INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 555
(iii) Pyroelectric detector
The pyroelectric detector is a thin wafer of material
such as, LiTaO 3 and LiNbO 3 , placed between two elec-
trodes to form a capacitor. The detector material is a non-
centrosymmetrical crystal whose internal electric field is
changed as a function of temperature when it is below its
Curie temperature. This detector is based on the change of
the capacitance of a substance with temperature and is sen-
sitive to the rate of change of the detector temperature. The
changing radiation is modulated by chopping or pulsing
because the detector ignores steady unchanging radiation.
Therefore this detector has a much faster response time than
those dependent on temperature directly. Depending on
the circuit parameters, the response times may be 1 msec
or 10 sec and the responsivity (detector output/incident
radiation) is 100 or 1, respectively. This detector has a large
ir range (see Figure 3).
(iv) Photoconductive and photovoltaic detectors
Photoconductive detectors are crystalline semicon-
ductor devices that experience an increase in conductivity
upon interaction with a photon. The increase of conduc-
tivity is due to the freeing of bound electrons by energy
absorbed from the radiation. A Wheatstone bridge is used
to measure the change in conductance (see following
Section III,B, 2,d. ). The semiconductor materials include
the metallic selenides, stibnides or sulfides of cadmium,
gallium, indium or lead.
Lead sulfide is commonly used as a detector in the near
infrared, 800 to 2000 nm, where it exhibits a flat response.
The cell consists of a thin layer of the compound on a thin
sheet of quartz or glass kept under vacuum.
Photovoltaic detectors have been discussed for uv/vis
spectroscopic instruments (see Section III,B,1, b.(4), (a), ( i )).
For ir applications the p -type indium antimonide detector,
cooled by liquid nitrogen, is available with a sensitivity limit
at 5.5 m. However the two types of lead in telluride detec-
tors extend the ir range. One detector, cooled with liquid
nitrogen, has a range of 5 to 13 m and a second one, cooled
with liquid helium, has a range of 6.6 to 18 m. A minimum
response time of 20 nsec (nanosec. 10^ ^9 sec) is achieved
with these detectors.(c) X-ray detectors
Gas-filled and semiconductor detectors and signal
processors and readout used in the measurement of
radioactivity (see Sections III,B, 3, b,( 1 ),(a) and (b) and
(2)) are the applicable in x-ray spectroscopy.(5) Instrument ensembles
The design of an instrument depends on its use and mon-
etary considerations. The several main modes of design are
designated temporal, spatial and multiplex. In turn each of
these are of the dispersive or nondispersive type.^24
In the temporal category the instrument scans sequentially,
in time, the wavelength in order to determine the intensity.
Dispersive systems employ monochrometers that are rotated
so as to position the selected wavelength on an aperature orslit preceding the sample or detector. Nondispersive systems
utilize a series of absorption or interference filters that are
interchangeable.
Spatial systems display the total spectrum with simul-
taneous determination of the radiation intensities. For a dis-
persive instrument a monochrometer provides the dispersed
radiation and a multichannel detector to detect their inten-
sities. Multichannel detectors utilized are a detector array
(silicon diode array or vidicon tube), a number of individual
detectors properly positioned, or a photographic plate. In
nondispersive systems the radiation beam is divided into a
number of beams and each passes through a unique filter
followed by a detector.
Multiplex systems employ a single data channel where
all the components of the signal are observed simultane-
ously. A Fourier transform is usually employed to resolve
the complex signal into its components requiring the use of a
computer. There are distinct advantages to Fourier transform
spectroscopy: namely, increased S/N ratio, increased energy
throughput, large precision in wavelength measurement, and
facility in its use. However, thus far the instrument is costly
to acquire and maintain. No further comment will be made
in this article about these instruments. 25,26
Dispersion instruments give more spectral detail because
the wavelength selected has a narrower bandwidth wave-
length spread. However non-dispersive instruments are usu-
ally cheaper, more rugged and have a higher signal to noise
ratio. Filter instruments are used frequently in monitoring
equipment.(6) Absorption instrumentation
In the absorption process, radiation passes through the
sample and a specific pattern of absorption of different
wavelengths occurs leading to a spectrum for that sample.
For each wavelength the amount of light absorbed will differ
and therefore the amount of transmitted light vary for each
wavelength. The intensity of transmitted light is inversely
proportional to the concentration of sample and is measured
in absorbance, Ab, units. The spectrum is the qualitative
factor of identification while the intensity of the transmit-
ted radiation is the quantitative measure. Light is absorbed
in the uv and visible region by electrons in the atoms or
molecules of a sample. Some elements are identified by
atomic absorption, AA, and some functional groups and
species by uv/vis spectroscopy. Absorption in the ri is due
to vibrational and rotational activity of atoms in molecu-
lar groupings, such as functional groups, double, triple, and
conjugated bonds, etc.
The spectroscopic curve or an instrument reading provides
an absorbance value for a chosen wavelength from which the
concentration of the absorbing substance can be computed.
The absorbance value represents the degree of attenuation of
the radiation of specific wavelength by absorbing substances
in the sample solution in the cell. A constant, the absorptivity,
a, or molar absorptivity, e, can be calculated for a pure sub-
stance for a given wavelength and solvent. The mathematical
relationship between the concentration of a substance, C, theC009_005_r03.indd 555C009_005_r03.indd 555 11/23/2005 11:12:22 AM11/23/2005 11:12:22