R E P O R T F O R A N A L Y T I C A L C H E M I S T S

Proper Utilization of Analytical

by S. Z. LEWIN, Department of Chemistry, New York University, New York 3, Ν. Υ.

^ ^ I - I E BEDROCK upon which all of -*- our chemical knowledge and ca­

pability rests is the reliability of the data supplied by analytical in­strumentation. I t therefore be­hooves every chemist to be as knowledgeable as possible concern­ing his instruments. He should know their characteristics and po­tentialities; their limitations and idiosyncrasies.

This is particularly important at the present time because of the widespread, and, indeed, indispen­sable use of relatively complex in­struments in routine laboratory work. The trend in modern laboratory instrumentation has been in the direction of making commercial instruments externally

ever simpler to operate and more pleasing to the eye, while they have become internally more sophisti­cated and complex in their elec­tronic and optical circuitry.

The character of chemical lab­oratory instrumentation has under­gone 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

istics were generally well under­stood 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 poten­tiometer and Wheatstone bridge circuits. The development of the vacuum tube and electronics dur­ing 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 characteris­tics through inverse feedback, and

Figure 1 . Schematic representation of a generalized instrument

VOL. 33, NO. 3, MARCH 1961 · 2 3 A

Instrumentation

REPORT FOR ANALYTICAL CHEMISTS

the automat ion of data-recording through application of the servo-mechanism 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 have greatly extended the range, versati l i ty, and precision of his analyt ical proce­dures.

These developments have taken place with such rapidi ty , 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 made commercially available, they have been quickly adopted and new analyt ical procedures have been elaborated based upon these instruments, even though the in­ternal circuitry and operating principles of the devices have been only vaguely appreciated, if, in­deed, they have been understood a t all, by the users.

In order to appreciate the nature of the problem of proper util iza­tion 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 the following basic functions :

A. A t ransducer (or sensor, or detector) takes the input signal, which may be temperature , pres­sure, concentration, p H , or any other parameter of interest to the chemist, and converts it to some other form, such as a current or voltage, t h a t can be more easily handled in the circuits of the in­strument.

B. The circuitry of the instru­ment is designed to t ake the out­pu t from the t ransducer and modify it in such a way as to create an output t h a t can be used to operate the read-out device. This modifi­cation m a y consist of:

converting the t ransducer output —e.g., a d.c. signal m a y be "chopped" into a . c ; a current m a y be transformed into a vol tage; etc.,

amplifying the signal to increase either its voltage ampli tude 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. The read-out device is the mechanism through whose agency the output signal is made visible or usable to the operator. I t may be a meter movement, an oscilloscope

screen, a pen-and- ink chart re­corder, a photographic emulsion, or the human sense organs.

In some instruments all of these features are clearly discernable; in others, more than one of the fea­tures cited m a y be displayed by a single physical component.

Thus , an instrument may be broadly defined as any mechanism t h a t converts a. proper ty of a sys­tem into a usable read-out .

Each stage of the instrument has its characteristic properties, and the over-all performance of the instrument is int imately related to the resolution, noise, drift, t ime constant, temperature stabil i ty, etc., of each of its constituent par ts . The 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 by a more sophisticated and elaborate net­work within the "black box" m a ­nipulated by the user than 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 lab­oratory work without proper re­gard for their l imitations and special characteristics. In fact, it appears t h a t the "black box philos­ophy" in the use of many types of

Figure 2A. Equivalent circuit of a glass electrode Figure 2C. Effect of connecting a high resist, voltmeter

Figure 2B. Effect of connecting low resist, galvanometer Figure 2D. Effect of connecting an electronic voltmeter

REPORT FOR ANALYTICAL CHEMISTS

recent instruments has become al­most universal. Black-box chem­ists accept the instrument as a mysterious, but nonetheless won­drous gift from the electronics en­gineer, 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 instru­ments arbitrarily from the rest is neither logical nor necessary.

Adherents of the black-box school often rationalize their ignor­ance of the basic functioning of a given device by recalling that, after all, they can operate their automo­biles and television sets with ade­quate success without understand­ing what exactly is going on be­neath 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 precipi­tous fall from grace awaits the sci­entist 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 signifi­cance is which of these two states it occupies. A measuring instru­ment, on the other hand, must re­spond to degree, quality, or inten­sity 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. I t 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 cir­cuit theory. All of the great, fun­damental 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 Michi­gan for graduate study, receiving an M.S. in 1942, and Ph.D. in 1950, following a three-year inter­val in the Army Chemical Corps during the war. He came to N.Y.U. as an instructor in 1950, was pro­moted to assistant professor in 1951 , associate professor in 1958, and ful l professor in 1960. His re­search interests are in instrumenta­t ion, physical measurements, and molecular structure. His monthly feature series on "Chemical Instru­mentat ion" has appeared continu­ously in the Journal of Chemical Education since January, 1959.

Figure 3A. Electrometer input

Figure 3B. Vibrating condenser input

Figure 3C. Electromechanical chopper input

Figure 3D. Photoconductive crystal chopper input

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2 6 A · ANALYTICAL CHEMISTRY

REPORT

emphasized, to the mat ter of the design of instruments, for which the mathematics of circuit theory is essential. T h e design of complex instruments is a highly specialized science tha t does not yield easily to the dilettante, but the quali ta­tive appreciation of even the most complex instrument is within the capacity of any normally intelli­gent person who has a scientific bent and has been exposed to a col­lege course in physics. A qual i ta­tive understanding of instrumenta­tion is completely adequate for the proper utilization of these tools.

