Real-Time Mass Spectrometry LOGOS-A Generalized Mass Spectrometry Computer System for High and Low Resolution, Gas Chromatography/Mass Spectrometry, and Closed-Loop Applications D. H. Smith,’ R. W. Olsen, F. C. Walls, and A. L. Burlingame2 Space Sciences Laboratory, University of California, Berkeley, Gal$ 94720 A flexible, general purpose laboratory mass spectrometry system called LOGOS employing a small digital computer for complete acquisition, processing, and display of low, medium, and high resolution mass spectral data from both single- and double-focusing mass spectrometers in real-time is described. The real-time computer system employs programming techniques whereby any number of individual programs may be called in any sequence to acquire or operate on mass spectral data. These programs, termed processors, include those necessary for routine work on individual samples or combined gas chromatograph/mass spectrometry (GC/MS) investigations, for general instrument or system evaluation and for feedback control. The monitor controlling these processors has been designed so that processors for specialized tasks may be easily added or modified. The performance of LOGOS in a variety of applications utilizing both low and high resolution techniques is described. MCMURRAYAND COWORKERS (1,2) were the first t o implement the utilization of high speed data acquisition for recording of GC/MS data by employing a frequency modulated magnetic (FM) tape for high resolution mass spectra. The application of computer techniques for low resolution GC/MS studies employing magnetic scanning, single-focusing instruments has been discussed (3-5). In addition, techniques for digitization of mass spectra from time-of-flight (6) and quadrupole (7) mass spectrometers have been described. Reviews of GC/MS techniques including proposed computer combinations have recently appeared (8, 9). When considering a real-time computer system t o acquire, process, and display mass spectral data from a magnetically scanned instrument under a wide variety of possible experimental conditions, we felt it mandatory t o retain as much mass spectral experimental flexibility as possible. 1 Present address, Department of Chemistry, Stanford University, Stanford, Calif. 94305. 2 John Simon Guggenheim Memorial Fellow 1970-71.
(1) W. J. McMurray, B. N. Greene, and S. R. Lipsky, ANAL. CHEM.,38, 1194 (1966). (2) S. R. Lipsky, W. J. McMurray, and C . G. Horvath in “Gas Chromatography 1966,” A. B. Littlewood, Ed., The Institute of Petroleum, London, 1966, p 299. (3) R. A. Hites and K. Biemann, ANAL.CHEM.,39, 965 (1967). (4) Zbid., 40, 1217 (1968). (5) C. C . Sweeley, B. D. Ray, W. I. Wood, J. F. Holland, and M. I. Krichevsky, ibid., 42, 1505 (1970). (6) M. A. Grayson and R.J. Conrads, ibid., p 456. (7) W. E. Reynolds, V. A. Bacon, J. C. Bridges, T. C. Coburn, B. Halpern, J. Lederberg, E. C. Levinthal, E. Steed, and R. B. Tucker, ibid., p 1122. (8) E. Ziegler, D. Henneberg. and G. Schomberg, ibid., (9) 51A. (9) D. Henneberg and G. Schomberg, International Conference on Mass Spectrometry, August 31-September 4, 1970, Brussels, Belgium. 1796
Serious consideration of flexibility in the strategy and conduct of experiments with single as well as double focusing mass spectrometers is important for the following reasons. For one, it would allow rapid adaption of the computer system to different design and operating conditions of various mass spectrometers including various modes of molecular ionization (10). For another, it would bring the full computational power of the computer t o bear on conventional sample introduction techniques such as batch, or direct, inlet methods, as well as GC/MS combinations. Advantages of such a system for handling the large amounts of data generated in low resolution GC/MS applications have been described previously (3, 4). Of course, the capability of LOGOS for high resolution GC/HRMS applications ( Z Z ) , while going beyond the high speed data acquisition task which is of primary importance, will bring unprecedented analytical power to bear in real-time on the resolution and identification of complex mixtures of organic materials from bio-organic, geo-organic and cosmo-organic sources. Finally, the software structure of the computer system could be designed such that “old” (existing) programs could be rapidly modified and new programs easily added t o handle new approaches in a n interdisciplinary research environment. Our goal was to implement such a real-time, computeraided research capability t o accomplish these tasks. In addition, this multi-instrument computer system would be designed around a small digital computer, making the