ANALYTICAL CHEMISTRY
1662 pump intake tube, and circulation will stop. This can be prevented by filling a 250-ml. volumetric flask with buffer and supporting i t above the apparatus in an inverted position with the open end of the neck just bplow the surface of the buffer in the
freedom from leaks, and compact arrangement of the assembled equipment. Their performance has been very satisfactory over extended periods of time. ACKNOWLEDGMENT
box. POWER PACK
A power pack, convenient for use with this apparatus, consists of a step-down transformer, yielding 35 volts, connected to a fullwave selenium rectifier of 3-ampere capacity. I n line with this are a variable series resistor and a direct current ammeter. In a typical fractionation with the large cell described here (window area, 102 sq. cm.), a nominal potential gradient (ignoring the area beneath the cell) of 4.6 volts per em. was produced by a current of 0.i5 ampere in phosphate buffer of 0.05 ionic strength. When the smaller cell was used (window area, 58 sq. cm.) a nominal potential gradient of 10.5 volts per cm. was produced by a current of 0.18 ampere in 0.01 ionic strength buffer. The advantages of these modifications of the apparatus for electrophoresis-convection are simplicity in the construction of the cell and in its assembly for use, flexibility of the cell volume,
The authors are indebted to Robert Sniffen for constructing this apparatus. LITERATURE CITED
(1) Cann, J. R., Brown, R. A., and Kirkwood, J. G.. J . Am. Chem. SOC.,71, 1609 (1949). (2) I bzd., p. 2687. (3) Cann, J. R., Brown, R. A , , and Kirkwood, J . G., J . B i d . Chem.,
181. 161 11939).
(4) Cann,'J. R., Kirkwood, J. G., Brown, R. .I.,and Plescia, 0. J., Ibid., 71, 1603 (1949). (5) Kirkwood, J. G., Abstracts of Papers, 120th Meeting, ~ M E R I C A N
CHEMICAL SOCIETY, September 1951. (6) Kirkwood, J. G., J . Chem. Phys., 9, S i 8 (1941). ( 7 ) Mathies, J. C., Science, 115, 144 (1952).
(8) Nielsen, L. E., and Kirkwood, J. G., J . Am. Chem. SOC.,68, 181
(19+6). RECEIVED for review
M a y 13, 1952.
Accepted June 30, 1952.
Decrease of N" to Ni4 Ratio Measurement as a Function of Pump-Out Time For Nier- Type Isotope-Ratio Mass Spectrometer JAMES GEORGE Diaision of Chemistry, Naval Medical Research I n s t i t u t e , National Naval Medical Center, Bethesda, &Id. D U R I N G the,course of several series of measurements of the natural abundance of i Y 1 5 in tank nitrogen with the K e r type isotope-ratio mass spectrometer (Consolidated Engineering Corp. 21-201), abnormally high value for the m / e 29/28 ratios were initially observed. Stein (6) also observed abnormal values for m/e 29 and proposed a correction for the true m/e 29/28 ratio for nitrogen gas. This phenomenon occurred sequentially whenever operation of the instrument was discontinued, either by a brief interruption resulting from power failure, or by a more prolonged intentional shutdown and venting of the analyzer tube with helium. As a consequence of these observations, a test of four shutdowns with helium venting was carried out. A decrease of the m / e 29/28 ratio with pump-out time (Table I and Figure 1) was found to coincide with a decrease in the m / e 29 background (bg.) peak (as shown for Experiment 4, in Figure 2). I n addition to
Table I. -Experiment Pump-out time. .~~~~ days 18" 25" 26 27 28 35 39 40 62 63 66
the abnormally high background peak, evidence suggestive of the presence of an orgmic vapor was obtained in the form of the m m i scan recorded in Tables I1 and 111. Since the ion patterns of nitrogen, carbon dioxide, and carbon monoxide show that thev contribute approximately 0.76, (1.2 of 9.0 = ) 0.11, and 1.2%, respectively, to the m/e 29 peak, these gases could not account for all of the m / e 29 peak. From the mass scan of Tables I1 and I11 where the m/e 29 background peak is 37.0 mv. and the m/e 28 background peak is 180.0 mv. one may calculate the m/e 29 peak t o be 20.6% of the m / e 28 peak, of lvhich about 18.5% represents organic background. Daily measurements of nitrogen gas samples for both the nile 29/28 ratio and the m / e 29 background peak showed fluctuations or scattering of the points on the curves such as shown in Figure 1 (Experiment 1, Figure 3). The nitrogen gas was dried and passed through a liquid nitrogen trap before collection in the
Measurements of m / e 29/28 Ratio for Nitrogen Gas, as a Function of Analyzer Tube Pump-Out Time for Isotope-Ratio Mass Spectrometer (C.E.C. 21-201) 1---
d e 29 be.. mv.
13 0 6 0
....
.... .... 3.0 3.0
....
