Mass Spectrometric Determination of Gas Impurities Separable by Condensation A7)IO.S S . NEWTON Radiation Laboratory, University of California, Berkeley, Calif. METHOD
of utilizing an internal standard in the mass spectro-
A metric determination of traces of noncondensable impurities
in condensable gases or condensable impurities in noncondensable gases is presented. The preparation and analysis of synthetic mixtures in the range of 30 to 200 parts of impurities per 100,000 parts of main component are described. The mass spectrometric determination of condensable impurities in noncondensable gases is well known. Recent examples of the use of this method are given in the papers of Washburn and Austin ( I O ) and Shepherd, Rock, Howard, and Stormes (8)on the concentration and mass spectrometric determination of condensable impurities related to smog in air. Happ, Stewart, and Brockmyre ( 4 ) have used the method for the determination of condensable vapors in room air. In these cases the combination of system volume and mass spectrometer sensitivity were used to determine the amount of impurity present, relating it to the volume of sample collected. This method is satisfactory for the study of condensable impurities in noncondensable gases, but does not work as well for the inverse case of noncondensable impurities in condensable gases, where the fraction of gas mechanically entrapped in the large bulk of condensate is unknown. To avoid the difficulty of mechanical entrapment of the noncondensable impurity, it has been found convenient to introduce into the mixture an internal standard of the same gas type as the impurities. The concentration of the added standard is arbitrary and need not be known, but the ratio of main component to internal standard should be easily and accurately measured on the mass spectrometer record, yet the concentration of the standard must be small enough so that after separation by condensation the ratio of impurities to standard can be accurately measured. For an unknown gas mixture this may require a trial blend before a desirable concentration of internal standard is added. Internal standards in mass spectrometric analyses have been used by Hindin and Grosse ( 6 ) ,who introduced neon as an internal standard in the determination of nitrogen, using a modified Dumas technique combined with the mass spectrometer. These authors suggested that the internal standard method might also be used for the analysis of trace impurities but did not extend the work to cover such cases. Grosse, Hindin, and Kirshenbaum ( 3 ) extended the method to use internal standards of oxygen-18, nitrogen-15, or carbon-13 for the determination of oxygen, nitrogen, and carbon, respectively, in the elementary analysis of organic materials. Thomas and Seyfried ( 9 ) have used an internal standard of benzene in the analysis of liquid oxygenated compounds. Selecting Internal Standards. I n principle, for the determination of noncondensable impurities in condensable gases the selection of the internal standard to be used is important if solubility effects are to be minimized. Since the system is one containing a small amount of gas in contact with a large bulk of condensate, the solubility characteristics of the internal standard and the impurities should be as nearly identical as possible. In practice, using liquid air or liquid nitrogen as the condensing medium, the choice of internal standard is rather limited, and the best one can do is strike a compromise, particularly if the properties of the impurities range from those of hydrogen to methane. At higher condensation temperaturesg., when dry ice or a carbon disulfide slush bath ( - 111' C.) is used-a n*ider choice is possible. In the work reported here, liquid nitrogen was the only condensation medium used. For noncondensable impurities, neon was used as an internal standard, chiefly because
it bridges the gap in properties between hydrogen and oxygen, nitrogen, and carbon monoxide and has an isolated peak of fair sensitivity a t mass 20, which simplifies the calculations. I n the case of condensable impurities in noncondensable gases this solubility problem does not arise and the choice of internal standards is much wider, the major consideration being complete condensation a t the temperature used with a minor consideration being the existence of an isolated peak of considerable sensitivity for simplification of the calculations. Any solubility of the gross noncondensable component in the condensate is easily taken care of in the analysis of the condensate spectrum. EXPERIMENTAL PROCEDURE
A411gases used were of the highest purity available, and all condensable gases used as major components were distilled three times in vacuum, pumping on the condensed material m t h a fast oil diffusion pump after each distillation. All gases were checked for purity with the mass spectrometer, and where the impuritiee were noninterfering-e.g., 0.2% argon in oxygenthe impurity peaks were considered as part of the gas spectrum. The tank carbon monoxide used in making mixtures was only 9 i . 5 % pure; both oxygen and nitrogen were present as impurities, and the amounts of these components were added to the oxygen and nitrogen contents of the gas mixtures. The carbon monoxide used for calibration purposes xas synthesized; the mass spectrometer pattern showed no extraneous peaks, and the purity was calculated to be better than 99.8%. Traces of impuritiee in the neon were considered part of the neon spectrum. Liquid8 used as condensable impurities viere reagent grade quality, used as received. Except for carbon monoxide, the calibration spectra were run with the same materials from which the mixtures were made.
