Detectores.pdf
- 1. Detectors • Detectors: • An infrared detector is a detector that reacts to infrared radiation. • It is simply a transducer of radiant energy. • Detectors are used to measure the intensity of unabsorbed infrared radiation.
- 3. 1. Thermal detectors: • • Can be used for wide range of wavelength. • Operate at room temperature. • Slow response time & lower sensitivity hence not use for FTIR. • Thermal detectors are better choice except in the near infrared where photoconductivity cells are generally used.
- 4. Thermocouple • Made up of 2 dissimilar metal like bismuth & antimony coated by metal oxides which produce small voltage proportional to temperature of junction. • If wires of 2 dissimilar metals joined head to tail, then a difference in temperature between head & tail causes a current to flow in the wire. • This current is proportional to the intensity of radiation.
- 6. Cont…. These are also called as thermopile detectors. • Materials should be thermally active. • These are used in dispersive instruments. • Response time is 30 milliseconds. • They give response for all frequencies. • The potential difference (voltage) between the junctions changes according to the difference in temperature between the junctions. • Heating capacity of absorbing element is small as to detect the change in temperature.
- 7. Cont…. • Potential varies in the 2 junctions with the difference in temperature. • One junction is cold junction which is reference junction kept at constant temperature not exposed to IR radiations. • The other junction is hot junction exposed to IR radiations which increases temperature of junction. • Temperature difference due to falling IR radiations generates the potential difference which amounts to IR radiations. • Hot junction is generally blackened to improve its heat absorbing capacity. • Advantage- Simple & used for short IR wavelength • Disadvantage- slower response time & sensitivity
- 8. Bolometers • Bolometer is derived from a Greek word (bolometron) Bolo- for something thrown Metron- measure Construction- • It consists of an absorptive element such as a thin layer of metal • Most bolometers use semiconductor or superconductor absorptive elements rather than metals. • Its resistance changes due to increase in temperature when IR radiation falls on it. • It is electrical resistance thermometer which can detect & measure feeble thermal radiation. • The electrical resistance increases approx. 0.4% for every Celsius degree increase of temperature.
- 9. It is made of 2 platinum strips, covered with lamp black, one strip is shielded from radiation & one exposed to it. • The strips form 2 branches of Wheatstone bridge.
- 10. Working • The circuit thus effectively operate as resistance temperature detector. • When IR radiations fall on the exposed strip, unbalance occur due to change in resistance , this causes current to flow through Galvanometer & the amount of current flowing is the measure intensity of IR radiation. • The response time is 4 seconds.
- 11. Golay cell • It consists of a small metal detector closed by a rigid blackened metal plate (2mm) , flexible slivered diaphragm at the other end filled with xenon gas. • Its response time is 20 milliseconds, hence faster than other thermal detectors • It is suitable for wavelengths greater than 15μ. • Also known as golay pneumatic detector. • Gas filled chamber undergoes a pressure rise when heated by radiant energy. • It is based on expansion of gas measuring device.
- 13. Working • The radiation falls on the blackened metal plate, this heats the gas which leads to deformation of flexible silvered diaphragm. • The light from a lamp inside the detector is made to fall on the diaphragm which reflects the light on to a phototube. • The signal seen by the phototube/photocell is modulated in accordance with the power of the radiation beams incident on the gas cell. • Advantage- • Wavelength is very wide. • Response time is much faster than bolometer or thermocouple.
- 14. Apllications of IR • APPLICATIONS OF IR SPECTROSCOPY • Infrared spectroscopy is widely used in industry as well as in research. It is a simple and reliable technique for measurement, quality control and dynamic measurement. It is also employed in forensic analysis in civil and criminal analysis. • Some of the major applications of IR spectroscopy are as follows: • 1. Identification of functional group and structure elucidation • Entire IR region is divided into group frequency region and fingerprint region. Range of group frequency is 4000-1500 cm- 1 while that of finger print region is 1500-400 cm-1. • In group frequency region, the peaks corresponding to different functional groups can be observed. According to corresponding peaks, functional group can be determined. • Each atom of the molecule is connected by bond and each bond requires different IR region so characteristic peaks are observed. This region of IR spectrum is called as finger print region of the molecule. It can be determined by characteristic peaks.
- 15. 2. Identification of substances IR spectroscopy is used to establish whether a given sample of an organic substance is identical with another or not. This is because large number of absorption bands is observed in the IR spectra of organic molecules and the probability that any two compounds will produce identical spectra is almost zero. So if two compounds have identical IR spectra then both of them must be samples of the same substances. IR spectra of two enatiomeric compound are identical. So IR spectroscopy fails to distinguish between enantiomers. For example, an IR spectrum of benzaldehyde is observed as follows. C-H stretching of aromatic ring- 3080 cm-1 C-H stretching of aldehyde- 2860 cm-1 and 2775 cm-1 C=O stretching of an aromatic aldehyde- 1700 cm-1 C=C stretching of an aromatic ring- 1595 cm-1 C-H bending- 745 cm-1 and 685 cm-1 No other compound then benzaldehyde produces same IR spectra as shown above.
- 16. Studying the progress of the reaction Progress of chemical reaction can be determined by examining the small portion of the reaction mixure withdrawn from time to time. The rate of disappearance of a characteristic absorption band of the reactant group and/or the rate of appearance of the characteristic absorption band of the product group due to formation of product is observed. 4. Detection of impurities IR spectrum of the test sample to be determined is compared with the standard compound. If any additional peaks are observed in the IR spectrum, then it is due to impurities present in the compound.
- 17. Quantitative analysis The quantity of the substance can be determined either in pure form or as a mixure of two or more compounds. In this, characteristic peak corresponding to the drug substance is chosen and log I0/It of peaks for standard and test sample is compared. This is called base line technique to determine the quantity of the substance.