Low response factor in new chromatograph

Discussions about GC and other "gas phase" separation techniques.

5 posts Page 1 of 1
Good morning everyone!

I'm having some trouble with a GC-FID method and I would like to share it with you in order to find a possible cause.

I'm trying to implement a GC-FID method in a new Thermo Trace 1300 we recently acquired in our lab, for two analytes (let's say A and B). It's a brand new chromatograph so I guess we shouldn't have leak or dirtiness or these kind of problems.

The method is tested in our old Agilent 6890 and apparently it works propperly: correct calibration linear curves (7 calibration level, ISTD method), daily controls yielding the target results and routine samples of known concentration yielding also valid results.
The instrumental methods in the new Thermo have the same "basic" conditions than used in the Agilent one (column flow, inlet temperature, split ratio, oven temperatures (two ramps for A, one ramp for B), FID gases flows and temperature, etc.
The thing is that after having injected both calibration curves in Thermo, A gives nearly identical results but B gives such a lower relative response factor and slope than for Agilent.

(aproximate) slopes for :
A: 0,56 --> 0,55
B: 0,64 --> 0,53

My question is that if there may be other method conditions appart from column flow, inlet and oven temperatures or detector conditions that I could be ignoring and may provoke a low response factor. I have started by testing with oven equilibration time and it doesn't seem to affect much (althought it has increased both responses of analyte and ISTD). I looked for dirtiness in the liner and it was quite clean so I discarded this cause too. I am thinking of other options such as injection pasrameters or others ... I'm not an expert and I'm a bit lost with parameters that don't appear in the Agilent method.

Both curves have been injected in both chromatographs within days/few weeks of difference, so I don't believe it's a problem of degradation of the calibration levels solutions.

I'm sorry for all this long writing, I wanted to be clear. If you need some method details such as flows or temp's, don't hesitate to ask.

Thank you
Since it is two completely different systems it is not unheard of to have a problem like this. I have not worked with the Thermo systems but unless the injection port is the same diameter and length as the Agilent, that can begin causing the differences. Also, can you be certain that the inlet temperature is the same for both instruments, such as calibration of the temperature sensors. If the placement of the sensor is in a different location of the inlet between the two instruments it can cause variations in the injector performance, just to name a few things that can be different.

Aside from temperature and physical dimensions of the inlet being different, the injection speed and timing can be different which could also affect the results. I am guessing that also detector design differences could cause such things also.

I say all this to show that you may not have the exact same results between two different designs of instrument. I have even seen small differences between two Agilent systems setup exactly the same. Experiment with temperature settings and maybe split flows at the inlet and see if they move the results closer to the same. If it doesn't then it just may be they will give different profiles and you should be able to calibrate the instruments and have different responses but the same final value, which is what matters the most.
The past is there to guide us into the future, not to dwell in.
Thank you for your extended response, James.

I have been using CG systems in the last few years with methods already created and validated (so I just had to prepare and inject samples); I did some maintenance to the Agilent chromatograph but just the basic (septum, liner, column changing ... so I'm a little familiar with it but at a pre-intermideate level haha). I hadn't delved much on the methods themselves and the variability of results a change on them can provoke.

You comment that a new device can provide different results just depending on its hardware characteristics, but the strange thing is that for one analite it is giving similar results but not for the other one. That is why I incline to the possibility of some method detail that I still haven't tested or detected, such as the injection time or the temperatures programmed, as you mentioned. You're right when you say that in the end, correct final values is what matters the most.. but it's not the case :(.

Thank you again :). I'll let you know what the problem was (I still have
remainig perseverance to keep on trying).

Laura
lmd_not wrote:
Thank you for your extended response, James.

I have been using CG systems in the last few years with methods already created and validated (so I just had to prepare and inject samples); I did some maintenance to the Agilent chromatograph but just the basic (septum, liner, column changing ... so I'm a little familiar with it but at a pre-intermideate level haha). I hadn't delved much on the methods themselves and the variability of results a change on them can provoke.

You comment that a new device can provide different results just depending on its hardware characteristics, but the strange thing is that for one analite it is giving similar results but not for the other one. That is why I incline to the possibility of some method detail that I still haven't tested or detected, such as the injection time or the temperatures programmed, as you mentioned. You're right when you say that in the end, correct final values is what matters the most.. but it's not the case :(.

Thank you again :). I'll let you know what the problem was (I still have
remainig perseverance to keep on trying).

Laura


My pleasure to help Laura,

The best way to learn is to make changes and observe what variation you have in the results. It may be you just need a little higher temperature at the injection port to solve the problem.

Does Analyte B have a higher boiling point that Analyte A? If so it may just not be getting onto the column as efficiently and changes in temperature or even split ratio can increase its response.

Is the injection port liner the same type in both instruments? If one has glass wool and the other doesn't that can also affect the response.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
lmd_not wrote:
Thank you for your extended response, James.

I have been using CG systems in the last few years with methods already created and validated (so I just had to prepare and inject samples); I did some maintenance to the Agilent chromatograph but just the basic (septum, liner, column changing ... so I'm a little familiar with it but at a pre-intermideate level haha). I hadn't delved much on the methods themselves and the variability of results a change on them can provoke.

You comment that a new device can provide different results just depending on its hardware characteristics, but the strange thing is that for one analite it is giving similar results but not for the other one. That is why I incline to the possibility of some method detail that I still haven't tested or detected, such as the injection time or the temperatures programmed, as you mentioned. You're right when you say that in the end, correct final values is what matters the most.. but it's not the case :(.

Thank you again :). I'll let you know what the problem was (I still have
remainig perseverance to keep on trying).

Laura


My pleasure to help Laura,

The best way to learn is to make changes and observe what variation you have in the results. It may be you just need a little higher temperature at the injection port to solve the problem.

Does Analyte B have a higher boiling point that Analyte A? If so it may just not be getting onto the column as efficiently and changes in temperature or even split ratio can increase its response.

Is the injection port liner the same type in both instruments? If one has glass wool and the other doesn't that can also affect the response.



I've been testing by changing different conditions of the method but some of them must remain the same than the "valid" method, such as the inlet temperature or the split ratio. As you suggested, B's boiling point is higher than A's.... added to possible differences on the chromatograph itself, could be the cause of the differences. I wish they let us introduce more changes in the method... However, with these little changes I've been trying, I've had better results.

Thankk you for your attention :)
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