Chromatography Theory I Flashcards
Stationary phase (SP)
The solid or liquid phase that is fixed in a column or on a surface of a plate
Mobile phase (MP)
(or eluent)
The phase (typically gas or liquid) that moves through or over the stationary phase
Elution
Process in which solutes pass through the chromatographic column by addition of fresh mobile phase
Distribution coefficient (constant), K
K = Cs / Cm
Describes partitioning of a solute with stationary and mobile phase
Measure of propensity (inclination) of a solute to distribute itself between the two phases
Retention volume, Vr
Volume of mobile phase necessary to elute a certain solute from a column
Retention time, t(r)
Time needed to elute a certain solute from a column
Void volume, Vm
Vr = Vm - KVs
(Vs = volume stationary phase)
The volume of solvent necessary to elute a solute that does not partition with the stationary phase
Void time, t(m)
Time to elute unretained solute (solute that does not partition with stationary phase)
Capacity factor, k’
Relates equilibrium distribution of the solute within the column to the thermodynamic properties of the column and to the temperature
k’ = (time in SP) / (time in MP)
k’ = (moles in SP) / (moles in MP)
Relationship between K and k’
k’ = K ( Vs / Vm )
Consequence of non-linear isotherm
Solute retention time varies significantly with sample size
Dependence of retention on sample size is a large disadvantage
Langmuir isotherm
Peak tailing
Solute saturates SP sorption sites
Fewer sites for retention
Usually result of active sites in SP
To fix: get rid of active sites or buy expensive columns
Anti-Langmuir isotherm
Peak fronting
Too much solute applied to column
Sample capacity too large
Retention time increases
To fix: reduce sample size or use a column with a different SP
