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.