H. V. Malmstadt, R. M. Barnes and P. A. Rodriguez
University of Illinois Urbano
II
A Multipurpose High Precision Recording Photometer An ionization detector-readout for gas-liquid chromatography
Accurate photometer systems find widespread use in flame photometers, calorimeters, absorption spectrophotometers, fluorimeters, spectrophotometric titrators, emission spectrometers, and other instruments of importance in chemistry or physics courses, as well as in research and control. Also, a welldesigned versatile photometer system can be directly applicable for use with ionization detectors in gas-liquid chromatography. Other laboratory applications will be apparent after considering the design principles and features for the versatile system described in the subsequent sections. For most photometer applications in the ultraviolet and visible regions the vacuum photoemissive tubes (phototubes and photomultiplier tubes) are a t present the best and most reliable detectors-providing very fast response and excellent reproducibility and linearity. However, the associated measurement readout systems are often inadequately designed to take advantage of the fine characteristics of these detectors, or less suitable detectors are sometimes substituted to keep the cost down for the readout system. The photometer system presented here is a compact recording module that combines the highest sensitivity for the detectors with optimum reliability, versatility, and convenience a t a low cost. Very small photocurrents can be displayed full scale on a 10-in. scale, or changes of photocurrent can be recorded continuously on the 10-in. ruled grid of a strip chart driven by a synchronous motor. For example, the photometer can be switched to a sensitivity of 5 X amperes full scale with a readability and precision equal to the line width of the pen (about 5 X 10-l3 amperes readability on this range). Many different full-scale spans are availahle over a wide range of current. Several scale lengths of suppression can be switched into the circuit so that small current changes on top of a larger current can be greatly expanded on the chart. The circuit design enables the difference in photocurrents from two phototubes to be recorded, which is desirable for double-beam operation. Also, the difference in ionization currents from two detectors on a dual-column gas chromatograph can be recorded. The pen travels across the full 10-in. chart in one second so that spectra with sharp peaks can be accurately recorded, and from day to day there is no measurable drift of the zero point. The module consists of the stable pH recording electrometer recently described by Malmstadt (1) and an inexpensive adapter unit and power supply (45, 90,
300, 600, 900, and 1200 v) for the different photo and ionization detectors. All units attach together to form a single compact module that is easily carried from one setup to another and integrated with other instrument n~odules. Many specific instrunlents that are very economical and yet provide the best attainable data can be assembled by interconnecting the high precision recording photometer module with suitable monochromators, light sources, chromatography columns, transducers, etc. Experimental data are presented that were obtained with the photometer module connected to other selected modules to provide a flame photometer, an atomic absorption spectrophotometer, an expanded range ultraviolet and visible spectrophotometer, and a spectrophotometric reaction-rate meter. Since the photometer module can measure very small currents (about 10-l2 amperes sensitivity in the present model) it can he applied as a recording ionization detector system in gas-liquid chromatography. The flame ionization detector and other ionization detectors (2) are noted for their fast response, high sensitivity and linearity. However, they require an accurate recording system with high signal-to-noise ratio for very low currents, fast pen response, and sometimes large-scale suppression (5). All of these requirements are met by the module described here. Therefore it can be connected without modification to the commercial (5) or homemade (2) ionization detectors. Chromatographic curves are presented that were obtained by connecting the photometer module to an available flame and column module. The recording module contains circuits for the accurate integration of photocurrents or ionization detector currents. The integrating feature is especially useful in gas-liquid chromatography because more significant results can often be obtained from the area under the peak than from the peak height (4). Also, the integrator is useful for averaging noise on signals from flame photometry, spectrophotometric reactionrate curves, etc. When switched to integration, the distance of pen movement is directly proportional to the area under the curve. A relay a t the end of the scale enables the pen to scan several times across the 10-in. chart for a single peak or selected time interval. A precision of about 0.1% is possible when the pen traverses the chart once during the integration period. Better then 0.1% precision is possible if the pen makes several full-scale excursions during a selected time interval. A plug is available so that both the peaks and Volume 47, Number 5, May 7964
/
263
the integral of the peaks can be recorded simultaneously by plugging in a second recorder. Voltages or integrals of voltage signak can be directly recorded as well as currents. Modular Instruments
The module described in the following sections meets the objectives of high sensitivity, versatility, precision, accuracy, convenience, and compactness that are important for experimental work in courses at a11 levels. It is one of a series of modules, some of which were recently described (1, 5). Since they are researchquality devices, new research developments are easily incorporated into student experiments without the handicap of inferior equipment. For most courses a few basic modules and several transducers can be interconnected in a variety of ways to provide the required measurements for the experiments. Ideally a set of high-quality modules and accessories for rapidly assembling all of the instruments to be used during a course would be available for each student. The opportunity for individual experimentation develops independence, and provides a chance for the better students to work a t a faster pace and to try new ideas; the ability to obtain research-quality data improves the students' attitude and interest in the laboratory. The data presented in the experimental section will provide some illustrations on how the photometer module can be interconnected with other devices for use in many chemistry courses and in research for a wide range of mcasurements.
point S is a t the same potential as point G. Therefore the anode-bcathode voltage E,, is equal to the supply voltage E,, regardless of the current, and nonlinear response cannot be caused by an IR drop in R,. If point G is connected to chassis ground then point S will be at ground potential, within the ability to set E, = E, (which is about 10 pv in the present model). The signal-to-noise ratio and response time of method 2 are generally better than for method 1, but both methods can give good results and both are possible with the recording module described here. Another unique advantage of method 2 is that the currents from several high-resistance transducers can be connected at point S (the summing point) and the sum of the currents determined. Also, the circuit arrangement in method 2 enables the current to be integrated by the simple substitution of a capacitor for the resistor RI. This method of integration will be discussed in a subsequent section. The general considerations of method 2 as a current-measurement and integration system are quite similar to those for operational amplifiers (6). From Figure l b it is apparent that the photocurrent passes through the effective resistance of the standard source E,. This can cause the actual value of E, to be different than its standard value, especially in the higher current ranges. The methods to eliminate this possible source of error will be considered under the specific circuit.