Hence, not only is a thorough understanding of modern instru­mentation within the chemist's grasp, it is actually a sine qua non for getting the best performance from these instruments. Essential to the most effective use of instru­ments is, ab initio, the choice of the most appropriate one of the available instruments for the task in hand. Once selected, the instru­ment must then be used in a man­ner dictated by its characteristics and limitations.

In the following sections we de­scribe (a) basic principles of the three great instrumentation tech­niques alluded to above, in order to demonstrate tha t they can be read­ily understood by the nonspecial-ist in electronics; (b) factors tha t need to be considered in making the most intelligent choice of the proper instrument for a given an­alytical problem; and finally, (c) factors involved in applying the instrument properly to the experi­mental situation.

The High Input Impedance Amplifier

One of the fundamental l imita­tions nature imposes on man in his probing of her secrets is the fact tha t the process of measuring a parameter inevitably alters it, so that what is actually measured is different from what existed be­fore the measuring device was applied. In most areas of chemical measurement, the relative error caused by this interaction can be made small enough to be neglected. However, in the fields of pH meas­urement, nuclear radiation dctec-

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tion, low-level light photometry, and several others, the factor limit­ing the accuracy of routine meas­urements was, until recently, the disturbance created by the meas­uring instrument.

Consider, for example, the prob­lem posed by p l l measurement us­ing a glass electrode as sensor of the pH. The glass electrode acquires a potential, relative to some refer­ence electrode, t ha t is a function of the hydrogen ion activity (plus cer­tain other potentials present a t junctions and interfaces in the cell). However, the glass mem­brane interposes a very large re­sistance (of the order of 108 ohms) in the system. Hence, the problem is to measure a voltage from a high internal resistance (or impedance) source. The nature of this problem can be appreciated by considering the examines shown in Figure 2, in which the pTI-sensing system is represented schematically as a voltage in series with a high resist­ance (Figure 2, / l ) .

If a low resistance meter, such as a galvanometer, is connected across the p H cell, as in Figure 2,7?, a current will flow tha t is limited by the resistance of the circuit, which in this case is essentially just tha t of the glass electrode. A current, 7. flowing through a resist­ance, 7?, generates a voltage differ­ence, E, across that resistance equal to IE. This is often simply re­ferred to as the 772 drop; its polar­ity is determined by the direction of the electron streams, since elec­trons flow from more negative to less negative locations. If the volt­age is to be measured with a pre­cision of one par t per thousand, the galvanometer must be able to measure the current of 10"° am­pere with a precision of ± 1 0 - 1 2

ampere. This requires a very deli­cate, shock-mounted, thermostated, shielded galvanometer, read pains­takingly through a telescope with a long optical lever to produce ade­quate magnification of the coil de­flections. Even worse, from the point, of view of practical meas­urements, is the fact t ha t a flow of even this small a current markedly polarizes the electrode surface— i.e.. changes the electrochemical conditions due to oxidation-reduc-

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REPORT FOR ANALYTICAL CHEMISTS

tion or ion-migration effects, so t ha t the electrode potential rapidly drops during the measurement.

If the a t tempt is made to meas­ure the voltage generated a t the electrodes in the pH cell by means of a high resistance voltmeter, as illustrated in Figure 2,C, it can be seen t ha t the correctness of the voltmeter reading is limited by the device's internal resistance. If, as shown in the figure, the voltmeter resistance is 108 ohms (which is extremely high for a nonelectronic, moving-coil meter) , the IR drop across the cell resistance is 0.05 volt, and since the voltage differ­ence measured on the voltmeter is tha t existing between points A and β—namely, (0.1 — 0.05) = 0.05 volt—the measured voltage is only half t ha t which existed between these points before the meter was connected. Hence, the measuring instrument has produced an un-acceptably large disturbance in the parameter being measured.

However, if the effective internal resistance of the measuring instru­ment is as high as 1012 ohms (typi­cal of modern electronic p H meters) , then the meter reading differs from the p H cell voltage existing before measurement by only 1 par t in 10,000, and the cur­rent drawn from the cell during measurement is so minute (10~ 1 3

ampere) tha t negligible polariza­tion occurs. These relationships are illustrated in Figure 2,D.

The high input impedance ampli­fier is an electronic device tha t gives an output signal of sufficient power to deflect a rugged, inex­pensive 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. The voltage to be measured is applied between grid and cathode of a vacuum tube, and influences (or modulates) the elec­tron stream flowing from cathode to plate. This current flows from the plate through the plate load resistor, RL, and generates an IR-drop in it proportional to the cur­rent. Variations in the tube cur­

rent due to the input voltage a t the grid are thus reflected as varia­tions 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 posi­t ion; the other is caused to vibrate by a surrounding coil through which alternating current flows. Thus, the spacing between the con­denser plates varies periodically; consequently, the electrical capaci­tance also varies in a periodic fashion. If the voltage to be meas­ured is connected across the con­denser, the latter tends alternately to charge up, and discharge, as its capacitance increases, then de­creases. This generates an alter­nating current in the transformer pr imary connected to the con­denser, which is stepped up in the secondary winding, and passed on to subsequent stages of amplifica­tion, such as the grid and cathode of a vacuum tube, as described pre­viously.

The amplification of an a.c. sig­nal is much easier and more reli­able than tha t of a d.c. signal. This is principally due to the fact that in the latter case, all drifts occurring in the tube voltages, tem­perature, etc. of the amplifier ap­pear as par 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 electro­mechanical converter, or "chopper," illustrated in Figure 3,C. A metal reed, carrying contacts, is caused by an a.c. winding to vibrate so tha t it al ternately 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 pr imary transformer winding in a direction determined by the polar­ity of the input voltage. When this connection is broken, and the reed touches the other contactor, cur­rent now tends to flow in the op­posite direction, through the other half of the primary winding. Thus, an alternating signal is generated

2 8 A · ANALYTICAL CHEMISTRY

NEW DB

in this transformer coil, and this signal can be passed on to subse­quent stages for further amplifica­tion.

