Transcript Chromatographic separations

Chromatographic separations
• Separation of species prior to detection
•
•
•
•
Description
Migration rates
Efficiency
Applications
17-1
Description
• Different components of chromatography
 column
 support
 stationary phase
 Different degree of reaction
 Chemicals separate into bands
* Characteristics of phase exploited to
maximize separation
 mobile phase
 Gas, liquid, supercritical fluid
17-2
Description
• Different methods available
 column chromatography
 paper chromatography
 gas-liquid chromatography
 thin layer chromatography (TLC)
 high-pressure liquid chromatography
 HPLC
* Also called high-performance liquid
chromatography
17-3
17-4
Column Chromatography
• chromatogram
 concentration versus
elution time
• strongly retained species
elutes last
 elution order
• analyte is diluted during
elution
 dispersion
• zone broadening
proportional to elution time
17-5
Column Chromatography
• Separations enhanced by
varying experimental
conditions
 adjust migration
rates for A and B
 increase band
separation
 adjust zone
broadening
 decrease band
spread
17-6
Retention Time
• Time for analyte to reach
detector
 Retention time (tR)
• Ideal tracer
 Dead time (tM)
• Migration rate
 v=L/ tR
 L=column length
 For mobile phase
 u=L/ tM
17-7
Retention time
• Relationship between
retention time and
distribution constant
 V (volume)
 c (concentration)
 M (mobile phase)
 S (stationary phase)
17-8
Capacity Factor
• Retention rates on
column
• k'A can be used to
evaluate separation
 Optimal from 2-10
 Poor at 1
 Slow >20
• Selectivity factor (a)
 Larger a means
better separations
17-9
Broadening
• Individual molecule undergoes "random walk"
• Many thousands of adsorption/desorption
processes
• Average time for each step with some variations
 Gaussian peak
like random errors
• Breadth of band increases down column
because more time
• Efficient separations have minimal broadening
17-10
Theoretical plates
• Column efficiency increases
with number of plates

N=L/H
 N= number of
plates, L = column
length, H= plate
height

Assume equilibrium
occurs at each plate

Movement down
column modeled
17-11
Theoretical Plates
• Plate number can be found experimentally
• Other factors that impact efficiency
 Mobile Phase Velocity
 Higher mobile phase velocity
 less time on column
 less zone broadening
• H = A + B/ u + Cu
• = A + B/ u + (CS + CM)u
 A
 multipath term
 B
 longitudinal diffusion term
 C
 mass transfer term
17-12
Efficiency
• Multipath

Molecules move through
different paths

Larger difference in path
lengths for larger particles

diffusion allows particles to
switch between paths quickly
and reduces variation in
transit time
• Diffusion term

Diffusion from zone (front
and tail)

