Contains research objectives andNational Institutes of Health (Grant 5 505 FR-07047-03

Glaser, J. I.

Steinbrecher, D. H.

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IX. ELECTRONIC INSTRUMENTATIONAcademic and Research StaffDr. D. H. Steinbrecher Prof. J. I. GlaserProf. K. Biemannt Dr. R. E. LovinstGraduate StudentsA. Y. ChenRESEARCH OBJECTIVESThe purpose of the Electronic Instrumentation Group is to fill the gap between state-of-the-art technology in electronics and the electronic instruments now being used inmany areas of scientific research. Our group is concerned not only with improving theprecision, sensitivity, and ease of operation of present electronic instruments but alsowith the creation of new instruments.The present effort is directed toward electronic instrumentation for high-resolutionmass spectrography. The specific goals are: to improve the resolution of the CEC21-110 mass spectrometer by placing it in a controlled environment, and to replace thepresent photographic plate detection scheme by an electronic interface so that the infor-mation contained in the ion-beam output can be read directly into a digital computer forprocessing. Any improvement in resolution will permit a more accurate determinationof unknown chemical compounds, and the electronic interface will eliminate the time nowrequired for removing, developing, and optically scanning the photographic plate.D. SteinbrecherA. OPTIMAL SINGLE AND DUAL DETECTOR ARRAYS FORTHE MATTAUCH-HERZOG MASS SPECTROMETER1. IntroductionThe Mattauch-Herzog high-resolution mass spectrometer normally records the massspectrum of a chemical compound on a photographic plate. It would be desirable toreplace the plate by an array of small electronic detectors whose outputs could becounted and stored digitally. Such an array and associated circuitry would provide themass spectrometer with a digital electronic output.1This report contains a description of a new detector array configuration which"matches" the inherent resolution of the instrument for each mass number. 2 The newarray achieves the inherent resolution of the instrument over the entire mass scale witha minimum number of detectors, and typically it requires 64% less detectors than anThis work was supported principally by the National Institutes of Health (Grant5 505 FR-07047-03).tProfessor K. Biemann and Dr. R. E. Lovins, of the Department of Chemistry,M. I. T., are collaborating with the Electronic Instrumentation Group of the ResearchLaboratory of Electronics in research under NIH Grant 1 505 FR-07047-03.QPR No. 92(IX. ELECTRONIC INSTRUMENTATION)array configuration of equi-spaced elements which also achieves the inherent resolutionof the instrument over the entire mass scale. The saving in number of detectors at noexpense in resolution also results in savings in counter circuitry and data-storagerequirements.A dual version of the new array eliminates the possibility of a narrow line escapingdetection. We shall also discuss possible fabrication techniques for detector arrays.2. Description of the Mass SpectrometerIn 1936, Mattauch presented the theory of a mass spectrometer that would double-focus simultaneously for all masses. 3 This instrument furnished a specialized applica-tion of the general theory of double-focussing presented two years earlier by Mattauchand Herzog. The instrument utilizes a combination of a radial electric field and a homo-geneous magnetic field to obtain both velocity and direction focussing over the entirefocal plane simultaneously for all masses. The schematic arrangement of the instrumentis shown in Fig. IX-1. A detailed description of the ion trajectories and factors deter-mining the resolution has been given previously.4 For the purpose of this report, it isnecessary to know the equation for the point where the ion trajectory strikes the focalplane as measured from 0, the mean entry point of the ion into the magnetic field, p,p = q (1)where m is the ionic mass, q is the ionic charge, B is the magnetic field, and V isthe potential through which the ion is accelerated. Equation 1 shows that the mass scaleFOCAL PLANEELECTRICSECTOR53 MAGNETICSECTOR\1SOURCEFig. IX-1. Mattauch-Herzog mass spectrometer.QPR No. 92dP/,,/e,Y/ /(IX. ELECTRONIC INSTRUMENTATION)in the focal plane is proportional to the square root of the mass for singly charged ions.The mass scale may be expressed incrementally in terms of the fractional change in p,Ap/p, for a fractional change in m,Amm (2)It is important to know that the theoretical resolution of the double-focussing instru-ment at any mass is only a function of the slit width S l and the radius of the electricsector r . The resolution is defined in terms of the smallest detectable fractionalemass difference.rm eAm 2S ' (3)1where m is the mass of the ion, Am is the smallest detectable absolute mass difference,re is the radius of the electric sector, and S1 is the slit width.3. Detection SchemeThe detection scheme considered here is a detector array fabricated by using solid-state integrated circuits and film techniques. A solid-state ion-multiplier plate mustbe developed. The plate must be 25 cm long and 0. 