The precision of the method was established by analyzing solutions containing (a) 1 x IO-BM, (b) 1 X lO-’M, and (c) 5 X lO-5M gold(1) cyanide. From seven 1-liter aliquots the standard deviations obtained were (a) 0.07 X 10-8 and (b) 0.01 X lO-7M and from five 20-ml aliquots the standard deviation for (c) was 0.05 x 10-6M. The accuracy of this method was gauged by comparison with the fire assay technique (Table 11). A duplicate determination may be performed within 10 minutes. The method is, in fact, so simple and rapid that it has been used as the basis for an automatic “on line” analyzer of gold-bearing cyanide waste solutions (IO). RECOMMENDED PROCEDURE
The most favorable spectrophotometric response, giving absorbance readings between 0.2 and 0.7 unit, may be obtained by selecting a suitable volume ratio of aqueous to organic phases for the solvent extraction of the gold. When diisobutyl ketone is used for this purpose, this ratio should be such that the concentration of gold in the organic phase lies within the range of 4 to 14 X 10-5M. However, adequate results may be obtained if this concentration range is expanded to 1 to 18 x 1 0 - 5 ~ . For most purposes a 1 solution of Aliquat 336 in diisobutyl ketone will be adequate for the extraction of the gold from the aqueous phase. A quantity sufficient for the determination of the gold may be collected if as little as 5 ml of (10) R. J. Calteaux, D. E. Naude, and A. J. Rossouw, Corner House Laboratories, P. 0. Box 1056, Johannesburg,private communica-
tion (1967).
the diisobutyl ketone solution is used for the solvent extraction. The required volumes of the aqueous gold-bearing solution and the diisobutyl ketone solution are agitated in a separating funnel for about 1 minute; when small volume ratios are used, complete extraction of the gold occurs within 15 seconds. After the phases have separated, the ketone layer is collected and aspirated into the flame of the atomic absorption spectrophotometer. Because absorbance readings are affected by slight changes in the flame, and by instrument settings, a series of standard solutions containing gold concentrations of the same order as those in the test solutions should always be used to construct a calibration curve. These standard gold solutions, made up to about 0.1M NaC1, may conveniently be prepared by dilution of a stock standard gold solution containing about 10-2M gold(1) cyanide. The solutions of gold in diisobutyl ketone, as prepared by this solvent extraction of the standard aqueous solutions, are stable, and may be retained without deterioration for calibration of the atomic absorption spectrophotometer at any time. ACKNOWLEDGMENT
The author is indebted to R. A. J. Wixley of the Mathematical Statistics Division of the Chamber of Mines of South Africa, Research Organisation, for devising the replicate statistical designs. He also thanks P. J. Lloyd for suggesting Aliquat 336 as an extractant of gold(1) cyanide and I. C. Stein, who was responsible for much of the experimentation. RECEIVED for review January 8, 1968. Accepted February 5, 1968. Permission to publish this manuscript granted by the Chamber of Mines of South Africa.
Direct Mass Spectrometric Determination of Atmospheric Carbon Dioxide Ernest E. Hughes and William D. Dorko Diaision of Analytical Chemistry, Institute for Materials Research, National Bureau of Standards, Washington, D. C . 20234 The direct mass spectrometric determination of atmospheric carbon dioxide cannot be performed with any useful degree of accuracy because of the high concentration of oxygen in the atmosphere. By removing the oxygen with phosphorus, it is possible to admit sufficiently high pressures of the remaining gas to determine the carbon dioxide with an inaccuracy of less than 1%. Because the argon content of the atmosphere is presumed constant, a direct comparison of the carbon dioxide to argon ratio should provide a fixed reference for future surveys. The method is rapid and requires little equipment other than a mass spectrometer. Results obtained on analysis of rural air in the vicinity of Washington, D. C., are presented.
THE DIRECT MASS SPECTROMETRIC DETERMINATION Of atmospheric carbon dioxide cannot be performed with an error of much less than 30% using conventional techniques. The reasons for this lie in the fact that inlet pressures sufficiently 866
ANALYTICAL CHEMISTRY
high to allow an accurate measurement of the ion current caused by carbon dioxide result in such high pressures of oxygen that filament failure can occur. Also, carbon dioxide is generated in the ionization region possibly through reaction between oxygen and the coating of tungsten carbide on tungsten filaments resulting in erroneously high results. It is possible to determine carbon dioxide in nitrogen by direct mass spectrometry employing inlet pressures considerably higher than those normally employed ( I ) . If oxygen is first removed from atmospheric air, it should be possible to employ a similar technique for the direct determination of atmospheric carbon dioxide without the need for concentration techniques or other techniques which might alter the carbon dioxide concentration. The method described here utilizes phosphorus to remove (1) E. E. Hughes and W. D. Dorko, ANAL.CHEM.,40,750 (1968).
