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mle Figure 5. HCIS sensitivity demonstrated for lead isotopes. 204Pb concentration, 60 ppba. Figure 2 conditions, except 3.0-kV mul-
tiplier voltage ratios can be obtained from a single scan. 2. Relatively nonselective ionization occurs. Interelement sensitivity factors appear to be quite small, within a factor of three. This can possibly be attributed to the Penning ionization process. The nonselective ionization is a significant advantage for both qualitative and quantitative analysis. 3. A small energy spread is observed. Background contamination can be seen as doublets and sometimes triplets which resolve even in the electrical scanning mode. Quadruplets may be observed with photoplate detection. Lines which normally interfere with inorganic spectra may not be a problem with this source. The Penning ionization
mode probably explains this low energy spread. 4. Good sensitiuity is obtained. Trace elements may be detected and determined to the low ppm level or below. 5. A large ion f l u x is produced. Monitor currents of up to 50 nanoamperes require replacement of the standard AEI readout monitor, which often pegs on its least sensitive position, or the use of beam chopping. 6. A variety of discharge gases ma3 be used. The inert gases vary in available ionization energy and sputtering capacity. Reactive gases might also be useful. 7. I t is a surface technique. Surface films, at least of conductors, may be sputtered and analyzed with relatively simple apparatus. 8. Solution residues m a y be examined. Prior experience with optical hollow cathodes has shown that thin films may be deposited reproducibly and sputtered for quantitative elemental analysis. Very little work has been done with the HCIS to optimize discharge parameters for optimum sensitivity, stability, and precision. It is unlikely that the conditions selected in these initial experiments will prove to be optimum. Thus, with further evaluation of such factors as tube current, discharge gas, gas pressure, and cathode-anode geometry, improved performance of the HCIS is expected. Another model has been constructed which allows simultaneous ion and optical sampling of the discharge to study the hollow cathode reactions. The HCIS has also been useful in our department as a filamentless source for chemical ionization mass spectrometry. W. W. Harrison C. W. Magee Department of Chemistry University of Virginia Charlottesville, Va. 22901
Received for review July 5 , 1973. Accepted October 4, 1973. Research supported in part by KIH Grant No. GM14569 and EPA Grant No. R-801829.
AIDS FOR ANALVTICAL CHEMISTS Digital Automatic Attenuator with Displays, Peak Memory, 1/0 Interface, and Scale Expansion-For Use with Potentiometric Chart Recorders Stephen R . Pareles’ Department of food Science, Rutgers-The
State University, College of Environmental Science, New Brunswick, N.J. 08903
Most analytical instruments, gas chromatographs, for example, provide an attenuation or range-change switch on the output of the detector which the operator uses to center both major and minor peaks on the potentiometric chart recorder with respect to the vertical axis. Thereby, component purity and retention time can be accurately ascertained in addition to an estimate of quantity, sometimes closely related to peak height. Frequently, the manipulation of the switch is an exercise in perception and dexterity and demands the operator’s attention for much of the analysis. This is especially so when the analyzed 1 Present address, project sponsored p r i v a t e l y .
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mixture is a complex unknown such as a natural product isolate. The result is an attenuated chromatogram that is nonreproducible and difficult to interpret. Automatic attenuators, intended to obviate these problems, have been described for several years. These require fixed mechanical attachments t o the recorder, such as microswitches (1-3) or gears ( 4 ) , to detect pen position; or ( 1 ) D. J. Darling, F. D. Miller, R . C. Bartsch. and F. M . Trent, A n a / .
Chem., 32, 144 (1960). (2) F. Bauman, F. A. White, and J. F. Johnson. Anal. Chem.. 34, 1331 (1962). (3) R . R Lowry, J . Chromatogr. Sci.. 7 , 383 (1969) ( 4 ) K . Abel and W. B. Dabney, A n a / . Chem.. 3 5 , 1335 (1963).
attachments to the range-change switch of the chromatograph or other instrument, itself, to effect attenuation (2). This limits their portability and speed of response. Occasionally, a peak that gains amplitude faster than the response capability of the attenuator will put the unit out of synchronization so that it must be reset a t inopportune times (3). Further, for each of these units, the state of attenuation can only be ascertained by careful inspection of the chromatogram. Sometimes the determination is impossible. Logarithmic (5) or fixed-offset (6) techniques have been described which suffer the disadvantage of nonlinearity.
