The introduction of different types of detectors for chromatography has transformed the dimensions and the scope of chromatographic analysis in an unprecedented way. High-resolution chromatographic techniques are fascinated with detectors to increase their analytical efficacy.
A detector is a device attached as the final component of a chromatographic system. Detectors provide real-time information about separated components for identification purposes.
Working principle of detectors
Analytical components eluting out of a column reach the detector. Chromatographic detectors function by turning a physical or chemical attribute of the analytical component into a measurable electric signal. This electric signal is called detector response. The detector response is then fed into a computer software called a recorder which plots a chromatogram.
Desirable features of chromatographic detectors
Detectors used for chromatography must possess maximum or all of the following features in order to yield a desirable response:
- Fast response.
- High sensitivity: The detector must respond to a small change in eluate concentration.
- High reproducibility: The detector response recorded with the same sample over specific time intervals must be consistent.
- Predictable response and specificity.
- Linear dynamic range: The detector response should increase linearly with an increase in solute concentration.
- The detector response should be unaffected by slight changes in temperature and/or mobile flow rate.
- The detector should not lead to peak broadening.
- It should be reliable and convenient to use.
- Non-destructive: It should not degrade the solute components.
- The detector should ideally provide both a qualitative as well as a piece of quantitative information about the analytical components.
Four main types of chromatographic detectors
Detectors can be categorized into four main types:
- Universal detector: A detector that can be used for all types of solutes.
- Selective detector: A detector that functions for specific types of solutes only.
- Bulk property detector: A detector that records changes in solute and mobile phase in combination.
- Solute property detector: A detector that measures the physical/chemical properties of the analytes independent of the mobile phase.
There are many different types of detectors that support a wide range of column chromatographic techniques. Since high-performance liquid chromatography (HPLC) and gas chromatography (GC) are two principal chromatographic techniques, so we have separated different detectors based on these two techniques. A detector for HPLC may be specifically beneficial for a reverse-phase or a hydrophobic interaction mode. But all these can be successfully applied for all the different modes of HPLC and likewise for GC detection.
Detectors for HPLC
The ultraviolet (UV) detector is the most common detector that is used for HPLC. It is a bulk property detector. As its name implies, the UV detector operates in the ultraviolet (195-370)nm region of the electromagnetic spectrum. It is useful for detecting compounds that absorb UV radiations.
The sample components eluted out of the chromatographic column with the mobile phase are passed through a colorless, glass cell called the flow cell. UV irradiation of a fixed (usually 254 nm) or multi-wavelength supplied through a photodiode array (PDA) scan is provided. The sample absorbs a part of the irradiated UV light and transmits the remaining. The detector records the transmitted wavelength. The intensity of transmitted light is recorded with and without the sample components in the mobile phase solvent. Difference between the two light intensities helps determine the amount/concentration of the targeted component.
UV detector is selective in nature because it only measures ultraviolet light absorbing components. It provides a good sensitivity up to a few picograms (pg).
Refractive index detector
The refractive index detector is a universal, bulk property detector. It is perhaps the oldest detector to be used for liquid chromatography. It functions by measuring a difference in refractive index between the mobile phase with and without the sample components.
It is based on a flow cell divided into two compartments. One compartment is called the reference cell while the other is known as the sample cell. The reference cell contains a static mobile phase trapped in it. On the other hand, eluate from the HPLC column passes through the sample cell. A beam of light is passed through both the compartments of the flow cell. When this light beam enters the sample compartment, it is refracted due to the greater density of components situated in this compartment.
The different pixels of the light beam refracted from the sample compartment as opposed to the reference cell are recorded and a detector response generated. A pair of photodiodes convert this signal into a measurable voltage output.
However, the refractive index detector has a low sensitivity up to a few milligrams (mg) only. It is sensitive to small changes in temperature and pressure. It cannot be used for trace analysis.
Evaporative light scattering detector
The evaporative light scattering detector (ELSD) functions on the principle of evaporation or nebulization of the mobile phase from the eluate, prior to analysis. It is a solute property detector. It is a universal detector type that can analyze all types of solute components. The solute components however should be less volatile than the mobile phase for detection via ELSD.
During ELSD analysis, the eluate is mixed with nitrogen gas and passed through a narrow bore to form a uniform dispersion of droplets called an aerosol. Heating is provided to evaporate the solvent from the aerosol. A beam of laser light is irradiated onto the resultant particles. The particles scatter this light beam which is then recorded. This light scattering property of analytical components depends upon the mass of the analyte.
ELSD has a high sensitivity in nanograms (ng). It is 10-100 times more sensitive than a refractive index detector.
Electrochemical detectors are selective for analytical components that can give an electrochemical reaction i.e., the analyte has an ability to undergo oxidation or reduction at metal electrode surfaces.
A fixed potential difference is applied between the working electrode and the reference electrode. Sample components reaching the working electrode undergo an electrochemical reaction to produce an electric current. The current is amplified and recorded to plot a chromatographic peak.
Electrochemical detectors exhibit a high sensitivity in femtograms (fg) and a high selectivity for liquid chromatographic analysis.
