5 minutes (cis reaction is complete or almost complete and trans reaction is largely incomplete) with the following reservations : 1,2-epoxides (monosubstituted) and trisubstituted epoxides react virtually completely, like the cis epoxides. alpha-Alkyl substitution slows the reaction of both cis and trans epoxides with the acid; however, the effect is not as marked as the differentiation between cis and trans. A hindered cis epoxide reacts more completely than does an unhindered trans epoxide. (Compare the last compound (hindered cis) of Table I1 with the trans com. pounds, which are subtracted partially or not at all.) Epoxide Reaction Product. The major reaction product of an epoxide is a polar material that migrates very little from the origin under conditions that cause appreciable migration of the epoxide. The following experiments indicated that the major product was the diol of the epoxide. About 0.75 mg of cis-9,10-epoxyoctadecane was reacted with 300 p1 of 10 H 8 0 4 on a preparative TLC plate; three developments with 1 :1 ether-hexane moved the polar products to Rp 0.40. This material was eluted from the gel and a portion of this
fraction, when reacted with periodic acid, produced a large amount of CS aldehyde as would be expected from the glycol, i.e., 9,lO-octadecanediol. cis-9-Octadecene was then treated with sulfuric acid and with KMn04 by procedures known to generate this diol. The diol product had the same R F value as the major product of the phosphoric acid reaction. (This R F value, as expected, was higher than those of the more polar compounds, 1,2-0ctadecanediol and 1,12-0ctadecanediol.) Finally the infrared spectrum of the major product showed strong OH absorption and was generallv consistent with that expected for 9,lO-octadecanediol. ACKNOWLEDGMENT
The authors thank Martin Jacobson and Rafael Sarmiento for supplying samples of some of the epoxides used in this study.
RECEIVED for review November 19, 1970. Accepted January 25, 1971.
Real Time Computer Control of a Reagent Addition System K. A. Mueller and M. F. Burke Department of Chemistry, University of Arizona, Tucson, Ariz. 85721 The purpose of this work was to develop and evaluate a system for on-line computer data acquisition and control of a chemical experiment. The system chosen for development included data acquisition from a spectrophotometer and control of a sampling system involving a motor driven buret. Experimental work included the construction of a spectrophotometer whose output could be conveniently digitized by means of an integrating digital voltmeter which was interfaced directly to the computer. Digital logic systems for controlling both a continuous drive and a stepper motor were also deweloped. AI orithms for data processing and software control of the logic systems were designed. A Hewlett-Packard 2115A computer with a 16-bit word and a 8192-word core memory was used. An assembler (a mnemonic machine language) was used for all of the software. This has the advantage of being the most efficient and flexible language for data acquisition as well as for control of peripheral equipment. The system was evaluated by following the color change of an indicator for an acid-base titration. Two programs were prepared, corresponding to the two types of burets, to evaluate the system. I n both of these, the computer determined the end point by checking to see when the derivative of the digital voltmeter readings exceeded a threshold typed in by the operator. The software also included noise rejection as well as the capability of changing the sampling rate during the experiment. Possible applications of this system are discussed.
USEOF DIGITAL COMPUTERS in chemical research is increasing at a phenomenal rate. There are three major modes of computer usage including: a) off-line data reduction and analysis (1-4); b) on-line data acquisition and data handling
(1) E. R. Brown, D. F. Smith, and D. D. DeFord, ANAL.CHEM., 38,1119 (1966). (2) G. L. Booman, ibid., p 1141, (3) J. C. Giddings and K. L. Malik, Znd. Eng. Chem., 59 (4), 18 (1967). ,-(4) J. E. Oberholtzer and L. B. Rogers, ANAL.CHEM.,41, 1234, (1969). - . I .
