Recording Mass Spectrometer for Process Analysis A. 0. NIER', T. A. ABBOTTa, J. I(. PICKARD, W. T. LELAND', T. I. TAYLOR3,C. M. STEVENS, D. L. DUKEY', A N D GERALD GOERTZEL4 The Keltex Corporation, New York, N. Y .
A recording mass spectrometer is described which has been used for making continuous analyses of the process gas stream in the uranium gaseous diffusion plant. The strategic distribution of instruments in the plant assures rapid detection of any inleaking contaminants. Modifications of the system described should prove useful in other large industrial installations.
A
Ability to measure each component of the process stream Negligible consumption of the extremely valuable material to be analyzed Rapid response to changes in the composition of the gas stream Continuous automatic recording of the principal components of the gas stream
LTHOUGH the mass spectrometer has found considerable use as a tool for performing both isotope and gas analyses in the laboratory (1, 3, 6),there is no record of its application to the continuous analysis of a process gas in an industrial installation. The present paper describes the essential parts of a mass spectrometer system which was employed for analyzing the contaminants in the process stream of the Oak Ridge gaseous diffusion plant for the separation of the uranium isotopes. A recording mass spectrometer was selected for making the analyses because of its many advantages over the other instruments considered. These advantages included:
.
In many respects the spectrometer resembled the conventional type instruments used in performing laboratory analyses. Figure 1 shows a schematic drawing of a typical installation. The g a t o be analyzed was allowed t o flow continuously past a specially designed adjustable Yeak." The flow through the leak was measured by a special Pirani gage flowmeter in the line leading t o the spectrometer tube. Conventional-type electronic stabilizing circuits (3) supplied the necessary voltages and currents to the spectrometer. Automatic switching equipment in the multipoint recorder varied the ion-accelerating potential through
1 Present address, Department of Physics, University of Minnesota, Minneapolis, Minn. 2 Present address, Standard Oil Co., Chicago, Ill. 8 Present address, Department of Chemistry, Columbia University, New York, N. Y. 4 Present address, Clinton Laboratory, Oak Ridge, Tenn.
.
-...r:,. ... i
I
jj
FROM INCOMING MANIFOLD
I
w
'PIRANI GAGE CONTROL
CIRCULATING
L PREAMPLIFIER MANIFOLD ASSEMBLY PURGING 6 EVACUATION EQUIPMENT
/\
1
I
1
1
-
~
v. eo CYCLE POWER SUPPLY
I I5
RECORDER
T O OUTGOING MANIFOLD
Figure 1.
I 4
Schematic Diagram of Recording Mass Spectrometer, Showing Alternate Gas Inlet System and Interconnections of Component Parts
188
V O L U M E 20, NO. 3, M A R C H 1948 a prearranged sequence of values in order t o select the masses
desired for making the analysis. The ion currents were measured by an electrometer tube preamplifier followed by an inverse feedback amplifier which supplied the necessary signal t o the recording potentiometer. M A S S SPECTROMETER TUBE
'4 drawing of the spectrometer tube is shown in Figure 2.
The sample gas enters the ionization chamber through the inlet lead. Because of the construction of the tube. the m-essure in the ionization chamber is higher than in the rest of the tube. The sample gas is ionized by electrons which are emitted from the heated filament, aligned by a magnetic field, and accelerated by a potential differencebetween the filament and the ionization chamber. Since the useful electron beam consists of a fine pencil of electrons which is directed into the trap, the trap current is an accurate indication of the electron current. Ions formed by collision of the electrons with the gas molecules are forced through the slit in shield S and are accelerated through plates J1, J 2 , J 3 , J5, and G by the voltages on them. Plates J1 and J 2 , which have independently adjustable voltages, serve to bend the ion beam t o one side or the other to compensate for imperfections in construction as well as for the slight bending caused by the magnetic field used in aligning the electron beam. The plates marked G are fastened to the tube body, which is electrically grounded. The ion beam is resolved in passing through the transverse magnetic field created by the permanent magnets.
