Tutorial 09 Overview of other chromatographic methods 1

Objectives 1.Describe separation mechanism of ion-exchange. 2.Differentiate ion-exchange chromatography and ion-pair chromatography. 3.Define "Size Exclusion Chromatography“. 4.Define "Supercritical Fluid Chromatography“. 2

1.1. Ion-Exchange Chromatography What is exchanged? Anion for anion and cation for cation How are the ions exchanged? Ions with higher affinity displace ions of lower affinity to the stat. phase Affinity increases as: 1.Charge increases 2.Size of (solvatized) ions decreases 3.Polarizability increases 3

1.2. Stationary Phases in Ion-Exchange Chromatography "Resins" or "Gels" that carry the ion-exchanger surface. Both are amorphous particles of organic material, but gels are softer. Polystyrene resins: made by co-polymerization of styrene and vinyl-bearing molecules Cellulose and dextran gels: Dextran, cross-linked to glycerin, is called (Sephadex TM ) Polyacrylamide gel 4

The benzene ring of the support can be modified to produce cation exchange resin containing sulfonate group (SO 3 - ) or an anion exchange resin containing ammonium groups (NR 3 + ). Ion exchangers are classified to strongly or weakly acidic or basic. SO 3 - is strong because it remains ionized even in strong acidic solutions. CO 2 - is weak because it becomes protonated at pH 4 and thus looses its ion exchange capacity. Strongly basic quaternary ammonium salts remain cationic at all pH values. Weakly basic tertiary anion exchangers are deprotonated in moderately basic solutions and lose their ability to bind anions. The resin becomes more rigid and porous as cross linking increases. Lightly cross linked resins permit rapid equilibration of solute between the inside and outside of the particle, however they swell in water which decreases the density of ion exchange sites and selectivity of the resin to different ions. Cellulose and dextran which are polymers of glucose possess larger pore size and lower charge densities than polystyrene resins. They are well suited to large molecules like proteins which may be irreversibly bound to resins because they are highly charged Stationary Phases in Ion-Exchange Chromatography

1.3. Examples of Ion-Exchange Resins 6

1.4. Ion-Exchange Chromatography for Water Treatment Obtaining soft water: Water is passed through a cation-exchange resin (Na + form). what happens to CaSO 4 if present in the water? Ca 2+ is replaced with 2 Na +,SO 4 2- is not affected.  cations of hardly soluble salts are removed (Ca 2+, Mg 2+ ). The resin can be regenerated with NaCl solution. Obtaining deionized water: Water is passed through an anion-exchange resin (OH - form) and cation- exchange resin (H + form), what happens to Cu(NO 3 ) 2 if present in the water? Cu 2+ is replaced with 2 H + and NO 3 - is replaced with OH -  H + + OH - = H 2 O is eluted while Cu(NO 3 ) 2 is "trapped“. 7

1.5. Elution in Ion exchange chromatography Gradient elution is a powerful technique in ion chromatography. 1.Concentration gradients: Eluents used in anion exchange contain an anionic compound in high concentration which competes with the analyte (an anion of course) for sites on the resin. Gradient elution is accomplished by increasing the concentration of the eluent anion during the run. 2.pH gradients: A fixed concentration of a weak acid (eluent A) is mixed with an increasing concentration of a strong base as NaOH (eluent B). pH gradients are a type of concentration gradients as the purpose of increasing the pH during an anion exchange run is to increase the concentration of the dissociated form of the weak acid eluent. 8

2. Ion-Pair Chromatography Stationary phase: common RP (no ion-exchange!) Mobile phase: contains surfactant that is “loosely attached” to stationary phase and so gives ion-exchange surface. Separation: counter-ions passing the column are attracted by surfactant and are exchanged. Separation mechanism: partition + ion-exchange Example: to separate a mixture of cations, an anionic surfactant is added to the mobile phase. The surfactant lodges in the stationary phase and effectively transforms it into an anion exchanger. When analyte cations pass through the column, they can associate with the stationary phase by electrostatic attraction to the surfactant anion. 9

3.1. Size (or Molecular) Exclusion Separation of organic polymers in non-aqueous systems is called gel permeation chromatography (GPC). Separation of biomolecules in aqueous systems is called gel filtration chromatography (GFC). Separation of mixtures according to different molecule sizes of the components: small molecules can intrude into pores of the stationary phase easily and are retained. But less easily large molecules which are too big to enter pores and are retained least. 10

column total volume volume occupied by solid matrix volume of the solvent held in the pores free volume outside particles V 0 : If no mixing or diffusion occurred, this is the volume of the solvent needed to transport components through the whole column that are too large to enter pores. V 0 + V i : Volume of the solvent needed to transport components that are small enough to intrude into pores easily 3.2. Chromatographic Parameters in SEC V e is the elution volume of a substance, i.e. the volume of mobile phase that is required for elution of this compound. At constant flow rate, it is proportional to retention time. 11

3.3. Determination of Molecular Weights by SEC Calibration graph for a certain molecular exclusion column. Note that the m. wt. can only be determined approximately because of the influence of the molecular geometry on the elution volume. 12

“molecular size” =“hydrodynamic volume” i.e. the volume created by the movement of the molecule in liquid (by adhesion of solvent, so depending on the surface area): 3.4. What is the "Size" do we talk about? Proteins tend to be globular molecules, while DNA or polysaccharides are more likely linear, the latter have a larger hydrodynamic volume and thus are eluted earlier. orig. volume hydrodynamic volume orig. volume hydrodynamic volume 13

4.1. Supercritical Fluid Chromatography Mobile phase: no gas nor liquid, but a supercritical fluid. 31,3 ºC, 73.9 bar ºC, 5.1 bar temperature pressure V phase diagram of carbon dioxide 14

4.2. Use of Supercritical Fluids Properties (density) between gas and liquid Speed and resolution better compared to liquid chromatography (faster diffusion inside the column cavity, thus faster equilibration)  mostly used: CO 2 compatible with common detectors (FID, UV). low critical temperature. non-toxic.  Supercritical fluids …possess lower surface tension than liquids (i.e. they spread faster over stat. phase). …dissolve non-volatile substances unlike gases …evaporate upon pressure reduction after passing the column: analytes are in gaseous phase and thus easily detectable 15

 What gradient? in HPLC: usually solvent gradient in GC: usually temperature gradient In ion exchange: concentration or pH gradient in SFC: pressure gradient Density increases as pressure increases. The denser the mobile phase, the bigger its capacity for a solute, the less distribution into stat. phase, the lower the retention Flow Rate and Gradient Elution in SFC SFC HPLC HETP flow rate u best (smallest) plate height at higher flow rate compared to HPLC min. plate height only 1/3 compared to HPLC 16