topic1 in chemical inftrument~tion
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FRANKA. SETTLE,JR. VlrginiaMiilWlnst~t~e Lexington. VA 24450
Choosing the Right Instrument: The Modular Approach Part I Howard A.
Strobel Duke University, Durham, NC 27706
In recent years two major developments have occurred in chemical instrumentation: first, the design and construction of instruments have b&me more complex; second, control and data-processing capabilities have heen revolutionized bv the introduction of microprocew,rs. As a further complirntic~n in the choiceofan instrument, manufarturmi haw now begun toemphasize the wntrul and data processing aspect to the exclusion of any adequate description of the basic design. Undisputedly, instruments are used more intelligently when this use is based on an understandine of their desien and ooeration. Such an iunders~andingis greatly enhanced through a modular o r systems approach, which 15 bawall) n cuncrptual approach. I t may be defined as the understanding of an instrument as a system, each module of which performs a needed function, such as detecting the signal from a sample, and not basically as a collection of integrated circuits, switches, optical slits, and other parts. I t may appear to those unfamiliar with the modular way of viewing instruments that such an approach might be impossibly mathematical and abstract. A homely example will dispel the notion. We grow up familiar with a modular approach to our gasoline-operated automobiles: we understand the automotive system to be a carburetor module through which a gasoline-air mixture is introduced to an internal combustion engine module whose crankshaft connects through a gear reduction module and drive shaft to a differential gear ~
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noward Strchl is a professor of chemistry at Duke University. He is a graduate of WashingIan State Univefsity and has waded on the Manhanan Project and at Brawn University where he earned his PhD. His bmk, "Chemical instrumentation," (Addison-Wesley) is slated to appear soon in a third edition. His research interests include nonaqueous ion exchange and study of solvent structure in mixed solvents.
and moving wheels that proprl the vehicle. A conceptual understandma s e m i ewe. cinlly impwtant for Instrument users, teachers of future instrument users, and for those writing instrument brochures, who also have a teaching role. This paper intends basically to present the modular approach in a fashion that will helo the user raise auestions that w~lllead 111 thprhoirr 01 themwt etfcclive instruments for partirular applications and then apply them mure rffertivrly. Such applications may involve characterizing chemical or biological systems, identifying substances, measuring concentrations, or determining microphysical or chemical structure.' The Modular Approach Taking a modular wew mems,nswggwted, that we cons~derinstrummti as rystPms. Incidentally, the w~despreatluse of blurk diagrams reminds us that such devices have long been conceived and explained in terms of modules (2-5). To move toward this view it is useful to consider how a measurement on a sample is made. In the classical sense it involves completing the following sequential steps: (1) generation of an energy flow ("signal"), (2) impinging the energy flow on the sample, (3) detecting the signal arising from interaction of the flow with the sample, (4) amplification of the signal as necessary, (5) processsing of the signal, (6) computation of analytical information about the sample, and (7) disolav. or read-out of the resultis). It isa modular approach that seems best to mesh thi.;ilescripti~mi,fmrasurctnent with actual
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' Appropriate
measurement techniques must, of course, also be identified h) thp user. I n selecting methods it is vdushk tu have access to an analysis of the potentialities of such methods. A fine example of such an evaluation is the brochure developed by research personnel of the Dow Chemical Company and published as "Modern Methods of Research and Analysis" ( 1 ) . Volume 61
instruments. In this context a module
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partrculor function, e.g., detection or amplification. Ifweopen an instrument case, we will always see a myriad of resistors, wires, integrated circuits, slits, mirrors, filters, phototubes, transformers, and other components. By moving conceptually to a higher level, however, we can in effect ignore them. If we keep the function of a module in mind, it should be possible to identify its principal components. Only if we must calibrate or test a module and do not have access totest points or circuits set up by the manufacturer are we likely to have to identify many parts. In the case of electronic modules our task is fortunately eased since many of them appear on individual circuit boards. Is the implication that most instruments are made from a standard collection of modules correct? Is it also true that only a fairly small set of modules is important enough to study extensively? If the answer to both queries is affirmative, an important question remains. Can one successfully trace instrument specifications to like specifications for individual modules? All these questions can be answered "yes." It is even true that often one module sets a specification. Furthermore, nearly all modules are types that will aooear in new instruments into the i n d e f ~ n ~ ft eu t u r ~ Imprnvements . in design will enhance thpir operation but the modults will cont#nuet v perfwn~the same functicjn. In many rases brlth the~rig~naland imprr,ved version continur in u w A g d example is the uwradinl: of the phototuhe t u a ph
