Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 551

the particular metal target. Metals such as chromium, copper,

iron, molybdenum, rhodium, silver, tungsten and others com-

prise the anode target. A number of radioisotope sources emit

useful x-rays, e.g., iron-55 yields manganese K radiation,

cadmium-109 gives silver K radiation, and cobalt-57 provides

iron K radiation.

(2) Sample interface

The sample is usually presented as a solution contained

in a cell made of material transparent to source radiation

for absorption and photoluminescence (see Figure 3). Solid

samples are also used. Potassium bromide disks containing

homogeneously distributed powdered analyte are used in ir

absorption methods.

However in atomic absorption spectroscopy the sample

is atomized in a flame, plasma or thermal heat source. In

effect the sample container is that volume of flame, plasma

or heat source.

Solutions, as well as solid samples in the form of pressed

disks, pieces of solids, or solid solutions in borax, are con-

veniently analyzed in an x-ray fluorescence instrument.

Solutions of sufficient thickness are the best sample prepara-

tions because of their homogeneity; they may be contained

in mylar cells. Obviously the solvent must not contain heavy

atoms that fluoresce. Sample surfaces are directly exposed to

the x-ray beam (see Figure 5).

In emission instruments the solid sample is placed in an

energy source environment, e.g., an electrical arc or spark, a

flame, or plasma.

(3) Wavelength selectors

The wavelength selector allows isolation of a particu-

lar wavelength segment of the source or transmitted beam.

A monochrometer is a selector comprising a grating or a

prism which disperses or separates the radiation continuously

over a considerable wavelength region. The effective band-

width of the wavelength, isolated by slits placed before the

sample, is quite narrow, 1 nm or less. The grating operates on

the principle of interference and the prism by dispersion.

Other wavelength selectors are interference and absorp-

tion filters. Their effective bandwidths are about 20 to 50 nm,

respectively; they are not continuous. An interference wedge

is continuous over a region with an effective bandwidth

of 20 nm.

The dispersing device, a single crystal mounted on a

rotating table or goniometer (see Figure 5a), is the wave-

length selector used in x-ray spectrometers. A specific

wavelength and its second and third orders of reflection are

diffracted at a given angle of the beam to a particular plane

of the crystal. The angle of diffraction depends on the “d” or

interplanar spacing of the crystal and the wavelength and is

defined by Bragg’s law. Some examples of diffracting crys-

tals with their unique wavelength ranges are topaz—0.24 to

2.67 Å, sodium chloride—0.49 to 5.55 Å, and ammonium

diphosphate—0.93 to 10.50 Å. (An angstrom, Å, is 10^ ^8

cm.) Unlike a prism or a grating that disperses a total spec-

trum in the spectral regions of the source of radiation, the

x-ray monochrometer diffracts a unique wavelength and its

orders of reflection depending on the angle of the beam to

the crystal plane.

(4) Detectors

In Figure 3 the variety of transducers are listed with their

wavelength range of detection. Following is a description

of the most commonly used detectors grouped according to

their wavelength range.

(a) Uv/visible

(i) Photovoltaic (barrier layer) cells

This detector, which generates its own signal, is sensitive

to radiant energy in the visible (350 to 750 nm) region. Light

shining on a semiconductor coating, such as selenium or

copper(I) oxide plated on an iron or copper electrode, gener-

ates a current at the metal–semiconductor interface. A second

electrode, a transparent coating of gold or silver on the outer

surface of the semiconductor, collects the electrons formed

by the action of radiant energy on the semiconductor. The

magnitude of the photocurrent is proportional to the number

of photons/sec impinging on the semiconductor. This detec-

tor is insensitive to low light levels, slow in response, shows

a tendency to suffer fatigue, and has a high temperature coef-

ficient. However, photovoltaic cells are rugged, require no

separate source of energy and are low in cost. They are used

in inexpensive filter photometers.

(ii) Vacuum photoemissive tubes^20

In a photoemissive detector two electrodes, a cathode

with an electron emissive coating and an anode, are enclosed

in an evacuated tube. When the saturation potential is applied

A. Absorptiona & Turbidity

Energy

Source

Wavelength

Selector

Sample Photoelectric

Detector

Signal

Processing

& Readout

Energy

Source

Energy

Source

Signal

Processing

& Readout

Signal

Processing

& Readout

Wavelength

Selector

Wavelength

Selector

Wavelength

Selector

Signal

Processing

& Readout

Photoelectric

Detector

Photoelectric

Detector

Photoelectric

Detector

Sample

Sample

Sample

Energy

Source

Filter

or

Wavelength

Selector

B. Infrared absorption

C. Emissionb

auv-vis and AA (flame and electrothermal)

barc, dc spark, inductively coupled & dc plasma, & flame

D. Fluorescencec & Nephelometry

cuv, x-ray

FIGURE 4 Outline of spectroscopic instrumentation.

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