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OPERATIONAL CHARACTERISTICS OF THE AUTOATTENUATOR The digital automatic attenuator described requires no mechanical attachments and so is easily moved from one instrument group to another. Its speed of response is controlled by an internal clock and unrelated to the relatively long 1-second full-scale response of the recorder pen; attenuation change can occur 6 or 12 times per second. The unit automatically resets itself if power is interrupted or suffers an unusual transient, and so cannot be put out of synchronization. Two digital displays are provided. The first display continuously presents attenuation status. The second display is a peak memory. It latches a t the maximum attenuation status of each peak and holds this reading until the next peak elutes that requires attenuation. Thereupon a new maximum status is automatically displayed for the new peak. A simple print command circuit is described that provides that only the highest attenuation state is printed for each peak, to effect economy in data output. The print command signal will also switch an alarm immediately after each peak maximum has occurred. In addition to these digital outputs, a 1000-fold analog scale expansion output is provided. Figure 1 shows a typical chromatographic peak attenuated by this instrument. The peak maximum is circled. The available steps are lx, 2 X , 4 X , 8 X , 16X, 32X, 64X, 128X, and 512X, which are sequential integer exponents, n. of the term, 2 - n , in the expression:
E , = E , x 2-n where n has the values 0 to 9 and E, and E , are input and output voltages, respectively, of the attenuator. Thus, each successive attenuation step upward or downward divides or multiplies t,he peak amplitude by a factor of 2. The peak in Figure 1 is attenuated to 32X or 2 - 5 . UNIQUE DESIGN ASPECTS OF THE INSTRUMENT Unlike previous designs, the attenuator uses TTL (transistor-transistor-logic) integrated circuits, monolithic operational amplifiers, and FET (field effect transistor) buffering for high input impedance and measurement accuracy. Signal voltage division is performed by a modified R / 2 R precision resist,or ladder network that offers advantages of accuracy and convenience for low-level signal attenuation over the voltage dividers commonly employed in analytical instruments. The use of recently available monolithic optical isolators between analog and digital sections ensures complete electrical isolation between them, eliminating digital glitches from interfering with the low-level analog signals ( 5 ) K . A . C h e n . A n a / . Chem., 40, 1171 (1968) ( 6 ) M P I A p p l i c a t m Notes. 6 ( 4 ) , 31 (1971).
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and reducing common mode and switching transient interferences. The logic scheme t h a t provides a sequential interrogation and validation of the output of two comparator amplifiers is also novel and may have extended applications in other servo designs. Unlike the attenuators found on several computing laboratory integrators of high cost, this circuit achieves similar efficiency without the use of A/D and D/A conversion processes. The cost of the attenuator, without power supplies, is about $125.
CIRCUIT DESCRIPTION Analog Section. The analog section (Figure 2) contains the modified R / 2 R precision Ladder network following the buffered input where the actual voltage attenuation in binary steps is realized when one of the ten form A miniature reed relays (or analog switches) is engaged by the logic. An Analog bus, common to one contact of each relay, leads to the output for a 1-mV strip-chart recorder. This output may be buffered if a recorder of low input impedance or noisy first stage must be driven. The Buffer is sometimes required for certain older recorders found in some laboratories. The bus also leads to the input of the Servo amplifier (Expansion amplifier) preceded by an FET-input buffer amplifier to overcome the marked loading effect from the input of the Servo amplifier upon the Ladder network. The Servo amplifier multiplies the signal voltage (after attenuation) by -1000 and leads to two Comparator amplifiers. The output of the Servo amplifier is available as a scale expander and can drive an inexpensive voltmeter built into the front panel of the unit if desired. The Up Comparator slews into positive saturation when the voltage seen by the Servo amplifier is higher than +0.9 mV which corresponds to 90% of chart on a 1-mV recorder. The Down Comparator swings into positive saturation when the voltage seen by the Servo amplifier is less