Measurement Principles
Two methods for measuring small (about to 10-I3 amps) photocurrents or ionization currents are illustrated in Figure 1. Both are null-point measurement systems (1, 6). Method I . The method shown a t the top of Figure 1 is similar in principle to the null-point procedure for measuring the emf of a high-resistance electrochemical cell as in pH determinations (1). For current measurements an unknown current I develops an unknown voltage drop EI = IR, across the load resistor R,, and a variable standard voltage Es is varied until E, = E, (as indicated by a null point), and consequently I = E,/R,. It is usually necessary to switch the values of the load resistor if a large range of currents is to be measured. The IR drop decreases the voltage E,, between the anode and cathode of the phototube or other transducer, and this can introduce nonlinear detection characteristics. Therefore, the resistor R, is switched in ten to one hundred fold increments so that the IR drop is not significant for a limited current range. The switch for R, becomes a multiplier switch for differentcurrent ranges. Method 2. The second method, illustrated in Figure lb, introduces a bucking voltage in series with the voltage drop across a resistor R,. The bucking voltage fl, is a standard variable voltage source that is varied until it is equal to the voltage drop E, = IR If E, = Efthen
.,
I
=
E./XI
(1)
and since the polarities of E, and El are opposite, 264
/
journal o f Chemical Educofion
I+
I AT MLANCE AND (1.5)
e,
i-S
-
si
-
e.
i 'e. /RL
iR' CDMFdRISON VOLTAGE SOURCE
NULL-POINT DETECTOR
Figure 1. Boric methods for accurate meowrement of small photocurrents in the range of 10-'to 10-" ompr
Automatic Self-balancing of E,. A variable voltage source is conveniently built with a linear slidewire, and its slider is readily coupled to an indicator or writing pen. Therefore, the distance of pen movement is proportional to the output voltage E, of the standard comparison source. At the null point the measured current is directly proportional to E, for both methods 1 and 2, and the movement of the pen provides a linear current scale. By using a servo system to automatically maintain a nuU point, the pen position
indicates the instantaneous current, and this is illustrated in Figure 2 for both measurement methods. I n Figure 2a, a chopper reed is connected to the input of the electrometer amplifier, and it alternately makes contact with chopper contacts b and d a t the rate of 60 cps. Any difference between the voltage drop across R, and the value of E, causes a potential difference between contacts b and d. 'This potential difference is seen a t the amplifier input as a 60 cps square wave. The amplified signal is fed to the control winding of a servo motor and the shaft turns to change E,: if the voltage drop IR, is greater than E., the servo motor shaft turns to increase E,; the motor reverses direction and decreases E, when 8, is larger than IR,. The motor stops when the controlwinding voltage is zero or of small magnitude. Since the amplifier gain is more than one million, a potential difference of less than 10 pv between contacts b and d yields sufficient output voltage for the servo motor to drive and accurately maintain a null point.
therefore the potential difference between contacts b and d is essentially zero. In other words, the summing point (contact U5 in Figure 2b) is automaticaUy held a t a virtual ground (6). Again, a pen is coupled to E, so that the pen position is directly proportional to the current. Since I = E,/R,it is obvious that the current span can be changed by varying either R, or the full-scale value of E,. For example, if the full-scale span of the comparison voltage is 100.0 mv and the resistor R, = 100 megohms, then I = lo-'/ amps full scale; by changing R, to 108 = 1 X 10 megohms the full-scale span is changed to 1 X lo-@amps.
AT BALANCE
e, = e,
Figure 3. Basic circuit for integration of the photocurrent and recording of the integral.
Currat Integration. It was shown above that the servo system operates in method 2 to maintain essentially zero potential between the chopper contacts b and d. With the contacts connected as illustrated in Figure 2b the summing point is maintained at a virtual ground as a necessary consequence of the servo operation. If R, is removed and replaced with a capacitor, as shown in Figure 3, the capacitor voltage E, must equal EE,to satisfy the above requirement. The capacitor voltage E, = Q,/C where C is the capacitance in farads and Q, is the total charge in coulombs on the capacitor. The total charge Q , is equal to the integral of the charging current during the charging period t . Since,
and
Q,
=
zdt
Figure 2. Avtomotic self-boloncing recording photometer. bored on the twomelhodr illvrtrated in Figure 1.
The circled and lettered points in Figure 2 are specific contacts within the working module. They show where the same contacts appear i n the two different measurement schemes and are convenient reference points when referring to the complete circuits. The servo for method 2 maintains a null point as in method 1. However, the chopper is connected so the amplifier sees the difference in potential between the summing point S and ground point G. The servo motor drives to vary E, so that it is always equal and opposite in polarity to the IR, voltage drop, and
Therefore the pen position is directly proportional to the integral of the input current. The integrator span is varied by changing the full-scale values of E,, as in the pH electrometer, and the value of the integrator capacitor. The relay switch across the capacitor is shorted when the pen reaches full scale, thereby discharging the capacitor. Voltage Measurements. In Figure 4 a resistor is connected between an input voltage source E, and the summing point S. Since point S is automatically held a t virtual ground and the positive end of the Volume 41, Number 5, May 7964
/
265
input voltage source E , is a t ground potential, the equivalent circuit would show R, across E, and the input current is I = E,/R,. From equation (1) it follows that,
and The full-scale span for the measured input voltage can be changed by varying the resistance ratio R J R , and the full-scale voltage span for E,. If e, = 100 mv full scale and R,/R, = lo8ohms/106ohms, then the full-scale inpl~tvoltage span B, = lo-' X lo8 = 100 v.
Figure 4.
