tude for each case. From these measurements on n-decane, we have concluded assumption (1) is valid. Since the average value of 6 in Table I is *0.05 m/e, it is evident that the readability error is the largest factor and the error of (c) may be disregarded. However, if this method is used with compounds of higher molecular weight and composed of only carbon and hydrogen, then the resulting large mass defect will probably have to be taken into consideration.
We have shown that metastable peaks in paraffins are not only abundant but can be determined t o considerably less than k O . 1 m/e unit. From a large family of mathematically possible transitions a computer can be programmed t o derive the necessary transitions in order to specify a reasonable mechanism by which the parent molecule breaks up in its flight through the mass spectrometer. The method shows great promise in general applicability to 811 Inass spectra in which metastable ions appear.
LITERATURE CITED
(1) Barber, Pyl., Elliott, R. bl., Twelfth Annual Conference on Mass Spectrom-
etry and Allied Topics, &fontreal, Canada, June 1964;, (2) B ~ J. H ~ hiass~ Spectrometry ~ ~ and Its Applications to Organic Chemistry,” P. 252, Elsevier, y e w Yo&. (3) Bieman, K., “>lass Spectrometry Organic Chemical Applications,” p. 154, McGraw-Hill, N~~ York. (4) Rhodes, R. E., Barber, M., unpublished results. RECEIVEDfor review August 4, 1965. Accepted November 12, 1965.
High Pressure Mass Spectrometry for Analysis of Trace Impurities in Helium ELMER T. SUTTLE, DAVID E. EMERSON, and DIANA W. BURFIELD Helium Research Center, Bureau of Mines, Amarillo, Texas
b
Trace impurities in helium in the parts per million range have been analyzed with a conventional mass spectrometer. To increase the sensitivity, the inlet sample pressure was increased from the micron to the millimeter range and the ionizing current to 100 pa. Twenty-eight cylinders of helium were analyzed by the Bureau of Mines trace impurity method, and these were used to determine the sensitivity of each impurity on the mass spectrometer. The sensitivities of the impurities, H2,CH4, Ne, Nz, 02,Ar, and COZ, varied among themselves from 3.1 to 26.9 divisions per p.p.m. Thirteen unknown samples were analyzed and the results compared with analyses by the Bureau of Mines trace impurity method. The results were within the accuracy of the Bureau of Mines method. The total impurities in these cylinders ranged from 10.5 to 189.9 p.p.m.
T
is a continuing need to develop better methods to analyze impurities in helium. Trace impurities found in Grade-A helium are currently analyzed by using the Bureau of Mines trace impurity apparatus (T. I.) (g), chromatograph (S), and electrolytic moisture analyzer (1). The concentrations of these impurities are so small that they cannot be satisfactorily analyzed on the mass spectrometer using the normal procedure. A study was made to increase the sensitivity without making major changes to the instrument. It was also necessary to retain the capability of switching the mass spectrometer back to routine operation. HERE
AUXILIARY M E T A L INLET V A L V E ASSEMBLY
Figure 1. Modifications to mass spectrometer inlet system
steel line to the auxiliary metal inlet valve assembly. T o resolve helium (He+’J) and hydrogen (Hz+),a 220-megohm, 1-watt resistor was installed in the scan rate 3 position. A rhenium filament was used for this study ( 4 ) . Instrumental. The inlet system is evacuated progressively to the cylinder valve. While a high vacuum is maintained on the inlet line, the cylinder valve, valve 9, and the stainlesssteel line are heated with an electric blower t o hasten outgasing. After a high vacuum is attained, valve 9 is closed and the cylinder valve opened. Valve 9 is next gradually opened to allow a purge of about 75 mm. of gas to sweep out any residual gases in the lines and valves. The purge is continued for 30 seconds or longer, then pumped out. Valve 9 is opened until a 25.0-mm. pressure read-
EXPERIMENTAL
Modifications. Only minor modifications to a Consolidated Electrodynamics Corp. (CEC) 21-103C mass spectrometer were necessary. The auxiliary metal inlet valve assembly was modified for the high pressure work by installing the mercury manometer (Figure 1) next t o valve 7. Also, the manometer connection normally furnished on the auxiliary metal inlet valve assembly was cut off near valve 8 and fitted with a stainless-steel connection. A Plexiglas sheet was placed on the inlet cabinet in front of the 3-liter sample volume for safety protection. T o eliminate air contamination, the sample cylinders were connected by the use of a standard stainless steel cylinder connection and a control valve 9 (vacuum to 3500 p.s.i.). The control valve outlet was fitted with a stainless
Table 1. Sensitivities and Contribution Patterns of Trace Impurities in Helium (25.0 mm. Pressure and 100 Ma. Ionizing
Component Hydrogen Methane Neon Nitrogen Oxygen Argon
Current) Sensitivity, divisions
Carbon dioxide Contributions : Carbon dioxide (44)
Nitrogen (28) Argon (40) Methane (15)
per p.p.m.
3.1 8.9
6.2
16.5 11.5 26.9 23.6
to mass 28 to mass 15 to mass 20 to mass 2
= = = =
0.2589 0.0008 0.1311 0.0183
VOL. 38, NO. 1, JANUARY 1966
51
,
4.54.2
-
I
14.0 -
-
3.9 3.6 3.3 3.0 2.7 2.4 2. I 18
I .5 i .2
.9 .6 .3 0
5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 PRESSURE, rnrn. mercury
Figure 2.
PRESSURE, rnrn. mercury
Figure 3.
