Chromatographic Troubleshooting
Retention time symptoms
2.Reproducibility is good early in the run but not toward the end:
C This may occur in temperature•programminga very densely packed column; as column contents expand with heating, resistance to flow may be so great that a mass flow controller cannot maintain constant flow. Try increasing carrier source pressure.
If this is the cause, the problem will either vanish or its onset will move later in the run.
CThis may indicate too low a starting temperature; many stationary phases have a minimum operating temperature, usually the melting point of the material. Below this temperature, gas•solid chromatography is performed; above it, gas•liquidchromatography is performed. If an oven temperature program passes through this range, results can be very erratic.
3.Reproducibility is good later in the run but not for the first few peaks:
CWhen earliest peaks elute very rapidly, they may not have had time to achieve chromatographic equilibrium with the stationary phase; they act like solvent peaks and are blown straight through the column.
A useful rule is that peaks of interest should require at least four times as long to appear as an unretained solvent or air peak. If this problem is suspected, try reducing oven temperature 30^C to approximately double the retention times.
4.Retention time changes with amount of sample:
CSample is overloading the column: When there is more sample than the stationary phase can handle, peaks will be deformed and shifted from correct retention times.
In gas•liquidchromatography an overloaded peak shifts to a longer retention time and tails on its trailing edge. With gas•solid chromatography the effect is the opposite. Try diluting the sample or injecting less.
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