Contents:• Definition• Ideal properties of a detector• Detectors used in GC- concentration dependent detectors- mass flow dependent detectors• Detectors used in HPLC- selective detectors- universal detectors
Detectors:• The detector senses the presence of the individualcomponents as they leave(elute) the column. The detector output after amplification is traced on a recorder.• The duration of the intervals is usually a single second or evenless than that.• Hence the detector is considered to be the brain of theinstrument.• The detector converts a change in effluent into an electricsignal that is recorded by data system
Ideal properties of a detector:The detectors used in both GC and HPLC should havefollowing ideal properties:1) High sensitivity.2) Good stability and reproducibility.3) A linear response to solutes.4) Negligible base line noise.5) Should be inexpensive.6) Capable of providing information on the identity of solute.7) A temperature range from room temperature to at least4000c.8) A short response time independent of flow rate.9) High reliability and ease of operation.10)The detector should be non-destructive.11)Response independent of mobile phase composition.
Detectors used in GC:Detection devices for a GC must respond rapidly andreproducibility to the low concentrations of the solutes emittedfrom the column.Concentration dependent detectors:- Thermal conductivity detector(TCD)- Electron capture detector(ECD)- Argon ionization detector- Helium ionization detectorMass flow dependent detectors:- Flame ionization detector(FID)- Nitrogen phosphorous detector(NPD)- Flame photometric detector(FPD)
• In addition to these other detectors used are:- Thermionic detectors- Photoionization detectors- Atomic emission detectors- Sulfur chemiluminescence detector• The most widely used detectors are TCD, FID, ECD.Thermal conductivity detector(TCD):• TCD was one of the old detectors for GC, is still widely in use.• It is also known as katharometer and hot wire detector.• Principle in TCD is change in thermal conductivity of gasstream.• Thermal conductivity of most of the samples is lesser thanmost commonly used carrier gases like H, and He.
• The thermal conductivity of He is 6-10 time greater than thatof the most organic compounds.• It is simple, inexpensive, non-selective, accurate, andnon-destructive type.Thermal conductivity detector
• Since the detector response depends upon the difference inthermal conduction between sample and carrier gas, a largedifference is essential.• An increase in temperature of the detector causes a change inthe resistance of thermistor and this resistance gives ameasure a measure of thermal conductivity of gas.• TCD consists of a temperature controlled metal block in whichtwo cylindrical chambers are present, which consists of twofilaments made up of platinum or tungsten.• Both the filaments are connected to the arms of Wheatstonebridge arrangement.• Resistance of filaments are constant as only the carrier gas ispassed through them, once the effluent passes through themthe change in conductivity is seen and is recorded.
Flame ionization detector(FID):• FID is the most widely used and generally applicable detectorfor GC.• With an FID the effluent from the column is directed into smallair-hydrogen flame.• Most of the organic compounds produce ions and electronswhen pyrolyzed at the temperature of air-hydrogen flame.• Detection invovles monitoring the current produced bycollecting this charge carriers.• A few hundred volts applied between the burner tip and acollector electrode located above the flame causes the ionsand electrons to move towards the collector.• The resulting current is then measured with high-impedancepicoammeter.
• The ionization of carbon compounds in the FID is not fullyunderstood, although the number of ions produced is roughlyproportional to the number of reduced carbon atoms in flame.• Because the FID responds to the number of carbon atomsentering the detector per unit time.• The detector is insensitive towards non-combustible gasessuch as H2O, CO2, SO2, CO.• Functional groups like carbonyl, alcohol, halogen, and amineyield fewer ions or none at all in a flame.• These properties make the FID a most useful general detectorfor the analysis of most organic samples, including thosecontaminated with water and oxides of nitrogen and sulfur.
• The FID exhibits a highsensitivity (10-13g/s),large linear responserange(107), and low noise.• It is generally rugged andeasy to useDisadvantages:• It destroys sample.• It requires additionalGases and controllers.
Electron-capture detector(ECD):• It is most widely used detector for environmental samplesbecause it selectively responds to halogen containing organiccompounds.e.g.: pesticides, polychlorinated biphenyls• In ECD, the sample eluate from a column is passed over aradioactive β emitter, usually nickel-63.• The ECD is selective in its response.• An electron from the emitter causes ionization of carrier gasand the production burst of electrons.• In the absence of organic species, a constant standing currentbetween a pair of electrodes results from this ionization.• In the presence of organic molecules containingelectronegative functional groups that tends to captureelectrons, hence the current decreases.
