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

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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

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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

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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

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Efficiency

17-15

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

17-21

Resin Synthesis

resorcinol

catechol

HO OH

HCOH

NaOH, H 2OHO OH

n

OH

OH

HCOH

NaOH, H 2OOH

OH

n

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

17-24

Resin Structure

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

17-26

Zeolite

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

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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

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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

17-30

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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

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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

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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

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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

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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

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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)

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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

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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

17-40(CH3CH2)2O Diethyl ether

17-41

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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

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TTA

• General ReactionMz+

(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