GAS CHROMATOGRAPHY detectors

Views:
 
Category: Entertainment
     
 

Presentation Description

DEFINATION ,CLASSIFICATION, ADV. DISADV, USE,CONSTRUCTION

Comments

Presentation Transcript

DETECTORS IN GAS CHROMATOGRAPHY: 

DETECTORS IN GAS CHROMATOGRAPHY Done By SONAM M. GANDHI I M Pharm ( Industrial Pharmacy )

GAS CHROMATOGRAPHY: 

GAS CHROMATOGRAPHY GC used for separation & analysis of volatile, gaseous substances.

PowerPoint Presentation: 

INSTRUMENTS FOR GC

DETECTORS: : 

DETECTORS: Devices used to or designed to respond to a very small quantity of sample present in the column gas physical and chemical properties of column gas differ from the sample vapours . 2/28/2013 RMR 5

Detection methods involve measurement of standard physical properties:: 

Detection methods involve measurement of standard physical properties: Thermal conductivity Light absorption Ionization potential Heat of combustion Gas density 2/28/2013 RMR 6

REQUIREMENTS:: 

REQUIREMENTS: High sensitivity Operated at maximum column temperature Ease of operation Not respond to undesired compounds Linear response corresponding to high concentration Insensitive to change in flow rate The detector volume must be small to prevent the remixing of components separated on the column. Do not destruct the sample. 2/28/2013 RMR 7

Types:: 

Types : Selective detectors: ( responds to range of compounds with common physical or chemical property that is single chemical compound) Nonselective detectors : (respond to all compounds except the carrier gas) Destructive detectors : destruct the sample Non destructive detectors : not destruct the sample 2/28/2013 RMR 8

PowerPoint Presentation: 

NON-SPECIFIC Thermal conductivity detector Atomic Emission detector (AED) Photoionization detector (PID) SPECIFIC DETECTORS Sulfur-phosphorous flame photometric detector (SP-FPD) Flame ionization detector Sulfur chemiluminescence detector (SCD) Nitrogen – phosphorus detector (NPD) Helium ionization detector (HID) Electron capture detector Destructive detectors Non destructive detectors NPD ECD FID PID SCD TCD

PowerPoint Presentation: 

conc. Dependent detector = conc. Of solute in the detector Mass flow dependant detector = signal is related to the rate at which solute molecules enter into the detector 2/28/2013 RMR 10 Detectors Type Support gases Selectivity Detect ability Dynamic range FID Mass flow Hydrogen and air Most org compds 100pg 10⁷ Thermal Conc Reference Universal 1 ng 10⁷ Electron capture Conc Make up Halides, Nitrates, Nitriles, Peroxides, organometals 50fg 10⁵

PowerPoint Presentation: 

Principle: The rate of heat loss from a heated wire placed in a gas stream depends on the thermal conductivity of the gas. Thus if the composition of the gas stream changes, the rate of heat loss from the wire will change and so the temperature and electrical resistance of the wire will change showing a signal in the chromatogram. Thermal conductivity detector

Construction: 

Construction Two cells of small volumes made within a metal block, termed as Reference cell and sample cell Each cell has a resistance wire or thermistor or filament That posses a high temp co- efficent or resistance i.e. resistance varies with slight variation in temp. i.e these 2-resistance namely reference cell + sample cell are included in 2 arms of wheat stone bridge surrounded by the carrier gas. 2/28/2013 RMR 12

working: 

working Also known as Katharometer or Hot wire device. It consists of a heated filament placed in the emerging gas stream. Within the cavity in the metal block, there extends a tightly coiled filament constructed of tungsten metal, tungsten-rhenium alloy or tungsten sheathed with gold. Filament is heated to constant temperature below dull-red condition by a regulated dc current supply .

PowerPoint Presentation: 

Heat loss from the filament to the metal block is constant when only carrier gas is flowing through the detector. During analysis, column effluent is passed through one pair of filaments and the second pair is placed in the gas stream ahead of the sample injection point. Imbalance between the pairs of filaments is recorded.

PowerPoint Presentation: 

Advantages: It is concentration dependant detector and it does not destroy the sample, hence it finds use in preparative scale. Simple and easy to use Disadvantages: Gets affected by flow rate and fluctuations. The response is only relative but not absolute. Biological samples cannot be analyzed.

PowerPoint Presentation: 

Representative Thermal Conductivity Values

Flame ionization detector(FID):: 

Flame ionization detector(FID): Most useful detector in GC Wide dynamic range High sensitivity Carrier gas= H 2 ,N 2 ,O 2 or air used as combustion gas 2/28/2013 RMR 17

Principle:: 

Principle: Eluent from the column ------ mixed with H2------ burned at the tip of the jet ( with air or O2) ions + free electrons are formed in the flame current produced is measured and recorded 2/28/2013 RMR 18

Design: 

Design Composed of sensor + electrode The body + electrode are made up of stainless steels These fittings connect the detector to gas supplies Jet + electrode is insulated from high temperature insulator ----- stable at elevated temperature Jet tip is constructed with metal ( it is not easily oxidized ) ex: stainless steel, Pt / rhodium 2/28/2013 RMR 19

PowerPoint Presentation: 

It consists of a flame jet and a cylinder positioned 0.5-1.0 cm placed above the tip of the flame which constitute the twin electrodes or two parallel plates arranged above the flame tip acts as electrodes. A potential of about 400 V is applied across the two electrodes, which lowers the resistance between electrodes and causes a current (~10 -12 A) to flow. Current arises from the ions and free electrons generated in pure hydrogen/air flame. working

PowerPoint Presentation: 

