1. P. Morgenthaler Georgetown University Washington, D. C. 20007
A Modular System for Teaching hstrumental Chemical Analysis
Teaching an undergraduate course in instrumental analysis poses a number of problems for the instructor. In its present state of development, instrumental analysis is expected to cover many subjects, but only a few of them can be covered in a one semester coune. In addition, the feeling has been growing steadily that for proper understanding of the instruments and their operation, it is essential that the electronics of the equipment be understood by the students. Unfortunately, most undergraduate physics courses present little or no practical electronics to the students. (This is not to criticize the physics departments, for they, too, have insu65cient time to cover all important topics in their own field.) To give the students enough electronics background for an intelligent discussion of the operation of a relatively simple instrument would require that a very large part of the course be devoted to electronics rather than instrumental analysis. Furthermore, many chemists do not feel that their own knowledge of electronics is sufficientto allow them to teach the subject competently. Another d i c u l t y facing the instructor of an instrumental analysis course is the lack of suitable equipment for use in the laboratory. I n smaller schools, especially those that are initiating or upgrading an instrumental course, the main problem is frequently money, specifically, the lack of it. The cost of equipping a basic instrumental laboratory is astronomical. A set of laboratory experiments could be designed around some dry cells, a few inches of platinum wire, some resistors, a galvanometer, and an old spectrophotometer. While this would bring equipment costs down, the experiments could but weakly represent the present state of chemical instrumentation. Even in a large, research-oriented school there may be difficulty getting equipment for use in the course. A researcher will usually guard his instruments jealously for fear that they will be harmed by an undergraduate's mistake or misuse. As aresult, the students frequently are allowed to stand by and watch whiie an experienced operator uses the instrument on their samples. The effect of this practice on the good students is to induce frustration while the average and poor students learn little, if anything, from such exposure. Ideally, the instrumental course should have its own equipment which the students are free to use. This statement obviously excepts some very major instru-
Presented at the Joint Svm~osiumon The Teaehine of Andvticsl Chemistry at the 152id ~ e e t i n gof the ~ m e r i c i nchemical Society, New York City, September, 1966.
ments such as nmr spectrometers and mass spectrographs. No matter what the size of the school, the cost of supplying the teaching laboratory with research quality commercial equipment is prohibitive and severely restricts the instructor in his choice of experiments. Any instrumental analysis course must be based on a series of compromises. We must compromise on the subjects discussed in lecture and we must compromise on the equipment that the students use in the laboratory. We must also compromise on the intelligence of our discussion of instrumental electronics as well as instrumental optics and mechanics because of the lack of student background in these areas. Still, we must give a course that supplies the students with a knowledge of the complexities and variables of the instrumental techniques. We should include material which will enable the student to extend the techniques that he has learned. After all, the purpose of education is not only to acquaint the student with what has been done, but to supply him with the tools necessary so that he might be able to extend old techniques and to devise new ones. The Modular Instrument Approach.
As a solution to these problems with the least number of compromises and with some very definite advantages, a modular equipment system is suggested for use in the instrumental laboratory. With this equipment a very large number of instruments can be synthesized by the students. Such a system has been adopted, and an instrumental analysis course designed to exploit its capabilities. The basic electronic component in the modular system is the operational amplifier, and several lecture periods (5 hr) a t the beginning of the course were devoted to operational amplifier circuitry. The circuits used around the operational amplifier were very simple and could be understood without a strong background in electronics. Other important factorssuchasimpedauce matching, noise level, and shielding were included in the discussion. The use of the amplifiers as adders, subtractors, multipliers, dividers, integrators, differentiators, logarithm generators, and non-linear function generators was discussed. The behavior of these circuits and their interconnection were emphasized by the programming of simple analog computer circuits. The students who had poor backgrounds in electronics were able to learn how to use the amplifiers in this length of time. Those who had reasonably good backgrounds were introduced to more complicated and more subtle operational amplifier circuits. Volume 44, Number 6, June 1967
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There are several excelleut sources of information available on the operational amplifier aud its circuits.' If an instructor has a modest understanding of electronics he can very readily familiarize himself with operational amplifier circuits in a short period of time. After this discussion of operational amplifier circuits, the students realized that they could actually design a working electronic device by themselves. This is important, because even if they had had a good course in basic electronics, few, if any, would have the courage or the knowledge t o attempt to design a complicated electronic device. The discussion of the operational amplifier and its circuits was reinforced by the first laboratory experiment in which the students constructed each of the basic circuits and verified their operation. The precision and accuracy of the circuits were examined as well as the signal-to-noise ratio. The limitations of the measurement of voltage and current were emphasized. After the operational amplifier circuits had been discussed, the remaiuder of the lecture portion of the course was devoted to instrumental chemical analysis. Only special electronic equipment was subsequently described and only when it arose in the normal course of the discussion. Slightly over one week was used to give the students a working kuowledge of operational amplifier circuits, so the problem of time was partially solved. This was a compromise because there was no attempt to discuss any basic electronics. As a result, the students occasionally made rather gross errors in their circuit designs because they did not fully understand the electronics of the circuits, but they usually constructed good, sound circuits. I n the laboratory, however, the operational amplifiers were further discussed aud used, and new circuits and more electronics were introduced with each succeeding experiment. The use of the modular system in the laboratory to synthesize instruments has several very important features. First, it partially solves cost and availability problems. Secondly, it permits instruction in a way that would be difficultusing commercial instruments. One of the chief cries of oppositiou to the use of the modular system for teachiug instrumentation is that ugly word: "black-box." Actually, for instructional purposes, the commercial instruments resemble blackboxes far more than the modular units. The commercial instrument appears to be an entity. It seems to perform only one function. The sample is placed in one end and, when all of the cont,rols are properly set, an answer as a chart or meter reading comes out of the other end. The modular system, on the other hand, must be arranged so that the proper electrical operatious are performed on a properly obtained detector sigual. If the electronics have more t,han a single function, then there
"Appli~ationsManual fur Computing Amplifiers," Philhrick Researches, Inc., Dedham, Mass., 1966. "Hsndhook of Operational Amplifier Applications," Blur-Bmwn Research Corp., Tucson, Ariz., 1963. MonnmoN, C. F., "Generalized Instromentation for Research and Teaching," Washington State University Press, Pullman, Wash., 1964. "NIP-System 1000, Operation and Application?," McKee-Pedersen Instrrxnents, llsnville, Calif., 1965.
