SOLUTION.To use the data logger in a current monitoring mode and to punch a data tape with readings at 10-second intervals for a period of 30 minutes. The data tape consisting of 360 data points is then batch processed on a small computer using Simpson’s rule of integration to determine the total number of coulombs. The computer also automatically makes plots of current us. time and log current vs. time. The total time to process a data tape and make the plots is about 3 to 5 minutes. PROBLEM. To automate the data handling from a hightemperature heat content experiment. The calorimeter temperature was measured as a function of time by measuring the value of a thermistor with a Wheatstone bridge and potentiometer obtaining one point at a time manually. Calculations for corrected temperature rise and heat content were done with a calculator and plotted by hand. Time to process data and plot curve from one 30-minute run was about two hours. SOLUTION.To automate the data collection by monitoring the unbalance of the Wheatstone bridge with the data logger in a voltage mode recording a data point every ten seconds for
a period of 30 minutes. The data tape is then processed on a small computer and the results are plotted automatically by the computer in several minutes. CONCLUSIONS
The data logger has proved to be an extremely valuable tool for automating the digital data collection from a wide variety of spectrometers and experiments which do not require control such as Monochrometers, gas chromatographs, IR, etc. The logging speed or data rate is limited to one data word per second or slower. This limits the systems use to most laboratory processes that are recorded on a conventional x-y or strip chart recorder. The data tapes are very easily batch processed on a small computer with a conversational interactive language. A negative of the printed circuit board may be obtained from the author. RECEIVED for review April 14, 1972. Accepted July 31, 1972.
Automatic Gap Control Unit for Spark Source Mass Spectrometry C. W. Magee and W. W. Harrison Department of Chemistry, University of Virginia, Charlottesville, Va. 22903
UNTILRECENTLY, spark source mass spectrometry (SSMS) has relied almost exclusively on the photographic plate for ion detection. Although this method has the advantage of integrating ion current fluctuations over the selected exposure (charge accumulation), it does not allow convenient study concerning effects of experimental parameters as is possible with an instantaneous type of readout. The electrical detection system (1) which is now available provides an immediate indication of the direction and magnitude of a particular effect induced by selected parameter variation. The increased precision and accuracy of the electrical mode over the photographic readout can make parameter control extremely important. Effects which contribute a 5 % variation may not be considered as significant in photographic detection, where 25-30Z error is common, as in electrical detection, where precision of 5 is obtainable by integration methods. Therefore, in studies which hope to exercise these greater electrical detection capabilities, it is incumbent upon the investigator to control closely as many experimental variables as possible. One of the more difficult parameters to maintain constant is the spark gap between the electrodes. Most investigators make no particular effort to monitor and control the gap. However, it has long been known that interelectrode gap width can be significant. Woolston and Honig (2) demonstrated that the energy distribution of ions in the beam is affected by gap. Bingham and Elliott ( I ) cited gap effects in high accuracy peak switching analysis, as have Colby and
(1) R . A. Bingham and R . M. Elliott, ANAL.CHEM., 43,43 (1971). (2) J. R . Woolston and R. E. Honig, R e i . Sci. Itistrurn., 35, 69 (1964). 220
Morrison (3). Konishi and Nakamura ( 4 ) have noted gap effects on ion ratios using photographic detection. This laboratory has also reported effects of gap width on sensitivity (5, 6), which showed marked changes in analytical ion yields with changing gap width for aluminum, copper, and steel matrices. More recent work with compacted graphite samples indicates an even greater gap dependence. The Autospark (A.E.I.), a standard part of the electrical detection system, is useful for maintaining spark ignition, but its cyclical nature continuously varies the interelectrode gap. Manual gap adjustment, as determined by oscilloscopic display of the breakdown voltage, has been our standard procedure in those studies calling for maximum precision, but this is tedious and keeps the operator constantly engaged. To obviate these difficulties, a unit has been designed and constructed in our laboratories which automatically maintains a preselected gap width, correcting for both short term and long term effects of electrode deterioration. It was brought to our attention during the preparation of this paper that another gap control unit for SSMS has recently been developed ( 3 ) . There are, however, significant differences in mode of operation and design considerations. Also, (3) B. N. Colby and G. H. Morrison. ANAL.CHEM..44, 1263 (1972). (4) F. Konishi and N. Nakamura, “Advances in Mass Spectrometry,” Vol. 5. A. Quale. Ed.. Institute of Petroleum, London, 1970, p 547. ( 5 ) C. W. Magee and W. W. Harrlson, Solids Workshop of the 19th Annual Conference on Mass Spectrometry, Atlanta, Ga, May 1971. (6) C. W. Magee and W. W. Harrison, ASMS Solids Workshop, St. Louis,Mo., October 1971.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
