V O L U M E 28, N O . 6, J U N E 1 9 5 6 of copper ions by copper dithio-oxamide. However, up to 40 p,p.m. of copper, this adsorption appears to be linear and reproducible. I n the preparation and use of several previous dithio-oxamide solutions, similar results were obtained. I n an ammonia-ammonium acet,at,esolut,ion whose pH was 9.6, the molarity obtained by titration of copper agreed more closely \Tith that calculated from the weight of dithio-oxamide. I n an alkaline solution a change in potential of about 300 mv. was observed during the titration. However, in alkaline solution, above 2 or 3 p.p.m. of copper, the determination is not very accurate or reproducible. This may be caused by adsorption of dithio-oxamide by the precipitate. I n any event the potentiomet,ric determination of copper in alkaline solution would be subject t o many more interferences. I n the reverse titration of neak solutions of dithio-oxamide in acid solution n-ith copper, the molarity of the dithio-oxamide as calculated from the n-eight taken agreed closelj- n-ith the molarity as calculated from the n-eight of copper required for the titration. T h e potentiometric method is rapid and as accurate and precise as colorimetric methods. Interferences are fen-er than in colorimetric methods and in some cases the method would be more applicable. OTHER APPLICATIONS
Using conditions similar to those used in the titration of copper with dithio-oxamide, and a glass electrode as reference, 100-ml. samples from 1 to 50 p,p.m. silver concentration rvere titrated with dithio-oxamide. Samples containing mercury(I1) ions in the concentration range from 5 to 35 p.p.m. were titrated similarly. K h e n the concentration of either t,he silver or mercury was plotted against milliliters of dithio-oxamide required
1049
for titration of 100-ml. samples, a straight line was obtained. The linear nature of the working curve indicated that a definite and reproducible relationship existed between the dithio-oxamide and the silver or mercury. From these results it rvould seem that the method may be satisfactori‘y applied to the determination of either silver or mercury. Using this method, it was possible to titrate mixtures of silver and copper and obtain two breaks in potential lvhich corresponded accurately to the amount of silver and copper present. Furthermore, it \\-as possible to eliminate completely the break in potential due to the copper (second brealcj by the addition of the disodium salt of (ethylenedinitri1o)tetraacetic acid. ACKNOWLEDGRZENT
The work described was supported in part by the Research Committee of the Graduate School from funds eupplied by the Kisconsin Alumni Research Foundat,ion. LITERATURE CITED
(1) Allport, X . L., Skriinshire, G. H., Quart. J . Pitarin. Pharmacol.
5, 461 (1932). ( 2 ) Center, J. E., lIacIntosh, R.ll.,ISD. E s o . CHEM.,- 4 x . t ~E . D. 17, 239 (1945). (3) Feigl, F., I b i d . , 8, 405 (1936). (4) Miller, W.L., Geld, J., Quatineta, ll.,- 1 s ~C~ HE .M .22, l 5 i 2 (1950). (5) Silsson, G., Analyst 64, 501 (1939). (6) Ray, P., Ray, 11. >I., Q u a r t . J . I n d i a n C‘/teui, S O C . 3, 118 (1926). ( 7 ) Tourky, -1.11..Wakkad, S. E . S., El. J . C‘hem. SOC.1948,740. (8) \-oznesenski, S.A , , Pazelskii, J., Tsiiln, I. 31.. Trans. Inst. P7ue Chem. Reagents (C-.S.S.R.) 16, 98 (19391. (9) West, P. W,, Compere, l 1 . , A x - ~ L C. H E x 21, G2S (1949). (10) Killard, H. H., Nosher, R.E., Boyle, -1J.. . Ihid., 21, 598 (1949).
RECEIVED for review
Deceriiber 28. 1955. -4cceI:ted I‘ehrunrj- 2 1 , 103ti.
Improved Techniques for the Isotopic Determination of Boron on the Mass Spectrometer C. E. M E L T O N , L. 0. GILPATRICK, RUSSELL B A L D O C K , and R. M. HEALY Stable Isotope Research and Production Division, Mass Spectrometer Department, O a k Ridge N a t i o n a l Laboratory, O a k Ridge, Tenn.
