Anal. Chem. 1981, 53, 549-550
analyzed with even greater accuracy. Sampling times for the preparation of samples from dilute solutions would be longer. The method could be extended to include analysis of SOlutions containing precipitates provided that filtration is performed before analysis or a suitable solvent is found for the water-insoluble fraction. The use of solvents other than water is also currently being studied to determine if finer dried aerosols can be successfully generated, which would further minimize the need for particle size corrections.
LITERATURE CITED ( 1 ) Berth, E. “Principles and Practice of X-ray Spectrometric Analysis”,
2nd ed.; Plenum: New York, 1975; p 763. (2) h u m , R. M.; Wlllls, R. D.; Walter, R. L.; Gutnecht, W. F.; Stiles, A. R. I n “X-ray Fluorescence Analysis of Environmental Samples”; Dzubay, T. G., Ed.; Ann Arbor Science: Ann Arbor, MI, 1977; pp 165-173.
549
(3) May, K. R. Aerosol SC/. 1973, 4, 235-243. (4) Wagman, J.; Bennett, R. L.; Knapp, K. T. “X-Flay Fluorescence MUMSpectrometer for Rapid Elemental Analysis of Particulate Pollutants”, Report No. 60012-76-033; Environmental Protection Agency: Washington DC, 1976. (5) O’Connor, 6. H.; Kerrljan, G. C.; Hlnchliffe, P. X-Ray Spectrorn. 1977, 6,83-85, (6) Hansen, L. D.; Ryder, J. F.; Mangelson, N. F.; Hill, M. W.; Faucette, K. J.; Eatough, D. J. An&d. Chern. 1980, 52, 821.
RECEIVED for review September 26,1980. Accepted December 1, 1980. This work was funded by the U S . Environmental Protection Agency under Contract 68-02-2566. The authors thank the Particulate Emissions Research Section, Stationary Sources Emission Research Branch, U S . Environmental Protection Agency, Research Triangle Park, NC, for their continued support of this project.
Preparation of High-Purity Volatile Acids and Bases by Isothermal Distillation Claude Velllon’ and D. C. Reamer USDA, SEA, Human Nutrition, BeltSvill8 Human Nutrition Research Center, Room 2 15, Building 307, Beltsviile, Maryland 20705
Several years ago, Robert Alvarez of the U S . National Bureau of Standards taught me a very simple, inexpensive, and effective way to purify volatile acids and bases by isothermal distillation. This procedure was never really described in any publication. The only references to the method which we have been able to find are in two Alvarez publications ( I , 2), where it is simply mentioned and not described in any detail. Also, we have never seen any analytical data published regarding the effectiveness of the method. Over the years, we have used this procedure with much success. Visitors to our laboratory are constantly amazed and impressed by this procedure, and it is evident that knowledge of it is not widespread. Consequently, we wish to describe the technique and present some data and observations we have accumulated. This purification method should prove valuable to many laboratories engaged in trace metal analysis a t very low concentration levels. Our experiences with this method are confined mainly to volatile acids and bases, such as hydrochloric acid, ammonia, and acetic acid, but particularly HCl. Each of these, in concentrated form, exhibits an appreciable vapor pressure at room temperature. The basis of the method is extremely simple. If a beaker of the concentrated acid or base is placed in a closed container along with a beaker of water, acid or base from the concentrated solution will “isothermally distill” and dissolve in the water until an equilibrium is reached. Theoretically, the equilibrium point would be where both concentrations are equal, but in practice the rate of transfer slows greatly as equilibrium is approached. The method is capable of preparing extremely pure acids or bases, based on trace metal content, primarily due to the facts that: water can easily be prepared in very high purity by ion exchange demineralization; there is almost no possibility of physical transfer of impurities from one container to another; and the system is essentially closed, thereby reducing airborne contamination.
PROCEDURE The procedure we have used for purifications is as follows. A large glass vacuum desiccator with porcelain plate is used as the closed system. The flange is not greased, and the vacuum port is opened slightly to prevent pressure buildup from lifting the desiccator cover. Only new, polypropylene beakers are used for reagent containers. These are cleaned by soaking in a 2% solution of detergent/decontaminant (“Isoclean”, Isolab, Inc., Akron, OH) for several hours, followed by thorough rinsing with ultrapure
18-MR water (3). The desired concentrated reagent grade acid or base is placed in one container and ultrapure water in another, and allowed to stand in the desiccator for the desired time. The product concentration can be increased by changing the crude material several times. Both the rate of concentration increase and the final product concentration can be raised by having a larger crude volume than the product volume. In the case of acetic acid, glacial acetic acid is used, and a substantial volume change occurs, in that some of the water distills into the acid.
