Topics in..
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Chemical instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serve the readers O ~ T H JOURNAL I ~ by calling attention to new developments in the theory, design, or availability of chemical laboratmy instrumentation, or by presenting useful insights and explanations of topics that are of practical importance to those who use, m teaeh the use of, modem instrumentation and instnmental techniques. The editor invites correspondence from prospective contributors.
XL.
Oxillostopes in Chemistry-Part
Two
Joseph E. Nelson, Chemfrix, Inc., Beoverton, Ore. 97005
THE TRIGGERED OSCILLOSCOPE When s. repetitive signal such aa a s i n e wave is applied to an oscilloscope and displayed on the screen it appears as a stationary waveform. A new waveform is superimposed on each scan and the net effect is a ststiorlsry-appearing display. This ability to exactly superimpose repetitive signals is one of the most important capabilities of the modern oscilloscope. I t is possible because each scan of the time base can be initiated by a trigger impulse that is derived from the signal of interest. A triggered oscilloscope is one in which each horizontal sweep is initiated or triggered by asignd that is external to the time base. Usually, the trigger signal is r e ceived from the vertical amplifier. However, it can also be applied directly to the time base from an external source. In the case of 8. single sweep, the trigger signal may he applied by actuating s. front panel switch. Almost all triggered oscilloscopes can also operate in a free-run, or nontriggered made. I n this case the sweep is continuous, one cycle after another, with no synchronized relationship to the signal applied to the vertical smplifier. This mode does produce a. trace on the CRT
screen and is useful for amplitude or dc mesaurement. However, the free-run mode will not produce a stable wrtveform. To produce a stable repetitive waveform the sweep voltage applied to the horizontal deflection plates of the CRT must start when the eignal of interest reaches a preset point in its rise or fall. I n Figure 6 the triangular wave is the signal of interest. The sweep starts when the triangular wave reaches point A. Once started, the sweep cannot be influenced by triggering signals. When the sweep is completed and retraced to the left side of the screen it will then wait until the triangular wave again crosses point A before scanning. Notice in Figure 6 that the display in each case is the same and when these successive traces are superimposed they appear as a stable display. The chemist will probably use single events more than repetitive signals and a good illustration of triggering concerns the capture of a. fleeting transient. In a flash photolysis experiment the sndden application of a. high energy flash produce8 chemical changes blmt can be followed electruchemiedly on the oscillascope. By using the pulse of light to start or trigger the sweep, the events that immediately follow
in the elect.rolysis cell can be recorded from the screen by camera, or preserved intact by use of a storage tube. The Time per Division switch set on the time base will determine the time period on which events will be recorded following the flash triggering. I n chranapatentiometry a constant current is suddenly applied to the electrolysis cell and the resultant potential is monitored. The change in potential upon spplication of the constant current can be used to trigger the time base. Since a trigger signal can be applied to the time base from an external source it is possible to couple electrical signala from mechanical devices. For example, in a. dropping mercury assembly one may wish to trigger an oscilloscope when a drop is dislodged. A mechanical striker can knack off the drop and simultaneously close smicroswitch to provide ashort pulse to the oscilloscope trigger circuit. Two additional types of triggering are of interest: (1) delay sweep after triggering and, (2) trigger after delay. I n the first a timing circuit (delay circuit) is started by the normal trigger. At the end of the preset time, the sweep scans a t the rate set bv the Time per Division
Figure 7.
between the input signal, the sweep voltage, and the display seen on Figure 6. The time the CRT screen; each rvccerrive display is superimpored over the previous one, ond the result appears as a steady waveform.
lo)
A triggered display with a 10-rec
Convention01 display, the orea d interest is encircled. lbl The central region oxp m d e d to RII the screen b y means of 0 delayed sweep; the *can is triggered a t the same point, but deloyed 4 rec followed b y o 0.5-sec scan. rcan.
(Continued on page A788)
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Chemical Instrumentation switch. To illustrate consider a sudden
the oscilloscope trace shows a, group of small fluctuations that are of interest. However, since the scan must be quite long, the fluctuations of interest are not too well defined. If the reeion between 3 and
oseilloscape that contains a delayed sweep feature. Thus, the delay would be set far 3 seconds and the sweep Time per Division to 0.05 sec. (0.5seconds per 10 divisions). The two different cases are shown in figure 7. The second type is similar except that a t the end of the delay period the trigger circuit will accept the next eligible trigger signal. This is useful in cases where some initial turbulence must be ignored before the signal of interest is produced. To sum uo trieeerine. each scan is
source.
