After reading this article you will learn about the principle of spectrophotometer in monitoring pollutants.
The principle of spectrophotometer is based upon the amount of light that a sample absorbs. The instrument operates by passing a beam of light through a sample and measuring the intensity of light reaching a detector. Spectrophotometer is commonly used to measure sulphur dioxide (SO2) concentrations.
In this method, dyes and chemicals are combined with a solution containing SO2. The colour of the solution results in different amounts of light being absorbed. The amount of light absorbed indicates the amount of sulphur dioxide present in the sample.
(A) U.V. Visible-Spectrophotometry:
Ozone can be analyzed using the monochromatic ultraviolet absorption spectrophotometry principle. As ultraviolet light at 253.7 nm is passed through the optic bench, a fixed quantity of “zero air” and ambient air are drawn into the bench.
The intensity of the ultraviolet radiation traversing the optic bench is attenuated by the ozone present in the ambient sample. This attenuated signal is detected and compared with the un-attenuated signal from the “zero air” cycle. This difference in intensity is electronically translated into a reading of ozone present in the ambient air.
(B) Gas Filters Correlation (GFC) Spectroscopy:
In this instrument the light of IR region is passed through the sample cell and into a receiving unit. In this unit, the beam is split and sent to two, parallel cells called the gas filter cell and the neutral density filter cell. The gas filter cell contains a high concentration of the gas being analyzed, and therefore removes nearly all of the energy of the predetermined wavelength (reference = 100% absorbance).
What remains of that beam (i.e., the wavelength not absorbed in the gas filter cell) passes of the detector. The beam from the neutral density filter cell also goes to the detector. The difference in energy of these two beams can then be related to the concentration of the gas of interest.
The advantage of splitting the beam is that low levels of particulate matter will not adversely affect the concentration reading. Particulate matter can further reduce the intensity of a light beam which is transmitted through a sample cell.
But because the beam goes to two separate cells and the signals from these cells are ratioed, the effect of particulate matter will then cancel and the concentration reading of the subject gas will be more accurate. Gas filter correlation spectroscopy is used to measure SO2, NO, CO, and CO2.
(C) Differential Absorption Spectroscopy Principle:
Certain gas molecules absorb unique wavelengths of light. Unlike non-dispersive IR spectroscopy which uses a sample cell and a reference cell, differential absorption analyzers employ a “measuring” wavelength and a “reference” wavelength.
The measuring wavelength corresponds to a region of the spectrum where the gaseous molecules of interest absorbs light energy, whereas the reference wavelength corresponds to a region of the spectrum in which the gaseous molecules of interest absorb little or no light energy.
The concentration of the identified gas is determined by finding the difference in energy for the measuring wavelength and the reference wavelength. The detected difference is then processed and sent to the readout instrument as a concentration value.
By changing the optical system it is possible to measure other gases. Certain filters can also be used to distinguish measuring and reference wavelengths.
(D) Second Derivative Spectroscopy:
In this technique a scanner moves back and forth across the absorption peak of the subject gas. The scanning is related to the second derivative of the absorption peak, with respect to wavelength. The signal from scanning is then used to determine the concentration of the selected gas.
It is mainly used for the detection of SO2 and NO air pollutant.