Determination of optimum inlet pressure 0 neon, 1.3,
Ionizing current = 10.5 pa. p.p.m.: water, 0.9; nitrogen, 7.1 ; A carbon dioxide, 0.6; total, 9.9.
ing in the 3-liter volume is accurately obtained. The ion source valve is next slowly opened over a period of 15 seconds. After the pressure reaches near mid-
Determination of optimum inlet pressure
Ionizing current = 32.5 pa. p.p.m.: 0 argon, 279.2; water, 2.0) 0 neon, 14.0; nitrogen, 91.6; hydrogen, 2.4; 0 oxygen, 13.6; h carbon dioxide, 0.2; total, 403.0.
range on the exhaust vacuum gauge, the sensitivity switch is set a t the ‘‘set zero” position to prevent filament cutoff. As the ion source valve is opened, the exhaust vacuum control
will read midrange in the low position, but the reading stabilizes in about 45 seconds. d scan is made for detecting impurities from masses 13 through 100. To make a hydrogen analysis, Hei2 and Hz+must be resolved. The collector is switched to the 0.007-inch slit width position. The ion accelerating voltage scan rate is switched to position 3 (slow scan rate), and a 1.5 minute scan is made for the m/e 2 peak.
::::y 52.0
48,0k44.0
E‘
40.0
p
36.0
‘5 ._
32.0
V
W (0
n
/
i
L L k v) z
RESULTS AND DISCUSSION
When the sample pressure is increased, there is an optimum increased sensitivity for each component. Two helium samples were used to determine the effects of varying pressure and ionizing current on the sensitivity of a CEC 21-103 mass spectrometer. They contained 9.9 and 403.0 p.p.m. total
24.0 20.0 16.0 i 2.0 8 .O 4.0 0
Figure 4.
10.0 20.0 30.0 400 50.0 60.0 70.0 80.0 90.0100.0 IONiZlNG CURRENT,
Optimum ionizing current determination
Pressure = 25.0 mm. of Hg. p.p.m.: W hydrogen, 2.4; water, 2.0; 0 neon, 14.0; nitrogen, 91.6; 0 oxygen, 13.6; argon, 279.2; A carbon dioxide, 0.2; total, 403.0.
52
ANALYTICAL CHEMISTRY
I\
-13 CHARGE
14 15161718 20
23
28
32
40 44
2
Figure 5. Typical high pressure, high ionizing current, mass spectrometer scan (25.0 mm. of Hg, 100 pa.)
impurities. Both of these samples of helium had been previously analyzed using the Bureau of Mines trace impurity method. Using the 9.9-p.p.m. sample, the ionizing current was kept constant a t 10.5 pa. (Figure 2). The pressure was gradually increased from the micron range to 50.0-mm. mercury pressure in approximately 5-mm. increments. The same procedure was followed a t 32.5 pa. ionizing current for the 403.0-p.p.m. sample (Figure 3). As the pressure was increased, the filament and high voltage plate gave increased cutoff problems. After several runs had been made a t the higher pressure settings, the instrument seemed to stabilize, and the samples could be analyzed with few cutoff problems. I t was found that 25.0 mm. was the optimum pressure for this method of analysis. Increasing the ionizing current also increases the sensitivity. The optimum setting (Figure 4) for the ionizing current was determined while using an inlet pressure of 25.0 mm. This was found to be the maximum available (100 pa.). Sub3equent analyses were run a t the optimum conditions of 25.0 mni. and 100 pa. (Figure 5). Specially selected cylinders of helium were analyzed twice on a CEC 21-103C mass spectrometer to determine the sensitivity (Table I) for each component. Preliminary calibrations showed the peak heights of the various impurities to be proportional to the concentrations of the impurities. Under these high pressure and ionizing current conditions, the peak for helium is off the oscillograph scan and is ignored. The contribution patterns in Table I were calculated from analyses of helium containing nitrogen, neon, methane, argon, and carbon dioxide. The p.p.m. of neon as determined by the T. I. method was multiplied by the neon sensitivity to give the peak height in divisions actually representing neon (Se+). The residual of the mass 20 peak on the analysis was considered to be a contribution from argon. The ratio of the argon peak (mass 40) to the residual mass 20 peak was calculated as the contribution of argon to the mass
Table II.
Precision Data
Impurity
Hz
CH4
Ne
N*
0 2
Ar
C02
Mean concn., p.p.m. Std. dev. Range
27.3
1.2
13.3
40.5
1.2
0.4
1.6
0.2 0.7
0.2 0.6
0.9 3.6
1.2 3.6
0.1 0.4
0.1 0.1
0.1 0.4
Table 111.
Comparison of Trace Impurity Method (T. I.) and High Pressure Mass Spectrometry Analyses
Analysis method Hz T. I.
s. T. I. hI. s. T. I. M.s. T. I. L1. s. T. I. 31.s. T. I. nr. s. T. I. 31. s. T. I. 31.s. T. I. 11. s. T. I. hI. s. T. I. AI. s. T. I. 31. s. T. I M.s. 31.
0.3 0.3 0.3 0.2 0.2 0.3 0.0 0.1 0.2 0.0 0.0 0.1 0.2 0.2 0.0 0.2 0.3 0.4 0.1 0.1 0.3 0.0 0.0 0.1 0.1 0.0
Component concentration, p.p.m. Nz 0 2 Ar
CH4
Ne
0.0 0.3 0.0 0.2 0.0 0.0 0.0 0.1 0.0 0.1 0.0 0.1 0.0