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than +0.3 mV. Thus, they are referenced a t -0.9 and -0.3 volts, respectively, or to limits set by the operator. The output of each of the two Comparators drives a LED (light-emitting-diode) in a corresponding integrated circuit Optical isolator (Figure 3), activating the base of a phototransistor which is mechanically encased with the LED but electrically isolated from it and the analog section. The phototransistor is the first stage of the logic section which interrogates the Comparator states 12 times per second. Subsequent clock and logic functions sequence the propagation of the Comparator signals, ultimately selecting one reed relay on the Ladder to effect the appropriate attenuation. Digital Section. Timing and control functions are performed by Decimal decoder 74141(1). It is preceded by Decimal counter 7490 driven by a 60-Hz clock function conveniently available. Faster rates can be implemented if the application requires it. The Decimal decoder enables, in turn, each One-shot 74121 to read, for lis0 second, its input derived from its associated Comparator in the analog section. Every 5/60 second, the opposite One-shot is so enabled. If triggered, either One-shot will produce a pulse for 3/60 second, thus leaving by difference from 5/60 second, a period of between and 2/60 second of quiescence when neither One-shot can possibly signal subsequent logic. This allows settling time for relays and amplifiers, and prevents logic that follows from seeing adjacent signals of opposite levels. We were a t first hampered by the fact that a One-shot will not trigger properly if its B input sees a pulse that is shorter than its propagation delay time, but will produce, instead, a short sporadic burst. Such bursts are related to chattering of the Comparators (Figure 2) operated openloop and in the absence of clamping circuitry which would 468
detract from the circuit’s streamlined construction, low package count, and cost. We easily treated this problem digitally in the following manner. To satisfy the requirement that valid Comparator signals be ,discriminated from these short bursts, the Q output of each One-shot is not read by subsequent logic until second after the ‘/60-second B input read period is over. Then it is read for ‘160 second. In other words, the output of a One-shot must last for several clock pulses. Otherwise it is not propagated by subsequent logic. Sequencing for this function is once again accomplished by Decoder 74141(1) in combination with YOR gates 7402 (Figure 3). This interlock, an alternating logical interrogate-holdpropagate sequence, implemented with One-shots instead of Latches or Flip flops, is particularly effective in preventing attenuator glitches. The NOR gate outputs lead each count down or count up signal to Bidirectional decimal counter 74192 through NAND gates 7400, considered forthwith. The 74192 must not see a simultaneous up and down signal (low logic level) to function properly. The 4/60-second quiescent period between counts described in connection with the previous logic provides this protection. The 74192 increments Decimal decoder 74141(2) operates, a t any time, one and only one of the relays on the resistor Ladder network (Figure 2) to effect an attenuation state. Branch lines from the “0” and “9” outputs of 74141(2) disable a down count when relay “0” is engaged or an up count when relay “9” is engaged. Otherwise, unwanted continuous cycling of the counter would occur. These important boundary conditions are implemented by NAND gates 7400. The 74192 can be preset and held for any attenuation value. The preset function overrides the count inputs as does the zero reset function. Thus external control is very
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, M A R C H 1974
and permits convenient expansion of attenuation steps, if desired, without altering the current drain from the preceding stage.
simple. The circuit shows the preset inputs hardwired for “9” which is useful for periodic calibration. The four BCD (binary coded decimal) lines from the output of the 74192, lead to two 7-segment Display circuits, each containing the decimal set, “0” through “9,” for n in the tern 2-n. The display with the Quad latch 7475 is permitted by the Q output of up One-shot 74121 to follow the up count sequence but never the down count. Therefore it serves to “remember” and display the highest attenuation status reached for a particular peak until the unit must reattenuate up for a new peak whereupon a new value is automatically “remembered” and displayed. The 7475 may be manually overridden to agree with the state of the first display. This is desirable a t the beginning of the analysis. The lines which drive the 7447 Decoders are also printer or minicomputer compatible, the latched lines being the most useful in this application. We have also worked out a simple scheme for using the second pen on a two-pen recorder to record attenuation status.