PLUG-IN AMPLIFIERS Since many of our modern oscilloscopes accept plug-in units for t h e X and Y axes, a large variety of vertical amplifier and time base combinations are possible. These units permit one to apply the latest developments available to a particular scientific problem.
Dual Trace Amplifler This vertical amplifier can accept two signals which are then displayed simultaneously on the CRT. The time relation between events is immediately apparent and one can think in terms of cause and effect. For example, let channel 1 show a n impulse signal applied to an electrolytic cell and let channel 2 show the current through the cell. With a slow scan of both traces moving across the screen, apply the impulse signal and see the response immediately in exact time relationship. . .
Figure 8.
A four-troce ornpliRer (Type 1804AI
used with o variable-persistence ordllorcope (Type 181Al. (Hewlett-Packord Co.) Dual trace is accomplished by internally switching each channel alternately (at 100 kHz) into the output amplifier. I t should not be confused with dual beam in which two separete CRT's are contained in one glass envelope although the results are airnilar. There are also four-trace units (Continued on page A790)
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for waveform amplitude measurements (Fig. 9).
Chemical Instrumentation
Operational Amplifiers
that accept and display four signals simultaneously, see Figlire 8.
Two special plug-in operational amplifiers are available (Fig. 10). These units each contain two programmable amplifier units as well as a common deflection amplifier. Thus, one can develop a variety of operationd systems by using external
Differential Amplifier This unit, also called a. difference amplifier, has the ability to amplify and display the difference between two signals. Because of this, many chemical instruments have been designed to use differential amplifier oscilloscopes as the readout system. By applying a blank signal to one input and the unknown to the second, only the difierencesignal is displayed. Thus, backernund or residual currents ineludine noise
called common-mode rejection measured by the common-maderejection ratio (CMRR), a n important parameter to consider when seleehg a difierential amplifier. I t is measured hv a ~ ~ l v i nidentical e sineuntil some measureable waveform 1s seen on the screen of the CRT. For example, assume that the Volts per Division switch is set to 0.001 v per division and that the input amplitude of the applied sine-wave must be increased to 10.0 v to produce a 1 division trace on thescreen. The ratio between the applied signal to the observed waveform is 10.0 v/0.001 v or 10,000 to 1. Thus, the CMRR of this amplifier would he 10,000 to 1. Amplifiers are available
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Figure 9. A differential mmporator plug-in module [Type W l that permits precise omplitvde meawements on di3played woveforms through use of a calibrated dc voltage applied to one channel. (Tektronix, 1nc.I
with up to 100,000 to 1 common-mode rejection. One additional function that can be accompliihed with a differential amplifier is the application of s. variable do voltage to channel 2 in order to offset a large signal in channel I. For example, to examine the peak of a waveform in detail would normally require a high sensitivity which would drive the trace well off the screen. By using s variable de voltage in the opposite channel one can move the peak of the trace back on screen and examine it in detail. Plug-in units that have this ability are called differential eompsrators and they provide an accurate tool
Figure 10. An operational-amplifier plug-in modvle [Type 3A8j that permih components to be applied externdy for a variety of onolog operations. (Tektronii, 1nc.I
(Continued on page A7981
Chemical Instrumentation -
components in the input and feedback positions. These unite d m contain internal components that can be switched into input and feedback positions. This type of plug-in can frequently oeeupy a dual role in an experiment. For example, one amplifier can be arranged to generate a. staircase function which can be applied to a n external system. The response of the system can then be impressed upon the second amplifier, connected as a. differentiator. The output of this rtmplifier is then internally applied t o the deflection amplifier and displayed as the derivative waveform on the CRT. Current measurements are often made by introducing a sampling resistor into the external circuit and then measuring the voltage drop with a voltage amplifier. Since the introduction of the sampling resistor may be objectionable, an operational amplifier can be used to measure small currents without introducing additional components when it is connected as a current to voltage converter.
SPECIAL OSCILLOSCOPES Under this heading one finds the storage oscilloscope, digital-readout, sampling, and signal-averaging oscilloscopes. Each of these oscilloscopes has special capabilities not possessed by eonventiand ininstruments. The following brief description is intended to introduce these instruments and point out their mast important features.
