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INSTRUMENTATION
Advisory Panel Jonathan W. Amy Jack W. Frazer G. Phillip Hicks
Donald R. Johnson Charles E . Klopfenstein Marvin Margoshes
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When the Computer Becomes a Parf of the Instrument
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MARVIN MARGOSHES 6806 Delaware St. Chevy Chase, Md. 20015
Computers have revolution ized ana lyt ica I instrumentation and the problems of instrumentco m puter i nterfac i ng a re continually confronting ana lyt ica I chemists. Making the computer a component part of the instrument should provide the most feasible means of realizing the maximum information content of an analytical method
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H a r r y L. Pardue Ralph E . Thiers William F. Ulrich
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analytical chemistry. Their influence on our instrumentation and methods will be as dramatic and pervasive as the earlier effect of electronics. Those who recognize and make intelligent use of these iie\y developments will reap considerable benefit. The field of X-ray crystallography provides an example of the likely effects. With computerized techniques, the same group of crystallographers was able to increase its number of publications sixfold (I). The accuracy and precision of the data are also improved. As a result, entirely new types of crystallographic studies are being made. I n analytical chemistry, computers are used mainly to automate existing instruments and conventional computations. These applicat,ions have been iiseful, hut the real benefits of computerizing will come with the development of i i e measurement ~ methods t>hatmay he possihle only with computerized instruments, and from techniques n-hich take advant'age of more of the information content of an analytical method. Kaiser ( 2 ) recently discussed, in this journal, the information content of an annlyticnl method. -4lthougli Kaiser stated that he was not necessarily referring to computers, it is apparent that routine realization of his sugge-t'ions requires the use of these machines a t least for the computations. I n this article, I will stress the instrumentation aspect, though one really Shoiild not, isolate the me2surement from how the data are used. I n particiilar, I n-ill stress types of measurements that, perhaps, can be accomplished n.ithout a compiiter, hut which nre most fcaqihlc n-hen the computer is made a pnrt of thP instrument. TOMPUTERS ARE CHANGING
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Attaching the Computer to an Instrument
Before going into the main theme of this article, it is worthwhile to take a brief look of the use of computers with conventional analytical instruments I n the first applications, the computer was used off-line only, and analog computers often were used because the digital machines were still expensive and unreliable Despite these limitations in early work, one of the first uses of computers in analytical chemistry is a good illustration of a technique that 1s greatly improved by being able to carry out fairh extensive computations. The application was the analysis of petroleum fractions by infrared spectrometry These fractions frequently are mixtures of similar compounds, and It is nearly impossible to find nn mfrared frequency wliicli allon s mensurement of the concentration of one component specifically, without interference from the other components Quantitative concentration data were computed 1~ equating the absorbances at each of qeveral wavelength. with the sum of the aborbnnces of the inch idual compounds For a ten-component miuture, measurements were made at ten wavelengths and ten simiiltaneous equations n ere solved on an analog computer I n principle, the analvtical computations could hive been made bv hand or with a desk calculator, hut the greater speed of the computer made the method much more attractiw for routine use The pa. cliromatograph erentiiall\ took over this type of analvw I t has the ad7 antage of giving a phv-icnl, rathrr than mithematical, vpirntion of the compounds in a rompleu mktiire Here, too, the computer ha;: hecome
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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Introducing the lowest priced laboratorycomputer: Th
On simple price comparison alone, the LAB 8 / e wins hands down. But that's only the beginning of the benefits. This is a general purpose computer system. Using any lab oscilloscope it becomes many instruments in one . . , signal averager, data correlator, histogram analyzer, frequency analyzer, and NMR data system. All for the cost of one good singlepurpose instrument. For the imaginative researcher, LAB 8 / e is an innova. tion machine - all of the software and hardware capability built into it lets you develop your own experiment control features, data reduction techniques, and analysis programs Lots of software is provided: FOCAL, BASIC, and FORTRAN languages.
