Encyclopedia of Environmental Science and Engineering, Volume I and II

550 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

The types of instruments can be characterized by some

simple diagrams regardless of wavelength range as given in

Figure 4. The basic difference between absorption and emis-

sion spectroscopy is the use of a transmitted emr energy

source in the former, while in the latter, the sample is stimu-

lated in a thermal or electrical energy source to emit radiation.

Photoluminescence (fluorescence and phosphorescence),

stimulated by emr, is observed usually perpendicular to the

stimulating beam. In nephelometry instrumentation similar

to photoluminescence is utilized; turbidimetry can employ

absorption instrumentation.

A brief description of the basic parts of the instruments

using these phenomena follows.

(1) Energy sources

As noted from the diagrams above, all but emission

instrumentation use energy sources that irradiate the sample.

In Figure 3 the sources are indicated as a function of the

wavelength range of their radiant emissions. Various lamps,

i.e., argon, xenon, H 2 (hydrogen) and D 2 (deuterium), and

solid state radiators give continuous emissions, i.e., a range

of contiguous wavelengths and are used in molecular spec-

troscopic instrumentation. Photoluminescence and nephelo-

metric instruments use these sources.

For atomic absorption instruments hollow cathode

lamps are utilized. They are line (discontinuous) sources

providing unique radiation with a narrow bandwidth char-

acteristic of particular element. An individual lamp is usu-

ally employed for each element. Some multielement lamps

are available.

X-ray sources include x-ray tubes or radioactive sources.

The x-ray tube consists of a tungsten cathode that emits elec-

trons when heated. The electrons accelerated by a large poten-

tial strike the metal anode generating x-rays characteristic of

(a) Sources

Continuous

Wavelength, nm

Discontinuous

Spectral region

100 200 400 700 1000 2000 4000 7000 10,000 70,000 40,000

VAC UV UV VISIBLE IR

Argon

lamp

(b) Wavelength

selectors

Continuous

Discontinuous

fluorite

prism

(c) Materials for

cells, windows,

& lenses

(d) Transducers

Photon

detectors

Heat

detectors Pyroelectric cell (capacitance)

Golay pneumatic cell

Thermocouple (volts) or Bolometer (ohms)

Photoconductor

Silicon diode

Tungsten lamp

Nernst glower (ZrO 2 + Y 2 O 3

Nichrome wire (Ni + Cr)

Globar (SiC)

Hollow cathode

lamps

Photomultiplier

Phototube

Photocell

TIBr - TII

KBr

NaCl

Silicate glass

Corex glass

Fused silica or quartz

LiF

filters

Glass absorption

Interference filters

Interference wedgers

Gratings with various number of lines/mm 50 lines/mm

KBr prism

NaCl prism

Fused silica or quartz prism

xanon lamp

3000 lines/mm

Glass prism

NEAR IR FAR IR

H 2 or O 2

lamp

FIGURE 3 Components and materials for optical spectroscopic instruments. (Courtesy

of Prof. A. R. Armstrong, College of William and Mary.)

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