FINE ,SINTERED DISC 2 0 m m
A
B
Vz
Figure 1. Gas Bottle for Making Mixtures, A, and Mixing Bulb for Adding Internal Standards, B
Samples of Noncondensable Impurities in Condensable Gases. A 500-ml. gas bottle (shown in Figure 1, A ) was used for making the mixtures containing known amounts of gases to be used as noncondensable impurities. An inlet sealed to the side of the flask was used for the introduction of the minor components. The inlet was closed with a mercury-sealed sintered disk. The volume of this bottle was accurately measured by comparing helium expansion in it with a standard 300-ml. gas bottle, which had been calibrated by weighing water contained. The volume measurement was accurate to within 0.5 ml. The stopcocks used both on this bottle and the mixing bulb were high vacuum stopcocks of the hollow-plug type, which eliminated trapping of unequilibrated gas in a stopcock bore.
1746
V O L U M E 25, NO. 1 1 , N O V E M B E R 1 9 5 3 After evacuation of the gas bottle to better than 10-5 mm. of mercury as measured on a McLeod gage, known amounts of each of the noncondensable gases were added through the inlet, using calibrated gas pipets of the type described by Charlet (I,!?). These gas pipets were volume calibrated against the metering volume and micromanometer of the mass spectrometer by making successive introductions of air into the mass spectrometer, first in the regular manner using the metering volume of the spectrometer toget the air sensitivity, and thenintroducingairwith the pipet through the mercury-sealed disk on the spectrometer. Since the metering volume on the spectrometer was calibrated by weighing mercury contained, and the expansion volumes were calibrated by comparison with a standard volume, the absolute pipet volume was known, with accuracy within 1%. The reproducibility of the pipets on successive introductions of air into the mass spectrometer through the mercury-sealed disk was within 0.2’%, this being measured by the reproducibility of the heights of the mass 32 and 28 peaks. Therefore, the absolute amounts of noncondensable gases added were known, within 1%. The condensable gas (main component) was added t o the 500ml. gas bottle from a storage bulb on the vacuum system into which it had been condensed. At the same time, a sample of the condensable gas was collected in another gas bottle on the vacuum line, to be used for calibrating purposes and run later as a check for purity by the same procedures used in analyzing the mixture. The main component was added by opening the cold storage bulb to the vacuum line and blank bottle, removing the cold trap, and allowing the bulb t o warm until the pressure in the system approached atmospheric, at which time the stopcock to the 500-ml. gas bottle was then opened slightly. When the pressure again approached atmospheric in the system, in rapid order the storage bulb was shut off, the cold trap replaced, and the stopcock to the 500-ml. gas bottle completely opened for a bare second for pressure equilibration, and then closed. The manometer then showed the total pressure in the gas bottle. Checks on this procedure showed that opening the gas bottle to the line a second time produced no change in the manometer reading. The gas bottle was then alternately gently warmed and cooled on one side t o aid in equilibration of the gas. (Caution: xarming must be very gentle to prevent loss of gas through the disk and mercury due to pressure increase.) For convenience these mixtures were allowed t o stand overnight before proceeding with the analysis though this was probably unnecessary for equilibration using convection mixing. Samples of Condensable Impurities in Noncondensable Gases. In making up mixtures with condensable impurities in noncondensable gases, liquids were used as condensable gases for convenience of handling. These were introduced into the evacuated mixing bottles via the mercury-sealed disk with Precision Capillary Dippers (2, 7 ) . The volume of the dippers used was calibrated in two ways. Dippers were filled with mercury and emptied into a weighing bottle by suction, weighing 40 dippers of mercury divided into 4 groups of 10 each. Only selected dippers which can be loaded n ith mercury and hold the load can be calibrated in this manner, but using these as primary standards others can be calibrated by comparison of pentane introductions on the mass spectrometer. In the second method of calibration the dipper volume was compared with the metering volume and micromanometer of the mass spectrometer, using n-pentane and introducing it alternately through the metering volume and with the dipper. -411 methods agreed to within 1% using npentane. In general, these dippers deliver reproducible amounts of lovi boiling hydrocarbons, the maximum deviation in 10 successive introductions of n-pentane to the mass spectrometer through a mercury-sealed sintered disk being not over 0.4y0 with a mean deviation of only 0.270. 