REPORT FOR ANALYTICAL
CHEMISTS
Proper Utilization of Analytical Instrumentation by S. Z. LEWIN, Department
of Chemistry, New York University, New York 3, Ν. Υ.
upon which all of -*- our chemical knowledge and ca pability rests is the reliability of the data supplied by analytical in strumentation. It therefore be hooves every chemist to be as knowledgeable as possible concern ing his instruments. He should know their characteristics and po tentialities; their limitations and idiosyncrasies. This is particularly important at the present time because of the widespread, and, indeed, indispen sable use of relatively complex in struments in routine laboratory work. The trend in modern laboratory instrumentation has been in the direction of making commercial instruments externally ^ ^ I - I E BEDROCK
Figure 1 .
ever simpler to operate and more pleasing to the eye, while they have become internally more sophisti cated and complex in their elec tronic and optical circuitry. The character of chemical lab oratory instrumentation has under gone a revolutionary change in the past three decades, and most of the more experienced scientists among us (and a great many of the more recent graduates, as well) have a considerable gap to close between their academic training in the use of the tools of their trade, and the current proper practice. As recently as twenty years ago, the principal tools of the chemist were relatively simple devices whose mechanisms and character Schematic representation of a generalized
istics were generally well under stood by their users. These tools included the classical lever-arm balance, volumetric glassware, colorimeters and photometers, and certain simple electrical devices based upon the Poggendorf poten tiometer and Wheatstone bridge circuits. The development of the vacuum tube and electronics dur ing the last two decades has led to revolutionary changes in analytical instrumentation. Particularly profound has been the influence on chemical practice of these three basic, instrumental advances: the techniques of high input impedance amplification, the stabilization of circuit characteris tics through inverse feedback, and instrument
VOL. 33, NO. 3, MARCH 1961
·
23 A
REPORT FOR ANALYTICAL CHEMISTS
the a u t o m a t i o n of data-recording through application of t h e servomechanism principle. B y exploiting these techniques, instrument makers have provided the chemist with a host of powerful and sensitive new tools t h a t h a v e greatly extended the range, versatility, and precision of his analytical procedures. These developments have t a k e n place with such r a p i d i t y , and continue to spawn new instruments in such profligacy, t h a t most analysts have been unable to keep pace with them. As new instruments have been m a d e commercially available, they have been quickly adopted a n d new analytical procedures have been elaborated based upon these instruments, even though t h e internal circuitry and operating principles of the devices have been only vaguely appreciated, if, indeed, t h e y have been understood a t all, by the users. In order to appreciate the n a t u r e of the problem of proper utilization of the instruments t h a t populate our laboratories a t present, it will be valuable to consider the features of instruments considered as a generalized class. An instrument can be represented schematically, as in Figure 1, as comprising t h e following basic functions :
A. A transducer (or sensor, or detector) t a k e s t h e input signal, which m a y be t e m p e r a t u r e , pressure, concentration, p H , or a n y other p a r a m e t e r of interest to t h e chemist, and converts it to some other form, such as a current or voltage, t h a t can be more easily handled in t h e circuits of the instrument. B. T h e circuitry of the instrument is designed to t a k e t h e outp u t from t h e transducer and modify it in such a w a y as to create an output t h a t can be used to operate the r e a d - o u t device. This modification m a y consist of: converting the transducer output —e.g., a d.c. signal m a y be " c h o p p e d " into a . c ; a current m a y be transformed into a voltage; etc., amplifying t h e signal to increase either its voltage amplitude or its available power, then subjecting it to a computing operation—e.g., comparing it to a reference signal ; or differentiating i t ; etc., finally, inverting it, changing it from a.c. to d.c. for more convenient readout. C. T h e r e a d - o u t device is t h e mechanism through whose agency the o u t p u t signal is m a d e visible or usable to the operator. I t m a y be a meter movement, a n oscilloscope
Figure 2A.
Equivalent circuit of a glass electrode
Figure 2C.
Figure 2B.