programming task more difficult in the sense that special programming techniques would be required t o process the data within the restricted core memory of a small computer under real-time display and feedback constraints. We have chosen t o call this system LOGOS (Lunar Organic Gas-analysis Operating System) in connection with the impetus provided initially by the NASA Lunar Sample Program (12, 13). The important programming features of LOGOS described here center around a n overlay technique. “Overlay” means that various programs can be executed sequentially in the same area of memory while simultaneously using undisturbed other areas of memory for communication linkage and/or data storage. Although the overlay concept is not necessarily unique to this system and some related systems (14) must of (10) H. D. Beckey and F. J. Comes, “Topics in Organic Mass Spectrometry,” A. L. Burlingame, Ed., Wiley-Interscience, New York, N. Y., 1970, Chap. 1. (11) B. R. Johnson, G. R. Waller, and A. L. Burlingame, J . Agr. Food Cliem., in press. (12) The Lunar Sample Preliminary Examination Team, “Preliminary Examination of Lunar Samples from Apollo 11,” Science, 165, 1211 (1969). (13) The Lunar Sample Preliminary Examination Team, “Preliminarv Examination of Lunar Samples from Apollo 12,” Science; 167, 1325 (1970). (14) R. J. Klimowsky, R. Venkataraghavan, F. W. McLafferty, and E. B. Delany, Org. Mass Spectrom., 4, 17 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
necessity use similar techniques, the method and sequence of overlays and the core memory area that must be allocated for various functions in small computers have not been discussed and merit further consideration in this work. The technique makes it possible t o operate on a set of spectral data at any stage of data reduction with several different processors, using parameter tables as constraints o n the operations. Variables used by the processors (which are assigned values either by default or by the operator) are stored in the parameter tables. Thus LOGOS has been implemented so as t o permit subdivision of the overall computational task into a sequence of relatively simple subtasks. Overlay, coupled with operator interaction with the on-line system, inherently provides a powerful facility for handling of mass spectral data and, further, is a necessity in utilization of a small computer with limited core size for applications in organic mass spectrometry. In conjunction with the overlay approach, the processor sequence is programmable. This means that the operator can specify any sequence of processors t o determine his desired information on a particular sample. In addition, the programmable sequence can be operated in a “DO” loop mode, where the operator can specify a certain processor sequence or groups of sequences t o be executed any given number of times. For example, in GC/MS work, the system can be cycled t o acquire, reduce, and display, e.g., normalized calibrated spectra, any number of times. When put into execution, this set of processors is executed automatically until completion. These procedures are described in more detail by examples in the subsequent section. The important features of LOGOS as regards t o applications are the provision of hardware and software facilities for the on-line acquisition, reduction, analysis, and presentation of both low and high resolution mass spectral data in realtime. Considerable attention has been paid t o viewing the acquisition and subsequent processing of both low and high resolution data as closely related problems as far as possible, thus obviating the requirement for separate programming approaches for handling each type of data. Determination of peak positions in time and peak areas occurs in the time interval both during and immediately after scanning a spectrum. LOGOS can be run using either external or internal massltime calibration, thus permitting processing of either low resolution data for nominal mass determination by prior calibration (external) or medium and high resolution data for accurate mass determination using a reference compound introduced with the sample (internal). Calibration and reduction to masses and relative abundances may be based on either a n increasing or decreasing exponential function of mass cs. time. Presentation of data t o the experimenter may be either through a C R T display for rapid review or through a n incremental plotter for hard copy output. Data presented may be as bar graphs of spectra, summary plots (e.g., selected mass, total ionization, etc.) of a complete run, both described in more detail below, or as heteroatomic plots (15, 16) or element maps (Z6-Z8).
