Chromatography Forum

I was wondering about the school of thought for constant pressure vs constant flow. Is the choice specific to detector, sample prep (headspace, spme, direct inject), type of analysis or just the chromatographers preference. Why the different options and what are the benefits of each applications. I hope someone can shed some light. Thanks in advance!

Jeni

Constant flow is better becuase the linear flow rate stays at its optimum despite changes in gas viscosity caused by temperature programming.

Peter

Thank you Peter. Would constant pressure cause any variability or change in response?

Constant pressure will give different column flows at different oven temperatures. Some detectors are very flow sensitive (TCD) and some will respond via mass per time calculations so a different flow can give a different response for the same amount of a particular analyte or a different LOD (but not always).

Constant flow can give you better (more consistent) separations but not necessarily faster separations.

Selection of constant flow and constant pressure all depend upon the cost and analytical considerdations of your analysis.

With constant pressure you can generally determine readily if a fault occurs but with constant flow it may not always be readily noticable.

best wishes,

Rod

When column temperature increases, the optimal flow rate of a carrier gas declines. In other words, in order to maintain the best efficiency at rising temperature, the flow should gradually decline.

If GC analysis starts with optimal flow and the flow is kept constant then the column gradually becomes over-optimized (actual flow becomes higher than the optimal one). The column efficiency gradually declines compared to the highest possible one. On the other hand, if pressure rather than flow is kept constant, the temperature increase causes the flow rate to decline. This decline is more rapid than the decline in the optimal flow. As a result, the actual flow gets lower than the optimal one, and the under-optimized column performs below its highest potential again. All in all, both operational modes – constant flow and constant pressure – are not optimal for the best efficiency. A detailed analysis shows that the deviation from the optimal efficiency at constant flow is slightly smaller than at constant pressure.

While all that might be important for understanding what’s going on, it is even more important to recognize that the loss in the column efficiency in either mode is relatively small. It is not why one should choose between the two modes.

The main reason for using constant flow is that detectors like it. Some (MS, TCD) like it more, others like it less, but all behave more consistently when the column flow is constant. And, if the constant flow mode is used with some detectors, why not to use it with all detectors? Just to have fewer options to worry about.

Constant flow mode has some disadvantages. One of them is that a constant flow mode with the same column and the same carrier gas requires higher pressure at higher temperatures. The required pressure might exceed the one that a GC instrument can provide.

More info can be found in the literature listed below. The first two items are a sort of a debate of the issue.

Literature:
1. K. Grob, Increasing GC Separation Efficiency by Electronic Pressure Control?, J. High Resolut. Chromatogr., 17 (1994), p. 556.
2. L. M. Blumberg, T. A. Berger, M.S. Klee, Constant Flow versus Constant Pressure in a Temperature-Programmed Gas Chromatography, J. High Resolut. Chromatogr. 18 (1995), p. 378-80.
3. L. M. Blumberg, W. H. Wilson and M. S. Klee, Evaluation of Column Performance in Constant Pressure and Constant Flow Capillary Gas Chromatography, J. Chromatogr. A, 842/1-2 (1999), p. 15-28.

Hi lmb

Before I go and get the references ..... are you referring to volume flow rate, or linear flow rate ?

Peter

Hi Peter,

Speaking of the flow rate, I was referring to volume of gas leaving a column every minute. The volume of the gas was supposed to be measured at atmospheric pressure and at 25 ºC no matter what the actual column temperature was.

Best regards,
lmb

Minor point:

Apparently, for gas phase analysis, pressure control is better, since it leads to less baseline upset at injection.

See: Basic Gas Chromatography by Harold M. McNair and James M. Miller. (AFAICT, this is the Varian "Green Book" in new clothes.)

pi3832,

You have a point for gas analysis when injecting with valves. The larger the loop size, the more important it is to use constant pressure. You also mention the 'Varian' green book. Have you ever looked closely at a flow diagram for a Varian GC using a gas sampling valve. They use a flow controller and a pressure controller in parallel, so obviously they see that there are advantages of both flow control and pressure control..........

Gasman

Apparently, for gas phase analysis, pressure control is better, since it leads to less baseline upset at injection.

Old packed column systems could have been flow controlled but most GCs were always controlled by pressure. Some manufacturers controlled via back pressure and some (Carlo Erba/Fisons) by head pressure. My understanding is that Thermo still control flow by measuring the back pressure and use different set flows and temperatures to calculate a K Factor for the column. This was then used to give constant flow and flow programming using just headpressure control. When the 6890 was launched it too abandoned back pressure regulation and changed to head pressure control with the operator entering the column dimentions to calculate the K factor. This has been later enhanced via retention time locking to adjust for viariation in the back pressure of the column.

CE Instruments

The comment that you make about Agilent abandoning backpressure control is not correct. The 6890 uses a backpressure regulator to control the head pressure when the capillary inlet is run in split mode. When the capillary inlet is run in splitless mode, the head pressure of the column is controlled by a forward pressure regulator.

The flow in a capillary column is always controlled by the head pressure, but you can have several ways to control the headpressure.

Gasman

I stand corrected re split injection