High Baseline
GC Troubleshooting Course
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
High Baseline (Column Bleed)
A rise in the baseline at the end of a GC run is common when an oven temperature ramp is used. Typically, it is a result of column stationary phase bleed. This is when the stationary phase on the column thermally degrades and releases components which are seen as noise in the chromatogram. Often the final oven temperature is held to flush out less volatile (high boiling) components and prevent carryover into the next run and it here the column bleed will be apparent. Adjusting the ramp timing can help ensure that this baseline increase does not interfere with peaks of interest.
But if the baseline is rising while your compounds of interest are still eluting, then accurate peak integration could be difficult. The rise in baseline can be compensated for by subtracting the results of a blank sample (although this doesn’t work if you’re using MS detection), and there are also integration parameters that can be used to account for a high baseline. But if you’re looking to resolve a high baseline issue at source, then the obvious thing to consider is improper column conditioning. This leads to components not being completely flushed through the column, causing a gradual build-up over time.
There are a few ways of tackling this:
- For new columns increase the conditioning time and/or temperature.
- Remove contamination by trimming the column by a few centimetres at both ends and recondition the column – typically overnight at a temperature close to the upper recommended limit of the column.
- You might also want to replace carrier gas and/or detector gas filters.
- And, for MS (mass spectrometry) detectors, you should clean the injector and ion source.
Trimming a column is a logical step to follow if you have a persistently high baseline,
because the high temperatures needed at the ends of the column
can cause gradual degradation and build-up of contaminants.
A less common cause of high baseline is oxidation of the stationary phase. This usually results from a leak in the system (column or septum) and might be indicated by a sudden change in baseline in a system that had currently been behaving well.
So, if your column is an older one, trimming or replacing it is an obvious course of action, along with a check of the column connections and septum. If you’re using MS detection, then you can perform a manual tune and look out for the presence of suspiciously large signals at m/z 18 (H2O), 28 (N2) and 32 (O2).
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