Depends. If Isocratic elution, then as long as there is no risk of late-eluting analytes from sample constituents, no need to wait for double (wasteful of solvent and produces more waste solvent to be disposed of at cost). If gradient elution is necessary to achieve required resolution from earlier eluting constituents, then perhaps the double waiting time til the end of the run increases organic solvent to elute any remaining analytes (reverse phase HPLC). If using an Internal standard, this may be chosen to elute as the last peak of interest.
Depends. If Isocratic elution, then as long as there is no risk of late-eluting analytes from sample constituents, no need to wait for double (wasteful of solvent and produces more waste solvent to be disposed of at cost). If gradient elution is necessary to achieve required resolution from earlier eluting constituents, then perhaps the double waiting time til the end of the run increases organic solvent to elute any remaining analytes (reverse phase HPLC). If using an Internal standard, this may be chosen to elute as the last peak of interest.
Using double the retention time of the compound may useful for avoid the carry over in next run having the same mobile phase conditions.
It may be necessary to elute any interfering compounds so they do not come off randomly in proceeding runs. For gradient it is often advantageous to run it up to maximum eluting conditions (95% acetonitrile for reverse phase) for each run or you may need to run a cleaning method every 5 to 20 samples (to be determined in method robustness testing).
Dear BS, generaly we follow this practice to avoid the chanc of carry over in next run. And second thning when we change our mobile phase to more organic solvent in case of reverse phase HPLC and gradient method, any thing like imprities may not stuck in the column and elutes in the same run. So, you need sufficient time to elute the complete compound whatever you have injected in the column.
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