A photoconductive chopper cir­cuit is shown schematically in Figure 3,Z). The photoconductive crystal has the property of pre­senting a resistance, in its circuit, tha t is a function of the light in­tensity incident upon the crystal. When dark, the resistance is great; when illuminated, the resistance decreases. In this circuit, the crys­tal is close to a neon bulb which is excited by the alternating cur­rent line voltage (60 c.p.s.) so tha 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.

Inverse Feedback Stabilization

Every measuring device is sub­ject to drift and to random fluctua­tions—i.e., noise—in its output, due to variations in ambient tem­perature, pressure and humidity, vibration and electrical pickup, Brownian motion, aging, fatigue and hysteresis effects in the struc­tural parts , etc. The precision of an instrument may be improved by either controlling its environment so closely that these disturbing ef­fects are reduced to a very low level, or designing the instrument so tha t its output is relatively in­sensitive to these variations. The first approach is cumbersome, ex­pensive, time-consuming, and un­certain. The second is obviously greatly to be preferred. One of the truly great achievements of mod c m electronics has been the de­velopment of circuitry capable of faithfully amplifying an input sig­nal without drift, distortion, or ex­cessive noise level.

Inverse, or negative, feedback is a technique t ha t is employed to make the output of a high-gain amplifier practically independent of

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VOL. 33, NO. 3, MARCH 1961 · 2 9 A

A NEW HEAVY

ALUMINUM CASE

and other improvements make this popular heavy-duty balance better than ever !

Troemner Model 195-B. Sensitivity 0.1 gram; capacity 3 kilograms. $100.

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Ne lewly redesigned for all-around laboratory use, Troemner Models 195-B and 2-89B are now set in a very sturdy aluminum case with chemical-resistant blue-gray finish. Choose according to desired capacity and sensitivity: Model 195-B has capacity of 3 kg., sensitivity of 0.1 g.; price $100. Model 2-89B has 6-kg. ca­pacity, 0.2-g. sensitivity; price $115. Weigh beams on both models are graduated either in grams or in ounces. Pans or flat plates are of polished stainless steel. A modern index tower protects the easy-read­ing pointer. Sliding and locking tare weight. For fast, accurate day-to-day deter­minations, these balances belong in every modern research laboratory. See your dealer or write us for information.

any fluctuations in the amplifier itself, such as might arise from variat ions in the supply voltage, filament emissivities, tube elec­trode spacings, s t ray leakage paths , etc. The fundamental principle of feedback circuitry may be under­stood qual i ta t ively by considera­tion of the diagrams in Figure 4.

An input signal of magni tude Ein is fed into an amplifier having an amplification factor, n, and appears as an output voltage of magni tude:

η χ Eia = U7„ut

A fraction, / , of this output is then fed back into the input circuit in opposition to t he original signal. Thus , the net input signal is now:

Ein — f X Rmt

-and the net output is :

η'χ Ein — } Χ η χ 7?„„t

Hence :

S o u t = Π X Ein — f Χ η X Eaut

Rearranging this equat ion:

-J^i- = ~= constant

Therefore, the output signal is pro­portional to the input signal, and independent of the amplification factor, or var ia t ions in it .

Figure 4 shows the significance of these simple equations in a graphic form. Figure 4,A shows the relation between input , output , and feedback voltages for a given am­plification factor. If the properties of the amplifier drift during use, so t h a t the amplification factor be­comes smaller, for example, as de­picted in Figure 4,-B the output sig­nal would tend to become smaller. However, this causes the feedback voltage to be smaller, which in tu rn causes the net input, (Ein — / X

Figure 4A. I l lustrating the principle of feedback stabilization of amplif ication. 1. Input voltage, 2. Net input to amplifier, 3. Amplifier, 4 . Output signal, 5. Feedback signal

Figure 4B. A decrease in the amplif ication factor is compensated by a cor­responding decrease in the magnitude of the feedback voltage, leaving the net output substantially constant. (Symbols have same significance as in part A above)

3 0 A · ANALYTICAL CHEMISTRY

E „ u t _ η

.Ει» 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:

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Ζϊ,,,,ι), to be larger. This tends to make the output larger, thereby compensating for the initial effect. Thus, the feedback constitutes a built-in correction factor tha t au­tomatically adjusts for variat ions in the amplifier's characteristics, and maintains an output tha t is a faithful and constant enlargement of the input function.

The Servomechanism

Many modern analytical instru­ments arc capable of providing the chemist with a wealth of data rapidly and continuously, in the main as a consequence of the ex­ploitation of the servomechanism principle. In fact, the servomecha­nism is largely responsible for the embarrassment of riches under which scientists currently labor, since it lias given them the means to acquire data faster than it can be assimilated.

An illustration of the servo-mechanism 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 and B. The direction of this current depends upon the direction of the potentiometer unbalance— viz., if the slidewire contactor is above the balance point, the cur­rent will be in one direction, if be­low the balance point, the current will be in the opposite direction. This unbalance current is converted to a.c. by the electromechanical chopper, and fed into the amplifier. The output of the amplifier is an a.c. voltage tha t is applied to one of the sets of coils—e.g., the stator —of a phase-sensitive, reversible motor. The other set of coils (e.g., the rotor) is connected to the line a . c , and always has a fluctuating magnetic field associated with it. Consequently, the rotor will turn only if both stator and rotor are energized by al ternat ing signals tha t are out of phase with each other. In the present instance, the rotor will turn only if there is an unbalance signal coming from the potentiometer. The direction in which the rotor turns depends

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X j LABORATORY APPARATUS CATALOG "62"

Publication of our new Catalog "62" is imminent. Mailings will be made shortly. And, necessarily, the quantity is limited. We want to put this new catalog in the hands of everyone w h o can make practical use of it. That is why we urge you not to delay in sending us your request, on your business or professional letterhead.