Charged aerosol detector
The charged aerosol detector works similarly to ELSD. It is a solute property detector. ELSD is based on eluate nebulization with nitrogen gas to remove the mobile phase. A secondary stream of positively charged nitrogen gas is sprayed onto the resultant analyte particles. The positive charge gets transferred to the analyte components. Analyte particles in turn transfer their charge to a collector which leads to an electrical signal production, proportional to the concentration of the targeted analyte.
The charged aerosol detector exhibits a good sensitivity, higher than the sensitivity of ELSD.
A fluorescence detector when coupled with an HPLC system offers selective detection of solute components that exhibit a native fluorescence. The solute components are exposed to a high energy wavelength. The molecules undergo electronic excitation followed by de-excitation to emit light. This optical light emission is then measured and recorded.
Chiral detectors are a selective type of HPLC detectors, helpful for the detection and identification of enantiomeric components separated through chromatography. A monochromatic beam of plane-polarized light is passed through the eluate, chiral components rotate the plane of vibration of plane-polarized light. The degree of rotation is measured through a polarimeter and recorded.
The direction of rotation identifies enantiomeric form of the chiral compound i.e., its molecular structure. Likewise, the degree and extent of rotation reveal the concentration of chiral components present in the eluate.
Detectors for GC
Mass spectrometer (MS) detector
A mass spectrometer (MS) is the first and foremost choice for detection in gas chromatography. Therefore, GC is often referred to as GCMS. The mass spectrometer is coupled to a gas chromatographic column using a special device called a GCMS interface. A gaseous mobile phase called a carrier gas is employed in gas chromatography which carries volatile solute components from the column to the detector. The mass spectrometer uses coherent electric and magnetic fields for detection.
The gaseous analyte components inside the MS detector are first subjected to an electric current. The electric current splits neutral molecules into electrically charged fragments. These charged fragments are then passed through a uniform magnetic field, applied by a series of magnets. The magnetic field deflects the fragments to the walls of the detector where they are neutralized. Consequently, an electric signal is recorded. Greater the number of fragments reaching the detector, higher will be the detector response thus giving strong peaks at the chromatogram.
One advantage of GCMS analysis is the in-built library which allows not only a qualitative identification but at the same time, it ensures adequate quantification of the sample components.
The MS detector can also be used for liquid chromatographic analyses such as HPLC. In that case, the chromatographic technique is called LCMS (liquid chromatography coupled with mass spectrometric detection).
Detector sensitivity: 1-10 pg
Operating temperature: 150-300°C
Flame ionization detector
The detector operates on ionization or pyrolysis of analyte components at a high temperature. The eluate is directed onto an air-hydrogen flame. Solute components undergo chemical decomposition, releasing ions and electrons for electrical conduction. An ammeter then measures the magnitude of this current which is proportional to the sample concentration.
Detector sensitivity: 100 pg
Operating temperature: 250-450°C
Thermal conductivity detector
The thermal conductivity detector as its name suggests measures the thermal/heat conduction ability of analytes. During GC analysis, the components eluted out of the column pass over a hot tungsten filament. The difference in the thermal conductivity of the carrier gas with and without the sample helps generate a detector response.
Detector sensitivity : 1.0 nanogram (ng)
Operating temperature: 150-250°C
Electron capture detector
The electron capture detector is specifically important for the detection of analytes having electronegative atoms such as nitrogen or oxygen. Carrier gas entering the detector is first ionized by the bombardment of high-energy electrons (β-rays) emitted from a radioactive Ni63 element. Electrons released from positively charged ions are then attracted to a negatively charged electrode, and an electric current is generated.
When the carrier gas containing analyte components enters the detector, high electron affinity analytes attract/capture some of the free electrons present. It causes a fluctuation in the electric current which is consequently recorded as the detector response.
Detector sensitivity: 50 fg
Operating temperature: 300-400°C
Nitrogen phosphorus detector
The nitrogen phosphorus detector is another example of a selective GC detector. It is specifically sensitive to compounds containing nitrogen (N) or phosphorus (P) atoms. It is a sub-type of a flame ionization detector. N or P-containing solute molecules can lose their lone pair of electrons to generate ionized species under high-temperature conditions. The ionized species migrate from the flame electrode to the collector electrode and generate an electric current which is then recorded as a signal and displayed as a chromatographic peak.
Detector sensitivity: 10 pg
Operating temperature: 250-300°C
In conclusion, the availability of a wide range of detectors for the detection, identification, and quantification of a plethora of compounds has increased the value and authenticity of chromatographic analysis multifold.
For more insightful information on chromatographic efficiency, we advise you to have a look at the following two articles in our chromatographic series.
1.Magnusson, L.-E., D. S. Risley and J. A. Koropchak (2015). “Aerosol-based detectors for liquid chromatography.” Journal of Chromatography A 1421: 68-81.
2. Scott, R. P. W. (2005). Essential oils. Encyclopedia of Analytical Science (Second Edition). P. Worsfold, A. Townshend, and C. Poole. Oxford, Elsevier: 554-561.
3. Sevcik, J. (2011). Detectors in gas chromatography , Elsevier scientific.