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Detectors in Gas Chromatography

Presented by: Samiksha Sawant

M.Pharm (IP), 2nd SemGuided by: Dr. Pratima

Tatke

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Gas ChromatographyGC used for separation and analysis of volatile, gaseous

substances.

Typical uses of GC include testing the purity of a particular substance, or separating the different components of a

mixture.

In some situations, GC may help in identifying a compound.

In preparative chromatography, GC can be used to prepare pure compounds from a mixture.

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Instrumentation of GC

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Detectors

Devices used to or designed to respond to a very small quantity of sample present in the column gas

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Types of detectors

Non specificThermal

conductivityAtomic emissionphotoionization

Destructive FID, NPD,PID, CD

Non destructiveTCD,ECD

SpecificFlame ionization

Nitrogen-PhosphorousFlame photometric

Helium/argon ionizationElectron capture

chemiluminiscence

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Ideal characteristics of detectorsAdequate sensitivity. Sensitivity should lie in the

range 10-8 to 10-15 g/ml

It should respond rapidly

It should respond to a wide variety of compounds

Good stability and reproducibility

It should produce linear response to the concentration of solutes in the eluate

A temperature range from room temperature to at least 400 degree celsius

Nondestructive to the sample

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Flame Ionization detector•The FID was invented by scientist Harley and Pretorious and separately by McWilliams and Dewer.•It makes use of an oven, wherein a flame is produced by burning hydrogen gas in presence of oxygen or air.•Effluent from the column is directed into a air/hydrogen flame.•A definite potential difference is maintained between the two electrodes with the help of a series of batteries.•Amplifier and recorder record chromatograms.

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Working • A portion of eluate coming from

the column is directed into the furnace through the wire loop.

• Solvent evaporates and organic compounds pyrolyses and forms ions.

• These ions are attracted towards the respective electrodes.

• This changes the potential difference between the electrodes and hence the current in the circuit.

• As electrical resistance of flame is high and resulting current is small, an electrometer is employed.

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Minute amount of solute can be detectedGives linear responseAs it responds to the number of C-atoms entering the detector per unit time, it is mass sensitive rather than concentration sensitiveInsensitive towards water, CO2, SO2, NO2

More complicated and more expensive Destructive of the sampleFunctionals groups like carbonyl, alcohol, halogen, amine are not detected

Advantages

Disadvantages

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Flame photometric detector• It is selective towards compounds containing

sulphur and phosphorous• The eluent is passed into the flame, which converts

phosphorous to HPO and sulphur into S2

• These excited species emit light as they luminescene in the flame.

• Compounds containing phosphorus are detectable with the 526nm filter, which is yellow on one side.

• The 394nm filter (blue on one side) allows detection of sulfur-containing compounds.

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• a combustion chamber to house the flame, • gas lines for hydrogen (fuel) and air (oxidant),• an exhaust chimney to remove combustion products, • thermal (bandpass) filter to isolate only the visible and UV

radiation emitted by the flame.

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Thermal conductivity detector• It works on the principle of

wheatstone’s bridge.• Out of four resistances in the

circuit, the magnitude of three resistances remains constant.

• But that of fourth resistance varies as per change in the temperature.

• This change is because of the difference in the capacity of the solute and the carrier gas to absorb heat (thermal conductivity differences).

• The change in the temperature changes the resistance and hence the current in circuit.

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• Mixture is passed over coil B, whereas only carrier gas passes through coil A.

• Both have different thermal conductivities.

• Hence when solid is eluted out in the carrier gas , this mixture removes differential amount of heat.

• The temperature of coil B now depends on the thermal conductivity of this mixture.

• So the temperature of coil B changes and hence the resistance R4.

• The change in R4 provides on formation about concentration of a solute in the eluate.

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• TCD is simple, rugged, inexpensive

• Non destructive to the sample

• Gives accurate results

Advantages

• It has low sensitivity• Detection of organic

compounds using N2 or Co2 as carrier gas is les sensitive.

Disadvantages

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Electron capture detector •This was invented in year 1954

•Consist of beta emitter such as nickel adsorbed on platinum or titanium foil

•Two electrodes are on either side of the emitter

•They are connected to anammeter in order to record the current in the circuit

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•A beam of electrons is produced by the beta emitter•When carrier gas passes over the emitter, the gas ionises producing electrons•In absence of compound, ionization of carrier gas produces a constant standing current•When solute is eluted out from the column, it captures electron towards it. Hence current decreases•This decrease gives idea about the concentration of a solute in the sample

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Advantages :• Highly sensitive towards compounds containing

electronegative functional groups such as halogens, peroxides, nitro etc.

• Detection and determination of chlorinated insecticides

• Does not alter the sample Disadvantages:• Non linear response unless potential across the

detector is pulsed

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Nitrogen Phosphorous Detector• NPD uses a Hydrogen/Air flame

through which the sample is passed.• NPD uses a rubidium/cesium bead

which is heated by a coil, over which the carrier gas mixed with Hydrogen passes.

• The hot bead emits electrons by which are collected at the anode and provides the background current.

• When a component that contains N/P exits the column, the partially combusted N/P materials are adsorbed on the surface of the bead.

• This then increases the emission of electrons.

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Advantages: • The specific response of NPD to nitrogen and

phosphorous, with relatively high sensitivity, makes it useful for analysis of pharmaceuticals

• In environmental analysis involving herbicides Disadvantages: • Performance deteriorates with time• Burning hydrogen vapours converts alkali silicate to

alkali hydroxide which has significant vapour pressure and causes loss of cesium/rubidium

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Photo ionization electrode• PID responds to all molecules whose ionization

potential is below 10.6eV.• It is often used in series with the FID. • The PID detector consists of a 10.6 electron volt

(eV) UV lamp mounted on a thermostatted, low-volume (100µL), flow-through cell.

