does generate a 25% difference. What concentration of what analyte in what solvent ?, what injection volume, inlet liner, temperatures and flows etc etc etc.

Peter