1 / 0 INTERFACE CIRCUIT Besides the preset capability previously described, an optional print command circuit is shown in Figure 4. This circuit prevents unnecessary events from being printed. It commands the printer to read and print the decimal integer, n, corresponding to the 4-bit BCD output of Latch 7475 and occupies only one printer column unless additional information such as time is to be printed. The command occurs on the first down count attenuation signal from NAND gate 7400 (Figure 3) after one or more up counts have occurred, Data transfer is bit-parallel from Q1 4 of Latch 7475. Only one value is printed for each attenuated peak, none for an unattenuated peak. An LED (or alarm) may be connected as shown which functions as a peak detector, reminding the operator to copy the attenuation value in the peak memory display. It should be noted that printing or signaling of every attenuation state can be easily effected if desired. ADVANTAGES OF MODIFIED R/2R NETWORK FOR VOLTAGE DIVISION The use of an R/2R Ladder network, along which binary voltage divisions occur, is a technique borrowed and modified from current digital/analog converter design technology (7, 8) applied in an inverted form. This is different from the purpose for which it was designed and commonly used. I t offers definite advantages over the traditional voltage divider of many unequal resistors used in most laboratory instrument range-change circuits. There is the obvious advantage of requiring only two values of resistance. Any convenient and practical value of R may be used. Only the ratio, rather than the absolute value of R is critical. Manufactured under identical conditions, the resistors are likely to have identical or similar temperature and aging characteristics and tend to maintain much better network accuracy than resistors of widely separated values. Such networks are even available in integrated circuit form although their internal terminal connections are often not suited to the present application. No matter how many attenuation steps are used, the resistance drop between the top of the ladder and ground is always 2R and the resistance drop from any point on the ladder to ground is always R. This simplifies calculations of output driving impedance of the preceding stage (7) “Analog-Digital Conversion Handbook,” D. H. Sheingold, Ed., Analog Devices, Inc., Norwood. Mass., 1972, p 11-36, (8) A. Corbin, “Computer Data Handling Circuits,” H. W. Sams & Co., Inc.. Indianapolis, Ind., 1971, p 138.
ATTENUATION ACCURACY AND PRECISION Inspection of all chromatograms automatically attenuated failed to show any nonlinearity that might be present. This was a good practical test of the circuit. By way of a more precise test of accuracy, we applied exactly 4.20 volts, using an available premeasured mercury cell, to the input of the attenuator. We expected the theoretical output voltage a t attenuation 512X (T9) of 8.15 mV. Our measurement of the 1000-fold expansion output was -8.20 volts in one instance and -8.30 volts ten minutes later. These measurements made with an RCA WV-38A multimeter represent worst case errors of less than 1--2% even after two stages of buffering and one of amplification. This result surprised us since we predicted an error close to 5% based on our experience with voltage dividers of differing resistor values. Moreover, during actual operation of the attenuator, only the first stage, if not nulled, has any effect on the accuracy of the output signal to the recorder. These tests, while not exhaustive, certainly demonstrate the usefulness of an R/2R Ladder network as a means for binary voltage division in analytical instruments. CONSTRUCTION OF THE INSTRUMENT The analog section is electrically and mechanically isolated from the digital section in its own metal cabinet connected to analog ground. This cabinet is contained within the main cabinet which also contains the digital section and the two power supplies. The outer cabinet is connected to earth ground. The optical isolators are located outside of the analog cabinet. Analog trim potentiometers are externally accessible without a sacrifice of shielding. The analog section requires *15 VDC and the logic section, f 5 VDC. The order of construction should proceed from completion and test of the digital section, and then the analog section. Dotted connections located on the diagrams labeled “test,” especially in the analog section, indicate necessary connections for testing, nulling, or debugging during construction. During preliminary testing, a very short iron-constantan thermocouple can be used to simulate the millivolt range output of a typical laboratory instrument.
APPLICATION OF THE AUTOATTENUATOR The unit was used between a Beckman GC-5 Gas Chromatograph with TC detector set a t lx and a Varian Model 20 Chart Recorder or a Heathkit Model IR-18M Multispeed Servo Chart Recorder. An integrator (9) was connected where indicated in Figure 2. This arrangement was found useful in the study of the complex volatile flavor spectrum of baked potato (10) where a record of component multiplicity large and small is essential. The attenuator was also used to great advantage between the electron multiplier output of our DuPont 21-490 Mass Spectrompter and a Sargent-Welch Recorder Model SRG during combined GC/single ion quantitative analysis of several organophosphate insecticides ( 1 1 ) . In this (9) S. R . Pareles. Anal. Chem., 45,998 (1973). (10) s. R . Pareles and S. S. Chang, J. Agr. Food. Chem., 22, (1974) in press. (11) J. D. Rosen and S. R . Pareles, “Proceedings, 165th Meeting, American Chemical Society. Division of Pesticide Chemistry,” Dalias. Texas, April 1973. A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 3, M A R C H 1974
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alarm or printer after the maximum of each peak and is readily compatible with most other 1/0 devices. A 1000-fold expansion analog output is provided. The circuit design contains several unusual features which should, in general, interest designers of new automatic laboratory devices. Efficient automatic attenuation is currently provided by much less simple means on a few sophisticated computing integrators of great cost.
application, although an operator was present, he was unable to reach the recorder range-setting switch while attending the other important controls of the mass spectrometer. The attenuator is also compatible with a repetitive scan demodulating unit recently designed in these laboratories for this mass spectrometer. On another occasion, the unit was used to attenuate the beam monitor output of the mass spectrometer. We also expect to use the attenuator on the output of a Technicon AutoAnalyzer modified to monitor an immobilized enzyme reaction. None of the above instruments are modified to use the autoattenuator. Further, all of the high impedance recorders mentioned could be driven directly from the unbuffered output of the attenuator.
