Chapter 26 Summary
One of the basic premises of chromatography is the comparison of mobile phase/stationary phase to multiple batch separations. In a batch separation involving two phases which don’t mix any analyte will “partition between the two phases” This partitioning can be quantified by the equilibrium constant K were K is equal to concentration in the organic phase over the concentration in the aqueous phase. As such, the concentration ratio between the two phases never changes while equilibrium is achieved. The amount in each phase can be varied by changing the concentration in each phase, but the concentration ration or “K” won’t change. In chromatography, the actual quantifying of how much is contained is difficult and a more useful term is used.
This term is the capacity factor k’
K’=(Tr-Tm)/Tm
The capacity factor describes the preference that the analyte has for each phase.
(The equations for the rest of the terms must be looked up)
The selectivity factor a describes difference in retentions times while accounting for the time it takes for an unretained analyte to elute. The bigger this term is, the easier to separate two compounds as they will have much different retention times
The plate count (N) describes how well a column separates . It refers to the number of imaginary separations that occur as an analyte moves down the column. The larger the plate count, the more narrow the peak.
The plate height simply refers to the length of column in that one separation takes place. This allows comparison of columns with different lengths.
There are three factors that affect plate height: Multiple flow paths, longitudinal diffusion, and resistance to mass transfer. The VanDeemter equation describes each contribution.
Column resolution tells how well two peaks are separated. Equation 26-20 tells us how to calculate this.
Three factors affect resolution: the plate height, the alpha value of two analytes and the capacity factor
This is summarized in equation 26-22.
Home work:1,2,7,10,11,14,15,16,18,19,20