Table I.
Variation of Sensitivity Factor for Carbon Dioxide Compared to Sensitivity for Argon Date 10/23 10/24 10/26 10/27 10/28 10/31 11/1 11/2 11/3 11/6 11/8 11/9
Sensitivity in scale division per micron 0.8253 0.8281 0.8234 0.8241 0.8344 0.8310 0.8417 0.8302 0.8351 0.8271 0.8248 0.8263
oxygen and, instead of comparing the carbon dioxide to nitrogen, it is compared to the argon in the atmosphere. EXPERIMENTAL
Samples of atmospheric air are collected in a bulb similar to that used in the determination of atmospheric oxygen (2). In this case, however, only phosphorus is suitable for the removal of oxygen from air. The bulb contains about 3 or 4 grams of white phosphorus with which the oxygen in the sample reacts immediately. Before the sample is drawn into the flask it is drawn through a filter of 0.2 p pore size to remove any solid organic particles which might react with oxygen to give carbon dioxide in the flask and then through a column of magnesium perchlorate in order to remove water vapor. This is necessary because high concentrations of water vapor cause the mass spectrometer to recall previous organic samples, probably by displacing adsorbed materials from the walls of the system. It has been observed that the mass numbers corresponding to C3 hydrocarbon are particularly enhanced when wet samples are introduced, thereby interfering with the accurate measurement of the essential ion currents at masses 40 and 44. Further, water vapor, if not removed, would react with the oxides of phosphorus formed in the bulb. Because the bulbs are used for many samples without changing the phosphorus, a layer of phosphoric acid would soon accumulate on the surface of the phosphorus. It had been observed that the presence of small amounts of the acid on the unreacted phosphorus greatly reduced the speed of reaction. The reaction vessels are made by enlarging the bulb below the barrel of a right angle stopcock and sealing it to a 200ml round bottom flask. The tightly packed glass wool plug in the base of the stopcock is included to prevent the inadvertent entrance of solid oxides of phosphorus into the mass spectrometer inlet system. The phosphorus is prepared by melting stick white phosphorus under hot water and separating it into globules small enough to slip through the bore of the stopcock. The flask is flushed with nitrogen through a small tube inserted in the bore while the phosphorus is added. After sufficient phosphorus has been introduced, the stopcock plug is inserted and the flask is evacuated. The flask is then gently heated with a soft flame until the phosphorus melts and the water which accompanies it is evaporated. The flask must be preconditioned by reacting several samples of air. Apparently some organic material is present either in the phosphorus or as a residue in the flask and the first results are usually (2) E. E. Hughes, Enuiron. Sci. Tech., 2, 201 (1968).
high by as much as a factor of three. The preconditioning is achieved simply by admitting air, allowing a short time for the oxygen to react, and then evacuating the flask. After repeating the procedure three times, the flasks are ready for use. After collecting the sample of atmospheric air in the flask, it is attached to the spectrometer and a portion of air is admitted to the inlet system at a pressure of about 1 mm. The mass region from 40 to 44 is scanned at a slow rate. The measured ion currents at masses 40 and 44 are converted to pressures by the division by a suitable factor called the sensitivity. The sensitivity is defined as the instrument response in scale divisions, or other units, per unit of pressure of the substance in the inlet system. Not only are the sensitivities of argon and carbon dioxide different but the difference betWeen them may differ from day to day. The reasons for these differences are not clearly understood but probably arise from changes in the character of the surface of the filament. Table I is an example of daily variations in the sensitivity for carbon dioxide when the sensitivity for argon is taken to be 1.000. Errors caused by these variations were avoided by a daily calibration with a mixture of argon and carbon dioxide compounded at about their atmospheric ratio (1/0.03). The mixture was prepared by carefully weighing a quantity of each gas into a cylinder. The weights used were calibrated to within 1 part in 100,000-NBS Class “S” calibration. The use of a calibrating mixture such as this eliminates errors caused by differences in the sensitivity of the pure substance and their individual sensitivity in a mixture. It also eliminates error arising from minor differences in linearity of the recording system because the ion currents for the two peaks of interest are read under the same conditions whether the calibrant or the sample is being analyzed.
