In the "precision instrument team" of high-performance liquid chromatography (HPLC) instruments, the detector is an indispensable member. It acts like a "signal converter," transforming the quantity of each component in the eluent into electrical signals. Different detectors have unique capabilities, with variations in working principles, application scopes, and detection targets.

An HPLC detector must be as sensitive as a sharp hound and "stable as a rock" with low noise (less susceptible to interference from temperature and flow rate changes). Additionally, it requires a wide linear range, good repeatability, and broad applicability to handle various detection tasks.

Classification by Measurement Principle

Optical property detectors: Detect based on the absorption, emission, and scattering of light by the measured substance.

‌Examples‌: UV-Vis, fluorescence (FLD), refractive index (RID), evaporative light-scattering (ELSD)

Electrochemical Detectors: Detect based on the electrochemical properties of the measured substance.

‌Examples‌: Amperometric, conductivity, coulometric, charged aerosol detector (CAD)

Thermal property detectors: Utilize thermal principles for detection.

‌Examples‌: Photoacoustic, thermal lens detectors

Radioactive detectors: Detect based on the radioactivity of the measured substance.

‌Examples‌: Scintillation counters, flow-through radioactivity monitors

Classification by Detector Nature

Universal detectors (bulk property detectors): Response values depend on the total change in certain physical properties of the effluent (including samples and mobile phase).

‌Examples‌: Refractive index detector, dielectric constant detector, conductivity detector, etc.

Selective detectors (solute property detectors): Response values depend on the physical or chemical characteristics of solutes in the mobile phase.

‌Examples‌: Ultraviolet-visible detector, fluorescence detector, chemiluminescence detector, mass spectrometer detector, amperometric detector, etc.

Classification by Signal Nature

Concentration-sensitive detectors: Response values are proportional to the concentration of solutes in the mobile phase, measuring instantaneous changes in solute concentration in the mobile phase.

Most commonly used liquid-phase detectors belong to this category, such as the ultraviolet-visible detector.

Mass-sensitive detectors: Response values are proportional to the mass of the substance passing through the detector per unit time, i.e., proportional to mass flow rate.

‌Examples‌: Coulometric detector.

In addition to the above classification methods, liquid-phase detectors can be divided into destructive and non-destructive detectors based on whether the sample is altered. Destructive detectors cannot be used in preparative chromatography.

‌Key Detector Types

1. Ultraviolet-Visible Detector (UV-Vis)

· Principle:

Based on the Lambert-Beer law, the content of sample components is determined by detecting changes in their absorption intensity of ultraviolet or visible light.

· Features:

High sensitivity and a wide linear range;

Insensitive to changes in flow rate and temperature, suitable for gradient elution separation;

Requires samples to have ultraviolet-visible absorption, and the mobile phase must have no absorption at the detection wavelength.

· Applicable objects:

The UV-Vis detector is the most widely used detector in HPLC, accounting for approximately 70% of usage. It is applicable for analyzing substances with ultraviolet absorption, such as proteins, nucleic acids, drugs, and aromatic compounds.

2. Photodiode Array Detector (PDA/DAD)

· Principle:

An ultraviolet detector based on flat-field concave holographic grating + photodiode sensor array technology. Its theoretical principle is consistent with the ultraviolet-visible detector, following the Lambert-Beer law.

· Features:

Can obtain chromatograms at multiple wavelengths simultaneously, allowing calculation of relative absorption ratios at different wavelengths;

Can real-time record absorption spectra at specified positions of each chromatographic peak during separation and calculate the maximum absorption wavelength;

Can perform point-by-point spectral scanning during operation, generating 3D graphs with time-wavelength-absorbance coordinates to intuitively display component separation and ultraviolet-visible spectra.

· Applicable objects:

Analysis of complex mixtures, compounds requiring multi-wavelength detection or spectral verification (e.g., multi-component drugs, natural products).

3. Fluorescence Detector (FLD)

· Principle:

Compounds emit fluorescence with a longer wavelength than the excitation light after being excited by ultraviolet light. The fluorescence intensity is proportional to the concentration of the substance solution.

· Features:

Extremely high sensitivity and good selectivity, with sensitivity approximately two orders of magnitude higher than that of the UV-Vis detector;

A relatively wide linear range, around 10⁴–10⁵;

Suitable for gradient elution, with minimal interference from external conditions.

· Applicable objects:

Naturally fluorescent substances such as organic amines, vitamins, and hormones, or derivatized compounds.

4. Refractive Index Detector (RID)

· Principle:

Determines solute concentration by continuously measuring changes in the refractive index of the chromatographic column effluent. The refractive index of a solution is the sum of the refractive indices of the solvent and solute multiplied by their respective molar concentrations. The difference in refractive index between the mobile phase containing the sample and the pure mobile phase indicates the sample concentration in the mobile phase.

· Features:

High universality (applicable to all substances), simple structure, and easy operation;

Sensitive to temperature, only supports isocratic elution, and cannot be used for gradient analysis.

· Applicable objects:

Compounds without ultraviolet absorption, such as carbohydrates.

5. Evaporative Light-Scattering Detector (ELSD)

· Principle:

First, the column eluent is nebulized into an aerosol. Solvents are evaporated in a heated drift tube, and the remaining non-volatile solute particles are detected in a light-scattering cell.

· Features:

Insensitive to the composition of the mobile phase, suitable for gradient elution;

Almost identical response to various substances, simplifying concentration determination;

Higher detection sensitivity than low-wavelength UV detectors and refractive index detectors;

Suitable for gradient elution but requires the mobile phase to be volatile solvents (e.g., acetonitrile, methanol).

· Applicable objects:

Substances without ultraviolet absorption and difficult to volatilize, such as phospholipids, saponins, alkaloids, and steroids.

6. Mass Spectrometer Detector (MS)

· Principle:

Substances are ionized and separated by mass-to-charge ratio (m/z), providing molecular weight and structural information.

· Features:

High sensitivity and selectivity, supporting simultaneous analysis of multiple components;

High cost, requiring complex operation and maintenance.

· Applicable objects:

Trace analysis and complex matrix samples (e.g., metabolomics, environmental pollutants).

7. Other Detectors

· Charged Aerosol Detector (CAD):

Based on the principle of aerosol charge changes, it has high sensitivity and is suitable for non-volatile/semi-volatile substances (e.g., carbohydrates, lipids, polymers) without the need for ultraviolet or fluorescent groups.

· Chemiluminescence Detector:

Based on the principle that certain substances undergo chemical reactions at room temperature to generate excited reaction intermediates or products. When they return to the ground state from the excited state, photons are emitted. It has a simple structure and high sensitivity.