Advantages: High sensitivity Stability Fast response Wide linear range not responds to moisture non ionisable gases 2/28/2013 RMR 21

PowerPoint Presentation: 

It responds proportionately to the no of -CH2- groups introduced into the flame. Eg : response to equimolar amount of butane is twice that of ethane. Insensitive to moisture and permanent gases (CO, CO2, CS2, SO2, NH3, N2O, NO, NO2, SiF 4 , SiCl 4 . This detector responds to all organic molecules. Ease of use and robust. Disadvantages: This is a mass dependent detector, hence it destroys the sample. 2/28/2013 RMR 22

PowerPoint Presentation: 

Need for Other Detectors than FID

PowerPoint Presentation: 

FID Response

PowerPoint Presentation: 

THERMIONIC EMISSION DETECTOR Principle: When a solution containing nitrogen or phosphorus is eluted, the partially combusted nitrogen or phosphorus materials are adsorbed on the surface of the bead. This adsorbed material reduces the work function of the surface and the emission of electrons is increased, which raises the anode current is recorded. The sensitivity of the detector is about 10 -12 g/ml for phosphorus and 10 -11 g/ml for nitrogen.

working: 

working This detector is sensitive only to organic compounds containing nitrogen and phosphorus, hence also called as Nitrogen- Phosphorus detector. This is a modification of flame ionization detector. It consists a bead of solid alkali metal salt placed just above (1.25cm)the flame tip. The bead is electrically heated (adjusted between 600-800ºC) hence bead’s temperature is independent of the flame. Hydrogen gas mixed with air is sent through the flame tip assembly. Heated alkali beads emit electrons by thermionic emission which is collected at the anode and thus produces anionic current.

PowerPoint Presentation: 

Advantages: Thermionic emission detector can be widely used in the analysis of phosphorus containing pesticides.

PowerPoint Presentation: 

The ECD uses a radioactive source such as Ni 63 which produces Beta particles which react with the carrier gas producing free electrons . These electrons flow to the anode producing an electrical signal. When electrophillic molecules are present, they capture the free electrons, lowering the signal. The amount of lowering is proportional to the amount of analyte present. Electron Capture Detector

Working – : 

Working – The ß ray source(foil containing Hᶟ or N⁶ᶟ) used to generate the ions that helps the ionization of the carrier gas(nitrogen) molecule to form + ve and free electrons C + radiation = C⁺ + e⁻, where The mobility of the lighter – ve ions is the electrons higher than the + ve ions ( i.e changed carrier gas molecules) The ECD has 2 electrodes, the column effluent passes b/n the two electrodes 2/28/2013 RMR 30

PowerPoint Presentation: 

One of the electrode is treated with radiation of ß ray which emits high energy electrons. Molecules that have affinity for thermal electrons, capture the electrons as they pass b/n electrodes reduces the steady state current. 2/28/2013 RMR 31

PowerPoint Presentation: 

Advantages: Halogens, peroxides, quinines etc compounds which have electronegative groups cab be easily estimated. Even nanogram quantities can be detected. Disadvantages: Applicable only to compounds which have affinity towards electrons. This detector is insensitive towards functional groups like amines, alcohols and hydrocarbons.

PowerPoint Presentation: 

Flame Photometric Detector Commercial instruments are limited to the detection of P and S The analytes are burned in a H 2 flame causing electrons to move to an excited unstable state. When the electrons return to the ground state, they emit a specific wavelength of light 526 nm for P and 394 nm for S These wavelengths are monitored by a photomultiplier, amplified, and turned into an electrical signal. For S, the response is non-linear.

PowerPoint Presentation: 

Pulse flame photometric detector

PowerPoint Presentation: 

advantage They allow for the immediate determination of the mass of the analyte . Disadvantage The tendency for samples to thermally degrade before detection and the end result of obliterating all the sample by fragmentation .

PowerPoint Presentation: 

Photo ionization detector (PID)

PowerPoint Presentation: 

Photo ionization detector (PID)

PowerPoint Presentation: 

Photo ionization Detector The photo ionization detector (PID) uses a UV lamp (xenon, krypton or argon) to ionize compounds. The ionization produces a current between the two electrodes in the detector. Used for aromatics and cyclic compounds. Limitation: Problems with its long term use,-especially with regard to contamination of lamp window & lifetime of lamp.

PowerPoint Presentation: 

Chemiluminescence (CS) detector Principle SCD is based on reaction between certain sulfur compound & fluoride. The resulting chemiluminescence intensity is proportional to the concentration of sulfur. This detector is particularly useful for determination of pollutants such as mercaptans.

PowerPoint Presentation: 

Mechanism: Compounds are mixed with a reaction gas and passed through a high temperature reaction tube. Specific reaction products are created which mix with a solvent and pass through an electrolytic conductivity cell. The change in the electrolytic conductivity of the solvent is measured and a signal is generated. Reaction tube temperature and solvent determine which types of compounds are detected. ELECTROLYTIC CONDUCTIVITY DETECTOR (ELCD)

PowerPoint Presentation: 

Atomic Emission Detector The strength of the AED lies in the detector's ability to simultaneously determine elements. It uses microwave energy to excite helium molecules (carrier gas) which emit radiation which breaks down molecules to atoms such as S, N, P, Hg, As, etc.These excited molecules emit distinctive wavelengths which can be separated by a grating and sent to the detector (typically a photodiode array) which produces the electrical signal .

PowerPoint Presentation: 

GC Detectors Comparison

PowerPoint Presentation: 

GC Detectors Comparison

REFERENCES: 

REFERENCES James w. Robinson. Undergraduate instrumental analysis. Pg no.775-782. www.chromatography-online.com

by