What is ion chromatography?
A liquid-chromatography technique that separates and detects ionic and strongly polar species by how they distribute between a moving liquid (mobile phase) and a fixed charged material (stationary phase).
IC vs. ordinary HPLC
| Feature | Classical HPLC | Ion Chromatography |
|---|---|---|
| Analytes | organic molecules | ionic / polar mixtures |
| Separation | reversed / normal phase | ion exchange |
| Column | silica-based | polymer-based (pH 0–14) |
| Flow path | stainless steel | metal-free (PEEK) |
| Detector | optical (UV/Vis) | conductivity |
Where it's used
Environmental water testing, food & beverage, pharmaceuticals, power/energy, semiconductors — it's written into official methods from the U.S. EPA, ASTM, ISO and others, largely for anions like fluoride, chloride, nitrate, sulfate and phosphate.
The IC flow path
Five stages in order. Press play to send a sample plug through the system.
Loop injection
In LOAD, sample fills a fixed-volume loop while eluent bypasses it. Switch to INJECT and eluent sweeps the exact loop volume onto the column — reproducible every time.
Ion exchange, animated
The column resin carries fixed positive sites (quaternary ammonium, R–N⁺). Sample anions compete with eluent anions for those sites. Stronger-binding ions lag behind — so they leave the column later and form separate peaks.
Why this order?
Retention rises with charge and polarizability. Singly-charged, poorly-polarizable F⁻ barely holds on and elutes first; larger monovalents (Cl⁻→NO₃⁻) hold progressively harder; the divalent HPO₄²⁻ and SO₄²⁻ bind two sites at once and elute last.
The suppressor — the trick that makes it sensitive
Conductivity detection has a problem: the eluent itself conducts, drowning the signal. The suppressor fixes this two ways at once.
Suppressor
OFF — raw eluent reaches the detectorWhat just happened
Placed just before the detector, the suppressor exchanges the eluent's Na⁺ for H⁺. The carbonate eluent (Na₂CO₃/NaHCO₃, ~70 µS) becomes weakly-ionised carbonic acid H₂CO₃ (~15 µS) → background and noise fall. Meanwhile analyte NaCl becomes strongly-conductive HCl → signal rises. Signal-to-noise jumps, so trace ions become measurable.
Detection & other mechanisms
Equivalent conductance (why H⁺ and OH⁻ matter)
| Anion | Λ (S·cm²/eq) | Cation | Λ (S·cm²/eq) |
|---|---|---|---|
| OH⁻ | 198.6 | H⁺ | 349.8 |
| Cl⁻ | 76.4 | Na⁺ | 50.1 |
| SO₄²⁻ | 80.0 | K⁺ | 73.5 |
| NO₃⁻ | 71.5 | — | — |
H⁺ conducts ~7× better than Na⁺ — exactly why converting analytes to their acid form (via the suppressor) amplifies the signal.
Three separation mechanisms
Ion exchange — charged analytes swap onto oppositely-charged resin sites. The workhorse for inorganic anions and cations.
Ion exclusion — a Donnan membrane repels fully-ionised species while weak acids (formic, acetic…) partition in and separate. Used for organic acids.
Reversed-phase ion pair — a bulky counter-ion (e.g. tetrabutylammonium) pairs with the analyte on a neutral hydrophobic resin. Useful when ion exchange won't hold the analyte.
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