Page 112 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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can isolate a narrow range of wavelengths (e.g. 1-0.01 nm) anywhere in a comparatively wide spectral
range (for atomic absorption spectrometry typically 190-900 nm). Most modern instruments use a
diffraction grating, which is a mirror having between 600 to 4200 lines per millimetre etched into it.
The commonest type of diffraction gratings in use are reflection gratings, which are manufactured by
depositing a layer of aluminium on the grating blank, then ruling the lines with a diamond tool, to form
a master grating from which replica gratings can be produced. A more modern method is used to
produce holographic gratings, obtained by coating the blank with a photosensitive material on to
which is projected the interference pattern of two coherent lasers to form the lines. The glass is etched
and a layer of aluminium deposited to form the reflective surface. Holographic gratings are less prone to
ghosting because the line pattern is more uniform, although they are less efficient.

A detailed discussion of the theory of a diffraction grating is beyond the scope of this book. However, a
simplified discussion of the mode of operation of a transmission grating will illustrate the general
principle which can be applied to a reflection grating. A transmission grating is a transparent plate
having 2000 lines mm ruled in it. Light transmitted through the grating behaves like light emanating
-1
through a series of regularly spaced narrow slits. From these, the light will emerge as a series of
intersecting wavelets like a series of semi-circles emanating from each slit, i.e. as if each slit were itself
a source of radiation. In a short time the wave systems will recombine to form the original wave-front,
the so-called zero-order. More interestingly, at a series of angles, 0 to the grating, there will be
constructive interference for light of a given wavelength. The position of these diffraction wavefronts is
given by the grating equation:

nl = d sinq

where l is the wavelength, d the space between the rulings on the grating and n an integer (0, 1, 2, 3,. .
. ) called the order. Thus, a beam of polychromatic radiation is diffracted into a series of spectra
symmetrically located on either side of the normal to the grating. It should be noted that for any given q
there will be several different wavelengths, albeit in different orders.

The problem of overlapping orders is reduced by blazing the grating. Figure 4.11 shows schematically a
blazed reflection grating, where the facets are tilted at an angle f, the blaze angle, from the surface of
the grating. A ray incident at an angle a will be reflected from the groove face at an angle ß, such that a
+ f = ß - f. Such a grating is known as an echellete grating and is highly efficient in the diffraction of
wavelengths for which specular reflection occurs (i.e. most of the radiation is reflected in a particular
order determined by the blaze angle, usually the first order). The wavelength at which specular
reflection and first order diffraction coincide is called the