Dwell volume is the fluidic volume between the point where solvents mix and the head of the column. When transferring an HPLC program from an older instrument to a modern UHPLC system, differences in dwell volume can shift retention times significantly.

The resolution ($R_s$) of two adjacent peaks in chromatography is governed by the fundamental Resolution Equation:

The current program is not fit for purpose for Aspirin quantification but can be made compliant with the simple modifications outlined above.

The program instructs the detector (UV-Vis, Photodiode Array, Mass Spectrometer, etc.) on when to start collecting data, which wavelengths to monitor, and when to stop. Step-by-Step HPLC Program Development

Assuming a mixture of Paracetamol (polar) and Ibuprofen (non-polar).

Once selectivity is achieved, efficiency is fine-tuned.