ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Industrial A p p k W. A. MORGAN AND G. JERNAKOFF General Elecfric Ca., Schenecfady,
N. Y.
K. P. LANNEAU Esso Sfandard O i l Co., lauisiana Division, Bafan Rouge, la.
The design and development of an ion resonance mass spectrometer for industrial applications has recently been completed b y the General Electric Co. This program was carried out in cooperation with Esso Standard Oil Co. which assisted in the application and performance evaluation phases of the program. The ion resonance mass spectrometer, exclusive of recorder, fits within a cabinet 24 X 30 inches on the base b y 47 inches high. The mass spectrometer tube and associated vacuum system are constructed of metal. All mechanical and electronic components are designed to meet industrial requirements. Qne o f the most important uses for this instrument will be as a continuous process monitor. Some of the other uses for this instrument are as routine gas analyzer, leak detector, and trace constituent analyzer. This paper describes the instrument, presents perfoi lllunce data, and discusses industrial applications.
s
INCE World \Tar I1 a growing interest has been developing in
continuous gas analyzers. The reasons for this trend are the increased competition in the field of process manufacturing and the development of an interest in completely automatic process equipment. Many types of continuous gas analyzers are already on the market. These instruments are satisfactory for the types of analysis for n-hich they were designed. Recently, however, a need has developed for a continuous gas analyzer which could be applied t o more diversified problems. Since mass spectrometers have been used for many years as versatile gas analyzers in the laboratory, i t was reasonable to believe that their inherent versatility could be incorporated i3to an industrial instrument.
sA'!pLE-7 4
OSCILLATOR
1
EMISSION REGULATOR
\ \ i t h this objective in mind, development work was started on a mass spectrometer t'o be used for continuous gas analysis. As the development and testing phases of the program progressed it became evident that the instrument was also suitable for routine gas analysis ~ o r in k t,he laboratory; for trace detection and reaction studies, and for special leak detection problems. Early in the development of the ion resonance mass spectronieter it vas desired that the instrument be not only technically excellent but also in a form applicable for customer problemt;. Therefore, a joint development program was established betwen the General Electric Co. and the Esso Laboratories of the Esso Standard Oil Co. of Baton Rouge, La. The spectra and analj-tical data presented in this paper were obtained with an instrument under test' a t Esso Laboratories. Figure 1,is a block diagram of t,heion resonance mass spect,rometer. The instrument consists of three major sections: the vacuum pumping sy5tem, the mass spectrometer tube! and the associated electronic components,
VACLUN
MAGNETIC FIELO
, olL&JLT,
jilt ititt f
OUT
AMPLIFIER
DIFFUSION
PUPP
MECHANICAL
I
ELECTRON PATH ION PATH BOOSTER PUMP
Figure 1.
9404
Block Diagram of ton Resonance Mass Spectrometer
-
8,
,
(~i't~i
lit,'
Figure 2. Ion Resonance Mass Spectrometer Operating Principle
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
PROCESS INSTRUMENTATION mahs spectrometer the mass spectrum is scanned by varying the frcquency of the applied r-f voltage. Several advantages result from the use of the ion resonance principle: 1. There are no high potentials applied to the tube, so the electrical leakage problem is virtually eliminated. 2. There are no slits for resolving the ion beam, so the effect of surface potentials and deposits as well as the problem of dimensional stability are minimized. The absence of slits also contributes to the high sensitivity of the instrument. 3. The resolution of the tube is variable by means of externally controlled potentials. This readily permits maximizing sensitivity for detection of trace constituents or maximizing resolution for special analysis problems. 4. Because of the spiral path of the ions in the strong magnetic field, high resolution can be obtained in a small compact tube that serves as both ion source and analyzer.
Figure 3.
Ion Resonance Mass Spectrometer Tube Assembly
Ion Resonance Tube A simplified representation of the ion resonance mass spectrometer ion source and analyzer is shown in Figure 2. The principle of operation is basically quite simple ( 1 ) . Ions are formed by bombardment of the molecules of the sample gas with electrons emitted from a hot filament. I n the older types of spectrometers, these ions are generally accelerated into another region where they are separated by a magnetic field according to their mass to charge ratio In the ion resonance tube the ions formed about the electron beam are immediately separated according to their mass to charge (m/e) ratio as they are accelerated by a radio-frequency (r-f) voltage toward a small collector inserted in the ion source. Only ions of a particular m/e ratio having a normal resonant frequency in the magnetic field equal to the frequency of the r-f voltage, can obtain enough energy to reach the collector. Thus, it is possible to detect ions of a given m/e ratio by application of an r-f voltage of the appropiiste frequency. In the ion resonance
Figure 4. Ion Resonance Mass Spectrometer Tube Element Assembly from Filament Side of Assembly July 1954
Figure 3 shows the complete tube and sample system assembly of the ion resonance mass spectrometer. The envelope and all internal parts, exclusive of insulators, are made of metal. The tube is attached to the vacuum pumping system by means of a flanged joint sealed with a Teflon gasket. During an analysis, mm. of the operating pressure within the tube is about 1 X mercury. The instrument is equipped with a simple, continuous sampling system which is suitable for analysis of process streams or bottled gas samples. The conventional type of expansion volume system may be attached when required. The tube element assembly shown in Figure 4 contains both the ion source and the analyzer. The portion of the tube element assembly in which the ions are formed and resolved is about the size of a penny matchbox. Because of the design of the various parts in the analyzer, precise alignment of the tube parts is easily obtained.