<Q Catalog "62" is, in no sense, "just another catalog," nor is it a compilation of manufacturers' literature. The selection, ar­rangement, and presentation of its contents have been newly organized from the point of view of the reader, rather than from the diverse viewpoints of our suppliers. It has been com­pletely rewritten, from introduction to index, to provide you with easily found, extremely detailed, and entirely factual information.

We feel sure that you will find this new catalog highly rewarding, both for reading and reference.

C H I C A G O A P P A R A T U S C O M P A N Y 1735 North Ashland Avenue, Chicago 22 , Illinois

KANSAS CITY —1827 McGee SI.. Kansas City 3, Missouri · ST. LOUIS —5147 Brown Ave., St Louis 15, Missouri

Circle No. 44 on Readers' Service Card

DRIERITE

Ca SCX, + % H,0

A CLASSIC COMBINATION FROM YOUR

LABORATORY SUPPLY DEALER

Circle No. 59 on Readers' Service Card

VOL. 33 , NO. 3, MARCH 1961 · 3 1 A

"The Versatile De sic cant"

QUALITY WITHOUT COMPROMISE ^^Performance

^^ Flexibility Reliability

GUARANTEED SENSITIVITY ΙΟ10 std. cc/sec VEECO'S MS-9 leak test stations are used to pressure, vacuum and inside out test hermetically sealed units. The helium mass spectrometer permits sealed objects to be certified leak proof at a sensitivity of 10_1° std. cc/sec.

V E E C O manufactures a complete line of high vacuum equ ipment . . . Components, Leak Detectors, Evapo­rators, Systems. . accepted as the quality line for over a decade.

Automatically or manually operated. For MS-9 Brochure or Complete Catalog write Dept.**

REPORT FOR ANALYTICAL CHEMISTS

upon the phase of the a.c. output of the amplifier; if this signal leads the line a.c. in phase, the rotor will turn in one direction, if it lags the line, the direction of rotation is re­versed. The rotor shaft is me­chanically linked to the slidewire contactor so that rotation of the shaft drives this contactor along the slidewire. The mechanical linkage is so arranged that the di­rection of drive of the sliding con­tactor is always in the sense that will reduce the amplifier output sig­nal. That is, if the contactor hap­pens to be above the balance point, the potentiometer unbalance signal will be of such a phase with respect to the line a.c. as to turn the motor in the direction that will move the contactor down. If the contactor is below the balance point, the un­balance signal will be opposite in phase to the previous case, and the motor drives the contactor upward, again toward the balance point.

Thus, the deviation of the poten­tiometer from balance produces a signal that sets in motion a chain of events the consequence of which is to move the slidewire contactor in a direction that reduces that signal. The contactor therefore continually seeks the balance point, for that is the only position in which the motor is not energized.

The basic principle of the servo-mechanism may be generalized in the schematic form shown in Fig­ure 6. The signal to be measured is compared with a reference stand­ard, and the difference is presented as an error signal to an amplifier, the output of which activates a sig­nal generator to produce a correc­tion signal that is applied to the source in such phase as to cancel the error signal.

The automatic, continuous-bal­ancing capability that the servo-mechanism provides has made pos-

(Continued on page 37 A)

Circle No. 163 on Readers' Service Card

3 2 A · ANALYTICAL CHEMISTRY

Figure 5. Schematic of a self-balancing potentiometer

VACUUM-ELECTRONICS CORP. Terminal Drive · Plainview, L. I. N.Y. HIGH VACUUM & LEAK DETECTION EQUIPMENT

Veeco

REPORT

(Continued from page SS A)

sible the high speed recording in­struments now so widely used in analytical work. Without it, all direct-reading data would still be obtained point by point, and only cumbersome photographic processes could be employed to give the type of continuous record needed for high resolution spectral, electro­chemical, and kinetic investigations.

The instrumentation principles described above are combined with each other, and with still other cir­cuit designs not considered here, in a variety of ways in the manifold instruments now in general use. These instruments have character­istics and limitations that are in­herent in their designs, and they cannot be used to best advantage unless these features are clearly understood.

Choosing the Instrument

Proper utilization of instrumen­tation starts with the proper choice of the instrument for the measure­ment problem in hand. Each situa­tion confronting the analyst has its own unique features, and the de­cision as to which instrument to use often must be made by taking into consideration noninstrumental factors, such as the local availabil­ity of instruments and operating personnel, need for certain kinds of records for legal purposes, etc.

A number of general considera­tions apply to all instruments, and should be in the forefront of the chemist's thinking when consider­ing an instrument for a given job.

Signal-to-Noise Ratio. Electri­cal noise arises in electronic circuits from the random motion of elec­trons in conductors, resistors, and vacuum tubes, generating sponta­neous voltage fluctuations that are superimposed on whatever directed voltages are present. The three types of noise commonly encoun­tered in electronic circuits are ther­mal-resistor, or Johnson noise, noise from fluctuations in plate and grid currents, and shot effect noise.

The random fluctuations in the directions and velocities of the free electrons in metals lead to differ­ences in electron density from one end to the other of the component;

Circle No. 46 on Readers' Service Card >·

in the first medium-priced MASS SPECTROMETER for analytical laboratories CEC's all-new 21-130 Mass Spectrometer offers the same accuracy, precision, sensitivity and scan speed you'll find only in the largest in­struments of its kind.