Table 11. Results of gravimetric and mass spectrometric analysis of a cylinder of air Carbon dioxide concentration, Method mole Gravimetric 0.0327 Mass spectrometric 0.0322 Mass spectrometric 0.0324 Mass spectrometric 0.0324 Mass spectrometric 0.0324
95 72
Confidence limit for average 0.0007 0.0001 o.Ooo1 0.0001
Determinations per sample
0.0001
5
10 10 10 10
After determining the pressure of both argon and carbon dioxide, the carbon dioxide content of the sampled air is determined by comparison with the argon. The value obtained using the ion current at mass 44 to determine carbon dioxide does not represent the total carbon dioxide in the atmosphere because it includes only the isotopes carbon-12 and oxygen-16. However, it is only these two mass numbers that are measured in the standard mixture thereby eliminating the need for adding all contributions from the isotopes, that is, masses 44, 45, and 46. In a situation where isotopic variations might occur it would only be necessary to add the ion currents at masses 45 and 46 to that at 44 in order to determine total carbon dioxide. The concentration of argon in the atmosphere is accepted as 0.9343 mol Z with a standard error of 0.0002 mol (3). Any carbon dioxide determination obtained by comparison with argon can always be adjusted if the accepted value, but not the actual concentration of argon, is revised. The concentration of “Ar on which these (3) E. Glueckauf, “Compendium of Meteorology,” T. F. Malone, Ed., American Meteorological Society, Boston, Mass., 1951, Chapter 1, p 4. VOL. 40, NO. 6, MAY 1968
e
867
measurements are based is 99.60 mol % of the total argon or a concentration in air of 0.9306 mol %. RESULTS
A cylinder of air was analyzed gravimetrically for its carbon dioxide content. Four samples from this same cylinder were then admitted to the reaction vessels and the ratio of the argon to carbon dioxide was determined. The transfer was affected using the manifold arrangement shown in Figure 1. This allows the sample to be transferred and the calibrants to be run without opening the system to the atmosphere. The results are shown in Table 11. The carbon dioxide content of the sample was further confirmed by comparison of the nitrogen to carbon dioxide ratio using National Bureau of Standards Standard Reference Material No. 1601 (0.0308 mol carbon dioxide in nitrogen) as a calibrant. The result was 0.0322 mol carbon dioxide with a standard error of 0.0003 mol The results indicate an inherent accuracy in the method which, when combined with its simplicity and the small and easily collected sample, would make it ideal for the analysis of samples collected outside of the laboratory. As a matter of further interest, 24 samples were collected in an isolated rural area well away from any source of combustion or motor vehicle traffic. All samples were taken at a height of 10 feet above ground and precautions were taken to prevent contamination by exhalations by the person while sampling. They were analyzed as described and the results are shown in Table 111.
x.
The variation in the results for a single sample is considerably less than that experienced from sample to sample, even of those in the same series. In view of the accuracy obtained on analysis of the cylinder air and because of the precaution taken to exclude organic dust, we must conclude that the variations are real. Because all four samples in
MASS SPECTROMETER INLET SYSTEM
x
U
E3
MERCURY BUBBLER ANDSAFETYVALVE
Figure 1. Manifold arrangement for admission of samples to reaction vessel and mass spectrometer
Table 111. Determination of Atmospheric Carbon Dioxide in Mole Per Cent Average
Standard deviation
limit for average
No
No No
0.03236 0.03207 0.03318 0.03748
0.0003 0.0002 0.0002 0.0003
0.0002 0.0001 0.0001 0.0001
Yes Yes Yes Yes
0.03207 0.03109 0.03135 0.03120
0.0002 0.0002 0.0001 0.0002
0.0001
10
o.Ooo1 O.ooOo9 0.0001
10 10 10
Clear, slight S.E.breeze
No
0.00009
No No
0.03562 0.03319 0.03391 0.03365
O.ooOo5 0.0001 0.0001
0.0002 0.0001 0.0003 0.0003
3 3 3 3
Clear, slight S.E.breeze
Yes Yes Yes Yes
0.03326 0.03370 0.03348 0.03240
0.0002 0.0001 O.ooOo8
0.0005 0.0003 0.0002 0.0002
3 3 3 3
Yes YeS YeS Yes
0.03988 0.03443 0,03448 0.03402
0.0001
0.0003 0.0002 0.0002 0.0005
3 3 3 3
Yes Yes Yes Yes
0.03201 0.03238 0.03293 0.03267
0.0001 0.0001
0.0002 0.0003 0.0002 0.0005
3 3 3 3
Conditions
10126
Clear, strong S. E.breeze
10127
10131
11/1
1112
11/3
868
Filter
Date
Clear, strong S.E.breeze
Heavy rain, no breeze
Clear, moderate N.E. breeze
ANALYTICAL CHEMISTRY
No
No
0.00009
0.00009 0.00009
0.0002
0.00009
0.0002
95
Confidence Determinations per Sample 10 10 10 10
any series were taken within minutes of each other, the differences must represent imperfect local mixing from sources of carbon dioxide unknown to the observers. CONCLUSION
A method has been presented which allows the rapid and accurate determination of atmospheric carbon dioxide. By comparing the carbon dioxide to nonvarying atmospheric
argon, a permanent index is available for future studies of variations in atmospheric carbon dioxide. Multiple results obtained by different methods on a single sample of compressed air in a cylinder indicate a high degree of accuracy for the method as well as an absence of systematic error. Samples can be conveniently collected in small containers and returned for later analyses. The sample size required is only a few milliliters.
