Detectors used in GC BY UPASHANA BANERJEE

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contains principle, operating parameters, working,construction,advantages,disadvantages,application and labelled diagrams of 7 major types of detectors used in GC

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Detectors used in Gas Chromatography: 

Detectors used in Gas Chromatography Upashana Banerjee M.Pharm . 1 st Semester. Pharmacology

detectors: 

detectors As solutes elute from the column, they interact with the detector. The detector converts this interaction into an electronic signal that is sent to the data system. The magnitude of the signal is plotted versus time (from the time of injection) and a chromatogram is generated. Most detectors require one or more gases to function properly like combustion, reagent, auxiliary and makeup gases. “Make up” gas is a gas flow that is used to sweep components through a detector to minimize band broadening.

Characteristic Of Ideal Detector: 

Characteristic Of Ideal Detector Adequate sensitivity: Range of 10 -8 to-15 g solute/s. Good stability and reproducibility. A temperature range from room temperature to at least 400˙c. High reliability and easy of use. A short response time that is independent of flow rate. Non destructive of sample. High predictable & selective response toward one or more classes of solute.

Types of detectors: 

Types of detectors Non-Destructive Thermal Conductivity (TCD) Electron Capture (ECD) Photo Ionization (PID) Destructive Flame Ionization (FID) Nitrogen-Phosphorus (NPD) Flame Photometric (FPD)

Thermal Conductivity (TCD): 

Thermal Conductivity (TCD) Principle : It is based upon thermal conductivity difference between carrier gas and that of component. Also called Katharometer . Sample can be recovered. Selectivity : All compounds except for the carrier gas Sensitivity : 5-20 ng Linear range : 10 5 -10 6 Gases : Makeup - same as the carrier gas Temperature : 150-250°C

CONSTRUCTION:- : 

CONSTRUCTION : -

WORKING :-: 

WORKING :- The TCD consists of four tungsten-rhenium filaments in a Wheatstone bridge configuration .  Electric current flows through the four filaments causing them to heat up. Carrier gas ( typically He ,which has very high thermal conductivity ) flows across the filaments removing heat at a constant rate.  When a sample molecule with lower thermal conductivity exits the column and flows across the two sample filaments, the temperature of the filaments increase unbalancing the Wheatstone bridge and generating a peak as the sample molecules transit through the detector.

Sensitivity: 

Sensitivity Sensitivity of TCD can be calculated as follows: S = A x C 1 x C 2 x C 3 / W S = Sensitivity (mV/mg) A = Peak area (cm 2) C 1 = recorder sensitivity (mV/cm) C 2 = chart speed (min /cm) C 3 = flow rate column exit corrected to column temp. & atmospheric pressure(ml/min) W = Weight of sample injected (mg)

advantages: 

advantages 1. Non-selective: Applicable to most compounds. 2.Linearity is good. 3.The sample is not destroyed and hence used in preparative scale. 4.Simple, easy to maintain and inexpensive. 5.Rugged. 6.Accurate.

DISADVANTAGES: 

DISADVANTAGES Low Sensitivity. Affected by fluctuations in temperature and flow rate causing detector noise. The response is only relative and not absolute. Biological samples cannot be analysed .

ApplicationS :: 

ApplicationS : It is used in measurement of gas mixtures. It has unique capability for the detection of water. It is applied to detect other solvent residues in samples which are presumed to be contaminated only with water. Determination of CO 2 in fuel gases.

Electron capture detector: 

Electron capture detector Principle : It is a form of Ionisation Detector. The amount of current loss due to capture of electrons generated from a radioactive source by the electronegative compounds is indirectly measured and a signal is generated. Selectivity : Halogens, nitrates and conjugated carbonyls Sensitivity : 0.1-10 pg (halogenated compounds); 1-100 pg (nitrates); 0.1-1 ng (carbonyls) Linear range : 10 3 -10 4 Gases : N 2 or Ar /CH 4 Temperature : 300-400°C

construction: 

construction

Working:: 

Working: It consists of two electrodes with the column effluent passing between them. One of the electrode is treated with a radioactive isotope which emits electrons . These electrons are collected by the anode when potential of 20 V is applied . The difference of current is amplified & recorded as output. The carrier gas can be Ar,N 2 ,H 2 etc.

advantages: 

advantages The electron-capture detector is selective in its response and is highly sensitive towards electronegative functional groups such as halogens, peroxides, quinones , and nitro groups. Electron-capture detectors possess the advantage of not altering the sample significantly (in contrast to the flame detector). It can be used to detect trace amounts of chemical compounds in a sample.