All this plus a totally new "human" engineered packaging concept that means: Greater accessibility (modular electronics on pull-down chassis) . . . Convenience in the grouping of operating controls by function, with each operable and adjustable from the front... Lighter weight because it's built on a welded extruded aluminum frame with formica-over -honeycomb cabinet panels.

Look at its features: A built-in direct writing oscillograph recording sys­tem using five galvanometers... a stainless steel inlet system... a built-in

micromanometer. And perform­ance? Mass range from m/e 2 to 230 continuous with unit resolu­tion up to m/e 200.

For complete information, call your nearest CEC sales and service office or write today for Bulletin CEC 21130-X5.

CONSOLIDATED ELECTRODYNAMICS / pasadena, California

A SUBSIDIARY OF B e l l & H o w e l l . FINER PRODUCTS THROUGH IMAGINATION

Analytical & Control Division CEC

a new human

engineering concept...

FAST . . . ACCURATE polarographic analyses

with the L & Ν Electro-Chemograph & Polarotron When analyzing metals, organics or bio-chemicals with an L&N Electro-Chemograph and Polarotron, you get a c c u r a t e resul ts—quick resul t s — because design and construction are u n m a t c h e d among po la rograph ic instruments.

Users f ind tha t the Electro-Chemograph prov ides:

Fast détermination—recorder balancing time for full scale travel is 1 second— assures delineation of current wave peaks in undamped operation.

Minimum error—polarizing is accurate to It mv—values are read directly from the scale . . . no interpolation required.

Flexible operation—reversible polarizing potential permits determinations of half-wave potentials. Four damping ranges, instead of three, allow optimum flexibility for accurate analyses.

Polarotron Dropping Mercury Electrode gives:

Time saving operation—unmatched speed and convenience provided by tilting sample cell—stable (reference) half-cell already prepared for immediate use. Minimum mercury required — automatic level control. . . needs only 50 grams.

For more detailed information, ask for D a t a Sheet E-97( l ) . Write Leeds & Nor thrup Company, 4906 Stenton Ave., Philadelphia 44, Pa., or check with your nearby L&N Field Office.

LEEDS NORTHRUP instruments | | | | | Automatic Controls · Furnaces

Circle No. 10 on Readers' Service Card

REPORT FOR ANALYTICAL CHEMISTS

these effects appear and disappear randomly—i.e., with all frequencies from zero to infinity. The magni­tude of the noise voltage associated with a resistance, R, is given by the equation:

e„2 = 4 k Τ R (U - A)

where e„ is the root-mean-square value of the noise voltage, k is the Boltzmann constant, Τ is absolute temperature, and (/2 — / i ) is the band width, or range of frequencies under consideration. I t is imme­diately evident t ha t the total ran­dom noise is greater the higher the temperature, and the wider the band width of frequencies passed and measured by the instrument.

Tube noise, consisting of those random fluctuations arising in the rate of emission of electrons from the cathode (shot noise), varia­tions in the division of current be­tween two or more positive elec­trodes (partition noise), variations in the number and distribution of positive gas ions (gas noise), etc., is similar in general character to re­sistor noise, in t ha t it consists of a broad distribution of noise frequen­cies and is temperature-dependent, but it is not amenable to as sim­ple mathematical t reatment .

The foregoing types of noise rep­resent the minimum noise possible in any instrument. Hence, the weakest signal tha t can be detected under ideal conditions is t ha t signal which can still be distinguished over this irreducible minimum noise.

In most instruments, the actual noise is much greater than the theoretical minimum. Wherever there are sliding contacts, me­chanically rotat ing components, or other moving parts , noise is gener­ated in the course of the motion. Noise also often arises in poor pres­sure contacts in switches, and in inadequately soldered connections. If the electrodes (cathode, grid, plate) of a vacuum tube are not perfectly rigidly mounted, they may vibrate slightly, producing a noise component in the output sig­nal. The test of the quali ty of an instrument is how close the actual noise level is to the Johnson noise.

In general, then, the instrument to be chosen for any given applica­

tion should be the one tha t can make the measurement with the best signal-to-noise (S /N) ratio. The following factors are of major importance in determining the sig­nal-to-noise ratio.

Rate of Read-Out. The more rapidly data must be obtained, the greater is the noise level and the poorer is the over-all instrumental accuracy. For example, a rapid-scan spectrophotometer used at its highest speed is excellent for quali­tat ive survey work, but is much in­ferior to a nonscanning spectropho­tometer for quanti tat ive measure­ments of absorbances at selected wave lengths.

Generalization. In choosing among alternative instruments for a given application, select tha t one which will read out the data at a rate as slow as is practicable, con­sistent with the time tha t can be devoted to getting the data.

Band Width. The wider the band of frequencies that pass through the instrument and affect the read-out, the higher is the noise level in the output. If the phe­nomena to be measured range over a wide spectrum of frequencies, then a wide-band amplifier must be used in the measuring instrument, and the operator must resign him­self to a relatively high noise level. On the other hand, if the input sig­nal corresponds to a single fre­quency—e.g., the frequency of the chopper in the input stage—a sharply tuned amplifier can be em­ployed, and the noise level in the output can be made extremely small.

Generalization. The instrument of choice is one in which the ampli­fier stages are tuned as sharply as is consistent with the frequency dis­tribution of the signal being ampli­fied.

Number of Components. Every additional resistor, vacuum tube, transformer, etc., is a source of noise and distortion in the output signal. Thus, if a one-stage ampli­fier gives an output of 10 μ&., and adding a second stage of amplifica­tion raised the output to 1 ma. it will generally be found t ha t the S / N of the one-stage instrument is greater than t ha t of the two-stage device, in spite of the weaker signal.