(15) A . L. Burlingame and D. H. Smith, Tetrahedron, 24, 5749
(1968). (16) A. L. Burlingame and H. K . Schnoes, in “Organic Geochemistry: Methods and Results,” G. Eglinton and M. T. J. M u r p h y , Ed., Springer-Verlag, New York, N. Y . , 1969, pp. 89-160. (17) K . Biemann. Pure Appl. Cliem., 9,95 (1964). (18) K . Biemann. P. Bommer, and D. Desiderio, Tetruhedro/r Lett., 1964. 1725.
The CRT display, because of its rapid drawing speed and storage capability, is a particularly powerful feature of LOGOS as it would be for any related system capable of driving a display. Its rapid speed allows a set of data (for example, the many individual spectra generated during a GC/MS run) t o be quickly viewed either in real-time or later by the experimenter. Its storage capability means that the data displayed need be drawn only once, thus freeing the computer for execution of additional processors while the display can be studied at leisure. LOGOS also provides the facility for logging the experimenter’s sample information by including a n experiment code and automatic sequencing of the scan numbers within a n experiment. Some low resolution GC/MS systems provide similar facilities (3-5, 7). The experiment code consists of a sample number input from teletype by the operator before data collection begins. Once data are recorded on magnetic tape, this logging feature permits operator recall of any particular scan or groups of scans within a given code by rapid search of the tape. By using magnetic tape as the primary output medium, the number of scans that can be recorded is extremely large (several thousand complete mass spectra) and has been no restriction on the type of experiment which has been performed with the system for the past two years. The primary requirement of LOGOS as far as a mass spectrometer is concerned is the need for a reproducible magnetic scan for applications using external calibration. The program is designed t o correct for uniform shifts of the mass/time function-Le., where all peaks are displaced in time by a n equal amount-but does not correct for nonuniform changes in the function-e.g., mechanical and/or electrical instability. In practice, we have demonstrated that the required reproducibility can be easily maintained by repetitive cycling of the magnetic field (19). This cycling can be carried out under feedback computer control by using the real-time clocks of the computer or a clock built into the computer/mass spectrometer interface, or alternatively, by triggering the beginning and end of scans using circuitry built into the mass spectrometer. EXPERIMENTAL
Instrumentation. A block diagram of LOGOS is presented in Figure 1. The mass spectrometer, as mentioned previously, may be any instrument capable of providing reproducible, exponential mass us. time magnetic scans. The scans may be either exponentially increasing o r decreasing in mass with a mass range limited only by the maximum range covered by the calibration compound employed. As an example of the advantage of the modularized software of LOGOS, any other mass-time function desired to enhance information derived from certain portions of the mass range can be incorporated using other processors-e.g., quadratic mass-time function (19). LOGOS has been operated with a GEC-AEI-MS-902 mass spectrometer, a Perkin-Elmer Model 270 GC/MS, both double-focusing instruments, and a special single-focusing Hitachi Perkin-Elmer RMU6-D. The latter instrument was used with LOGOS in conjunction with the preliminary organic analysis of returned lunar samples a t the Lunar Receiving Laboratory, Manned Spacecraft Center, Houston, Texas (12, 13). The interface and computer configuration shown in Figure 1 is independent of the mass spectrometer employed, with (19) A. L. Burlingame, R. W. Olsen, and D. H. Smith, Proc. 15th Amual Confereme Mass Spectrom. Allied Topics, Denver, Colo., May 14-19, 1967, p 568.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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I
WA INTERRUPT
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CPU
I INTERRUPT (2)
SCAN CONTROL
I
]TELETYPE]
4
I
ENAELEHFREPUENCY DISABLE DIVIDER
I
Figure 1. LOGOS hardware configuration
BOOTSTRAP LOADER
PROCESSOR COMMUNICATION SYSTEM CONTROL MONITOR U T I L I T I E S OVERLAY LOADER MONITOR RECALL
NON- RESIDENT MONITOR OR PROCESSORS
USER
AREA 7
7 - 3 TIME/INTENSTTY
DATA
DATA
-I/
8191
Figure 2. Core memory allocation with functions and respective word boundaries the exception of the manner in which the feedback for scan control is implemented and minor changes in the isolation amplifier. The output of the mass spectrometer electron multiplier/amplifier system is first fed to an isolation amplifier. This is a differential amplifier placed in the path between the mass spectrometer and the analog-to-digital (A/D) converter input to accomplish three things. First, the differential input of this amplifier eliminates the sometimes considerable noise in the grounding system of the mass spectrometer. Second, the amplifier acts to match the impedances of the mass spectrometer amplifier and the A/D input. Third, by adjusting the feedback of the amplifier to set the proper gain, the output level of the mass spectrometer amplifier is matched to the 0-10 V range of this A/D. The A/D-multiplexer is an Adage Corporation, Model VT13AB/VMX55. This is a 14-bit (13 plus sign) converter with a conversion time of 10 psec, or a maximum digitization rate of 100 KHz for a single channel. The computer employed in this system is a Xerox Data Systems (XDS) 2-2. The XDS 2-2 is a n older version of the current XDS 2-3 computer which is program compatible. 1798