3Zeasurements with alcohols and other oxygenated compounds are not as reproducible, and successive introductions show deviations as large as 1%. Thus the error on the amount of liquid (condensable component) added was about 1%. The noncondensable component was then added from a large storage reservoir on the vacuum system in the same manner as described above. Adding Internal Standards. The gas mixtures were blended with the internal standard gas in a mixing bulb of about 100-ml. capacity shown in Figure 1, B. This was placed between the gas bottle and the vacuum line, both mixing bulb stopcocks were opened, and the bottle was evacuated to less than 10-6 mm. of mercury. The lower stopcock was then closed, and the internal standard gas was added through the vacuum line to a pressure of 8 to 10 mm. (only roughly measured), which represents a suitable concentration of standard for impurities in the range 0.3 t o 2 parts per thousand, and for the bottle sizes described. The stopcock to the vacuum line was then closed, that of the gas bottle was opened, and finally the stopcock to the mixing bulb was opened and then closed. It is not necessary to completely
1747
equilibrate the pressures, nor is diffusion between the two bottles critical a t this point. The mixing bulb was then gently warmed on one side for a few minutes t o ensure equilibration of the internal standard and gas mixture in the bulb. I n the case of noncondensable impurities in condensable gases, the bulb was ready for the mass spectrometer a t this point, but for the condensable impurities in noncondensable gases, part of the blend of internal standard and gas mixture was transferred t o a fingershaped gas bottle, which could be immersed almost completely in liquid nitrogen. Mass Spectrometric Analysis. The mass spectrometer used was a Consolidated Engineering Corp. model 21-102 converted to model 21-103 except for the inlet system, although the 21-102 inlet system used was provided with a Consolidated Engineering Corp. type 23-105 micromanometer. The spectrometer was operated essentially in accordance with the manufacturer’s recommendations. The sensitivity of the instrument can be changed a t any time by a switch increasing the ionizing current by a factor of 5, which increases the sensitivity by the same factor, and by a switch changing the grid resistor in the d.c. amplifier from 1Olo to lo9 ohms, which decreases the sensitivity by a factor of 10. In the work reported here on1 the first method of changing the sensitivity was used, althougg in principle the analytical range of the method could be extended by using a combination of the two sensitivity changes. Calibration runs on the pure components and internal standard were made at both normal and increased sensitivity ( x 5 ) , although on the spectrometer used no significant pattern changes resulted from increasing the electron emission as long as the ion source temperature was not allowed t o rise appreciably during the run at increased emission; thus the sensitivity factor was the same for all peaks in the spectra of the component gases. Mass spectrometer runs were made according t o the following schedule: (1) Calibration runs on main component gas a t normal and increased sensitivity. (2) Total gas run, all peaks being recorded at normal sensitivity except those of the internal standard used (neon mas8 20 or sulfur dioxide mass 64) which were run a t increased sensitivity. (3) Calibration runs on internal standard a t normal and increased sensitivity. (4) Separated gas run, the separation of noncondensable gases from condensable being made by very slowly bleeding the gae into the mass spectrometer through the liquid air trap on the inlet system. This allowed a trace of the condensable gas to go through the liquid air trap, but this did not interfere in the analysis. The condensable-impurity type mixture was separated by immereing almost thp entire finger-type gas bottle in liquid nitrogen for 15 to 20 minutes, then the noncondensable gases were pumped off, the stopcock was closed, and the entire trap was allowed to warm to room temperature before introducing the condensable gas into the mass spectrometer. Trial ex eri of ments showed no appreciable fractionation (less than components using freezing times of 5 minutes to 1 hour and impurities and internal standard concentrations in the range of the sample used. The separated components were run, scanning all peaks a t increased sensitivity except the internal standard peaks which were scanned a t normal sensitivity. (5) Calibration runs on each of the impurity gases at increased sensitivity.