DISADVANTAGES: 

DISADVANTAGES It can be used only for compounds with electron affinity . It has a limited dynamic range. It is insensitive to compounds such as amines,alcohols , and hydrocarbons. It is more prone to contamination. It has high cost. Its linearity is poor.

APPLICATIONS: 

APPLICATIONS Its greatest application is in analysis of halogenated compounds . An important application is detection and determination of chlorinated insecticides and organometallic compounds. The major pharmaceutical applications of the ECD are in selective determination of sub micro drug levels in biological specimens such as blood ,urine & tissue. Determination of Griseofulvin & Hydrochlorthiazide in human plasma.

Photo ionisation detector: 

Photo ionisation detector Principle : This device uses ultraviolet light as a means of ionising an analyte exiting from a GC column. The ions produced by this process are collected by electrodes. The current generated is therefore a measure of the analyte concentration. It is a nondestructive GC detector. Selectivity : Depends on lamp energy. Usually used for aromatics and olefins (10 eV lamp). Sensitivity : 25-50 pg (aromatics); 50-200 pg (olefins) Linear range : 10 5 -10 6 Gases : Makeup - same as the carrier gas Temperature : 200°C

construction: 

construction

working: 

working Compounds eluting into a cell are bombarded with high energy photons emitted from a UV lamp. Compounds with ionization potentials below the photon energy are ionized. The resulting ions are attracted to an electrode, measured, and a signal is generated.

advantages: 

advantages Selective and highly sensitive detection of specific compounds. Efficient and inexpensive. Produce instantaneous readings and operate continuously. PIDs are used as monitoring solutions for: Lower explosive limit measurements Ammonia detection Hazardous materials handling

disadvantages: 

disadvantages The PID is not suitable for the detection of semi-volatile compounds . Detection limits are lower. High concentrations of methane can hinder performance. Rapid variations in temperature at the detector and strong electrical fields may affect instrument response. The PID must be re-calibrated frequently

applications: 

applications Its most important application is the selective determination of aromatic hydrocarbons or organo -heteroatom species. They can be used before other detectors in multiple-detector configurations as they are non-destructive in nature. They are widely used in military, industrial, and confined working facilities for safety .

FLAME IONISATION DETECTOR: 

FLAME IONISATION DETECTOR Principle : An FID consists of a hydrogen/air flame and a collector plate. Compounds are burned in a hydrogen-air flame. Carbon containing compounds produce ions that are attracted to the collector. The number of ions hitting the collector is measured and a signal is generated. Sample cannot be recovered. Selectivity : Compounds with C-H bonds Sensitivity : 0.1-10 ng Linear range : 10 5 -10 7 Gases : Combustion - hydrogen and air; Makeup - helium or nitrogen Temperature : 250-300°C,and 400-450°C for high temperature analyses.

CONSTRUCTION: 

CONSTRUCTION

working: 

working An FID consists of a hydrogen/air flame and a collector plate. The carrier gas effluent from the GC column is mixed with hydrogen and then routed through an unbreakable stainless steel jet.  The hydrogen supports a flame at the tip of the jet, which breaks down organic molecules ,thus ionizing the analyte molecules.  A collector electrode attracts the negative ions to the electrometer amplifier producing an analog signal which is connected to the data system input.

advantages: 

advantages The FID is the most commonly used GC detector , responding linearly from its minimum detectable quantity of about 100 picograms to almost 100%. The FID response is very stable from day to day, and is not susceptible to contamination from dirty samples or column bleed . The FID is extremely sensitive with a large dynamic range.

disadvantages: 

disadvantages Its only disadvantage is that it destroys the sample.

applications: 

applications Universal GC detector. FIDs are best for detecting hydrocarbons and other easily flammable components.