The central processor has a 8192 X 16-bit word memory with a cycle time of 960 nsec. The computer has as peripheral devices a random access disk (RAD), a magnetic tape, a CRT display, teletype, and an incremental plotter. The time base for the system is the 1.024 MHz crystal clock of the central processor (CPU). The interface consists of the circuitry necessary to acquire data through the A/D, control, or monitor the scanning of the mass spectrometer and output data to the CRT display. A frequency divider controlled by switches allows a selection of digitization rates for data acquisition. The scan control, which depending on the mass spectrometer may be activated by relay contacts representing the beginning and ending of scans or by direct feedback computer control of the scans as with the Hitachi Perkin-Elmer RMU6-D, arms and enables the A/D interrupts during data acquisition and disarms and disables them during re-scan of the magnet. The computer thus senses the state of the scan by determining whether or not it is receiving interrupts. The interface also contains data registers to store data both for transfer of the A/D data to the computer and for transfer of computer data to the CRT. The address decoder simply determines which device is to be serviced. The CRT is a Tektronix Model 611. It is capable of operating in either a nonstore or a storage mode. In the case of LOGOS, it is generally operated in the storage mode so that once a particular drawing is traced on the display it remains stored for any desired length of time. Drawings are constructed by means of data presented to the interface from the computer in the form of x , y coordinate pairs. The interface then moves the light beam (either turned on or blanked off) from the present position to the new x,y coordinate using the digital to analog converters. In a sense, then, construction of a drawing on the CRT is similar to construction of a plot on an incremental plotter. The differences are primarily the extremely fast writing speed of the display and additional features available such as automatic erase. In addition, the CRT operates on a second interrupt as shown in Figure 1. This interrupt is activated when the CRT has finished the move to the new x,y coordinates and is free to accept a new pair. This frees the computer for other operations even when construction of a display is in progress. Programming. The task of data handling from acquisition to presentation involves far more programming and calculations than can be carried out with one large program in the limited memory of this computer. For this reason, as was alluded to previously, this task is broken down into relatively simple subtasks. This not only allows the entire task to be carried out, but permits flexible operation and facile modification of the software. The processors that carry out these tasks, for example, a processor to acquire data, one to reduce data, one to display data, etc., will be described in some detail, but it is important to realize that this method of processing mass spectral data depends heavily on the program used to keep track of and sequence these processors
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
and to put them into execution. This sequencing and control program is called the LOGOS monitor. By use of the overlay techniques, this monitor is capable of sequencing processors, either manually or automatically. The sequence and choice of processors is determined by the operator according to the requirements of a particular experiment. The LOGOS monitor and its processors reside in a random access disk (RAD) file indexed by function names. The first two characters (which are input from the teletype) of each function name serve as control commands which cause either the LOGOS monitor or a processor to be loaded into core memory and executed. Core memory is partitioned into areas reserved for system control, a user area, and data arrays, as shown in Figure 2. Normally, the LOGOS monitor and one of a sequence of processors alternate as exclusive residents of the user core area. Loading of the monitor or a processor into the user core area overlays and destroys the previous resident but leaves the system control area and data arrays unchanged. The structure of the LOGOS monitor is presented in Figure 3 which also indicates its primary functions. The monitor consists of an executive program, a processor file directory, a table for command lists, and a set of system generation and