• Compounds such as halogens, peroxides, quinones and nitrogroups are detected with high sensitivity.• The detector is insensitive to functional groups such asamines, alcohols and hydrocarbons.• An important application of ECD is for the detection andquantitative determination of chlorinated insecticides.• The advantage of ECD is that it does not alter the sample.
Flame photometric detector(FPD):• FPD is widely used in the analysis of air and water pollutants,pesticides, coal hydrogenation products• It is selective towards compounds containing sulfur andphosphorous.• In this detector the eluent is passed in to a low temperaturehydrogen-air flame, which converts part of phosphorous in toHPO species that emits bands of radiation at about 510 and 526nm.• It also converts sulfur in to S2 which emits a band at 394nm.• Suitable filters are used to isolate appropriate bands and theirintensity is recorded photometrically.• FPD is used to detect elements like halogens, nitrogen andmetals like tin, chromium, selenium and germanium.
Photoionization detector:• In this the molecules eluting from GC column are photoionizedby ultra-violet radiation from a 10.2 eV hydrogen or a 11.7 eVargon lamp.• This source ionizes species with an ionization potential belowthe lamp energy.• compounds with a higher ionization potential do not absorb theenergy and thus they are not detected.• The ions and electrons produced by Photoionization are thencollected at pair of biased electrode.• The detector is most sensitive for aromatic hydrocarbons andorganosulfur or organophosphorous.
Atomic emission detector(AED):• In AED, the effluent from GC column is introduced in to amicrowave induced plasma(MIP), an inductively coupledplasma(ICP) or a direct current plasma.• Among these MIP has been most widely used and is availablecommercially.• The MIP is used in conjunction with diode array or chargecoupled device atomic emission spectrometer.• The plasma is sufficiently energetic to atomize all ofelements in a sample and to excite their characteristic atomicemission spectra.• Hence, the AED is an element selective detector.
• The sample in this case consists of gasoline containing smallconcentrations of methyl tertiary butyl ether(MTBE), as well asseveral aliphatic alcohols in low concentrations.• By using oxygen(777nm) rather than carbon(198nm) in lampwe can obtain a chromatogram peaks for alcohols and forMTBE which are readily identifiable.
Mass spectrometry detectors:• one of the most powerful detectors for GC is the massspectrometer.• The combination of GC with Mass spectrometry is known asGC-MS.• Currently, nearly fifty instrument companies offer GC-MSequipment.• Currently, capillary columns are invariably used in GC-MSinstruments and no other separators are needed.• Thermal degradation of components can be difficulty in GC-MS, lowering the temperature can minimize degradation.• However the mass spectrometer can be used to identifydecomposition products, which can lead to chromatographicmodifications that solve degradation problem.
• The most common ion sources used in GC-MS are electron-impact ionization and chemical ionization.• The most common mass analyzers are quadrapole and ion-trap analyzers.• In GC-MS, the mass spectrometer scans the massesrepetitively during a chromatographic experiment.• GC-MS instruments have been used for identification ofcomponents present in natural and biological systems.e.g.: water pollutants, breath components and drugmetabolites.• MS can also be used to obtain information about incompletelyseparated components.• It is extremely powerful tool for identifying components in themixtures.
• Several other types of GC detectors are useful for detection ofspecific components, they are thermionic detectors and sulfurchemiluminescence detectors.• Thermionic detector is selective towards organic compoundscontaining phosphorous and nitrogen.• The response to a phosphorous atom is approximately tentimes greater than a nitrogen atom and 104-106 times largerthan to a carbon atom.• Sulfur chemiluminiscence detector is specific to the sulfuratom.• It is based on the reaction between certain sulfur compoundsand ozone.• This detector has proven particularly useful for thedetermination of the pollutants such as mercaptans
Detectors used in HPLC:• The function of the detector used in HPLC is to monitor themobile phase it emerges from column.• The detectors used in HPLC are of majorly two types:selective detectors(solute property):• Absorbance detectors• Fluorescence detectors• Electrochemical detectors• Mass spectrometric detectorsUniversal detectors(bulk property):• Refractive index detectors• Evaporative light scattering detectors
UV-Visible Absorption Detectors:• In this a Z-shape flow through cell for absorptionmeasurements on eluents from a chromatographic column.• To minimize extracolumn band broadening, the volume ofsuch a cell should be kept as small as possible, typically 1-10µl• Many absorption detectors are double-beam devices in whichone beam passes through eluent cell and other beam isreference beam.• Matched photoelectric detectors are used to compare theintensities of the two beams.• Single beam instruments are also used, were the intensitymeasurements of the solvent system are stored in a computermemory and used for the calculation of absorbance.
