Contains research objectives and reports on four research projects.National Institutes of Health (Grant 1 505 FR07047-01

Biemann, K.

Glaser, J. I.

Lovins, R. E.

Steele, J. M.

Steinbrecher, D. H.

Zisk, S. H.

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VI. ELECTRONIC INSTRUMENTATIONAcademic and Research StaffDr. D. H. Steinbrecher Dr. J. I. Glaser Dr. R. E. LovinstProf. K. Biemannt Dr. S. H. ZiskGraduate StudentsJ. M. SteeleN. T. TinRESEARCH OBJECTIVESElectronic instrumentation plays a significant role in almost every branch of scien-tific research. Similarly, progress in electronic instrumentation is increasingly depen-dent on advances in chemistry and physics. The formation of the ElectronicInstrumentation Group is an effort to stimulate instrumentation research and develop-ment with state-of-the-art needs in other areas of science.The activities now under way in our group are centered on instrumentation for massspectrography. Profile analyses and spectrographic data-reduction methods alreadyrequire extensive use of high-speed digital computers. Data from a Mass Spectrographare collected, however, on photographic plates that must be processed by time-consuming methods to achieve a form acceptable to a computer. Many experiments,which are impossible to perform with present methods, could be conducted with theaddition of a real-time interface between a Mass Spectrograph and a computer withon-line processing capability. Reports on progress in areas related to the developmentof suitable interface hardware and software appear in Sections VI-B, -C, and -D.Since much of the present and future Mass Spectrograph instrumentation is and willbe environmentally sensitive, a semiclean, temperature- and humidity-controlledshielded room is being provided. The instrument will be mounted within the room on a1-Hz air piston antishock and vibration assembly. Nearly complete environmental con-trol will permit an investigation of system-produced instabilities that can be correctedby sophisticated instrumentation. The resulting influence on next generation equipmentswill allow spectrographic analyses to be carried out on low-vapor-pressure compoundsthat require very long (several hours) exposures. It is virtually impossible to makeanalyses of some of these compounds with present methods. Progress on the shieldedroom facility is discussed in Section VI-A.D. H. Steinbrecher, K. BiemannA. SHIELDED ROOM FACILITYThis report outlines briefly the current status and the rest of the work to be done onthe wrought-iron screen room that is being constructed in Room 56-031, M.I.T. Theiron walls, floor, and ceiling have been erected and the continuous seam welds areThis work was supported principally by the National Institutes of Health (Grant1 505 FR07047-01)._Professor 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. 85(VI. ELECTRONIC INSTRUMENTATION)approximately 80% complete. Also, the electrical ground for the room has been com-pleted.We estimate that the completion of the basic room (i. e. , welding of iron) will soonbe accomplished. In addition to the basic-room design, additional time will be neededfor (a) installation of an air-conditioning system, (b) interior finishing of the room(i. e. , acoustical walls, ceiling, etc.), (c) making available electrical power and waterto the room through pierce points in the roof of the room, (d) exterior finishing of thearea surrounding the room (i. e. , block wall in the corridor, etc.), (e) installation ofa motor generator set for the mass spectrometer primary power, (f) installation of linefilters and other accessories to provide services for the room.R. E. LovinsB. MASS SPECTROMETER DETECTORThe simplest detection scheme, that of collecting each ion beam on a metal plateand amplifying the resulting current, would require that we push the present capabilityto measure low current levels downward by at least two orders of magnitude. Thereis an alternative, on the other hand, which involves use of state-of-the-art components,and thus affords a better chance of success in building the final instrument.We are considering replacement of the photographic plate by a slit mask containing300-1000 slits, each 1 mm long and with an expected width of 5 microns. The maskcould be made of metal foil on a rigid frame, with slits formed either by high-resolutionphoto-etching (by Bendix Research Laboratories) or by multiple drilling with a pulsedlaser (Raytheon Company, among others). Since the slits would have fixed positions onthe mask, either the beam of the spectrometer or the mask itself could be scanned insteps through a total distance somewhat greater than the spacing between adjacent slits.The ions issuing from the slits would be counted by continuous-channel photomultipliersmounted behind each slit. These novel devices are made by the Bendix Corporation, andconsist of a glass tube (as small