modification programs. The executive program interprets control commands and sequences the loading and execution of the monitor and processor overlays. Its essential parts are a mode controller, a control command interpreter (CCI), a command list processor, an overlay loader, and a monitor recall program. The operations of the executive are described in Figure 4. The mode controller determines whether the mode of operation is normal, program, or sequence. In the normal mode, the system may be operated in a stepwise fashion at the operator’s discretion. The monitor simply accepts a two-character code (control command, CC) from the teletype (TTY). This code is checked by the control command interpreter (CCI) to determine if it is valid. If it is not, it is rejected. If it is, the code is passed to the overlay loader, the selected processor location on the RAD determined from the processor file directory, and the processor loaded into the user area of memory, overlaying the monitor, and executed. When the processor’s task is completed, execution is passed to the monitor recall program, which brings the monitor back into memory, again using the overlay loader. As indicated in Figure 2 , the overlay loader and monitor
I ) MODE CONTROLLER
21 CONTROL COMMAND INTERPRETER I C C I ) 31 COMMAND L I S T PROCESSOR 4) OVERLAY LOADER 5 ) MONITOR RECALL
1
PROCESSOR FILE DIRECTORY
I
u
1
COMMAND LIST TABLE
1 I 1
SYSTEM ON G E N E R I T I ON AND MODIFICATION
I I
Figure 3. LOGOS monitor functions recall programs reside in the undisturbed portion of memory. The monitor is then ready to accept the next control command. In the program mode, no loading or execution of processors takes place. Rather, the operator is requested to select a two-character name for the subsequent program. Subsequent TTY inputs of legitimate, two-character, processor codes build up a list in the command list table, until the END PROGRAM command is input. When the computer is requested to execute this program, LOGOS transfers control to the sequence mode of operation. From this point the monitor uses the command list table for this program in a stepwise fashion as outlined for the normal mode. A processor is found, loaded, and executed. When execution is terminated and the monitor is reloaded, the next entry in the command list is taken, the next processor is found, loaded, and executed, and so forth until the sequencer finds the E N D PROGRAM command, whereupon the monitor returns to the normal mode. The program mode of operation is capable of setting up D O loops, which permit automatic sequencing and execution of a given set of processors expressing an experimental strategy any desired number of times. Many different programs may be stored in the command list table under different two-character names. The monitor also has the facility to clear, or erase, the current programs in the command list table. The final features of the monitor are the system generation and modification capabilities. If a processor has an error OVERLAY
1 PROCESSOR
ERROR MESSAQE
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giyt&
EXECUTE PROCESSOR
(NORMAL
COMMAND
DIRECTORY
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INITIALIZE COMMAND LIST
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OVERLAY
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MONITOR
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COMMAND
L
1
Figure 4. LOGOS monitor executive operation flow-chart ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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PROCESSOR
TTY CODE FUNCTION
PROCESSOR
TTY CODE FUNCTION
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sc
SCAN
ACQUISITION
AC
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PT
CALIBRATE
CA
PF CALIBRATE
PF
REDUCE
RE
DELETE CAL
DC
COMPOSITION
co
COMPOSITION SET
cs
PARAMETERS
PA
START SUMMARIZE
ss
SUMMARIZE
su
Acquires data during an increasing or decreasing exponential magnetic scan and determines peak centers of gravity (c/g) and intensities (areas). Maximum data rate 20 KHz. Acquires peak profile data during scan and outputs to tape between scans. Maximum data rate 33 KHz. Reads profile tape and determines c/g and area. Calibrates spectrometer mass/ time curve using hydrocarbon spectrum. Calibrates spectrometer mass/ time curve using perfluorokerosene spectrum. Calculates masses based on calibration data. Deletes all the peaks of the calibration compound within tolerances set by the operator. Determines the elemental composition of each peak within tolerances set by the operator. Output may be selected to either the TTY or Tape. Sets the elements and limits for the COMPOSITION and DELETE CAL processors. Inputs variables for processors which require them. Initializes the pointers and data arrays prior to operating the SUMMARIZE processor. Calculates the total ion current cs. scan number, the total minus deleted ion current 6s. scan number, the total series ion current (20 series), 10 single mass ion currents us. scan number, and 2 external channel readings us. scan number for up to 1800 scans.
