Abstract: This article provides a detailed introduction to five commonly used detectors in liquid chromatography – UV, fluorescence, evaporative light scattering, refractive index, and diode array detectors. It not only explains their working principles and suitable samples but also offers key selection criteria, practical precautions, and common troubleshooting solutions to help users select the right detector and solve real problems efficiently.
I. Detailed Introduction to Core Detectors and Selection Guide
In liquid chromatography analysis, the choice of detector directly determines the success or failure of the analysis. Below is an in‑depth comparison of the five core detectors.
Suitable samples: Most organic compounds that absorb in the UV‑Vis region, such as aromatic compounds and those containing double bonds or carbonyl groups. It is the mainstream detector in pharmaceutical analysis and content determination.
Reasons for selection: The most widely used detector (about 80% of applications); high sensitivity (up to 10⁻⁹ g/mL); insensitive to flow rate and temperature changes, providing good stability; excellent repeatability.
Suitable samples: Substances that are naturally fluorescent or can be derivatised to become fluorescent. Typical applications include aflatoxins, pesticide residues (e.g., polycyclic aromatic hydrocarbons), amino acids, vitamins, etc.
Reasons for selection: Extremely high sensitivity and selectivity, far exceeding UV detectors, especially suitable for trace component analysis in complex matrices.
Suitable samples: Any sample that is less volatile than the mobile phase, especially non‑UV absorbing substances such as carbohydrates, lipids, polymers, surfactants, and some traditional Chinese medicine components.
Reasons for selection: Truly universal detector; compatible with gradient elution, overcoming a major limitation of refractive index detectors.
Suitable samples: Substances that differ in refractive index from the mobile phase; widely used in sugar analysis (e.g., sucrose, glucose), as well as for polymers and oils.
Reasons for selection: Detects any compound with a different refractive index; however, sensitivity is relatively low, and it cannot be used with gradient elution.
Suitable samples: Same as UV detector, but with more powerful capabilities.
Reasons for selection: Simultaneously scans the entire UV‑Vis region, providing peak purity assessment and spectral library searching – a powerful tool for method development and substance identification.
II. Key Precautions and High‑Frequency Problem Troubleshooting Guide
Proper use of the detector is essential for obtaining reliable data. Below are core precautions and common problem solutions for each detector.
Key precautions:
Solvent cutoff wavelength: The detection wavelength must be greater than the UV cutoff wavelength of the mobile phase (e.g., methanol 205 nm, acetonitrile 190 nm).
Deuterium lamp lifetime: Keep track of lamp usage time; replace when energy is insufficient.
Common problems:
Question: High baseline noise?
Answer: Check whether the mobile phase is thoroughly degassed, whether there are bubbles in the flow cell, and whether the solvent purity meets requirements.
Question: Ghost peaks appear?
Answer: Usually caused by mobile phase contamination or system residues; flush the system and use high‑purity reagents.
Key precautions:
Fluorescence quenching: Avoid solvents containing halogens or heavy metal ions, as they reduce fluorescence intensity.
Wavelength setting: Optimising excitation and emission wavelengths is critical for achieving high sensitivity.
Common problems:
Question: Weak or no signal?
Answer: First confirm that the wavelength settings are correct; then check whether the sample is intrinsically non‑fluorescent or has undergone quenching.
Key precautions (very important):
Volatile mobile phase: Never use non‑volatile buffer salts (e.g., phosphate, sulfate) – they will severely clog and damage the drift tube. Only volatile additives such as ammonium formate or ammonium acetate are allowed.
Gas and temperature: Ensure a pure and stable nitrogen supply, and optimise the drift tube temperature.
Common problems:
Question: Persistent baseline drift or high noise?
Answer: Check whether the gas pressure is stable, whether the mobile phase contains non‑volatile impurities, and whether the drift tube temperature is set appropriately.
Question: Instrument error, high pressure?
Answer: Stop immediately! This is very likely caused by salt crystallisation blocking the drift tube; professional cleaning is required.
Key precautions:
Constant temperature and flow: Extremely sensitive to temperature fluctuations; must be thoroughly preheated (often >1 hour) and the ambient temperature kept constant.
Gradient not allowed: Can only be used with isocratic elution.
Common problems:
Question: Baseline never stabilises?
Answer: Ensure the system has been thoroughly preheated; check that the column and detector temperatures are consistent and stable.