3 8 A · ANALYTICAL CHEMISTRY

On the other hand, a milliammeter is more rugged, and much cheaper, than a microammeter of the same relative accuracy, and the two-stage amplifier might be more prac­tical to use, even with its smaller signal-to-noise ratio.

Generalization. Other things be­ing equal, choose that instrument which has the fewest stages be­tween input and output, provided the output signal is large enough to be measurable with suitable pre­cision.

Time Constant. The presence of capacitance and inductance in elec­trical circuits introduces time de­lays between the instant the input signal is applied, or undergoes a change in magnitude, and the time this is reflected in the output. These time delays have a filtering action, and tend to prevent the high frequency (rapidly changing) noise from coming through into the out­put. In this sense, some time delay is desirable. However, too much time delay will distort the signal, and give an incorrect output. The amount of time delay that can be tolerated is smaller, the more rapidly the signal is varying, or is being read out.

Generalization. Choose that in­strument in which the time delay is small enough to produce negli­gible distortion of the signal, but no smaller.

Moving Parts. Mechanically moving parts in an instrument are not only sources of electrical noise ; they also tend to wear out, change alignment, corrode, or otherwise undergo deleterious changes. Such parts are: rotating mirrors, elec­tromechanical converters, relays, motors, etc. In many cases, these moving parts play an indispensable role in the instrument design, and cannot be replaced by all-electronic devices. However, new techniques continue to be developed that suc­ceed in many cases in eliminating all moving parts from an instru­ment design.

Generalization. Other things be­ing equal, choose the instrument that has the fewest mechanically moving parts, and pay especial at­tention to the quality of construc­tion of those it has.

for ukra-hieh sensitivity \. GAS CHROMATOGRAPHY

THE AEROGRAPH Η Ρ m DET-CTOR This hydrogen flame ionization kit readily adapts to any Aerograph. It will also fit other commercial or laboratory built gas chromatography units.

features... • Temperatures to 300°C. • Excellent for Temperature

Programming.* • Extraordinary stable

base line.* • Sensitivity 10 1 1 moles.

(0.001 micrograms) • Attenuation 1 to 1000 with

1 % accuracy. • High impedance

transistorized electrometer. • Silky, 10 turn zero control. • Excellent operation with

VB" packed columns.

to the chemist... Note the ease with which paraffin wax components from C2i to C!4 are separated. A 0.5 lambda sample of a 1% carbon disulfide solution (5.0 micrograms) was injected at 100°C. The furnace power was turned on ful l to program the temperature up to 265°C in 62 minutes. A 5 ft. χ Va in. SE-30 silicone column was used at 16-" attenuation.

FOR ALL YOUR GAS CHROMATOGRAPHY NEEDS

The Aerograph Hydrogen Flame Kit Model A-500 is complete and ready to use with stainless steel flame head, high impedance electrometer, shielded cables, air pump, air filter, gas con­nections, hydrogen restrictor, and one 5 ' χ Va" s.s. column. Delivery from stock $495.00.

W I L K E N S I N S T R U M E N T A N D R E S E A R C H I N C . P. O . B o x 3 1 3 W a l n u t C r e e k . C a l i f o r n i a

Circle No. 104 on Readers' Service Card

VOL. 33, NO. 3, MARCH 1901 · 3 9 A

v e r s a t i l e r e c o r d e r for l a b o r a t o r y

a n d p lan t u s e

The new Beckman Potentiometric Strip-Chart Recorder is low-cost and simple to operate. Compact, push­button unit features continuously adjustable input voltage span of 10 to lOOmv full-scale, facilitating its use with pH meters, spectrophotometers, and many other laboratory or process control instruments. Pen response is 1.0 second full-scale, and pen zero can be set to any point throughout the 5" pen travel. Chart speed is 1 " per minute, and additional speeds are available with plug-in units. S3 Other accessories include a circular chart recording adaptor and an external circuit controller. Unit may be wall-mounted or placed on laboratory bench. Ask your Beckman laboratory apparatus dealer for complete information, or write for Data File 15-3-21.

B e c k m a n ® Scientific and Process J Instruments Division

Beckman Instruments, Inc. SSOO Fullertmi Road, Fullerton, California

Circle No. 13 on Readers' Service Card

Transfer Function. The most vital organ of every instrument is its transducer, or sensing element. This is the component tha t converts the parameter of interest into an •electrical, mechanical, or optical signal t ha t is then fed into the amplifier. For example, a glass electrode is a transducer tha t con­verts hydrogen ion activity into a voltage.

The relationship between the in­put to the transducer and its output signal is called the transfer func­tion, and this may be linear or nonlinear. In general, the more linear the transfer function, the greater is the precision with which the instrument can be calibrated, the greater the slope of the transfer function, the more sensitive the in­strument is to the property being sensed.

Generalization. Choose the in­strument having the most linear and steepest transfer function, pro­vided the transducer has adequate stability and rate of response.

Calibration. The read-out of every instrument must be cali­brated against s tandards or refer­ences at more or less frequent inter­vals, the frequency depending upon the inherent stability of the elec­tronics and optics (including light sources, batteries, etc.). Also, the greater the accuracy required in the results, the smaller must the t ime interval be between measurement and standardization. Thus, in the double-beam design of spectro­photometers (in which the rate of read-out of data is rapid) , or the internal s tandard technique used with spectrographs and flame pho­tometers (in which the signal sta­bility is poor) , the sample is con­tinuously compared—i.e., many times per second—with the refer­ence throughout the course of the measurement.

Generalization. Choose tha t in­strument in which the separation, both in space and in time, between the measurement and calibration functions is as small as is consistent with the stabili ty and rate of read­out of the output signal.

Sound Design. There are a num­ber of additional features t ha t char­acterize a well-designed instrument.

