Has anyone used Constant Flow for GC carrier gas instead of Constant Pressure? I've always used Constant Pressure but would like to hear what others think about the Constant Flow mode. Specifically for Agilent 6890's. Thanks
By Anonymous on Monday, December 1, 2003 - 07:30 am:
I've used constant flow to compensate for flow drop off at higher temperatures on 6890s, with fine results.
By Anonymous on Monday, December 1, 2003 - 04:29 pm:
I remember constant pressure mode is recommended
when capillary column is used.
By Anonymous on Monday, December 1, 2003 - 08:41 pm:
I have always used constant flow on the 6890. At 1.5 ml/min I can't see allowing the flow to become too reduced as the temperature increases, as it would have to under the constant pressure mode.
By Anonymous on Wednesday, December 3, 2003 - 03:17 am:
Anonymous (Dec 1): Please elaborate on your statement that constant pressure is recommended for capillary columns.
By Anonymous on Wednesday, December 3, 2003 - 01:40 pm:
You can check out this article by Blumberg, Wilson and Klee: J. Chromatog A. 842:15-28 (1999). The authors use an Agilent 6890 to demonstrate no appreciable difference between constant flow and constant pressure mode with the analysis of normal hydrocarbons.
The correct protocol is what gives you good results, not what somebody remembers reading somewhere.
By Jason Ellis on Wednesday, December 3, 2003 - 02:29 pm:
Generally speaking, I lean toward the use of constant flow mode. If you're doing a relatively minor temperature program (say from 50C to 150C), then you won't notice much difference between constant flow and constant pressure modes. If you're doing a more "dramatic" temperature program (say from 35C to 350C), then the differences will be significant due to the relatively large reduction in column flow as you reach higher oven temperatures. Personally, I've just become used to using constant flow mode and that's what I normally default to when doing method development now.
With a more "dramatic" temperature program (larger temperature range), then you will see a reduction in retention time for late eluting compounds in constant flow mode. Additionally, you may see an improvement in resolution for late eluters in const flow mode as well, depending upon initial conditions. This is because there can be very significant changes in carrier gas linear velocity in constant pressure mode for temp programs with a large temperature range.
As the previous poster suggested, ultimately whatever protocol works for you and gives good results is what you should use. Plus, if you're developing a method for distribution to other instruments, it is important that you consider the capabilities of the other instruments the method will be run on (whether or not they have electronic pressure control).
GC Application Engineer
By Anonymous on Thursday, December 4, 2003 - 06:19 am:
What is wrong with the following reasoning? If the temperature range is large, then maximum resolution is required when the high boilers elute. This is because the number of isomers increase exponentailly with the carbon number (boiling point). For constant pressure this means then that optimal flow (Van Deemter curve) is required towards the end of the chromatogram and hence the operating pressure is set for this. This further implies that such a set constant pressure will provide a too high linear flow rate for the early eluters - thus a relatively poor separation for the low boilers. This cenario can be even worsend of a mass spectrometer (EI-mode) is used. I can imagine that a high flow early in the chromatogram may lead to not only to EI-spectra, but also to CID (collision induced spectra) which can be problematic when mass spectra interpretations needs to be done.
All in all, it seems to me constant flow is the better choice. Or am I wrong?
By GCguy on Thursday, December 4, 2003 - 11:57 pm:
Wow ! Thanks. I enjoyed all the comments.