Encyclopedia of Environmental Science and Engineering, Volume I and II

560 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

relaxation emit fluorescence emission. The wavelengths in the

fluorescence emission are unique for different elements. This

information may be delineated by wavelength or energy dis-

persion instruments (see Figure 4C). Wavelength dispersion

is carried out with a crystal as a diffraction grating with sub-

sequent detection by a gasfilled detector (see Figure 5), while

energy dispersion may be accomplished by a lithium-drifted

silicon detector and energy-discriminative electronic circuits

(see Figure 16). Non-dispersive instruments use filters.

Elements with atomic numbers greater than 7

(oxygen  8) fluoresce on irradiation with x-rays. Useful

irradiating wavelengths extend from 0.5 to 2.5 Å. Due to

the large absorption of wavelengths greater than 2.5 Å by

air and spectrometer windows, elements of atomic numbers

below 22 (titanium) cannot be detected. Elements with lower

atomic numbers can be detected with a change of atmo-

sphere; down to aluminium (atomic no.  13) in helium and

to boron (atomic no.  5) in a vacuum.

(9) Nephelometric & turbidimetric devices^36

The presence of turbidity, suspended matter and col-

loids, in liquid results in the scattering of a beam of incident

light passing through the liquid. The scattering process is

elastic; the wavelength of incident and scattered light is the

same. Particle shape and size distribution, size relative to the

wavelength of the incident light, concentration of particles,

and molecular absorption effect the angular distribution of

scattered light intensity. Since the scattering phenomenon is

so complicated analytical results are empirical depending on

the use of standards. However, differences in the design of

instruments leads to different values for the same standard.

Turbidimetry refers to the measure of the decrease in the

intensity of a beam of light undergoing scatter by suspended

or colloidal particles in a liquid. If the transmittance is less

than 90%, this method is effective. A filter photometer illus-

trated in Figure 9 is a suitable instrument.

If the intensity of the scattered beam is measured at

an angle to the transmitted beam, then the phenomenon is

known as nephelometry. Right angle scatter is commonly

used for a number of readings, although forward scatter

is more sensitive to large particles. Stray light caused by

scratches, dirt or condensation on cell walls leads to a positive

error. Figure 14, a simple fluorometer, measuring scattered

light at 90 can be employed for nephelometric measure-

ments. A surface scatter instrument shown in Figure 17 is

used to eliminate stray light. No cell is employed since the

flowing water sample surface is directly illuminated with the

light beam. Figure 18 illustrates a low range turbidimeter.

Electron

column

Si(Li)

detector

Electrons

Sample

X-rays

Preamp

Pulse

processor

Energy-to-

digital

converter

Video

Keyboard

Multi-

channel

analyzer

Disk

storage

system

Mini-

computer

FIGURE 16 Components of a typical energy-dispersive microanalysis system. The Si(Li) detector is cooled in

a liquid nitrogen cryostat. The charge pulse from the Si(Li) detector is converted in the preamp to a step on a volt-

age ramp. The pulse processor converts the signal to a well-shaped voltage pulse with an amplitude proportional

to the energy of the x-ray. (Courtesy of the Kevex Instruments, Inc.)

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