•One step further à Selectivity factor (a) describes differential migration
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•For two                             KB          kB’
•                                a = ------  =  -------
•components                       KA          kA’
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•And from                              (tR)B - tM
•                                  a = ---------------
•chromatogram                      (tR)A – tM
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•Allows calculation of the resolving power of
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•a chromatographic system (i.e. column with A & B)