Effect of connecting low resist, galvanometer
Figure 2 D .
screen, a p e n - a n d - i n k chart recorder, a photographic emulsion, or the h u m a n sense organs. I n some instruments all of these features are clearly discernable; in others, more t h a n one of the features cited m a y be displayed by a single physical component. T h u s , an instrument m a y be broadly defined as a n y mechanism t h a t converts a. p r o p e r t y of a syst e m into a usable read-out. E a c h stage of t h e instrument has its characteristic properties, and the over-all performance of the instrument is intimately related to t h e resolution, noise, drift, t i m e constant, temperature stability, etc., of each of its constituent p a r t s . T h e special character of modern instruments, as contrasted with those of several decades ago, is t h a t currently the input and readout stages are separated b y a more sophisticated and elaborate network within the " b l a c k box" m a nipulated b y the user t h a n was formerly the case, and the relationship between individual components and the final signal is more subtle. I t has become common to use these black boxes in chemical l a b oratory work without proper regard for their limitations and special characteristics. I n fact, it appears t h a t the "black box philoso p h y " in the use of m a n y types of
Effect of connecting a high resist, voltmeter Effect of connecting an electronic voltmeter
REPORT FOR ANALYTICAL CHEMISTS
recent instruments has become al most universal. Black-box chem ists accept the instrument as a mysterious, but nonetheless won drous gift from the electronics en gineer, and are content (or, more charitably, resigned) to learn merely the proper sequence of throwing the switches and turning the knobs. Most of the currently offered courses in colleges and universities that go by that gross misnomer "instrumental methods of analysis" are composed simply of analytical chemistry with the help of black boxes. Such courses are misnamed because all analysis is instrumental in nature, depending upon some mechanism, whether it be a balance or a photocell or an electrode, to give a read-out that is proportional to amount of substance, and that is free of subjective interpretation. To separate one group of instru ments arbitrarily from the rest is neither logical nor necessary. Adherents of the black-box school often rationalize their ignor ance of the basic functioning of a given device by recalling that, after all, they can operate their automo biles and television sets with ade quate success without understand ing what exactly is going on be neath the decorative exterior. There is, however, a yawning
chasm between the driving of an automobile as a mean's of getting somewhere, and the utilization of an analytical instrument to yield reliable, objective data. A precipi tous fall from grace awaits the sci entist who ignores this pitfall ! An auto or TV set is basically a go/no-go, or yes-no device, which either runs or doesn't run, and the principal consideration of signifi cance is which of these two states it occupies. A measuring instru ment, on the other hand, must re spond to degree, quality, or inten sity of a signal, and the reliability of the measurement is a direct function of the intelligence with which it is applied. A second rationalization often adumbrated is that understanding many modern instruments is so difficult that the average chemist had best not even try. This is, however, most certainly not true. It is an interesting fact that all the electronic and optical circuity encountered in modern laboratory instrumentation can be explained without recourse to the complicated mathematics of cir cuit theory. All of the great, fun damental principles are basically simple concepts which can be stated with adequate rigor in graphic terms. This does not apply, it must be
Dr. S. Z. Lewin is professor of chemistry at New York University, Washington Square, Ν. Υ. He was born in New York City and received a B.S. from the College of the City of New York in 1 9 4 1 . He then moved to the University of Michi gan for graduate study, receiving an M.S. in 1942, and Ph.D. in 1950, following a three-year inter val in the Army Chemical Corps during the war. He came to N.Y.U. as an instructor in 1950, was pro moted to assistant professor in 1 9 5 1 , associate professor in 1958, and full professor in 1960. His re search interests are in instrumenta tion, physical measurements, and molecular structure. His monthly feature series on "Chemical Instru mentation" has appeared continu ously in the Journal of Chemical Education since January, 1959.
Figure 3A.
Electrometer input
Figure 3C.
Electromechanical chopper input
Figure 3B.
Vibrating condenser input
Figure 3 D .
