GC Technical Tip
Level: Basic
Retention Times Prolonged or Shortened
In this technical tip we will examine the possible causes of prolonged or shortened retention times and suggest some remedies. We will consider first retention time shifts on the same column. After that we will examine cases where retention times shifted after replacing the column.
The current column in use, which previously provided good results:
| Possible Cause | Suggested Remedy |
|---|---|
| Column temperatures higher (lower) than previous analyses. | Adjust temperature. |
| Carrier gas flow rate lower (higher) than previous analyses. | Adjust flow rate. |
| Carrier gas leak at septum or column connection. | Find and fix leaks. Replace septum or tighten connection if necessary. |
| Column contaminated or deteriorated. | Rinse column (bonded phase only) or remove 1-2 coils at inlet. If column performance is not restored, replace column. |
| Some nonbonded phases: rapid temperature changes cause phase to puddle (form droplets within column), causing drastic loss of efficiency. | Replace column. Important: Always heat and cool column gradually to avoid thermal rearrangement of phase. Use bonded phase columns whenever possible. |
| Incorrect recorder chart speed or recorder defective. | Adjust chart speed. |
| Column overloaded. | Reduce sample size. |
| Column flow varies from run to run. | Ensure pressure at gas source exceeds pressure needed at head of column by at least 15psig at maximum temperature. Replace cylinder before pressure falls below 300psig. Replace regulator or flow controller if defective. |
New column (previous column of same composition provided good results):
| Possible Cause | Suggested Remedy |
|---|---|
| Incorrect column ordered (phase or dimension). | Compare column tags or other manufacturer’s information. Column lengths, IDs, film thicknesses, etc. must be identical. Contact manufacturer if you find differences. |
| New column with same description not identical to previous column (phase or dimension). | Use a certified column from a reliable manufacturer. |
| Incorrect linear velocity. | Adjust linear velocity. |
| Carrier gas different from that used with previous column. | Change carrier gas. |
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