detectors used in HPLC


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DETECTORS (used in HPLC & GC) Manish kumar


Contents Detector characteristics & classification GC detector HPLC detector

Detector classification:

Detector classification Universal They respond to everything eluting from the column TCD Selective They may be element selective, structure/functional group selective or selective to other properties FID (very broad selectivity) ECD PID Specific They are so selective to distinguish particular structures or elements NPD FPD

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Destructive Common destructive: FID NPD FPD Non-Destructive Common non-destructive: TCD PID ECD D etectors classification

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Detector Response Ch aracteristics Sensitivity (or Response Factor RF) Detector efficiency to convert the sample in an electrical signal Linear Dynamic Range Range of sample concentration for which the detector can provide a detectable signal variation with analyte amount Selectivity The ratio of the detector sensitivities of a given compound over a potentially interfering compound Minimum Detectability Minimum amount of sample for which the peak height is three times the noise (S/N=3 )

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Detectors dynamic range 10 -15 ppt fg 10 -12 ppb pg 10 -9 ppm ng 10 -6 0.1% ug 10 -3 GRAMS 100% mg TCD FID ECD PID NPD (N) NPD (P) FPD (S) FPD (P)

GC Detector:

GC Detector Flame ionization detector Nitrogen phosphorus detector Electron capture detector Thermal conductivity detector Flame photometric detector Photoionization detector

Flame ionization detector:

Flame ionization detector Selective response (broad range) Ionization detection Destructive


FID Hydrogen is mixed with gas stream at bottom of jet and air or oxygen is supplied axially around the jet Hydrogen flame burns at the tip, which also functions as anode and it is electrically insulated from the body Collector electrode is above the burner tip


FID Principle of operation Combustion of organic compounds in a oxidizing flame CH + O CHO + + e - Electric field between the jet and the collector electrode Voltage -300V Collection of the ions generated into the flame A good combustion step is the prevailing factor to get the best performances

FID characteristic:

FID characteristic Minimum detectability: 10 -11 gm (50ppb) Response: organic carbon containing compound Temp limit: 400°C Gases: carrier gas: 1 ml/min makeup gas: 30ml/min hydrogen gas: 30ml/min air: 300ml/min

Nitrogen phosphorus detector:

Nitrogen phosphorus detector Specific detector Ionization detector Destructive


NPD Show enhance sensitivity to nitrogen & phosphorus compound Main difference is the rubidium silicate bead which is heated to emit thermionic electron. Hydrogen flow much lower than in FID- too low to sustain flame at jet tip. Fuel, carrier, make up, oxidizer partially combust around the bead heating filament. Partially combust N2 & P compound adsorb on the bead surface. This reduce the beads ‘work function’,increase the electron density at a given temp.


NPD Increased electron density= increased current= detector response Performance decrease with time. Moisture form during combustion hydrolyses the alkali silicate bead to alkali hydroxide and silica. Rubidium hydroxide is volatile and constantly lost from the bead.

NPD characteristic:

NPD characteristic Minimum detectability: 10 -11 gm Response: selective to nitrogen & phosphorus containing compound Temp limit: 260-350°C Gases: carrier gas: 2 ml/min makeup gas: 15ml/min hydrogen gas: 4ml/min air: 60ml/min

Electron capture detector:

Electron capture detector Non- destructive Selective detector Measure electrical conductivity of the effluent after exposure to ionizing radiation. Sensitive to ‘electron capturing species’- halogens. Radioactive 63Ni foil emit low energy electron (beta particle) These β particle collide with carrier to produce high energy electron


ECD This establishes a high current flow between detector body (foil) and centrally located collector. Halogenated analyte elute and ‘capture’ some of the electrons Negative ions formed are low energy and are not collected on the electrode. Reduction in the current flow. Characteristic: Minimum detectability : Response: selective to halogen containing compound

Thermal conductivity detector:

Thermal conductivity detector universal response non-destructive TCD detector consists of an electrically-heated wire. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when analyte molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance.


TCD classic thermal conductivity detector design utilizing a wheatstone bridge circuit. Characteristic: Minimum detectability: 10 -9 gm (10ppm) Response : all compound

Flame photometric detector:

Flame photometric detector Specific for phosphorus and sulfur containing compound. Compounds are burn in a hydrogen-air flame same as in FID. After burning sulfur and phosphorus produce light emitting species. A monochromatic filter allow only one of the wavelength to pass & photomultiplier tube use to measure the amount of incident light & a signal is generated. Minimum detectability:

Photoionization detector:

Photoionization detector Compound eluting into the cell are bombarded with high energy photon emitted from a lamp. Compounds with ionization potential below the photon energy are ionized. Resulting ions attracted towards opposite electrode, measured and signal is generated.


PID Minimum detectability: 25-50 pg (aromatics) Response: depend on lamp characteristic, conventionally used for aromatics.

HPLC Detectors:

HPLC Detectors UV/UV-VIS PDA (Photodiode array) Fluorescence detector Refractive Index detector Evaporative light scattering detector

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UV detector Fixed wavelength detector Ex: mercury vapor lamp(253.7nm) 24 A = e · C · l = – log ( E out / E in ) ( A : Absorbance) l C : Concentration Cell E in E out A C D 2 / W Lamps

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Disperse UV detector

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PDA detector multi wavelength detector. Deuterium lamp

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Fluorescence detector Use for trace analysis + h n 1 * Excitation wavelength Emission wavelength Fluorescence * h n 2 + h n 1 h n 2 Excitation state Quasi-excitation state Ground state

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Fluorescent Detector

Refractive index detector:

Refractive index detector Useful in detection of analyte which are nonionic,nonfluorescence & cant adsorb UV. Polymer detection. Non-destructive Universal detector

Refractive index detector:

Refractive index detector 1.Mobile phase 2.Heat exchanger 3.Sample cell 4.Reference cell 5.Sample photomultiplier 6.Beam splitter 7.Lens & slit 8.Refrence photomultiplier 9.Waste container 10.Recycle valve 11.Purge valve 12.lamp


RID When beam light passes from one media to other both the wave velosity and direction change. RI detection principle is based on the comparison of RI of sample with the RI of reference. Change in light intensity is directly proportional to change in refractive index. Higher sample concentration higher refraction.

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Evaporative light scattering detector The column effluent is continuously evaporated and the light scattering of the resulting aerosol is measured. Only used in LC


ELSD Nebulization of the effluent Evaporation of the effluent Detection Detection range: nanogram range Intensity of light scattered from solid suspended particle depend on their particle size.

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