Chromatography Forum

Hi,

I am using a constant pressure, isothermal method with TCD as a detector, N2 as carrier gas to analyze H2. Whenever i increase the pressure, the area goes down.
Is it due to the characteristic of TCD?

For Packed column, why does the reference flow rate has to be 1.5 times the carrier gas flow?


Thank you

I am using a constant pressure, isothermal method with TCD as a detector, N2 as carrier gas to analyze H2. Whenever i increase the pressure, the area goes down.
Is it due to the characteristic of TCD?

yes

For Packed column, why does the reference flow rate has to be 1.5 times the carrier gas flow?

You must be talking about a specific system, really that is not
a generally.

When you increase the pressure you increase the flow, giving less residence time of the gas impurity in the carrier (your analyte) which cools the TCD filament less than if the flow was slower.

Usually the reference is one-half to one and a half times the flow of the carrier in the column, as a general rule. This keeps the filament temperatures approximately the same for both reference and detector.

best wishes,

Rod

When you increase the pressure you increase the flow, giving less residence time of the gas impurity in the carrier (your analyte) which cools the TCD filament less than if the flow was slower.

The peak height is proportional to the square root of the plate number and the inverse of the retention volume, but don't matter if you have a TCD detector fast enough to follow this variation, the peak area is proportionality to the inverse of the flow rate.

Hi,

Thanks for the input, becoz I am using micropacked column.
So wat happen when then reference flow is more than 1.5 times the carrier flow? will it cause a problem in the analysis?

Currently I am experiencing non-repeatable area accompany with drift in retention time for my hydrogen. The area is higher when retention time shift to left. I am not sure if it is due to the detector or the epc not giving a consistent pressure

Punt,

A change in reference flow will certainly change your zero value. It shouldn't really change your sensitivity. Moreover, a change in retention time is a chromatographic problem, not really a detector problem so that is where I would start looking first. Leaks in upstream plumbing, injectors, etc... Once you get steady retention, then focus on steady detection...

Best regards.

Hi AICMM,

Thanks for the input. If there is a leak upstream, will the retention drift in one direction? be coz currently the drift in retention is moving left and right for different injection. It is not moving in one direction.

Thank you

Punt,

A shift both ways is sort of unusual. Usually it just shifts one way. One thing I am wondering, based on this and your other post, is if you are not returning your valves to the original state within your run. That is, you move your valve one way in the first run and don't return it back to the load state within the same run.

Can you post your method?

Best regards.

Hi AICMM,

The valve return to its original position after hydrogen peak eluted from molsieve column.

Carrier gas : N2
Detector: TCD
Constant Pressure and Isothermal temperature at 70C
Column: HaysepQ follow by Molsieve5A
Origin Position (Valve Off)
Sample injected (Valve On),
After 0.8mins backflush, flow reverse (Valve Off) to detector.
RT of hydrogen peak is around 0.6mins

The problem i encounter is the peak is shifting from left and right within repeated injection accompany with area increase when the peak shift to right, area drop when it shift to left.
The RT shift between 0.58 to 0.61.

We actually check for leak in system (column, fitting, valve) but could not find any.

Thank you

Punt,

Try a much shorter valve time. Something like 0.3 minutes comes to mind. See what that does.

Also, per your other post, with nitrogen carrier you won't see a nitrogen peak upon injecting air.

Best regards.

Hi AICMM,

What do you mean by try a shorter valve time (0.3mins)?

at 0.1 mins sample injected (valve ON)
The RT of Analyte peak (H2) is 0.6mins.
So the backflush time (valve OFF) has to be after 0.6 mins inorder for the H2 peak to elute from the molsieve column.

So how can the valve timing be 0.3mins?

Thank you for your reply
Regards

Punt,

I don't know how your GC is configured. I will make an assumption that it is either backflush or sequence reversal to detector. Either way, the hydrogen comes off the pre-column and onto the analytical column very quickly (and then back through the pre-column..) A very short valve time will still accomplish what you want and may decrease flow vagaries to improve reproducability.

Best regards.

Hi AICMM,

Thanks for your feed back,

the Hydrogen travel from precolumn (Haysep) to analytical column (Molsieve) then to detector in about 0.6 mins time, after that flow reverse and the heavier compounds which are still retained on haysep column are backflush.

You mention a short valve time will improve reproducibility. Waht does it mean?

Thank you

Punt,

Please tell AICMM if you are backflushing to vent or backflushing to detector, it makes a difference in your answer and in his understanding or not understanding of the problem.

Thanks,

Rod

Hi Rod,

It is backflushed to detector. What kind of difference does it impact between backflushing to detector or vent?

Thank you