Tailing solvent peak during split injection? Check your column installation distance!

When performing splitless injections, it is not uncommon to witness a tailing solvent peak, given the large amount of solvent introduced onto the column over a relatively long period of time.  For split injections, however, only a small portion of the solvent loads onto the column and it occurs very quickly. This results in a relatively sharp solvent peak.

Recently, I set up a method with a 20:1 split and upon my first injection I noticed a badly tailing solvent peak, which I wasn’t expecting (Figure 1 red trace).  My gut feeling told me something was occurring within the inlet and the solvent wasn’t split properly.  I removed my column from the inlet to find that the installation distance was too low. The recommended installation distance for an Agilent split/splitless inlet is 4-6mm above the ferrule and my column was only 1-2mm above it.  This would lead to the column being below the inlet seal surface, creating an area of dead volume, where residual solvent would not get swept away by the higher split flow.  After correcting the installation distance, my problem disappeared (Figure 1 blue trace)!

Figure 1: Improper installation distance of column can create excess dead volume, leading to tailing.

Figure 2: Capillary installation gauge

 

Generally, I pre-swage my ferrule to the column at the proper distance using a capillary installation gauge, but I was in a rush and didn’t do that this time. I probably bumped the column while installing, inadvertently pushing it down.  I highly recommend using a capillary installation gauge, which allows you to pre-swage your ferrule to the column at the exact specified distance for installation (Figure 2).  It only takes a few seconds and greatly improves consistency of installation.  The table below shows the capillary installation gauges that Restek carries for various instrument makes.

 

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3 Responses to “Tailing solvent peak during split injection? Check your column installation distance!”

  1. James Ball says:

    I have had this happen before also. Without the tool I found that I can get a good measurement by placing and old gold seal on top of the ferrule that I have tightened in the inlet first, removed and then set the old seal on top and adjust the column end is 1-2mm above the seal. Then just have to reinsert.

    Another way if you don’t have old seals, and especially if you are using the Restek replacement inlet bottom fitting that uses a standard 1/16″ Swagelok nut is to remove the inlet liner, flatten the end of a cotton swab by tapping it on the bench then dropping it tip first into the inlet. Insert the column until the swab stick rises 1-2mm then tighten the nut. It gives the correct height no matter what thickness the seal or the ferrule is.

  2. Wayne Halozan says:

    Another trick/technic is to measure the length of the retainer nut (normally 20mm) then add 5mm to get a total of 25mm of column past the end of the retainer nut, I record the 25mm on the wall inside the GC. Insert the column through the nut and ferrule, trim to length then mark the column with either a black sharpie (light colored columns) or with “Silver Metallic” sharpie for darker columns.
    This also works on the detector side especially Mass Specs, measure once and record on the measurement on the GC wall (Agilent GC/MS 175mm). Works every time.

  3. Thanks for sharing, Wayne. It’s always interesting to hear different techniques and tricks!

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