The answer seems fairly straightforward: Use a narrow internal diameter (ID) liner to keep peak shapes sharp by preventing band broadening; but this isn’t always the correct answer. There has to be enough internal volume in the liner to contain all of the sample. Let me explain.
Let’s first look at liners when using purge & trap (P&T). All instrumentation that I am familiar with uses a transfer line from the P&T unit to the GC injection port. In this case, you do want to use a narrow ID liner to minimize dead volume and prevent band broadening, especially for the compounds which will not condense and refocus within the capillary column.
Now let’s look at liners when using headspace (HS). Headspace units can transfer the sample to the injection port two ways:
(1) Using a transfer line (like those used on a P&T unit).
(2) Using a Gas-Tight syringe.
If the headspace unit uses a transfer line, then a small ID liner should be used. However, if a Gas-Tight syringe is used to inject the gas sample directly into an injection port liner, then a larger ID liner is commonly used. I briefly discussed liner selection for gas samples in the post below.
Liners Every Lab Should Own (in my opinion)
In this post, I stated:
The liner below is Restek 23302.1 Sky® 4.0mm ID Single Taper Inlet Liner , which is typically the best choice for 1-2µL injections of non-polar solvents (like hexane), 0.5-1µL injections of polar solvents (like methanol), and gas injections approximately > 250µL to 1mL.
The liner below is Restek 23315.1 Sky® 2.0mm ID Single Taper Inlet Liner , which is typically the best choice for 1µL or less injections of non-polar solvents (with low expansion volumes), or gas injections less than approximately 250µL.
Note: The liners above are typically used for splitless (or split/splitless) injections. If you are splitting your sample, choose an open-bottom liner (with no gooseneck or other restriction at the bottom), like 23301.1 or 23333.1 (shown below).
.Sky® 4.0 mm ID Straight Inlet Liner (23303.1)
If you are not using a syringe for sample introduction, but instead the sample is from a purge & trap unit or gas sampling valve, then the liner I usually recommend is 23333.1 (photo below). Sky® 1.0mm ID Straight Inlet Liner
Please note that these are representative liners for an Agilent GC with a split/splitless injection port; the catalog number of the liner you would select depends upon your specific instrument and injection port.
In summary, liner selection for gas samples will depend on your particular instrumentation and how the sample is introduced into the injection port liner. If your instrument uses a transfer line, then a small ID liner will likely provide you the best chromatography. If a Gas-Tight syringe is used to inject the gas sample, a larger internal diameter liner will usually provide you the best results.