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The Storage Oscilloscope I n the past, the chemist has been somewhat hampered in using oscilloscopes since m m v of his ex~eriment.3were not reoeti-
graph. The storage oscilloscope solves this problem in that images can be retained on the screen for indefinite periods. When operated in the storage mode every trace is stored on the screen until erased by the operator. This includes all traces regardless of scan speed up to the limit of the CRT voltage writing rate. A recent variation of the storage oscilloscope is termed variable persistence. This system permits the operator to select the fading time for a trace and extends from almost immediate erasure to complete storage without fade. This technique can he useful in following slowly changing repetitive signals since the previous waveform will still be visible although partially faded with respect to the new waveform. With regard to the storage oscilloscope as a working tool, it is the writers opinion that eventually all low and medi~tmfreq~rency laboratory oscilloscopes will be of the storage type.
The Digital Readout Oscilloscope Each new development responds to the need for greater accuracy, ease of operation or new areas of measurement. The digital readout oscilloscope was created for all t,hree reasons.
The trace on this oscilloscope cantsins two small brightened spots called intensified zones that can be moved across the trace to any selected point. The readout will then provide the amplitude between those two points. For example, if one zone is set a t the baseline of a square wave and the second zone on the top of the wave, the readout will indicate digitally the amplitude of the squarewave. Thus, the possible ambiquity of reading the position of the trace with respect to graticule divisions is resolved. One significant aspect of this instrument may appeal to those chemists who enjoy automating a. series of measurements. For example, since the digital readout is well suited far peak amplitude measurements, a series of peak rneasurenlents of a multiple peak waveform could be made in seqnence by designing a device that would automatically move the zones to each successive peak sequentially. In addition, most digital readout instruments have auxiliary output signah that can be used to actuate a printer far a permanent record of the peak values.
The Sampling Oscilloscope Although most chemists rarely deal with signals a t frequencies of a gigahertz or higher, they should be aware of the existence of oscilloscopes that can display these microwwe frequencies. In the sampling oscilloscope 'real time' (our usual concept of time) is converted to equivalent time. A good analogy here is the moving picture (Continued o n page A796)
Chemical Instrumentation photography of 8, speed event such as a horse race that is viewed as it occurred ired time) and also viewed in slow motion frequency of one gigshehertz the real time frequency of the displayed waveform may he only ten kilohertz. Sampling is based on taking voltage samples from many input signals with each sample taken a t s. slightly later point on sahseaaent sienals. As each samole is
way a replica of the input signal is constructed point by point and displayed on the CRT. In real time the replicas me formed repetitively a t perhaps up to 10.000/sec. The oscilloscooe Time Der
The Signal-Averaging Oscilloscope This oscilloscope has as its prime application to extract meaningful repetitive signals that are buried in noise. This latter condition esn be visualized by considering a one volt sinewave signal completely covered with 5 v of atmospheric random noise. The conventional oacilloscope display would he simply a five volt hand of noise with no visible evidence of the one volt sinewave signal. When thii repetitive signal is introduced to the signalaveraging oscilloscope one finds that the sinewave signal can he displayed with almost no evidence of noise. in a. memory system during successive scrtns of the sweep system of the oscilloscope. Since the noise is completely random, t,he average naise level when sommed over many scans is practically zero. The sienal. ,. . on the other hand.. is reoetitive nod cnch sran %Ids to ttw prwin~isone a d presents . l t t avrragc that ia n true replim of ~ h origird. r The s i g d must be repetitive hut i t can be of arelatively slow nature since the previous scans are held in the memory hank and will wait for subsequent signals to arrive. Due to the mathematical nature of noise averaging, the
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Figure 1 1 . A single-sweep polorographic onolyrer (Type SSP-31 that we. o storage tube for anobg dirploy, coupled with digital mod-out of polamgraphic peak height. (Chemtrii, Inc.)
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(Continued on page A7QY) Circle No. 163 01 Readers' Service Card
Chemical instrumentation greater the number of sweeps, the closer the net weversge of the noise is to zero. Much like tossing a coin in which a 507' heads figure will be reached if enough tosses are made. Accordingly one might consider the averaging oscilloscope in situations where data might he submerged in a high noise level and s. repetitive acan is possible. Thus, new levels of sensitivity in analytical techniques may he possible.
