Chemistry Reference
In-Depth Information
White
Red
First order
spectrum
Violet
Zero order spectrum
(no diffraction)
Violet
Red
First order
spectrum
Figure 7.14
A diagram of a diffraction grating.
Detector
After light has passed through the sample, any decrease in intensity, due to
absorption, is measured by a detector. This is usually a clever piece of elec-
tronics called a
photomultiplier tube
(see Figure 7.15), which acts to convert
the intensity of the beam of light into an electrical signal that can be
measured easily, and then also acts as an amplifier to increase the strength
of the signal still further. Light enters the tube and strikes the cathode; this
releases electrons, which are attracted to an anode above. When the elec-
trons strike this anode they release more electrons, which are, in turn,
attracted to the anode above that, where the process is repeated. In this way
a cascade of electrons is generated and the signal is amplified.
Once the electrical signal leaves the photomultiplier tube, it is fed to
a recorder if a printout is required, or, more usually, to a screen where the
absorption spectrum can be displayed. Most modern spectrophotometers
are now interfaced to a personal computer to allow storage of large
amounts of data, or to allow access to a library of stored spectra on the
hard drive of the machine. This allows comparison of stored spectra with
the experimentally derived results from the laboratory and aids in the
identification of unknown compounds.
Experimental measurement of absorbance
The sequence of events in making a measurement with a spectrophotometer
is as follows.
The monochromator is set to the wavelength of measurement, the shutter is
closed to prevent light reaching the detector, and the instrument is set to infinite
1.