Ion Resonance Mass spectrometer Figure 5 shows an interior view of the instrument with the rear door of the cabinet open. The vacuum system which is in the foreground of this picture consists of a high speed oil diffusion pump, booster pump, and mechanical fore pump. The system is all metal in construction and incorporates a high vacuum valve
Figure
5. Ion Resonance Mass Spectrometer
Rear view with door open showing vacuum system and general layout of instrument (leff); front view with door open showing arrangement of electronic components (right)
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT
20 c -
1
Figure 6.
8
1
le
1
1'
I
16
I
" I
Spectrum of Water and Heavy Water
isolating the tube from the pumping sj-st,eni and a charcoal trap rather than a liquid nitrogen or dry ice acetone cold trap. These features result in simplified maintenance and permit long periods of unat'tended operation of the vacuum system. The electronic components are mounted a t the front of the inst,rument. The electronic circuits are conventional in design and have been subdivided into four separate panels to facilitate trouble shooting, to simplify maintenance, and t o permit customizing particular cir cuits to meet unusual applicat,ion problems. The panels consist of an oscillator, amplifier, emission regulator, and vacuum metering panel.
t'rum of n-butane. The cracking patterns of hydrocarbon gases on the ion resonance mass spectrometer appear to, be more or less conventional. The sensitivity to hydrocarbon gases is high and the resolution of the instrument is more than adequate for hydrocarbon gas analysis. There is a 100% separation betn-een masses 57 and 58. Figure 8 s h o w the spectrum of a liquid hydrocarbon, n-hept,ane. There is complete resolution between masses i o and 7 1 . The instrument possesses a high degree of stability in the presence of hydrocarbon samples. A number of other compounds such as organic halides and oxygenated materials have been tested and likewise produce no undesirable effects in the instrument. Figure 9 shows a spectrum of ethyl ether that was obtained on the ion resonance mass spectrometer. A complete scan of the spectrum from mass 12 to mass 80 is covered. This type run requires 7 to 10 minutes. One of the interesting features of the ion resonance principle is the fact that resolving power is very high a t low mass numbers. This permits it to perform in an entirely routine manner a special analysis which is normally somewhat difficult with a mass spectrometer of this size or by other methods of analysis. Figure 10 shows the spectrum of a mixture of helium and deuterium in the mass 4 region. Here a complete separation of atomic helium and molecular deuterium has been accomplished, permitting analysis of mixtures containing these gases. The two peaks shown have a mass separation of only 0.026 mass unit. Resulk of analytical studies conducted on different models of the ion resonance mass spectrometer are very satisfactory. Table I shows the analysis of simple synthetic blends. The analysis for the helium-deuterium mixture Fas performed by means of measurements a t the mass 4 doublet shown in t,he last figure. The analyses of the second and third samples in Table I show good results on simple hydrocarbon mixt'ures. The data indicate that the ion resonance mass spectrometer is capable of producing analyt,ical results comparable t'o those obtained on larger and more expensive laboratory instruments. Table I1 shorn the analysis of a i-component hydrocarbon mixture. O 6 n g to the good resol\.ing power of the instrument, this analysis has been made using t'he same masses which would normally be chosen when setting up the matrix for this mixt,ure.
Performance Data Spectra of a number of gaseous and liyuid materials have been obtained on the ion resonance mass spectrometer Figure 6 shows an actual photograph of a spectrum of a a t e r and heavy water. Instrument stability was not adversely affected bq the water sample. In all the spectra which are presented in this paper the mass peaks which exceed full-scale reading on the recorder are automatically kicked back and attenuated by a factor of 3 euch as the mass 18 peak on this spectrum. The full-scale sensitivity after attenuation i s 100 mv. on all charta. A considerable amount of the early test work on this instrument has been devoted to studies with hydrocarhon gases. Figure i shows a typical spec1406
Figure 7.
Spectrum of n-Butane
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46, No. 7
PROCESS INSTRUMENTATION
N. w e
Figure 8.