Photoconductive crystal chopper input
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ANALYTICAL CHEMISTRY
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emphasized, to t h e m a t t e r of t h e design of instruments, for which the mathematics of circuit theory is essential. T h e design of complex instruments is a highly specialized science t h a t does n o t yield easily to the dilettante, b u t the qualita tive appreciation of even the most complex instrument is within t h e capacity of a n y normally intelli gent person who h a s a scientific bent a n d has been exposed to a col lege course in physics. A qualita tive understanding of instrumenta tion is completely adequate for t h e proper utilization of these tools. Hence, n o t only is a thorough understanding of modern instru mentation within t h e chemist's grasp, it is actually a sine qua non for getting t h e best performance from these instruments. Essential to the most effective use of instru ments is, ab initio, t h e choice of the most appropriate one of t h e available instruments for the t a s k in hand. Once selected, the instru ment must then be used in a m a n ner dictated b y its characteristics and limitations. I n the following sections we de scribe (a) basic principles of t h e three great instrumentation tech niques alluded t o above, in order t o demonstrate t h a t they can be read ily understood b y t h e nonspecialist in electronics; (b) factors t h a t need t o be considered in m a k i n g the most intelligent choice of t h e proper instrument for a given a n alytical problem; and finally, (c) factors involved in applying t h e instrument properly t o the experi mental situation.
One of t h e fundamental limita tions nature imposes on m a n in his probing of her secrets is t h e fact t h a t t h e process of measuring a parameter inevitably alters it, so t h a t what is actually measured is different from what existed b e fore t h e measuring device w a s applied. I n most areas of chemical measurement, t h e relative error caused b y this interaction can be made small enough to be neglected. However, in t h e fields of p H meas urement, nuclear radiation dctec-
REPORT
tion, low-level light photometry, and several others, t h e factor limit ing t h e accuracy of routine meas urements was, until recently, t h e disturbance created by t h e meas uring instrument. Consider, for example, t h e prob lem posed by p l l measurement u s ing a glass electrode as sensor of t h e pH. T h e glass electrode acquires a potential, relative to some refer ence electrode, t h a t is a function of the hydrogen ion activity (plus cer tain other potentials present a t junctions a n d interfaces in t h e cell). However, t h e glass m e m brane interposes a very large r e sistance (of t h e order of 10 8 ohms) in t h e system. Hence, t h e problem is to measure a voltage from a high internal resistance (or impedance) source. T h e n a t u r e of this problem can be appreciated b y considering the examines shown in Figure 2, in which the pTI-sensing system is represented schematically as a voltage in series with a high resist ance (Figure 2,/l). If a low resistance meter, such as a galvanometer, is connected across t h e p H cell, as in Figure 2,7?, a current will flow t h a t is limited by t h e resistance of t h e circuit, which in this case is essentially just t h a t of t h e glass electrode. A current, 7. flowing through a resist ance, 7?, generates a voltage differ ence, E, across t h a t resistance equal to IE. This is often simply re ferred to as t h e 772 d r o p ; its polar ity is determined by t h e direction of t h e electron streams, since elec trons flow from more negative to less negative locations. If the volt age is to be measured with a pre cision of one p a r t per thousand, t h e galvanometer must be able t o measure t h e current of 10"° a m pere with a precision of ± 1 0 - 1 2 ampere. This requires a very deli cate, shock-mounted, thermostated, shielded galvanometer, read pains takingly through a telescope with a long optical lever t o produce ade q u a t e magnification of the coil d e flections. Even worse, from t h e point, of view of practical meas urements, is t h e fact t h a t a flow of even this small a current m a r k e d l y polarizes t h e electrode surface— i.e.. changes t h e electrochemical conditions due to oxidation-reduc-
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ANALYTICAL CHEMISTRY
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REPORT FOR A N A L Y T I C A L CHEMISTS