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the unit described in this paper is specifically designed to coordinate with a commercially available electrical detection package currently in use in many laboratories. EXPERIMENTAL
Circuit Description. The automatic gap controller utilizes the amplitude of the R F voltage developed between the electrodes as the controlling signal. The amplitude to which the R F builds before breakdown is directly proportional to the spark gap width. Our Automatic Gap Unit (AGU) continuously adjusts one electrode in order to maintain a fixed R F voltage between the electrodes, and thus a constant gap. The AGU was designed and constructed t o be used with a n AEI MS-702 spark source mass spectrometer and t o utilize certain AEI Autospark components. The entire schematic of the AGU and associated Autospark components is shown in Figure 1. The output circuit of the Autospark consists of a current limiting transistor TR1 in series with the electrode deflection coil. The position of the electrode is thus depen-
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dent upon the signal magnitude a t the base of T R l , the gap decreasing with increasing base voltage in the normal mode of operation. The control circuitry of the AGU provides that a n increasing gap, with correspondingly larger R F spark gap voltage, will produce an increasing output voltage which is coupled to the base of T R I , decreasing the gap width until the pre-set R F level is reacquired. The unit also contains associated switch circuitry allowing for oscilloscopic monitoring of pertinent voltages and wavcfornis uithin the circuit, as well as panel meter display of voltages representing gap width, AGU output, and Autospark output. The R F voltage pickup is oblained with a coil probe mounted in the source inspection port of the mass spectrometer. The induced RF voltage is fed via a shielded coaxial cable to the input of the control circuit. The input is a twostage high pass filter feeding a high impedance noninverting amplifier A1 of variable gain which serves as a buffer between the input signal and the rectitication elements. The A I output signal, after rectification and filtering, is fed to the input of the inverting second stage, A2, a variable gain amplifier which allows compensation for variations in R F pulse length and
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
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Figure 2. Discharge current us. gap width for vibrating electrodes (recorded directly from AGU recorder output, Autospark mode)
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repetition rate, both of which affect the level of the second stage input voltage. The output of A2 is filtered to prevent oscillation of the electrode arising from very short term perturbation of the gap width, and the signal fed to a non-inverting amplifier A3. The A3 output is directed to one input of the summing amplifier A4, the output of which is dependent upon the difference between the output of A3 and the reference voltage as adjusted by the G a p Width Control RV3. The output from A4 is directed t o the base of TR1, thus controlling electrode position. Alternatively, S1 can feed TR1 the output from the Autospark, thus enabling that device t o operate in its normal manner. Switch S2 determines the meter display. The operator can continuously monitor (a) the output of A2, which is analogous t o the spark gap width, or (b) the A4 output which represents electrode position, or (c) the output of the Autospark, in either the manual or servo mode. S3 determines the meter sensitivity (1, 3, or 10 V.FSD). S I , the oscilloscope display switch, allows for convenient voltage monitoring within the AGU circuit. With appropriate voltage dividers, this output may also be coupled to a strip chart recorder. Operating Procedure. The stationary electrode (right) is positioned on the ion axis using the optical viewing system previously described (7). The controlled electrode is initially positioned about 5 mm above the fixed one in a slightly overlapping configuration. (To facilitate precise initial adjustment, our instrument has been modified to retain the manual micromanipulator controls in conjunction with the Auto(7) C. W. Magee, D. L. Donohue, and W. W. Harrison, ANAL. CHEM., 44,2413 (1972). 222
spark.) With the spark on, and S4 switched to the AI output, the first stage gain is adjusted until the spark envelope is 8 t o 10 volts peak-to-peak as monitored by the oscilloscope. The Autospark output from the manual adjust, monitored by the AGU meter switch S2, is set at about 1 volt in order to have the Autospark output circuit operating in the middle of its dynamic range. The manipulator for the controlled electrode is adjusted until the sample is sparking a t the desired gap as monitored by the optical viewing system, or by oscilloscope display. The meter is then switched to monitor the AGU output, and the gain of A2 is adjusted to yield a n output of 4 to 8 volts. Gap stability and response time are dependent on this setting. When the voltage has stabilized, usually within 10 to 15 seconds after spark initiation, the reference voltage is adjusted with RV3 until the AGU output reaches the level of the Autospark output, as determined by switching (S2) between the AGU and Autospark outputs. When no significant difference is shown (
