ning or photoplate techniques. A variety of similar volatile samples has been run routinely. It is not necessary to lower the ion source temperature for such samples. The 70-volt mass spectrum of pyruvic acid, obtained a t different ion source temperatures (Figure 2j, indicates the catalytic decomposition of this compound which can occur at elevated temperatures. This behavior would make a conventional metal reservoir system unsatisfactory for such a compound. The spectra shown were obtained by varying the ion source temperature while maintaining a constant sample temperature of -20’ C. and a constant
source pressure. Cooling the ion source from 200’ to 130’ C. and using the same sample restored the normal pyruvic acid spectrum (Figure 2Aj. These data show that significant transfer of thermal energy between the source walls and the molecular beam can occur, as has been noted previously (3). For samples that decompose at ambient temperatures it is necessary t o use an ion source of open construction, such as the source of the Bendix T-0-F spectrometer. The low temperature probe has general applicability for obtaining mass spectra from submicrogram volatile samples in a convenient fashion, avoiding thermal and catalytic effects on the
sample encountered with conventional inlet systems, and studying the effects of temperature on samples and on mass spectra over a wide temperature range. LITERATURE CITED
&I., Baitinger, TV. E., ICIcLafferty, F. W., ANAL.CHEM.
(1) Amy, J. W., Chait, E.
37, 1265 (1965). (2) McGee, H. A., hlalone, T. J., Martin, W. J., Rev. Sci. Instr. 37, 561 (1966). (3) Spiteller-Friedmann, hl., Eggers, S., Spiteller, G., Monatsh. Chem. 95, 1740 (1964). PRESENTED in part at the 14th Annual hIeeting on Mass Spectrometry, ASTILL E-14, Dallas, hIay 1966. Work supported by a research grant from the National Institutes of Health (GM 12’7%).
Sample Preparation for Low-Level, Alpha-Particle Spectrometry of Radium-226 Bernard Keischl and Arnold S. Levine, Nuclear Science & Engineering Corp., Pittsburgh, Pa.
PARTICLE
spectrometry with solid-state detectors is an excellent method for measuring low levels of alpha radioactivity (1j because very low backgrounds can be obtained by observing only those energies of interest. However, in order to obtain meaningful spectra, sample thickness must be kept to a minimum. In our laboratories, attempts to prepare “weightless” counting samples of radium-226 by means of removing all other solids from a solution and evaporating the solution on a counting planchet were unsuccessful. At best, small deposits of solid material weighing several milligrams were obtained, even when using ion-exchange methods to purify the solution. Because of the low levels of activity involved, reduction of solids merely by taking small aliquots was unacceptable. Subsequently, a method was developed in which the radium-226 was coprecipitated with a very small mass (-0.1 t o 0.2 mg.) of BaS04 and filtered on a membrane filter. By using this method not only were good spectra and excellent radium-226 recoveries obtained, but in addition the descendant activities could be allowed to grow in because of excellent radon retention. In effect, this resulted in quadrupling the overall efficiency of the measurements. PROCEDURE
In our case, the sample solutions contain macroquantities of lead in a dilute nitric acid solution. A series of steps to remove the lead via chloride precipitation and sulfide precipitation precedes the final steps in the preparation of the counting sample. In other Present address, hIellon Institute, Pittsburgh, Pa. 15213.
25
15236
R
20
-
0) C
6
r
15
V \ VI e
C
2 1c
V
E
0 Pulse Height Units
Figure 1. Alpha-particle spectrum of descendants
cases, any other metal ions capable of precipitating as the sulfate would have to be removed. The resulting leadfree, dilute nitric acid solution of about 20 ml. in volume is then evaporated to a small volume, transferred to a 5ml. beaker, and evaporated just to dryness. Two to three milliliters of 1N nitric acid are added to redissolve the material, and 0.1 mg. of barium carrier (as the nitrate) is added. The solution is warmed, a few drops of 10% sulfuric acid are added to it, and the mixture is digested for 15 niinutes at a temperature just below boiling. After cooling, the solution is filtered through a membrane filter (1.2-micron pore size) in a special holder, and the filter is then mashed with a few milliliters of 1% sulfuric acid and a few milliliters of water. The holder consists of a filter-support disk (-25 mm. in diameter), to which the edges of the filter are lightly ce-
Ra-226 and
its
mented, and a chimney which fits closely over the disk. The filter support and chimney are products of Control Molding Corp., Staten Island, N. Y. The all-but-invisible precipitate of Bas04 is collected on the surface of the filter, and self-absorption of the emitted alpha particles is thus minimized. After drying under a heat lamp, the filter on its support disk may be stored for a t least two weeks if it is desired to allow the ingrowth of the radon, polonium, and bismuth descendants to approach equilibrium. An alpha-particle spectrum shows the four main peaks (Figure 1): Ra-226 a t 4.8 Mev., a t 5.5 Mev., Po-218 a t 6.0 Mev., and Po-zld a t 7.7 Mev. RESULTS
The observed counting rates for each peak in the alpha spectrum of a typical low-level radium standard (prepared VOL 38, NO. 13, DECEMBER 1966 e
1969
from a National Bureau of Standards solution standard) were as follows: 3.8 i 0.2 c.p.m.; Em-222, 3.9 i 0.2 c.p.m.; Po-218,3.7 0.2 c.p.m.; 3.8 i 0.2 c.p.m. Based upon. the initial activity of the radium used, the overall efficiency including chemical recovery and counting geometry was an average of 17.2% for each peak for this detector. Because this is within 5oj, (relative) of that
obtained with a source of known alpha activity with a similar counting geometry, the chemical recovery of the radium is assumed to be approximately 95%. Following some improvements in the source-detector geometry and using a 450 sq. mm., silicon-surface-barrier detector (Oak Ridge Technical Enterprises Corp., Oak Ridge, Tenn.), the overall efficiency for radium-226 measurement has consistently been on the
order of 85% with a typical background activity level of approximately 0.01 c.p.m. for the four peaks combined. LITERATURE CITED
(1) Yavin, A. I., de Pasquili, G., Baron, P., Nature 205, 899 (1965). WORKsupported by a grant from the National Gallery of Art, Washington,
D. C., in association with the National Gallery of. Art Research Project at Mellon Institute.
Decomposition of Organic Fluorine Compounds Using Sodium-Biphenyl Reagent Phyllis P. Wheeler and M a e I. Fauth, Research and Development Dept., U. S. Naval Propellant Plant, Indian Head, Md.
