SPECTROPHOTOMETER INTRODUCTION: A spectrophotometer is an analytical instrument that measures the intensity of light as a function of its wavelength. A common tool in physics, spectrophotometers are commonly used to measure light absorption. There are two major measurement classes of spectrophotometers-spectrophotometers-single beam and double beam spectropho spectrophotomete tometers. rs. There are a few major differences between the single beam and the double beam spectrophotometers. A single beam spectrophotometer measures absolute light intensity, while the double beam spectrophotometer spectrophotometer measures the ratio of light intensities on two separate light paths--the reference standard and the sample. Although double beam spectrophotometers were popular in the early days of spectroscopy, it is now thought that the single beam spectrophotometer is more advantageous. This is because it becomes difficult to recombine the light beam prior to reaching the monochromator with the double beam spectrophotometers. The monochromator is a device for selecting light from a narrow band of wavelengths. The monochromator monochrom ator splits the light into its component wavelengths, wavelengths, so that only the light from the desired wavelengths reach the sample. One example of a monochromator is a prism. The single beam spectrophotometers also have a larger dynamic range than the double beam spectrophotometers.The oldest form of spectroscopy is ultraviolet/visible spectroscopy. An ultraviolet/visible ultraviolet/ visible spectrophotometer spectrophotometer works by placing a sample in the spectrophotometer spectrophotometer and shining ultraviolet and/or visible light through. Measurements are made depending on how much light was absorbed by the sample.
PRINCIPLE: The basic components of a spectrophotometer spectrophotometer are a light source, a wavelength monochromatic, focusing devices, a cuvette, a photo detector, and a display device. Light passes through a monochromatic that separates the light into its component wavelengths. Slits then isolate a band pass (the segment of the spectrum isolated by the monochromatic) of the wavelength needed for measurement, improving its purity. Next, the light passes through the sample, and a portion of the radiant energy is absorbed; the amount depends on the nature and concentration of the sample. The portion of the band pass that is not absorbed is transmitted to a photo detector, which converts light energy to electrical energy that can be registered on a meter or digital readout. COMPONENTS OF SPECTROPHOTOMETER: All spectrophotometer spectrophotometer instruments designed to measure the absorption of radiant r adiant energy have the basic components as follows 1. A stable source of radiant energy (Light). 2. A wavelength selector to isolate a desired wavelength from the source (filter or monochromator) 3. Transparent container (cuvette) for the sample and the blank.
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. A radiation detector (phototube) to convert the radiant energy received to a measurable signal. 5. A readout device that displays the signal from the detector. 4
Components of a spectrophotometer
The energy source is to provide a stable source of light radiation, whereas the wavelength selector permits separation of radiation of the desired wavelength from other radiation. Light radiation passes through a glass container with sample. The detector measures the energy af ter it has passed through the sample. The readout device calculates the amount of light absorbed by the sample displays the signal from the detector as absorbance or transmission.The spectrophotometers which are used for such measurements may vary from simple and relatively inexpensive colorimeters to highly sophisticated and expensiveinstruments that automatically scan the ability of a solution to absorb radiation over a wide range of wavelengths and record the results of these measurements.One instrument cannot be used to measure absorbance at all wavelengths because a given energy source and energy detector is suitable for use over only a limited range of wavelengths. Spectrophotometers are instruments equipped with monochromators that permit the continuous variation and selection of waveiength.The sample containers, cells or cuvettes, must be fabricated from material that is transparent to radiation in the spectral region of interest. TYPES OF SPECTROPHOTOMETER: There are two major classes of devices: single beam and double beam. A double beam spectrophotometer compares the light intensity between two light paths, one path containing a reference sample and the other the test sample. A single beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is inserted. Although comparison measurements from double beam instruments are easier and more stable, single beam instruments can have a larger dynamic range and are optically simpler and more compact.
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Additionally, some specialized instruments, such as spectrophotometer built onto microscopes or telescopes, are single beam instruments due to practicality.
SINGLE BEAM SPECTROPHOTOMETER: A single beam spectrophotometer has one light path that passes from the light source through the monochromator system and sample cuvette and then to the detector.A blank is used to set the instrument to 100%T(0 A),then the samples are read.
DOUBLE BEAM SPECTROPHOTOMETER: A Double beam spectrophotometer has two light paths,both originating from the same light source.One path is for the sample and other for the blank or reference.The beam from the source strikes a vibrating or rotating mirror that alternate directs light through the reference cell and the sample cell.Light passing through each cell is sent to the detector.
Uses of a Spectrophotometer
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Spectrophotometers are directly used to measure light intensity at different wavelengths, and this can be represented as percent of incident light transmitted or absorbed.
Using this
information and comparing it to other data obtained or known, spectroscopy can be used as a tool. One example is comparing spectra to determine concentrations of a solute in solution. This can be done by recording transmittance/absorbance at a specific wavelength (a wavelength that the solute absorbs) and known concentration.Then analysis of a solution of unknown concentration can be compared to the known data, and be interpolation the concentration can be calculated. This can even be done with solutions containing multiple solutes, however is it most accurate when the different solutes absorb different wavelengths. Spectrometers that do not have a light source, but generate spectra based on the incoming light can be used in a similar way to identify light sources. The spectra graph obtained from an unknown light source (or mixture of sources) can be compared to a database of graphs for different known light sources to identify the unknown light source. Another application of the spectrophotometer is to determine the equilibrium constant of a reaction involving ions, which takes place in aqueous solution.
Starting a solution
containing only one reactant, the spectrum is measured. Then small, measured amount of the other reactant is added and after each addition, the spectrum is measured again. This method works best if there is a known wavelength that the product absorbs. Then, as more products are formed from adding more reactant, more light will be absorbed.
When the solution
becomes saturated and the reaction reaches net equilibrium, the increase in light absorption will level out, indication equilibrium.
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Reference: http://en.wikipedia.org/wiki/Spectrophotometry http://popular.ebay.com/business-industrial/spectrophotometer.htm http://www.google.com.pk/imgres?imgurl=http://www.chemguide.co.uk/analysis/uvvisible/spectromet er.gif&imgrefurl=http://www.lookchem.com/Chempedia/Chemical-Instruments/ApplicationNote/8929.html&usg=__qeJe0NyBZCZQcUOwfoQqXrz_AY=&h=269&w=401&sz=5&hl=en&start=10&zoom=1&tbnid=AYPRHtToRr1DyM:&tbnh=83&tbn w=124&ei=rmqjTfugAYPEsAPd7_35DA&prev=/images%3Fq%3Ddouble%2Bbeam%2Bspectrophotomete r%26um%3D1%26hl%3Den%26biw%3D1280%26bih%3D656%26tbm%3Disch&um=1&itbs=1 http://www.chemguide.co.uk/analysis/uvvisible/spectrometer.html http://www.hunterlab.com/appnotes/an11_97.pdf http://www.rrcap.unep.org/male/manual/national/11Chapter11.pdf
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