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Increasing in Retention time

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

11 posts Page 1 of 1
Hello,
1- I have GC-FID 8890 and's problem suddenly appeared, the increasing shift in retention time for concentrated samples and for some of the diluted samples that I previously injected, and the retention time didn't have any shift.
Please consider, that I don't change any parameters of the method such as flow gas or temperature. The shifting is more increasing in early peaks than later.
I did some procedures but the problem still persists:
1- I replaced septum
2- I performed inlet and septum leak tests and passed the tests.
my second question about the ramped flow rate in column I see the operator adjusts the rate 10ml/min per min and value 4ml/min but how the value of the rate is more than the value ?!
ahmed ismail wrote:
Hello,
1- I have GC-FID 8890 and's problem suddenly appeared, the increasing shift in retention time for concentrated samples and for some of the diluted samples that I previously injected, and the retention time didn't have any shift.
Please consider, that I don't change any parameters of the method such as flow gas or temperature. The shifting is more increasing in early peaks than later.
I did some procedures but the problem still persists:
1- I replaced septum
2- I performed inlet and septum leak tests and passed the tests.
my second question about the ramped flow rate in column I see the operator adjusts the rate 10ml/min per min and value 4ml/min but how the value of the rate is more than the value ?!


Do you have the instrument configured to constant flow, constant pressure or a variable ramped pressure?

With constant flow, the instrument will calculate the increase in head pressure to keep the flow rate constant through the column. With constant pressure the head pressure will remain constant but the flow through the column will decrease as temperature increases. Ramped flow or ramped pressure is not commonly used unless necessary for very difficult separations.

One thing to check is if the incoming pressure is high enough for the instrument to maintain it's flow. If the pressure from the tank becomes too low then the flow will lag behind what it should be as it can't increase at the rate necessary. If the difference is too great the instrument should be giving a warning sound or message on screen. Another problem can be a ferrule that doesn't seal at cool oven temperature but seals at higher temperatures which would cause worse problems for early analytes than for later ones, that one can drive you crazy :)
The past is there to guide us into the future, not to dwell in.
There's a lot you are not telling us, like your column details and the types of compounds you're injecting and how you're injecting them.
It is quite possible that all you need is to trim your column to remove active sites that have formed with use and are proving to be "sticky" for your earlier eluters.
Thanks,
DR
Image
DR wrote:
There's a lot you are not telling us, like your column details and the types of compounds you're injecting and how you're injecting them.
It is quite possible that all you need is to trim your column to remove active sites that have formed with use and are proving to be "sticky" for your earlier eluters.

Sorry for the late reply.
My Column is DB 642 and the compounds are organic solvents like IPA, alkyl acetate, and glycols. I inject 1ul by syringe ( manual injection ).
James_Ball wrote:
ahmed ismail wrote:
Hello,
1- I have GC-FID 8890 and's problem suddenly appeared, the increasing shift in retention time for concentrated samples and for some of the diluted samples that I previously injected, and the retention time didn't have any shift.
Please consider, that I don't change any parameters of the method such as flow gas or temperature. The shifting is more increasing in early peaks than later.
I did some procedures but the problem still persists:
1- I replaced septum
2- I performed inlet and septum leak tests and passed the tests.
my second question about the ramped flow rate in column I see the operator adjusts the rate 10ml/min per min and value 4ml/min but how the value of the rate is more than the value ?!


Do you have the instrument configured to constant flow, constant pressure or a variable ramped pressure?

With constant flow, the instrument will calculate the increase in head pressure to keep the flow rate constant through the column. With constant pressure the head pressure will remain constant but the flow through the column will decrease as temperature increases. Ramped flow or ramped pressure is not commonly used unless necessary for very difficult separations.

One thing to check is if the incoming pressure is high enough for the instrument to maintain it's flow. If the pressure from the tank becomes too low then the flow will lag behind what it should be as it can't increase at the rate necessary. If the difference is too great the instrument should be giving a warning sound or message on screen. Another problem can be a ferrule that doesn't seal at cool oven temperature but seals at higher temperatures which would cause worse problems for early analytes than for later ones, that one can drive you crazy :)


yes, Dear. I configured it to constant flow 4 ml/min for DB 624 Column.
do you suggest altering Ferrule with another type?
How different are the concentrations? Some of the compounds or all of the compounds? Earlier or later? Really high concentrations of components can shift retention times because they overwhelm the phase. Especially true for a compound opposite the phase of the column, that is to say a polar compound on a non-polar phase for example.

Also, you provide a flow but no column dimensions....

Best regards,

AICMM
AICMM wrote:
How different are the concentrations? Some of the compounds or all of the compounds? Earlier or later? Really high concentrations of components can shift retention times because they overwhelm the phase. Especially true for a compound opposite the phase of the column, that is to say a polar compound on a non-polar phase for example.

Also, you provide a flow but no column dimensions....

Best regards,

AICMM

Earlier solvents such as IPA, methanol, ethanol, methyl, and ethyl acetate.
My column is DB-624 0.53 mm X 3 µm.
4 ml/min is really low for a megabore. Only 19.5kpa or 2.8 psi according to my math (if a 30 meter column) on the pressure controller. Have you looked at a higher flow rate? Rule of thumb for me is about 8 - 40 for a megabore.

Best regards,

AICMM
AICMM wrote:
4 ml/min is really low for a megabore. Only 19.5kpa or 2.8 psi according to my math (if a 30 meter column) on the pressure controller. Have you looked at a higher flow rate? Rule of thumb for me is about 8 - 40 for a megabore.

Best regards,

AICMM

I have been working with a flow rate of 4 ml/min since I commenced work on the instrument ( from 19/7/2023 to 14/8/2023 ) and the retention time was stable but suddenly the retention time increased and is still fixed at this shift till now.
[quote=" the increasing shift in retention time for concentrated samples and for some of the diluted samples that I previously injected, and the retention time didn't have any shift.[/quote]

How big is the shift - in time units, and what are the usual retention times?
Peter Apps
Peter Apps wrote:
[quote=" the increasing shift in retention time for concentrated samples and for some of the diluted samples that I previously injected, and the retention time didn't have any shift.


How big is the shift - in time units, and what are the usual retention times?[/quote]
the bigger shit for the early compounds from 4.20 min to 16 min is 0.5 min.
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