Created with MindGenius Business 2005Created with MindGenius Business 2005®®

Ion Exchange Ion Exchange Chromatography Chromatography

Principles Small K means low affinity – so

low RT K is often quoted vs. a common

reference ion (e.g. H+) K affected by:

Charge of ion: charge gives K Size of hydrated ion

Uses displacement of an ion from the stationary phase by a solute ionK is dependent on relative affinity of the analyte for the surface over the solvent ions

Created with MindGenius Business 2005Created with MindGenius Business 2005®®

Cation Exchange Cation Exchange Chromatography Chromatography

Use organic acid functional groups as ionic species “Strong acid” type uses sulphonic acid (RSO3H) (more common) “Weak acid” type use carboxylic acids (RCOOH)

xRSO3H + Mx+ (RSO3-)xMx+ + xH+

Stationary Mobile Stationary Mobile

Ion SeriesFor a strong acid column, (e.g. RSO3H) size of K :

Ag+ > Cs+ > Rb+ > K+ > NH4+ > Na+ > H+ > Li+

andBa2+ > Pb2+ > Sr2+ > Ca2+ > Ni2+ > Cd2+ > Cu2+ > Co2+ > Zn2+ > Mg2+

Created with MindGenius Business 2005Created with MindGenius Business 2005®®

Anion Exchange Anion Exchange Chromatography Chromatography

Use amine groups as ionic species “Strong base” type uses quaternary amines (e.g. RN(CH3)3+OH-) “Weak base” type uses secondary or tertiary amines

xRN(CH3)3+OH- + Ax- (RN(CH3)3)x+Ax- + xOH-

Stationary Mobile Stationary Mobile

Ion Series

For a strong base column, affinity series for K is usually :

SO42- > Cr2O4

2- > I- > NO3- > Br - > Cl- > HCO2

- > CH3CO2- > OH- > F-

Created with MindGenius Business 2005Created with MindGenius Business 2005®®

Ion Exchange Ion Exchange ChromatographyChromatography(optimisation) (optimisation)

Stationary phase: “Strong” or “weak”, different pKa’s and pKb’s of ionic stationary phase groups give different strengthsMobile phase pH: can affect both

a) the proportion of protonated stationary phase sites,b) the proportion of ionised analyte,

but b) is more influential so in mobile phase pH gives: in retention time for cation exchange in retention time for anion exchange

Ionic Strength: (concentration of the mobile phase), mostly increases selectivity between ions of different charge Buffer salt: i.e. the relative position of the competing counter ion in the series for K Organic modifier: resin supports have a non-ionic component so reverse-phase interactions can occur. Affected by adding an organic solvent Temperature: affects buffer solubility, viscosity and mass transfer kinetics