ACKNOWLEDGMENT The author wishes to thank the Barbers, Hazlet, N.J., for assembly tools; teachers, R. I. Edenson, D. E. Smith, F. G. Bordwell, S. S. Chang, J. D. Rosen, and R. Killops, for guidance and incentive, past and present; Williams Electronics, Edison, N.J., H. Cohen of Harwood-Sandler Associates, Woodbury, N.Y ., representing Analog Devices, Inc., and Automatic Laboratory Devices, Bridgeport, Conn., for components, technical suggestions, and printed circuit board artmasters, respectively. Diagrams were executed by James Kiss.
SUMMARY A digital automatic binary attenuator has been described, for use between a number of laboratory instruments and a potentiometric chart recorder, that is portable, efficient, versatile, and of low cost. One of its two displays continuously latches a t the attenuation state of greatest significance for each peak while the other displays the present attenuation state. The unit signals an
Received for review August 9, 1973. Accepted November 2, 1973.
Solvent Selection in Adsorption Liquid Chromatography D. L. Saunders Union Oil Company of California, Research Department, Brea, Calif. 92621
During the past several years, the field of liquid chromatography has been growing at an increasingly rapid rate. Recently, a variety of short courses and monographs (1-5) have appeared describing the theory and practice of the subject. These have gone a long way toward introducing the newcomer to the field; nevertheless, the selection of a solvent for a new problem remains a matter of trialand-error for most newcomers as well as some experienced practitioners. A working theory of adsorption chromatography was developed in the last decade by the prolific work of Snyder who summarized the details in a monograph on the subject ( 5 ) .With this theory, it is possible to determine approximately the appropriate solvent mixture for a specific compound or mixture of compounds. In practice, the application of this theory requires the assembly of a number of sample, adsorbent, and solvent parameters, and a set of somewhat tedious calculations. We have condensed some of Snyder's basic concepts of adsorbent activity, group adsorption strength, and mixed solvent strength into a simplified graphical form. The application of these graphs is rapid and provides a reasonable first approximation to a solvent mixture appropriate for a given sample. It must be stressed that the results are approximate and, in some special cases, the solvent (1) J. J. Kirkland, Ed., "Modern Practice of Liquid Chromatography," Wiley-Interscience, New York, N.Y., 1971. (2) P. R. Brown, "High Pressure Liquid Chromatography,'' Academic Press, New York, N.Y., 1973. (3) S. G. Perry, R. Amos, and P. I. Brewer, "Practical Liquid Chrornatography," Plenum Press, New York, N.Y., 1972. (4) F. Baumann and N. Hadden, Ed., "Basic Liquid Chromatography,'' Varian Aerograph, Walnut Creek, Calif., 1972. (5) L. R. Snyder, "Principles of Adsorption Chromatography," Marcel Dekker, New York, N.Y.. 1968.
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mixtures indicated will be in error. Yet, in practice, we have found the technique presented below to be a timesaving alternative to both the calculation and the trialand-error approach.
DISCUSSION The sample partition ratio k' is defined as the ratio of the amount of sample in the stationary phase to the amount in the mobile phase. It can be calculated from retention data using Equation 1. tr #3-
- to to
where tr is the sample retention time and t o is the retention time of an unadsorbed component. In adsorption chromatography, the value of k' for a given component can change by a factor of -lolo from a very strong solvent to a very weak one. Nearly always we seek a solvent mixture which will give 12' > 1 to obtain separation of the component of interest from other sample components of similar composition. Generally, we also seek a solvent mixture which will give k' < 10 in order to reduce the total separation time and minimize the sample dilution which occurs at high k'. Snyder (5) defines the solvent strength parameter eo as the adsorption energy per unit area of standard adsorbent. For a given sample and adsorbent, log k' varies linearly with eo. We define the E3 value of a sample as the solvent strength required to give 12' = 3. Figure 1 shows the E3 values for a variety of compounds on a typical silica (300 m*/gram). We will describe how to approximately determine the solvent strength for a very broad variety of mono- and poly-func-
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