Page 36 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GJB Revised Pages
Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

Analytical Chemistry 575

and can be used to picomolar detection limits for many aging, statistical analysis, integration, and data storage.
immunochemical analyses. Often these functions can be handled by a micro- or mini-
computer with a minimum of 8-bit word lengths, though
greater word length and greater capability of microproces-
3. Piezoelectric Devices
sor addressing allow for increased processing speeds and
It is well known that certain crystalline substances such greater on-line data storage capability. Data acquisition is
as quartz generate an electrical voltage when subjected to usually performed by specialized analog-to-digital circuit
physical compression. Similarly, it is possible to induce packages, which may receive polling requests and timing
a constant frequency of physical oscillation of the crystal signals from the microprocessor. Two common devices
planes when a regular alternating voltage is applied to such for transducing analog instrument measurements into dig-
a crystal. Piezoelectric crystals coated with thin films of ital form are known as counter converters and successive
selectiveadsorbenthavefoundapplicationasconventional approximation counters. Some instruments acquire com-
gas detectors when employed as physical oscillators. The plicated and extensive data sets at rapid rates. Such data
mass changes caused by the adsorption of gases to the may be difficult to interpret manually, but with the aid of a
crystal cause the oscillation frequency to change, f,in reference library, aspects such as compound identification
proportion to adsorbed mass as can readily be established by computer. Such reference li-
braries are used extensively for infrared and mass spectral

f =−2 f q 2 (P q V q )m f , analysis.
where f q is the quartz crystal frequency, P q is the quartz
density, V q is the velocity of the wave in the quartz, and 2. Microprocessor Control
m f is the mass per unit area for the deposited matter on
Due to its great speed, a computer can often control an in-
the quartz surface. It has been demonstrated that mass
strument more efficiently than is possible by manual oper-
responses and also microviscosity responses associated
ation. Such active instrument control must occur in at least
with surface reactions can be obtained from enzyme–
two steps. The microprocessor must first ascertain the state
substrate and immunochemical complexation on bulk
of some variable, and then action must be taken to activate
acoustic wave piezoelectric quartz crystals. Furthermore,
an instrument control to adjust the value of the variable.
both bulk acoustic and surface acoustic wave devices can
Many instruments employing microprocessor control con-
be used directly in aqueous media in certain configurations
tain more than one processor and can perform a series
for quantitative analysis.
of complicated feedback functions. These processors are
considered to be “dedicated” to a particular series of tasks
and receive their operating commands from a hard-wired
III. COMPUTERS IN
ANALYTICAL CHEMISTRY instruction set as well as the instrument operator. The sim-
plest microprocessors operate with instruction sets based
on 8-bit word lengths. Memory increments of 256 words
A. Instrument Control and Data Handling
are available for such systems, and often only one or two
Since 1975 there has been a tremendous revolution in ca- of these memory increments are required for instrument
pability and cost of computers and microprocessors, re- control. Control usually takes the form of activating a stan-
sulting in the incorporation of such technology into al- dard electrical relay device (which may drive a motor or
most every modern analytical instrument available today. switch) by toggling an active line connected to the relay
Computers are fundamental to two areas of analytical and controlled by the microprocessor between electrical
chemistry: (1) the applications of mathematics to analysis ground and a standard 5-V high level. More complicated
(chemometrics), encompassing such topics as factor anal- control systems involve more control relays and necessi-
ysis, multiple regression, pattern recognition, optimiza- tate the availability of more control lines and microproces-
tion, and statistics, and (2) interactions with analytical sors. The time-sharing capabilities of a minicomputer are
instruments. The latter can actually be considered to be often used in controlling a number of microcomputers or
either passive or active interaction, as determined by com- microprocessors in complex instruments. Significant ad-
puter control of instrument operation. vances in speed and therefore “real time” complexity of
instrument operation have been achieved as new 16- and
32-bit microprocessors have entered the market. The ad-
1. Data Acquisition and Processing
vantages of speed and increased memory have aided in the
This area usually implies a passive function involving the development of expert systems and parallel processing. A
actual collection of data, simple calculations, data aver- revolution in instrument control and data processing is