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Study Notes: GC Retention Time and Peaks

Separation of sample components on a GC column depends on the time it takes for each individual component to move along and elute from the column. Faster moving (migrating) components will be eluted first and slower moving components will be eluted later.

The time it takes for an individual component to elute from the column is called the retention time. It is defined as ‘the time from the point of sample injection to the peak maxima’. It is also sometimes called migration time.

A chromatogram showing retention time and area under a peak.

Notice that retention time is the period from injection (start) to the top of the eluting peak. Also note the area under the third peak.

Retention time is affected by all of those factors that have an impact on resolution of peaks as discussed previously. Two factors are the rate of flow of carrier gas through the column and temperature.

Carrier gas rate
Increasing carrier gas rate decreases the retention time – this can be thought of ‘sweeping’ analytes out of the column at a faster rate. Conversely, decreasing the carrier gas rate increases the retention time.

Column temperature
Increasing column temperature decreases the retention time – this can be thought of as reducing the analytes solubility in the stationary phase. Conversely, decreasing the column temperature increases the retention time.

The peak and its shape/size
The retention time is measured from the peak maxima, but what is a peak? The peak area represents the amount of electronic charge (for an FID detector) being measured by the detector over a given period of time. As the component comes off the column, the component is detected via an electrical output from the detector.

This output is directly related to the amount of component coming off the column.

The component comes off the column as a band ie not all at once but over a short period of time depending upon the amount of analyte being detected. This results in the component having a characteristic distribution (eg tall sharp or flat squat peak) rather than as a single line.

In general, the longer the retention time the broader and flatter the peak (band broadening effect).

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