Technical Tips

A Technical Tip by Kory Kelly
GC Product Manager

When scaling down column dimensions for Fast GC, there are a few parameters that can be optimized. The results that you were achieving on larger ID columns can be transferred to a smaller ID column and also give faster run times, sharper peaks, possibly better resolution, and improved sensitivity. Though not all of the below are necessary, they are recommended to get the best results:

Smaller ID columns have a smaller internal volume and will require a lower flow rate (mL/min) to achieve the same linear velocities. Most new instruments use Electronic Pressure Control (EPC) and can account for this – as long as the correct column dimensions have been entered into the instrument software and the correct linear velocity of carrier gas (in cm/sec) is used!

The lower flow rate through the column will have an effect on the total flow through the inlet. In some cases, this can require correction of distorted peaks. There are a couple of ways to do this:

  1. 1. Use an inlet liner with glass wool in the middle of the liner. This results in one site of vaporization and helps restrict the sample to one area in the liner for sharper peaks in the detector. Alternatively, you can use a smaller ID liner (with or without wool) which will compensate for the lower column flow. Phenomenex offers several options, below are just a few examples:
    Liner Benefits Dimensions
    ID x L x OD (mm)
    Part No.
    Split Liner with Wool
    Good for general use or
    dirty samples
    4 x 78.5 x 6.3 AG0-8172
    Split/Splitless Liner with Wool
    Good for large injections and trace level analysis 4 x 78.5 x 6.3 AG0-4655
    Split/Splitless FocusLiner™ with Wool
    Good for trace analysis of dirty samples 4 x 78.5 x 6.3 AG0-7515
    Split/Splitless Liner with Wool Good for small injection and trace analysis of dirty samples 2.3 x 78.5 x 6.3 AG0-8379
  2. Temporarily increase the flow through the inlet only during the injection. This can be done using the pressure pulse option available in EPC based instruments. This option increases the inlet pressure and therefore the flow through the inlet, keeping similar transfer rates onto the column. To avoid affecting the separation, only keep the pressure pulse operational for the duration of the injection (Example: 0.5 minutes for a splitless injection with a 0.5 minute splitless hold time).

There are two things to account for in the oven. The first is related to the slower flow rate through the inlet and is used to help focus analytes on the column that may not be transferred onto the column fast enough. The second is related to the shorter column length.

  1. If analytes are still being smeared onto the column because of slower overall flows in the inlet, it may be necessary to focus them onto the column thermally by lowering the initial oven temperature. If possible, use an initial temperature at least 40 °C below the boiling point of the earliest eluting compound. Lowering the initial oven temperature further sometimes even improves the peak shapes of early eluting compounds.
  2. Smaller ID columns are usually shorter and require less time to elute an analyte from the column. The temperature ramps can usually be increased to keep analytes coming off the column at the same elution temperatures.

Though there isn’t much to optimize in the detector, keep in mind that the column flow should have decreased, so the detector make-up flow may need to increase to account for the overall lower flow. Also, pay attention to the signal-to-noise more than the overall intensity of the peaks when looking at peak responses. One way to increase inlet flow is to increase split ratios. This can lower peak heights, but may still result in higher signal-to-noise, because shorter and smaller ID columns offer less noise. Therefore, the overall sensitivity of the method may improve even though the absolute intensity may decrease.

Scaling to Fast GC can deliver many chromatographic benefits if parameters are optimized properly!
If you’re considering moving to fast GC, there are many column options available. If you want more information on Fast GC, below are some additional resources and as always, feel free to contact your GC Specialist with questions!