Ion Chromatography

Common IC Support

Polystyrene

Why is polystyrene a good IC support?

-stable at pH’s from 0-14

-phenol groups (resonance) provide great functionalization

-mixed with divinylbenzene (co-polymer which creates tremendous stability with cross-linking)

Microporous Resin

-swells with water

-never dry pack

-low surface area, minimal impact on separation

-Functionalization happens in the stationary phase on the surface of the resin

Macroporous Resin

-exposed to several thousand psi

-needs lots of crosslinking to avoid collapsing of pores

-high surface area, separation of ions takes place IN pores

-can dry pack

SCX (strong cation exchanger)

• SO3-

• adjust pH to deprotonate (autoproteolysis reaction)

WCX (weak cation exchanger)

• carboxylate group

• has to have a specific pH range to get the charged form

Anion Exchange resin

• Quaternary Ammonium or Amine

• Contains great leaving groups

• Or use the partial positive from a CH2 group after the Cl leaves

SAX (Strong Anion Exchange) vs. WAX (weak anion exchange)

• binds to anions

• gets weaker as H+ are the groups.

• [N+R4OH]

Resin Capacity

A description of the # of functional groups per weight unit of resin

High/Low Resin Capacity

-Many stationary phases groups bound to the support

or

-Has sparser labeling of the stationary groups

Old Methods IC

Used high capacity resin

Modern Methods

Used low capacity resin

Ion Exchange

Remove one ion and replacing it with another

Ion Chromatography

Separating one type of ions from others, retaining different ions differing amounts on a stationary phase

Elution of Analyte from column

• Need’s “pusher” ion to force the analyte ions off the SCX/SAX resin

• Analyte ion is holding on stronger than the solvent can push off so the pusher ion puts a stronger cation/anion to allow the analyte to come off.

Why do we want low resin capacity?

• HRC takes too long for the analyte ion to get pushed through the system

• HRC requires a very high pusher concentration; bad for detectors and separation

Detectors

Absorbance

-UV-Vis absorbance

• only a few ions absorb

• derivatize (change the structure of the molecule) them to improve conductivity

Complexation of Cations

• need equipment

• pre-column (may not work, will complex have charge?) and post-column (needs fast kinetics) needed

Indirect Abs. Detection

• more practical

• analytes don’t absorb, pick a pusher ion that does

Universal Detectors

-detect everything

-low in sensitivity (small signal for a given concentration of analyte

A voltage placed across a stream and the electrical conductance is measured

Equivalent Ionic Conductance

-ions are not the same in conductivity

-charge and speed and size are factors

-pusher and analyte need to have different EIC, so baseline conductivity changes

EIC Extremes

Highest Cation - H+

Highest Anion - OH-

Troft features happen when high speed ions are switched with the normal ions that were used

Dionex Approach

Suppressed IC

Flow→seperation column→suppression (cat/anion exchange) column→detector

Suppressed IC

Opposite charged ion- higher resin capacity in suppressor column

High capacity column (so you can do multiple trials) for opposite type of ion as the separator

Peaks would get bigger after suppression column
Greatly reduced baseline conductivity

Figure gets dipped without suppression

Single Column IC

-Pusher ion is more dilute, and the resin capacity is lower, lower conductivity pusher ions. Don’t need the suppression column

Dionex 2nd Generation Suppressor

-Hollow fiber membrane suppressor

-minimized peak broadening, regeneration issue

-no stationary phase, continual flow of concentration gradient flushes out waste and regenerates column

-dialysis membrane permeable to cations/anions

Use of IC

• rapid separation of standard anions

• hard to detect, can do multiple in one common experiment

• useful for more complex anion separation

Equipment needed

-column

-suppressor

-conductivity detector

-AC design, temp control

Alternating Current

As ions begin to move to the opposing charges in the electrodes, the voltage begins to change and get smaller, so conductivity changes.

______ causes the voltages to switch, and the charges flip flop and the ions won’t sit on the electrode.