14.
Calculations. The spectra obtained were analyzed by the usual mass spectrometric analytical methods. The calculations of the composition were made as follows: letting P I , fix, P A , PB, etc., be the microns of pressure observed for the internal standard gas, I , main component, M , and impurity gases, A , B , etc., respectively, from run 1 and the calibration runs, the mole ratio of internal standard to main component, R l , ,M, is calculated. = P.U
RIIM
From run 2. and the calibration runs: ?! PI
=
R A / I ; L!? = RBI I; etc. PI
where Ra/ 1, RE,I , etc., are the mole ratios of impurity gases to the internal standard. Combining these mole ratios, the mole ratio of each impurity to the main component is calculated.
R I I MX R A I I= R A I M ;etc.
(3)
ANALYTICAL CHEMISTRY
1748 Table I.
Noncondensable Impurities in Condensable Gases
for these components by removing all other cornponenta from the spectra and adjusting the respective contributions of carbon monoxide and nitroeen to the 12. 14. 16. and 28 Deaks to give the best fit to the resi&al spectrum on all these peaks. The spectrometer used shows a background at masses 12, 16, and 28, which is believed due to carbon monoxide produced on the filament, and the amount of this background varies with the composition of gas being run. The existence of this background makes an absolute carbon monoxide-nitrogen split difficult, although the split on any given sample of gas is reproducible. In the case of methane in sulfur dioxide, the use of neon as an internal standard for methane and use of liquid nitrogen separation is probably not a justifiable procedure considering the relative vapor pressures of neon and methane a t this temperature. Assuming Henry’s law can be applied under these conditions, the mole fraction of solute being very low and there being no obvious interactions ( 5 ) , methane would have an ideal solubility about 16 times as great in sulfur dioxide as oxygen, about 20 times as great as argon, about 40 times as great as carbon monoxide, and about 76 times as great as nitrogen, if all were present a t the same partial pressure: while hydrogen and neon would have negligible solubilities by comparison. For samples containing methane impurities the use of a higher condensation temperature and different internal standard should produce better results. The absolute accuracy of condensable impurities in air is about the same as that of noncondensable impurities, as is shown in Table 11. In this example results for all the condensable impurities are a few per cent low. This is probably due to adsorption on the glass disk and walls of the mixing chamber in the case of oxygenated impurities, and absorption in stopcock grease in the case of pentane and ethyl bromide, resulting in the actual composition being lower than that calculated from the dipper volumes. ,
hlole % Added Found
A
Mixture I
COP(by difference)
Air
co
Ne-internal standard Mixture I1 Propane (by difference) He Hz 0, Ne-internal standard Mixture 111 COZ(by difference)
n.
Cz co
Se-internal standard Mixture IV SOP(by difference) 0. ~. Nz
99 66 0 127 0 208
99 67 0 129 0 197
+ O 002 -0 011
99.74 0.112 0.112 0.034
99.74 0,114 0.117 0.032
+0.005
99 45 0 111 0 222 0 215
99.46 0.109 0 222 0 205
-0 002 0 000
99.46
99,50 0.034 0.045 0.102 0.198 0.086 0.034
0.036
0.037 0.116 0.206 0.114
co
Ha CHc A Ne-internal
0.035
standard
co
Ne-internal
-0 002
-0 010
.....
-0
002
+o 008 -0 014
-0,007 -0 028 -0 001
Analysis. % I I1 99.847 99,842 0.102 0.102 0.017 0.018 0.034 0.038
hlixture V (unknown) CO? (by difference) On Nn
..
, .
+0.002
standard
Table 11. Condensable Impurities in Noncondensable Gases Mole % Added Found
hliuture I Dry COS free air (by difference) Ethanol Acetone Ethvl ether n-Pintane Ethyl bromide SO-internal standard
99.62 0.087 0.032
99 66 0 072 0.028 0 043 0 074 0 125
0.048 0.084 0.I30
APPLICABILITY OF METHOD
.....