as 0.25 mm O.D. by 1 cm long) coated on the inner sur-face with a secondary-emitting film of high resistivity. A high-voltage applied acrossthe ends of the tube accelerates the electrons as they carom down the tube and build uptheir numbers to produce, finally, a constant-amplitude space-charge-limited outputpulse for each incident ion (with at least 60% ion-collection efficiency).1. Block DiagramThe essential components of the interface system between computer and ion beamsare sketched in Fig. VI-1. The diagram is a peliminary description of one form oflogic that transfers the complete spectrum from the focal plane of the spectrograph intoQPR No. 850viSCALER OVERFLOW INDICATIONMAS KELECTRONSOFTYPICALe-m E EOUTPUTR R" TRANSFER D,ION ,GATEBEAMS" INHIBIT - COUNT " RESET " STORE SUM "GATE BUS TOCOMPUTERBEAM-SHIFT [t I 01 1tELECTRONICS " SHIFT- BEAMS "AND " RESET " tFig. VI-1. Logic diagram of the spectrometer focal-plane-to-digitalcomputer - a typical channel (of 300-1000 channels).(VI. ELECTRONIC INSTRUMENTATION)the computer. The ion counts are accumulated in the scaler until the time to, at whichtime the counting is stopped; the sum is transferred to the buffer an instant later, attl, and, if it is desired, the beam is shifted to align the next samples of the spectrumwith the slits. The scaler is disconnected from the buffer and is reset at t ; and thecounting begins at t 3 , after a total delay of a few milliseconds. Meanwhile, the countin the buffer remains available, for transfer to the computer memory, until thebeginning of the next cycle at (t +T).2. Sampling and Retrieval of the SpectrumThe interactions among timing, slit width, and resolving power can be solved by con-sidering that the ion beam intensity is an unknown function of the distance x along thefocal plane. The profile of an isolated line is approximately Gaussian with a well-defined half-width. We may therefore consider the whole spectrum to be a function thatconsists of a sequence of Dirac delta functions with different areas and x coordinates,which has been convolved as a whole with a Gaussian. (In reality, the width of theprofile changes by approximately 5 times from one end to the other of the 30-cm focalplane, so that a one-dimensional transformation of the spectrum must be performedfirst if mathematical rigor is to be preserved.) In this way, the problem is reduced toclassical terms. Ideally, then, each slit in the mask obtains a sample of the smoothedspectrum; however, because of the nonzero width of the slit, we must consider that thesmoothed spectrum is convolved again by the slit, and the output from an electron mul-tiplier then represents a sample (in the mathematical sense) of the resulting spectrum.3. Anticipated DifficultiesThe assembly of slits and multipliers must be located at the edge of the magnet gap,whithin a magnetic field of approximately 10, 000 Gauss. We do not yet know whetherthe multiplier tubes can be operated in such a high field. In order to prevent the firstemitted electron from striking the wall too soon, before it has acquired enough energyfrom the electric field to dislodge secondaries, we shall probably align the axes of themultipliers with the magnetic field; other problems and/or experimental solutions willundoubtedly turn up.The second problem is the unknown response of the active film in the electron mul-tipliers to ions at the heavy (hence low-velocity) end of the spectrum.S. H. ZiskC. RESOLUTION OF MASS SPECTROMETERA proposed analog-to-digital converter for the ion-beam outputs should not signif-icantly decrease the mass spectrometer's resolution. 1 It is important, then, to deter-mine quantitatively the factors that degrade the resolution: (i) energy dispersion in theQPR No. 85(VI. ELECTRONIC INSTRUMENTATION)ion source; (ii) defocussing in the ion-optics systems; and (iii) electronic noise in thedetector. A search is being made for an electrical equivalent circuit for an ion-beamoutput in which these 3 factors will be lumped in noise sources.J. I. GlaserReferences1. K. Biemann, Mass Spectrometry: Organic Chemical Applications (McGraw-HillBook Company, Inc., New York, 1962).D. SAMPLING AND RETRIEVAL OF THE SPECTRUMTo increase the speed with which results may be obtained from the Mass Spectrom-eter, the data will be gathered digitally and processed in a computer. The ions will bedetected by electron multipliers, on related devices, and the pulses counted and storedfor transfer to the computer.Digital acquisition of data from the Mass Spectrometer poses the problem of howwell a discrete number of digital samples characterizes the continuous spectrum. Acomputer program has been written to simulate the sampling process on a normal Gaus-sian curve and to reconstruct the curve from the samples for comparison.J. M. SteeleQPR No. 85

Electronic Instrumentation

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