(5-8); and c) on-line control of experiments in which the computer interacts directly to modify the course of the experiment (9-12). While all three of these uses represent valuable contributions to the researcher, the majority of the work published thus far has dealt with the first two modes of usage. The third mode, that of experimental control, represents a more complete use of the capabilities of the small on-line digital computer in the chemistry laboratory. The speed, time resolution, and decision making ability of the digital logic when used for real-time interaction with the experiment can provide the chemist with more reliable and meaningful results with less expenditure of time and work. Much of the work reported thus far concerning control systems has been in the area of electrochemical studies in which the control involves simply a potential, resistance, or current source (12). However, many of the present systems used in automated instrumentation involve measurements where control of the sampling system would be of great interest (8). In these cases, the control involves mechanical linkages which have additional problems of timing and noise not found with the totally electrical systems. The use of the computer for real-time control also requires the development of software algorithms which allow the (5) H. R. Felton, H. A. Hancock, and J. L. Knupp, Jr., Znstrum. Confr. Syst.. 40(8), 83 (1967). (6) S. P. Perone, J. E. Harrar, F. B. Stephens, and R. E. Anderson, ANAL.CHEM., 40,899 (1968). (7) J. Frazer, ibid. 40 (8), 26A (1968). (8) C. P. Hicks, A. A. Eggert, and G. C. Toren, Jr., ibid. 42, 729 ( 1970). (9) S. P. Perone, D. 0. Jones, and W. F. Gutnecht, ibid., 41, 1154 (1969). (10) M. F. Burke and R. G. Thurman, J. Chromatogr. Sei., 8, ' 39 (1970). (11) R. G. Thurman, K. A. Mueller and M. F. Burke, ibid., in press. (12) L. Ramaley and G. S. Wilson, ANAL.CHEM., 42,606 (1970). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
641
A
A
SPECTROPHOTOMETER
p%% I ] CONTROL
EVETEU
T I
I TELETYPE
i--L1
F l
1
SAMPLES
Figure 1. Block diagram showing hardware and functional relationships Arrows indicate the flow of information and control. Solid lines indicate computer control. DVM refers to the digital voltmeter and CPU to the central processingunit
computer to make decisions to alter the course of the experiment. The requirements for the software depend very much on the particular experiment; however, certain general algorithms allow the evaluation of the hardware systems developed. This paper describes a photometric system which has been placed under closed-loop control of an on-line computer. The system includes data acquisition from a phototube and control of a motor driven sampling buret. Algorithms for data processing and experimental control are also described. EXPERIMENTAL
A block diagram of the system used is shown in Figure 1. As can be seen from this figure, the system may be subdivided into smaller units. The spectrophotometric system consists of the spectrometer and an FET input amplifier; the reagent addition system contains the motorized buret and is run by the control interface. The computer is responsible for conditioning information used in conjunction with these two systems and for relaying information to and from the experimenter. Spectrophotometric Instrument. A spectrophotometric measurement system was set up to be used in conjunction with a digital computer for the purpose of developing and evaluating a sampling system to be operated under computer control. A Bausch and Lomb Spectronic 20 Spectrophotometer was modified in a manner similar to that of Pardue et ul. (13) for use in the system. The modifications were necessary since the spectrophotometer was designed for convenient analog output using an ammeter while a voltage output is necessary to use digital voltage measuring equipment. The internal amplifier was, therefore, disconnected from the phototube and an FET input amplifier circuit was connected in its place. In this case, the output is linear with transmission. The zero control on the electrometer provided a means of using an expanded scale technique on some of the measurements. The cell holder of the Spec 20 was modified to allow the sample to be introduced by means of a hypodermic syringe. The hinged door was removed and replaced by an aluminum housing which fits tightly over the cell holder and has a hole for the introduction of samples centered above the sample tube. The syringe barrel was mounted to the cell housing by a short piece of rubber tubing. (13) R. L. Habig, H. L. Pardue and J. B. Worthington, ibid.,39,
600(1967). 642
ANALYTICAL CHEMISTRY, VOL. 43, NO, 6, MAY 1971
ENCODE
0 s.