Ordinary mass spectrometer methods for the isotopic determination of boron, using boron trifluoride gas, are not adequate for the routine determination of enriched boron isotopes because of interference by residual gases or compounds from previous samples measured in the spectrometer. A new method for reducing “niemor? ” has been developed, in which boron trichloride gas is used to remove the adsorbed boron trifluoride from the mass spectrometer.
I
SOTOPIC detei mination of boron, with boron trifluoride used as the sample gaq, has long been a problem in the mass spectronieter brcause of a very strong “memory” ( 3 ) of the instrument. Mass spectrometer memory is caused primarily by the adsorption of boron tiifluoride gas on the inner surfaces of the apparatus. There is also isotopic exchange betneen the boron in the boron trifluoiide gas and the boron in the glass of the apparatus. Ordinan- methods for the isotopic determination of boron in boron trifluoride consist in saturating the inner surfaces of the apparatus with the sample under consideration by repeatedly flushing and evacuating it. This is a time-consuming operation and is wasteful of ralriable material; therefore, it is desirable and
in many ins:ances necessary to find a method n-hich n-ill more effectively reduce the memory. Previous investigators have suggested the use of a number of gases to reduce memory (3). Preliminary experiments in this laboratory with several of these gases-hydrogen. helium, water vapor, ammonia, and nitrogen-did not indictate useful memory reduction; h o w v e r , an incidental observation suggested that boron trichloride might give the desired effects. The method described here consists in removing the adsorbed boron trifluoride by exposing the surfaces of the apparatus to boron trichloride. APP 4R 4TUS
Isotopic abundance analyses were pel formed on a modified, G-inch radius, 60”-sector type of analvtical mass spectrometer. -2 three-nay stopcock was placed betxveen the gas leak and the expansion volume. Sample reservoirs (250 cc ) were constructed from new, soft glass. They were chemically cleaned, evacuated to beloiv lo-’ mm. of mercury, and baked a t 350’ C. for a t least 1 hour. The surface-to-volume ratios of the sample reservoirs were made taice as large as those normally used in a mass spectrometer, so that memory effects due to surface adsorption would be amplified. T h e sample reservoirs n e r e attached to the mass spectrometer (Figure 1) through a 12/30 standard-taper ground-glass joint. Stopcocks and joints of the sample reservoirs and of the sample-handling system were lubricated n i t h poly-
1050
ANALYTICAL CHEMISTRY Table I.
Isotopic Content of Boron Standards O R N L Values.
Other Values,
Standard
hlass
cc
%
.I
10 10
93 95 It 0 04 10 01 0 0.3
957fO1 109fO3
n
G , AS
RESERVOIR
*
SAMPLE RESERVOIR,
?-1
I
I
u
LEAK
@ STOPCOCKS I Figure 1.
I
SPECTROMETER SOURCE
Schematic diagram of sample-handling system
ethylene stopcock grease, which does not react chemically with boron trifluoride. GEXERAL PROCEDURE AND EXPERIhIENTAL RESULTS
Normal boron consists of the two isotopes, boron-10 and boron-11. The adopted value for the isotopic abundance of boron-10 lies in the range of 18.45 to 18.98% (1, 2, 6). Recently, Osberghaus ( 4 ) has reported a value of 19.57% for the isotopic. abundance of boron-10. This value is more nearly in agreement with the value of 19.98% which has been obtained a t the Oak Ridge National Laboratory in an extensive investigation which has extended over the past 2 years. For simplicity in the following discussion, the data are confined t o one isotope, boron-10. Enriched stable isotopes of boron utilized in this study were obtained from the Oak Ridge National Laboratory Stable Isotopes Research and Product,ion Division. Two standards were used in the investigation: standard A, which contained 95.95% boron-10, and st,andard B, which contained 10.91% boron-10. The isotopic ahundance measurements of these two enriched standards were ohtnined by removing the adsorbed boron trifluoride from the mass spectrometer with boron trichloride and then snturating the surfaces of the spectrometer with each of the standards. As an independent check on the values obtained, the standards were submitted t o another laboratory for analysis. A comparison of the values obtained a t the two laboratories is given in Table I. Values for the errors represent the 9.5% confidence interval and were obt,ained from 10 sets of (1:tt:i; they are a measure of the precision of the method. The procedure consisted in oxposing t,he inner surfaces of two, clean, soft-glass sample reservoii s to boron trifluoride of lonboron-10 content, thus producing a layer of adsorbed boron trifluoride. After one of the exposed reservoirs was treated with boron trichloride, the rate of isotopic exchange of boron in the treated reservoir was compared lvith the rate of exchange for boron in the untreated reservoir by exposing each reservoir to boron trifluoride of high boron-10 content. Two, new, soft-glass reservoirs were cleaned by the methods previously described. They were filled with boron trifluoride to a pressure of 30 mm. of mercury from standard B, low boron-10 content. After 12 hours, both reservoirs \\-ere evacuated to below 70-7 mm. of mercury in order to wniovc~the boron trifliioride;