decomposition Of Organic fluorine compounds is often extremely difficult. Fusion with metallic sodium or potassium in a nickel bomb is often recommended as the most universally effective method (6). The oxygen flask combustion has also been applied successfully (3). Carbonbonded halogen may be converted to halide by treatment with a solution of the diphenyl-sodium-dimethoxyethane complex (sodium-biphenyl reagent) (1, 4). Because both the fusion and the oxygen flask methods require heating the sample, the sodium-biphenyl method is more convenient and less hazardous. Chambers et al. (8) found that there is a limiting vapor pressure above which this technique cannot be used without modification. UAKTITATIVE
EXPERIMENTAL
Materials and Reagents. Sodiumbiphenyl reagent (Southwestern Analytical Chemicals) was purchased (ts the premixed solution in 15-ml. vials. Toluene and isopropyl ether (Rlatheson, Coleman and Bell) were used without further purification. 1,2=-Dibromoperfluoropropane, perfluorodimethylcyclobutane, and 3,6dioxa - 2 - hydryl . perfluoro - 5methylnonane were obtained from the DuPont Co.
Procedure. A weighed sample of a size expected t o yield 1 to 10 mg. of fluorine was dissolved in an inert solvent in a 125-ml. separatory funnel. Solid and high-boiling liquid samples were weighed in gelatin capsules. Low-boiling liquids presented some difficulty. Weighing the sample in a glass capillary with the open end drawn to a fine tip proved successful with some substances. The capillary was then broken under the surface of the solvent. A vial containing 15-ml. of the sodium-biphenyl reagent is emptied into the separatory funnel containing the sample dissolved in toluene or isopropyl ether. After two minutes, excess reagent is destroyed by 2 ml. of isopropyl alcohol. The fluoride is then extracted with four 20-ml. portions of water and titrated with 0.04N thorium nitrate using sodium alizarin sulfonate as the indicator (6). RESULTS AND DISCUSSION
Several types of fluorine-containing organic compounds were analyzed. These include organic acids, halocarbons containing bromine, ring compounds, phenylmethane derivatives, and one compound, 1,l-difluorourea, in which N-F bonds occur. Data for these compounds are given in Table I. Results for research compounds which have not been completely characterized are presented in Table 11.
20640
Table 11. Results Obtained for Research Compounds
Formula
Fluorine, % , Calcd. Found
The method is applicable to a wide variety of organic compounds and may be used for compounds containing elements other than carbon, hydrogen, and fluorine, such as other halogens, nitrogen, sulfur, and oxygen. The fact that the method is suitable for the decomposition of 1,l-difluorourea indicates that it may be used for classes of fluorine compounds other than those containing only C-F bonds. The principal limitations of the method are low solubility of some compounds in the types of solvent required and handling problems arising from high volatility of sample. For those compounds which are free from the above limitations, the sodium-biphenyl decomposition of organic fluorine compounds is a rapid, convenient, and safe method of preparing the sample for subsequent determination of fluorine. LITERATURE CITED
(1) Bennett, C. E., Debbrecht! E. J.,
Table 1.
Fluorine Analysis of Various Classes of Organic Compounds
Compound p-Fluorobenzoic acid 1,l-Difluorourea 1,2-Dibromoperfluoropropane
Perfluoropropanoic acid
Formula CeHdFCOOH NH2CONF2 CF&FBrCFzBr CFsCFzCOOH
CFsCF&F( CFs)Ck( CFa) C&?&HZC~E Trisrpent afluoropheny1)-methane ( C~FS)&H Phenylpentafluoro henylmethane C ~ H ~ C H ~ C B F ~ Perf3uorodimethylcyclobutane Bis( entafluoropheny1)-methane
3,6-Dioxa-Z-hydry~perAuoro-j-
methylnonane
1970
ANALYTICAL CHEMISTRY
Fluorine, 70 Calcd. Fourid 13.56 13.68 39.57 38.38 36.79 37.01 57.91 57.87 56.87 56.61 75.98 54.57 55.43 36.80
75.64 53.96 55.65 37.03
CFsCF&F*OCF(CFs)CFzOCHFCFa 71.45 71.27
131st Meeting ACS, Miami, Fla., April 1957, Abstracts, 24B. (2) Chambers, R. D., Musgrave, W. K. R.,Savory, J., Analyst 86, 356 (1961). (3) Fernandopulle, M. E., Macdonald, A. Rf. G., Microchem. J . 11, 41 (1966). (4) Johncock, P., Xusgrave, W. I(. R., Wiper, A., Analyst 84, 245 (1959). (5) Ma, T. S., ANAL. CHEW30, 1557 (1958). (6) Steyermark, A., "Quantitative Qrganic Microanalysis," 326-32, 2nd ed. Academic Press, New York, 1961. Division of Analytical Chemistr 152nd Meeting, ACS, New York, N. %!,1966. Research supported by the Foundatlonal Research Program of the Naval Ordnance Systems Command.