III. How to Quickly Choose a Detector?
Sample has UV absorbance → UV detector is the first choice.
Trace analysis and the sample fluoresces → choose fluorescence detector.
Sample has no UV absorbance (e.g., sugars, lipids) and gradient elution is required → choose evaporative light scattering detector.
Sample has no UV absorbance, budget is limited, and isocratic analysis is sufficient → consider refractive index detector.
Method development or peak purity assessment is needed → choose diode array detector.
IV. Comparison of Five Common Liquid Chromatography Detectors
Below is a comparison table of five commonly used liquid chromatography detectors, covering detection principles, suitable sample types, sensitivity, advantages, disadvantages, and typical applications.
|
Detector Type |
Detection Principle |
Suitable Sample Types |
Sensitivity |
Advantages |
Disadvantages |
Typical Applications |
|
UV Detector (UV) |
Based on absorption of UV light (190‑400 nm), following the Lambert‑Beer law. |
Compounds with UV‑absorbing groups (e.g., conjugated double bonds, aromatic rings). |
High (ng/mL level), most versatile. |
Wide linear range, good stability, insensitive to flow rate and temperature, moderate price. |
Cannot detect non‑UV‑absorbing substances (e.g., sugars, lipids, some organic acids); requires high solvent purity. |
Determination of active pharmaceutical ingredients, food additive testing, environmental pollutant analysis. |
|
Fluorescence Detector (FLD) |
Substance emits longer‑wavelength fluorescence after excitation; intensity proportional to concentration. |
Naturally fluorescent or derivatisable compounds (e.g., polycyclic aromatic hydrocarbons, amino acids, vitamins). |
Very high (pg/mL level), 2‑3 orders of magnitude higher than UV. |
High selectivity, extremely high sensitivity, suitable for trace analysis. |
Narrow applicability; most substances are not fluorescent and require derivatisation; fluorescence affected by solvents, pH, etc. |
Aflatoxin detection, trace PAH analysis, amino acid analysis. |
|
Evaporative Light Scattering Detector (ELSD) |
Aerosolisation and solvent evaporation; non‑volatile sample particles scatter laser light; scattered light intensity proportional to sample mass. |
Non‑UV‑absorbing or non-fluorescent substances (e.g., sugars, lipids, polymers, surfactants). |
Moderate (sub‑μg/mL level), depends on sample volatility. |
Universal; compatible with gradient elution, overcoming a major limitation of RID. |
Lower sensitivity than UV/FLD; response non‑linear, requires special data processing; consumes carrier gas. |
Carbohydrate analysis, natural product detection, oil and phospholipid analysis, polymer molecular weight distribution. |
|
Refractive Index Detector (RID) |
Continuously measures the difference in refractive index between the reference and sample cells. |
Universal substances lacking UV absorbance or fluorescence (especially polymers, sugars, alcohols). |
Low (μg/mL level), highly sensitive to temperature and pressure fluctuations. |
Universal; response proportional to change in refractive index, no derivatisation needed. |
Low sensitivity; cannot be used with gradient elution (baseline drifts severely); very sensitive to temperature and flow rate. |
Polymer GPC molecular weight determination, sugar analysis, oligomer detection. |
|
Diode Array Detector (DAD) |
Based on UV absorbance, but uses a grating and diode array to simultaneously acquire full‑spectrum information (190‑800 nm). |
Same as UV detector, but provides 3D (time‑wavelength‑absorbance) data. |
High (comparable to UV). |
Simultaneously acquires chromatograms at multiple wavelengths; supports peak purity assessment, spectral library searching, stopped‑flow scanning; excellent for method development. |
Higher instrument cost and data processing complexity than a conventional UV detector; sensitivity sometimes slightly lower than fixed‑wavelength UV. |
Peak purity checking for complex samples, unknown identification, metabolomics research, multi‑residue pesticide analysis. |
Supplementary notes:
The UV detector and diode array detector share the same principle; the latter is a “full‑spectrum upgrade” of the former, suitable for method development and complex sample analysis.
If the sample has neither UV absorbance nor fluorescence and gradient elution is required, the evaporative light scattering detector is the first choice. If gradient is not needed and the concentration is relatively high, the refractive index detector can be considered.
The choice of detector should be made based on sample properties, sensitivity requirements, and the analytical method (isocratic/gradient).
