Why do only some of my peaks tail (GC)?

In this post, I would like to provide some troubleshooting tips for resolving tailing peaks issues. However, unlike most published troubleshooting information which discusses when all GC peaks tail, the focus of this post will be when only some of the peaks tail, and other peak shapes are fine. Please note that the information provided below does not include particle-based columns, such as packed/micropacked or PLOT, which, if overloaded, will produce a tailing peak.

 

Case 1: Only early eluting peaks tail.

When this happens while performing volatile analysis (using purge & trap, headspace, or sample loop for gas sample introduction), carrier gas flow disruption is usually to blame. What can cause this flow disruption? Leaks and/or dead volume, which are upstream of the analytical column, are two common causes. A poorly cut column and/or improper column installation distance (into the injection port/liner) are two more. Having something stuck inside the analytical column, like a piece of a ferrule or septum, is another. So why aren’t later eluting peaks affected? It is because these higher boiling point compounds are able to condense and refocus within the analytical column.

When this happens while performing semi-volatile analysis (using syringe injection for liquid sample introduction), the sample/standard solvent may also play an important role. There are two common injection techniques when using a split/splitless injection port; one is solvent focusing and the other is analyte focusing. Both are discussed below.

If your injection technique incorporates solvent focusing, make sure that the GC oven temperature is set low enough so that the solvent and all compounds condense inside the analytical column. If not, then a scenario called the “Solvent Effect Violation” may occur, which can cause early eluting compound peaks to be broad, rounded, or even tail. If you suspect that this may be happening, try lowering the GC oven temperature 20°C to 40°C below the solvent boiling point to make sure all the solvent condenses inside the analytical column. This should help refocus the compound peaks. If you continue to notice any chromatography issues using this technique, there may be a polarity mismatch between the capillary column and solvent. If this happens, you may need to use a different polarity analytical column, or attach a retention gap (guard column) of the correct polarity. 

Alternatively, you may be able to try analyte focusing (sometimes referred to as “cold-trapping”), where the solvent stays in vapor form while all of the compounds condense at the head of the analytical column. However, you will need to use a solvent with a much lower boiling point than your compounds of interest. Generally speaking, analyte focusing may require as much as 150°C difference between the initial GC oven temperature and the boiling point of the first eluting compound.

Finally, do not overlook the autosampler as the potential cause.   Most autosamplers inject quickly to help prevent chromatographic issues such as tailing peaks, but sometimes just the opposite occurs and a quick injection can cause peak shape issues.  Or, for a previous analysis, autosampler injection parameters were changed to improve peak shape (or reproducibility or sensitivity) and not reset after the analysis was finished.  For example, if an autosampler was programmed for hot-needle injections* for one analysis and not reset to current analysis parameters (which did not incorporate hot needle injection parameters), peak shapes of the current analysis may be different than previously obtained.

To fix: a) Confirm that the correct liner is being used. If you suspect dead volume, try using a smaller internal diameter liner, or incorporate an injection pressure-pulse into your instrument program. b) Use an electronic leak detector to locate leaks. c) Remove the column from the injection port, trim, and reinstall. d) Lower the injection port temperature to help focus your sample/standard (solvent and/or compounds) onto the head of the analytical column. e) Try using a guard column which matches the polarity of the solvent. f) Verify autosampler injection parameters are properly set. 

* Hot needle injections usually involve a two or three second injection delay after the syringe needle enters the injection port.  This delay allows the needle to become very hot which helps to improve vaporization of high-boiling point and/or viscous compounds/matrix. 

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Case 2: Only later eluting peaks tail.

When this happens, usually a cold spot, column contamination, and/or low carrier gas flow rates are to blame. If using a mass spec, a low ion source temperature and/or low transfer line temperature may be the cause. In addition, the transfer line may have become oxidized or contaminated, which may lead to tailing peaks.

To fix: a) Verify all zones are properly heated. b) Bake-out the column/instrument to remove contamination. c) Increase the carrier gas flow rate. d) Increase the mass spec source and transfer line* temperature. e) Remove the transfer line section of the analytical column.

* Caution - Increasing the temperature of the mass spec transfer line above 300°C for an extended period of time may cause the polyimide coating to flake off and the column to become brittle.

 

Case 3: Only certain peaks throughout the chromatogram tail.

This is usually caused by activity issues in the system, or inadequate instrument sensitivity for particular compounds.

To fix: a) Increase the on-column concentration of the problem compounds. b) If the peaks still tail, trim the column, replace the injection port liner, bake-out the system (to remove any contamination) and/or “prime” the system by injecting a high concentration standard of the problem compounds to neutralize any remaining activity (don’t forget to run a solvent blank afterwards to confirm there is no carryover). c) If tailing remains, replace the analytical column with a new one, or one that you have confirmed works well for your particular analysis.

If none of the fixes listed above work, you will need to start ruling-out what is not causing the peak(s) to tail. You may want to start by injecting the standard on another instrument to confirm nothing is wrong with the analytical standard, and that compound interaction with the column/liner is not the issue. If no tailing peaks are observed on the second instrument, you will need to begin troubleshooting the current instrument. Try not to get overwhelmed – begin by focusing on one specific area at a time. It may take a while, but eventually you will find and resolve the issue. Like an old boss of mine once said, “You don’t learn anything when things go smoothly, but you learn a lot when they don’t”.