Anal. Chem. 1986, 58,2589-2591
Registry No. Ca, 7440-70-2; K,7440-09-7. LITERATURE CITED Bates, R. G. Determination of pH, Theory and Practice, 2nd ed.;Wiley: New York, London, Sydney, Toronto, 1973. Illingworth, J. A. Blochem. J. 1981, 195, 259-262. Winkelman. J. W.; Merrltt, C.; Scott, W. J.; Kumar, A.; Baum. G. Clin. Chem. (Winston-Salem, N . C . ) 1984, 3 0 , 482-484. Mldgley, D.; Torrance, K. Analyst (London) 1976, 101, 833-847. Slggaard-Andersen, 0.; Fogh-Andersen, N.; Thode, J. Scand. J. Clin. Lab. Invest. Suppl. 1983, 43 (Suppi. 165), 43-46. Brezinski, D. P. Analyst (London) 1983, 108, 425-442. Brezinski, D. P. Anal. Chim. Acta 1982, 134, 247-262. Covington, A. K.; Whalley, P. D.; Davison, W. Anal. Chlm. Acta 1985, 169, 221-229. Alner. D. J.; Greczek, J. J.; Smeeth, A. 0.J. Chem. SOC. A 1967, 1205-1211. Covington, A. K.; Rebeio, M. J. Ion-Sel. Electrode Rev. 1983, 5 , 93-126. Osswald, H. F.; Dohner, R. E.; Meier, T.; Meier, P. C.; Simon, W. Chimia 1977, 3 1 , 50-52. tjlils, G. J.; Ives, D. J. G. J. Chem. SOC. 1951, 311-318. Stefanac, 2 . ; Simon, W. Microchem. J. 1987, 12, 125-132. Anker, P.; Wleland, E.; Ammann, D.; Dohner, R. E.; Asper, R.; Simon, W. Anal. Chem. 1981, 5 3 , 1970-1974. Jenny, H.-B.; Ammann, D.; Dijrlg, R.; Magyar, B.; Asper, R.; Slmon, W. Mikrochim. Acta 1980, I I , 125-131. Janz, G. J. I n Reference Nectrodes; Ives,D. J. G., Janz, G. J., Eds.;
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Academic Press: New York and London, 1961; p 205. Wuthier, U.; Pham. H. V.; Zund, R.; Weltl, D.; Funck, R. J. J.; Bezegh, A.; Ammann, D.; Pretsch, E.; Simon, W. Anal. Chem. 1984, 5 6 , 535-536. Meler, P. C.; Morf, W. E.; Laubll, M.; Simon, W. Anal. Chlm. Acta 1984, 156, 1-8. Meier, P. C.; Ammann, D.; Morf, W. E.; Simon, W. I n Medical and Blolcgical Appllcatlons of Electrochemical Devices ; Koryta, J., Ed.; Wiiey: Chichester, 1980; pp 13-91. Morf, W. E. The Principles of Ion-Selectlve Nectrodes and of Membrane Transport; Akademlai Kiadb: Budapest, Elsevler: Amsterdam, 1981. Meier, P. C. Anal. Chim. Acta 1982, 136, 363-368. Bates, R. G. Pure Appl. Chem. 1073, 3 6 , 407-420. Miiazzo, G. Elektrochemle I , Grundlagen und Anwendungen ; BirkWuser: Basel, 1980. Salling, N.; Slggaard-Andersen, 0. Scand. J. Clln. Lab. Invest. 1971, 2 8 , 33-40. Pickard, W. F. Math. Blosci. 1972, 13, 113-123. Henderson, P. Z . Phys. Chem. 1907, 5 9 , 118-127. Henderson, P. Z . Phys. Chem. 1908, 6 3 , 325-345. Hefter, G. T. Anal. Chem. 1982, 5 4 , 2518-2524.
RECEIVED for review January 7,1986. Accepted June 1,1986. This work was partly supported by the Swiss National Science Foundation, by Eppendorf Geratebau, Hamburg, and by NOVA Biomedical Corp.
