At the pH of the measured solution, 12.7, only about half the sulfide is complexed and available for detection by the electrode ( 4 ) . The EDTA served to adjust the ionic strength and to facilitate dissolution of the zinc sulfidezinc hydroxide precipitate, but its buffer capacity at this high pH is minimal. However, in analysis of most natural waters, the effect of' sample acidity on the unbuffered system (particularly with the preconcentration step) would be negligible. Ascorbic acid was included as antioxidant following the recommendation of Bock and Puff ( 5 ) . The method has been used in the field for determining sulfide content of environmental waters a t the Savannah River Plant. Successful determination of these low-level ( 4 ) Orion Research Incorporated Instruction Manual, "Sulfide Ion Electrode, Model 94-16." (1966) p 10. (5) R. Bock and H. J. Puff, Fresenius' 2. Anal. Chem., 240,381 (1968).
sulfides depends strongly upon the handling of the samples prior to and during the determination. Therefore, to eliminate loss of sulfide through volatilization or oxidation, the sample should be quenched with the alkaline antioxidant reagent or by zinc acetate-sodium carbonate addition as soon as possible, and handled thereafter with minimum exposure to air. Elizabeth W. Baumann E. I. Du Pont de Nemours & Co. Savannah River Laboratory Aiken, S.C. 29801 Received for review December 17, 1973. Accepted April 5 , 1974. The information contained in this article was developed during the course of work under Contract AT(07-2)-1 Atomic Energy Commission. with the U.S.
AIDS FOR ANALYTICAL CHEMISTS _I
P. H. Daum and Peter Zamie Department of Chemistry, Northern lllinois University, DeKalb, 111. 60 7 75
The typical method for obtaining data from fast kinetic experiments such as a stopped-flow, in the absence of an interfaced laboratory computer, is to record the data photographically from an oscilloscope trace. This has distinct limitations, not the least of which is the time consuming process for the accurate transcription of the data from this form to a digital form suitable for computer processing. In an attempt to alleviate this tedious and time consuming task, and to increase the accuracy of the experiment, we have designed and constructed an apparatus for obtaining and storing data from a stopped flow apparatus. The instrument is constructed primarily from readily available components and systems, is easily fabricated, and should prove versatile enough for a variety of laboratory applications. The instrument is similar in concept to commercially available instruments such as Biomation and Nicolat. The instrument IS designed to measure and store 32 data points a t evenly spaced time intervals upon receipt of a trigger signal from the stopped flow apparatus. The acquisition system consists of a variable gain buffer amplifier, to invert and amplify the negative signal of the stopped flow detector output, a sample and hold amplifier, and a 12-bit analog-to-digital converter. The data points are stored in a memory which was constructed from two MOS-LSI shift registers connected in parallel ( I ) . The timing of the unit is controlled by a crystal oscillator and scalers. The instrument was constructed using a 3-module Heath cabinet (SU-800-RC) containing a Digital Power Module Heath (EU-801-11), Auxiliary Module (EU-80113), and Decimal Readout Module (EU-801-15). IC cards not available from Heath were constructed using Heath Dual Inline cards (EU-50-MC).In the case of the S/H amplifier, A/D converter, and memory, cards were made by cutting and etching commercial PC board to fit the Heath Chassis. Hard wire connections between cards were made using the Heath Permanent Patch Accessory (SU-50-JA). ( 1 ) P H Daurn and D F
Nelson
Anal Chem , 4 5 , 4 6 3 (1973)
EXPERIMENTAL Apparatus. The design and operation of the instrument can be readily understood by dividing the instrument into three basic sections, time base, data acquisition, and sequencing as shown in Figure 1. The time base consists of a crystal oscillator and scalers (Heath EU-800-KC), the modulo-8 counter, and the selector gates marked A, B, C, and D. The Heath oscillator provides the basic time base of the instrument, and consists of a 1-MHz crystal oscillator with seven decade scalers to give frequencies which vary in decades from 1 MHz to 0.1 Hz. These frequencies are available from a multiplexed output which is controlled by the state of three "Data Select" inputs available a t the top of the card. The desired decade frequency is selected with a front panel BCD switch connected to these inputs. To increase the timing flexibility of the instrument, the decade frequency is further divided by 2, 4, and 8 using a binary scaler and the logic gates A, B, C, D, and E. The desired time base output (appearing a t the output of gate E) is chosen by supplying an 11-1 signal to the control input of the appropriate gate while maintaining a n 11-0 a t the control inputs of the remaining gates. For example, if the logic levels a t Ac, Bc, Cc, Dc were 0, 1, 0, 0, respectively, the frequency a t the output of gate E would be one half the decade frequency. The logic levels are chosen using an appropriately wired front panel switch. The frequencies available from the time base range from 1 MHz to 0.0125 Hz in a 1-2-4-8 sequence. The data acquisition system consists of a wide band pass amplifier (Analog Devices 149A) connected in the inverting configuration. The gain of the amplifier is controlled through a range of 1 to 10 in a 1-2-5 sequence by varying the magnitude of the feedback resistor with a front panel