Chromatography Separates components in mixture: Based on - polarity - boiling point - ionic strength - size
Dec 28, 2015
Chromatography
Separates components in mixture:
Based on- polarity- boiling point- ionic strength- size
Chromatography
• Mobile phase: phase which sample is dissolved in may be gas, liquid, or supercritical fluid
• Stationary phase: phase which mobile phase is forced through
• Mobile and stationary phases are chosen so the analyte will distribute itself between the two phases
Partition Chromatography
• Movie • Used in GC & LC• Molecules will partition
into the stationary phase based upon affinity for stationary phase & eventually partition into mobile phase again
• Thin layer is coated onto inside of GC column or on small particles on LC column
Adsorption Chromatography• Very similar to
partition chromatography
• Adsorption just on surface, partition into thin layer
• Not used as widely as partition used mainly in TLC & very small particles in LC
• Movie
Ion Exchange Chromatography• Movie • Separation of either
cations or anions• Separtion based on
relative strength of ionic bond
• Anion exchange has cations on surface
• Used in LC exclusively
Molecular Exclusion Chromatography• Separation based
on size• Small molecules
get trapped in pores & take longer to get out
• Movie
Gel Electrophoresis
• Separation based on size and charge
• Smaller molecules will migrate further, less tangled
• Movie
Affinity Chromatography
• Very selective• Specific binding
site is used to concentrate analyte on column
• Used a lot in biological applications
• Movie
Typical Gas Chromatogram
Typical Liquid Chromatogram
Introduction to Chromatography - Theory
General Relationships1. Distribution constant
a. Craig counter current experiment2. Retention time3. Relationship between distribution constant and retention time4. Capacity factor k’5. Selectivity factor
Introduction to Chromatography - Theory
Peak Broadening1. Shapes2. Column efficiency
a. plate heightb. number of plates
3. Kinetic factors – Van Deemter equation
Craig counter current
• movie
2. Retention time tr
• Time it takes for analyte to reach detector after sample injection
• Tm = retention time for material to come through column which is not retained also called dead time or void volume
tm rate of migration is the same as the average rate of motion of the mobile phase molecules u = L/tm
3. Distribution constant & retention time
v = u x moles of analyte in mobile phase
total moles of analyte
v = u x CmVm = 1
CmVm + CsVs 1 + CsVs/CmVm
v = u x 1
1 + KVs/Vm
4. Capacity factor k’• Describes migration rates of analytes in columnFor a species A
• k’ = KAVs v = u x 1/(1 + k’)
• kA’ = (tr- tm)/tm
• For separations involving few components ideal capacity factors are between 1 - 5
What is k’ for this peak?
5. Selectivity factor
• Ability to distinguish between 2 species, A & B
Purpose of Chromatography
• Achieve separation
Elution movie
Peak Broadening
Peak Broadening
• Is peak broadening a good or bad thing?
BAD Why?
Column Efficiency• Plate height (H)• # theoretical plates
(N)• N = L/H• Efficiency of a
column goes up as N increases and H decreases
• Typical 250 – 10,000 plates
Plate Height
3. Kinetic Factors: The Van Deemter Equation
Reality: column efficiency is affected by kinetic factors
What variable do you think are important in determining the efficiency of a separation?
In your notebook predict what the effect of increasing linear velocity (flow rate) will have on column efficiency (H)
Van Deemter Equation
Van Deemter EquationH = A + B/u + Cu
A = Eddy diffusion: Due to different paths molecules can take as they go through particles
B/u = longitudnal diffusion Band diffuses in and against direction of mobile phase movement
Cu often broken into 2 terms Csu + Cmu
Mass transfer coefficient: Time it takes for analyte to diffuse into stationary phase
How can band broadening be reduced? (and thus column efficiency be enhanced)
1. Decrease particle diameter2. Decrease column width3. Lowering temperature in GC
(reduces diffusion coefficient)4. Minimize thickness of liquid
stationary phase
Resolution
Rs = 2((tr)B – (tr)A) wA + wB
This is called General Elution Problem