189
Potential between S and J 3 Potential between J 5 and G #
Potential between 5 and T Potential between filament and S Accelerating potential between S and G Total electron emission Total trap current Electron aligning magnet strength Main magnet strength Mass 28 accelerating voltage
120 volts 0.6 potential between S
and G
90 volts
75 volts
Adjuatable 0 to 2500 volts 100 microamperes 100 microamperes 250 gauss
3250 gauss 1050 volt8
Although the ions impinging on the collector and nearby surfaces produce secondary electrons, these do not contribute to the collector current, inasmuch as a weak magnet field is present. This field is produced by a small instrument magnet. ELECTRICAL COMPONENTS
The electrical components for the recording mass spectrometer consist of switching, regulating, and amplifying equipment. These components are separated on a functional basis and mounted on separate relay rack panels. The interconnections which are made with plug-in type cables are shown iri Figure 1. All circuits, with the exception of the high-voltage supply which is self-stabilized, are operated from stabilized 115-volt, 60-cycle power. The ionizing and accelerating voltages of the spectrometer ion source are produced by the emission regulator and high-voltage The operating conditions for a typical recording mass specsupply and are controlled by the automatic switches in the recorder which operate relays located on the main control panel. trometer are as follows: The emission regulator supplies (1) regulated current to the Potential between S and J 1 Approximately 100 volts spectrometer filament, (2) stabilized voltage to the electron Potential between S and J2 Approximately 100 volts trap, and (3) stabilized voltages to the accelerating plates, S, J1, J2, and J3. Potentiometers and meters are used in each circuit for adjusting the voltages. The ion accelerating voltages for plates S and J 6 are provided by the high-voltage supply. Two potential dividers, one for manual and one for automatic operation, on the main control panel permit the selection GLASS ENVELOPE of the different accelerating voltages required to tune the spectrometer to the different mass numbers, although several specific voltages, corresponding to various components of the process gas, may be used in automatic operaMANENT MAGNET tion. FOR A L I G N8EAM ING The ion currents to the collector ELECTRON plate are amplified by an inverse feedback direct current amplifier. This amplifier has four stages, the first two of which, consisting of two 964 tubes and AL TO GLASS SE4L a 5 X 109 ohm input resistor, make up the preamplifier and are mounted in an aluminum box. This box is mounted SLIT WIDTHS on vibration insulators and connected INPLATE S t o the spectrometer tube by zi bellows I N P L A T E J3 . assembly. I N PLATE J5 The amplified ion currents are measI N PLATES C ured by a self-balancing potentiometerCOLLECTOR S L I T 22MM type recorder. A motor-driven switch in the recorder, operating in synchronism with the siritch used to change the O U T L I N E OF accelerating voltage, automatically selects any one of seven sensitivity factors between 1 and 100. The recorder has 16 channels; it records the amplifier zero and Pirani gage reading as well as the several mass ions on IRON POLE F4CES a 24-seconds-per-channel cycle. The COLLECTOR SLI entire 16 channels are used; thus some measurements are recorded more freCOLLECTOR PLAT€ quently than others. An ionization gage is used to measure the pressure in the spectromR E Y I B L E COUPLING eter tube. X special protective feature has been added to the ion gage control u hich prevents the filaments of the gage and spectrometer tube IO 15 20 from burning out by automatically SCALE I N CM cutting off their currents when the pressure exceeds a piedetermined value. Figure 2. Detail of Recording Mass Spectrometer Tube
-
ANALYTICAL CHEMISTRY
190 GAS INLET SYSTEMS
Two gas inlet systems are shown in Figure 1. The system chosen depends on the relative quantities of uranium hexafluoride in the sample stream. Since samples with high concentrations of uranium hexafluoride cause the rapid formation of insulating coatings on those electrodes in the spectrometer tube which are bombarded by electrons, they are not admitted directly into the tube but are f i s t passed through a chemical trap which removes the uranium hexafluoride, In this system the pressure in the spectrometer is adjusted in such a way that the ion current of each constituent a t the appropriate mass number is proportional to its concentration in the original mixture. This is done by building into the gas-inlet system a set of resistances and a Pirani gage; when the flow is adjusted so that the Pirani reading is held constant, the ion current of each constituent in the spectrometer is proportional to its concentration in the original mixture. This relation is strictly true only for low concentrations of impurities, and for high concentrations special calibration is required. If the samples contain small amounts of uranium hexafluoride, the second inlet system may be used. In this system the gases are admitted directly to the tube through the alternate leak without absorption of uranium hexafluoride.