however, strongly adsorbed boron trifluoride remained on the surfaces of the reservoirs at this pressure. Reservoir 1 was subsequently exposed to normal boron trichloride at a pressure of 10 mm. of mercury, so that the adsorbed boron trifluoride would be removed. The boron trichloride, plus any desorbed boron trifluoride, was removed from reservoir 1 by evacuation to below 10-7 mm. of mercury. This pressure was maintained for 48 hours, after which time the evacuated reservoir was placed on the mass spectrometer (Figure 1). Boron trifluoride of high boron-10 content, standard A, was admitted to the expansion volume (Figure 1). The reservoir was then filled to a pressure of 0.1 mm. of mercury from the expansion volume and was isolated from the expansion volume by the three-way stopcock. Isotopic abundance measurements were made on the Bl0F2+and B1lF2+ ions; the resultant data are shown in Table 11. Reservoir 1 was replaced on the mass spectrometer with reservoir 2, which had had no boron trichloride treatment It was also filled with boron trifluoride from standard A to a pressure of 0.1 mm. of mercury, and isotopic abundance measurements were made for 50 minutes. Experimental data are shown in Table 11. Two additional soft-glass sample reservoirs, 3 and 4, were cleaned, and the described procedure \vas repeated with the isotopically enriched standards reversed. The surfaces n-ere conditioned initially with boron trifluoride of high boron-10 content rather than uith the boron trifluoride of low boron-10 content from standard B. The treatment for reservoir 3 was identical with that for reservoir 1; reservoir 4 had no treatment with boron trichloride. The values for the determinations are shown in Table 111. The data in Table I11 suggest that the difference in the rate of isotopic exchange for reservoirs 3 and 4 was due to isotopic exchange with boron trichloride rather than t o a desorption effect. Since reservoirs 3 and 4 m-ere exposed t o boron trifluoride from standard A (9G% boron-10) and since the rate of isotopic exchange \vas studied with boron trifluoride from standard B (11% boron-IO), the evposure of reservoir 3 to normal boron trichloride (20% boron-IO) could lower the value for the isotopic abundance of the adsorbed boron tiifluoride from 96% boron-10 to approximately 20% boron-10. T o clarify this point, two additional 432-cc. sample reservoiis, 5 and 6, were cleaned, evacuated, and exposed t o normal boron trifluoride. The change in isotopic abundance was studied with boron trifluoride from standttrd A. The data are shown in Table IV. The volume of reservoirs 5 and 6 was larger than that of reservoirs 1, 2, 3, and 4; hence, the exchange was not so pronounced.
Table 11. Time, Bfin. 0
10 20 30
:;
Isotopic Exchange Rates for Reservoirs 1 and 2 BID in Reservoir 1 ,
B1Qin Reservoir 2,
96 81 76 72 69 68
96 54 51 49 47 45
%
%
Table '111. Isotopic Exchange Rates for Reservoirs 3 and 4 Time, Min. 0 10 20
30
2; Table IV. Time, Min. 0 10 20 30 40 50
B'o in Reservoir 3 ,
%
11 13 14 15 16 18
BID in Reservoir 4 ,
% 11 24 33
36 37 38
Isotopic Exchange Rates for Reservoirs 5 and 6 B'a in Reservoir 5,
% 96 91 90
89 88 87
B'O in Reservoir 6 ,
75 96
84 82 81 80 78
V O L U M E 28, NO. 6, J U N E 1 9 5 6
1051
The data indicate that the memory is significantly reduced because of the desorption of boron trifluoride by boron trichloride. .4 337-cc. soft-glass sample reservoir was cleaned and then exposed to normal boron trifluoride a t a pressure of 10 mm. of mercury for 5 hours, thus permitting boron trifluoride t o adsorb on the surface. It was evacuated to below 10-7 mm. of mercury for 36 hours and then placed on the mass spectrometer (Figure 1). Boron trichloride was introduced into the sample reservoir a t a pressure of 0.148 mm. of mercury t o demonstrate further that boron trichloride will remove adsorbed boron trifluoride. Ions a t mass 45 (BloCl36+),46 (B11C13j+),47 (B10Cla7+),48 (B11C137+ R'OFZ-), and 49 (B"F*+) were monitored t o determine the rate of desorption for boron trifluoride. A mass spectrogram of the HCl+ and BF2+ ions is shown in Figure 2. T h e intensity of HC1+ and of BF2+ is shown as a function of time in Figure 3.