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will be several circuits, each circuit. performing a single, understandable function. As an example, consider a modular synthesis of a scanning polarograph. The signal is to be recorded on a Y-T recorder. It is necessary therefore, to have a scau voltage which changes liriearly with time. The source of the scan voltage must have a very low output impedance as the polarographic cell has a resistance of only a few hundred ohms. If the voltage source impedance is not extremely low, we will not know the true voltage that is being applied to the dropping mercury electrode (DR'IE). A suitable source of the scan voltage would be an integrat,or circuit having a constant input voltage (or current). The output impedance of the integrator circuit is less than 1.0 ohms and so will maintain the DR4E a t a lcnown voltage with respect to ground. The parameter to be measured in a polarographic circuit is the current that flows through the cell. Any current that flows through the DR'IE must also flow through the reference electrode. If we wish to measure currents as accurately as possible, our measuring device must have an input impedance which is as low as possible. The operational amplifier is basically a current sensitive device, and when used iu the operational mode the negative input is a t virtual ground which means that the input resistance of the amplifier is approximately zero. Therefore, the reference electrode may be connected directly t o the negative input of the amplifier. Since the negative input is maiutained a t virtual ground, the potential of t,he DME with respect to the reference electrode is known, the output voltage of the amplifier is proportioual to the current flowing through the reference electrode, and the proportionality constant is simply the feedback resistance which can be varied t o change the curreut sensitivity. I n the design and construction of this circuit, the instrumental requiremeut,s can he pointed out t o the student since he is using circuits with which he is already familiar and simply arrauging them t,o perform the objectives of the experiment. Compare this analysis of the modular unit with the instructions for operatiou of a commercial polarograph. The voltage span is fixed by setting one dial; the initial potential by set,ting a second; the current seusitivity by settiug a third. The culve is obtained by turning a switch. The dials and knobs and switches become mysteriously operating entities which really do not briug the impact of the circuit requirements or functions to the students as does the modular unit. The commercial polarograph will be a black-box to most students. The modular unit is a composite of "little black-boxes," but the function of each of the little black-boxes is lcnown and can be understood. The ability of the modular unit to teach the functions of the instrument is its most important property. Finally, and not the least important, is the training that the modular unit supplies t o the student in modifying or originating electronic apparatus. This training will be excellent preparation for future scientific work including possible research in graduate school. If he knows the functions that, he wishes a circuit t o perform, he can design and construct such a circuit in a very short time. The modular equipmeut that is used a t Georgetown is manufactured by McKee-Pedersen Instruments of Dan-
ville, Calif., and is designated as the MP-System 1000.2 The basic part of the equipment is the electronics console. This unit contains power supplies and a number of interchangeable plug-in modules. The modules include a potentiometer, four operational amplifiers, an electrometer, a second potentiometer for spectrophotometric work. a wheatstone bridee. - , a 5 and 10 KHz oscillator, an integrating capacitor and resistor matched to give RC = 60 sec, and a set of precision high-value resistors and capacitors mounted in a shielded case. Connections between these modules are made via patch cords. Individual precision resistors and capacitors mounted on standard double banana plugs are supplied and these can be plugged directly into the amplifiers to give the desired input networks and feedback loops. Separate items, not mounted on the console, include a 10-mv recorder, a potentiostat capable of delivering 35 w., a a a t i n e monochromator with resolution better than 1 nm, photomultipliers, phototubes, light sources, sample holders for spectrophotometric work, and a magnetic stirrer. The equipment is designed to be essentially studentproof. Each of our units has had over 120 hr of student use in the course. There have been no repairs necessary on the instruments. Maintenance of the units involved replacement of about $5 worth of mercury batteries. All of the equipment is transistorized. Except for the photodetectors, a maximum of 20 v is available from the equipment, making it safe for completely inexperienced students, unlike some operational amplifiers sold for instructional purposes that are designed so the student can easily poke his fingers into 250 v on the front panel of the unit. In the AiIP system, the high voltage for the photodetectors is available only from properly connected Cannon connectors or phone jacks and is completely inaccessible to the students. All of the equipment has current limiting devices so it is possible to short an output directly to ground without any damage to theinstrument.
