Contains research objectives and reports on four research projects.Electronic Instrumentation Group of the Research Laboratory of Electronics in research under NIH Grant 1 505 FR07047-01National Institutes of Health (Grant 1 505 FR07047-01

Glaser, J. I.

Lovins, R. E.

Steele, J. M.

Steinbrecher, D. H.

Tin, N. T.

Tung, R. Y.

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IX. ELECTRONIC INSTRUMENTATIONAcademic and Research StaffDr. D. H. Steinbrecher Prof. J. I. GlaserProf. K. Biemannt Dr. R. E. LovinstGraduate StudentsJ. M. SteeleN. T. TinR. Y. TungRESEARCH 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 CECZ1-110 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.During the past six months significant progress has been made both in the environ-mental control problem and the electronic interface problem. The instrument has beeninstalled in Room 56-031, M. I. T., which is electrically shielded and isolated from theexternal enviroment and has a vibration- controlled instrument mount, as well as tem-perature and humidity controls. The success of the electrical isolation has been veri-fied by noise measurements taken in the room; these are described in Section IX-A.An attempt is being made to provide a stable DC power supply for the electric-fieldsector voltage; this progress is described in Section IX-B. The interface developmenthas had two phases: (i) investigation of the feasibility of using some commercially avail-able miniature electron-multiplier tubes mounted in the focal plane as individual iondetectors, and (ii) design and construction of low-noise miniature integrated circuitamplifiers for each electronic multiplier tube, as well as associated logic circuitry forcounting ions and reading the counts into a digital computer. The construction of aprototype for the amplifier and logic circuitry is under way and is described inSection IX-C. The theoretical problems associated with the effects of electrical noiseon the performance of the system, as well as the limitations on resolution imposed bythe ion-generation method and ion optics have been studied with the aid of computersimulation of the instrument. Progress in this area is described in Section IX-D.D. Steinbrecher, J. GlaserThis work was supported principally by the National Institutes of Health (Grant1 505 FR07047-01).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 FR07047-01.QPR No. 88(IX. ELECTRONIC INSTRUMENTATION)A. SHIELDED ROOM FACILITYThis report briefly describes the completed shielded room facility in Room 56-031,M. I. T., as well as the noise measurements that verify the effectiveness of the shielding.The room was designed to provide maximum environmental control and maximum elec-trical isolation for the instrument.1. DescriptionThe room was constructed by the Sprague Electric Company from 3/8" wrought ironplate, chosen for its magnetic shielding properties; the physical exterior dimensionsare: floor 17' 7" X 16' 3", ceiling 8' 3/4". Structural stability is maintained by 6"I-beams. The walls and ceiling are covered with acoustical tile, and the floor con-sists of light weight 2" cement fill on top of the wrought iron. Four 6" X 24"metal ducts provide air intake and exhaust vents between the interior of theroom and a 7 1/2 ton capacity air conditioner that controls the temperature andhumidity. Five 12" X 3" wrought iron pipes in the wall act as feed-throughs forthe rubber vacuum leads between the instrument and the pumps, which are locatedoutside the room. Five 8" X 1" wrought iron pipes in the ceiling act as waterlines to the instrument and electrical feed-throughs. The entire instrument restson a mount supplied by Barry Controls, Inc., which damps out vibrations above1 Hz through a nitrogen pneumatic control system. The room is Cd-welded toa wrought iron conductor that is connected to the 40' steel piling of Building56, which provides a low-resistance ground. The room itself rests on a fiberglas mat,which was initially 2" thick. Electric power for all equipment in the room is supplied bya 5 KVA synchronous motor-generator set that is mounted above the room. The motoris electrically isolated from the generator by a fiber shaft. The generator has a brush-less permanent magnet rotor and supplies 3-phase 120-VAC output. Each of the 3phases is isolated by means of 3 autotransformers connected in a Y-star configuration,which provides six-wire 3-phase output. Each phase is then separately filtered andregulated by a General Variac motor-driven regulator. The telephone lines leading tothe room are also filtered, and the lighting consists of 12 150-Watt 150-Volt lights thatare operated from a special DC power supply.2. Noise MeasurementsIn order to ascertain the effectiveness of the power-line isolation, several measure-ments were made. Spot-frequency measurements with signal generators indicated totalisolation in excess of 150 dB from 10 kHz to 1 GHz. Measurements of power-linenoise inside the shielded enclosure were made with the instrument "off" and theQPR No. 88(IX. ELECTRONIC INSTRUMENTATION)instrument "on."The results of the measurements are shown in Fig. IX-1. The vertical axis showsthe measured noise power in dB relative to kT 0 (-204 dBW/Hz) measured by a Singer-Metrics NF-105 receiver at an impedance level of 50 0. The frequency range is from180SENSITIVITY LIMIT(INSTRUMENT OFF)0.01 0.1 I 10FREQUENCY (MHz)100 I000Fig. IX-1. Normalized noise power vs frequency.14 kHz to 1 GHz. Note that the mass spectrograph contributes all of the measurablenoise over the entire frequency range.D-C magnetic field measurements were made and recorded on a strip chart over aperiod of a few days. Fluctuations caused by the local subway facilities werereduced almost one order of magnitude (to 3 mG) from those observed outsidethe room.Future plans include the measurement of electrical noise from DC to 15 kHz,which requires the modification of existing test equipment.R. E. Lovins, D. SteinbrecherQPR No. 88(IX. ELECTRONIC INSTRUMENTATION)B. CONTROL OF INSTABILITIES1. Electric SectorThe effect of instabilities in the DC voltage supplied to the radial electric sector isbeing investigated. A comparison measurement will be made on the stability of the ionbeam output, by substituting for the present DC supply two precision DC power supplieswith calibrated and monitored