matrices are almost constant, and cesium K a and iodine K a are hard radiations, so that any matrix effects are small; and in the wave length region of cesium K a and iodine K a , background, which is mainly scattered white radiation, varies greatly with angle, so that a value a t the cesium K a angle would have to be obtained by interpolation. Supporting the soundness of this approach is the fact that results for rubidia were identical whether individual background measurements were made or a common zero correction from a synthetic blank was used. The working curve for cesia, using synthetic standards with a quartz matrix, is illustrated in Figure 1. As the curve for rubidia is a straight line, only one standard and blank were required. The curve for cesia is completely empirical; its curvature may be due to incomplete resolution or to excitation of iodine by cesium K,!?. Replicate counts had about twice the range expected from counting statistics alone. The precision indicated was 10% of the amount present or 0.1% for cesium and 0.02y0 for rubidium, whichever is larger. COMPARISON WITH OTHER ANALYTICAL METHODS
The lack of dependably analyzed natural materials, which necessitated the use of synthetic standards, also required the analysis of samples by independent methods to demonstrate accuracy. Gravimetric analyses (4, 5 ) involved such difficult and repeated
cannot show whether one method is more accurate than the others. The sensitivities of x-ray spectroscopy and flame photometry are about the same, and poorer than that of radioactivation. The time required for a large series is about 1 hour per sample for cesia and rubidia by x-ray spectroscopy, and 0.5 hour per determination by flame photcmetry. The three methods are all reasonably accurate, and the choice among them will depend on convenience, cost, and required sensitivity. ACKNOWLEDGMENT
2
4
Yo
cs,o
6
8
Figure 1. Relation of intensity ratio to per cent cesia
separations that their accuracy-probably the greatest possible by such methods-was questioned even by the chemist who made them, and they were of little use for comparisons. J. I. Dinnin, U. S. Geological Survey, cooperated by developing a flame photometric method and analyzed many samples, and A. A. Sniales and AI. J. Cabell, Atomic Energy Research Establishment, Harwell, England, analyzed eight samples by radioactivation. Results of analyses by two or more methods are compared in Table 11. Agreement is within the precision of the x-ray spectroscopy. The comparison does not indicate that any method is less accurate than the other two, and
The authors gratefully acknowledge permission to use the analytical results of J. I. Dinnin, U. S.Geological Survey, and of A. A. Smales and ha. J. Cabell, Atomic Energy Research Establishment a t Harwell. Inspiration and samples were provided by IT7. T. Schaller and R. E. Stevens, and help in the actual measurements by Michael Milton, all of the U. S. Geological Survey. LITERATURE CITED
(1) Adler, I., NoTelco Reporter 3, 54-7 (19,56). \ - - - - ,
(2) Adler, I., Axelrod, J. N., ANAL. CHEM.27, 1002-3 (1955). (3) hdler, I., Axelrod, J. hl., Spectrochkn. Acta 7, 91-9 (1955). (4) Stevens, R. E., Am. Xineralogist 23, 607-28 (1938). ( 5 ) Stevens, R. E., Schaller, R. T., Ibzd., 27,525-37 (1942).
RECEIVED for reviem January 24, 1957. Accepted April 22, 1957. Publication authorized by the Director, U. S. Geological Survey.
Reaction of Oxygen in a Mass Spectrometer to Form Carbon Monoxide G. F. CRABLE and N. F. KERR Gulf Research & Development Co., Piffsburgh, Pa. ,An investigation has been made of the formation of carbon monoxide by the reaction of oxygen with carbonaceous materials in the ion chamber of a mass spectrometer. Carbon monoxide production was directly proportional to the oxygen partial pressure in pure oxygen samples and blends containing oxygen. The carbon monoxide is believed to be formed by oxygen decarbiding the tungsten filament.
C
was formed by the reaction of oxygen with carbonaceous materials in the ion chamber of a Consolidated Electrodynamics Corp. m a s spectrometer, a modified Model 21-102. When the earlier ion sources used with this instrument were replaced with the Isatron, the mass 12 and 28 peaks observed in oxygen calibration runs became large enough to instigate an investigation of their origin. -4 typical mass spectrometer ARBON MOXOXIDE
analysis of a sample of oxygen shown to contain less than 0.06 mole yo by an infrared examination resulted in a calculated value of 5 mole % of carbon monoxide. A series of oxygen runs was made a t different oxygen pressures in the inlet reservoir of the mass spectrometer. These results are shoivn in Figure 1 after background and carbon dioxide contributions to the mass 12 and 28 ion currents were removed. The ratio VOL. 29,
NO. 9 , SEPTEMBER 1957
1281
Table
1.