• The temperature is adjustable from ambient to 250oC.

• The PID lamp is held in place by a spring-loaded plate, so that the lamp may be quickly removed for cleaning and replaced without any special tools.

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• PID design uses a 10.6eV lamp with a high voltage power supply.

• Sample laden carrier gas flows from the analytical column into the PID sample inlet.

• When sample molecules flow into the cell, they are bombarded by the UV light beam.

• Molecules with an ionization potential lower than 10.6eV release an ion when struck by the ultraviolet photons.

• These ions are attracted to a collector electrode, then sent to the amplifier to produce a signal

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Helium ionization detector• HID advantage to use helium as both the carrier gas and the

ionization gas. HID uses β-emitters as radioactive source to create metastable helium species.

• The radioactive source ionizes helium atoms by bombarding them with emissions. The metastable helium species have an energy of up to 19.8 eV.

• As components elute from the GC's column they collide with the metastable helium ions, which then ionize the components.

• The ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.

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Argon ionisation detector

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Atomic emission detector• The strength of this detector lies

in its ability to simultaneously determine elements

• It uses microwave energy to excite molecules

• This emits radiations which breaks down molecules to atoms such as S,N,P,Hg, etc.

• These excited molecules emit distinctive wavelength which can be separated by a grating and send to photodiode array which produces the electric signal

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Chemiluminescence detector• Nitrogen- and sulfur-containing

compounds commonly occur as trace-level analytes in complex samples and highly selective detectors have been developed.

• The nitrogen chemiluminescence detector and the sulfur chemiluminescence detector have emerged as powerful tool in GC,HPLC

• Nitrogen/sulfur analysers can be based on the same chemiluminescence reactions.

• Detectors of either element are based on the same ozone-induced gas phase chemiluminescence

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• Chemiluminescence is preceded by high temperature pyrolysis which oxidizes the nitrogen in the sample (RN) to nitric oxide (NO):

Oxidation: RN + O2 → NO + CO2 + H2O• The sulfur in the sample (RS) is converted first into sulfur dioxide

(SO2), which is then reduced in the presence of hydrogen to sulfur monoxide (SO):

Oxidation: RS + O2 → SO2 + CO2 + H2O Reduction: SO2 + H2 → SO + H2O Overall: RS + O2 + H2 → SO + CO2 + H2O• These reactions produce the species that react with ozone, producing

excited nitrogen dioxide and excited sulfur dioxide respectively NO + O3 → NO2* + O2

Chemiluminescence: NO2* → NO2 + light (~ 1200 nm) Reaction with ozone: SO + O3 → SO2* Chemiluminescence: SO2* → SO2 + light (~ 360 nm)

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References • Introduction to analytical gas chromatography by Raymond P. W. Scott,

2nd edition,171-199• Fundamentals of Analytical Chemistry by Douglas A. Skoog,Donald

West,F.James Holler, Stanley R. Crouch, 8th edition, 947-972• Instrumental Methods of Analysis by Dr.S.S.Mahajan, 288-290• Gas Chromatography Mass Spectroscopy(GC-MS) (http://www.bris.ac.uk/nerclsmsf/techniques/gcms.html)• Electron Capture Detector

(http://www.bucksci.com/atomic-absorption-spectrophotometers/details/3127/3127/electron-capture-detector-ecd.html)

• Flame Ionization Detector (http://en.wikipedia.org/wiki/Flame_ionization_detector) • Flame Ionization Detector (http://www.cambustion.com/products/hfr500/fast-fid-principles)

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• The Thermal Conductivity Detector(http://www.ecs.umass.edu/eve/facilities/equipment/Agilent6890/The%20Thermal%20Conductivity%20Detector.pdf)

• The Thermal Conductivity Detector(http://www.bucksci.com/atomic-absorption-spectrophotometers/details/3119/394/gas-chromatographs/gc-detectors/thermal-conductivity-detector-tcd.html)

• Thermal conductivity detector (TCD)(http://hiq.lindegas.com/en/analytical_methods/gas_chromatography/thermal_conductivity_detector.html)

• Flame Photometric Detector (http://www.airproducts.com/industries/analytical-laboratories/analytical-lab-applications/product-list/gc-with-flame-photometric-detector-gc-fpd-analytical-laboratories.aspx?itemId=5E12372B85BB4009A5425A215D56C92A

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• Photo Ionization Detector (http://www.srigc.com/PIDman.pdf)• Helium Ionization Detector (http://www.srigc.com/HIDman.pdf)• Nitrogen/Phosphorus Detector (http://www.sri-

instruments-europe.com/en/pdf/npd-mdi.pdf)• The Nitrogen-Phosphorus Detector

(http://www.ecs.umass.edu/eve/facilities/equipment/Agilent6890/The%20Nitrogen-Phosphorus%20Detector.pdf)

• Atomic Emission Detector (http://www.shsu.edu/chm_tgc/primers/pdf/AED.pdf)

• A chemi-luminescence detector (http://www.cambustion.com/products/cld500/cld-principles)

• Analytical_Chemiluminescence/Chemiluminescence detection in gas chromatography (http://en.wikibooks.org/wiki/Analytical_Chemiluminescence/Chemiluminescence_detection_in_gas_chromatography)

• Gas chromatography mass spectrometry (http://en.wikipedia.org/wiki/Gas_chromatography%E2%80%93mass_spectrometry)

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Thankyou!!