Figure 2. Stepper motor logic FF JKllip-flop OS. Monostable multivibrator Schmitt trigger for shaping flag pulse S.T. Power ampliRers used to drive the motor coils P.A. Astable multivibrator used to provide a pulse train to reA.M. wind the motor at a rate of about 1 rps
A Standard Spec 20 cuvette which had been shortened to fit below the housing was used for all measurements. The stirring was accomplished by means of a small magnetic stirring bar and a magnetic stirring motor placed beneath the spectrophotometer. Care was taken to ensure that neither the stirring bar nor the syringe needle interfered with the light path. The cell as described had a useful sample size range of 2to4ml. Digitizer and Computer. A Hewlett-Packard Model 2401C Digital Voltmeter was used as an analog to digital converter for this system. Unwanted high frequency noise is eliminated by the integrating characteristic of the digital voltmeter (DVM). The voltmeter used had been modified in order to eliminate the built-in time delays which permit the use of the Nixie tube display. With this modification, data may be taken a t rates of one, ten, or sixty data points per second. In this case, a sampling rate of 0.1 sec was used. Therefore the precision of the digitized word was *O.l mV. A Hewlett-Packard Model 2115A computer (16 bit word size) equipped with an 8192 word core memory, a high speed paper tape reader, a General Purpose Register (GPR), and a teleprinter were used for control of the experiment, data acquisition, and data processing. The digital voltmeter was interfaced to the computer using a Hewlett-Packard Number H-P 12604A interface kit. Syringe Drive Systems. Two systems were used for driving the sample syringes. The first system was a MicroMetric Instrument Co. Model Number SB2 syringe microburet connected to a synchronous motor by means of a chain drive. The drive was set up to give the minimum speed of rotation of the buret micrometer possible with the set of gears available (cu. 0.8 rpm). A Hamilton Co., Inc. “Gastight R” No, 1710 syringe with a 100-pl capacity was used in the micrometer buret assembly. The delivery of liquid from the syringe to the cell in the spectrophotometer was made by a piece of polyethylene tubing that had been stretched to a fine tip. The fine tip was necessary to minimize diffusion of titrant into the sample solution. (Because of surface tension effects, the tip of the titrant delivery tube had to be submerged in the sample solution.) In order to minimize the amount of stray light entering the spectrophotometer cell, the tip was covered with black tape at the point where it emerged from the ceIl cover and was also taped onto the cell cover. This provided a sufficiently light tight seal. This type of system could also be used with a stepper motor.
This was not done, however, since better resolution could be obtained with the system described below. The second system was developed to provide greater resolution in the introduction of titrant increments by means of a smaller sample size and a faster sampling rate. The same syringe was connected to a 4-40machine screw which turned in a nut that was permanently mounted above the sample cell. The drive screw was connected to the motor by means of a splined shaft which allowed the screw to advance or retract as the motor turned. The drive screw and splined shaft were made as long as the plunger of the syringe so that its full capacity would be used. The syringe plunger was spring loaded to ensure that it remained tight against the drive screw. The buret assembly was fixed and the spectrophotometer assembly was raised and lowered in order to allow for refilling of the buret and cell. The cell was filled with a 2-ml pipet. The syringe was refilled from a septum capped bottle by using the rewind capability of the motor controller. A Clifton (Clifton, Division of Litton Industries, Clifton Heights, Pa.) MSA-15-AS-1 stepper motor was used to drive the buret. The stepping angle was 90 & 3" and had a holding torque of 3.2 oz. in. per phase which was sufficient to drive the loaded syringe. Motor Interface Logic. A logic diagram for the stepper motor logic unit is shown in Figure 2. The encode from the computer is immediately used by this device to send a flag back to the computer through the use of the one shot (OS.) and Schmitt trigger (S.T.). This clears the encode but leaves a pulse of sufficient duration to operate the clock inputs of the two J-K flip-flops. The flip-flops are wired in such a manner that the two outputs alternately change states. These two outputs are each used to drive a pair of power amplifiers (PA), one non-inverting and the other inverting. The outputs of the controller are shown in Table I. This sequence of operation results in the forward operation of the motor. When an output is a logical 0, the corresponding cojl of the stepper motor is energized. The switch from Q or Q for the output of the second flip-flop is used to determine the motor direction. The other switch runs the motor in the direction opposite to that of the setting of the direction switch at a speed of about 1 rps. This may be used to rewind the buret screw. With the exception of the flip-flops, which were both in one MC663 integrated circuit, discrete wiring was used for this logic unit. Software. All of the programming was done in assembly language since this has the advantage of being the most efficient and flexible language for data acquisition as well as for control of peripheral equipment. A simplified flow chart of the software program named SPC 30 used for the experiments using the stepper buret is shown in Figure 3. Program (SPC 30) was designed for use with the stepper motor sampling system. This program gives the operator the choice of computer controlled titrations where the computer is used in a closed-loop system or simple manual titrations where titrant is added at a constant rate and the computer is used only for data acquisition. The software also has the capability of noise rejection by means of a simple averaging algorithm, as shown in Figure 3. In addition to noise rejection, the averaging routine provides a means of having the computer wait until the solution is well mixed by the stirrer. In this way, the computer varies the sampling rate to meet the requirements of the experiment. The program also has a rapid titrant addition capability. In this case, the buret is operated several times for each reading until the end point is neared. The approach of the end point is determined by a rapid addition threshold typed in by the operator at the beginning of the program. When the data point is taken, the value of the derivative of the data is compared to the rapid addition threshold. If it is over this threshold, the flag for the rapid additions is set. The next time the buret is to be operated, this flag is checked and, if set, the
Table I.
