Poor Peak Resolution

GC Troubleshooting Course

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1 - Basic Steps

2 - Tailing Peaks

3 - Broad Peaks

4 - No Peaks

5 - High Baseline

6 - Unstable Baseline

7 - Carryover/Ghost Peaks

8 - Fronting Peaks

9 - Poor Peak Resolution

10 - Split Peaks

11 - Response Variability

12 - Retention Time Variability

13 - Course Summary & Test

Poor Peak Resolution

Resolution

Peak resolution is critical to accurate quantitation and compound identification. Poorly resolved peaks can result in overlapping signals, misidentification, and incorrect integration — particularly when using detectors like FID or TCD that lack mass-selective capabilities.

What is Poor Resolution? Two peaks are considered poorly resolved when:

- They are not separated to baseline.

- The valley between them is shallow or non-existent.

- Integration becomes ambiguous, especially for minor components adjacent to major peaks.

 

Common Causes & Fixes

1. Non-selective or Suboptimal Column

Cause: The stationary phase lacks selectivity for your target compounds, or the column dimensions are not optimal for your separation.

Fixes:

  • Choose a column with a stationary phase tailored for your analytes. This is particularly important for: Isomers (e.g., xylenes, PAHs) and sulphur compounds, which often require special deactivation or selectivity.
  • Adjust column dimensions: Longer columns improve resolution (at the cost of runtime); narrower internal diameters provide sharper peaks; use application-specific phases when available.
 

Application notes and chromatographic literature can be invaluable for selecting appropriate column chemistries. 

 

2. Non-Optimised GC Method Parameters

Cause: The carrier gas velocity or oven temperature ramp is not well matched to the analyte volatility and separation needs.

Fixes:

  • Reduce carrier gas linear velocity to increase interaction time with the stationary phase.
  • Modify oven program: Use a shallower ramp or isothermal hold in the region where compounds are co-eluting; lower the initial temperature for better focusing.

Slowe ramps and lower flows increase resolution but can also increase runtime.

 

3. Column Overload

Cause: Injecting too much sample causes peak broadening and overlap, particularly when small peaks elute near large ones.

Fixes:

  • Dilute the sample or increase the split ratio.
  • Ensure injection volumes are appropriate for the column dimensions and film thickness.

Be cautious when small peaks are masked by overloaded larger peaks — reducing the injection volume may improve both visibility and resolution but will reduce sensitivity.

 

4. Challenging Analyte Chemistry

Some compounds are notoriously difficult to separate due to:

  • Structural similarity (e.g., isomers like ethylbenzene and xylenes).
  • Adsorption issues (e.g., sulphur compounds).
  • Close boiling points or high polarity.

 

Fixes:

  • Use specialist columns (e.g., PAH-selective).
  • Use deactivated components in the sample flow path (e.g. Sulfinert fittings).
  • Consider derivatisation during sample prep to improve chromatographic behaviour.

 

Case Example - Chromatogram Comparison:

A common column phase fails to separate Benzo[b]fluoranthene, Benzo[k]fluoranthene, and Benzo[j]fluoranthene, which are successfully resolved using a PAH-specific column.

This highlights the impact of selecting the right stationary phase — a small change in chemistry can dramatically improve separation.

chromatogram 1

 

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