Figure 5b should be useful in most situations. It is a hybrid of the methods illustrated in Figure 2. One detector is connected as in Figure 2a and all other parts of the circuit and the second detector are connected as in Figure 2b. With the circuit connected as shown in Figure 5b, the summing point S will be maintained a t a potential determined by the potential drop across R,. If the potential drop across R , is 0.1 v, then contact d will be -0.1 v with respect to ground. For equal photocurrents and equal resistance values for R, and R, the potential a t contact b (point S) is equal to the potential a t contact d. For different photocurrents from the two t,nbes the instantaneous potentials a t contacts b and d are different and the servo system will operate to adjust the potential of contact b to the potential of contact d. The distance of pen travel is a measure of the difference of intensity on the two tubes. The zero adjust control and voltage index control (I) of the comparison voltage source can be used to set the desired reference point on the chart.
Boris sirwitfor meorvrement of high v>ltages.
Voltage Integration. As shown above, when a fixed resistor R, is connected from an input voltage E, to the summing point, the input current I = EJR,. This current, instead of the photocurrent, can be fed into the integrator circuit of Figure 3. By substituting E,/R, for the current I in equation (2), it follows that
It is apparent that the distance of pen travel, which is a linear function of Es, is directly proportional to the integral of the input voltage E,. The time required for the pen to travel full scale for a specific value of the integral can be adjusted by changing the values of R,, C, and the full-scalevalue of E,. Dm1 Detector Methods. Without using additional circuitry, such as isolation amplifiers or two electrometer channels, it is possible to connect two separate detectors of high internal resistance into the recording module and determine the difference of their outputs. Two methods are illustrated in Figure 5 with phottubes, but other transducers can be used. In Figure 5a, the connections of the two phototubes are reversed so the anode of one and the cathode of the other are connected to the summing point. For perfectly matched phototubes the photocurrents will be equal when the intensities of incident radiation on each tube are equal. The flow of electrons in the two tubes will he in opposite directions so the net input current into R, will be zero. The distance of pen travel is directly proportional to the difference in photocurrents. More than two phototube detectors could be connected to the summing point. The method of connecting transducers in Figure 5a is not desirable for identical detectors that have large internal current amplification (photomultipliers) or have pounded shields and other design and construction features that make the reversing of connections impractical. However, the method illustrated in 266
/
Journal o f Chemical Education
Figure 5. Two photodetectors connected far measurement of 'difference in photocvrrentr (Method o a t thetop, ond mehod b ot the botbrnl.
Specific Circuits and Characterislicr
Because of the many specific applications for the pH recording electrometer (I), it seemed worthwhile
to design a simple adapter that would plug into the pH unit rather than to use a completely new adapter
to perform the functions of current and voltage measurements and integration. The desirability of a specific adapter for these functions, instead of the pH adapter, is being investigated, but the present approach is more economical and versatile, especially where many units are required in a chemistry laboratory. Only a switch, a few resistors, capacitors, and outlets are required in conjunction with the pH adapter, as shown in Figure 6. By referring to Figure 2 and the above general measurement principles, Figure 6 is selfexplanatory. A few specific comments on each function are pertinent. Current Measurements. The current measurements are made with switch S2 in positions 1-3. Current is fed to input connector A or D or both. In switch
positions 1-3 the input is connected directly to point S. Full-scale current spans are switched by selecting different full-scale E, spans and R , values. Resistors R1, R2, and R3 are the specific R, resistors, and they provide tenfold changes in current range. The voltage divider of the comparison voltage source (1) provides full-scale spans of E , equal to 50, 100, 200, 500, 1000, and 1400 mv. The selector switches are labeled so the unknown current is equal to the product of the span setting times the multiplier of switch 52. For example, when set on the lo-" multiplier position and a span of 50 mv, the full-scale current span is 50 X lo-" amps; on a span of 1000 mv and the lo-" multiplier position, the full-scale current spau is 1000 X lo-" or 1 X amps: etc. For the present adapter the full-scale current spans vary from 1.4 X lo-' to 5 X 10-lo amps. Other ranges could be easily added by providing other values of R, or E,. A readability of 0.1% and a sensitivity of 0.2% of full scale are possible on all current ranges. The damping control needs to be adjusted only when switching the span control and not when switching the current multiplier. On the most sensitive current spans it is important to prevent much movement of the input coaxial lead because flexing the cable will induce small current pulses that will appear as noise. Even on the most sensitive current spans the zero reference point is rock steady because of the circuit design. The value of R , can be calculated from the ratio E J I , where E, is the calibrated full-scale value across the voltage divider of the comparison voltage source when the divider is not loaded, as in Figure 2a. However, when E , is connected as in Figure 2b, the input current I flows through the effective resistance Rd of
I
t
VOLTS MULTIPLIER
PLUG 'urnIN RECORUNN$ ELECTROMETER
Inn. n A m
VOLTAGE INT. RATE
x l d 1I
~ ~ ~ I o - ~
I
~~
CURRENT M~LTIPLIER
6.