Energization Logic for Stepper Motor Coils t state
Initial state 1st pulse 2nd pulse 3rd pulse
4th pulse
1 0 0 1 1
0 1 1 0 0
1 1 0 0 1
0 0 1 1 0
buret is run only once. If it is not set, however, the buret is run the number of times that has been specified at the beginning of the program. The threshold used for the rapid addition threshold must, of course, be smaller than the end-point threshold. The end point is detected by determining when the derivative of the data points exceeds the end-point threshold that has been typed in by the operator at the beginning of the program. The operator may choose to have the program terminate either at the end point or 10 points after the end point. The computer will also calculate either the weight or the concentration of the unknown in the sample cell. All of the data in memory may be dumped or plotted or both as the operator chooses. This program also has the capability of repeating titrations without requiring that the initialization information be re-entered by the operator. RESULTS AND DISCUSSION
The use of a computer to change the course of an experiment in progress is not very widely used. The photometric system is evaluated first so that it may be used later in testing the rest of the system. The reagent addition system hardware was evaluated next, followed by the evaluation of the software. Photometric System. A typical titration took about 10 minutes. The short term stability of the amplifier was 1.3 mV at zero T where the amplifier zero was adjusted to give an output of zero volts and 3 mV at 100% T where the output was adjusted to 1 volt with the light control of the Spec 20. The values were obtained with the FET input amplifier. The choice of amplifiers is not very critical in this application; the use of an FET input amplifier for the input stage does, however, improve performance of the device by increasing its input impedance. The spectrophotometer and FET input amplifier Combination were, therefore, sufficiently stable to permit the use of the data obtained from this to evaluate other parts of the system. It was found that if the stirring bar was kept sufficiently far below the light beam, no noise was noticeable in the DVM output. Evaluation of the Synchronous System. SOFTWARE.Acidbase titrations using sodium hydroxide and sulfuric acid were used to evaluate the synchronous system. The sodium hydroxide solution used was 0.109M. Thymol blue was used as the indicator with the wavelength set at 615 nm. This indicator was chosen for convenience and its high molar absorptivity. The limit of this system is f one titrant addition. One of the greatest limitations with this system is that it is necessary to assume that the acceleration and deceleration of the syringe drive is reproducible. The type of motor is, therefore, the greatest problem with the system. It is because of the limitations of this system that the stepper buret system was designed. Evaluation of the Stepper System. HARDWARE.The stepper motorized buret system was found to work very well ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
643
-
~~
-
Averoge N DVM Roodings Compare lost Reading-Repoat If Not Quo1
>Store Doto Point
TO
Monwl
Titrotion 1
t o t oStore WM Point Wing
I
No Rapid Addition
I
Average N WM Readings-Com n To Lost boding,- Repoat If NoPOEquol Store Doto Point
I
-I
I Store Endpoint Data I
IPrint
Endpoint Doto
I
I
Dump Doto
J Plot Doto
I
1
& Figure 3. Flowchart for software used with stepper motor system
without some of the limitations of the synchronous buret system. The buret delivers 0.266 + 0.009 111 per addition where the error is noncumulative. The delivery per addition was determined by using the following information: The stepper motor had an index angle of 90' + '3 noncumulative, the machine screw used to drive the syringe had forty 644
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
threads per inch, and information on the number of microliters per inch of the syringe. For very precise work a better method of calibration would be required. This could be done by weighing a quantity of mercury delivered by it. SOFTWARE.As a test of this system, titrations were run using program SPC 30. In using this system, it was found
E out,
Concn.a
end point 760 732 725 768 815 a
that the sulfuric acid-sodium hydroxide system had a toosharp color change at the end point for use with the rapid addition option of the program. These could, however, be done with the single addition option of the program. This is a significant improvement over the synchronous system in terms of the smaller sample size which may be delivered and the increased precision of the addition of the titrant (the error in this case is noncumulative). An acetic acid-sodium hydroxide titration was therefore used for evaluation of the rapid addition system since it has a slower color change at the end point. The results of some of these titrations are shown in Table 11. The titrations were run with the rapid addition option of the program where 5 additions were made per reading until the end point was near. Figure 4 shows a typical titration curve obtained with this system. The average sample concentration calculated from the end point was found to be 4.89 X lO-3M with a standard deviation of 1.17 X 10-5M. This rapid addition method provides a great improvement (about a factor of 1/4 difference) in the amount of time needed to do a titration.