Spectrum of Liquid Hydrocarbon n-Heptane
Table I11 shows the results of a IO-component gas analysis. The accuracy, reproducibility, and stability of the instrument are clearly indicated by the data shown in Table IV. Here the peak heights for several masses in the n-butane spectrum have been recorded on successive days with no intervening adjustments made on the instrument. The over-all sensitivity stability is excellent, and only very minor changes in the cracking pattern have occurred during the 9-day period. Table V shows the sensitivity of the instrument to various gases. (All data were taken with a sample pressure of 30 microns and a 0.001-inch diameter molecular leak.) Part A of the table
Table I.
Spthesis,
KO.
Analysis, %
%
Compound Helium Deuterium
50.0 50.0
49.6 50.4
... ...
...
41 43
Isobutylene n-Butane
50.0 60.0
50.0 50.0
50.0 50.0
50.7 49.3
16 44 58
Methane Propane %-Butane
33.3 33.8 33.3
32.3 33.1 34.6
32.8 32.8 34.4
32.6 32.9 34.5
...
...
...
. ..
Mass NO.
Compound
4 16 26 30 40 44 53 56 43 58
Helium, H e Methane, CHa Ethylene, CzH4 Ethane, CzHs Propylene, CsHe Propane, CaHs Butadiene C4He Butylene, ’CaHs Isobutane, i-C4HIO n-Butane, n-CdHio
Table IV.
Ion Resonance Mass Spectrometer Typical Gas Ana Iyses
Xiaes
Ion Resonance Mass Splectrometer Typical Gas An a lyses
Table 111.
Mass No.
1st
27 29 41 43 58
66.1 82.1 70.4 206.0 32.4
’
Synthesis,
Analysis,
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
9.7 10.3 10.3 10.4 10.1 10.2 9.6 9.9 9.7 9.8
70
%
Ion Resonance Mass Spectrometer n-Butane Cracking Pattern Log Peak Heights Recorded on Successive Days 2nd 4th 7th 8th 66.0 82.0 70.5 205.4 32.1
66.8 83.0 72.1 209.0 33.2
68.0 85.0 72.6 210.4 33.3
67.6 84.1 71.8 209.0 32.6
9th 67.7 84.3 72.0 207.2 32.6
... 32.3 33.4 34.3
Table V.
Ion Resonance Mass Spectrometer Sensitivity Data
(All d a t a taken with a sample pressure of 30 microns)
Table 11. Mass NO. 16 30 44 54 56 43 58
July 1954
Ion Resonance Mass Spectrometer Typical Gas Analyses Compound Methane, CHI Ethane, CzHe Propane, CaHs Butadiene CaHa Butylene, b H s Isobutane, ~‘-CPHIO n-Butane, n-CdH~o
Synthesis,
Analysis.
7.7 18.8 47.5 0.0 0.6 15.1 10.3
7.7 18.5 46.8 0.1
%
Gas
A.
%
0.8
14.5 11.6
Ion Current, Amp.
Normal operation with complete resolution between adjacent peaks n-Butane
B.
Peak
43
5
x
10-11
Instrument adjusted for maximum sensitivity with reduced resolution Argon Methane Helium Hydrogen
40 16 4 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
3.5
x
10-10
1.0 1.5
x
10-10 10-10
2 . 5 X 10-10
x
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ENGINEERING, DESIGN, AND PROCESS DEVELQPMENT
I
l i /
'
1 I
x
1
:+-Figure 10. Spectrum of Mixture of Helium and Deuterium in Mass 4 Region
indicates t'he ion current which reaches t,he collect,or from the n i a e 43 peak of n-butane. This ion current. was recorded while the iiistrument \vas adjusted for normal operation Bith completc resolution of the mass 13 peak from the adjacent mass 1 2 and 14 peaks. Part B of the table shows the ion currents which reach the collect,or when the instrument is set up for maximum sensitivity. Resolution between adjacent peaks is not obtained under these conditions a t the highrr mass numbers.
