Why is my chromatogram showing peak tailing, ghost peaks, fronting peaks, split peaks / shoulder peaks or rounded peaks?
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Peak tailing can occur due to numerous reasons. The problem can be identified according to the following scheme:
- Mass overload: when injecting less sample amount (mass) either the peak becomes more symmetrical or resolves into two separate peaks. Use a more dilute injection sample to correct the situation. In fact, rounded peaks with a fronting end generally are a typical manifestation of an overloaded column.
- Secondary interactions: when injecting a neutral compound (acetophenone, toluene) the peak becomes symmetrical . Adjust the mobile phase pH in order to neutralize charged analytes.
For larger ID columns (>10 mm) radial temperature gradient can also cause peak tailing. In order to avoid such a problem, it is recommended to use a column oven.
If neither of the above applies, the tailing can also be caused by irregularities in the column packing process (preferential path) , a void at the column inlet or by a partially clogged inlet frit leading to an irregular flow profile/path. If it is a badly packed column, empty and repack with a different slurry/solvent/pressure combination and retest to check if that corrected the problem. If it is a partially blocked frit, carefully change the frit making sure not to disturb the column bed surface and retest. The frit can also be removed, sonicated in Methanol and replaced in column fitting.
Ghost peaks usually come from a late elution of an analyte from a previous injection, column contamination, unproper sample preparation or mobile phase contamination.
Fronting peaks are generally symptomatic of an overloaded column or a column that wasn’t properly packed and that shows an uneven silica bed density. Generally, using lower slurry density solvents both for the slurry and the packing solvent and less gel to pack these columns will solve that problem. When the packing process is at fault all peaks in the chromatogram are showing a fronting end.
Split Peaks / Shoulder Peaks
Peak splitting or double peaks is usually symptomatic of a void at column inlet, a partially blocked inlet frit (not necessarily leading to a pressure increase) and anything else that causes a disruption of the sample path where the sample follows multiple paths throughout the column. This phenomenon also occurs when the column is poorly packed and that the packing bed settles under the system pressure or that the mobile phase pH is too high and dissolves the silica thus creating a void at the column inlet.
If all the peaks in the chromatogram have shoulders and the problem has gradually been getting worse, then a void at the column head has to be considered as a strong possibility. Not much can be done to save the column and you are better off replacing the column.