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Chromatographic separations

Chromatographic separations. Separation of species prior to detection Description Migration rates Efficiency Applications. Description. Different components of chromatography column support stationary phase Different degree of reaction Chemicals separate into bands

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Chromatographic separations

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  1. Chromatographic separations • Separation of species prior to detection • Description • Migration rates • Efficiency • Applications

  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

  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

  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

  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

  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

  7. Retention time • Relationship between retention time and distribution constant • V (volume) • c (concentration) • M (mobile phase) • S (stationary phase)

  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

  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

  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

  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

  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

  13. Efficiency

  14. Ion Exchange Resins • General resin information • Functional Groups • Synthesis • Types • Structure • Resin Data • Kinetics • Thermodynamics • Distribution • Radiation effects • Ion Specific Resins

  15. 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

  16. 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. 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

  18. Organic Resins • Functional group • Functionalize benzene • Sulfonated to produce cation exchanger • Chlorinated to produce anion exchanger

  19. Resin Synthesis HO OH HO OH NaOH, H O 2 HCOH n resorcinol OH OH OH OH NaOH, H O 2 HCOH n catechol a a a

  20. 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

  21. Organic Resin groups Linkage group Cation exchange Anion exchange Chloride

  22. Resin Structure

  23. 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

  24. Zeolite

  25. Inorganic Ion Exchanger • Easy to synthesis • Metal salt with phosphate • Precipitate forms • Grind and sieve • Zr can be replaced by other tetravalent metals • Sn, Th, U

  26. 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

  27. 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

  28. 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

  29. 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

  30. OH HO OH OH n n Catechol Formaldehyde Resin Resorcinol Formaldehyde Resin OH OH OH N x m n x = 0, Phenol-8-Hydroxyquinoline Formaldehyde Resin x = 1, Catechol-8-Hydroxyquinoline Formaldehyde Resin x = 1, Resorcinol-8-Hydroxyquinoline Formaldehyde Resin

  31. 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 ICP-AES or radiochemically • Distribution coefficient Ci = initial concentration Cf = final solution concentration V= solution volume (mL) m = resin mass (g)

  32. 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) Selectivity Resin Li Na K Rb Cs Cs/Na Cs/K PF 10.5 0.01 8.0 13.0 79.8 7980 10 RF 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

  33. Cesium Column Studies with RF pH 14, Na, Cs, K, Al, V, As

  34. Eu-La Separation

  35. 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

  36. 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

  37. (CH3CH2)2O Diethyl ether

  38. Keto Hydrate Enol 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)

  39. 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

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