Setting the splitless injection purge valve time in gas chromatography

In splitless injection GC we set the purge valve time to sweep the inlet liner of excess solvent so that it doesn’t “tail out” through the whole chromatogram.  But what time do we set?  A good “rule of thumb” is to sweep the inlet liner 1.5 to 2 times with carrier gas prior to turning on the purge valve so that all sample analytes are completely (as possible) transferred from inlet liner to GC column.  GC inlet flow can be estimated using a flow calculator and liner volume can be calculated using the inside diameter, pi, and liner length.  After that, it’s a simple matter to calculate the 1.5 to 2x sweep time.

FC 01

You can see the effect of splitless purge valve time on  peak response for a volatility range of PAHs (naphthalene being relatively volatile, benzo[ghi]perylene being relatively involatile), including the EFSA PAH4, in the chart below.  A properly set purge valve time becomes even more important when analyzing dirty “real world” samples.  More on that later…

PAHs Purge Valve Time 03

4 Responses to “Setting the splitless injection purge valve time in gas chromatography”

  1. jaap de zeeuw says:

    Jack
    You may add to the discussion that we deal with a gas under pressure.
    That means that at a 100kPa pressure, and a 1 mL/min flow, the flow in the liner is only 0.5 mL/min.

    For estimating time to “empty” the liner we have to consider INLET flow, not OUTLET flow. Problem is, that we only see “outlet flow ” values.

    You know I am not a fan of “flows” and this is another reason (:))..

  2. Jack Cochran says:

    Jaap, you need to check your inlet flow value. It depends on several things as regards inlet flow. But the Restek beta flow calculator I use indicates, for example, that a 30m x 0.25mm column with 1 mL/min outlet flow at 40°C and vaccum and a 1 mL volume 250°C inlet liner has a flow of approx. 1.2 mL/min, NOT 0.5 mL/min. At atmospheric outlet pressure, it’s about 1 mL/min through the liner.

    The only way I can get 0.5 mL/min in this example at atm press outlet for the inlet flow is to set the starting oven temp at 580°C! That’s a bit hot for even our best metal 100% dimethylpolysiloxane column!

  3. Yuki says:

    Hi Jack,

    >a 30m x 0.25mm column with 1 mL/min outlet flow at 40°C and vaccum and a 1 mL volume 250°C inlet liner has a flow of approx. 1.2 mL/min

    How can the liner flow be higher(1.2mL/min) than the outlet(1mL/min)?? I know the initial oven temp is only 40C but the gas should still be compressible?

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