A spectroscope is an instrument which separates the regions of light into a spectrum (an ordered arrangement of radiation according to wavelength). A very simple spectroscope is one using a prism which separates the visible range of light into its colours for each colour has a different wavelength. When a meter for reading wavelength is attached, the instrument is called a “spectrometer”.
A spectrophotometer, in addition to a monochromator, will have a part, called a detector, that will detect the amount of radiation that is absorbed by the sample.
A double beam spectrophotometer is used for the study of absorption in uv-visible region. It consists of following essential components
(1) Source of radiation
(3) Beam splitter
(4) Sampling area
(5) Light chopper
(7) Amplifier and
A schematic diagram of a double beam uv-visble spectrophotometer is shown in the Fig.-2.6.
( I )SOURCE
The source of visible radiation is usually a tungsten filament lamp, which provides radiations in the visible region. A hydrogen discharge lamp or a deuterium discharge lamp is used as an ultraviolet source.
A monochromator consists of the entrance and the exit slits, a prism and mirrors or lenses to deflect or focus the radiation. The prism resolves the polychromatic radiation into monochromatic radiations. The wavelength of the radiation to be used can be selected by rotating the prism. Diffraction grating provides better resolution than prism.
The monochromator radiation is split into two beams of equal intensity by means of a beam splitting device called beam splitter. One beam passes through the reference cell and the other through sample cell.
The sample holders are the cells commonly called covets, usually made of quartz, and have rectangular walls through which radiation passes must be transparent.
(5) LIGHT CHOPPER
The two beams coming out of the reference and the sample cells are reflected to the radiation-chopping device. This device consists of a rotating semicircular mirror, which rotates at a frequency of approximately 10 cycles per second. As the chopper rotates, it causes the reference beam and the sample beam to be reflected alternately to the detector.
The commonly used detector is the photomultiplier tube. After the absorption of radiation of a particular wavelength by the sample and reference. the detector will receive from the chopper alternately an intense reference beam and a weak sample beam. This will cause an alternating current to flow through the detector.
From the detector, the current is passed to the amplifier, which is designed to amplify only the alternating current.
The ratio of intensities of the ‘reference and the sample beams is amplified and fed to a self-balancing potentiometric pen recorder. The radiation that falls on the detector creates a voltage proportional to the intensity of the radiation, which strikes the detector. The off-balance voltage resulting from absorption of the sample beam is balanced by an equivalent voltage tapped from a portion of a slide wire. The recorder pen travels with the contact of the slide wire and records absorbance (A) against wavelength .
Ultraviolet spectra of compounds are usually determined either in the vapour phase or in solution. The solution must be diluted and its concentration is known. Before putting the solution in the cell, the quartz cell should be thoroughly rinsed with the solvent. The solvent used for making solution must be transparent within the wavelength range to be examined. The most commonly used solvents are: cyclo-hexane, 95% ethanol and 1,4 -dioxane. The solvent should be inert to the solute. For example, the spectra of aldehydes should not be determined in alcohol because of occurring chemical reaction.