A chromatogram is obtained for aspirin (acetylsalicylic acid) by “reverse phase”

Question

A chromatogram is obtained for aspirin (acetylsalicylic acid) by “reverse phase” HPLC. The aspirin peak has a retention time of 14.15 minutes using isocratic (constant mobile phase composition) conditions with 20% methanol/80% water mobile phase and a flow rate of 1.0 mL/min.   Molybdate is injected as an un-retained component; it does not interact with the stationary phase, and comes out of the column at 2.42 minutes.

Calculate k’, the capacity factor, for aspirin.

Suppose the analyst wanted to decrease the retention time of aspirin. What change could be made to the experimental conditions?

Explanation / Answer

Based on information provided...

The capacity factor is calculated as:

k' = (tR - t0)/t0  

where tR is the retention time of the peak, and t0 is the dead time of the column.
k' = (14.15-2.42)/2.42

k' = 4.84

You can decresed retention time of aspirin by increasing % methanol or either adding some acidic buffer also help.

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