Chromatography techniques using in chemistry
- 1. DISCOVER . LEARN . EMPOWER UNIVERSITY INSTITUTE OF SCIENCES DIVISION-CHEMISTRY Masters in Science (M.Sc. Chemistry) Dr. Kashif Raees E11790 Subject Name – Chromatographic Techniques Subject Code – 24SHT-629
- 2. 2 This lecture is mapped with CO4 Interpret the instrumentation of various chromatographic techniques including TLC, paper chromatography, GC, HPLC and supercritical fluid chromatography.
- 3. 3 CONTENT Introduction Types of detector & Principles Advantages & Limitations a) FID ( flame ionization detector ) b) TCD ( thermal conductivity detector ) c) ECD ( electron capture detector ) d) FPD ( flame photometric detector ) e) NPD ( nitrogen-phosphorus detector ) f) PID ( photo ionization detector )
- 4. 4 . • Chromatography detectors are utilized to identify mixture components that are being eluted from the chromatography column in gas chromatography (GC) or liquid chromatography (LC). • A crucial part of the chromatograph is the detector. The detector converts data collected by the chromatograph into a format that the technician can use. Then, computers and recording devices may save and utilize this information, enabling technicians to analyze the data and make important judgments. • Destructive as well as non-destructive detectors are the two major types. • Destructive detectors : When the column effluent is constantly converted by the destructive detectors (by burning, evaporating, or combining with reagents), and the surface texture of the new substance is then measured (plasma, aerosol, or reaction mixture). • Non-destructive detectors : The non-destructive detectors are directly measure the sample or without any change in sample’s physical properties.
- 5. 5 The Ideal Detector • The ideal detector should have adequate sensitivity (Sensitivity refers to the change in detector response as a function of the change in the amount or concentration of the analyte) - range 10–18 to 10–15 g analyte/s • Good stability and reproducibility • A linear response to analyte that extends over several orders of magnitude • A temperature range from room temperature to at least 400°C • A short response time that is independent of flow rate, high reliability and ease of use • Similarity in response toward all analytes • A higher predictable and selective response toward analytes
- 6. 6 Flame Photometric Detector The reason to use more than one kind of detector for gas chromatography is to achieve selective and/or highly sensitive detection of specific compounds encountered in particular chromatographic analyses. 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. The lambda max for emission of sulfur compounds is approximately 394 nm. The lambda max for emission of phosphorus compounds 526nm. In order to selectively detect compounds as the sample elutes from the column, it interference with the filter which is used between the flame and the photomultiplier tube (PMT) to isolate the appropriate emission band.
- 7. 7 Instrumentation 1) A combustion chamber to house the flame 2) Gas lines for hydrogen (fuel) and air (oxidant), 3) An exhaust chimney to remove combustion products, 4) A thermal filter to isolate only the visible and UV radiation emitted by the flame 5) The PMT is also physically insulated from the combustion chamber by using poorly (thermally) conducting metals to attach the PMT housing, filters, etc. Specific: phosphorous or sulfur Destructive LOD: 20 pg S /sec, 0.9 pg P / sec Linear range: ~104 P, ~103 S Combustion – Hydrogen and air Makeup – Nitrogen Advantages: 1. Universal detector for organics 2. Does not respond to common inorganic compounds 3. Mobile phase impurities not detected 4. Carrier gases not detected 5. Limit of detection: fid is 1000x better than TCD 6. Linear and dynamic 7. Destructive detector
- 8. 8 NPD ( nitrogen-phosphorus detector ) Also known as thermionic specific detector (TSD) or Alkali flame ionization detector Sensitive to nitrogen- and phosphorus-containing compounds and is highly selective. Inside the NPD detector body, an electrically heated thermionic bead (NPD bead) is positioned between the jet orifice and the collector electrode. NPD uses a Rubidium or Cesium chloride bead which is heated by a coil which promotes the ionization of compounds that contain nitrogen or phosphorus. Hydrogen and air flows create a hydrogen plasma around the hot NPD bead. When molecules containing nitrogen or phosphorus enter the plasma from the column and jet orifice, they undergo a catalytic reaction, producing thermionic electrons. The resulting ions are attracted to a positively charged collector electrode, then amplified and output to the data system.
