WHAT IT IS
In ion chromatography (IC), the detector measures analytes as they elute from the column, converting physical or chemical signals into electrical output. The type of detector used affects sensitivity, selectivity, compatibility with suppression, and overall analytical scope. Most IC detectors are optimized for ionic species, though others can support organic or redox-active compounds.
HOW IT WORKS
Separated compounds pass through a flow cell or detector interface. As analytes interact with the detector, they generate a response based on conductivity, light absorbance, redox activity, or mass. The signal is converted into peaks in the chromatogram.
Sensitivity – Determines how low a concentration can be reliably detected.
Selectivity – Reflects the detector’s preference for specific ion types or chemical functionalities.
Baseline Stability – Affected by temperature, flow rate, and eluent quality; crucial for trace analysis.
Suppression Compatibility – Some detectors (like conductivity) require suppression for optimal performance.
Application Fit – Detector choice depends on whether the analytes are inorganic ions, organics, or electroactive compounds.
GENERAL TYPES OF IC DETECTORS
Conductivity Detector:
Working Principle – Measures changes in electrical conductivity of the eluent as ionic species pass through.
Application – Universal for inorganic anions and cations; standard detector in suppressed and non-suppressed IC.
Strengths – High sensitivity; excellent for charged species; compatible with aqueous eluents.
Limitations – Requires stable flow and temperature; background eluent conductivity must be minimized.
UV/Vis Absorbance Detector:
Working Principle – Detects absorbance of ultraviolet or visible light by analytes with chromophores.
Application – Organic acids, transition metals, aromatic compounds, and pharmaceuticals.
Strengths – Non-destructive; can be used alongside conductivity; useful for non-ionic or weakly ionic compounds.
Limitations – Ineffective for species without UV/Vis absorbance; requires transparent eluents.
Electrochemical Detector (Amperometric):
Working Principle – Monitors current generated by oxidation or reduction reactions at an electrode surface.
Application – Carbohydrates, alcohols, amines, sulfur species, and other electroactive compounds.
Strengths – Highly selective; very low detection limits; ideal for redox-active analytes.
Limitations – Sensitive to contamination and mobile phase purity; requires electrode maintenance.
Mass Spectrometer (IC-MS):
Working Principle – Ionizes analytes and separates them based on mass-to-charge ratio.
Application – Speciation, isotope ratio, structural confirmation, and trace analysis in complex matrices.
Strengths – High selectivity and sensitivity; provides structural and elemental information.
Limitations – Requires volatile eluents or interfaces; high cost and complexity.
Refractive Index Detector (rare in IC):
Working Principle – Measures differences in refractive index between analyte and mobile phase.
Application – Occasionally used for non-ionic species or organics in hybrid IC systems.
Strengths – Non-destructive; universal response.
Limitations – Low sensitivity; unstable with gradients; rarely used in standard IC workflows.
IMPACT ON PERFORMANCE
Sensitivity and Detection Limits – Conductivity and electrochemical detectors are well suited for trace analysis of ionic and redox-active species.
Selectivity – Conductivity enables general ionic profiling; electrochemical and UV/Vis detectors provide compound-specific response.
Quantitative Accuracy – Suppressed conductivity and UV absorbance offer good linearity and reproducibility for quantitation.
Identification and Confirmation – MS and UV/Vis (with PDA) provide additional molecular or spectral information.
Workflow Flexibility – Many systems combine detectors (e.g., conductivity + UV or MS) for broader detection coverage in a single run.
CHALLENGES AND LIMITATIONS
Detector Specificity – Each detector type is selective for certain analyte classes; no universal option exists.
Eluent and Suppression Constraints – Detectors may require specific eluents (e.g., volatile for MS) or suppression conditions.
Baseline Instability – Temperature shifts, flow pulsation, or contamination can degrade signal quality.
Maintenance Demands – Electrochemical and MS detectors require regular cleaning, calibration, and hardware upkeep.
System Complexity and Cost – Advanced detectors increase system cost and operational complexity, especially with hyphenated configurations.