laboratory gas analyses the problem may be eliminated by employing samples whose pressure has been reduced to a fixed value of 100 microns or less ( 2 ) and allowing the gas flow to take place through small holes whose diameter is small as compared with the mean free path, thus ensuring pure molecular flow. In the present instance this solution was not practical, since the manifold pressure was not always the same and not necessarily . in the proper absolute range. The problem was solved by using a length of capillary on the high-pressure side of the flowing adjustment part of the leak. The lower portion of Figure 3 shows the gas flow through the leak. There is practically no pressure drop in the capillary tube itself, essentially all taking place in the region clamped by the colletlike jaws. The capillary tube has a sufficiently large diameter so that the mean free path of gas molecules in it will always be greater, and viscous and not molecular flow will take place. It was chosen sufficiently long so that the gas just above the pinched region containing a slight excess of heavy molecules due to the partial fractionation produced in the pinch, could not diffuse back into the circulating sample stream. The capillary diameter, while sufficiently great to emure viscous flow within it, kept the mass flow velocity large enough to prevent backdiffusion. Thus the gas leaving the leak on the low-pressure side was representative of that in the circulating sample stream. It is apparent that the design of the leak involved a compromise between obtaining an accurately representative sample and
The adjustable leak with its capillary tube is an extremely low-flow nonfractionating valve. Its details are shown in Figure 3. It has been designed so that a flow rate of 25 cc. micron per second may be obtained when the absolute upstream pressure varies between 1 and 60 cm. of mercury with a downstream pressure of a few microns of mercury. The leak is adjusted by turning the handle which forces the shoes to move in and out along the inclined planes in the clamp arms, thereby closing and opening the annular space between the leak tube and the leak plug. In meeting the range requirements, the jaws move approximately 0.003 inch (0.0075 cm.) for 35 turns of the handle. A force of approximately 5000 pounds (227 kg.) is necessary to compress the leak tube. The performance of the leak depends on, the accuracy with which the parts are made. The critical dimensions and their associated tolerances are given in Figure 3. Whenever gas is allowed to flow from a high-pressure t o a low-pressure region as in a mass spectrometer, extreme care must be taken to ensure that the composition of the gas in the lowpressure region-Le., spectrometer region-bears a known relation t o that on the high-pressure side-Le., in the sample. Conventional capillary "leaks" consisting of pieces of drawnout glass tubes such as are commonly employed in the laboratory, needle valves, or other adjustable flow devices (4) will, in general, produce fractionation which depends upon both the composition of the gas and the pressure on the high-pressure side. In making
CIRCULATING SAMPLE STREAM
HIGH TENSILE s m L BOLT
SCALE
0 b
&" .i"
2"
1''
3"
ISOMCTRIC OF LEAU AND LEAK CLAMP c e i r i c A L oiMms/oyz 0.0 OF LEAK r u a E A T J A W . J ~ ~ O /. D. OF JAW .3695'?.0005" I D . OF LEAX TUBE A T J A W . J I Z 5 " O.D. OF LEAX PLUG AT JAW.3llS"i.OOOZ" CAPILLARY TUBE .OOSY.OO/ I.D , .O,?S'+O 0. 5 V 4 ? . I P S ' L O N G
GAS INLET
LEAK PLUG
A
U P I L L A R Y PLUG CIRCULATING SAMPLE S T R E A M 1
Figure 3.
Schematic and Isometric Drawing of Adjustable Leak and Capillary Tube Used on Recording Mass Spectrometer
V O L U M E 20, NO. 3, M A R C H 1 9 4 8 having a reasonable time response for the entire pressure and composition range. Had a longer capillary been chosen, a n even more accurately representative sample would have been obtained, but a t the expense of time response. Figure 4 shows how for a given mixture of nitrogen with uranium hexafluoride the composition of the gas stream leaving the leak would vary with upstream pressure with and without the use of a capillary.
