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Gas Chromatography


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  • 1. GAS CHROMATOGRAPHY DETECTORS Presented By : Amul Chahar
    • Introduction
    • Principle of Gas Chromatography
    • Classification of Detectors
    • Ideal Characteristics of Detectors
    • Different Detectors Used in GC
    • Comparison of Sensitivity & Selectivity of Detectors
    • Application
    • References
    • Chromatography comprises a group of methods for the separating molecular mixtures that depend on the differential affinities of the solutes between two immiscible phases.
  • 4. Cont….
    • Gas Chromatography is a technique for separation of volatile substances by percolating gas stream over a stationary phase.
    • Gas chromatography is one of the most versatile of all the chromatographic techniques, was first describes by James & Martin in 1952.
    • If the stationary phase is a solid, we speak of Gas-Solid Chromatography . This depends upon the adsorption properties of the column packing to separate samples, primarily gases.
    • Common packing used are silica gel, molecular sieves & charcoal.
  • 5. Cont….
    • If the stationary phase is a liquid, we speak of Gas-Liquid Chromatography.
    • Gas chromatography is valuable technique for analyzing compounds of pharmaceutical interest.
    • The great sensitivity & specificity of GLC are outstanding characters.
    • Because of the high sensitivity of the detectors available nano gram quantities of the sample are sufficient.
    • It is used to analyze gases, liquids & solids.
    • The principle of separation in GC is partition.
    • Gas is used as a mobile phase & liquid which is coated on to a solid support is used as a stationary phase.
    • The mixture of component to be separated is converted to vapour and mixed with gaseous mobile phase.
    • The component which is more soluble in stationary phase travel slower & eluted later. The component which is less soluble in stationary phase travel faster & eluted out first.
    • No two components has same partition co-efficient for fixed combination of stationary phase, mobile phase and other condition.
    • So the components are separated according to their partition co-efficient.
    • Partition co-efficient is the ratio of solubility of a substance distributed between two immiscible liquids at a constant temperature.
  • 7. Instrumentation
    • It is the brain of GC. Its function is to detect & quantify the different components of the sample as they emerge out from the column.
    • The choice of the detector depends upon the type of analysis being performed.
    • Requirement of an Ideal Detectors :
    •  High sensitivity
    •  Physically suitable
    •  Capable of operable upto a maximum column
    • temperature. E.g. – 350 o C
    •  Ease of operation
    •  No response to undesirable compounds.
    •  Response to compounds for which analysis is required.
    •  Non destructive to sample in case of preparative work
    •  Inexpensive
    • Detectors can be classified into two types ,
      • The Bulk Property Detectors measure some bulk physical property of the eluent (such as dielectric constant or refractive index)
      • The Solvent Property Detector , measures some physical or chemical property that is unique to the solute (such as heat of combustion or fluorescence).
    • Detectors can also be classified as
      • Concentration Sensitive Devices such as the katharometer or
      • Mass Sensitive Devices such as the flame ionization detector (FID).
  • 10. Detectors used in Gas Chromatography
    • Thermal conductivity Detector (Katharometer) (TCD)
    • Flame Ionisation Detector (FID)
    • Electron Capture Detector (ECD)
    • Flame Photometric Detector (FPD)
    • Argon Ionisation Detector (AID)
    • Nitrogen-phosphorus Detector (NPD) or Flame Thermionic Detector
  • 11. Thermal Conductivity Detector
  • 12.
    • The thermal conductivity detector is based upon change in thermal conductivity of the gas stream.
    • Because the thermal conductivity of different gases are different, a change in composition of the gas causes the thermal conductivity to change.
    • The detector is a hot wire through which a current is allowed to flow. As the thermal conductivity of the gas environment changes, the temperature of the wire also changes.
    • The resistance of the wire or thermistor gives a measure of the thermal conductivity of the gas.
    • Advantages :
    •  Simple
    •  Rugged
    •  Inexpensive
    •  Non-selective
    •  Accurate
    •  Non-destructive to sample
  • 13.
    • Disadvantage :
    •  Low sensitivity
    •  Affected by fluctuation in temp. & flow rate
    •  Response is only relative & not absolute
    •  Biological samples can not be analyzed
    • Application :
    • These detectors have considerable application apart from gas chromatography but one of the most extensive applications of these detectors is in the determination of CO 2 in gases.
  • 14. Flame Ionisation Detectors (FID)
  • 15.
    • Flame Ionization Detector (FID) is the most useful GC detector.
    • The FID has a very wide dynamic range, a high sensitivity & will detect all substances that contain carbon.
    • FIDs are mass sensitive rather than concentration sensitive.
    • Working :
    • The effluent from the column is mixed with hydrogen & air, & ignited at a small jet.
    • Organic compounds burning in the flame produce ions & electrons which can conduct electricity through the flame.
    •  Surrounding the flame is a cylindrical electrode & a relatively high voltage is applied between the jet & the electrode to collect the ions that are formed in the flame.
  • 16.
    • The current resulting from the Pyrolysis of any organic compounds is amplified by a high impedance amplifier & the output fed to a data acquisition system or a potentiometric recorder.