+
The moles of adsorbed boron trifluoride removed from the surface by one exposure to boron trichloride can be calculated from the data shown in Figure 3. rlssiiming that all BFt+ ions were from boron trifluoride, 1 X 10-9 mole per sq. cm. was released by the boron trichloride. Boron trichloride was adsorbed t o a greater extent than was boron trifluoride. The same parameters from Figure 3 were used to calculate the moles of boron trichloride adsoibed on the surface, and 6 X 10-Qmole per sq. cm. was ob-
200
400
1 45
46
47
40
IWF
de-
The sample-handling system of the mass spectrometer is flushed twice with 0.2 cc. of boron trichloride a t standard temperature and pressure. The gas used for flushing should be ~i isotopic composition near that of the samples to he annlyz~d. After the sample-handling system is evacuated, it ia flushed three times with 0.2 cc. of the boron trifluoride at st:indard temperature and pressure from the sample under considrration. Data are taken after the fourth flush. At least five boron trifluoride samples of approximately the same isotopic, composition can be analyzed before it is necessary t o flush again with horon trichloride. There is an isotopic exchange of boron betncen adsorbed boron trichloride and boron trifluoride, biit the rate of eschange is s l o and ~ can be neglected when the isotopic composition of the adsorbed boron trichloride is near that of the ),oron trifluoride sample.
B40c135 t
0
T K O sample reservoirs, a 250-cc. borosilicate glass sample reservoir a-ith a surface area of 280 sq. em. and an identical softglass reservoir, were filled with boron trifluoride from sta1id:trd A :tt a pressure of 20 mm. of mercury. The isotopic eschange between the boron in the boron trifluoride and the boron ill the borosilicate glass was then determined. The gas remained in the container for 120 hours before an isotopic measurement was made. The abundance of boron-10 had decreased from 95.95 to 9 3 . 4 0 ~ o . h soft-glass sample reservoir used as a control showed no (Ietwtable decrease in the abundance of boron-10. The following method for the isotopic analysis of boron veloped from thme studies:
n
i
taiiied. Other aspects of the adsorption problem are still under investigation. The possibility t,hat the apparent adsorption of boron trifluoride was caused by a reaction with the stopcock grease was investigated. A soft-glass sample i~servoirwhich had been exposed to boron trifluoride was flamed under vacuum for 30 minutes to facilitate the removal of the adsorbed boron trifluoride. Boron trichloride was t.hen introduced into the reservoir on the mas6 spectrometer, and the rate of desorption of boron trifluoride was studied. The amount of boron trifluoride liberated from the surface was compared with that liberated from the surface of a rcservoir which had had similar treatments b u t no heat, and was found t o be 63% less. These observations indicate that most of the boron trifluoride is liberated from the glass rather than from the stopcock grease.
C O S C LU SION
49
45
46
47
48
49
Figure 2. JZass spectrogram of BF2" and BCl+ ion currents showing decay of BCl+ and growth of BF2+
This method greatly reduces the memoyy effect in consecutive analL-ses, and it has been applied successfully a t this laboratory for the routine determination of enriched stable isotopes of various isotopic composition. Thc results shown in Table I11 indirate that the memory is minimized when t,he surfaces are exposed t o boron trichloride containing boron of isotopic abundance near that of the saniplc under consideration. After the sample rcservoir is exposed twice t,o boron trichloride, the residual absorbed boron trifluoride is no longer detected. This method is very advantageous for analyses of small samples, as an analysis ran be performed with approximately one third the material required for ordinary methods. ACKNOWLEDGMENT
The nrithors wish to express their appreciation t o H. 11. Itosenstock of this laboratory for his constructive crit,icism and siigg~stionF roncerning this work.
A
(30
75
;
