What is the cause of variable retention times, loss of resolution or selecticity change?
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Variable retention times are often due to leaks or changes in mobile phase composition (small changes in mobile phase can lead to large changes in retention time).
Trapped air in pump heads can also increase or decrease retention times in a totally random fashion.
Column temperature fluctuations, especially in ion exchange chromatography, can also lead to retention time changes.
Retention times usually decrease when the mass of solute exceeds the column capacity and have a tendency to increase with volume overload.
As a column ages retention times usually decrease.
A contaminated mobile phase will also cause the column retention times to shift and the selectivity to change.
An obstructed guard or analytical column will change your retention times.