Computer-Controlled Detection System for High-Precision Isotope Ratio Measurements Bruce R. McCord and James W. Taylor* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706 In the measurement of heavy atom isotopic ratios, it is essential to distinguish between very small differences in isotopic abundances. T o make these measurements to the required precision, it is often necessary to use an isotope-ratio mass spectrometer with a double collecting detection system in order that the two isotopic ion currents can be measured simultaneously. Isotope ratio mass spectrometers also permit making all measurements relative to a standard gas. In this manner instrumental effects can be reduced to a minimum. The detection system of a standard isotope ratio mass spectrometer collects two ion currents, amplifies them, and then uses null detection to obtain the isotopic ratio ( I ) . The precision of this type of measuring system is limited by the large amount of Johnson noise associated with the high value resistors used, 10" 0, as well as their large dependence on temperature and voltage (2, 3). Because a system of this type produces one part in lo4 precision, we found it marginal for measuring the subtle differences arising from small changes in reaction conditions ( 4 ) . The key to improvement was to eliminate the high-ohm resistors from the system in the manner advanced by Jackson and Young (5). A capacitive integrating detection system avoids the use of high value resistors by integrating the ion currents using low leakage capacitors across highly stable amplifiers. The two output voltages are ratioed by use of a digital voltmeter. Our implementation of this system in which the precision of the instrument was improved to one part in lo5 has been described in a previous paper ( 4 ) . There were, however, some problems with this system. Because only a single data point was taken at the end of the integration period, there was no way to monitor the instrument stability during the measuring period. This is necessary as a certain amount of time is needed to allow for stabilization of gas pressure inside the mass spectrometer. Another problem with the system was that it required a digital volt-
meter that had the ability to measure the ratio of two steadily increasing voltages in .real time. Many commercial digital voltmeters require a stable and constant voltage reference in order to perform ratio measurements or do not have the ability to make a real time ratio measurement a t all. In this paper we describe a detection system that avoids these problems. In this new system, the requirement for a ratioing digital voltmeter has been eliminated, and a standard digital voltmeter interfaced to a computer is employed. Instead of measuring the ratio of the two steadily increasing output voltages simultaneously, the digital voltmeter alternately samples the outputs a t a precise rate over a certain period of time. The data are sent to the computer which calculates the rate of charge of each amplifier and divides the two rates to obtain the isotopic ratio. These results simulate a coincident measurement of the output of both integrators. The charge rate is calculated by using a linear regression method, and the standard error of the slope gives a measure of the stability of the system at the time the measurement was taken. This approach has an advantage over other systems (4-7)in that i t avoids the need for simultaneous measurement of the two voltages or for a memory capacitor to store one reading while the other is being sampled. Another advantage inherent in the design of this system is its unique ability to monitor itself during the sampling process. The system described here is specifically for application to an isotope ratio mass spectrometer; however, the suggested circuitry could be employed for any two signals that need to be compared in real time to improve the precision of the measurement.
EXPERIMENTAL SECTION The measuring system is shown in Figure 1. The capacitive integrator and the mass spectrometer have been described previously ( 4 ) . The voltmeter used was an HP3456A 6-1/2 digit
0003-2700/86/0358-2589$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
Table I. Typical Results in the Measurement of Chlorine Isotopic Ratios
1
Flgure 1. Block diagram of detection system.
ratio"
corr I*
corr 2
0.756 870072 0.758374090 0.754 128308 0.758 385 997 0.754 145904 0.758 383 903 0.754 132 178 0.758 396 324 0.754 140 162 0.758407 905
0.999 675089 l.OOOOOO000 l.OOOOOO001 0.999999995 1.000000001 0.999999999 1.000000000 1.000000000 1.000000001 1.000000000
0.999999 999 1.000OOO001 1.00OOOO001 1.000000001 l.OOOOOO001 1.000000001 1.000000000 1.000000000 1.000000001 1.000000002
std error
std error
1
2
2.36473-3 1.56353-6 1.19383-6 2.97713-6 3.35343-6 5.28043-6 1.89393-6 4.01413-6 1.22973-6 8.95833-7
2.34213-6 1.55633-6 1.79633-6 2.32083-6 2.44123-6 3.40803-6 1.46343-6 2.77313-6 1.21893-6 1.50423-6
"Ratio obtained by dividing the slopes of lines in Figure 2. *Correlation coefficient for upper line in Figure 2.
W
m m 0
3
1
1
2
.
1
4
.
1
.
6
t
.
S
1
.
10
1
.
I2
1
.
14
1
.
16
1
.
1
18
Seconds Figure 2. Output of integrator.
digital voltmeter (Hewlett-Packard,Palo Alto,CA), which is fully programmable over an IEEE-488 interface bus. The voltmeter is also provided with an external trigger and an output pulse that indicates when the voltmeter has completed its measurement. The computer used was an IBM PPC (International Business Machines, Boca Raton, FL) equipped with a National Instruments GPIB-PC IEEE488 interface bus (National Instruments, Austin, TX). The operating system of the mass spectrometer is controlled by a fixed timing sequence. The gas valves and capacitive integrators are controlled by a cam-actuated clock (4).This clock cycles every 135 s. At the start of the cycle the sample gas valve is opened, and the gas is admitted into the mass spectrometer giving an ion current of 2 X A. The capacitive integrators are shorted at this time. The gas is allowed to stabilize for 75 s, after which the short is removed via a reed relay and the integrators begin to charge. Approximately 30 s after the integrators are turned on, the output voltage of one integrator reaches a value of -3 V. At this point the computer switches the voltmeter to external trigger control. This permits the voltmeter to begin sampling the outputs of the integrators at a rate determined by the frequency of the external crystal clock. The change from using the computer to trigger the digital voltmeter to using an external clock was done because it was not possible to obtain the necessary precision (1ppm), using the computer clock driven by a BASIC program. Such precision was necessary in order to obtain accurate ratio readings to six places. The crystal clock circuit consists of a 200-kHz piezoelectric crystal constrained to oscillate at TTL logic levels. The signal frequency is then divided down and used to trigger the digital voltmeter to take a reading. The voltage obtained is stored in the memory of the digital voltmeter, and the voltmeter sends a measurement complete pulse to the toggle flip-flop which switches the relay to the second capacitive integrator. Figure 2 shows the values of the gradually increasing output voltages which are obtained from the two integrators. The measuring process continues until at least 20 readings are taken. The computer then switches the digital voltmeter back to remote control and reads in the last 20 voltages which were stored in the memory of the voltmeter. The readings are sorted according to which isotope they represent, and the slope, standard error of the
slope, and correlation coefficient are calculated by using 10 data points for each line. After this is done, the gas valves are switched, and the process is repeated for the reference gas. To determine the slope and other statistical information, a nonweighted linear regression is performed on the data using a procedure adapted from Miller (8). It should be noted that these calculations are susceptible to roundoff error when using a microcomputer. Modifications were made in the statistical program to correct this potential problem. In particular the standard errors of the slopes were calculated by summing each individual residual. The output from the program lists the isotopic ratio, and the standard error, correlation coefficient,and intercept for each line. In the present protocol, the measuring process is repeated until 16 ratio readings are printed out, eight for the sample gas and eight for the standard gas.
RESULTS AND DISCUSSION The result from a typical run in which the isotope ratio of a sample of methyl chloride is measured is shown in Table I. The standard ratios (odd run numbers), and the sample ratios (even run numbers), are printed out in the order in which they were taken. The data clearly show the high precision obtainable with this system. The low standard deviation of the slopes reveals the extreme linearity of the output of the two integrators and the correlation coefficients provide a quick visual check of the quality of the fit. Unstable values, such as the first entry in Table I, can be quickly spotted by using the correlation coefficient. Such results are often seen when a sample is first admitted into the mass spectrometer and are due to oscillations in inlet pressure and instabilities resulting from the warmup of the electrometer amplifiers. The correlation coefficient can be used as an indicator of when the instrument is stable enough to begin taking measurements. The extent of the instability in the output of the integrator is revealed by examining the standard error of the slope of the line. For this reading the standard error is 1C-3 vs. 10-6 for the other standard errors. Such poor standard errors can be used as a criterion to reject bad data. This can be justified by using a Dixon test, which reveals that the initial ratio, 0.756 870, can be rejected at the 95% confidence level (9). After deviant ratios were rejected, statistical analysis was used to correct for base line drift. The result gave a 6 value of 4.236 ppt with a standard deviation of 0.005. This compares favorably with standard deviation of 0.007 obtained for similar data using a ratioing digital voltmeter ( 4 ) . Thus this new system can obtain similar precision to a continuously ratioing digital voltmeter with the added advantage of diagnostic information on the stability of the measurement.
LITERATURE CITED (1) Mook, W. G.; Grootes, P. M. Int. J . Mass Spectrom. Ion Phys. 1973, 12, 273. (2) Habfast, K. 2. Instrumentenkd. 1960, 68, 82.
Anal. Chem. 1986, 58, 2591-2592 (3) Peterson, D. W.; Hayes, J. M. "Signal-to-Noise Ratios in Mass Spectroscoplc Ion-Current-Measurement Systems". I n Contemporary Topics in Ana/ytica/ and C/inica/Chemistry; Hercules, David M.,HiefJe, Gary M., Snyder, Lloyd R., Evenson, Merle A,, Ed$.; Plenum Press; New York, 1978; Vol. 3. (4) Wiley, J. F.; Taylor, J. W. Anal. Chem. 1978, 5 0 , 1930. (5) Jackson, M. C.; Young, W. A. Rev. Sci. Instrum. 1973,4 4 , 32. (6) Halas, S.;Skorzynsky, 2. J . Phys. E 1981, 1 4 , 509.
