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Short answer
Anion chromatography separates negatively charged ions, such as chloride, nitrate and sulfate, on a column that carries fixed positive charges. In the analytical form usually called ion chromatography, the separated anions pass through a suppressor and are detected by conductivity, then identified by retention time and quantified by peak area or height.
Definition and scope
Anion chromatography, also written anion exchange chromatography or anion-exchange chromatography, is the branch of ion-exchange chromatography in which the stationary phase holds positive charges and retains anions. The same words cover two quite different practices. In analytical chemistry it means measuring small inorganic anions in water and similar samples. In protein science it means separating biomolecules by charge, which is a different workflow with different columns, buffers and detectors (see the section on misunderstandings below). This guide covers the analytical practice.
How anions are separated, suppressed and detected
Standard Methods 4110 B, Ion Chromatography with Chemical Suppression of Eluent Conductivity, published by APHA, AWWA and WEF, describes the arrangement. A water sample is injected into an eluent stream and passes through a guard column and an analytical column packed with a low-capacity, strongly basic anion exchanger, where the anions separate from one another. The USGS National Water Quality Laboratory method I-2057, Anions, ion-exchange chromatographic, automated, likewise pairs an anion-exchange column with a conductivity detector.
What the suppressor does
Between the column and the detector sits a suppressor. According to Standard Methods 4110 B, it converts the separated anions to their acid forms and reduces the conductivity of the eluent. Thermo Fisher describes the same step: the suppressor removes counter ions (cations, when anions are of interest) and converts the ion of interest into a more conductive form, which raises sensitivity. Anions are typically eluted with hydroxide or carbonate eluents, and hydroxide can be produced by an eluent generator from potassium hydroxide.
The reason this matters is practical. Without a suppressor, in Thermo Fisher’s words, there would be tiny or negative peaks sitting on a large background of conductivity from the eluent. Thermo Fisher dates the introduction of the suppressor to 1975 and credits it with bringing ion chromatography to the forefront.
Identification and quantitation
Standard Methods 4110 B identifies each anion by comparing its retention time with standards and quantifies it from peak area or height.
Types and related concepts: which anions, at what levels
Standard Methods 4110 B lists eight analytes: bromide, chloride, fluoride, nitrate, nitrate/nitrite, nitrite, phosphate and sulfate. The USGS method covers the same seven individual anions. The two methods publish very different low-end numbers for them, which shows that a detection or reporting level belongs to a method, a sample loop and a matrix, not to anion chromatography in general.
| Anion | Standard Methods 4110 B detection level (mg/L, reagent water, 25 µL loop) | USGS I-2057 minimum reporting level (mg/L) |
|---|---|---|
| Bromide | 0.014 | 0.100 |
| Chloride | 0.004 | 0.200 |
| Fluoride | 0.002 | 0.010 |
| Nitrate | 0.003 | 0.050 |
| Nitrite | 0.004 | 0.020 |
| Phosphate | 0.014 | 0.060 |
| Sulfate | 0.018 | 0.200 |
The columns are not interchangeable: one is a detection level in reagent water, the other a minimum reporting level. Standard Methods adds that natural waters may give higher detection levels than reagent water.
Where it matters for labs: an interference checklist
Both method summaries describe the same pattern. Peaks that sit close together, or a very large peak beside a small one, are where results go wrong. The items below come directly from the two published methods.
- Bromide and nitrate elute close together. USGS I-2057 states that the bromide-to-nitrate peak ratio should not exceed 1:10 or 10:1 if both ions are to be quantitated.
- Formate and acetate elute between fluoride and chloride. USGS lists this as an interference for those two anions.
- Low-molecular-weight organic acids, bromate and chlorite may interfere with chloride and fluoride, per Standard Methods 4110 B.
- High concentration of one ion affects the resolution and retention of others. Standard Methods notes that dilution or gradient elution overcomes many of these cases; USGS adds that high chloride can prevent resolution of closely eluting peaks.
- The water dip. USGS notes that a sample can produce a negative peak because its conductivity is lower than that of the suppressed eluent.
- Late-eluting peaks from one injection can appear in a later run (USGS I-2057).
For choosing and specifying the instrument itself, see the ion chromatography equipment guide. For general method-building habits shared with liquid chromatography, see HPLC method development workflow.
Common misunderstandings
Anion chromatography is not the same as anion-exchange chromatography of proteins
Both rely on a positively charged stationary phase, and both are called anion exchange. The analytical form measures small inorganic anions with suppressed conductivity detection. Protein anion exchange is a separation based on the net charge of biomolecules, and the site’s ion-exchange HPLC guide explains why that is not ion chromatography in the analytical sense.
A published detection limit is not a property of the technique
As the table shows, chloride is listed at 0.004 mg/L in one method and 0.200 mg/L in another. Quote the method, the injection volume and the matrix along with any number.
The suppressor is not a cosmetic add-on
It lowers the eluent background and raises analyte response. Without it, conductivity detection of anions gives tiny or negative peaks on a high background, as described above.
FAQ
What anions can anion chromatography measure?
The two published water methods cited here cover bromide, chloride, fluoride, nitrate, nitrite, phosphate and sulfate; Standard Methods 4110 B also lists combined nitrate/nitrite.
Why is a suppressor used in anion chromatography?
It converts the separated anions to a more conductive form and removes the eluent’s counter ions, so the analyte peaks stand out against a low background instead of tiny or negative peaks on a high one.
What interferes with anion chromatography results?
Closely eluting pairs such as bromide and nitrate, formate and acetate between fluoride and chloride, organic acids, bromate and chlorite, and very high concentrations of a single ion such as chloride. See the checklist above.
How are the separated anions identified and quantified?
By retention time compared with standards, and by peak area or height.
Sources
- APHA, AWWA, WEF, Standard Methods 4110 B, Ion Chromatography with Chemical Suppression of Eluent Conductivity, method summary on the National Environmental Methods Index: nemi.gov/methods/method_summary/7428 (checked 2026-10-29).
- USGS National Water Quality Laboratory, method I-2057, Anions, ion-exchange chromatographic, automated (rev. 1985), method summary on NEMI: nemi.gov/methods/method_summary/5536 (checked 2026-10-29).
- Thermo Fisher Scientific, AnalyteGuru, Suppression unravelled: understanding the role of suppression in ion chromatography (manufacturer technical article): thermofisher.com/blog/analyteguru/suppression-unravelled (checked 2026-10-29).