Titrations with Stepper Motor
Table 11.
x
Addn No. 339 339 340 338 338
103~
4.89 4.89 4.91 4.88 4.88
Calculated concentration was 4.92 X 103M.
CONCLUSIONS
0.1
1 0
I
IO
20
I
I
30
PO
50
60
70
80
It has been demonstrated that a great aid to the automation of analytical experimentation is the computer controlled sampling system. The use of the computer to make decisions, and on the basis of this to modify the progress of the experiment, has also been demonstrated. The obvious application to this is in auto-analysis such as for amino acid analysis. This type of sampling system would also be of use in kinetic studies as was suggested by Hicks, Eggert, and Toren (8).
TiTRAHT AOOTllON NUMBIR
Figure 4. Typical titration curve of an acid with a base using Thymol Blue indicator with the spectrophotometerset at 615 nm
RECEIVED for review August 3, 1970. Accepted February 8, 1971.
Gel Permeation Chromatographic Separation of Petroleum Acids T. E. Cogswell, J. F. McKay, and D . R. Latham Laramie Petroleum Research Center, Bureau of Mines, U. S . Department of Interior, Laramie, Wyo. 82070 Gel permeation chromatography (GPC) was used to separate the components of an acid concentrate. This concentrate was prepared from the 400-500 O C distillate of Wilmington (Calif.) petroleum. The separation was made with a cross-linked polystyrene gel, using methylene chloride as a solvent; and the fractions obtained were characterized by infrared spectra and by molecular weight data. A carboxylic acid fraction obtained was essentially free of phenolic and nitrogen-containing compounds. Results indicate that molecular association of some compound types is responsible for the separation.
RECENTLY, GEL PERMEATION CHROMATOGRAPHY (GPC) has been used as a highly effective tool for separating high-molecular-weight materials. The early progress in this area has been summarized by Moore (I), and extensive model-compound studies have been carried out to clarify the basic separation mechanism (2). The first polymer gels available for (1) J, C. Moore, J. Polymer Sci., C21, 1 (1968). (2) T. Edstrom and B. A. Petro, ibid., p 171.
GPC were limited in use to aqueous systems, but the development of rigidly cross-linked polymer gels has extended the uses of GPC into systems requiring organic solvents ( 3 , 4 ) . Many of the important applications of GPC using organic solvents are in the field of petroleum where GPC has been used to characterize total crude oils (5, 6) and crude oil fractions (6-11). Structural investigations of petroleum fractions,
(3) J. C. Moore, J . Polymer Sci., A2, 835 (1964). (4) K. H. Altgelt and J. C. Moore in “Polymer Fractionation,” M. J. K. Cantow, Ed., Academic Press, New York, N. Y . , 1967. ( 5 ) H. H. Oelert, D. R. Latham, and W. E. Haines, Preprints, Dic. Petrol. Chem., ACS, 15 (2), A204, Feb. 1970. (6) J. N. Done and W . K. Reid, ibid., A242. (7) J . G. Bergmann and L. J . Duffy, ibid., A217. (8) H . J. Coleman, D. E. Hirsch, and J. E. Dooley, ANAL.CHEM., 41, 8 (1969). (9) K. H. Altgelt, Mukromol. Chem., 88, 75 (1965). (10) K. H. Altgelt, J. Appl. Polymer Sci., 9, 3389 (1965). (11) R. J. Rosscup and H. P. Pohlmann, Preprints, Diu. of Petrol. Chem., ACS, 12 (2), A103 (1967). ANALYTICAL CHEMISTRY, VOL. 43,
NO. 6, MAY 1971
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