.... 2.0-
7 -
29/28 ratio
mle
Pump-out time, days 2a
3'7
22 28 30 37 40 46 49 54
-
Experiment -2 m / e 29 bx., m f e 29/28 mr. ratio 20 0 5 0 3 0'
3 O+ 3 0 2 o+ 2 0-
0.008400 0.007835 0,007730 0.007680 0,007620 0.007610 0,007600 0.007592 0.007665 0.007530
7 -
Pump-out time, days 2n 3 8 1" 8 2 8.3 8 4 11 0 15 0 24.0
Experiment --3 m / e 29 bg., m / e 29/28 mv. ratio 27 I5 20 14 14 12 5 4 4
0 0 0 0 OT
0 0 O+ 0-
0 0 0 0 0 0 0 0 0
021930 008280 008610 008120 008125 008085 007700 007629 007600
--Experiment Punip-out time. days 2a 3 la 3.2 3.4 6 1Q 6 3 7 1 7.2 7.3 8 9 10,lG 10.4
13 14 15
16.2 16.3 22 0:
Measurements made after baking out analyzer tube.
-
4 m / e 29
bg.,
mv.
32.0
32 0 14 0 14 0
--
29/28 ratio 0.009195 0.008200 0.008530
m/e
4 0
0.008200 0.007790 0,007726 0.007682 0,007683 0.007660
5 3 3 3 3 2 2 2 2 2 2
0.007633 0.007606 0,007600 0.007600 0.007560 0.007545 0.007550 0.007530 0.007525 0.007530
6.4 5 6 5 0 0 8 5334 4 4 1 1 0'
1663
V O L U M E 24, NO. 10, O C T O B E R 1 9 5 2 Table 11. Mass Scan of Background after Analyzer Tube Pump-Out Time of 2 Hours Mass, mie 26 27
28
2'3
30 3% 3Y 40 ..
41 43 44 54-59 66-69 89-93 100 200
Acceleration T'oltage, Volts 2000 1936 1871 1806 1740 1642 1354 1318 1287 1256 1244 930-965 766-794 620-640 524 262
Peak Voltage, MV. 12.0 40.0 180.0 37.0 8.0 6.0 32.0 9.0 72 .O 19.0 68.0 37.0 21.0 12.0 68 145
Gas Contributing to Peak
c,02
but also a noticeable decrease in the range of scattering of these values about the curve (Figure 3). A more rapid rate of decrease of this ratio may, of course, be attained by baking out the analyzer tube (footnote, Table I). Under the optimum conditions indicated, values found for the abundance of N15in tank nitrogen, and in thermally decomposed ammonium dichromate, were 0.3710 and 0.3'74'7, rrspectively. This compares with the Ernerally accrpted avcrage value of 0.38 ( 4 ) .
-
- 30 - 28
Magnetic strength, 2150 gauss Total ionizing current, 430 microamperes Trap current, 50 microamperes Ultimate varuum of analyzer tube, >0.001 micron of mercury Inlet pressure of Sz ( m / e 281, 18 mm. of mercury per 20 volts output Manifold pump-out time (32). 7 5 seconds Resolution -0 01 % Correction'factor, 0.997 Represents organic background peaks. Small peaks in the ranges of m / e 48 to 60, 66 to 72, 86 t o 100 were also observed.
-26
p
-24
$
-22
2
-10
::
- 8
9
I.
0
5
15
IO
20
DAYS
Figure 2. Relationship of m / e 29/28 Ratios and m / e 29 Background Peak Values to Pump-Out, Time, as Found in Experiment 4 I:
EXPERIMEhT
I
0 : EXPERIMENT 2
A : EXPERIMENT 3 : EXPERIMENT
Measurement of m / e 29 bg. after baking out is indicated by point B +
4
0,0082
0
A
:EXPERIMENT I lWlTHOUT C02 T R 4
n: EXPERIMENT 4 DAYS
Figure 1. Relationship of m / e 29/28 Ratio Values to Analyzer Tube Pump-Out Times for Nitrogen Gas saniple bulbs. The m / e 20 background peak was measured before and after the ni/e 29/28 measurement of the gas sample. On the assumption that organic vapor was present only in the analyzer tube, a steady decrease of both the ni/e 29/28 ratio and the m/e 29 background values would be expected as the pump-out time increased. However, the fluctuations in these values were indicative of the presence of organic vapor in the manifold system; therefore, a dry ice trap was installed between the manifold pump and the capillary leak (Figure 4). With the trap a t room temperature, an ultimate pressure of approximately 6 microns was attained in the manifold system, whereas a t dry ice temperature it was approximately 2 microns. These measurements were made with a Pirani gage. No attempt was made to distinguish between the presence of oil vapors, and of impurities in the mechanical pump oil, as a cause of these organic background peaks. The operating temperature of the pump oil was 50" C. After 3 months of continuous operation, almost 1 ml. of mechanical pump oil was collected in the dry ice trap. The fluctuations in the m / e 29/28 ratio measurements observed in the course of shutdown Experiment 1 were independent of variation in gas handling and pump-out procedures. Measurements obtained with a dry ice trap in the manifold system in Bhutdown Experiment 4 showed not only an expectedly more rapid attainment of constancy of m / e 29/28 ratios with time,
A
C
00075
I
I
0
O
O
B
I
During the initial stage 01 evacuation of the analyzer tube, as a precaution against oxidation of the mercury, a pressure of less than 25 microns should be attained before the mercury diffusion pump is turned on. Hou-ever, if the time required to attain the proper pressure is longer than about 0.5 hour, the oil temperature in the forepump rises to about 50" C. and its oil vapor diffuses throughout the vacuum system ( 3 ) . The presence of water vapor is usually responsible for a long initial pump-out time for this particular instrument. Blears ( 8 ) has shown that it is very difficult to rcmove oil vapor from the fine side of a vacuum system even when it is baked out a t a temperature of 250" C. for several days, with the cold trap maintained at a liquid nitrogen temperature. Diffusion of oil vapor into the analyzer tube of the isotope-ratio mass spectrometer may occur within 6 minutes after a power failure. In an attempt to prevent this effect a small dry ice
1664
ANALYTICAL CHEMISTRY
trap wab installed between the forepump and the merrury diffusion pump. A series of simulated power failure tests was conducted with and without a dry ice trap between the forepump and the mercury diffusion pump. The method by which diffusion of oil vapor was detected in the analyzer tube following power failure was as follows.