-0.015 -0.004
-0.005 -0.010 -0.005
RA,.$r
(1
Mole % M = 100
+ RAIM+ RB/M+ . . . )
(1
+
(4)
1 RAIM
f RBIMf
,
A
From the mole ratios of impurity to main component the molar composition is calculated by simple algebraic methods, Mole % A = 100
I
e)
By this method one does not independently calculate the main component percentage but only the difference between the perthe same result being achieved centages of impurities and 100~o, by direct subtraction. BLANK. In each case cited here except the unknown Mixture V, Table I, a blank was run on the pure main component gas using the identical procedure. I n a few cases 1 or 2 parts per 100,000 of impurities were found. These amounts were subtracted from the total in calculating the percentages found in the mixtures shown in Table I. RESULTS
The results on several synthetic mixtures are shown in Tables I and 11. The accuracy of the method for noncondensable impurities in the range of 30 to 200 parts per 100,000is within 5%, based on the impurity added, except for carbon monoxide, which seems to be uniformly low by about 10 parts per 100,000, and methane in sulfur dioxide (Mixture IV), which is low by about 25%. I n mixtures of carbon monoxide and nitrogen, the analyses were made
The method is applicable in principle to the determination of much lower concentrations of impurities than those used in the experiments here. I n practice the method will be limited by the linear range of the spectrometer used; the sensitivity ratios of the internal standard and gas components; the amount of sample available or which can be handled; and differential solubility effects of the internal standard and impui-ities in the condensed phase, which will become more pronounced and may lead to serious errors in the determination of very small amounts of noncondensable impurities. KOattempt was made to extend the method to the range of parts per million when it was found difficult to prepare samples of condensable gases free of noncondensable impurities to much better than one part per hundred thousand. Therefore, no lower limit on the applicability of the method can be set from the experiments reported here The extension of the method to other condensation temperatures and the use of other internal standards is obvious. The main component need not be a pure gas but can be any mixture which can be analyzed by ordinary methods either on the total gas run with the internal standard or in a separate run using the original gas mixture, Thus in Mixture I, Table 11, the results could have been reported in terms of oxygen, nitrogen, and argon contents plus the condensable impurities just as well as in terms of air and the impurities. I n certain cases, as suggested by Hindin and Grosse ( 6 ) ,it might be convenient to add two internal standards to cover impurities in different concentration ranges or use two standards and two condensation temperatures to achieve the maximum sensitivity on components cliff ering widely in properties. Each case encountered must be considered separately, and the procedure adapted to fit the needs of the mixture under consideration. ACKNOWLEDGMENT
This work was performed under the auspices of the U. S. Atomic Energy Commission. The author wishes to express his thanks to
V O L U M E 2 5 , N O . 11, N O V E M B E R 1 9 5 3
1749
Laurin Tolman for aid in running the samples on the mass spectrometer. LITERATURE CITED
(I) Charlet, E.
“-4xewT~~~ l \ ~ a s sspectrometer sample
Introduction System Employing a New ‘Mercury Sealed ~carp. orifice’ Iralve,” presented at consolidated ~ Group Meeting, Yew Orleans, May 18,1950. (2) Consolidated Engineering Corp., Pasadena, Calif., Bull. 1817, September 1950. (3) Grosse, A. Hindin, S . G . , and Kirshenbaum, A . D., h . 4 ~ CHEM.,21,386 (1949). (4) Happ, G. P., Stewart, D. W., and Brockmyre, H. F., Ihid., 22.1224 (1950).
v.,
.
(5) Hildebrand, J. H., and Scott, R. L., “The Solubility of Noneiectrolytes,” 3rd ed., pp. 26, 239, New York, Reinhold Publishing Corp., 1950. (6) Hindin, S. G., and Grosse, A . Tr., ANAL.CHEM.,20, 1019 (1948). ( 7 ) Purdy. K.11..and Harris, R. J.. Ibid.. 22. 1337 (1950). ( 8 ) Shepherd, hf., Rock, S.X , Howard, k., and Stormes, J., Ibid., 23,1431 (1951). ~(9) Thomas, i €3. w., and ~ SeYfried, ~We D., Iba.,~ 21, 1022 (1949). ~ i (10) Washburn, H. W., and Austin, R. R., “Some Instrumentation Problems in the Analysis of the Atmosphere” (proceedings of the First Sational Air Pollution Symposium, Pasadena, Calif., November 1949),p. 69, Stanford, Calif., Stanford Research Institute, 1950. RECEIVED for review May 13, 1952.