F~QY.~
the divider, which is different for each specific voltage span. The magnitude of the input current I relative to the divider current Id determines whether the standard value of E. will be significantly effected. Obviously, by using smaller divider resistors the divider current will be higher and the I/Id ratio more favorable. However, the decrease in resistances of the divider resistors is limited by the loading effect on the bridge. It is practical to decrease the resistances of the divider resistors to one-half the values previously reported (I), and this change is incorporated in the latest design. The divider resistances now range from 2250 ohms to 45K in going. from 50- to 1000-mv full-scale spans. All full-scale current spans from 5 X 10-lo t o 2 X lo-' amps. are accurate within 1% of the nominal values. Any specific current spau can be calibrated to about O.lyo with a known input current source. On the 5 X lo-' and 1 X 10-6 amp spans, the input current through the divider lowers the current spans from the nominal values by about 2 and 4y0, respectively. Either span could he calibrated with the MV CAL adjustment (1) but this would cause the other spans to be in error. Compensated values R,, of R, could be switched in for these two spans, where R,, = R, - Rd. For measurement of higher currents than one microampere a different design is recommended and such a design will be considered in a future article for a differenttype of module. Voltage Measurements. The voltage ranges are selected by switching to positions 4, 5, and 6 of switch S2 in Figure 6 , which connects a 1000-, loo-, or 10megohm resistor R, between the input connector and the summing point S. I n these positions full-scale voltage spans ranging from 0.5-1400 v are available. 1he six millivolts ranges are the same as for the recording electrometer, and they are selected by switching to position 7 and then using the span selector switch. Also, the input leads are the same as for the pH measurements (terminals H and B), rather than hetween terminal A and the cable shield for the volts measurements. I n positions 4, 5, or 6 the full-scale input voltage (in volts) is read by multiplying the 50, 100, 200, 500, 1000, or 1400 from the span control by the multiplier on switch 52. For example, in position 4 the multiplier is "1" so that in the "1000" span position the full-scale voltage input span is 1000 v ; etc. It is impressive that in nosition 4 the iunut resistor is 1000 .-megohms, as compared to about 10 meg~10.001ut ohms on most VTVMs. Therefore, the loading effect on the measured circuit is extremely small. Also, an accuracy a t least ten times greater than most VTVMs is readily obtained. I n position 7, outlet P R is switched in for future applications. It could be used to facilitate the measurement of very high resist,ancrs. Voltage and Current Integration. Posi""it. tions 8 and 9 of switch S2 are used for
circvih for
adopter to be
with PH remrding
Volume 4 1 . Number 5, May 1964
/
267
voltage integrations. The voltage source is connected between terminal A and the cable shield. The 10-megohm resistor connects the input to point S. In position 8, the servo varies E, to keep it equal at any moment to the voltage developed across a 10-pf capacitor. Inposition 9, a 1-pf capacitor changes voltage a t a rate ten times faster than the 10-pf capacitor for the same signal, so that the pen moves ten times faster across the scale. Positions 10 and 11 of switch 52 are used for current integrations. The current source is connected to the input terminal and the input terminal is connected by switch 52 to point S. The same 1- and 10-pf capacitors are used for current integration as are used for voltage integration. They are high-quality Difilm metallized capacitors (Sprague Type 118P, 200 v). The tenfold change in capacitance together with the span values for E. provide more than a two hundred fold change in the rate of pen travel across the scale for either current or voltage integrations. An outlet, labeled PC, has terminals connected across the integrator capacitors. Microswitches can be mounted on both ends of the 10-in. chart and the contacts connected by a cable to the PC terminals. Therefore, the capacitors are discharged each time the pen reaches the end of the scale. Simultaneous Readout of Transducer Signal and I t s Integral. I n some applications it is important to record simultaneously the output current (or voltage) from a transducer and also the integral of the output current. Examples of specific applications are given in the experimental section. The physical layout of the module is not convenient for recording both signals on one chart, as is common with several potentiometric recorders that are equipped with mechanical integrators or dual servo channels. It is very convenient and economical, however, to record the current or voltage on one chart and its integral on another synchronized chart. This has the disadvantage of requiring two charts but the definite advantages of versatility with the interchangeability of basic multipurpose units and the utilization of a full 10-in. chart to display data for each function without overlapping curves. When two identical modules are used (Fig. 6), the one is switched to the desired current or voltage range and the transducer is connected to input connector A or D. A cable is connected between the OUTPUT TO ~ N D RECORDER outlet and the input terminal A or D of the second module, which is switched to one of the VOLT INT. RATE positions. The time required for the pen to travel full scale for a given output voltage from the 6.rst module can be varied by changing the integrator capacitance (selector switch) by a factor of ten and the E. span by a factor of twenty-eight, so as to provide a range of 280 for any given current or voltage span set on the other module. Instead of utilizing an identical recording photometer module for performing the integration it is possible to construct a simple and compact adapter that plugs directly into the EUW20A recorder rather thau into the reccrding electrometer. The output from the circuit of Figure 6 is directly across the bridge circuit of the comparison voltage source in the electrometer (1). Therefore, on all current or voltage spans there is a full-scale output 268
/
Journd of Chemical Education
voltage of about 1800 mv. This output voltage is connected directly into the adapter unit shown in Figure 7, which is connected in place of the standard plug "B" in the EUW-2OA recorder. A 10-megohm resistor is connected a t the input when in the voltage integration position. Therefore, the full-scale input current is 1.8 X lo-' amps for all current or voltage ranges on the module that is connected to the transducer. The 1.8 X lo-' amps is a sufficiently large current so that the input leakage currents in the recorder are not significant, and the use of the electrometer adapter is not required. OUTLET TO SHORTING RELAY INPUT FROM PHOTOMETER
PHOTOMETER l P FOR HANGING ADAPTER BOX
0 " s " FLUE IN HEATH U W - 2 0 & RECORDER
Figure 7. recorder.