RESULTS AND DISCUSSION Hydrochloric Acid. Our greatest amount of experience with purification by isothermal distillation is with HC1. We routinely use this acid to dissolve ashed samples of biological materials for trace metal analysis, particularly for zinc, chromium, and selenium. With the system described above, when equal volumes of concentrated HCl (11.3 M) and water in identical beakers (equal surface area) are equilibrated, the product molarity increase vs. time is as shown in Figure 1 (at a temperature of approximately 26 “C). Here, small aliquots were removed periodically and titrated against standardized 1 M NaOH. The slight deviations from a smooth curve are believed due to fluctuations in the room temperature. At the end of the 13.5h time period, the molarity of the crude HC1 had dropped to about 6.2 M, while the product reached about 5.1 M. When equal volumes of HCl and H 2 0 are used as before, but the crude HC1 replaced every day, the results shown in Figure 2 are obtained. A much sharper rise in product molarity to nearly that of the crude HC1 is seen. A similar effect is observed when the crude HC1 volume greatly exceeds that of the product. For example, when one beaker of water and five beakers of crude HC1 are equilibrated (Le., HC1 volume and surface area is 5 times that of the HzO) the results shown in Figure 3 are obtained. Here, the crude HC1 was not changed each day. Purity. Over the years, we have had occasion to check the purity of the acids and bases prepared in this manner with respect to several trace elements by extremely sensitive spectroscopic methods, as well as radiotracer studies. By use of an extremely sensitive atomic fluorescence system (4,5), Zn, Cu, and Bi were found to be less than 0.0003,0.03, and 1 ng/mL, respectively, in the product. By use of a low-pressure microwave-induced plasma emission spectrometer (6-8), product concentrations of Cd,
This article not subject to U S . Copyright. Published 1981 by the American Chemical Society
550
Anal. Chem. 1981, 53, 550-552
r
TIME, DAYS
Concentration increase with time for equal volumes and surface areas of HCI and H20. Figure 1.
r
I
10>-
k 8a a J
P o I3
n
0
a a
I
I
I
I
2
3
4
I
TIME, DAYS
Flgure 3. As in Figure 1, but HCI volume and surface area Sfold greater than that of H20.
ferred to the product. In the latter experiment, the spike amounted to 4 pg of 75Sewith 7 ng of 75Setransferred after 7 days. Similar experiments were performed with esZn and SICr. After 7 days, no detectable counts above background were observed in the product. This purification method is simple, inexpensive, and very effective. Since the system can operate unattended, the relatively slow rate is not a serious disadvantage. Batches can be in progress continually, and no harm will come if left for long time periods. With modest control and timing, batches with reasonably well-known concentrations can be prepared. In our hands, this method has worked extremely well, and it is hoped that it will benefit others having a need for these reagents in high purity with respect to trace metals.
T I M E , DAYS
Flgure 2. As
LITERATURE CITED
in Figure 1, but HCI renewed daily.
Zn, Pb, Fe, Co, Mg, Mn, Cu, and As were found to be less than 0.5, 0.3, 10, 2, 7, 0.05, 0.3, 0.2 and 10 ng/mL, respectively. By use of a combined gas chromatograph/mass spectrometer system and stable isotope dilution techniques (9),and a continuum source, echelle monochromator, wavelengthmodulated atomic absorption spectrometer with graphite furnace (IO),products were found to contain less than 0.05 ng of Cr/mL. While collecting the HC1 molarity data shown in Figures 1-3, we also performed radiotracer studies with ‘%e, @Zn,and W r . Placing 60-mL crude HCI spiked with 75Seand 60 mL of H20 in the desiccator and equilibrating for 2.5 days resulted in approximately 0.1% of the Se being transferred to the product. The initial spike had a count rate of 225200 counts/min, as measured with a well-type y spectrometer, and the product had 268 net counts/min above background. On repetition of the experiment with a 527 400-counts/min spike and a 7-day equilibration time, 0.18% of the Se was trans-
(1) Alvarez, R.; Paulsen, P. J.; Kelleher, D. E. Anal. Chem. 1969, 47, 955-958. (2) Paulsen, P. J.: Ahrarez, R.; Kelleher, D. E. Spectrochim. Acta, Part 8 1089, 248, 535-544. (3) Velllon, C.; Vallee, 8. L. “Methods In Enzymlogy”; Academic Press: New Yo*, 1978; Vol. 54, pp 448-484. (4) Murphy, M. K.; Clyburn, S. A.; Velllon, C. Anal. Chem. 1073, 45, 1488- 1473. (5) Clyburn. S. A.; Serlo, G. F.; BartschmM. B. R.; Evans, J. E.; Vellbn, C. Anal. Blochem. 1975, 83, 231-240. (8) Kawaguchl, K.; Vallee, B. L. Anal. Chem. 1075. 47, 1029-1034. (7) Atsuya, I.; Kawaguchl, H.; Velllon, C. Val&, B. L. Anel. Chem. 1977. 49, 1489-1491 (8) Atsuya, I.; Alter, G. M.; Velllon, C.; Vallee, E. L. Anal. 8bchem. 1977, 79,202-211. (9) Veilion, C.; Wolf, W. R.; Cuhrle, B. E. Anal. Chem. 1979, 57, 1022-1024. (10) Guthrie, 8. E.; Wolf, W. R.; Velllon, C. Ana/. Chem. 1978, 50, 1900- 1902.
RECEIVED for review September 8,1980. Accepted November 7,1980. D.C.R. is Research Associate, Children’s Hospital, Boston, MA, and is supported in part by General Cooperative Agreement No. 58-32U4-0-127.
Townsend Discharge Ionization Probe for a Mass Spectrometer Ralph C. Scheibel, 0. P. lanner, and Karl V. Wood” Monsanto Company, 800 North Lindbergh Blvd., St. Louis, Missouri 63166
Townsend discharge (TD) ionization for mass spectrometry has become an increasingly important ionization technique 0003-2700/81/0353-0550$01 .OO/O
in recent years, particularly with the growth of negative ion chemical ionization (1-7). The Townsend discharge is used 0 1981 American Chemical Society
