Gas Chromatography

GAS CHROMATOGRAPHY DETECTORS Presented By : Amul Chahar

CONTENTS 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

INTRODUCTION Chromatography comprises a group of methods for the separating molecular mixtures that depend on the differential affinities of the solutes between two immiscible phases.

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.

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.

PRINCIPLE OF GAS CHROMATOGRAPGY 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.

DETECTORS 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

CLASSIFICATION OF DETECTORS 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).

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

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

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.

Flame Ionisation Detectors (FID)

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.

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.

Electron Capture Detector (ECD)

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

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.

Flame Photometric Detector (FPD)

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.

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.

Argon Ionisation Detector (AID)

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

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

Nitrogen-phosphorus Detector (NPD)

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).

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

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 www.google.com

Gas Chromatography

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Gas Chromatography