Packed/Micropacked Column Troubleshooting – Part 2 – Poor Peak Shapes

This second installment of my packed/micropacked column troubleshooting series will focus on poor peak shapes; common causes and how to improve their shape.

A variety of poor peak shapes you may observe are described below.

Tailing peaks.

Rounded or flat top peaks.

Fronting peaks.

Split peaks.

Unresolved peaks.

Tailing peaks

Common causes of tailing peaks when using packed columns are:

Overloaded column.

Decrease the on-column concentration and/or injection volume.

Dilute the sample/standard.

 

Column/instrument surface activity.

Install a new or different column that you know works well and is very inert.

If a new/inert column does not improve peak shapes, you may need to rinse/clean the injection port, replace the injection port liner, and clean/replace any transfer line(s).

If using a syringe to inject a sample/standard, make certain the needle is not touching the packing (and is far enough away from the packing for there to be enough expansion volume).

 

Carrier gas flow disruption or improper make-up gas flow rate.

Instrument leak. Check for leaks using an electronic leak detector.

Dead volume. Replace sample transfer line(s) with smaller internal diameter tubing.  If using a gas valve/sample loop to inject a gaseous sample/standard into an injection port liner, confirm the proper injection port liner is installed.  Which GC injection port liner to use for gas samples (restek.com)

Debris (like a piece of septa in the injection port or column). Inspect the injection port liner and column inlet for debris.

Detector make-up gas flow rate is not optimized. Using a flow meter, set and/or confirm the make-up gas flow rate.

 

Rounded or flat top peaks

Often associated with overloading the detector. Decrease the on-column concentration and/or injection volume.

 

Fronting peaks

Common causes of fronting peaks

Overloaded column and/or detector. Decrease the on-column concentration and/or injection volume.

More than one peak is eluting closely together. Inject a small volume (1µL) of non-target (non-contaminating) solvent headspace vapor or gas (like butane) instead of the sample/standard and observe peak shape.

Temperatures set too low. Increase temperatures, but do not exceed the column’s maximum isothermal temperature limit.

Compound degradation/breakdown. Inject a different compound which is known to be stable, like a straight-chain hydrocarbon.

 

Split peaks

Split peaks may be caused by the following:

Polarity mismatch between packing and solvent. Try using a different solvent, one which better matches the polarity of the column’s packing.

Injection port temperature not optimized. Lower the injection port temperature to determine if needle flashing (results resemble a double-injection) is an issue.

Column/detector overloaded. Decrease the on-column concentration and/or injection volume.

 

Unresolved peaks

Peaks which no longer separate may indicate the column needs replacing, compound concentration has changed, or the instrument is no longer optimized.

GC columns are consumable products and will eventually need replaced.

Columns which contain a liquid phase will eventually show signs of degradation, whether in the form of loss of theoretical plates (loss of separation power) or high bleed (phase damage from oxygen, moisture, or high heat).

Uncoated packings (no liquid phase) may lose their ability to separate compounds because of contamination or damage to the particles.

Increased on-column compound concentration may create broader peaks leading to reduced compound separation.

If retention times have shifted, you should verify the carrier gas flow rate is correct by measuring the gas exiting the column. You should also confirm heated zones are at the correct temperatures.

 

Thank you for reading this second installment.