Accepted June 3, 1953.
Separations with a Micro Mercury Cathode RICH.IRD B. H.4HhT1 ‘4nalytical Chemistry Dicision, Oak Ridge Xational Laboratory, Oak Ridge, Tenn. conventional mercury cathode (9, 4 ) used for separations Tof metals is not very satisfactory with small volumes of soluof the mercury in contact with the sohHE
tion, because the surface tion is limited. Keeping the solution thoroughly stirred and avoiding spray losses are difficult in electrolyzing small volumes. These difficulties can be eliminated by using a rigid metal cathode plated with liquid mercury. The mercury surface is thus increased twofold, as both surfaces of the electrode are in contacat
A 5-mil silver foil is used, spot-welded to a platinum wire. The foil is cleaned by rubbing with fine-gage steel wool, then bent into a semicircular shape and immersed in 6 N nitric acid for a minute or two to clean the surface further. Mercury is applied to the surface of the electrode by electrolysis of a 10% solution of mercurous nitrate containing a few drops of nitric acid. Electrolysis is continued until the surface of the silver foil is completely covered with liquid mercury and a noticeable deposit of liquid mercury hangs on the bottom of the electrode. The electrode is then removed from the mercurous nitrate solution and washed with water. The electrode is conditioned by placing it in a dilute sulfuric acid solution and electrolyzing a t a current of 1 ampere for about 15 minutes. It is then ready for use. Procedure. 4 3- to 5-ml. volume of the solution to be analyzed (0.5 to 1.0 N in sulfuric acid and free from other anions) is placed in the electrolytic cell, a flat-bottomed tube of dimensions shown in Figure 2. The prepared cathode is introduced to the bottom of this tube. .4 platinum wire, approximately 12-gage, serves as the anode. The mercury-coated cathode should be completely immersed in the solution; otherwise mercury will dissolve a t the liquid-air interface. The cell is suspended in a water bath a t room t.emperature to effect cooling during the electrolysis. The solution is electrolyzed 1 hour, the current being 1 ampere during the first 45 minutes and 0.5 ampere during the last 15 minutes. Higher current will cause mercury to be lost from the cathode. The sides of the cell should be washed down a t 15-minute intervals with a small amount of water. At the end of 1 hour and with the current still on, the cathode is lifted from the soluI + tion and washed immediately with a small amount of water, the washings going into the electrolysis cell. The solution remaining in the electrolysis cell is now ready for analysis.
-
-I
CM-I
Figure 1. Construction of Cathode with the solution, and stirring is accomplished by the evolution of gases during electrolysis. A rigid mercury cathode of brass wire mesh coated with mercury was first used by Pawcek ( 5 ) . 4 more detailed investigation was made later by Boettger (f). These investigators used large electrodes and were interested primarily in weighing the deposited metals, rather than in separations. I n the author’s investigations an electrode of silver foil 1.5 X 2 cm., coated with mercury, was used. CONSTRUCTION OF ELECTROLYSIS CELL
Preparation of Electrode. shown in Figure 1. 1
The cathode is constructed as
Present address, Wayne University, Detroit 1, Mich
Figure 2. Electrolytic Cell
The efficiency of the cathode was tested by removing iron and/or c o p p e r f r o m p u r e s o l u t i o n s of aluminum(III), uranium(VI), and zirconium(1V) ions (Table I). These data indicate t h a t the micro mercury cathode made separations as effectively as the conventional mercury cathode (3, 4), the separable metal ions being completely removed as shown by colorimetric and radiochemical tests. Iron and copper were determined colorimetrically with 1,lO-phenanthroline and diethyldit h i o c a r b a m a t e , respectively. The quantitative recovery of the remaining metal ions was shown by precipitation of the aluminum as the hy-
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