Ion Chromatography Analysis: Anion, Cation and Inorganic Ion Testing

Ion chromatography (IC) is a branch of high-performance liquid chromatography (HPLC) that separates and quantifies ionic and polar species in solution. Ions in the sample are separated by their differential affinity for an ion-exchange resin (stationary phase) as they flow through the column with an eluent (mobile phase). Anions are separated on an anion exchange column; cations on a cation exchange column. Detection is typically by suppressed conductivity. A suppressor module reduces background conductivity of the eluent so that analyte ions are measured with high sensitivity.

Ion chromatography can detect and quantify: common inorganic anions (fluoride, chloride, nitrite, bromide, nitrate, phosphate, sulphate); inorganic cations (lithium, sodium, potassium, ammonium, magnesium, calcium); organic acids (citric, lactic, acetic, oxalic acid); oxyhalides such as bromate, chlorate, and chlorite (regulated disinfection byproducts in drinking water); carbohydrates and sugars; amino acids; and certain pharmaceutical impurities and counter-ions. It is particularly powerful for anion analysis where other techniques (AAS, ICP) have limited sensitivity.

In water testing, IC is routinely used for: fluoride determination (IS 10500 limit 1.0 to 1.5 mg/L); nitrite and nitrate quantification (health critical; nitrate limit 45 mg/L per IS 10500); sulphate and chloride measurement (taste and corrosion parameters); bromate analysis in ozonated drinking water (a carcinogenic disinfection byproduct, WHO guideline 0.01 mg/L); and phosphate monitoring in wastewater and surface water for eutrophication control. IC provides simultaneous multi-anion analysis in a single run, making it highly efficient for routine water quality monitoring.

In pharmaceutical testing, IC is used for: counter-ion determination in salt-form drugs (for example, sodium, potassium, chloride, sulphate in API salts); inorganic impurity profiling in APIs and excipients; cleaning validation of manufacturing equipment (detection of trace residues of ionic cleaning agents); determination of preservatives such as benzoate and sorbate in liquid formulations; analysis of phosphate, citrate, and acetate buffers in biologics formulations; and water quality testing for purified water and WFI systems per USP 1231.

Suppressed conductivity detection in ion chromatography typically achieves detection limits in the range of 1 to 10 µg/L (ppb) for common inorganic anions and cations without sample pre-concentration. With inline concentration (preconcentration column), detection limits can be extended to 0.01 to 0.1 µg/L. For trace bromate in drinking water, the WHO guideline value is 10 µg/L (0.010 mg/L), routinely measurable by IC with suppressed conductivity. For pharmaceutical applications, method-specific validation per ICH Q2(R1) establishes the LOD and LOQ.