How is the capacity factor k' calculated?

How is the capacity factor k' calculated?

• A. tR / tM
• B. (tR - tM)/tM
• C. tM / tR
• D. tR - tM

Answer: (B)

Capacity factor k' expresses how much of the time a compound spends in the stationary phase relative to the time it spends in the mobile phase. The retention time tR is the total time from injection to detection, while tM (the void or dead time) is the time for unretained solutes to pass through the column in the mobile phase. The portion of time the analyte spends in the stationary phase is tR minus tM. To compare retention across runs, this stationary-time is placed over the mobile-phase time tM, giving k' = (tR − tM)/tM. This yields a unitless value that increases with greater retention. The other forms don’t reflect this ratio: tR/tM mixes the total time with mobile-time in the denominator; tM/tR reverses the relationship; and tR − tM gives a time difference without normalizing by tM.

Capacity factor k' expresses how much of the time a compound spends in the stationary phase relative to the time it spends in the mobile phase. The retention time tR is the total time from injection to detection, while tM (the void or dead time) is the time for unretained solutes to pass through the column in the mobile phase. The portion of time the analyte spends in the stationary phase is tR minus tM. To compare retention across runs, this stationary-time is placed over the mobile-phase time tM, giving k' = (tR − tM)/tM. This yields a unitless value that increases with greater retention.

The other forms don’t reflect this ratio: tR/tM mixes the total time with mobile-time in the denominator; tM/tR reverses the relationship; and tR − tM gives a time difference without normalizing by tM.