IA CH 26

Define elution
The analytical process in which analytes are seperated from one another on a column, owing to differences in time that they are retained on the column.
Define mobile phase
a liquid or gas that carries analytes through a liquid or solid staitionary phase
Define stationary phase
a solid or an immobilized liquid on which analyte species are partitioned during passage of a mobile phase
Define distribution constant
the equilibrium constant for the distribution of an analyte in two immisible solvent, which is approx equal to the ratio of the equilibrium molar concentration
What is the eq for the distribution constant?
Kc= (aA)stationary/(aA)moble
Define retention time
the time between sample injection on a chromatographic column and the arrival of an analyte peak at the detector.
Write the equation for retention time
tr= ts+tm
ts=time in stationary phase
tm=time in mobile phase
Define retention factor
used to compare the migration rates of solutes in columns
Write the eq for retention factor


tr=retention time

tm=void time

Define selectivity factor
the ratio of retention factors of two species.
Write the equation for selectivity factor
alpha=Ka/Kb or
alpha= ((trb)-tm)/((tra)-tm)
Define plate height
Describes the efficiency of a chromatographic column. Efficiency in terms of variance per unit length
Write the equation for plate height

H= L/N or


Define longitudinal Diffussion
a process in which species migrate from a more concentrate part of the medium to a more dilute region. (Common source of band broadening)
Write the equation for longitudinal diffusion.

B= 2?*Dm 

?= constant that depends on quality of packing

Dm= diffusion coefficient of mobile phase

Define eddy diffusion
A diffusion of solutes that contributes to chromatographic band broadening, it is the result of differences in pathways of solutes as they transverse a column
Write the equation for eddy currents
A= 2 lambda*dp
lambda=constant that depends on quality of packing
dp=diameter of packing particles
Define column resolution
Measures the capability of a column to seperate to analyte bands.
Write the equation for column resolution


(tr)a and (tr)b are retention times

Wa and Wb are peak widths

Define Elutent
A mobile phase in chromatography that is used to carry solutes through a stationary phase
Describe the elution problem
Problem found when different distribution constants for solutes are separated by chromotography. When conditions are optimized for late eluting peaks then good results are not obtained for early peaks and vice versa. Problem is fixed by using gradient elution in LC and temp programming in GC
List the 7 variables that lead to zone broadening in chromatography
1)High viscosity in LC
2) Slow injection of sample onto packed column
3) Increased diameter of the particles for column packing
4) Dense layers of liquid stationary phase
5) Large diameter of the column
6) High or slow flow rates of mobile phase
7) High temp in GC
Define gas-liquid chromatography
separation technique which gas is a mobile phase that gets adsorbed over the liquid stationary phase. The equilibrium established is the adsorption of anaylte between liquid stationary phase and gas mobile phase
Define liquid-liquid chromatography
Separation technique in which liquid is mobile phase that gets adsorbed over the liquid stationary phase. The equilibrium is a partitioned equilibrium between the two phases which are immscible.
What 4 variables are likely to affect the selectivity factor for a pair of analytes?
1) composition of mobile phase
2) Characteristic feature of stationary phase in column
3) Set temp in column chromatography
4) Use of different chemical reagents such as ion pairing, surfactants and complexing agents
Describe how the retention factor for a solute can be manipulated in GC
1) Changing length of column
2) Changing temp of column
3) Changing the nature of the stationary phase
Describe how the retention factor for a solute can be manipulated in LC

1) Change the composition of the mobile phase

2) Change the nature of the stationary phase

3) Change the length of the column

4) Using different chemical reagents, ion pairing, complexing, or surfactants

What are to methods to improve resolution
1) increase column efficiency
2) Change column selectivity
What is gradient elution?
Gradient elution is the the change in flow rate of mobile phase or change in composition of mobile phase. Mainly used in LC