Principles and advantages and disadvantages of other commonly used detectors in liquid chromatography

Other commonly used detectors for second-hand liquid chromatographs include refractive index detector RID, evaporative light scattering detector ELSD, fluorescence detector, and electrospray detector CAD
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What are the principles and advantages and disadvantages of other commonly used detectors in liquid chromatography? The RIDRID is a universal detector that is responsive to all samples
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The principle differential detector is based on the continuous measurement of the change in refractive index between the sample flow path and the reference flow path to determine the sample content
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When light passes from one medium to another, refraction occurs due to the difference in the refractive indices of the two substances
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As long as the refractive index of the sample component and the mobile phase are different, they can be detected.
The greater the difference between the two, the higher the sensitivity.
The output of the detector is proportional to the solute concentration within a certain concentration range
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Advantages and disadvantages The advantage is that it is universal, and there is nothing that cannot be detected
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Disadvantages: low sensitivity, generally speaking, it cannot be used for the detection of related substances, and RID is generally used for content determination; another disadvantage is determined by its principle, which can only be isocratic, so there are problems in the separation of multiple components
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In addition, the equilibration time is too long, and the high environmental requirements (especially the fluctuation of room temperature) are also troublesome factors for the operator
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Evaporative Light Scattering Detector ELSD A new general purpose detector for less volatile components
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Sensitivity is generally between UV detectors and refractive index detectors
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Principle The eluent is atomized with inert gas, the mobile phase is evaporated in the heating tube (drift tube), the sample particles enter the detector and scatter the light emitted by the laser light source, and the optical signal is converted into an electrical signal and output
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The principle seems to be very high, and the sensitivity is also acceptable.
In the face of so many compounds without UV response, in theory, it should be possible to unify the rivers and lakes
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There are always things but, the next chapter reviews the pros and cons of ELSD in detail
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Advantages and disadvantages The advantage is that it is universal, whether it has UV response or not, it has a response, as long as the volatility is weaker than that of the mobile phase
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And can take gradient, can be used to separate complex mixtures
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But.
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there are many drawbacks
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An unacceptable disadvantage is that the concentration and responder are not linear, the logarithm of the two is linear
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The linearity is very poor, the linear range is not large, the correlation coefficient is very poor, and the intercept is very large
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Because of this shortcoming, I think everyone already understands that the single-point external standard method of this detector is not feasible, so this detector is generally used for the external standard two-point method (the response value of the test sample needs to be between the two reference substances, and no extension is allowed.
) to detect the content; this detector is rarely used for related substances
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Everyone should be able to understand that two reference solutions of different concentrations should be prepared for each impurity, and the response value of the test product is between the two controls (which cannot be done, the amount of impurities cannot be predicted, unlike the content).
What a frustrating thing it was
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Another disadvantage is that the mobile phase needs to be volatile, which means that there can be no non-volatile components, which means that the phosphate series cannot be used
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There is also a need to increase the gas, the operation is cumbersome, and harmful gases will be generated
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Fluorescence Detector A fluorescence detector is a highly sensitive, selective detector that detects compounds that produce fluorescence
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Some non-fluorescent substances can be chemically derivatized to generate fluorescent derivatives, and then fluorescently detected
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Its detection concentration can reach 0.
ng/ml, which is suitable for trace analysis.
Generally, the sensitivity of fluorescence detector is about an order of magnitude higher than that of UV detector, but its linear range is not as wide as that of UV detector
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In recent years, laser-induced fluorescence detectors produced by using lasers as the light source of fluorescence detectors have greatly enhanced the signal-to-noise ratio of fluorescence detection, so they have high sensitivity and are widely used in trace and ultra-trace analysis
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Principle After the compound is excited by ultraviolet light, it emits light with a longer wavelength than the excitation light, which is called fluorescence; the relationship between the fluorescence intensity (F), the excitation light intensity (I0) and the concentration of the fluorescent substance (C) is: F= .
3QKI0εCl; F=KC, Q is the quantum yield, K is the fluorescence efficiency, ε is the molar absorption coefficient, and l is the optical path length
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From the point of view of electronic transition, fluorescence refers to the transition of some electrons in the atom from the vibrational energy level in the ground state to a higher vibrational energy level after some substance absorbs light of the same frequency as its own
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The collision of electrons in the same molecule or other molecules consumes a considerable amount of energy, thereby falling to the vibrational energy level in the electronic excited state.
This form of energy transfer is called *radiative transition
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Fluorescence occurs when the vibrational energy level drops to some different energy levels in the ground state, and at the same time emits a light with a lower frequency and a longer wavelength than the original absorption
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The light absorbed by the compound is called excitation light, and the resulting fluorescence is called emission light
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The wavelength of fluorescence is always longer than the wavelength of ultraviolet light absorbed by the molecule, usually in the visible range
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The nature of fluorescence is closely related to the molecular structure, and not all molecules with different structures can emit fluorescence after being excited
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Advantages and disadvantages The advantage of fluorescence detectors is that the sensitivity is very high, which can reach the ppt level
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The disadvantage is that the scope of application is narrow, and it is only suitable for substances that can generate fluorescence after being excited, which limits the application of fluorescence detectors
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Electrospray Detector CAD A new general-purpose detector from Thermo Fisher Scientific is currently under conservation
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Sensitivity is better than ELSD
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Principle Step 1: The detector converts the analyte into solute particles
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Particle size increases with analyte content
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Step 2: The solute particle collides with the positively charged nitrogen particle, and the charge is transferred to the particle - the larger the solute particle, the more charged it is
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Step 3: The solute particles transfer their charges to the collector, and the charged amount of the solute particles is measured by a high-sensitivity electrostatic detector, and the resulting signal current is proportional to the solute content
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Advantages and disadvantages The advantages are the same as ELSD, but the sensitivity is higher
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Thermo Fisher's propaganda promotes it as godlike, but it also has buts
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The disadvantage is also the same as ELSD, with poor linearity, but better than ELSD
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The response is nonlinear over the full scale range, but the CAD is more reproducible and the response is basically linear in the small concentration range
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Although CAD changes the principle to make the particles charged to detect the collected electrical signal, so that the concentration and the responder are directly linear within a certain range
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However, the formation process of particles is a process determined by the properties of non-material itself, which means that the response is unstable
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Unlike UV detectors, UV absorption is a property of the substance itself, which does not change and has good reproducibility
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