WHAT IT IS
In Liquid Chromatography (LC), the detector is the component that identifies and measures analytes as they elute from the column. LC detectors convert physical or chemical properties of analytes into electrical signals that can be recorded and quantified.
Each detector relies on a different principle – such as light absorption, fluorescence, refractive index, conductivity, or molecular mass – and is chosen based on the analyte type, required sensitivity, selectivity, and the analytical application.
HOW IT WORKS
As separated compounds exit the LC column, they pass through the detector’s flow cell or interface. The detector responds to a measurable property of the analyte and produces an electrical signal. This signal is processed by the data system and appears as peaks in the chromatogram.
Key aspects include:
Sensitivity - Ability to detect low concentrations.
Selectivity - Preference for certain compounds or functional groups.
Linearity - Range over which the detector’s response is proportional to analyte concentration.
Response Time - Speed at which the detector registers a change in analyte concentration.
Compatibility - Each detector may have specific requirements for solvents, gradients, or flow rates.
GENERAL TYPES OF LC DETECTORS
UV/Vis Absorbance Detector:
Working Principle – Measures absorption of ultraviolet or visible light by analytes, typically at 190–400 nm.
Application – Widely used for compounds with chromophores, including many pharmaceuticals and organics.
Strengths – Reliable, inexpensive, and compatible with gradient elution. PDA/DAD models provide spectral data for identification.
Limitations – Ineffective for analytes lacking UV/Vis absorbance.
Fluorescence Detector:
Working Principle – Excites compounds with specific wavelengths and measures emitted light.
Application – Trace detection of naturally fluorescent molecules or derivatized compounds.
Strengths – Extremely sensitive and selective.
Limitations – Only applicable to fluorescent species.
Refractive Index (RI) Detector:
Working Principle – Detects differences in refractive index between mobile phase and analyte.
Application – Suitable for carbohydrates, lipids, and polymers.
Strengths – Universal for many compounds; non-destructive.
Limitations – Low sensitivity; incompatible with gradient elution; sensitive to temperature fluctuations.
Conductivity Detector:
Working Principle – Measures changes in electrical conductivity of the mobile phase.
Application – Common in ion chromatography for inorganic and ionic species.
Strengths – High selectivity for charged analytes.
Limitations – Requires strictly controlled mobile phases.
Electrochemical Detector (ECD):
Working Principle – Monitors current changes from oxidation or reduction at an electrode surface.
Application – Sensitive for electroactive compounds such as neurotransmitters, vitamins, or drugs.
Strengths – Excellent selectivity and sensitivity for redox-active analytes.
Limitations – Sensitive to impurities and requires clean mobile phases.
Evaporative Light Scattering Detector (ELSD):
Working Principle – Nebulizes and evaporates the mobile phase, then detects scattered light from analyte particles.
Application – Lipids, sugars, surfactants, and non-volatile compounds without chromophores.
Strengths – Broad applicability; suitable for gradient runs.
Limitations – Less sensitive than MS; response varies between analytes.
Charged Aerosol Detector (CAD):
Working Principle – Similar to ELSD but charges aerosol particles and measures the resulting current.
Application – Pharmaceuticals, lipids, excipients.
Strengths – More uniform response across analytes than ELSD.
Limitations – Mobile phases must be volatile; higher cost.
Mass Spectrometry (LC-MS):
Working Principle – Ionizes analytes and separates them by mass-to-charge ratio (m/z).
Application – Provides both qualitative and quantitative analysis across small molecules, biomolecules, and metabolites.
Strengths – High sensitivity and selectivity; structural information; trace-level detection.
Limitations – High cost; requires volatile mobile phases and extensive maintenance.
IMPACT ON PERFORMANCE
Sensitivity and Detection Limits: Detector choice sets the lowest concentration measurable. Fluorescence, CAD, ELSD, and MS are preferred for trace analysis.
Selectivity: UV/Vis is general, while fluorescence and electrochemical detectors offer compound-specific selectivity. Chosen detector depends on analyte chemistry.
Quantitative Accuracy: Wide-linear-range detectors such as UV/Vis and RI provide strong reproducibility for quantitative work.
Data Quality and Identification: PDA detectors give spectral fingerprints; MS delivers molecular mass and structure, enabling compound confirmation.
Operational Flexibility: Non-destructive detectors (UV/Vis, RI) allow samples to be routed to multiple detectors in series.
CHALLENGES AND LIMITATIONS
Detector Specificity: No single detector is universal; choice depends on analyte properties.
Maintenance and Calibration: Detectors require cleaning, calibration, and consumable replacement (lamps, flow cells, electrodes).
Cost and Complexity: Advanced detectors such as MS and CAD are expensive and demand expertise.
Solvent and Gradient Constraints: RI cannot be used with gradients; MS requires volatile mobile phases; ECD requires ultra-pure solvents.
Baseline Stability: Temperature fluctuations, impurities, or pump noise can reduce sensitivity.