Encyclopedia of Environmental Science and Engineering, Volume I and II

556 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

path length of the sample cell, b, and its absorbance units, Ab,

is expressed as follows:

molar absorptivity, e  Ab /( b, cm)( C, moles/L) (9)

absorptivity, a  Ab /( b, cm)( C, g/L). (10)

The absorptivity, a, can be expressed in a number of con-

centration terms (g/L, grams/Liter and mg/ml, milligrams/

milliliter) and path length terms (cm, centimeters and mm,

millimeters).

Equations 9 and 10 are a statement of Beer’s Law indicating

a linear relationship between the absorbance and the con-

centration for fixed conditions. In analytical determinations

the concentration can be calculated using equations 9 or 10.

Also used in analysis is a calibration curve, whose slope is

absorptivity. It can be drawn using concentrations and cor-

responding absorbance values. Beer’s Law prevails for many

substances, but there are deviations for some substances due

to chemical, instrumental, and physical phenomena.

(a) Uv/visible instrumentation^27

A uv/visible instrument consists mainly of uv and visi-

ble energy sources (lamps), a wavelength selector (grating,

prism or filter), reference and/or sample cell, a detector

(photodetector or photomultiplier) and a readout device

(recorder, analog or digital meter, etc.) (see Figure 4A).

Figures 9 and 10 illustrate the arrangement of these parts

for photometers and spectrophotometers, respectively. The

distinction between the two types of instruments is that a

photometer uses a filter and a spectro photometer a grat-

ing or prism as a wavelength selector. In Figure 9 the dif-

ference between a single and double beam instrument is

shown and the arrangement also refers to a spectrophotom-

eter. The various parts that transmit the light beam such as

lens, cells, mirrors, transmission gratings and prisms must

be transparent to uv light and be fabricated of fused silica

or quartz. Flint glass can be used in the visible region.

(b) Infrared instrumentation^28

Figure 11 is a schematic of a double beam ir spectropho-

tometer. The parts and functions are similar to a un/visible

instrument, however the arrangement differs—the light beam

passes through the sample and then the wavelength selector

in contradistinction to the uv/visible instrument (see Figures

4A, 9, and 10). Materials transparent to ir are the alkali metal

chloride, bromide, and iodide salts.

Filters

Grating

Visible

source

UV Source

Slits

Sample

Compartment

Focal

Point Detector

Tungsten

lamp

Deuterium

lamp

Concave

grating

Sector

mirror

Sample

Reference

Photomultiplier

Grid

mirror

(a)

(b)

FIGURE 9 Single- and double-beam uv-visible spectrophotometers.

(a) Beckman DUR Series 60, single-beam. (Courtesy of Beckman Instruments,

Inc., Fullerton, CA.) (b) Hitachi Model 100–60, double-beam. (Courtesy of

Hitachi Instruments, Inc., Danbury, CT.)

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