Proportional to mobile phase
diffusion coefficient

Inversely proportional to
flow rate
 high flow, less time for
diffusion
17-13
Efficiency
17-14
17-15
Ion Exchange Resins
• General resin information
 Functional Groups
 Synthesis
 Types
 Structure
• Resin Data
 Kinetics
 Thermodynamics
 Distribution
• Radiation effects
• Ion Specific Resins
17-16
Ion Exchange Resins
• Resins
 Organic or inorganic polymer used to
exchange cations or anions from a solution
phase
• General Structure
 Polymer backbone not involved in bonding
 Functional group for complexing anion or
cation
17-17
Resins
• Properties
 Capacity
Amount of exchangeable ions per unit quantity of
material
* Proton exchange capacity (PEC)
 Selectivity
Cation or anion exchange
* Cations are positive ions
* Anions are negative ions
Some selectivities within group
* Distribution of metal ion can vary with solution
17-18
Resins
• Exchange proceeds on an equivalent basis
 Charge of the exchange ion must be neutralized
Z=3 must bind with 3 proton exchanging groups
• Organic Exchange Resins
 Backbone
Cross linked polymer chain
* Divinylbenzene, polystyrene
* Cross linking limits swelling, restricts cavity
size
17-19
Organic Resins
 Functional group
Functionalize benzene
* Sulfonated to produce cation
exchanger
* Chlorinated to produce anion
exchanger
17-20
Resin Synthesis
HO
OH
HO
OH
NaOH, H 2 O
HCOH
n
resorcinol
OH
OH
OH
OH
NaOH, H 2 O
HCOH
catechol
n
17-21
Resins
• Structure
 Randomness in crosslinking produces disordered
structure
Range of distances between sites
Environments
* Near organic backbone or mainly interacting
with solution
Sorption based resins
• Organic with long carbon chains (XAD resins)
 Sorbs organics from aqueous solutions
 Can be used to make functionalized exchangers
17-22
Organic Resin groups
SO3 H
Linkage group
CH2 Cl
Chloride
Cation exchange
CH2 N(CH3 )3 Cl
Anion exchange
17-23
Resin
Structure
17-24
Inorganic Resins
• More formalized structures
 Silicates (SiO4)
 Alumina (AlO4)
Both tetrahedral
Can be combined
* (Ca,Na)(Si4Al2O12).6H2O
Aluminosilicates
* zeolite, montmorillonites
* Cation exchangers
* Can be synthesized
 Zirconium, Tin- phosphate
17-25
Zeolite
17-26
Inorganic Ion Exchanger
OPO(OH)2 OH
OH
Zr
O
Zr
O
Zr
OPO(OH)2
O
Zr
OPO(OH)2 OPO(OH)2 OPO(OH)2 OPO(OH)2
• Easy to synthesis
 Metal salt with phosphate
 Precipitate forms
Grind and sieve
• Zr can be replaced by other tetravalent metals
 Sn, Th, U
17-27
Kinetics
• Diffusion controlled
 Film diffusion
On surface of resin
 Particle diffusion
Movement into resin
• Rate is generally fast
• Increase in crosslinking decrease rate
• Theoretical plates used to estimate reactions
Swelling
• Solvation increases exchange
• Greater swelling decreases selectivity
17-28
Selectivity
• Distribution Coefficient
 D=Ion per mass dry resin/Ion per volume
• The stability constants for metal ions can be found
 Based on molality (equivalents/kg solute)
 Ratio (neutralized equivalents)
Equilibrium constants related to selectivity
constants
• Thermodynamic concentration based upon amount of
sites available
 Constants can be evaluated for resins
Need to determine site concentration
17-29
17-30
Ion Selective Resins
• Selected extraction of radionuclides
 Cs for waste reduction
 Am and Cm from lanthanides
Reprocessing
Transmutation
• Separation based on differences in radii and ligand
interaction
 size and ligand
• Prefer solid-liquid extraction
• Metal ion used as template
17-31
Characteristics of Resins
• Ability to construct specific metal ion selectivity
 Use metal ion as template
• Ease of Synthesis
• High degree of metal ion complexation
• Flexibility of applications
• Different functional groups
 Phenol
 Catechol
 Resorcinol
 8-Hydroxyquinoline
17-32
OH
HO
OH
OH
n
n
Resorcinol Formaldehyde Resin
OH
Catechol Formaldehyde Resin
OH
OH
N
x
n
m
x = 0, Phenol-8-Hydroxyquinoline Formaldehyde Resin
x = 1, Catechol-8-Hydroxyquinoline Formaldehyde Resin
17-33
x = 1, Resorcinol-8-Hydroxyquinoline Formaldehyde Resin
Experimental
• Distribution studies
 With H+ and Na+ forms
 0.05 g resin
 10 mL of 0.005-.1 M metal ion
 Metal concentration determined by ICPAES or radiochemically
 Distribution coefficient
Ci  Cf V
D
Ci = initial concentration
Cf
m
Cf = final solution concentration
V= solution volume (mL)
m = resin mass (g)
17-34
Distribution Coefficients for Group 1
elements.
All metal ions as hydroxides at 0.02 M, 5 mL solution, 25 mg
resin, mixing time 5 hours
D (mL/g (dry)
Na
K
Rb
Resin
Li
PF
RF
CF
10.5 0.01
93.9 59.4
128.2 66.7
8.0
71.9
68.5
13.0
85.2
77.5
Cs
Selectivity
Cs/Na
Cs/K
79.8
229.5
112.8
7980
3.9
1.7
10
3.2
1.6
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Cesium Column Studies with RF
pH 14, Na, Cs, K, Al, V, As
Eluant Concentration (g/mL)
40
0.1 M HCl
1.0 M HCl
35
30
25
20
Cs
Na
K
Al
15
10
5
0
0
2
4
6
8
10
Volum e Eluant (m L)
12
14
16
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Eu-La Separation
12
10
D Eu/D
La
8
6
4
CQF
PQF
RQF
2
0
0
20
40
60
80
100
Mixing Tim e (Hours )
120
140
17-37
Solvent Extraction
• Based on separating aqueous phase from organic phase
• Used in many separations
 U, Zr, Hf, Th, Lanthanides, Ta, Nb, Co, Ni
 Can be a multistage separation
 Can vary aqueous phase, organic phase, ligands
 Uncomplexed metal ions are not soluble in organic
phase
 Metals complexed by organics can be extracted into
organic phase
 Considered as liquid ion exchangers
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Extraction Reaction
• Phases are mixed
• Ligand in organic phase complexes metal ion in
aqueous phase
 Conditions can select specific metal ions
oxidation state
ionic radius
stability with extracting ligands
• Phase are separated
• Metal ion removed from organic phase
 Evaporation
 Back Extraction
17-39
(CH3CH2)2O Diethyl ether
17-40
17-41
Reactions
• Tributyl Phosphate (TBP)
 (C4H9O)3P=O
 Resonance of double bond between P and O
 UO22+(aq) + 2NO3-(aq) + 2TBP(org) <->UO2(NO3)2.2TBP(org)
 Consider Pu4+
• Thenoyltrifluoroacetone (TTA)
O
O
O
CF3
S
CF3
S
Keto
Enol
O
OH
HO
OH
CF3
S
Hydrate
17-42
TTA
• General Reaction
 Mz+(aq) + zHTTA(org) <-->M(TTA)z(org) + H+(aq)
 What is the equilibrium constant?
Problems with solvent extraction
• Waste
• Degradation of ligands
• Ternary phase formation
• Solubility
17-43