You don't have to be a computer expert to use them Lab instrumentation simply plugs into LAB 8 / e without extra interfacing or power supplies And the mobile LAB 8 / e allows you to move your lab computer as easily as you move your oscilloscope LAB 8 / e is the lowest cost entry into laboratory computing yet complements the more powerful PDP-12 And DIGITAL the world s largest manufacturer of mini-computers, backs this versatile research tool with the training and service that a large company can provide But start by thinking of the LAB 8 / e as a bargain Remember it costs no more than a single singlepurpose instrument ' T i e c s i l I l 3 s c m e c 5 3 ay s r 3 ? % " s a 1 0 0 fled T~K:.CIIX Mode' 6 3 2 a i a i l a b l e Tron D I G I T A L at a i a d c i t i o i a l cnarge
Circle No. 43 on Readers' Service Card
prominent. T h e original hardware integrating circuits have been replaced by computer attachments of varying capability, providing for one or many chromatographs and for different data reduction techniques. The instrument industry has developed these many types of systems in response to the widely differing needs of analytical laboratories. The chemist is faced with the task of selecting b0t.h the best chromatograph and the best data system for his present and projected needs. This cost-benefit analysis requires time, but there is an obvious advantage in having flexibility of choice. Sumerous types of computation devices also have been used in emission spectrometry over t'he past two decades. These include analog and digital devices: the digital computers were used in the dedicated, time-sharing, and batch-loading modes. Mostly, t,he computers were programmed to automate the usual types of computations. I t has been proved ( 3 ) that improvemcnts in analytical accuracy and precision are possible when more complex calculations are made and/or when inore of the information in an emission ,spectrum is used. Until quite recently, there were no instruments vhich could innkc w e of more than n small fraction of this information. -4utomatic, highspeed microphotometers now have been developed ( 4 , to read all of the information on a photographic plate and convcrt it into machine-readable form. -4 spectrometer capable of mea.suring photoelectrically a t up to 2048 wavelengths: is under development ( 5 ) . This spectromcter requires that a computer lie designed in : it cannot be operated in :iny other na!-. The automatic microphotoinctcr docs n o t require a computer as a component, but it produces more data than can be hnndled manually. It is snfc t o predict that these inst,riimental dewlopincnts, together with computational improvements, will have a drainntic impact on methodi of emission spectrochemical analysis far beyond mere automation of existing methods. Piilse-height analysis is done routincly for collcction of data in such analytical methods as nuclear activation and X-ray spectrometry. Signal averagers are less common, but they are used in several methods n.here the signal mensured in a short time is accompnnicd by excessive noise. I t is most common to use specinl pulse-height annlyzers and signal averagers, but, in fact, t h c e are forms of computers. The data collectcd by these instruments are often fed to a larger computer, and it, is logical to iise a general-purpose computer to combine the tasks of d a h collection and aiialj-sk. The general-purI ~ S P rompliter is ii~uallyless expensive
than the special-purpose device, though savings here must be balanced against programming cost. The great advantage of the general-purpose computer in these applications is flexibility in data collection and processing through changes in software. The major disadvantage is that general-purpose computers are normally slower in carrying out a function than are machines wired for tlie specific operation. It is still necessary to use special-purpose signal averagers and pulse-height analyzers when the data input rates are high.
Building the Computer In
When it IS recognized that the instrument vi11 always be used with a computer, it becomes logical to design the instrument with the computer as a component rather than as an add-on attachment. Two major adhantages come from this approach. (1) I t may be possible to make new types of measurements when the computer is used to operate tlie instrument and/or to convert raw instrument output into a useful form. ( 2 ) The computer can serve several purposes and can replace costly cpecialized circuits Fourier transform spectrometers i1lustrate both advantages Both nmr and infrared spectra are now being measured by Fourier transform methods The ir measurements are probably easier to explain, so they will be described fir>t
I n f r a r e d Fourier T r a n s f o r m Spect r o m e t r y The essential components of an infrared Fourier transform spectrometer are the interferometer and the dnta wstem T h e latter consists of a computer, interface circuits, a plotter or other device for displaying the spectrum, and a Teletype or similar unit for input and output of nonspectral information
Figure 1 is a schematic representation of a Michelson interferometer. T h e beam splitter sends beams of the incoming light to the two mirrors. Light reflected from these mirrors is recombined a t the beam splitter and emerges a t right angles to the incident beam. We may now consider what happens when the incoming light is monochromatic and one of the mirrors is moved toward the beam splitter at a constant velocity. .4t the starting point, the two mirrors are equidistant from the beam splitter, so that both light beams travel the same distance and arrive back a t the beam splitter in phase. These beams interfere constructively and the intensity of the beam emerging from the interferometer is at a mayimum. When the mirror has moved a distance equal to one-fourth of the wavelength of the incident light, the distance traveled by the beam in that path is one-half wavelength shorter than the path to and from the fixed mirror The two beams then interfere destructively and give a minimum in the output intensity. When a detector senses the intensity of the beam from the interferometer and its signal is plotted against tlie distance moved by the mirror, the curve is a cosine function. This is the interferog r a m for monochromatic light. K h e n monochromatic light of another wavelength is used, the interferogram is identical except that it has a different period. The interferogram for polvchromatic light IS the sum of the individual cosine functions which result from all of the discrete wavelengths. Each cosine function is weighted by the intensity a t that wivelength. This interferogram is simply the Fourier transform of the spectrum, and computing the Fourier transform of the interferogram gives the desired spectrum. T h e basic functions of the data .ystem in this case are to store measurement. made at equal interrals of mirror
Fixed mirror Beam splitter
Figure 1. Michelson interferometer ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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