Wherever possible, the design should be such that the instrument user can visualize the location and functioning of the various compo­nent parts inside the black box. A feature tha t is heartily recom­mended is the practice some manu­facturers have recently adopted of inscribing a schematic diagram of the instrument circuitry on its con­trol panel, so tha t the function of each of the operating controls is clearly evident.

The use of electronic stabilization wherever possible is also generally desirable, except where the extra complexity this introduces into the circuitry raises the noise level too much. Power supplies can be sta­bilized by the use of voltage regu­lator tubes, constant-voltage t rans­formers, or electronic regulator cir­cuits. Amplifiers can be stabilized by incorporation of negative feed­back.

Wherever light is passed through windows or lenses, it is desirable to use coated surfaces to reduce the amount of light lost in unwanted reflections. Photocells should be shielded from stray, scattered light through the use of appropriate fil­ters, when possible.

Using the Instrument

Fundamental ly the same consid­erations as were involved in deter­mining the choice of the best in­strument for a given measurement problem also determine the proper

Figure 6. Fundamental principle of the servomechanism

40 A · ANALYTICAL CHEMISTRY

REPORT FOR ANALYTICAL CHEMISTS

manner of applying the instrument to the problem.

These may be summed up in the following general rules.

Read out the output signal as slowly as is practical. Never use a rapid rate of scan for analysis if a slow rate will do. Never use an automatic scanning instrument if a nonscanning instru­ment will do.

If the instrument provides a choice of time constants—i.e., time delays— use the largest time constant that does not appreciably alter the output signal (or, the smallest time constant that does not make the output excessively noisy).

In calibrating the output signal, choose standards or references which are as close as possible in properties to the samples to be measured. This re­sults in the use of only a small portion of the transfer function in interpolation between sample and reference, and per­mits this interpolation to be made with good precision.

Wherever possible, operate the in­strument so that some noise or "jitter" is visible in the read-out device. This ensures that the read-out indicator is active, and that the read-out is not dis­turbed or distorted due to frictional and incrtial effects.

Devise and use in situ checking pro­cedures on the instrument to ensure that it is functioning properly. For instance, a standard signal injected periodically into the input circuit can serve to give a frequent, rapid indica­tion of instrument sensitivity, time con­stant, and stability. Never start up an instrument that has been out of use overnight or longer without running a standard sample on it.

Always give instruments adequate warm-up time. Electronic circuits con­taining vacuum tubes are, in fact, best left on continuously if the instrument is used daily. The frequent turning on and off of filament and plate voltages causes more rapid deterioration of the tube than does continuous operation. Tube life is also lengthened by arrang­ing the circuitry so that the filaments have time to warm up to the operating temperature for thermionic emission before the cathode-to-plate voltage is applied.

Typical Instrumentation Problems

I t may be instructive to conclude this general discussion of the factors involved in the proper utilization of analytical instrumentat ion by citing a few specific instances of instru­mentat ion problems frequently en­countered in practice.

Problem. It is desired to determine the pH of a certain solution. Two dif­ferent makes of pH meters give read-

Here's how you can MEASURE OPTICAL PATH DIFFERENCE

with the AO-Baker Interference Microscope

ln First, as shown in the photomicro­graph* above, the microscope ana­

lyzer was rotated until the background was brought to extinction. Readings were taken directly from the analyzer scale. Averaged settings resulted in read­ing of 70.4°.

2 . Next, the analyzer was rotated until the nucleus of the cell was brought

to extinction. Average settings resulted in reading of 138.2°.

3 The Optical Path Difference, in degrees, is tuii\- the difference between the two readings: / 1 3 5 . 6 ° \

OPD = 2 (138.2C-70.4C) =135.6° ; or OPD = { ~——- j .546 = .206 Microns.

Optical path difference measurements can be made to an optimum accuracy of 1/300 wavelength. This unique ability to measure optical path thick­nesses is in itself of great importance. But even more important, these meas­urements can be converted into a vari­ety of quantitative information of great potential value. Water and protein con­tent of a cell, for example, may be meas­ured. Materials such as glass, plastics, emulsions, textiles can be examined.

While the AO-Baker Interference Microscope is primarily a quantitative instrument, it also offers unique advantages for qualitative observations through variable intensity contrast and dramatically effective variable color contrast.

'Photomicrographs taken by Mr. Lynn C. Wall, Medical Division, Eastman Kodak Co. Data: Epithelial Cell.' AO-Baker Interference Microscope, 40X Shearing objective, 10X eyepieces. Corning filter CS4-120 with AO

Mode] 630 Pulsarc Illuminator to transmit monochromatic light at .546 microns.

WRITE FOR COMPLETE INFORMATION

American Optical Company

Dcpt. Q-'i8. Please Send me your NEW COLOR BROCHURE SB7 on the AO-BAKER INTERFERENCE MICROSCOPE.

NAME-

ADDRESS.

CITY -ZONE STATE-

Circle No. 31 on Readers' Service Card

VOL. 33, NO. 3, MARCH 1901 · 4 1 A

•»»i-iii.mjii.iin»i.»wiiijii.iaMi...u.iiii BPZHZif

Optically and mechanical ly , the M-5 offers unequalled precision, con­venience and versatil ity for the most diverse research applications. Uniform, m a x i m u m sharpness throughout the field, with no change in accommodation, is one of the m a n y exclus ive features of the instrument.

With a constant working distance of 96 mm., standard magnif icat ions are 6x, 12x, 25x , and 50x, convenient ly selected on the rotatable horizon­tal drum. Us ing 8x, lOx, 15x, and 2 0 x eyepieces and attachment objec­tives, a total power range of from 5x to 2 0 0 x is obtainable. Accessories not il lustrated include other wide field eyepieces, various l ight sources, s tages and polarizing, photographic and measuring attachments.

