Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 557

(c) Atomic absorption instrumentation^29

A schematic for a single beam atomic absorption

spectrophotometer is given in Figure 12. Both flame and

electrothermal atomizers may be utilized in this instrumenta-

tion (see Figure 4A). The hollow cathode lamp is the energy

source generating uv or vis radiation that passes through

the sample. The flame or thermal area of the electrothermal

device acts as the sample cell where the sample solution that

has been nebulized (formed into a fine aerosol) is atomized.

(Atomization is the formation of free atoms through thermal

energy in the flame or thermal area.) The flame is generated

from various fuel mixtures: acetylene and the oxidants air,

oxygen or nitrous oxide; hydrogen and the aforementioned

oxidants; and natural gas and air or oxygen. Each of these

mixtures as well as the fuel/oxidant ratio determines the

flame temperature, a critical condition for atomization.

(7) Emission instrumentation 30,31

Emission of emr by elements and some chemical enti-

ties, energized by flames, plasmas, and arc, is the basis of

emission spectroscopy. The electrons are energized and

move to higher energy levels on absorption of the energy.

On relaxation, the electron returns to a lower energy level

and the absorbed energy is emitted as radiation.

Emission methods give rise to atomic spectra by a series

of atomization techniques: namely, flame, inductively cou-

pled argon plasma (ICP), electric arc and spark, and direct

current argon plasma, DCP (see Figure 4B). An emission

spectrophotometer capable of using plasma and arc and

spark sources is illustrated in Figure 13.

(8) Photoluminescence instrumentation

The occurrence of fluorescence and phosphorescence

(photoluminescence) refer to substances which on excitation

by radiation emit light on relaxation of the excited species.

In resonance fluorescence the wavelengths of excitation and

emission are the same. However, in many cases the wave-

length of emitted radiation is longer than that of the exciting

radiation. The difference in fluorescence and phosphores-

cence is the time delay between excitation and emission. The

former is quite small (
 10 ^6 sec), while the latter has a time

delay of several seconds or longer. Fluorescence can occur in

a number of organic and metal-organic complex molecules,

and gases on excitation with uv light.

(a) Uv/visible^32

A general schematic for this instrumentation is given in

Figure 4C. Fluorescence of molecular substances is measured

Field lens

Entrance slit Objective lens

Grating

Wavelength

Light control cam

Exit

slit

Occluder

Sample

Filter

Lamp

Measuring

phototube

FIGURE 10 An example of a simple spectrophotometer. The SPECTRONICR 20. (Courtesy of the Milton Roy Co.,

Rochester, NY)

Reference

Beam

Sample

Beam

Source

M5 Comb

M6

M8

M7

M9

G2

G1

M12

M13

S2

S1

Filters

Thermocouple

M14

C

+

Sampling

Area

Attenuator

M2

M1

M

M

M11

M10

FIGURE 11 Schematic diagram of a typical double-beam infrared spectrophotometer. The sym-

bol M1 through M14 indicate mirrors; S1 and S2 indicate slits; and G1 and G2 indicate gratings.

(Courtesy of the Perkin-Elmer Corporation.)

C009_005_r03.indd 557C009_005_r03.indd 557 11/23/2005 11:12:22 AM11/23/2005 11:12:22