6 cm wide. Each detector in thearray must have a pulse output proportional to the number of ion impacts on its surfacewhen connected to suitable circuitry.One approach toward producing the plate might be to grow the structure on a suitablesubstrate as is done in the manufacture of transistors by the epitaxial method. The platewould then be divided to form an array of ion detectors by cutting partially through theplate with a precision electron beam. Chemical or laser methods would not be suitablebecause some of the dimensions required may be as small as 1 micron. Electrical con-nection to the individual detectors in the array would be through deposited aluminumcontacts which fan out to allow mechanical connection with the associated circuitry.An alternative approach may be to use electron-beam deposition methods for thewhole manufacturing process. Experimental transistor structures have been fabricatedwith element dimensions as small as 1 i through the application of advanced electron-beam technology. 5We shall assume that all arrays discussed are to be placed in the focal plane of theinstrument and will be 25 cm long, extending from p = 5 cm to p = 30 cm. Consequently,a scale along the array, f, will go from 0 to 25 cm andf= p- 5 cm 5 cm <p <30 cm. (4)QPR No. 92(IX. ELECTRONIC INSTRUMENTATION)It has been shown that with a Maxwell-Boltzmann energy distribution of ions leavingthe source, a Gaussian-like mass peak results in the focal plane. 4 This assumption willbe used in determining the smallest detectable separation between mass peaks in thespectrum.4. Resolution and Linear ArraysThe smallest resolvable fractional mass difference, I/R, is a constant for theinstrument. On the other hand, the smallest detectable fractional mass differencedepends on the spacing between the detectors and the size of the detectors. Theoptimal array is achieved when the smallest detectable mass difference is equalto the smallest resolvable mass difference over the entire mass scale; this meansthat the array is matched to the instrument, since it is capable of achieving thetheoretical resolution of the instrument for all masses. Equation 1 can be substi-tuted in Eq. 3 to yield the smallest resolvable fractional distance between masspeaks in the focal plane.AP= _2 (5)p R'Equation 5 shows that the smallest resolvable incremental distance, Ap, increaseslinearly with p; therefore, larger mass differences are resolvable for higher mass ionsthan for lower mass ions. Before presenting the optimal array, the linear arrays willbe discussed.The single linear array is shown in Fig. IX-2a. An equal length is assumed for thewidth of each detector and the associated space that separates it from the adjacent detec-tor, and the width of each detector-space pair, 6, is constant along the array. Thesingle linear array will not detect narrow spectral lines that strike a space. In orderto eliminate this difficulty, a dual linear array as shown in Fig. IX-2c could be con-structed. The smallest detectable separation between two mass peaks in the focal planefor the single linear array will be taken as 36/2 and for the dual linear array as 6; thisis a reasonable assumption for Gaussian-like mass peaks. The resolution available fromthe arrays will then beSingle DualAvailable P P (6)Resolution 36 26Equation 6 shows that the resolution available from the linear arrays increases linearlywith distance p along the focal plane (see Fig. IX-3). Therefore, the array resolutionat the smallest p must match the theoretically attainable resolution, R, so that theQPR No. 92(IX. ELECTRONIC INSTRUMENTATION)then must be less than a value given by2 2-2 2' n /(C)Fig. IX-2. Detector array configurations (not drawn to scale).theoretical limit is detectable for all p (or for all masses). The detector-space widththen must be less than a value given bySingle Dualp1  p16 1< - (7)3R ZRin order that the instrument resolution be achieved for all masses. p 1 is the distancefrom the mean ray entry point into the magnetic sector 0 to the beginning of the detec-tor array.The minimum number of detectors then required by the linear arrays isSingle DualN k N p1  (8)The number of detectors for the linear arrays as a function of 6 is plotted in Fig. IX-4.It is clear that the resolution available from the linear array having a 6 given by theequality in Eq. 7 is equal to the theoretically attainable resolution, R, for p = p 1 , butQPR No. 92400Zz- 300Oz6 =2p1 2 3 4 5S(pi)0 Ep 5L 06 lo=5p8 = 20P10 15 20 25 30 (cm)5 10 15 20 25Fig. IX-3.Linear arrays. Resolution, R, as a function ofposition along the array for various values of 6.Dual array.