control. The inversion was necessary because the output of the photomultiplier was negative. The output of the variable gain amplifier is fed to the input of a sample and hold amplifier, (Analog Devices SHA 1x1) which samples the input signal a t times specified by the time base. The output of the S / H is then sent to an A/D conveter (Analog Devices ADC12QMBCD). The converter has a BCD coded output to provide logic compatibility with the Heath Decimal Readout Module, which requires BCD input. The 12-bit conversion system thus provides a resolution of 1ppt. The output of the A/D converter is sent to the memory, Figure 2, which consists of two MOS-LSI shift registers (Texas Instruments TMS-3112-JC) connected in parallel. Each LSI contains six 32-bit shift registers in parallel. Thus, the memory is capable of storing thirty-two 12 bit numbers. The memory circuit boards
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9 , AUGUST 1974
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Figure 1. Data logger circuit diagram 0
8
'6
24
32
40
40
56
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-
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I
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-
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Figure 2. Memory schematic
--
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96
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Figure 4. Stopped flow data for reaction of BudNAuCll with tri phenyl phosphine
Upper curve Raw data from Data Logger Lower curve Plot of A vs time for reaction
were designed to include one register and additional logic circuitry to enable the data to bypass the memory so that either the output of the A/D converter or the output of the memory could be observed on the Heath readout module. This additional circuitry, consisting of the gates marked G, H, I, and J is controlled from a front panel switch a t the point marked In/Out. The output of the memory is displayed when the control is a t a 11-1, and the output of the A/D converter when a t a logic level 0. Data can he written into the memory hy causing the circulate/ write control to have a logic level 0. This causes the input gates of the memory to open, and new data will he transferred into the registers when a 1-0 transition occurs at the clock input. The data stored in the memory can he read by making the circulate/ write control a 11-1. The data then appear sequentially a t the output a t a rate which is determined by the frequency of the signal appearing a t the clock input. Provision is made to provide clock pulses to the memory either from a manual pushbutton or from the time-base section of the instrument through a front panel man/auto control. It should be noted that reading the memory is done non-destructively since, when the circulate/write control is a t an 11-1, the output of the memory is fed back to the input. Thus, it is possible to read the memory as many times as necessary to record the data. The sequencing of the instrument can be understood with reference to Figure 1 and the timing diagram Figure 3. When a 1-0 logic level transition is sent to the clear of flip-flop-1 ( F F - l ) , either from a front panel switch or from the stopped flow, the 8. 1348
ANALYTICAL CHEMISTRY, VOL. 46,
output of the FF undergoes a 0-1 transition. This releases the counters and scalers in the instrument and allows timing pulses to be transmitted. Exactly one period of the timing pulse after a trigger is received, the timing pulse undergoes a 1-0 transition. The output of monostable-1 (MS-1) then produces a short duration pulse (ca. 5 rsec) which is fed to the convert command input of the A/D converter. Simultaneously both the STATUS and STATUS outputs of the converter undergo transitions of 0-1 and 1 4 , respectively. The STATUS output is fed to the logic input of the S / H amplifier and causes the amplifier to go into the hold mode. The signal a t the output of the amplifier a t this time is then converted to a digital form. When the conversion is complete (after ca. 25 gsec), the output returns to 11-1 causing the S/H amplifier to return to the sample mode, and the STATUS output returns to zero causing a 1-0 transition to occur a t the input of-MS-5. As a result of the 1-0 transition occurring a t its input, the Q output of MS-2 also undergoes a 1-0 transition. This causes the data from the output of the A/D converter to be shifted into the memory. The output of MS-2 is also fed to the input of the modulo-32 counter where the pulse is counted. This timing cycle is repeated 32 times; on the 32nd time, when the memory is filled, the Q output of FF-1 undergoes a 1-0 transition. This clears all of the counters and resets the scaler on the crystal oscillator card. These are held a t zero until the next trigger pulse is received. Data which have been stored in the memory can he read out either manually or automatically. In either case, the circulate/ write control on the memory is set a t 11-1, to avoid destruction of
NO. 9, AUGUST 1974
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the contents of the memory. The In/Out control, which determines whether the output of the memory or A/D converter is displayed, is set a t 11-1 so t h a t the output of the memory is displayed. For automatic operation, FF-1 is cleared manually, and the data will then recirculate a t a rate determined by the timing section of the instrument. For manual operation, the Man/Auto control is set a t 11-1, FF-1 is cleared, and the data are shifted manually by feeding pulses into the Man. input from a front panel pushbutton.