Phommeter adopter for direct use with H e a h EUW-20A
The comparison voltage E. in Figure 7 is obtained from a divider connected between the slider of the zero adjust pot (B5) and the slider of the servo pot (Bl), which are in the bridge circuit of the attached recorder. By decreasing the full-scale value of E, the relative sensitivity of the integrator is increased; i.e., the smaller the full-scale value of E. the less time required for the capacitor to charge to the full-scale voltage, and the pen moves across the chart a t a faster rate for a given charging current. The span selector on the EUW-20A recorder must be in the EXTernal position and a shorting wire connected between input terminals Hand B when the circuit of Figure 7 is used. The complete adapter circuit of Figure 7 is mounted in a mini-box that attaches to the back of the EUW20A recorder. A switch at the input enables the 10megohm resistor to be shorted for applications where it might be desirable to integrate current directly from a transducer, rather than from a voltage output as above. An outlet provides connections to the leads of the integrator capacitor so that it can be discharged by
external microswitches or relays. It is convenient to mount microswitches a t either end of the chart so that the capacitor is discharged when the pen reaches the full-scale value. The pen immediately flies back to the opposite side so the microswitch remains closed less than 0.005 see. If an input current remains applied when the capacitor is discharged, it mill be partially charged before the pen reaches the zero-line because of the 1-see full-scalepen response time. A circuit similar t o the one shown in Figure 7, but with a resistor R, to replace C, enables the EUW-20A recorder to be used for low-current or high-voltage measurements without the additional expense of the electrometer input. However, the adapter is not as sensitive or versatile as the recording photometerelectrometer described above. Power Supply. Most phototubes, photomultipliers, and ionization detectors require operating voltages ranging from about 45 to 1200 v, and they require very small currents from the supply. Many regulated electronic power supplies are suitable, but the simple compact dry-cell supply and switchmg arrangement shown in Figure 8 have been found widely applicable, and it can be quickly assembled.
voltage can be taken between 2 and 1, where terminal 1 is negative with respect to ground. Both plus and minus 600 v can be obtained at the high-voltage outlet by switching to the 1200-v position and connecting pins 3 and 4 together. Terminal 2 is now 600 v negative and terminal 1 is 600 v positive with respect to ground. The "featherweight," type U-15,22.5-v dry batteries, are used in the low-voltage supply. At the lowvoltage outlet, 90 v are available between terminals 1 and 3 on all switch positions, but the negative end of the supply is conuected to ground only when the switch is in positions 2-5. I n position 6, terminal 2 of the low-voltage outlet is 45 v positive and terminal 3 is 45 v negative with respect to ground. The two 45-v supplies make it possible to connect two phototubes as shown in Figure 5.
LOW VOLTAE OUTLET
P U G "U"
IN
RECORDING
45
l BLACK
RED
Figure 9.
I
45-45
\
OFF
Figure 8. Power supply for phototuber, photomvltipliers, and ionization detecton.
I n switch positions 2, 3, 4, and 5, either plus or minus 300, 600, 900, and 1200 v are available at the high-voltage outlet. Four of the small 300-v, type U-200, dry batteries are used. Terminal 4 of the "U" plug is connected to ground, and in switch positions 2, 3, 4, and 5 it is connected t o terminal 4 of the high voltage outlet. Therefore, by connecting a shorting wire between terminals 2 and 4, the high voltage can be taken between 1 and 2, where terminal 1 is a positive with respect t o ground. By connecting terminals 1 and 4 together the supply
Auxiliary zero suppression circuit.
Amiliary Bucking Potential. The recording photometer module has a largezero adjustment over several scale lengths because of the index and zero adjustment controls. However, it is sometimes convenient to have a suppression voltage adjustment connected as shown in Figure 9. The application of the bucking potential unit is referred to in the experimental section. It plugs into terminals H and B of the electrometer unit. By turning the Knobpot from one end to the other, contact d of the chopper (Figure 26) is changed continuously from plus to minus 125 mv with respect to ground G. The mercury cell is connected into the circuit by a third section Slc of the power-supply switch (Fig. 9). The shorting wire across the binding posts at the right can he removed and an input voltage source can be connected in series with the bucking potentiel unit. If the H 2nd B plugs for the circuit of Figure 9 are disconnected, a shorting cable must be connected between terminals H and B of the electrometer, or an input source connected. Terminals H and B must Volume 4 1 , Number 5, Moy 1964
/
269
not be left floating or the circuit will not operate. Also, if the circuit of Figure 9 is disconnected the H and B plugs should be shorted to the ground terminal GND, to prevent feeding noise through the leads to other circuits within the same chassis.
INPUT
+L 45
-r -
PLUG
REVERSING SWITCH Figure 10.
Connections for phototubes.
HIGH-VOLTAGE INPUT
and the Knobpot on the right side. It might he more convenient t o mount the chassis at the top edge of the back cover so the switches and pot are available from the top and just in back of the electrometer controls (1). Also, it might be more convenient to construct the circuit of Figures 6 in one box and the power supply in a separate box, with both chassis attached to the back cover of the electrometer section. Caution: Care must be exercised not to touch the switch for the high-voltage power supply when the cover is removed. If students are apt to get into this circuit it should be plainly marked DANGER H. v. Housing and Connections for Phototubes and Photomultipliers. The connections for a typical phototube (929, 935, 1P39, etc.) are shown in Figure 10. The reversing switch connects either the anode or cathode to the coaxial lead that goes to input A of the module, and connects the proper polarity of the supply voltage to the opposite electrode. The photomultiplier tubes with nine dynodes (1P28, 1P22, 1P21) are connected as shown in Figure 11. Tubes with ten or more dynodes (6217, etc.) can be connected in the same way as in Figure 11 except that more divider resistors are required (one more than the number of dynodes). The relative sensitivities of the tubes for different wavelength regions and their relat.ive merits have been discussed (7). MOUNTING BRACKET
t
220K
I I
PHOTOTUBE MOUNTING BbSE Figure 12.
Figure 11.
Photomultiplier connections
Housing for Measurmnat Circuits and the Power Supply. A 3- X 6- X 13-in. aluminum box will hold all circuits and batteries shown in Figures 6, 8, and 9. By attaching the box to the back of the electrometer unit the whole system becomes a compact portable recording module. The complete operating module can be assembled in a couple of minutes from the three basic sections. The present model has the selector and powersupply switches available at the left side of the module 270
/
Journal o f Chemical Education
Housing for phototuber and photomultiplier%
A convenient housing for mounting different sizes of phototubes and photomultipliers is shown in Figure 12. The housing is fitted with a diaphragm-shutter assembly (from an old camera) and an interchaugeable bracket to fit on optical systems, such as the Beckman DU, or Jarrell-Ash monochromators, or Sargent Spectro unit. The bases for mounting the phototnbe or photcmultiplier are made in different depths to accommodate differentlengths of tubes. For phototubes both the reversing switch and cables (Fig. 10) are mounted in the base. Therefore, base assemblies with different tubes can be readily
interchanged. A housing of another shape would be required for tubes with the cathode on the end, such as the RCA 6217 red-sensitive photomultiplier. Specific Applications with Experimental Resulls
The following data illustrate several important chemical applications of the recording module described above. Many other applications are possible. These applications demonstrate how one inexpensive modular building block can be used repeatedly for a wide range of chemical measurements t o provide results comparable to the best research instruments.
