Chemistry Reference
In-Depth Information
a plot of the whole absorption spectrum of the sample, switch lamps
automatically.
Monochromator
For most quantitative measurements, light must be
monochromatic
, i.e. of
one particular wavelength. This is achieved by passing the polychromatic
light (i.e. light of many wavelengths) through a monochromator. There are
two types of monochromator in modern spectrophotometers:
prisms
or
diffraction gratings
.
A prism is a triangular piece of quartz that refracts (or bends) light
passing through it. The extent of the refraction depends on the wavelength
of the light, so a beam of white light can be split into its component colours
by passage through a prism. The prism is then rotated to select a particular
wavelength required for the assay (Figure 7.13). This effect is identical to the
formation of a rainbow when light from the sun is split into its seven
component colours (red, orange, yellow, green, blue, indigo and violet) by
refraction through droplets of rain.
Angle of
refraction
Figure 7.13
A diagram of a prism.
A diffraction grating is a small piece of mirrored glass onto which a
large number of equally spaced lines have been cut, several thousand per
millimetre of grating, to give a structure that looks like a small comb. The
spaces between the cuts are approximately equal to the wavelengths of
light and so a beam of polychromatic light will be resolved into its compo-
nent wavelengths by the grating. The grating is then rotated to select the
wavelength desired for assay (Figure 7.14).