PRESSURE UPSTREAM OF LEAK (CM. Hg ) Figure 4. Effect of Capillary Tube i n Compensating for Fractionating Property of Adjustahle Leak Measurements m a d e o n sample gas mixtures of IV? and UFO A. Leak without capillary t u b e B . Leak w i t h capillary tube
Ordinarily in a mass spectrometer the gas passing through the leak would pass directly into the mass spectrometer. However, because of the adverse effect of uranium hexafluoride on the ion source, this component is not permitted t o enter the source unless its concentration in the process stream is low. When the concentration is high this component is first absorbed by employing a “chemical trap” consisting of a small mercury reservoir shown in Figure 1. The mercury reservoir, n-hich is kept a t room temperature, produces mercury vapor which diffuses to the cold spots, condenses, and falls back into the trap. When uranium hexafluoride is flowing into the system it meets the mercury stream a t the first cold spot and a reaction takes place in which solid products are formed and accumulate on the walls of the tubing. The cold spots are held a t approximately 0” C. The one in the lead to the spectrometer tube limits the mercury pressure in the tube, whereas the one in the lead t o the leak confines the reaction region. To make certain that the mercury does not condense before reaching the cold spots, the warming fin is attached as shown in the diagram. The reading of the Pirani gage depends on the composition of the gas stream as well as the pressure a t the gage. For a flow of pure uranium hexafluoride the pressure is determined by the flow resistance of that portion of the gas line between the Pirani gage and the chemical trap. For a flow of nitrogen or air the pressure is determined by the resistance of that portion of the line between the Pirani gage and the diffusion pump. These resistances have been chosen so that when the Pirani gage reading is held constant the output of the amplifier corresponding to the nitrogen peak is approximately proportional to the concentration of nitrogen in the gas stream being analyzed. The instrument is calibrated with mixtures of nitrogen and uranium hexafluoride of known composition and a correction curve is provided to relate amplifier output t o nitrogen concentration. The pressure-sensitive element of the Pirani gage forms one arm of a bridge circuit. The unbalance of the bridge circuit is
191
read on an indicating millivoltmeter as well as on the recorder. The pressure-sensitive element, consisting of a 0.003-inch nickel wire, is encased in a None1 tube and has been found stable despite the corrosive nature of the sample gases. The normal operating pressure a t the gage is approximately 10 microns of mercury. PERFOR>IANCE OF RECORDING M A S S SPECTROMETER
Most of the recording mass spectrometers in the diffusion plant are used for automatic analysis of the process stream and are equipped with Pirani flowmeters and chemical traps for handling high uranium hexafluoride concentrations. A Spectrometer chart is shown in Figure 5. Referring to the chart, a nitrogen surge is observed a t approximately 1:55 A X . Concentrations of the gases for this test are indicated in the figure. The accuracy of their measurement is approximately 5% of reading, depending on the care taken in calibrating the instrument. The concentration of hydrogen fluoride cannot be determined accurately with the standard instruments, as this gas is strongly absorbed by the gas inlet system, and hence the response of the spectrometer is est reniely sluggish. The average consumption of uranium hexafluoride is less than 40 mg. per instrument per day. Thirty seconds after the composition of the gas in the sample stream has changed, the spectrometer tube ion current reflects this change. Rfost of this delay is caused by the time required for the gas to pass through the capillary tube a t the adjustable leak. The appearance of this change on the recorder depends on the printing cycle; for nitrogen which is printed every other point on the recorder, a change in concentration is recorded within a minute after it occurs in the saniplc stream. Several of the mass spectrometers are provided with the alternate inlet system as shown in Figure 1. These are used t o measure low concentrations of uranium hexafluoride and hydrogen fluoride. Concentrations of uranium hexafluoride as 2:30 2:20 2:lO
t-
1:30
I: 20,
Figure 5 . Typical Recording .\lass Spectrometer Chart, Showing Normal Concentrations of Process Stream Components in Granium Gaseous Diffusion Plant Amplification zero Oz m a s s 32, 0.270 Nz m a s s 14, 670 H F maes 2 0 , 2 70 C o t m a s s 44, 0.170 Fluorocarbon m a s s 69, 0.270 N2 mas8 28, 670 H. Total gas flow as measured hy Pirani gag? .4.
