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range from a broad band polychromatic source. The transmission range may be 50–300 nm for

typical absorption filters. Absorption filters are limited to the visible region of the spectrum and

the X-ray region. The second type of filter is the interference filter, constructed of multiple layers
of different materials. The filter operates on the principle of constructive interference to transmit

selected wavelength ranges. The wavelengths transmitted are controlled by the thickness and
refractive index of the center layer of material. Interference filters can be constructed for

transmission of light in the IR, visible, and UV regions of the spectrum. The wavelength ranges

transmitted are much smaller than for absorption filters, generally1–10 nm, and the amount of
light transmitted is generally higher than for absorption filters.

Monochromator

A monochromator consists of a dispersion element, an entrance slit and an exit slit, plus
lenses and mirrors for collimating and focusing the beam of radiation. The function of the

dispersion element is to spread out in space, or disperse, the radiation falling on it according to

wavelength. The two most common types of dispersion elements are prisms and gratings. You
are probably already familiar with the ability of a prism to disperse white light into a rainbow of

its component colors. The entrance slit allows light from the source to fall on the dispersion
element. The dispersed light falls on the exit slit of the monochromator. The function of the exit

slit is to permit only a very narrow band of light to pass through to the sample and detector. One

way to accomplish this is to rotate the dispersion element to allow dispersed light of different
wavelengths to fall on the exit slit in sequence. For example, a white light source is dispersed

into violet through red light by a prism or grating. The dispersion element is rotated slowly,
allowing first violet light through the exit slit, then blue light, and so on all the way to red light.

In this way, the monochromator sorts polychromatic radiation from a source into nearly

monochromatic radiation leaving the exit slit.

Detectors
The most common detector that is used for AAS is the photomultiplier (PM) tube . A

sketch of a photomultiplier tube is shown in Fig. The tube consists of a cathode, an anode, and
several additional electrodes that are called dynodes. All of the electrodes are enclosed in an

evacuated glass envelope that is similar to an old radio tube. In the PM tube design shown in Fig.

The electrodes extend nearly the entire length of the tube. Electric connections to the electrodes
are made through pins that protrude from the bottom of the tube.

If the PM tube is to be used in the uv region, a window of quartz or some other uv

transparent material is mounted in the wall of the glass tube to allow radiation to enter the tube.
EMR that enters the tube passes through a metal grill and strikes the cathode, or photocathode as

it is often called. The cathode is coated with some substance that has a low ionization potential,

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