Encyclopedia of Environmental Science and Engineering, Volume I and II

554 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

photomultiplier tube is the most widely utilized detector in

optical spectroscopic instruments.

(iv) Photodiodes

The photodiode, PD, is a small wafer of silicon dioxide

with a shallow layer of p and n material of the top and bottom

surfaces, respectively, to which are attached electrodes. The

device is reverse biased. When photons impinge on the

optically-active p diffusion region, electrons promoted to the

conduction band generate a photocurrent that is proportional

to the intensity of the optical light beam. The PD detectors

are about ten times more sensitive than the vacuum photo-

emissive tube and are mostly responsive in the visible and

near ir regions. Some tubes are sensitive to the uv region at

about 200 nm. A lens is optically coupled to each small PD

wafer.

Linear arrays or vidicon tubes of these multichannel

detectors allow nearly simultaneous detection of a spectrum

of wavelengths in instruments operated in the spatial mode

(see Section III,B,1, b,5 Instrumental ensembles), where

the detectors are swept electronically. Optical multichannel

analyzers consisting of a monochromater, a multichannel

detector and a computer are used in flame emission and uv/

visible spectrophotometers.^21

(b) Infrared detectors 22,23

There are two categories of detectors used for the spec-

tral region above 1.2  m (micrometer, 10^ ^6 m) namely, heat

and semiconductor detectors.

(i) Thermocouples and thermopiles

A thermocouple is formed when two wires of a metal

are separately joined to the opposite ends of a wire of a

dissimilar metal. If the two dissimilar metal junctions are

maintained at different temperatures, a thermoelectric cur-

rent will flow in the circuit. Therefore, if one junction is

maintained at a constant temperature, a thermoelectric cur-

rent will be generated proportional to the temperature of the

second junction. The changes in the intensity of incident

ir radiation can be detected in ir spectrophotometers using

this type of detector. The sensitivity is 6 to 8 microvolts per

microwatt and a temperature difference of 10^ ^6 C is detect-

able (see Figure 8). A thermopile, consisting of a number

of series-connected thermocouples, may be miniaturized

through thin film techniques to provide an effective ir detec-

tor. It has an 80 msec (millisec, 10^ ^3 sec) response time with

a flat response below a frequency of 0.35 Hz (Hertz).

(ii) Golay cell

The Golay cell is a pneumatic device similar to a gas

thermometer. The ir radiation shining on the blackened sur-

face of a sealed cell containing xenon gas causes the gas to

expand and distort a diaphragm, a part of the cell wall. The

moving diaphragm may be coupled to one plate of a capaci-

tor transducing an ir intensity to a capacitance. In another

mechanism the beam of ir radiation is reflected from the mir-

rored diaphragm surface to impinge on a photocell. The area

of coverage of the beam on the photocell changes as a func-

tion of the movement of the diaphragm. The intensity of the

ir radiation affects the area of the beam that impinges on the

photocell and ultimately the magnitude of the photocurrent.

The ir beam must be optically focused on the detector. Its

response time is 20 msec. Sensitivity is about equal to that

of the thermocouple detector. In the far ir it is an excellent

detector (see Figure 3).

+15V

Irradiated junction

Reference junction

–15V

Negative

feedback

To

+ amplifier

FIGURE 8 Thermocouple and preamplifier (Reprinted from Ref. (180). With permission from

the Journal of Chemical Education.)

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