B. C. D. E. F. 6.
low as 0.1 mole yc and those of hydrogen fluoride of 1 mole % may be measured with an accuracy of loyo. The response time of these instruments is appreciably less than that of the others. The performance data cited here are by no means necessarily typical of what is t o be expected in all mass spectrometer installations. For example, laboratory spectrometers which can be checked and calibrated frequently ordinarily give analyses t o an accuracy of 1q or better. The present paper does demonstrate that by employing special methods such as the L1chemical trap” and Pirani gage flowmeter, a mass spectrometer may even be used with highly corrosive gases under adverse circumstances. No doubt there are other commercial processes in which mass
192
ANALYTICAL CHEMISTRY
-
1.60
-
140
-
120
-
100
-
>
I
W 9 4
5 0
>
I-
2
80-
t
3 0
a
w k 2I
60-
n
W+
40-
4
20
RC*
-
0 ;
k'
I
I
I
I
I
4 Ip I
I
,
I
A
,-,.dub cc;
I _ l
o+
M,L
MASS NUMBER
Figure 6. Typical Automatic Mass Scan of Air Taken on a Recording Mass Spectrometer Installed in Uranium Gaseous Diffusion Plant
spectrometers could be applied if modifications or extensions of
the principles employed here were used. When it is desirable to obtain a complete analysis of all impurities in the sample stream, the multipoint recorder is replaced by a single-pen recorder and the ion-accelerating voltage is varied continuously with time, so that the recorder draws a mass spectrum from mass 12 t o mass 500. Figure 6 shows a portion of such a spectrum when dry air is introduced into the instrument. The spectrum included several masses such as 20 (HF), 200 (HG'), and 100 (HG++) which are from residual gases in the system. ACKIYOWLEDGMENT
The Original design Of the recor'ng mass spectrometer Was made by members Of The Kellex and after modifications t o improve the reliability and facilitate the manufacture,
a large number of the instruments were built by the General Electric Company. The authors are indebted to 1. K. Brenholdt for his design and construction of many of the electronic components. Much of the mechanical design, especially of the mass spectrometer tube, is due to R. B. Thorness. LITERATURE CITED
(1) Hipple, J. A., J.A p p l i e d Phys., 13,551 (1942). (2) Honig, R.E., Zbid., 16,646 (1945).
(3) Nier, A. O.,Rev. Sci.Instruments, 11, 212 (1940); 18,398 (1947). (4) Nier, A. O.,Ney, E. P., a n d Inghram, M. G., Ibid., 18,191 (1947). (5) Washburn, H.W., Wiley, H. F., a n d Rock, S. M.. IXD. E m . CHEM.,ANAL.ED., 15,541(1943). RECEIVEDSeptember 20, 1946. Based on work done for the Manhattan Engineer Project under contract KO.W-7405 Eng. 23 a t The Kellex Corporation.
Determination of Potash in fertilizers E$ect of Saturation of Acid-Alcohol with Potassium Chloroplatinate 41. A. EWAN, 0. W. FORD, AND E. D. SCHALL Pccrdue University Agricultural Experiment S t a t i o n , Lufayette, Znd.
I
N T H E official A.O.A.C. method for determining potash in
fertilizers the potassium chloroplatinate is treated with acid-alcohol and washed with 80y0alcohol (3). A question has arisen whether this step causes low results due t o the solubility of the salt in the alcohol. Although the literature contains a number of references to the solubility of potassium chloroplatinate in alcohol of various strengths ( I , 2, 6, 7), no data have been found on the solubility in acid-alcohol. Accordingly, the purpose of the present work was to determine the practical significance of this solubility by using acid-alcohol previously saturated with potassium chloroplatinate and comparing with the official method in concurrent analysis of identical samples.
In addition to the study with acid-alcohol prepared from 80% alcohol as used in the official procedure (200 ml. of 80% alcohol plus 20 ml. of hydrochloric acid), this study includes experiments with acid-alcohol prepared from 95% alcohol (200 ml. of 95% alcohol plus 20 ml. of hydrochloric acid) which had previously been saturated with potassium chloroplatinate. The alcohol used in subsequent washing in each case was 80 and 95%, respectively. (This study also included the examination of the purity of the potassium chloroplatinate obtained with the two strengths of alcohol under the varying conditions.) At the suggestion of the general referee on fertilizer of the A.O.A.C. to the associate referee on potash, a comparison was made on one
