Ion Exchange and Electrophoretic Separations
Charged species need a separation that works on charge. Ion exchange holds them on an oppositely charged phase and releases them with a competing ion, while electrophoresis moves them through a gel or a capillary under a field.
What a learner can do afterwards
- Explains how an ion exchange resin retains an analyte and what an eluent competes for
- Predicts how changing pH changes the retention of a weak acid on an exchanger
- Explains what sets electrophoretic mobility and why size and charge both appear in it
- Describes how a suppressor makes conductivity detection possible in ion chromatography
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Ion exchange holds charged analytes on an oppositely charged phase and releases them with a competing ion. The column carries charged resin beads: a cation exchanger has negative beads that grip positive ions, while an anion exchanger has positive beads that grip negative ions. Bound ions are washed off by raising the salt level or shifting the pH, which disrupts the grip.
Changing pH changes the charge of a weak acid and therefore its retention. At a pH where the acid loses its proton it turns negative and sticks to an anion exchanger; at a pH where it keeps its proton it stays neutral and washes through. So to predict binding, first work out the charge of each species at the given pH, then match it to the opposite resin.
Electrophoresis sorts by motion instead of sticking. Each charged molecule is pulled toward the oppositely charged electrode, and its speed depends on three things: size, shape, and net charge. Small highly charged molecules zip through the gel while big lightly charged ones lag. A suppressor then makes conductivity detection possible in ion chromatography by lowering the background signal of the eluent, so the analyte peaks stand out.
Two special cases remove one variable each. In SDS PAGE, soap like SDS unfolds proteins and coats them evenly negative, so charge per mass goes uniform and the gel sorts almost purely by size. In DNA gels every fragment already carries similar charge per mass, so short fragments outrun long ones through the mesh. Stain the bands and compare against a ladder of known sizes to read the result.
Opposite charges grip on the resin, fields pull by charge and size in gels, and the eluent or coating decides what remains.
More to read
Protein Electrophoresis and SDS-PAGE
Electrophoresis is a way to separate charged molecules by making them move through a gel under an electric field. A protein carries a net charge, so the field pushes it toward the electrode with the opposite charge. How fast it travels depends on three things: its size, its shape, and how much net charge it carries. A small, highly charged protein zips through the gel, while a big or lightly charged one lags behind. That is why both size and charge show up in electrophoretic mobility.
SDS-PAGE is a special version of this trick. SDS is a soap-like chemical that unfolds proteins and coats them with a steady negative charge. After this coating, every protein has about the same charge per unit of mass, so charge stops mattering much. Now the gel separates them almost purely by size: small proteins travel farther, large proteins stay near the start. Scientists then stain the gel to see the separated protein bands.
Learn and try: Gel electrophoresis
Gel electrophoresis is a technique that separates DNA fragments by size. First, tiny wells are cut at one end of a soft gel, and the DNA samples are loaded into those wells. Then an electric current is switched on across the gel.
DNA fragments are negatively charged because of their phosphate groups, so the field pulls them toward the positive electrode at the far end. Every DNA fragment carries roughly the same charge per unit of mass, so the charge is not what separates them. Instead, size decides the race. Small fragments slip through the gel's mesh quickly, while long fragments get tangled and move slowly.
After some time, the fragments form separate bands at different distances. The gel is then stained with a dye that sticks to DNA, so the bands glow under UV light. By comparing band positions to a ruler of known fragment sizes, called a ladder, you can estimate how long each fragment is.
Applications of Magnetic Forces and Fields
Being able to sort charged particles lets scientists learn what matter is made of. One powerful sorting tool is the mass spectrometer. It first gives atoms an electric charge so they become ions, then fires them into a magnetic field. The magnetic field pushes on each moving ion, bending its path into a curve. The tighter the bend, the more the ion is deflected.
The bending depends on the ion's charge-to-mass ratio. Ions with the same charge but less mass curve more sharply, while heavy ions curve gently. By measuring where each ion lands on a detector, the instrument can tell particles apart and even identify unknown substances. This is the same big idea as other charge-based separations: give species a charge, apply a field, and let how they respond to the field sort them. The section also explains the cyclotron, a machine that uses electric and magnetic fields together to accelerate charges to very high speeds for experiments.
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