UV absorption detectors with filters:• These are used earlier with a mercury lamp as the source.• Most commonly, the intense line at 254nm was isolated byfilters, by substitution of filters lines at 250, 313, 334, and365nm can also be used.• Because of this reason, these type detectors are restricted asmost of the organic and inorganic species exhibit broadabsorption bands.UV absorption detectors with scanning capabilities:• A detector with scanning spectrophotometer with gratingoptics is most widely used.• Several operational modes can be chosen with thesedetectors.
• For example, the entire chromatogram can be obtained atsingle wavelength or different wavelengths can be chosen fora single peak.• When entire spectra are desired for identification purposesthe flow of eluent can be stopped for a sufficient period topermit scanning the wavelength region of interest.
Fluorescence detectors:• Fluorescence detectors for HPLC are similar in design to thefluorometers and spectrofluorometers.• In most, fluorescence is observed by a photoelectrictransducer located at 900 excitation of beam.• The simplest detectors use mercury excitation source and oneor more filters to isolate a band of emitted radiation.• More sophisticated instruments use a xenon source and agrating monochromator to isolate the fluorescence radiation.• Laser-induced fluorescence is also used because of itssensitivity and selectivity.• An inherent advantage of these detectors is their highsensitivity, hence used in LC for separation and determinationof components of samples that fluoresce.
• Fluorescent compounds can be obtained by treating withreagents that form fluorescent derivatives.• For example, dansylchloride forms fluorescent compoundswith primary amines, secondary amines, amino acids andphenols, hence widely used for detection of amino acids inprotein hydrolyzates.Fluorescent detector
Refractive-index detectors:• The ability of a compound or a solvent to deflect light providesa way to detect it.• RI is a measure of molecule’s ability to deflect light in aflowing mobile phase in a flow cell relative to a static mobilephase contained in a reference cell.• The amount of deflection is proportional to concentration.• The RI detector is considered as a universal detector but it isnot very sensitive.• To achieve high sensitivity, in practice solvents are selectedthat have a very high or very low refraction index.• The detection limit is in the range of 10-6-10-8 g/ml.
Electrochemical detectors:• This is based on amperometry, voltammetry, coloumetry, andconductometry.• Electron transfer processes offer highly selective andsensitive method.• Easily adaptable for use with micro columns.• As background noise is dependent on mobile phase conditions,it is difficult to utilize these detectors with gradient elutionseparations.• It is of two types:Amperometric detection:• Fixed potential is applied to the electrode (glassy carbon) anda solute which will oxidize at that potential yields an outputcurrent
Coulometric detection:• 100% of the solute species is converted, which offersadvantages of no mobile phase flow dependence on signal andabsolute quantitation through Faraday’s law.
• ELSD can outperform traditional detectors when analysing non-chromophoric samples by HPLC.• Traditional HPLC detectors such as UV and RI have limitedcapabilities.• UV and RI are not compatible with a wide range of solventsRI detection is not gradient compatible• Different analytes produce different UV responses depending ontheir extinction co-efficient.• ELSDs can detect anything that is less volatile than the mobilephase• ELSD is universal and compatible with a wide range of solventsEvaporative Light Scattering Detector:
• Three steps are involved in detection:- nebulization- mobile phase evaporation- detectionNebulization:• Column effluent passes throughnebulizer needle.• It mixes with the nitrogen gas.• Dispersion of droplets are formed.Mobile phase evaporation:• The above formed droplets arepassed through a heating zone.• Mobile phase evaporatesfrom the particles.
Detection:• Sample particles pass through an optical cell.• Sample particles interrupt laser beam and scattered light.• Photodiode detects the scattered light.• ELSD is an effective replacement or a perfect complement toexisting LC detectors.
Conclusion:Detector is the key element that is present in anydevice that is used for the identification and estimation of anycompound. It detects in a faster rate i.e. with in seconds henceit is considered as brain if the instrument. With out a detectorno one can analyze the compound. Hence, it attains such animportance in the field of analysis.
References:• Instrumental analysis by Skoog, Holler, and Crouch.• Instrumental methods of chemical analysis by B. K. Sharma.• Pharmaceutical analysis by David G. Watson.• Principles of instrumental analysis by Douglas A. Skoog andDonald M. West.• Basic gas chromatography by Harold M. McNair andJames M. Miller.