GC Inlet Liner Selection, Part IV: Liner Volume and Diameter

In parts I and II of this blog series I discussed the various liner configurations available for both split and splitless analyses.  One thing I did not mention was liner volume and inner diameter.  Most of the liners I compared are available in multiple inner diameters.  For the comparisons I did, I used a 4 mm inner diameter, which is relatively common for several instrument manufacturers, including Agilent.

Since many of Restek's liner configurations are available in multiple inner diameters, what would make you choose one over the other?  One of the primary advantages of smaller inner diameters (which translates to smaller liner volumes), is the linear velocity of the carrier gas will be higher on the smaller I.D. liner, given the same flow setpoints.  This translates to faster column loading and less time for the analytes to spend in the inlet.  For volatile compounds this produces a narrower analyte band, which improves both resolution and sensitivity.  Figure 1 illustrates an example of peak shape differences for volatile compounds on a 2 mm liner vs a 4 mm liner.  Note that on the 4 mm liner the poorer focusing/peak shape significantly reduces resolution between these two peaks.  Another potential advantage of a smaller ID liner is that the active compounds will spend less time in the inlet, reducing the chances of adverse interactions, such as adsorption and reactivity.

Figure 1: Splitless injection of volatile compounds on a 2 mm ID single taper liner with wool vs. a 4 mm ID single taper liner with wool.

Ok great, so smaller ID’s lead to sharper peaks and less time for adverse interactions to occur in the inlet, so it’s better to always go smaller?  Well…not exactly.  When you inject a solvent into a hot inlet it is going to rapidly expand into a much larger vapor cloud.  The actual size of this vapor cloud will depend upon the solvent itself, as well as the temperature and pressure conditions found in the inlet.  Some solvents, such as water and methanol, will expand more than others (i.e. hexane, dichloromethane).  In fact, water can expand more than 1000 times in volume as it's transferred to a vapor. Figure 2 gives an example of the difference in expansion volume when injecting 1 µL of hexane vs 1 µL of water under the same inlet conditions.

Figure 2: Comparison of solvent vapor expansion volumes for hexane and water, injected at 1 µL under the same inlet conditions.

If you do not have enough liner volume to accept the total size of the vapor cloud as it expands, the vapor can expand beyond the confines of the liner into carrier gas lines, purge vent lines, etc.  This phenomenon is known as backflash and can potentially cause issues.  For one, backflashing can lead to contamination of the system, as your solvent will possibly carry sample material into carrier gas lines and purge lines, which can later off-gas, leading to contaminant peaks.  In addition, if you are vaporizing outside of the confines of your liner, you are adding variability to your injections, making them less consistent and potentially leading to higher injection to injection %RSD’s.  Backflash can also produce poor peak shapes, including fronting, tailing, or split peaks.

There are some ways to mitigate backflash, even when injecting large volumes.  The use of wool can help to prevent backflash, since the wool provides a large surface area for solvent evaporation. This creates an evaporative cooling effect, trapping the analytes on the wool, keeping them within the liner even if some solvent may ultimately backflash.  Using an injection technique such as pulsed splitless can also help, as it uses a temporary increase in inlet pressure to load the analytes onto the column faster.  This pressure spike will reduce solvent expansion volume.  Care must be taken when using a pulsed injection mode, though, as sometimes poor chromatography of volatiles may result from the sudden pressure changes.

Beyond the issue of backflash, a smaller ID liner will also have less surface area and can become contaminated faster if injecting dirty samples.  This could result in more frequent liner changes compared to a larger volume liner.

So ultimately, liner volume can be a balancing act when injecting liquid samples.  Smaller volumes can help with peak shapes of volatiles, as well as reduce inlet residence time, but can also lead to backflash or quicker fouling of the liner from sample matrix.  Restek has a solvent expansion calculator, which can help you determine whether your solvent injection will stay within the confines of the liner.  Figure 3 gives some common liner configurations with the physical and effective volumes listed.  Effective volume takes the volume of carrier gas into consideration and is half of the physical volume of the liner.

Figure 3: Common liner physical and effective volumes.

A Word on Gas Analyses

The above discussion focused on liquid injections in split and splitless mode.  If you are introducing your sample as a gas, with techniques such as headspace, purge and trap, and SPME, you no longer have to worry about solvent and you are also working with more volatile analytes.  In this case, a small ID liner such as a 1 mm ID (or smaller) will lead to the quickest column transfer and therefore sharp, narrow peaks.  Also, since there is no need to evaporate the sample or protect the column from matrix material, the use of wool or another obstruction is not necessary when introducing gaseous samples.