Your own evaluation of this superb stereomicroscope wi l l prove most rewarding. Write for Booklet M-5.

"the FIRST name in a complete line of Surveying Instruments, Photogrammetrlc Equipment and Microscopes.

REPORT

ings with this solution that differ by over U.3 pH unit, yet both instruments give proper readings, within the stated tolerance of ±0.01 pH unit, with standard buffers.

Interpretation. A poorly poised solu­tion-electrode combination—i.e., one in which the electrode potential is sensi­tive to small flows of current—will give different readings with pH meters hav­ing different input currents. That is, the magnitude of the leakage current that flows between glass and reference electrodes across the insulating sur­faces of the instrument, together with the grid current in the tube due to the residual gas content, may markedly affect the electrode potential of a glass electrode in a poorly buffered system. In this instance, the closest approach to the correct pH will be obtained by the pH meter having the smallest input current. Since standard buffers are ex­tremely well-poised electrochemical sys­tems, the performance of the pH meter with these buffers gives no indication of the magnitude of the input current. The input current should be separately measured. This is an excellent example of the fact that it is often possible to get an instrument to read out data which may be quite inaccurate, in spite of the fact that the instrument is func­tioning entirely properly.

Problem. The infrared spectrum of a certain substance appears to change when the spectrophotometer is run at different scanning speeds.

Interpretation. The ability of the recording pen to follow a rapidly vary­ing output signal is limited by the mechanical inertia of the motor and pen-linkage system. Sharp absorption bands present a signal to the motor that rises and falls more rapidly than the pen can move as the instrument scans through the band. The faster the rate of scan, the weaker the peak absorbance appears to be. Sharp bands should be scanned slowly, or a low-inertia read-out, such as an oscil­lograph or a photographic plate should be employed.

In employing infrared spectra for qualitative identification or quantita­tive estimation, proper attention should be paid to noting and reproducing the rate of data read-out.

Problem. It is necessary to deter­mine the concentration of a minor con­stituent in a mixture by gas chroma­tography. The concentration is so small that the signal of the transducer is close to the noise level and drift is also a serious disturbing factor. It is difficult to distinguish the peak from the background.

Interpretation. If elution conditions are chosen so that the constituent of interest comes off the column after a long retention time, the chromato­graphic peak will be broad and shal­low. If the retention time is short, the same peak will be much sharper and deeper. In this case, the short retention time is generally best, for the

42 A ·

Circle No. 21 on Readers' Service Card

ANALYTICAL CHEMISTRY

HEERBRUGG

REPORT

increase in signal more than compen­sates for the higher noise level accom­panying faster read-out, especially if a galvanometer is used as the read-out device, to eliminate the effect of pen inertia. Also, at the faster elution, the drift in background signal is less seri­ous.

This is an example of a special type of problem in which fast read-out is preferable to slow read-out, because of the unique requirements of the physi­cal-chemical system. In the absence of such special considerations, slow read-out gives the best signal-to-noise ratio.

Problem. The transfer function re­lating concentration to diffusion cur­rent in a polarographic determination of a metal ion is found to be different for a manual instrument than it is for a recording instrument, all solution conditions being held constant. I t is necessary to decide what type of in­strument to use for control purposes, and what kind of standards to run.

Interpretation. In the absence of other electroactive constituents with half-wave potentials within a few-tenths of a volt of the ion of interest, and in the absence of pronounced maxima, the manual instrument will give the best precision because of a more favorable signal-to-noise ratio. If there are nearby waves that con­tribute to the current at the voltage being monitored, an automatic record­ing instrument will be more useful, particularly if it is capable of provid­ing a derivative polarogram. These latter instruments make it possible to distinguish the wave of interest from the interfering waves.

Thus, if the analytical problem is simple, a few data should suffice and these can be read out at a slow rate; if the problem is complex, many more data are necessary, and the read-out must be faster, with a correspondingly poorer signal-to-noise ratio and less precision in the final results.

If practical, an internal standard would be chosen. To be practical, the internal standard must consist of an ion that reacts at the polarographic electrode at a potential close enough to the ion being determined to give assurance that instrumental factors are substantially constant in the time in­terval between the reaction of the two species, yet far enough removed from each _ other so that the two electrode reactions do not interfere with each other.

If the proper instrument has been chosen, and if it is properly used in at tacking an analytical problem, the chemist may have confidence t ha t the precision of his results will, in nearly all cases, be limited by the chemical operations and manipulat ions involved in the pro­cedure, and not by the measuring instrument.

ACE interchangeable REACTION APPARATUS

for research & production

Bottoms, Tops are available in stainless steel, Monel and other metals as well as glass. The 1 to 72 litre range permits use for bench top appl i ­cations as well as pilot plants and small batches; the 50 to 72 litre size proven ideal for specialized pharmaceutical and biological compounds.

Al l tops are interchangeable with all bottoms regardless of size. Experiments can be carried out with only one top and a series of bottoms ranging from 1 litre to 72 litres. Necks are wide, facil i­tate cleaning of viscous material. In addition to standard heads. Ace will custom make to your specifications.

Standard clamp adaptable to entire series of reaction flasks. Top section removable without disturbing lower section.

Convenient arm for clamping to supporting frame. Hammertone finish.

Full line of accessories: Glas-Col Heaters Powerstats Trubore Stirrers Stirring Motors.

Ace Reaction Apparatus is fully described in Ace Brochure f58.

Write Dept. A for your copy.

Circle No. 107 on Readers' Service Card

VOL. 33, NO. 3, MARCH 1961 · 4 3 A

ACE GLASS I N C O R P O R A T E D

LpuisvHIe, Ky., V i n e l a n d N . J . Springfield, Moss.