- 10. 10 PID ( photo ionization detector ) • A photo ionization detector (PID) is a gas detector that uses ultraviolet (UV) rays to detect the presence of volatile organic compounds and other gases like methane and benzene in a workplace. • A PID can instantly indicate the presence of VOCs even if they are present in low concentration. 1. It is Non-destructive type of detector 2. Used for the identification of aromatic HCs, Org heteroatom, Inorganic/organic Comp. 3. The Photo Ionization Detector (PID) responds to all molecules whose ionization potential is below 10.6eV. 4. The PID detector consists of a 10.6 electron volt (eV) UV lamp, only those sample will ionize which have lower ionization potential than UV photons.
- 11. Working • Sample laden carrier gas flows from the analytical column into the PID sample inlet, where it is streamed through a 100µL flow-through cell. • 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 an analog signal, which is acquired by the data system. • Limitations: 1. Not suitable for detecting semi-volatile compounds 2. Only indicates if volatile organic compounds are presents. 3. Frequent calibrations are required
- 12. 12 TCD ( thermal conductivity detector ) • Also known as Katharometer and is commonly used in GC. • Termed as the universal detector since it detects air, hydrogen, CO, N, sulfur dioxide, inorganic gases and many others. • In oil industry Katharometer are used for hydrocarbon detection.
- 13. . • Each gas(pure carrier gas from reference column & sample + carrier gas from analytic column) enters the TCD separately. • Electrically heated resistant wires are located in the chambers inside the TCD. • The power supply provides a current to the resistance wires which causes the wires to heat up. • As the gas flows through the TCD the physical properties of the reference and the sample gases will allow the wires to cool at different rates. • This change in temperature will result in the change in the resistance for both the reference as well as the sample gas which will produce an electrical signal that we ultimately need for the compounds to be analyzed. • This signal is proportional to the concentration of the sample components. • TCD therefore provides a direct means of measuring a particular component in the sample. • Further a chromatogram is received digitally for the sample analyzed. • Advantages: 1. Simplicity 2. Large linear dynamic range 3. Non-destructive • Disadvantages: 1. Low sensitivity
- 14. 14 ECD ( electron capture detector ) • The electron-capture detector is selective in its response being highly sensitive to molecule containing electronegative functional groups such as halogen, peroxide, quinones and nitro groups. • It is insensitive to functional group such as amine, alcohol, and hydrocarbons. • Electrons are supplied from a 63 Ni foil Lining the detector cell. • As current is generated in the cell. Electronegative compound capture electron resulting in a reduction in the current. • The amount of current loss is indirectly measured and signal is generated. • Selectivity : Halogen, nitrates, conjugated carbonyls • Temperature : 300-400 c • Gases : Nitrogen/Argon/methane
- 15. 15 . Advantages: 1. Useful for environmental testing detection of chlorinated pesticides or herbicides; polynuclear aromatic carcinogens, organometallic compounds. 2. Selective for halogens-(I, Br, Cl, F), nitro-, and sulfur-containing compounds. 3. Detects polynuclear aromatic compounds, anhydrides and conjugated carbonyl compounds. Disadvantages: Could be affected by the flow rate.
- 16. 16 MS (Mass spectrometer) • Mass spectrometry is used to accurately measure the mass of the various molecules within a sample. The four stages of mass spectrometry are – ionization, acceleration, deflection, and detection. Ionization • The sample is vaporized before being passed into an ionization chamber where it is bombarded by a stream of electrons emitted by an electrically heated metal coil. The forceful collisions knock off one or more electrons from the particle, resulting in a positively charged ion. Most of these have a +1 charge because of the inherent difficulty in removing a second electron from an ion that is already positive. Acceleration • The positively charged ionization chamber repels the positively charged ions, which accelerate towards three negatively charged slits with progressively decreasing voltage. The speed at which they accelerate depends on their mass so the lighter ions move faster than the heavier molecules.
- 17. 17 MS (Mass spectrometer) Deflection • In this stage, the stream of positively charged ions are deflected by a magnetic field. The extent of the deflection depends on the mass and charge of the ion. The lighter the mass of the ion, the more the deflection. Ions with a charge greater than +1 will also be deflected more. Detection • In this final stage, the beam of ions passing through the mass analyzer is detected by a detector on the basis of the mass-to-charge ratio (m/z). When an ion hits the detector, the charge is neutralized by an electron jumping from the metal onto the ion. This generates an electrical current which is proportional to the abundance of the ion. The mass spectrum generated on completion of these four stages is sent to a computer for analyses, where it shows the different m/z values of the ions present and their relative abundance.
- 19. 19 REFERENCES • J. M. Miller, Chromatography : Concepts and contrast, wiley, 2009. • P.W. Scott, Chromatography theory-Chromatographic Science, CRC Press, 2002. • G. D. Christian, Analytical chemistry, wiley, 2003.
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