tion or ion-migration effects, so t h a t the electrode potential rapidly drops during the measurement. If the a t t e m p t is made to meas ure the voltage generated a t the electrodes in the p H cell by means of a high resistance voltmeter, as illustrated in Figure 2,C, it can be seen t h a t the correctness of the voltmeter reading is limited by the device's internal resistance. If, as shown in the figure, t h e voltmeter resistance is 10 8 ohms (which is extremely high for a nonelectronic, moving-coil m e t e r ) , the IR drop across the cell resistance is 0.05 volt, and since the voltage differ ence measured on the voltmeter is t h a t existing between points A and β — n a m e l y , (0.1 — 0.05) = 0.05 volt—the measured voltage is only half t h a t which existed between these points before the meter was connected. Hence, the measuring instrument has produced an unacceptably large disturbance in the parameter being measured. However, if the effective internal resistance of the measuring instru ment is as high as 10 12 ohms (typi cal of modern electronic pH meters), then the meter reading differs from the p H cell voltage existing before measurement by only 1 p a r t in 10,000, and the cur rent drawn from the cell during measurement is so minute (10~ 1 3 ampere) t h a t negligible polariza tion occurs. These relationships are illustrated in Figure 2,D. The high input impedance ampli fier is an electronic device t h a t gives an output signal of sufficient power to deflect a rugged, inex pensive meter without drawing appreciable current from the input voltage being measured. The principles of the input stages of several types of these devices are illustrated in Figure 3. A basic electrometer circuit is shown in Figure 3,A. T h e voltage to be measured is applied between grid and cathode of a vacuum tube, and influences (or modulates) the elec tron stream flowing from cathode to plate. This current flows from the plate through the plate load resistor, RL, and generates an IRdrop in it proportional to the cur rent. Variations in the tube cur
rent due to the input voltage a t the grid are thus reflected as varia tions in the voltage drop across the plate resistor, and these can be passed on to another vacuum tube for further amplification. The principle of the vibrating condenser input circuit is illustrated in Figure 3,B. One plate of the condenser is fixed rigidly in posi tion; the other is caused to vibrate by a surrounding coil through which alternating current flows. Thus, the spacing between the con denser plates varies periodically; consequently, the electrical capaci tance also varies in a periodic fashion. If the voltage to be meas ured is connected across the con denser, the latter tends alternately to charge up, and discharge, as its capacitance increases, then de creases. This generates an alter nating current in the transformer primary connected to the con denser, which is stepped up in the secondary winding, and passed on to subsequent stages of amplifica tion, such as the grid and cathode of a vacuum tube, as described pre viously. The amplification of an a.c. sig nal is much easier and more reli able t h a n t h a t of a d.c. signal. This is principally due to the fact t h a t in the latter case, all drifts occurring in the tube voltages, tem perature, etc. of the amplifier a p pear as p a r t of the output signal. Means for converting a d.c. signal into an a.c. input to the device is often an essential ingredient in these high input impedance amplifiers. One such means is the electro mechanical converter, or "chopper," illustrated in Figure 3,C. A metal reed, carrying contacts, is caused by an a.c. winding to vibrate so t h a t it alternately makes contact to one or the other of two fixed contactors. When contact is made to one of these, current tends to flow through the center-tapped primary transformer winding in a direction determined by the polar ity of the input voltage. When this connection is broken, and the reed touches the other contactor, cur rent now tends to flow in the op posite direction, through the other half of the primary winding. Thus, an alternating signal is generated
in this transformer coil, and this signal can be passed on to subse quent stages for further amplifica tion. A photoconductive chopper cir cuit is shown schematically in Figure 3,Z). T h e photoconductive crystal has the property of pre senting a resistance, in its circuit, t h a t is a function of the light in tensity incident upon the crystal. When dark, the resistance is great; when illuminated, the resistance decreases. In this circuit, the crys tal is close to a neon bulb which is excited by the alternating cur rent line voltage (60 c.p.s.) so t h a t it goes on and off 120 times each second. As a consequence, the effective resistance in the input circuit varies a t this frequency, causing the input voltage to be chopped into an a.c. signal, which can then be amplified as before. In all these cases, the current drawn from the input source can be kept extremely small, in the best case (vibrating condenser) as low as 10 , 0 ampere.
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Circle No. 75 on Readers' Service Card VOL. 33, NO. 3, MARCH 1961
·
29 A
A NEW HEAVY ALUMINUM CASE and other improvements make this popular heavy-duty balance better than ever !