Table 111. Optimum Background after 4 Months of Continuous Pump-Out of Analyzer Tuhe d e Peak Voltage, M Y . 15.0 0.3 2.0
28 29 32
4.0
44
17.0 15.0
100 200
A series of shutdown experiments was carried out in two groups of 25-minute shutdowns a n d 12-hour shutdowns with and without a dry ice trap between the forepump and mercury diffueon pump. The analyzer tube dry ice trap was maintained at - 18” C . during the shutdown period and during evacuation i n each experiment. When the maximum peak of the m/e 54 to 59 range was measured with respect to pump-out time, the results indicated only a partial prevention of oil vapor diffusion with a dry ice trap between the forepump and mercury diffusion pump (Figure 6). However, the time required to reach optimum operating conditions was much shorter than when a dry ice trap was not nsed.
Mechanical Vacuum
pump
E
1
ANALYZER
>001
I
001
I
01
l
03
l
04
1
\
v)
= WITHOUT TRAP :WITH
o
TUBE PRESSURE IN MICRONS
401
X
X = 0 5 HOUR SHUTDOWN-WITH CUT TRAP i 0 5 HOUR SHUTDOWN-WITH TRAP
\
Figlire 4 . Vacuum System Used in Shutdown Experiments (Figure 1, Table I)
> 001
12 HOUR SHUTDOWN-WITH OUT TRAP A = 12 HOUR SHUTDOWN-WITH TRAP
0 s
B
TRAP
X
\
I
3ot-
\
1
\ \
\
/
ANALYZER TUBE
PUMP-OUT TIME IN HOURS
Figure 6. Relationship of m / e 57 bg. w - i t h Respect to Analyzer Tube Pump-Out Time Following Shutdown Experiments Measurement of m/e 57 after baking o u t is indicated bv B+ 1
. I
TIME IN MINUTES FOREPUMP AND Hg DIFFUSION PUMP OFF
Figure 5. Rate of Diffusion of Forepump Oil Vapor into Analyzer Tube Following Shutdown of Pumping Systetn
B e c k and Jaycox ( I ) have reported on the use of activated charcoal traps in a very high vacuum system. Their results indicate that this type of trap is very effective in preventing oil vapor from diffusing into the fine side of R vacuum s ~ s t e m . LITERATURE CITED
Becker, J. S.,and .Jaycox, E. K., Re#. Sci. Znslruments, 2, 773 (19311. (2) Blears, J., J . Sci. Instrutnents, Supplement 1, 36 (1950). (3) Consolidated Englneerlng Corp., Operation and hfaintenance Manual S o . 2012a, Pasadena, Calif., 1948. (4) Mattauch, J., “Nuclear Physics Tables,” Yew York, Interscience Publishers, 1946. ( 5 ) Stein, F. S., paper presented at annual meeting of mas8 spectrometry, sponsored by Consolidated Engineering Corp., Sew Orleans, 1950. (1)
With the instrument in optimum operating condition the accelerating voltage was adjusted to focus the maximum peak of the m / e 54 to 59 range on the S o . 2 collector. The forepump and mercury diffusion pump were then shut off and the maximum peak (m/e 57) was measured with respect to time until the analyzer tube pressure increased to 0.04 micron (Figure 5 ) . At this point the filament cut off, the high voltage was turned off, and both pumps were turned on again. The pressure in the analyzer tube a t this stage was approximately 25 microns. After a pumping period of about 8 minutes the ultimate pressure was again less than 0.001 micron.
RECEIVED f o r review January 11, 1952. Accepted August
4 , 1952.
