Detection is part of the separation design
A detector does not report everything leaving a column or capillary equally. It responds to a property—such as absorbance, emitted light, conductivity, electrochemical activity, refractive index, aerosol mass, or ion mass-to-charge ratio. Response selectivity, calibration sensitivity, noise, acquisition rate, dynamic range, cell geometry, and interface requirements determine which peaks are visible and what evidence they provide.
The separation and detector should be designed together. A background electrolyte ideal for electrophoresis may be incompatible with an ion source. An LC gradient can change optical background or ionization. A compact detector may trade away path length, spectral range, collection efficiency, or alignment stability.
Sensitivity is not a detection limit
In metrological usage, sensitivity describes how much the measured response changes when the relevant analyte quantity changes—the slope of a calibration relationship under stated conditions. A method can have high sensitivity yet still suffer a poor detection limit if variability or background is large.
The limit of detection concerns the smallest signal or quantity that can be detected with a stated decision rule and error probabilities. The limit of quantification concerns the lowest level that can be quantified with fit-for-purpose performance. Neither is a timeless property of an instrument: matrix, preparation, calibration, blanks, and data treatment matter.
Common detector families
- Absorbance: responds when an analyte absorbs at the selected wavelength; molar absorptivity, path length, and background determine performance.
- Fluorescence: can be highly sensitive and selective, but requires native fluorescence or a suitable label and can be affected by quenching or photobleaching.
- Conductivity: responds to ionic conductivity and suits compact systems; background-electrolyte design is critical.
- Electrochemical: responds to electroactive species at an electrode under controlled conditions and supports very small detection cells.
- Flame ionization: provides a broad carbon-dependent response for many GC analytes, consumes the effluent, and is not universal.
- Aerosol and evaporative LC detectors: respond to nonvolatile material remaining after mobile-phase removal; volatility and mobile-phase composition constrain them.
- Mass spectrometry: measures gas-phase ions by mass-to-charge ratio and may add isotope or fragmentation evidence, depending on the experiment and instrument.
Some detectors leave most of the effluent available for another detector or collection; others consume or transform it. This destructive/nondestructive distinction matters when detectors are placed in series or material must be recovered.
Hyphenated methods
CE–MS, LC–MS, and GC–MS are not simple sums of two instruments. Flow, pressure, voltage, gas, solvent, ionization, transfer efficiency, and time alignment must work as a system. Interfaces and connecting volumes can introduce dilution, extra-column dispersion, transfer loss, adsorption, ion suppression or enhancement, and time offsets.
Interpretation traps
- a steep calibration slope does not compensate for poor selectivity or unstable background;
- spectral or mass-library matches are evidence, not automatic identity;
- detector response factors can differ substantially among analytes;
- a low detection limit in a standard may not describe a real matrix;
- sampling rate must be adequate for narrow peaks;
- compactness does not establish robustness, calibration stability, or field readiness.
A useful design question
Instead of asking which detector is “best,” ask what evidence is required for the decision: presence, concentration, identity, structure, elemental composition, spatial distribution, or real-time change. That answer should shape both the separation and the detector.