....................... FLAME : IONIZATION i
could he obtained from the power supply of Figure 8, but the presented data were obtained when the Microtek 300-v polarizing voltage was connected. The output across the bridge of the recording photometer was connected t o an integrating recorder consisting of the adapter in Figure 7 connected to a Heath EUW-20A recorder. The integrated chromate grams of Figure 15 were recorded simultaneously with the curves in Figure 14. On three successive runs the ratio of peak areas as read from the recording integrator reproduced within 0.2%. For the curves in Figure 15 the areas are proportional to the number of divisions of pen travel during the period represented by each peak. I n counting the number of divisions of pen travel it should be recalled
O-
MICROTEK i
RECORDING
Figure 13. The recording photometer and recording integrator connected for use in gar-liquid chromatogrephy with o Rome ionization detector.
Figure 14. Chromotogrom obtained from the setup of-Figure-13. The 10-fl column wor 20y0 (by weight) Acon Palor 50HB 2 8 0 X (Microtek Inc.1 on Analabr ABS 80/9O Mesh. An 0.8-pl sample was wed.
Gas-Liquid Chromotogrophy
The basic blocks are illustrated in Figure 13 that Kere used in obtaining the gas chromatograms shown in Figure 14. The flame ionization detector was used because of its high sensitivity, simplicity, linearity, and reproducibility. The sample was 0.8 of a mixture of cyclohexene, cyclohexane, and benzene injected by a syringe into a column (see Fig. 13) controlled a t 100°C. The column jacket and temperature controller constitute a major block in gas chromatography experimentation. ?he specific controller used to obtain the curves in Figure 14 was from the Microtek GC2500. The specific flame detector and all gas valves were from the same Microtek instrument. All other functions, including the direct recording of the chromatograms and the integration of the peaks to provide also a direct readout of the areas under the peaks, are performed by the modules described in this paper. The recording photometer was connected directly t o the Microtek flame detector housing by a cable fitted with standard twistlock coax connectors on both ends, one end to the housing and the other end to input A. Other flame detectors of simple design could be readily assembled (Z) or purchased (5). The 300-v polarizing voltage commonly used for flame detectors
Figure 15. Direct readout of the integrals for the chromatograms in Figure 14. Cyclohexene reads96.3,syclohexane 103.2, ond benzene 116.9 ""it..
Volume 41, Number 5, Moy 1964
/
271
that the pen would have reached the zero base line if there were no charging current during the one second required for the pen to return from full-scale to zero after the capacitor is discharged. This one second full-scale pen response time is not lost integration time, and the counting of divisions is always from the zero base line after capacitor discharge. The only integration time that is lost is the brief time that the microswitch remains closed, which is about 0.005 sec. Atomic Absorption Spectroscopy
The absorption of monochromatic resouance radiation by an atomic vapor provides a relatively new laboratory tool for the quantitative determination of many elements (7, 8, 9). At the present time more than 35 elements can be determined, and for some elements the method is considerably more sensitive and readily applicable than conventional emission spectroscopy, or other analytical techniques. For example, zinc can be detected a t about 0.01 ppm with atomic absorption spectrochemical methods; calcium and magnesium can be detected in the parts-per-billion range and can be accurately determined in a wide
INTENSITY
Figure 16. The recording photometer connected for use in atomic abrorpti& spectroscopy.
Figure 17.
range of materials that are advantageously studied in chemical laboratories. Also, the atomic absorption procedures are less susceptible to interference than most techniques. The high sensitivity, inherently high selectivity, and relative freedom from quantitative interferences (and therefore high accuracy) are the important features that have spurred widespread interest in the method. The important components that make up an atomic absorption spectrophotometer are illustrated in Figure 16. The atomic vapor is easily produced from a solution sample by a commercial atomizer and acetyleneair burner (Optica, Inc.). A hollow-cathode tube that radiates monochromatic resonance radiation for the element of interest is set in position and a narrow beam is isolated with slits so that it passes through the atomic vapor. The specific source shown in Figure 16 and whose spectra is given in Figure 17 is applicable for both calcium and magnesium determinations. It is powered by an inexpensive regulated dc voltage supply (variable from 180-380 v) that maintains a constant current through the hollow-cathode tube which is in series with a 100-W 2-K resistor. A monochromator is used to isolate an analytical wavelength for the sought-for element. The photomultiplier housing (Fig. 12) is attached a t the exist slit of the monochrom:ttor and the 1P28 PM is connected to the photometer module. If magnesium is to be determined, the characteristic resonance radiation of 285.2 mp from the source is isolated by the monochromator. The zero transmission line is established by using a shutter to block the source radiation, but allowing background radiation from the Rame to enter the monochromator. The 100% line is set by adjusting photometer sensitivity and monochromator slit width. The per cent transmission T is read directly from the chart or scale. A plot of absorbance ( A = log 100/T) versus concentration provides a working curve for quantitative determinations. With the setup of Figure 16, a monochromator slit width of 0.014 mp, and a photometer sensitivity of 5 X 10-lo amps full scale, a lmear working curve for @I0 ppm magnesium was obtained and samples determined with a relative error of about 0.05 ppm Mg. The high sensitivity of the photometer is useful because a narrow slit can then be used so that the radiation reaching the detector is low in background and the
Spectrum of a hollow-cothode discharge tube recorded by the setup of Figure 16
272 / Journol of Chemicol Education
relative sensitivity for changes in the selected resonance radiation is high. In cases where the emitted resonance radiation from the flame for the sought-for element is high relative to that from the source, a modnlation system is required so that the detection system will only respond to the hollow-cathode radiation (8). By using an air-propane flame to produce the atomic vapor, a gas-discharge lamp (Osram) to provide the resonance radiation, an interference filter to isolate the resonance radiation from background, and a suitable phototube detector, either sodium or potassium or both can he determined (9, 10) with very high sensitivity and relative freedom from interferences.