Nitrogen-Phosphorus (NPD): 

Nitrogen-Phosphorus (NPD) Principle :- It is also called as thermionic detector. Thermal energy is used to ionize an analyte . Selectivity : Nitrogen and phosphorous containing compounds Sensitivity : 1-10 pg Linear range : 10 4 -10 6 Gases : Combustion - hydrogen and air; Makeup – helium Temperature : 250-300°C

construction: 

construction The design is based on the FID to a large extent although its main difference from FID is rubidium or cesium chloride bead .The bead is situated above the H 2 jet and heated by a coil, over which the nitrogen carrier gas mixed with hydrogen passes.

working: 

working Compounds are burned in a plasma surrounding a rubidium bead supplied with hydrogen and air. Nitrogen and phosphorous containing compounds produce ions that are attracted to the collector. The number of ions hitting the collector is measured and a signal is generated.

PowerPoint Presentation: 

Advantages:- It is a very sensitive, specific detector. Disadvantages:- The main disadvantage of this detector is that its performance deteriorates with time.

APPLICATIONS:-: 

APPLICATIONS:- NPD is commonly used to detect pesticides, herbicides, and drugs of abuse because it responds to N-P very selectively. NPD selectively detects the organo -phosphate pesticides.

Flame Photometric (FPD) : 

Flame Photometric (FPD) Principle :- Compounds burned in a hydrogen-air flame. S and P containing compounds produce light emitting species (S at 394 nm and P at 526 nm). Selectivity : Sulfur or phosphorous containing compounds. Only one at a time. Sensitivity : 10-100 pg (sulfur); 1-10 pg (phosphorous) Linear range : Non-linear (sulfur); 10 3 -10 5 (phosphorous) Gases : Combustion - hydrogen and air; Makeup – nitrogen Temperature : 250-300°C

construction: 

construction

working: 

working The eluent from the column is fed into a hydrogen-rich flame and produces light emission. Optical filters are used to select the wavelength range of the emission which is characteristic of specific atoms (usually sulfur or phosphorus). A monochromatic filter allows only one of the wavelengths to pass. A photomultiplier tube used to measure the amount of light and a signal is generated. Different filters required for each detection mode.

advantages: 

advantages Sensitive detection of specific compounds. The detector is very specific, depending on the choice of optical filters. It can detect the S- and P-containing compounds down to10 –3 ppmv , but the detector is non-linear.

disadvantages: 

disadvantages The drawback here being that the filter must be exchanged between chromatographic runs if the other family of compounds is to be detected.

applications: 

applications It can be used for the simultaneous detection of sulfur and phosphorus in different hydrocarbons.

Helium ionization detector : 

Helium ionization detector Principle :- An HID is an ion detector which uses a radioactive source to produce ions. The radioactive source ionizes helium atoms by bombarding them with emissions.

construction: 

construction

working: 

working As components elute from the GC”s column they are mixed with the 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.

advantages: 

advantages HIDs are sensitive to a broad range of components. HIDs are non-destructive detectors. They do not destroy/consume the components they detect. No requirement of hydrogen and air, only helium is needed. Robust and easily serviceable electrode.

disadvantages: 

disadvantages The drawback to HIDs are that they contain a radioactive source.  Discharge ionization detectors have generally supplanted them.

Application : 

Application They can be used before other detectors in multiple-detector configurations. It is a universal detector which responds to all molecules except neon. It is specially useful in combination with the TCD detector to provide more sensitivity. It is particularly useful for volatile inorganics like NO X ,CO,CO 2 ,N 2 ,H 2 ,O 2 ,etc.

Applications:-: 

Applications:- It finds use in the analysis of purgeable halogenated compounds found in drinking water, wastewater and solid waste.

Characteristics of detectors: 

Characteristics of detectors Detector Type Support gases Selectivity Detectability Flame ionization (FID) Mass flow Hydrogen and air Most organic cpds. 100 pg Thermal conductivity (TCD) Concentration Reference Universal 1 ng Electron capture (ECD) Concentration Make-up Halides, nitrates, nitriles, peroxides, anhydrides, organometallics 50 fg Nitrogen-phosphorus Mass flow Hydrogen and air Nitrogen, phosphorus 10 pg Flame photometric (FPD) Mass flow Hydrogen and air possibly oxygen Sulphur, phosphorus, tin, boron, arsenic, germanium, selenium, chromium 100 pg Photo-ionization (PID) Concentration Make-up Aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulphurs, some organometallics 2 pg

Thank you: 

Thank you