RESULTS An example of the data obtained using a Model DllO Durrum Stopped Flow system with a D150 modification is shown in Figure 4. The reaction is that of Bu4NAuC14 with triphenylphosphine in acetonitrile at a 20:1 concentration ratio a t 15 "C. The reaction is pseudo-first order under these conditions ( 2 ) . The data logger was triggered from the oscilloscope trigger output of the stopped flow. The time base for the measurements was 4 msec. The points of the upper curve are the raw data as recorded by the data logger. The dip in the curve is the result of mixing, and could be removed by resetting the micro-switch on the stop-syringe. The lower curve is a plot of absorbance us. time for the reaction. Only those points which occurred after mixing were included in the curve. The rate constant which was measured was 91.7 sec-l. This compares favorably with results obtained using photographic techniques ( 2 ) . P Fenske. Chemistry Department, Northern Illinois University, personal communication. 1973
(2) G
CONCLUSIONS The versatility and speed of the data logger is hardly shown by the above example. In its present form, the instrument is capable of recording data at precisely timed intervals which range from 0.1 msec to 80 sec. With some modification of the sample and hold system, rates as high as 30-40 Fsec could be readily attained. The resolution of the instrument is controlled by the 12-bit BCD converter. In combination with the buffer amplifier a t its maximum gain of 10, the resolution is 1 mV. Obviously, the total resolution of the system could be increased by using a 12-bit binary converter, storing the number in the memory in a binary form, and using a binary to BCD decoder to drive the decimal readout module. However, the increased resolution would come a t the expense of additional complexity in the design and construction of the instrument. The instrument should prove to be useful for many applications where it is not feasible to use a computer, yet where the time consuming process of transcribing data from photographs is to be avoided. If a larger memory, increased resolution, or speed are necessary, the design can be readily modified to meet these more stringent requirements without changing the basic concept of the instrument. Received for review August 9, 1973. Accepted February 11,1974.
Instrumental Determination of Molecular Weight by Gas Pressure F. W. Karasek and R. J. Laub Department of Chemistry. University of Waterloo, Waterloo, Ontario, Canada
The determination of molecular weights is necessary in a wide variety of industrial and academic applications. Many methods have been used, including cryoscopic, ebullioscopic, and isopiestic methods-ie., boiling point elevation, freezing point depression, and the Dumas technique. Several of these are accurate to only 5% a t best, and are too time-consuming for routine use in a modern analytical laboratory. One method that does meet the criteria of speed and exactness is the determination of the mass of a molecular ion by mass spectrometry. Molecular ions must have a lifetime of approximately sec to be detectable. Many organic compounds (notably alcohols, ethers, and esters), however, do not show measurable molecular ions in a normal electron impact mass spectrum. Several methods are available for overcoming this limitation, including electron impact mass spectrometry at low ionizing potentials, and the techniques of field ionization and chemical ionization ( 1 ) . The disadvantages of mass spectrometry and related methods are the large investment required and concomitant operational difficulties when complex instrumentation of these types is used. Other less expensive methods which are capable of 1-270 accuracy include gas chromatography and vapor-phase osmometry ( 2 ) . (1 ) J. Roboz. "Introduction to Mass Spectroscopy," Interscience Publishers. New York. N . Y . . 1968. ( 2 ) R. C. Crippen, "Identification of Organic Compounds with the Aid of Gas Chromatography." McGraw-Hill Book Co., New York. N.Y., 1973, p 152.
The instrument and method described here provide a simple, fast, and accurate method of determining molecular weights for compounds which are volatile at 200 "C in a millitorr vacuum. The principle on which it is based is the ideal gas law, PV = nRT, written in the form
MW
=
K(g/P)
(1)
where
K = -RT V and g is the weight of sample, R is the gas constant, P, V, T, and M W are pressure, volume, temperature, and molecular weight, respectively. A known amount of volatile sample is injected into an evacuated volume held at constant temperature. The increase in pressure is measured and the molecular weight of the sample calculated uia Equation 1. Conditions are chosen such that the system remains in the ideal range of the gas law equation. A constant volume method was first proposed by Bleier and Kohn in 1899 ( 3 ) and later improved by Lumsden ( 4 ) . In I947 Young and Taylor ( 5 ) constructed an apparatus in which the sample was introduced into an evacuated vol(3) 0. Bleier and L. Kohn, Monatsh. Chem., 20, 505 (1899) (4) J. S. Lumsden,J. Chem. SOC..(London), 83, 342 (1903). (5) W. S. Young and R . C. Taylor, Ind. Eng. Chem.. Ana/. Chem.. 19, 135 (1947).
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