APPLICATIONS Within the chemists' own domain one
Single Sweep Polarography The instrument shown in Figure 11 uses a storage tube for visual readout of the polerographic waveform as well as a digital readout of individual peak heights. High speed polaragraphy a t 1 millisecond per polarogrsm is possible with the oscilloscope storage readout. I n this application, the time hase waveform that drives the C R T horizontal scan also serves, after attenuation, to drive the polarogrsphic current through the cell. The cell current is applied to the vertical amplifier which in turn drives the vertical plates of the CRT. The resultant palarogram is a cmrenb voltage plot where the horizontal axis of the CRT represents the potential applied to the cell. I n the example shown the time hase waveform was a triangular wave; the combined cathodic and anodic scan is called s cyclic polamgram (also called a cyclic voltammogram). I n the past some objection had been raised due to the cathode-ray tube's limited vertical area for peak height measurements (8 centimeters). I t appeared difficult to resolve small differences in peak height. This has been overcome through use of the digital reedout since each vertical centimeler can be read to one part in a hundred.
Spectrophotometry In instruments of this type the major function of the oscilloscope concerns a display of radiant power versus wavelength. The spectrum can be scanned a t a variety of scan rates from as little as one millisecond up to several aeconds. The plot in this case is wavelength on the horizontal axis with either absorbance or emission on the vertical axis. Since the horizontal scan fallows exactly the wavelength scan, precise wavelength measurements are passihle through pdat-ion markers within the spectrum. Several commercial instmments that use oscilloacoppa in their system are described by Lott, THIS JOURNAL, 45, A89, A169, A273(1968). In view of the oscilloscope's ability to he triggered; and also to be triggered after a delay, one might wish to try to display a small segment of the spectral scan. For example, assume a spectral acan of one millisecond that covers 10 mp. To examine a portion of this scan in detail, one (Continued on page A800) Volume 45, Number 10, October 1968
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would trigger the oscilloscope delay circuit a t the start and then set the delay to end just before the occurance of the peak of interest. The actual ascilloseo~esesn of magnification.
Waveform Monitor For classroom instruction the usual fiveinch cathade-ray tuhe is often difficult to see, especially in large lecture halls. T o overcame this problem Welch Scientifio Company has produced s 12 in. lecture table oscilloscope. The electron beam of the C R T can be brightened to g i v e s welldefined trace that will he visible from any part of the classroom. And as one further improvement, even the instructor is removed from the front of the display tuhe, since all of the controls plus a small 3 in. C R T monitor are located on the rear of the instrument. I n this way the instructor sees the same waveform as the students hut does not block m y portion of the large screen display. Additional information on modern oscilloscopes can he obtained from the manufacturers. I n the case of Tektronix and Hewlett-Packard their oradoct
that will he especially helpful to those not acquainted with oscilloscope terminology.
They are (1) A Primer of W a v e f o ~ m sand their oscilloscope displays, and (2) Undcrstanding Operational Amplifiers. For oscillorrcope applications in polarography write for 50 Questions on Single Sweep Polarography from Chemtrix. Manufacturers o f Oscilloscopes
There are a t least 30 manufacturers of oscilloscopes. The following is a selection which seems pzwticulerly significant in the present context. Code: S T = storage 'scopes, S P = sampling, SA = signal averaging. Mast have various other models besides those coded here. Annlnb (Div. of Benrus) [ST, SP] Wnterbuw, Conn. 06720 Fnbti-Tek. Inc. [SAI IMndison. Wis. 53711 Hewlett-Pneknrd Co. [ST, SA, SF] Pnlo Alto, Calif. 94304 Hughes Instruments (Div. of Hughes Aircraft Co.) [ST, SPI Culver City. Calif. 90230 Measurement Control Devices. Inc. [ST] Philndel~hin,Pa. 19125 Northern Scientific, h e . [ST, SA] M~dison.Wis. 53711 Nuelenr D&, Ino. lSAl Palatine. Ill. 60067 Technioal Instruments. Inc. [SA] 06473 North H ~ v e nConn. . Tektranin. Inc. [ST. SPI
Corrections: Line 22. ,page AB42. September liavea rise time of 0.35/ 500 kH,,= 0.7 rseo." Also line 21 should have read: of 0.1 m e e wouldappear to ihsve 0.7 psee."
iasile should have read:
