V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4 tion of groups with respect to their neighbors and is an invaluable aid in assigning structimil formulas, where this is possible. Certain
group^,
suclr
:I-
c.tli!-l, isopropyl, and
\ /'F C
a:: \vel1 as
/ \I€ many others give elid)- !,cc.ogniztd niultiplet patterns Phosphorus-31 signal< :ire :ilso adequate for high rc~olution nol,k in liquid samples :ind studit,+OE the pliosphorus-:3J high resolution nuclear niagnctic re.wiianw 3pectra should prove fruitful. Silicones and boranrls i,t~pros(~nt still other possible : ~ ~ c n u cofs iiivrstig:~tion. The, prot o!i y)eotr:i of these c.ompouiid~ are already revealing, n n d the, .~ilicon-2Uand boron-1 1 spc.c+ix, particul:uly the latter, are Iironiising possibilities. AIethotla of st:~iidardi~ing and classifying spectra a1 e I jriiig worltrtl out a t the present time. Tlie American Pctrolwrn Institute Iteeearch Projwt 44 is Ptudying ~ i ~ h e n i efor s nuc,lr;ir ni:rgnctic* reson:ince ( c:ition, and its recomiiic~iitl:itioir is exjieried to be issued as soon :IF rc~nsonalilycompkte :igrrenicnr c;tn Iw reached as to tlir nioct suitable form. Intensive study of liotli the theoretical aspects anti pr;ic,tic:tl of nucleur magnetic resonance high i,cpolution ap~~lications qiwtroscopy ZIP well as worli with broad resonances is going on i n both the universitios and in industrial laboratories. l ' r o g r w ~ is being nia& in improving resolution and stability to a dcyree not believed 1Jossibk only a short time ago Spectra of many ('ompounds are being cataloged and classified. AIethods of \volliirig n-ith liquefied gases and inelted solids are already well rst:iblished. A s tlic tccahiiique advaiires, many new applications of thi- esciting new xpecti~oscopicmcthod of investigation, both to tlicorctic~al a n d applied problems, slioultl appcar.
1403 (21 Bloch, I'., P h y s . Reo., 70, 460 (1940). (a) Bloch. F., Harisen, IT. W,, and Packard, .\I. E.. Zhid., 70, 474 (1946). (1) Rloembergen, S . ,Purcell, E. AI,,and Pound, It. V., Ibid., 73, 079 (1948). (,5) Gutowsky, H. 9.. and Hoffman, C. J.. J . C'iicm. P h g s . . 19, 1259 (1951). (6) Guton-sky, H. S., 3IcCall, D. K.. lIcGarvey, R.R . . and Aleyer, L. 13.. ,J. A m . Cheqn. Soc.. 74, 4809 (1952). ( 7 ) Gutowsky. H. S., hleyer. I,.H., and llcClure, I:. E., Ret. Sei. in st^.. 24, 044 (1953). (9) ,Jacobsohn. € 3 1., and Yangsness. R. K., P h , u . Re?'.,73, 942 (1945). (!ij lIeyer. L. H.. and Cutowsky, II. S., J . Ph!/s. Cheni., 57, 451 (1953). (10) Ateyer, L. H.. Saika. -1..and Gutowsky, H.S., .J. drn. C h e m S o c . , 75, 4 3 7 (1933).
Rollwitz, IT, L., "Deterniination of lloisture By Suclear lIagnetic Resonance, Experimental Results on I'rorlurts of the Corn Wet-liilling Industr ('arimhydrate and .\nalytical Chemistry, ;\nalytic~al IIethods and Instrumentation -4pplied to Sugars and Other Carbohvdratei. 124th 3Ieeting. AM. C ' m : x . Soc., Chicago. Ill. Illi'd, "Theory and Desizn Considerations for a Practical Instrument." Packard. Sf. E., Rea. Sei.I n s f r . . 19, 439 (1948). Pound, 11. V., and Knight, W.D., Ibid., 21, 219 (1951). Proctor, IT. G., Phys. Rw.,79, 35 (1950). Purcell, E. li., Torres, H. C., and Pound, R . V., I b i d . , 69, 37
(1 1 ) O ' l l c a r a , J. P.. and
(12' (1X (14) (1.5) (16)
(1940). (17) Ross. I. 11.. and Johnson. F.B.,Sattire, 167, 286 (1931). (18) ShaK, T. AI., Elsken. R. H., and Iiunsrnan, C . FI., J . A4~soc. O ~ CA.g r . Chemists, 36, 1070 (1953). (19) Shoolery. J. K,, J . Chem. P h y s . , 21, 1899 (1953). (20) Taylor, I-clisrxteristic of misture components. Thus, this method utilizw the relstionBhip existing between the chosen relative abundnnce ratios of mass ion currents and sample composition. Therefore, it is independent of several of the mentioned variatlilcs, notably, instrument sensitivity change and pressure mt::tsurcnicrit. I n order to minimize the unpredictable effect xhich changing temperature of the ion source may have on analytical detcrniinatioris) both the sample inlet system and spectromrtrr gun were electrically heaied for uniform temperature control. Furthermore, electron emission from the tungsten filament was automatlcall. controlled so that variation in emissivity due t o oxygencontaining sample compounds was practically negligible. THEORY
Tlie ratio measuring mass spectrometer is similar in prrformance to a conventional directional focusing analyzer. I t differs in three main features which are the use of an interrupting device for modulating the accelerating potential, a dual high voltage source, and a special ratio measuring circwit for determining the ratio of two ion currents. Referring to Figure 1, the analyzer is a conventional 60-degree
1404
ANALYTICAL CHEMISTRY
sector-type spectrometer nearly identical to the original Niertype instrument (5,6). Neutral sample molecules in the gaseous state are subjected to electron bombardment in the ionization chamber with the resulting formation of positive ions. By means of a square wave modulated ion accelerating potential furnished by two high voltage sources coupled to a modulator and applied to the ionization chamber, ion gun lens, and collimating slits, these positive ions arc accelerated down the spectrometer tube as a modulated beam.
VOLTAGE
VOLTAGE
UODVLATOR
HO 2 NO 1 0 2
-1WV IONIZ4TION CH4MBiR
ION L E N S COLLIUATINC
SLITS
Sier-type instrument but redesigned and built especially for use in analyzing oxidized hydrocarbon compounds. Considerable modification of Nier’s spectrometer was made in the construction of the spectrometer tube, leak, control circuits, and sampling system. I n addition, the spectrometer includes miscellaneous monitoring and automatic protective devices, as well as an automatic recording system. One of the major changes nccessary to adapt t’he spectrometer for ratio determination is the use of a modulated accelerating potential supply whereby two pulsed direct current potentials, of one second duration each, are alternately furnished to the accelerating plates of the ion gun. Figure 2 shows the essent,ial components in block form needed to accomplish this. The arrangement was developed from two basic circuits: ( a ) An electronic circuit devised by Serfass, Muraca, and Schmitt ( 7 ) which employs a lasser-triode tube as a means of varying high voltage output; ( b ) A conventional decade resistance voltage divider network by Nier (6) which supplies the proper potentials to the ion gun elements. .4s indicated in Figure 2, the out u t of a well-filtered high voltage source is placed across SationaYUnion Type NU2C53 tubes in parallel. These tubes are n-ell insulated off-ground, and the output across the subsequent, divider network load of 25 megohms is taken from the cathode circuit. .4 grid bias supply of -18 volts direct current for the tuhes is obtained from a separate soui*ce which utilizes a t,en-turn Helipot ( 1), automatically controlled by a Brown (4)amplifier and servomotor, to vary the bias between 0 and 18 volt,s, wit,h a corresponding high voltage change of 0 to 2500 volt,s. Accelerating potentials from two sources can be placrd altcrnately across the volt,age divider network B associated with the ion gun elements. One source, Hir S o . I , originating from the cathode circuit of the SU2C53 tubes is switched to this network by means of the relay. The other source, Hi7 S o . 2, from voltage divider network, A , is likpn-ise fed to the B network by the Same relay. Since the voltage output from divider network A must’ always be a fraction of the cat,hode circuit output, its value will always be dependent upon the total voltage output from this circuk If thc contact positions of the relay are alternat’ely opened and closed for one-second intervals, two direct current pulses of different amplitude arp alternately applied to the ion gun elements. In this manner, a modulated accelerating potent’ial of any desired value may be obtained. It, is the function of the modulator to actuate the relay for the open and closed positions. .4 corresponding modulated out,put fmm the electrometer and direct current amplifier circuits is separated by the demodulator into two distinct pulsating direct current signals. Subsequently, these signals are directed to two filter networks which change them into steady direct currrnt signals, and finally, the ratio measuring circuit determines thc ratio of the two signals with respect to each other, or with respect to some external standard. The ratio measuring circuit, consists of a resistance bridge having a variable arm utilizing a 500,000-ohm, 40-turn Helipot, and a fixed arm having a 500,000-ohm precision resistor. A high sensitivity direct current vacuum tube millivoltmeter (3) serves both to measure the potential difference across the Helipot and fixed resistor, and to function as a null galvanometer. The adjustment of the Helipot, which has 4000 scale divisions, is directly related to the ratio of the potentials across the arms of the resist’ance bridge, and is thus used as a method of ion current ratio measure-
-
__-
’
0
DC
1:
Figure 1. Block Diagram Illustrating Theor) of Ratio Measuring AIass Spectrometer
I n the analyzer, the beam of p o d i v e ions is acted upon by the magnetic field in a way that onlv those ions pass through the exit slit which satisfies the directional focusing equation. Beyond the exit slit a Faraday cup serves as a collector for the beam. The positive ions are detected as a modulated ion current by means of an electrometer circuit. A synchronized demodulator then feeds the ion current alternately into each of two filter networks to form two ion currents R-hich are subsequently directed to the ratio measuring circuit for ratio determination. APPARATUS AND PROCEDURE
A 60-degree Nier-ty e mass spectrometer ( 5 ) was modified to measure relative abun&nce ratios by incorporating into the basic design three major supplements:
-4dual high voltage source capable of furnishing two widely differing accelerating potentials for the ion gun plates. An interrupting device, or modulator, for approximate squarewave modulation of these potentials, and including provision for demodulating the resultant ion current signals produced by the direct current amplifier. A special ratio measuring circuit for determining the ratio of two ion currents which are detected and amplified by the single collector-cup and amplifier system. With the exception of these added features, the mass spectrometer used to accomplish ratio measurements is a conventional
ment.
EXPERIMENTAL
Analytical Performance Tests. I n order to evaluate instrument performance for subsequent ratio determinations, a series of tests were made to ascertain the optimum values of the spectrometer variables, such ae magnetic-field strength, accelerating potential, electron current, and sample pressure. Data from these tests provided the correct individual component settings necessary to ensure the full utilization of instrument capabilities, as well as to confirm spectrometer alignment and linear response to variations in electron current and sample pressure. The ultimate test of the spectrometer was made by performing analyses of four binary mixtures of known composition. The mlxtures were acetone and methyl alcohol, benzene and cyclohexane, carbon tetrachlofide and toluene, and methyl alcohol and isopropyl alcohol. For each mixture, samples were prepared which ranged in
1405
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4
-
3 - c 3
A
’ i -
&!/Mass iV is additively composed of the ratio Mass M/Mass N from the binary mixture, and the Mass M/Mass .V ratio from component A of the mixture. Because the latter value is known, or is easily determined, it can be subtracted from the measured ratio t o yield a corrected ratio. CONCLUSIONS
composition from 0 to 100 mole %for one component with respect t o the other. Large peaks could be selected in each mixture as being characteristic of but one of the two components. Since it wae expedient to work l$-ith only those mass numbers reflecting ion currents of high relative intensities, the principal peaks of the pure compounds were usually used for purposes of analysis. The samples were anal>-zed in the specwometer according to conventional procedures, and their composition computed by means of the well-known partial pressure method. The results vf ana lyse,^ agreed with the known d u e s within l.Oyo. Analysis by Measuring Relative Abundance Ratios. The same binary mixture samples ~vereaiialJ-zed by measuring the relative abundance ratios of ion currents. Data obtained from ratio measurements of ion currents (’ail be plot.tcd in one of two ways: 1. Concentration of component, A (or B ) versus the ratio of ion currents from selected masses. 2 . Ratio of concentration of components .UB versus the ratio of ion currents from selected 111 Empirical curves necessary for performing analyses are drawn from plots of the data. The curves resulting from using method ? hsve the equation CAICB = LI.C.A/I.C.B, in which CA and C B are the concentration of A and B, respectively, and Z.C.A and I.C.B are the measured ion currents for A and B. The slope of the straight line is k . An analysis accomplished by ratio measurements depends upon empirical data, and in this respect is similar to other spectrometric methods. Compositions of a niisture may be determined by measuring the ratio of ion currents from selected mixture masses, and then reading the corresponding composition from a working curve of the mixture. Thirty-two analytical tests on the binary mixture samples gave re>ults n-hich agreed within 1% of their known composition values. IVhen analyzing binary mixtures in which the ion current peaks measured reflect mutual contributions from each component, ratio measurement,s may still be made, provided corrections are npplied to negate the contrihuting effect of one of the compoiwnts. A method similar to that of Johnsen ( 2 ) may be used to ol3t:liii correct,ed peak ratio measurements or, alternatively) the following rela,tion may be employed: >[ass -11= R Mass11 observed I i n mixture AIass S AIas. -’I component A of mixture ~Mass ‘V
rn
whir*his equivalent, to stating that the measured ratio of Mass
The accuracy of an analysis by ratio measurement can be expected to be within i l . O % of the true values. To attain an accuracy on the order of O . l % , it is necessary to measure ratios precisely to the 3rd or 4th significant figure depending upon how small the ratio value may be. The sensitivity of the galvanometer in the direct current millivolt vacuum-tube voltmeter then becomes the limiting factor in exact potential measurement. The ratio method of performing mixture analysis makes no claim of superior accuracy over conventional pi-essure-volume methods in which sensitivity considerations limit analytical accuracy. However, several advantages of the ratio method, as compared with conventional methods, are evident and are enumerated belOW:
I S D E P E S D E N T O F INSTRU3fEST SESSITIVITY. The sensitivity of the spectrometer for different mass peaks changes daily. In making ratio measurements, instrument sensitivity fluctuations were equally reflected to all peaks measured. In conventional analyses, a slight error in recording sensitivity change results in a considerable error in analytical accuracy. ISDEPEXDEST OF SAMPLE PRESSURE.There is practically no need for making a record of sample pressure \Then using the ratio method if the quantity of sample introduced will not cause a pressure in excess of that allowed for linear sample pressure-ion current variation. EASE OF C o m w T A T I o x . After empirical working curves have been constructed for a particular mixture, the composition of mixture samples may be determined quickly from these curves. ISDEPEXDEST OF ELECTRON CURREST. Small changes of catcher currents, as well as other electronic circuit component fluctuations, were found to be equally reflected to the mass spectral peaks measured. The chief disadvantage of the instrument lies in the difficulty of its direct application to systems containing more than two components.
ACKNOWLEDGMENT
The authors wish to express their appreciation to the Lehigh Institute of Research for providing the facilities and funds necessary for this project. LITERATURE CITED
(1) Beckman Instruments, Inc., Helipot Division, South Pasadena, Calif., Bull. 104 (1948). (2) Johnsen, S. E. J., A.v.4~.CHEM.,19, 305-6 (1947). (3) Lyons, W., and Heller, R. E., Electronics, 25 (Kovember 1939). (4) Minneapolis-Honeywell
Regulator Co., Philadelphia, “Instrument Data Sheet,” No. 10, 20-3 (1952). (5) Xier, A4.O.,Rev. Sci. Instr., 1 1 , 212-16 (1940).
Pa.,
(6) Ibid., 18, 398-441 (1947).
(7) Serfass. E. J . , Nuraca, K . F., and Schmitt. S. R., Jr., Ibid., 24, 11524 (1953). R E C E L V Efor D review J u n e
4, 1952. Accepted hIag 2 7 , 10.54