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(7) Halas, S.; Skorzynsky, 2 . J . Phsy. E 1980, 13, 346. (8) Miller, A. Basic Programs for Scientists and Engineers; Sybex: Berkeley, 1981; pp. 115-140. (9) Dixon, W. J. Biometrics 1953, 9, 74.
RECEIVED for review April 7,1986. Accepted June 12, 1986.
Evaluation of Alumina Furnace in Vacuum Fusion Determinations of Gases in High-Temperature Materials
-
Edward K. Pang
GTE Electrical Products, Lighting Research and Engineering, Syluania Lighting Center, Danuers, Massachusetts 01923 Knowledge of the outgassing characteristics of materials and the determinations of gases in materials used in a hightemperature and high-vacuum environment is important. Pang ( I ) recently reported a system for fast mass spectrometric characterizations and quantifications of high-temperature outgassing, but the quartz used in the system has a relatively low softening temperature. It has been noted that there is a substantial carbon monoxide background associated with the high-temperature reaction between the graphite insulation and the quartz furnace ( 2 ) . Dallmann ( 2 )suggested using a pyrolytic boron nitride furnace, but the nitrogen background is quite significant and can interfere with the carbon monoxide peak a t rnlz 28. A general program has been undertaken to design and evaluate different systems for vacuum fusion determinations of gases in high-temperature materials used in lighting products. This paper describes a high-purity alumina furnace for such application. The vacuum integrity and the outgassing properties of the assembly are reported.
EXPERIMENTAL SECTION The mass spectrometer and inlet systems have been described by Pang and co-workers ( I , 3) elsewhere. The modified furnace assembly using high-purity alumina is shown in Figure 1. The alumina tube (Omegatite, Omega Engineering, Inc., Stamford, CT) was connected to the glass inlet system by applying a very low vapor pressure resin (Torr Seal, Varian Associates, Palo Alto, CA), but the resin has to be maintained at temperatures below 100 "C by air cooling. The furnace system is attached to the all-glass inlet system by glass blowing. The experimental system is shown in Figure 2. The alumina is of high purity and typical analysis supplied by manufacturer shows 99.8% alumina, 0.07% silica, 0.05% magnesia, 0.05% calcium oxide, and 0.03% iron(II1) oxide. The electric furnace is microprocessor-based and programmable with a maximum temperature of 1650 "C (Applied Test Systems, Inc., Butler, PA). The tube temperature was measured with chromel-alumel and Pt/Pt-13% Rh thermocouples which were held against the alumina furnace tube. Pressure rise during measurement was monitored by a thoria-coated iridium ionization gauge (Fil Tech, Boston, MA) and a MKS Baratron capacitance manometer (10 mmHg full scale with ranges X l , 0.1, 0.01). Characterization of the gas was made possible by opening an all-metal variable leak valve that maintains a constant inlet preasure for mass spectrometric analysis. The as-received alumina tubes were stored in an inert atmosphere at room temperature before experiments. Before measurements were made, the asreceived tube assembly was baked at 1600 "C under a vacuum of 8 X lo4 mmHg for 200 h to detect any sign of deterioration. The pressure dropped to 3 x mmHg after the alumina tube
WRMGLASS
O.D. = 20 MM
1,l
12MM
ALUMINA FURNACE I.D. = 25.5 MM O.D. = 31.8 MM LENGTH = 305 MM
I
Figure 1. Design of alumina furnace. TO W A D R U W C E MASS SPECTROMETER AND "*C"UU SYSTEM
+
I l l CALIBRATED
TO
.
ffi ICU OAUGE CM .CAPACITANCE MANOMETER ABSQLUTE OR MFFERENiUL
LEAK VALYF
VACON PUMP
I
-
TGRBOMOLECIJLAR PUMP
AlummdTube
Figure 2. Schematics of experlmentai system. cooled to room temperature. Pressures during experiments were typically 4 X lo-' mmHg.
RESULTS AND DISCUSSION Figure 3 shows the quantity to gas with various temperatures of the alumina tube (volume, 0.57 L; inside geometric surface area, 244 cm2). Each measurement takes place in 30 min. Because of the minute quantity of the gases evolved a t high temperatures, the tube assembly is suitable for micro-
0003-2700/86/0358-259 1$01.50/0 0 1986 American chemical Society