Figure 18.
Conversion of setup in Figure 16 to flame emission photometer.
Flome Emission Photometry
broad flame with a &in. path length is especially desirable for increased sensitivity in atomic absorption, it is more popular in flame photometry to use the small total consumption burner (Beckman, Inc.), illustrated in Figure 18. The high sensitivity of the recording photometer module makes it possible to maintain very narrow slit widths and high resolution, even when the radiant power is very low. The iron spectrum in Figure 19 was recorded a t a photometer current sensitivity of 1 X amps fullscale, a slit width of 0.014 mm on the Beckman DU monochromator, 600 v on the 1P28 PM, and a 500-ppm iron solution sprayed into an OrHt flame. When the emitted radiation from the flame is blocked from the detector the base line is a perfect straight line; the observed fluctuations of the base line in Figure 19 are from flame background and low intensity iron lines. If the high-voltage supply for the 1P28 PM is increased to 900 or 1200 v the signal-to-noise ratio is decreased. Therefore, it is best to operate the 1P28 at 600 v and increase the detection limit by changing the current measurement sensitivity. An example of scale expansion for an emission line is shown in Figure 20. The strontium 421.55 mp line, shown as a small pip inside the circle, is expanded 50 times by switching from 5 X amps to 1 X amps full-scale sensitivity.
To convert from atomic absorption spectrophotometry to flame emission photometry it is only necessary to turn off the radiation from the hollow-cathode tube and provide some means of efficiently collecting the emitted radiation from the flame and focusing it a t the entrance of the monochromator; all other components can remain the same as in Figure 16. Whereas a IRON
a
3n.99 I
..c
IRON 5 0 0 PPM SLIT 0.014 mm SENSITIVITY 1 x 1 0 " ~ ~ ~ ~ 0.12 psi
H, 1.5
Figure 20. mp line.
WAVELENGTH, millimicrons Figure 19. Iron rpeetrum recorded illustrated in Figures 16 and 18.
by Rome photometer with setup
Stmntivm spectrum rhowing 50-fold expansion of the
421.55
Quantitative determinations of many elements are possible in the ppm range by making a working curve of relative intensity versus concentration at a preselected characteristic wavelength for the sought-for element. For high accuracy at high sensitivity the fluctuations in emitted radiation are recorded and the average used to estimate the relative intensity. An alternative method is to switch the recording photometer to the current integrator position and measure the slopes from the integrator for each sample; a plot of slope versus concentration gives the working curve. The integrator averages the fluctuations and a smooth integral curve is obtained. Both methods Volume 41, Number 5, May 1964
/ 273
were tested on 0-10-ppm calcium solutions; .the maximum deviation from a linear working curve was 0.1 ppm by estimating the average peak height from recorded data, and the deviation was 0.05 ppm calcium from a linear working curve by the integration procedure. Expanded-scale Absorption Spectrophotometry
To convert from the flame photon~etric setup of Figure 18 to an absorption spectrophotometer it is only necessary to remove the flame and connect a light source and cell holder as shown in Figure 21. The sensitivity and stability of the recording photometer are sufficient so that it is not necessary to use the typical high-current, high-intensity bulb for work in the visible region and down to 350 mp or less. At a slit width of 0.012 mm on the Beckman DU monochromator, it was found that a small bulb operated directly from a Sola constant-voltage transformer provided exceptional stability after a warm-up period of about 1 hr; fluctuations were then equivalent to less than 0.001 absorbance unit.
b p
TUNGSTEN LAMP
........... .........: !
............. m..%g TRLNYORMER ..........
Figure 21. Conversionof setup in Figure 16 to expanded-scale absorption rpectrophotometer.
Figure 22 is the relative response curve for a GE64 tungsten bulb as a function of wavelength, using a 1P28 PM at a fixed slit of 0.0125 mm. The curve shape depends on the relative sensitivities for both the tungsten bulb and photon~ultiplier as functions of wavelength. It can be seen in Figure 22 that fullscale response is obtained with a narrow slit and the low-current source by increasing the sensitivity of the recording photometer. Since the low-current source is very stable and utilizes an inexpensive care-free constant-voltage transformer, the recording photometer provides indirect benefits in improved performance with less expense and care. Another benefit from using the photon~etermodule can be derived from the calibrated bucking potential
Figure 22.