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The Experiments
Utilizing this equipment, the studentssynthesized and used a large number of instruments in the laboratory. The laboratory experiments were designed to emphasize the functions of the instrument and the instrumental requirements for measuring the signal of interest. In addition to the period spent in becoming acquainted with the equipment, the following experiments were performed using the modular equipment: I.
A Study of pH Meters.
The input impedance ofthe elect,~ometer, the output impedance of the glass electrode, the effects of proper temperature compensation and adjustment oi the compensator, acid error, and sodium ion error were measured. Throughout this experiment the proper use of buffers was stressed.
For photographs of the equipment and a brief description of the unit see M%LER, R. H., Anal. Chern. 37, 113A (1965). The gas chromatograph and the hydrogen flame detector were constructed from Swage-Lok fittings, rtn alumlnum plate, and copper tubing. Excluding the cost of the gas reduction valves for nitrogen, hydrogen, and air, the total cost of building this chromatograph was less than $50. Full scale recorder response could be obtained from less than 10 nanograms of sample.
2. Potentiometric Titrations. A Mariotte bottle was used to obtain a eonsbnt titrant flow rate. Ag+ was titrated with C1; snd the patential of 8. Ag,AgC1 electrode was recorded. The exact potential a t the inflection point was determined. Using constant titrant flow with the preceding titrat,ion, differentiator circuits were introduced and first and second derivatives of the titration curve were recorded. An automatic titrator of the preset endpoint potential type was set up for the above titration using a hounded operational amplifier voltage comparator circuit, a relay, and a solenoid type buret valve.
3. Polarography A ramp circuit was constructed to act rts a. scan voltage source. A current sensitive amplifier with known gain was set up and the output directed to the recorder. Polsrograms were made which examined the oxygen wave, the effect of a maximum supressor, the parameters of the Ilkovic equation, and the reversibility of the electrode reaction. Standards were made and a qnantitative analysis was performed.
4. Anodic Stripping Analysis An appropriate circuit was set up for the analysis (the circuit is essentially t,he same as that for polarography). The components of a. very dilute solution were identified. The reproducibility of the method and its applicability to quant,itative analysis were examined.
5 . Coulometric Titrant Endpoint Detection
Generation-Amperometric
A four electrode syiitem was set up. Using a, constant current source bromide was axidi~edto bromine (arsenite is then oxidized by bromine). The circuit was constructed for amperametric endpoint detection using a fixed voltage source and a current sensitive amplifier.
6 . Single Beam Spectrophotometry A visual observation of absmption hands and a direct observat,ion of stray light in the monochromator was made. Overall response of source, monochromator, and photomultiplier was measured. Reaction kinetics were measured spectrophotometrically. A logarithmic amplifier was added to the measuring circuit, and a soectrurn was measured direetlv in absorbance. The ;esolutian of the monochromator and the effect of the slit width on the resolution was observed.
7 . Fluorometric Endpoint Detection Can+ was titrated with EUTA a t pH = 12 using calaein indicator in a photometric titrator and using a mercury lamp as an excitation source and a photomultiplier detector.
8. Gas Chromatography Electronics were arranged with the electrometer to s ~ ~ p pthe ly proper circuit for use with a hydrogen flame detector.= Retention times were measured as a function of carbon chain length for a series of norms1 hydrocarbons. The effect of chain branching on retention time was observed. A qualitative and then quantitrttive analysis on a commercial petroleum ether was performed. The effect of carrier gas flow rate on resolution was observed.
There were two other experiments performed in the course: atomic absorption and nmr spectrometry. Both of these experiments were performed on commercial instruments. However, if time had allowed, it would have been possible to use the MP System for the atomic absorption experiment. In the next instrumental analvsis course that will be taught a t Georgetown, ~xperimint4 will be deleted and in its place will be an experiment on differential thermal Volume 44, Number 6, June 1967
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nnalysi.. Both thc linear temperature programruerand thed~fftrrnrinldetectorvdl hr huilt with r l ~ .\ll'equipe ment. Many additional instruments can be synthesized with the modular unit. The number appears to be limited only by the imagination of the experimenter. Conclusion
The use of the modular system for the instrumental laboratory has shown itself to be of great value as it makes possible equipment suitable for a large variety of meaningful experiments with a relatively small equipment budget. This equipment is completely available for course use. At times when the course is not being
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taught, the equipment is available for use in research, so that it is a valuable addition to the department as well as to the course. The use of the modular system strongly emphasizes the function($ of the electronics in an instrument and therefore strongly aids in understanding the operation of the instrument. However, it does not require that large amounts of lecture time be devoted to a discussion of basic electronics. Instruction in the use of the modular equipment gives the students a background in the construction of electronic devices that they can immediately put to use in their own (undergraduate) research. Using the electronic equipment whets the student's appetite and induces him t o study basic electronics on his own.