REPORT FOR ANALYTICAL CHEMISTS
a n y fluctuations in t h e amplifier itself, such as might arise from variations in t h e supply voltage, filament emissivities, t u b e elec trode spacings, s t r a y leakage p a t h s , etc. T h e f u n d a m e n t a l principle of feedback circuitry m a y be under stood q u a l i t a t i v e l y b y considera tion of t h e diagrams in Figure 4. An i n p u t signal of m a g n i t u d e Ein is fed into a n amplifier having an amplification factor, n, a n d appears as a n o u t p u t voltage of m a g n i t u d e : η
χ
Eia
=
U7„ut
A fraction, / , of this o u t p u t is then fed back into t h e i n p u t circuit in opposition t o t h e original signal. T h u s , t h e n e t i n p u t signal is n o w : Ein — f X R m t
-and t h e n e t o u t p u t i s : η'χ
Ein
— } Χ η χ
7?„„t
Hence: Sout
=
Π X
Ein
—
f
Χ
η
X
R e a r r a n g i n g this equation:
Eaut
E„ut
.Ει»
_
η
1 + / Χ η
If n is large, and / is not too small— i.e., if both the amplification factor and feedback are considerable— (1 + / X n) can be reduced simply to / X n, and the equation becomes: -J^i-
= ~=
constant
Therefore, t h e o u t p u t signal is p r o portional t o t h e i n p u t signal, a n d independent of t h e amplification factor, or v a r i a t i o n s in it. Figure 4 shows t h e significance of these simple equations in a graphic form. Figure 4,A shows t h e relation between input, output, a n d feedback voltages for a given a m plification factor. If t h e properties of t h e amplifier drift during use, so t h a t t h e amplification factor b e comes smaller, for example, as d e picted in F i g u r e 4,-B t h e output sig nal would t e n d t o become smaller. However, this causes t h e feedback voltage to be smaller, which in t u r n causes t h e n e t input, (Ein — / X
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ANALYTICAL CHEMISTRY
Figure 4A. Illustrating the principle of feedback stabilization of amplification. 1. Input voltage, 2 . Net input to amplifier, 3. Amplifier, 4 . Output signal, 5. Feedback signal Figure 4 B . A decrease in the amplification factor is compensated by a cor responding decrease in the magnitude of the feedback voltage, leaving the net output substantially constant. (Symbols have same significance as in part A above)
Ζϊ,,,,ι), to be larger. This tends to make the output larger, thereby compensating for t h e initial effect. Thus, the feedback constitutes a built-in correction factor t h a t a u tomatically adjusts for variations in t h e amplifier's characteristics, and maintains an output t h a t is a faithful a n d constant enlargement of the input function.
The
Servomechanism
M a n y modern analytical instru ments arc capable of providing t h e chemist with a wealth of d a t a rapidly and continuously, in t h e main as a consequence of the ex ploitation of t h e servomechanism principle. I n fact, the servomecha nism is largely responsible for t h e embarrassment of riches under which scientists currently labor, since it lias given them the means to acquire data faster t h a n it can be assimilated. An illustration of t h e servomechanism principle is given in Figure 5, which shows, in a very schematic form, the basic features of a self-balancing potentiometer. If the potentiometer is unbalanced, there is an unbalance signal—i.e., a current—flowing between points A a n d B. T h e direction of this current depends upon the direction of the potentiometer unbalance— viz., if the slidewire contactor is above the balance point, t h e cur rent will be in one direction, if be low the balance point, t h e current will be in the opposite direction. This unbalance current is converted to a.c. b y the electromechanical chopper, a n d fed into t h e amplifier. The o u t p u t of the amplifier is a n a.c. voltage t h a t is applied t o one of t h e sets of coils—e.g., t h e stator —of a phase-sensitive, reversible motor. T h e other set of coils (e.g., the rotor) is connected to t h e line a . c , a n d always has a fluctuating magnetic field associated with it. Consequently, t h e rotor will t u r n only if both stator a n d rotor a r e energized b y alternating signals t h a t are out of phase with each other. I n t h e present instance, t h e rotor will turn only if there is an unbalance signal coming from t h e potentiometer. T h e direction in which t h e rotor t u r n s depends
-bem ι UMZU until ify toa
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