274
/
(INDEXcontrol). With a recent modification of the pH index control (1) it is possible to obtain fonrteen 50-mv increments of bucking potential on the M V positions of the electrometer. Therefore, with the SPAN set a t 50, the MULTIPLIER and SLIT are set for full-scale O - l O O ~ o T , as in "ordinary" spectrophotometry (11); the MULTIPLIER can then be switched to a tenfold greater sensitivity and the INDEX control is used to select any 10% T as full scale over the entire transmittance range. A full-scale per cent transmittance of any 207& etc., could also be selected. To take full advantages of the high sensitivity from scale expansion it is necessary that the light source be very stable (or a unique double-beam system employed), and many sample variables must be carefully controlled. To illustrate the sensitivity and adaptability of the module for expanded-scale absorption spectrophotometry, Ce(IV) ammonium nitrate solutions were measured at 315 mp with the small tungsten bulb as a light source. The scale was expanded full scale with 70-ppm Ce(1V) a t one end and 78-ppm Ce(1V) a t the other end of the 10-in. chart. Each chart division represented 0.08 ppm so that when experimental conditions are controlled to provide a reproducibility of one division a precision of about 0.1% in Ce(1V) concentration is attained. Absorption Spectrophotometric Reaction-rote Methods
Kinetic data are readily obtained by following absorbance changes for one of the reactants as a function of time. It is most convenient to record these changes continuously at some pre-selected wavelength. Since the absorptivity and concentration of a measured species might be small, and the reaction proceed over a long period, it is essential that the recording photometer have high sensitivity and stability-characteristics which are inherent in the present recording module. A reliable interference-filter instrument for following the rates of reactions by absorbance measurements is illustrated in Figure 23. The integrating recorder is not necessary for obtaining kinetic data, but it is a useful readout for an analytical procedure described below. For studies involving an absorbing species in the range of 340-800 mp, the interference filters are generally as suitable as a prism or grating monochromator. I n addition to recording kinetic data, the iustrument in Figure 23 is suitable for routine, sensitive, accurate, and rapid quantitative deterolinations by a
Relative response curve for o tvngrten bulb and 1 P28 photomultipliertube wilh expansion of full-scale curred span at appropriate intervals.
Journol o f Chemical Education
latter factor. The reproducihility of instrumental conditions allows the use of the same working curve for many days, the only change being in the ordinate intercept, which is easily found by running standard samples.
reaction-rate procedure. The sensitive measurement system enables the rate of a reaction to he observed within seconds after the start, before significant change in the concentration of reactants, and before products build up in concentration and side reactions commence. Several recent analytical methods have been based on these rate measurements ( I f , 15). One of the most sensitive is the important determination of iodide in the parts-per-billion (ppb) range (14). I n the range of &loo-ppb iodide the precision and accuracy are ahout 0.5 ppb.
PHOTOMETER MODULE
i!f so
A
CONSTANTVOLTAGE TRANSFORMER
FILTERS
hi360mp.
Figure 24. Working curves obtained by t w o memodr for determinotion of iodide in the porh-per-billion range.
RECORDING INTEGRATOR
All of the above methods using the recording photometer module are now used for student experiments or in research. I t is planned that specific details and tested experiments for all of the above instrumental techniques will he made available in future articles. Figure 23. The recording photometer and recording integrator for use in rpectmphotometric readion-rote methods.
Acknowledgment
The help of Mr. Verle Walters and Mr. Korm Greaves in constructing equipment is gratefully acknowledged.
The reaction between Ce(1V) and As(II1) proceeds a t a very slow rate in the absence of iodide, requiring many hours to go to completion. However, in the presence of iodide, the rate of the reaction is greatly increased. Under controlled conditions the reaction can be shown to be "first order" in Ce(1V) and I-. A working curve can be obtained by measuring the time required for the Ce(1V) concentration to change a preselected amount, determined in most cases as a fixed change in absorbance (about 0.024.0.5 A A ) . A plot a reciprocal of time versus iodide concentration gives a straight line with a slope of k (reaction-rate constant), as shown by curve 1in Figure 24. A working curve can also be obtained by connecting a recording integrator (Fig. 7) and measuring the integrals over a fixed time interval (fi&90 sec), as shown by curve 2 in Figure 24. The integrator method has the advantage that it could he made direct-reading in ppb of iodide. Also, it has been found to he somewhat more precise for iodide determinations than the reciprocal time method, especially a t high concentrations where the relative time error becomes large in the first method. The working curves for iodide do not go through the origin; the intercept is a function of the rate of uncatalyzed reaction, iodide impurities in the reagents, and in the case of the integrator method the small input signal that is present previous to the point where the integration is started. It is easy to correct for the
+
Literature Cited (1) M.AI.Ms.'A~T, H. V., J. CHEM.EDUC.,41, 148 (1964). (2) PURNELI.,H., " G a Chrumatogmphy," John Wiley and Suns, Inc., New York, 1962, p. 296. (3) LEWIN,S. Z., J. CHEM.EDUC.,39. AllO, A161 (1962). 141 , , 11.a NOOARE. , S.. , AND JUYET. R. 8.. JR.. , "Gas-Liouid
.
Chromatography," Interscience Publishers, New Yurk, 1962, p. 214. (5) ENKE,C. G., AND BAXTER,R. A,, J. CHEM.EDUC.,41, 202 (1964). (6) MALMSTIDT, H. V., AND ENEE,C. G., "Electronics for Seientists," W. A. Benjamin, Inc., New York, 1962, pp. 251, *n, -a*.
(7) GILBERT, P. T., "Symposium on Spectm,scopy," ASTM, Philadelphia, Penna., 1959, p. 93. (8) MALMSTADT, H. V., AND HARRISON, W. W., "McGraw-Hill Yearbook of Science and Technology," McGraw-Hill Book Co., New York, 1963, p. 131 (9) WALSH,A., "Proceedings oi the Xth Colloquium Spectroand M . M. scopicum Internationale," E. R. LIPPINCOTT MAROO~HES, Editors, Spartan Books, Inc., Washington, D. C., 1963, p. 127. H. V., AND CHAMBERS, W. E., Anal. Chem., 32, (10) MILMSTADT, 225 (1960).
(11)
REILLEY, C. ti.,.4ND CRAWFORD, C. M., Anal. Chem., 27, 716 (lR551~ \----,-
(12) MALMSTADT, H. V., AND HADJIIANNOU, S. I., Anal. Chem., 34, 452 (1962). (13) MALMSTADT, H. V., AND HADJIIANNOU, T. P., Anal. Chem., 34, 455 (1962). (14) MALMSTADT. H. V.. AND HADJIIANNOII. T.P., Anal. Chem.,
+
+ Volume 41, Number 5, May 1964
/
275