    • Advantages :
    •  Respond to most organic compounds
    •  Linearity is good
    •  High sensitive
    •  Stable & insensitive to change in flow rate
    • Disadvantages :
    • The presence of "Heteroatom" in a molecule, such as oxygen, decreases the detector's response.
    • Methane response (CH 4 ) is fabulous but formaldehyde's (CH 2 O) is quite poor.
  • 17. Electron Capture Detector (ECD)
  • 18.
    • The ECD uses a radioactive  emitter (electrons) to ionize some of the carrier gas & produce a current.
    • The ECD is sensitive to compounds with high electron affinities (halogen-containing compounds).
    • Gas entering the detector is ionized by a high energy radioactive source that gives off electrons (often 63 Ni).
    • When organic molecules that contain electronegative functional groups, such as halogens, phosphorous, & nitro groups pass by the detector, they capture some of the electrons & reduce the current measured between the electrodes.
    • Advantages :
    •  Simple & reliable
    •  Sensitive to electronegative groups
    •  Non-destructive
    •  Does not have any effect on sample
  • 19.
    • Disadvantages :
    •  Insensitive to amine, alcohol & hydrocarbons
    •  Limited dynamic range
    •  Compounds which have more electron affinity groups
    • can only used.
    • Application :
    •  Used for detection and measurement of trace environmental pollutants.
    •  Due to high sensitivity for halogenated compound, used for detection of pesticides, herbicides & polynuclear aromatic carcinogens.
  • 20. Flame Photometric Detector (FPD)
  • 21.
    • The determination of sulfur or phosphorus containing compounds is the job of the flame photometric detector (FPD).
    • This device uses the chemiluminescent reactions of these compounds in a Hydrogen/Air flame as a source of analytical information that is relatively specific for substances containing these two kinds of atoms.
    • In order to selectively detect one or the other family of compounds as it elutes from the GC column, an interference filter is used between the flame & the photomultiplier tube (PMT) to isolate the appropriate emission beam.
    • The final component necessary for this instrument is a thermal filter to isolate only the visible & UV radiation emitted by the flame.
  • 22.
    • Disadvantages :
    •  Its selectivity
    •  Poor commercial availability.
    •  the filter must be exchanged between chromatographic runs if the other family of compounds is to be detected.
    • Application :
    •  Use to detect heavy metals (Fe, Sn, Pb) in organometallic compounds, with nanogram (nm) sensitivity.
  • 23. Argon Ionisation Detector (AID)
  • 24.
    • This type of detector depends on the excitation of argon atoms to a metastable state by using radioactive energy.
    • This can be achieved by irradiating the carrier gas with either alpha particles or beta particles.
    • Alpha particles can be obtained from radium-D.
    • Beta particles can be obtained from 90 Sr or tritium.
    • These high energy particles ionize the argon atoms and hence they are exited to metastable state.
    • These molecule collide with the effluent molecules and ionize them.
    • These ions when they reach the detector will cause an increase in current.
    • Thus the compounds can be detected.
    • Argon Argon e - Metastable state of argon
    • Increase in Ionization
    • current
    Irradiation Collision Collision of Substances
  • 25.
    • Advantages :
    •  Responds to most of the organic compounds.
    •  High sensitivity
    • Disadvantages :
    •  Response is not absolute & it is relative
    •  Sensitivity is affected by water and much reduced for
    • halogenated compounds
    •  The response varies with the temperature of the detector
  • 26. Nitrogen-phosphorus Detector (NPD)
  • 27.
    • Also known as Flame Thermionic Detector
    • The NPD is a specific type of FID that is particularly sensitive for compounds that contain N or P.
    • It may also be termed as a nitrogen detector, sulphur detector, phosphorus detector and halogen detector.
    • It is widely known as NP-FID because it is employed for carrying out the analysis of N- or P- containing organic compounds.
    • Alkali metal salt like Ruthenium silicate beads in flame increase sensitivity of compound containing halogen ,nitrogen ,phosphorus
    • (500 factor).
  • 28. Detectors Type Support gases Selectivity Sensitivity Dynamic range Flame ionization (FID) Mass flow Hydrogen & air Most organic compounds Highly sensitive 10 7 Thermal conductivity (TCD) Concentration Reference Universal Low sensitive 10 7 Electron capture (ECD) Concentration Make-up Halides, nitrates, nitriles, peroxides, anhydrides, organometallics Sensitive to electronegative group 10 5 Flame photometric (FPD) Mass flow Hydrogen & air possibly oxygen Sulphur, phosphorus, tin, boron, arsenic, germanium, selenium, chromium High sensitive 10 3 Argon Ionisation Detector (AID) Concentration Make-up Organic compound Highly sensitive 10 7
  • 29. References
    • Text book of Pharmaceutical Analysis by Ravi Shankar.
    • Principles of Instrumental Analysis by Skoog.D.A; Holler.F.J.
    • Quantitative Analysis of drug-III edition by P.D.Sethi.
    • Pharmaceutical analysis- vol II by Dr. A.V.kasture
  • 30. THANK U