17-1
Chromatographic separations
• Separation of species prior to detection
• Description• Migration rates• Efficiency• Applications
17-2
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
17-3
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
17-5
• chromatogram concentration versus
elution time
• strongly retained species elutes last elution order
• analyte is diluted during elution dispersion
• zone broadening proportional to elution time
Column Chromatography
17-6
Column Chromatography
• Separations enhanced by varying experimental conditions adjust migration
rates for A and B increase band
separation adjust zone
broadening decrease band
spread
17-7
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
17-8
Retention time
• Relationship between retention time and distribution constant V (volume) c (concentration) M (mobile phase) S (stationary phase)
17-9
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 () Larger means
better separations
17-10
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
17-11
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
17-12
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
17-13
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
17-16
Ion Exchange Resins• General resin information
Functional Groups SynthesisTypes Structure
• Resin DataKineticsThermodynamicsDistribution
• Radiation effects• Ion Specific Resins
17-17
Ion Exchange Resins
• ResinsOrganic or inorganic polymer used to
exchange cations or anions from a solution phase
• General StructurePolymer backbone not involved in bondingFunctional group for complexing anion or
cation
17-18
Resins• Properties
CapacityAmount of exchangeable ions per unit quantity of
material* Proton exchange capacity (PEC)
SelectivityCation or anion exchange
* Cations are positive ions* Anions are negative ions
Some selectivities within group* Distribution of metal ion can vary with solution
17-19
Resins• Exchange proceeds on an equivalent basis
Charge of the exchange ion must be neutralizedZ=3 must bind with 3 proton exchanging groups
• Organic Exchange ResinsBackbone
Cross linked polymer chain
* Divinylbenzene, polystyrene
* Cross linking limits swelling, restricts cavity size
17-20
Organic Resins
Functional groupFunctionalize benzene
* Sulfonated to produce cation exchanger
* Chlorinated to produce anion exchanger
17-22
Resins• Structure
Randomness in crosslinking produces disordered structureRange of distances between sitesEnvironments
* Near organic backbone or mainly interacting with solution
Sorption based resins• Organic with long carbon chains (XAD resins)
Sorbs organics from aqueous solutionsCan be used to make functionalized exchangers
17-23
Organic Resin groups
aa
SO3H
Linkage group Cation exchange
Chloride
aa
CH2Cl
aa
CH2N(CH3)3Cl
Anion exchange
17-25
Inorganic Resins• More formalized structures
Silicates (SiO4)Alumina (AlO4)
Both tetrahedralCan be combined
* (Ca,Na)(Si4Al2O12).6H2OAluminosilicates
* zeolite, montmorillonites* Cation exchangers* Can be synthesized
Zirconium, Tin- phosphate
17-27
Inorganic Ion Exchanger
• Easy to synthesisMetal salt with phosphatePrecipitate forms
Grind and sieve
• Zr can be replaced by other tetravalent metalsSn, Th, U
aa
OH
OPO(OH)2
O
OPO(OH)2
OPO(OH)2
O
OH
OPO(OH)2
O
OPO(OH)2
OPO(OH)2
Zr ZrZrZr
17-28
Kinetics• Diffusion controlled
Film diffusionOn surface of resin
Particle diffusionMovement 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
17-29
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 availableConstants can be evaluated for resins
Need to determine site concentration
17-31
Ion Selective Resins• Selected extraction of radionuclides
Cs for waste reductionAm and Cm from lanthanides
ReprocessingTransmutation
• Separation based on differences in radii and ligand interactionsize and ligand
• Prefer solid-liquid extraction• Metal ion used as template
17-32
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 CatecholResorcinol8-Hydroxyquinoline
17-33
n
HO OH
Resorcinol Formaldehyde Resin
n
OH
OH
Catechol Formaldehyde Resin
OH OH
N
n m
OH x
x = 0, Phenol-8-Hydroxyquinoline Formaldehyde Resinx = 1, Catechol-8-Hydroxyquinoline Formaldehyde Resinx = 1, Resorcinol-8-Hydroxyquinoline Formaldehyde Resin
17-34
Experimental• Distribution studies
With H+ and Na+ forms0.05 g resin10 mL of 0.005-.1 M metal ionMetal concentration determined by ICP-
AES or radiochemicallyDistribution coefficientCi = initial concentration
Cf = final solution concentrationV= solution volume (mL)m = resin mass (g)
D Ci Cf
Cf
V
m
17-35
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) SelectivityResin Li Na K Rb Cs Cs/Na Cs/K
PF 10.5 0.01 8.0 13.0 79.8 7980 10RF 93.9 59.4 71.9 85.2 229.5 3.9 3.2 CF 128.2 66.7 68.5 77.5 112.8 1.7 1.6
17-36
Cesium Column Studies with RF
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14 16
CsNaK
Al
Elu
an
t C
on
ce
ntr
ati
on
(g
/mL
)
Volume Eluant (mL)
0.1 M HCl 1.0 M HCl
pH 14, Na, Cs, K, Al, V, As
17-37
Eu-La Separation
0
2
4
6
8
10
12
0 20 40 60 80 100 120 140
CQFPQFRQF
DE
u/D
La
Mixing Time (Hours)
17-38
Solvent Extraction• Based on separating aqueous phase from organic phase• Used in many separations
U, Zr, Hf, Th, Lanthanides, Ta, Nb, Co, NiCan be a multistage separationCan vary aqueous phase, organic phase, ligandsUncomplexed metal ions are not soluble in organic
phaseMetals complexed by organics can be extracted into
organic phaseConsidered as liquid ion exchangers
17-39
Extraction Reaction• Phases are mixed• Ligand in organic phase complexes metal ion in
aqueous phaseConditions can select specific metal ions
oxidation stateionic radiusstability with extracting ligands
• Phase are separated• Metal ion removed from organic phase
EvaporationBack Extraction
17-42
Reactions
• Tributyl Phosphate (TBP)(C4H9O)3P=O
Resonance of double bond between P and OUO2
2+(aq) + 2NO3
-(aq) + 2TBP(org) <--
>UO2(NO3)2.2TBP(org)
Consider Pu4+
• Thenoyltrifluoroacetone (TTA)
Keto Enol Hydrate
aa
S
O O
CF3S
O OH
CF3S
OOH
CF3
HO