output.2. Magnetic Field InstabilitiesAny 60-cycle magnetic leakage fields from transformers in the instrument maydegrade the resolution of the instrument by modulating the ion trajectories. In orderto detect 60-Hz magnetic fields in the shielded room, a battery operated magneticfield detector is being designed. A small loop will act as the field probe and the inducedvoltage will be detected by a sensitive 60-Hz phase-locked receiver.D. H. Steinbrecher, R. Y. TungC. ELECTRONIC INTERFACE1. DetectorsAt the present time, an investigation of the Bendix "channeltron," a miniature elec-tron multiplier tube, as a possible ion detector is being made. A "channeltron" is beingmounted in a vacuum chamber and will be tested in order to determine whether or not itwill function in vacuum; the extent to which it will detect ions instead of electrons;whether its performance will be degraded by ion bombardment; the stability of itsresponse; and the effect of magnetic fields on its response. The possibility of employinga small low-noise integrated circuit to amplify the output of each ion detector will alsobe investigated.2. Sampling and Retrieval of the SpectrumA description of a possible interface system between the electron multipliers andthe computer has been presented elsewhere in Quarterly Progress Report No. 85(pages 36-37). A tentative design has been worked out with commercially available inte-grated circuitry. A small prototype is being built and will be tested soon. Thefinal interface will work as follows: Initially, the magnetic field and the radial electricfield of the instrument will be electronically preset; therefore, ions entering the instru-ment of zero initial energy will exit at points in the focal plane corresponding to theirmass-to-charge ratio. In other words,, each point in the exit focal plane correspondsto a given mass-to- charge ratio. (Because the ions will have an initial energy, therewill be some spread to the exit distribution for an input of ions having the sameQPR No. 88(IX. ELECTRONIC INSTRUMENTATION)mass-to-charge ratio.) Each ion detector will generate a pulse when an ion strikes itsaperture. These pulses will be counted during some preset integration time, and theindividual counts will be stored in the computer. Then, the magnetic field and theradial electric field will automatically be set to new values so that each point in the exitfocal plane will correspond to a new mass/charge ratio of input ions of zero energy.The counting will be repeated during the same preset integration time and the newcounts stored in the computer. The procedure will be repeated a given number of times.Finally, the computer will take the counts obtained during each "scan" and mathemati-cally reconstruct the distribution of ions in the exit focal plane. A theoretical studyhas been made concerning the number of scans and number of detectors necessary forthe computer to reconstruct the distribution of ions in the focal plane. The processingis essentially the reconstruction of a function from sampled data points and will bedescribed briefly. The sample taken by each ion detector is an integral of the iondistribution over distance in the focal plane corresponding to the effective aperture ofthe detector. If the detectors are equispaced by X s and each has an aperture X w, thes thnumber of ions collected during the integration by the k detector is given bykX + Xw/2S(k) = SX / N(x) dx,X -X /2where N is the distribution of ions in the focal plane. N(x) presumably will be aGaussian-like distribution centered about each discrete mass/charge point in the focalplane for a given magnetic and radial electric field setting. A computer study indicatesthat a single Gaussian distribution can be reconstructed when X and X are approxi-mately equal to the standard deviation of the Gaussian. For a 250-mm focal plane andstandard deviation of 2.5 ri, 105 data points are necessary. 105 ion detectors would notfit in the 250-mm focal plane, and their cost would be prohibitive. Two hundred ofthe Bendix "Channeltron"-type detectors would fit in the focal plane, but 500 scanswould be needed to obtain the required number of data points. On the basis of empiricaldata, each scan would require at least 0. 2 sec, so that 100 sec would be required tocomplete the spectrum.N. T. Tin, J. SteeleD. MASS SPECTROMETER RESOLUTION1. Present StatusThe object of this study is the improvement of the resolution of the mass spectrom-eter by eliminating many of the factors that degrade the resolution and by mathe-matically processing the detected ion beam output of the instrument in order to extractQPR No. 88(IX. ELECTRONIC INSTRUMENTATION)the maximum amount of information. To determine the factors affecting resolution andthe mathematical character of the ion-beam output distribution, a computer simulationof the instrument has been programmed. The simulation allows the user to determinethe distribution of ion positions in the exit focal plane for random or nonrandom distribu-tions of ions in position, mass, and velocity in the input focal plane. The output distri-bution appears as a histogram. A description of the program and theory appeared inQuarterly Progress Report No. 87 (pages 17-24).2. Future PlansIn order to use the computer simulation in conjunction with the actual instrument,the simulation must correspond closely to the instrument in its performance. Becausefringing fields were neglected in the analysis used for the simulation, it remains tomodify the actual parameters given to the computer in order to account for any discrep-ancies between the real and simulated instrument.The effect of 60-cycle hum on resolution will be included by calculating the trajec-tory of ions in the radial electric section, under the assumption that a small humvoltage is added to the electric sector voltage.The technique of determining mass locations by crosscorrelating the output distri-bution with a theoretical distribution will be investigated, and the improvement in reso-lution achieved by this method will be evaluated.An attempt will be made to vary the parameters supplied to the computer, such asfield strengths and dimensions, in order to determine whether or not the instrument istruly a "double-focusing" mass spectrometer.J. I. GlaserQPR No. 88

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