RECEIVED January 18,1968. Accepted March 4,1968.
Trace Element Survey Analysis of Biological Materials by Spark Source Mass Spectrometry C . A. Evans, Jr., and G . H. Morrison Department of Chemistry, Cornell University, Ithaca, N . Y. The applicability of spark source mass spectrometry to the quantitative determination of trace elements in biological materials has been evaluated. The method is capable of providing information simultaneously on 50 or more trace elements in the concentration range from 100 ppm on down to a few ppb with reproducibilities on the order of *10-25% and with comparable accuracy when standards are employed. Applicability to the analysis of a variety of biological materials (blood serum, kidney tumor, lung tissue, bone, and plant leaves) is demonstrated. A low temperature ashing technique is employed to remove organic ion interference and a simple method of quantitation is described.
WITHTHE RAPiDLY GROWING AWARENESS of the importance of trace elements in biological processes, a strong need has developed for survey methods of analysis permitting the simultaneous determination of many elements with high sensitivity. To date, the only method that has been applied to the survey analysis of biological materials with reasonable success has been optical emission spectroscopy (I). More recently, neutron activation analysis coupled with gamma spectrometry employing lithium drifted germanium detectors (2) has shown some promise. Spark source mass spectrometry is one of the most sensitive and comprehensive techniques of trace analysis of inorganic systems (3); however, its applicability to biological materials has never been fully explored. Sasaki and Watanabe ( 4 ) have examined several different biological tissues employing a semiquantitative approach, and Wolstenholme (5) has reported some preliminary work on the determination of trace elements in dried blood plasma but the results of both of these studies are presumed to be accurate to only within a (1) W. H. Allaway, in “Trace Analysis: Physical Methods,” G. H. Morrison, Ed., Interscience, New York, 1965, p 67. (2) G. W. Smith and D. A. Becker, Proceedings of the Symposium on Nuclear Activation Techniques in the Life Sciences, Amsterdam, May 1967, Paper SM 91/61. (3) J. Roboz, in “Trace Analysis: Physical Methods,” G. H. Morrison, Ed., Interscience, New York, 1965, p 435. (4) N. Sasaki and E. Watanabe, Thirteenth Annual Conference on Mass Spectrometry and Allied Topics, St. Louis, Mo., 1965, p 474. ( 5 ) W. A. Wolstenholme, Nature, 203, 1284 (1964).
factor of three. Brown and coworkers (6)were able to obtain qualitative information on very small amounts of ashed human hair and synthetic fibers by a tipped electrode method. No detailed investigation has been made on the quantitative feasibility of the survey approach. This study demonstrates that spark source mass spectrometric analysis of biological materials is capable of providing trace element information simultaneously on 50 or more elements with reproducibilities on the order of +10-25x and with comparable accuracy when comparative standards are employed. Although elements present in high concentration can also be determined, emphasis has been placed on the determination of trace elements present in concentrations from 100 ppm on down to a fewppb. Thus, those elements such as P, Ca, K, S, and Na, which can be done easily by other methods were ignored. Applicability to the analysis of a variety of biological materials is demonstrated and includes such widely divergent materials as human blood serum, human kidney tumor, sheep lung, sheep metacarpal bone, and dried plant leaves. Because of the time required for analysis (2-6 hours) and the complexity of the technique, quantitative mass spectrometry is primarily a research tool and not presently amenable to mass production clinical analyses. EXPERIMENTAL
Mass Spectrograph. The experimental conditions for the mass spectrograph are given in Table I. Samples. A variety of samples from plant leaves to human blood and tissue were examined mass spectrographically. For the determination of precision, accuracy, and sensitivity discussed in this study, the following samples were used: freeze-dried sheep bone, sheep lung, and sheep liver obtained from the U. S. Soil and Nutrition Laboratory, Cornell University; and freeze-dried human kidney tumor and a pooled human blood serum from Dr. Lutwak, Sage Hospital, Cornell University. The samples were dried at 110’ C, and stored in a desiccator until ashed. (6) R. Brown, W. J. Richardson, and H. W. Somerford, Fifteenth Annual Conference on Mass Spectrometry and Allied Topics, Denver, Colo., 1967, p 157. VOL. 40, NO. 6, MAY 1968
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