Applications The high versatility of the ion resonance mass spectrometer if perhaps best utilized in the continuous analysis of gas streams on pilot plant instaIlations, where a ma,simum amount of analytical data is usually obtained. The analytical accuracy of the instrument and its ability to monitor continuous streams make it suitable for this application. I n many cases 11-here analytical informnt,ion is required for process control purposes, but not neceswrily on an entirely continuous basis: the ion resonmce m spectrometer Fill be useful as an easily operated and maintained instrument readily available for spot analyses of process streams. For some time a less expensive routine laboratorj, analytical mass spectrometer has been needed that ~vouldbe easier to maintain and t,o operate than present-day analytical spectrometer$. The ion resonance mass spectrometer meets these requirements. The data shonn in Tables I through IV indicate the applicability of the instrumeiit t o complex gas analysis. Several features of the instrument make it attractive for laboratory installat'ion. The simplicity of both the vacuum and electronics system as m l l as the analyzcr tttbc itself should reducc the maintenance cost. The problem of training personnel t,o operate the Ppectronietcr i p minimized because of the simple cont,rol panel and :sample system. In addition to t'he use of the ion resonance mass spectrom1408
I N D U S T R I A L A N D E N G I N E E R IN G C H E M I S T R Y
Vol. 46, No. 7
PROCESS INSTRUMENTATION eter for routine gas analysis in the laboratory, it may be used for the analysis of light liquids because of its good resolving power. However, only limited data have been obtained on this type sample. The ion resonance mass spectrometer is also applicable to a number of special problems. The stability of the instrument is such that various types of gaseous and liquid materials which often have deleterious effects on mass spectrometers can be analyzed in the ion resonance mass spectrometer without difficulty. The high speed of response of the instrument permits its use in the analysis of product gases in a study of process kinetics. Because of the variable resolving power, the instrument may be set
up in a few minutes for extremely high sensitivity to detect trace constituents or leaks. This mass spectrometer has been designed to be built a t reasonable cost to meet the requirements of several applications. Major emphasis has been placed on producing a simple rugged instrument that is easy to operate and to maintain, and that provides high sensitivity and good resolution throughout the mass range 2 to 100.
Literature Cited (1) Sommer, H., Thomas, H. A , and Hipple, J. A., Phus. Rev., 82,
697-702 (1951). RECEIVED for review September 7, 1958.
ACCEPTED February 22, 1964.
Recording Differential Refractometer Continuous Plant Stream Monitoring D. N. CAMPBELL, C. G. FELLOWS, S. B. SPRACKLEN,
AND
C. F. HWANG
lnsfrumenf Division, Carbide and Carbon Chemicals Co., So. Charlesfon, W. Vu.
A
recording differential refractometer is described that measures continuously the change of refractive index of flowing liquid streams. The instrument is constructed of readily replaceable integral components that are designed for long term maintenance-free operation. A transistor amplifier having a gain of approximately 40,000 i s used for perhaps the first time in an instrument of this type. A sealed beam head lamp i s used for the light source and the liquid cell system comprises in effect a prism and lens, which serve as the optical system. The instrument has a sensitivity of 3 X 1 0-6 refractive index units.
THE
measurement of one component, in a multicomponent liquid stream or even the measurement of one component in a binary mixture is often difficult. The problems involved in plant stream measurement are far greater than commonly encounte:ed in the laboratory. Many of these difficult,ies are due to the inherent nature of the plant stream itself, which, as commonly encountered, is composed of chemical species that are very similar. Furthermore, these substances quite often contain the same functional groups and have nearly identical chemical and physical properties. To measure quantitatively one component in such a stream requires an instrument of either high Reneitivity or high selectivity. In the case of refract'ive index measurements, selectivity is out of t,he question, but' sufficient sensitivity in refractometers is at'tainable although not, to the author's knonledge, available in presently produced commercial instruments. Variations in temperature of both plant stream and instrument present one of the most difficult problems of continuous refractive index mpasurement. The average kmperature coefficient expressing the variation of refractive index ( n )with temperature, degree Cent,igrade (' C.) is for most liquids
dn' - = dt
0.0004
Therefore, to measure the refractive index of a liquid continuously and reproducibly to a few parts in the sixth place requires that the temperature of the liquid either be controlled to better than =!c0.0lo C. on an absolute basis, or else develop a refractometer in nhich this absolute temperature effect is reduced to a negligible degree. Temperature control to this precision is most difficult to maintain and particularly so for instruments used for the continuous monitoring of process streams. July 1954
Various types of refractometer have been described in the literature. In general, those described have been either of the conventional laboratory type 4.2, 4),standard type of recording refractometer (1, 6, 7 , 8), or of the differential type ( 3 , 6). This type possesses some outstanding advantages which are of paramount importance to an instrument to be used for the continuous monitoring of plant streams. These advantages are: 1. The instrument produces a differential measurement not based on absolute quantities. 2. The temperature of the instrument and sample need not be constant but may vary as long as the reference and sample liquids have approximately the same temperature. 3. The instrument is relatively insensitive to pressure, color change at the equicomposition point, spectral purity, and small amounts of dirt that may accumulate in the sample cell.
In an effort to minimize the layge errors due to temperature accompanying the ordinary type of refractometer and to improve the relative temperature insensitivity of existing differential refractometers, the design and development of a plant stream differential refractometer were undertaken. Further plant requirements for this instrument were:
1. Long term-week after week-maintenance-free operation. 2. Continuous-week after ~~,eek-reproducibility of measurement to a t least 10-5 refractive index unit. 3. Easily adaptable to automatic process control. 4. Type of construction that provided a vaportight case that would be continuously purged with an inert gas. As developed and reduced to practice, the instrument functions as follows: Chopped light emerging from a rectangular slit as a wedgeRhaped beam passes through the cell system and is then reflected from a plane mirror onto a pair of selenium photronic cells. Be-
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