Mass Spectra of Oxygen Runs at a Series of Ion Source Temperatures
Heater Connected Disconnected
e
Temp., O
c.
250 a
198 192 186
mle 28 127. i div. 49.8 51.5 52.4 54.3
Corrected m/e 28
127.7 div. 49.8 48.5 48.4 49.8
Run made immeditrtely after disconnecting heater.
of the mass 12 t o 28 ioii currents was in good agreement with the ratio for pure carbon monoxide. Although carbon dioxide was found in all runs, the maximum concentration was always less than 0.2%. The linear relation found for the mass 28 ion current, or the carbon monoxide produced, and the oxygen pressure is what would be expected for the formation of carbon monoxide by the reaction of oxygen molecules striking a heated surface containing carbon. Spectra obtained from blends of oxygen and argon and blends of oxygen and n-butane showed the same relationship between carbon monoxide formation and the partial pressure of oxygen as the pure oxygen runs. If carbon monoxide forms on the filament by the reaction of oxygen with carbon, probably in the form of tungsten carbide, an increase in carbon monoxide formation should occur with an increase in the carbon on the filament. Oxygen runs made before and after a 2-hour treatment of the filament with 2butene showed an increase in the carbon monoxide to oxygen ratio of approximately 20%. An oxygen spectrum obtained with a new and untreated filament in the ion source had a carbon monoxide to oxygen ratio 14% lower than that of a treated filament which had been in use for some time. I n addition to the ion source filament there were two other possible sites for the production of carbon monoxide from oxygen, the heater and the ion source block. The heater is a small coil of Nichrome wire heated electrically to maintain the ion chamber a t a temperature of 250" C. while the ion source block refers to the surfaces surrounding the ionizing region. To determine the role of the heater, an oxygen run was made at the normal ion source temperature of 250" C.; then the heater was disconnected and a second oxygen run was made immediately. With the heater disconnected, the carbon monoxide to oxygen ratio dropped 62.4%, indicating that the major portion of the carbon monoxide was formed either on the heater or on a surface very near the heater. 1282
ANALYTICAL CHEMISTRY
The effect of the source block temperature on carbon monoxide formation w&s studied by disconnecting the heater and allowing the ion source to cool. Oxygen spectra were obtained
after disconnecting the heater. Carbon monoxide molecules formed in the ion source were assumed to be in no more favorable position for ionization than molecules entering the ion chamber through the leak. The decrease in the observed carbon monoxide was calculated as an increase in the oxygen partial pressure in the inlet system on the assumption that one molecule of oxygen produced one molecule of carbon monoxide. The calculated increase in the m/e 32 ion current was 60.1 divisions as compared with the observed increase of 60.0 divisions. This result indicates that the correct expression for the reaction is X C O2 + X O CO where X represents the heated metal surface. It is expected that the introduction of any highly oxygenated and reactive
+
+
/+32 /O
/'
/J2*
7" /o
O X Y G E N PRESSURE (MICRONS)
Figure 1 . Oxygen spectral data obtained from runs made at a series of oxygen pressures
a t several temperatures as measured by a thermocouple built into the ion source. To correct the observed data for temperature effects on the ion currents, as reported by Fox and Hipple (Z), Stevenson (3), and Berry ( I ) , pure carbon monoxide was run a t a series of source temperatures. From a plot of these data as a function of !P1I2 correction factors were obtained to convert data observed a t any temperature to equivalent data of 250' C. Data from oxygen runs made with the heater disconnected and a t a series of ion chamber temperatures are shown in Table I. The corrected m / e 28 data show little change, indicating that the source block plays no part in the reaction. A material balance was calculated for the oxygen runs made before and
compounds, such as the oxides of nitrogen or sulfur, into a similarly designed ion source containing a carbided filament could result in contamination of the original material and result in spurious analytical results.
LITERATURE CITED
(1) Berry, C. E., J. Chem. Phys. 17, 1164 (1949). (2) Fox, R. E., Hipple, J. A., Zbid., 15, 208 (1947). (3) Stevenson, D. P., Zbid., 17,101 (1949).
RECEIVED for review June 8, 1955. Ac-
cepted April 18, 1957. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 28-March 4, 1955.
