SOURCE:- The source depends upon the wavelength range that is required for the study. Intense source are normally used in order to maximize the output signal and to increase the signal-to-noise ratio. Tungsten filament lamp have been used in the spectral range from about 600 to 1000nm. The most popular is high pressure(50 to 70 atm) xenon arc lamp in spectral range of 250 to 2500nm. Hydrogen and dueterium arcs are useful from about 165 to 250nm. Krypton lamp can be used from 125 to 165nm. Open carbon arc lamp from 350 to 700nm can be used but the disadvantage is that carbon electrode in the source are consumed during emission and must be replaced periodically. 3
Lasers are available for use throughout most of the spectral range used for PAS. the lasers can be operated in either the continous or pulsed mode. both lasers that emit light and lasers that emit continuum are used. E.g argon ion laser, nitrogen laser, Nd:YAG laser, co2 laser, co laser etc. The intensity of the source is usually modulated, at a single wavelength, either by pulsing the source or by using a constant- intensity source with a 4 chopper.
WAVELENGTH SELECTOR:- In instruments that use a continuous source, such as a xenon-arc lamp, the radiation is usually made monochromatic with a diffraction grating monochromator. Sources that emit line radiation, such as some lasers, can use a monochromator or filter to select the proper line. 5
CELLS:- Cell design depends on the physical state of sample. The cell must posses one transparent window through which incident radiation can pass and acoustically insulated to prevent interference from external signals. The body is constructed from stainless steel or polished aluminum, that has large thermal mass. A substance that has a large thermal mass requires absorption of a relatively large amount of energy before its temperature is significantly raised. In addition cells are also constructed from copper, silver, and gold. 6
Because of large thermal mass, scattered incident radiation that strikes the cell body causes an insignificant temperature rise and PAS signal relative to those that occur when radiation strikes an absorbing samples. The cell windows are constructed from material that is transparent. Sapphire and quartz window are most commonly used in UV region. In the visible region glass and plastic windows have been used. In IR region – ZnSe is used. 7
When analyte is a gas, the cell is most often cylindrical.1. Single-pass cells: In those cells that have two windows on the end of the cylinder, radiation makes a single pass through the cell. In some cells acoustic baffle are used to decrease noise within cell.2. Multiple-pass cells: In this type both ends of the cylinder are coated with a multilayer of a highly reflective dielectric material. A hole through the dielectric substance allows radiation to enter the cell. 8
The radiation is reflected through the sample several times by dielectric material. This type can be used for assays at lower concentrations than single-pass cells.3. Differential cell: Often consist of two sequentially connected cells within the same cylinder. Each cell use separate detector. The incident radiation sequentially passes through the cell. The combined cell and detector is a spectrophone. PAS in the IR region has been used as detector for gas chromatography. 9
The distance between the sample and the window should be greater than the length the length of thermal diffusion in order to prevent loss of thermal energy from the sample through the window. The volume of the filler gas within the cell is kept as small as possible in order to maximize the PAS signal. The filler gas chosen to be non absorbing. Often air or an inert gas is used. Used of an inert gas generally results in greater analytical sensitivity. If the sample is a thin layer of a thermally thin substance, incident radiation can strike the sample without first passing through the filler gas because heat can rapidly travel to inner surface of the sample. If the sample is not a thin layer of thermally thin substance, radiation usually strikes the sample from 11 above after passing through a filler gas.
DETECTORS:- Microphone and piezoelectric transducer(PZT) have been used as detector for PAS. Of the several type of microphones, the condenser microphone is most common used.1. Condenser microphone: Consist of circular, thin metallic diaphragm that is mounted by its edge near a rigid stationary metallic plate. The two plate serve as the plate of capacitor in an electric circuit. The thin diaphragm is forced closer to the stationary plate whenever a pressure wave strikes it. When pressure is no longer exerted on the diaphragm by the wave, the diaphragm is returned to its initial position by tension in the diaphragm. 12
The result is diaphragm that vibrates at the frequency of the modulated radiation that strikes the sample. The distance through which the diaphragm modulates is the function of amplitude of pressure wave that is emitted from the cell. Microphone cannot be used in liquids. Piezoelectric transducer are primarily used for detecting PAS signal in solutions.2. Piezoelectric detector: Is a solid material such as lead zirconate titanate which has a structure that is altered with application of pressure. Structural alteration causes a rearrangement of charge within a substance and an alteration of the potential difference across the material. A piezoelectric transducer changes a pressure wave into an electric signal which subsequently is amplified and monitered. 13