-, Single array.0 F5 10 15 20 25 30 35 40 45 506 (p)Fig. IX-4.Linear arrays. N, number of detectors, counters ordata storage units as a function of 5 for a 25-cm array.Dual array.- , Single array.80,0006=1I(IX. ELECTRONIC INSTRUMENTATION)exceeds R for p greater than pl . For this reason, linear arrays are wasteful innumber of detectors.5. Constant Resolution or Optimal ArrayThe optimal array achieves the theoretically attainable instrument resolution foreach mass number by having a detector-space width 6 that varies with p (or massnumber). For this reason, the optimal array is called a constant resolution array. It0 Ep 5L 0/ R = 5,000R =10,000R =25,000R 100,00010 15 20 25 30 (cm)5 10 15 20 25Fig. IX-5. Constant R arrays. Wiof position along the arr, Dual array.-- --- , Single array.idth of detector-space pair, 6 n , as a functionay for various values of resolution, R.is called optimal because it achieves the ultimate resolution with the minimum numberof detectors. Small detector-space widths compared with pl , the desired incrementalspacing for the optimal single and dual arrays, can be obtained from Eq. 6.QPR No. 92(IX. ELECTRONIC INSTRUMENTATION)SingleP63R=3RDual(9)6= R'2R'where R is the theoretical resolution given by Eq. 3. Equation 9 now defines a con-stant resolution single and dual array. Equation 9 is plotted in Fig. IX-5 for variousvalues of resolution.Since the array is made up of discrete elements, as shown in Fig. IX-2b, Fig. IX-5will only give a close approximation to 6 n . The exact values are obtained from thefollowing relations.Single DualP16 1 3Rpl +61+ . + n- 16 3Rn 3RP11 2RPi + 1 + ... + 6 n-n 2Rwhere pl is the distance from the mean ray entry point into the magnetic sector 0 to thebeginning of the detector array. A simple exact expression can be determined for 6nas well as for the total number of detectors in the array of lengthSingle Dualn 13RN= log +log (i += = 1 + nn N =2log ( +log I +For R > 10, the number of detectors is given very closely bySingleN= 3Rln( + 1)DualN= 4R ln(.- + 1)Equation 12 is plotted in Fig. IX-6. The actual saving in number of detectors for theconstant R array over the linear array for f = 25 cm and pl = 5 cm isLinearConstant RSingleN = 15RN = 5. 375RDualN = 20RN = 7. 167RQPR No. 92(10)(11)(13)(12)(IX. ELECTRONIC INSTRUMENTATION)0 10 20 30 40 50 60 70 80 90 100R (THOUSANDS)Fig. IX-6. Constant R arrays. N, number of detectors,counters or data storage units as a function ofR for a 25-cm array extending from p = 5 cmto p = 30 cm., Dual array., Single array.The constant R array represents a 64. 2% saving in number of detectors both for thesingle 'and dual arrays.6. ConclusionThe constant R single and dual arrays have been shown to achieve the theoreticallyattainable resolution of the Mattauch-type mass spectrometer in an efficient mannerwhen compared with the linear arrays. For example, for an instrument with a reso-lution of 10, 000 the 64% saving translates into 90, 000 sets of counters and data storageunits for the single arrays and 128, 000 sets for the dual arrays.The dual constant R array has been shown to achieve the theoretical resolution andalso eliminate the possibility of a narrow spectral line escaping detection.J. GlaserQPR No. 92(IX. ELECTRONIC INSTRUMENTATION)References1. S. H. Zisk, "Mass Spectrometer Detector," Quarterly Progress Report No. 85,Research Laboratory of Electronics, M. I. T., April 15, 1967, pp. 34-36.2. This was first reported by J. Hirschel, "An Optimal Detector Array for theMattauch-type Mass Spectrometer," S. B. Thesis, Department of Electrical Engi-neering, M. I. T., September 1968.3. J. Mattauch, "A Double-Focussing Mass Spectograph," Phys. Rev. 50, 617 (1936).4. J. I. Glaser, "Mass Spectrometer Resolution," Quarterly Progress Report No. 88,Research Laboratory of Electronics, M. I. T., January 15, 1968, pp. 39-40.5. R. Landauer and J. J. Hall, "Solid State Physics as a Source of Modern Electronics,"Science 160, 740 (May 17, 1968).QPR No. 92

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