INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 547
the measurement of voltage. Resistance may also be mea-
sured with the meter. One weakness of this device is its low
internal resistance causing loading errors by high impedance
signals.^11
A meter is used in the analysis of single samples or
samples analyzed, serially, at a slow rate on a spectroscopic
instrument at one frequency or wavelength. Meters also
are employed to indicate proper adjustment of potentials,
currents, temperatures, etc. for various instruments.
An electronic voltmeter, EVM, is more sensitive and
accurate than the D’Arsonval-based meter previously
described, particularly for signals with high impedance.
The internal resistance is 10 Mohms (megaohms, 10^6 ohms)
or more for d.c. (direct current) signals and 1 Mohm for
a.c. (alternating current) signals. The circuits use solid state
devices compared to the earlier device, a VTVM (vacuum
tube voltmeter). Current and resistance is measurable with
the EVM. Its application parallels those for the D’Arsonval-
based meter.^11
A photographic plate or film may be used to collect data
in the time domain where all the data are displayed simul-
taneously, that is a spectrum in emission spectroscopy. The
radiation in the dispersion pattern of the sample reflected or
transmitted from the prism or grating impinges on the pho-
tographic plate.
Electronic integrators determine the area under a curve
and are superior in precision to the ball and disk integra-
tor and the several hand methods widely utilized. They
may be based on operational amplifier or transistor cir-
cuitry. Some potentiometric recorders have a second pen
controlled by an integrator and the density of the pen’s
excursions determine the area under the curve. This last
type is not as convenient as the electronic integrators that
can correct for baseline changes. Chromatographic peak
areas for GC and HPLC (high performance liquid chroma-
tography), anodic stripping analysis peaks, spectroscopic
curves, etc. are integrated as a means of quantitation and
analysis of an analyte.
Analog computers are available but are not used now to
any great extent.Digital Devices The digital computer or microprocessor
interfaced to the instrument brings a broad capability to the
display and processing of instrumental data. Data reception
and storage is convenient when real time computation and
display are not required. Mathematical calculations, includ-
ing the areas under curves, graphic and tabular displays,
correlation with previously collected data, and many other
operations can be carried out at one’s convenience. Real
time processing can be accomplished on a time-sharing
basis or with a dedicated computer. The visual display is at
a video monitor and a printer provides a hard (printed) copy
of the raw and calculated data, graphs, and other informa-
tion. Computer devices include microprocessors and micro-,
mini-, and mainframe computers. The instrument must be
carefully interfaced to the computer and this task requires
much electronic skill. Instruments providing spectral read-
outs, the need for number crunching and repetitive analysescan benefit greatly from a computer interface. Some instru-
ments that utilize Fourier transform analysis require a com-
puter capability and many instrumental techniques have been
revolutionalized by computer use. The use of the computer^12
in the reduction of noise in instrumental signals by ensemble
and boxcar averaging has greatly improved the quality of
instrumental data.^12
Digital meters measure analog signals and provide
a digital readout. A/D conversion of the analog input is
accomplished electronically. The digital data is displayed
as numeric images using solid state devices such as LEDs,
light emitting diodes, and LCDs, liquid crystal displays, and
lamps such as, NIXIE, neon, and incandescent bulbs. The
LED is the more convenient device because its seven seg-
ment readout display uses lower currents and voltages than
the lamp displays. The LED’s red image, due to the semi-
conductor gallium arsenide doped with phosphorus, may
be increased in intensity by using more semiconductor in
the LED. The image color of LEDs may be fabricated to be
green or yellow, also. 11,13 LCDs operate by means of polar-
izing light. They use reflected light for viewing, a seven-
segment and dot matrix readout display, an a.c. voltage,
consume very little power and are more fragile than LEDs.^14
The LCDs and LEDs are the newest and most convenient
display devices.
Digital meters can be used in place of the analog variety.
The former are more accurate and easier to read.Instrumental Parameters and Definitions Instrumental
characteristics of operation and data treatment and statistics
are defined by a number of parameters. A definition of each
term is as follows:- The range of frequencies (information) in the
signal is called the bandwidth. During amplifica-
tion, some amplifiers cannot respond to the range
of frequencies in the signal producing an amplified
signal with a narrower bandwidth. - The baseline is the signal obtained when no
sample is being examined and reflects the noise
inherent in the instrument. - Calibration is the process relating instrument
response to quantity of analyte. In general a
series of standard solutions or quantities of ana-
lyte are analyzed on the instrument taking reagent
blanks into account and using a similar matrix
as the sample under consideration. The quantity-
response data are plotted to provide a calibration
curve where error bars indicate the precision of
the method.^15 Other calibration procedures such as
the methods of standard additions^16 and of internal
standards^17 have advantages in specific situations.
The former is helpful in ameliorating interferences
from the sample matrix and the latter in correcting
for changes in instrument response particularly in
GC, and ir (infrared) and emission spectroscopy.^18 - The gain refers to the ability to amplify a signal
and is the ratio of the output to input signal. The
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