Constant flow mode versus pressure ramp

[kaka_munda]

Hi guys,

I am using a GCxGC-ToFMS system. Unfortunately, the operating software does not allow to have a constant flow mode. So, I can only use pressure ramps. I use HP flow calculator to estimate pressures at min and max temp for a given flow rate (1.5 ml/min) for each segment of columns (transfer line, 2nd column, modulating deactivated column, 1st column). Later, I simply divide the difference of press at min and max temp by total run time to figure out the pressure ramp rate.

I am just wandering if I am doing this properly or there may be other approach?


Best,
kaka

[larkl]

I can't comment on GCxGC, but for traditional GC, most would just set a constant pressure for the entire run. The flow is going to slow as the temperature increases. All the old pneumatic instruments ran this way, so there's a long history of using instruments in this manner.

[Don_Hilton]

This may depend on the instrument you are using. The LECO systems that I am familiar with compute pressure ramps appropriate for the column and thermal zone combinations -- as long as it is not too complex for the GC. Otherwise, it will fall back to allowing the GC to attempt to keep the flow as constant as it can. Given that you typicall do not need to do a complex temperature gradiend, the pressure ramps computed by the ChromaTOF software do just fine.

[emi_r]

Hallo Kaka!

Could you solve your problem?
I have recently been working with a GCxGC system (also under constant pressure) and we are struggling with shifting RT (on the first column) ...

I have also observed that the flow that the device sets differs slightly from measurement to measurement - what can be the cause of the fluctuations. But why is the flow different? Maybe you have a hint?

Thank you in advance!

[lmb]

Hello Kaka. Why do you need constant flow?