END SUMMARIZE
ES
DATA SELECT
DA
TYPEOUT
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IN
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EO DE RD
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EXTERNAL B
EB
Terminates the summarize process. Selects and inputs the SUMMARIZE data into core. Outputs to the TTY the data in core. (Time-intensity, m/eintensity, scan numberintensity). Outputs to the TTY the deleted ion currents and the series ion currents. Displays c/g or reduced data on CRT. Plots c/g or reduced data on plotter. Plots summarize or sum mass data, ion current us. scan number. Displays SUMMARIZE or SUM MASS data, ion current L‘S. scan number. Plots the SUMMARIZE data in core on the same graph as a previous SUM PLOT. Searches for and inputs a selected scan from the disk. Searches for and inputs a selected scan from tape. Inputs one spectrum from tape to memory. Outputs one spectrum from memory to tape. Inputs one spectrum from disk to memory. Outputs one spectrum from memory to disk. Writes end of file on tape. Writes end of file on disk. “Rewinds” disk, i.e., sets RAD pointer back to zero. Inputs and stores the reading on external Channel A. Inputs and stores the reading on external Channel B.
Figure 5. Current list of processors in it, or if a modification of a current processor is desired, it can be loaded into memory and changed by teletype control. The modified version is then written out on the disk, and subsequent monitor requests for this processor will access the modified version. A request for system generation writes the current LOGOS system out on magnetic tape with only the updated processors included. In this manner, several versions of LOGOS to d o specialized tasks may be stored on tape and read into the computer as needed. In a similar fashion, additional processors may be added t o LOGOS when desired, and this updated LOGOS system written back out on magnetic tape for future use. A list of the most frequently used processors for mass spectrometric data processing, including a brief description of the function of each processor is presented in Figure 5. There are some important features of certain of these processors that warrant a more detailed description. The SCAN processor is programmed to monitor the state of the mass spectrometer magnetic scan. If this processor is entered during a scan or rescan of the magnet, the processor waits for the beginning of the next scan before acquiring data. During data acquisition, information necessary to determine the center of gravity and intensity of each peak is stored in memory. This information consists of the peak area (A), the summed areas (S), and the time a t the end of the peak 1800
0
(T). When the end of scan signal is received, the integer data in core is converted into floating point notation and peak centers of gravity (C/G) are determined by the formula T - S/A (20). Because a n exponential scan function results in peak areas being already normalized throughout the mass range, the peak area is used as the peak intensity (21) rather than the less accurate peak height (3, 4). The PARAMETER processor allows the operator to set a threshold to eliminate base-line amplifier noise and t o set a minimum number of data points per peak criterion to eliminate possible noise spikes. An alternative data acquisition processor can be used to acquire and store on magnetic tape the thresholded peak profile data. These peak profiles can then be treated in a variety of ways t o yield more detailed information (22). (20) D. H. Smith, R. W. Olsen, and A. L. Burlingame, Proc. 16th Aiiiiirnl Coilfereiice Mass Spectrom. Allied Topics, Pittsburgh,
Pa., May 13-17, 1968, p 101; Burlingame. D. H. Smith, and R. W. Olsen, ANAL. CHEM., 40, 13 (1968). (22) A. L. Burlingame, D. H. Smith, T. 0. Merren, and R. W. Olsen. in “Computers in Analytical Chemistry,” C. H. Orr and J. A.’ Norris, Ed., Progress in Analytical Chemistry Vol. 4, Plenum Press, New York, N. Y.,1970, Chap. 111. (21), A. L. \-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
The methods of mass-time function calibration and reduction of peak occurrence times t o masses are the most important features of LOGOS. There are two CALIBRATE processors that can be used, one based on the spectrum of a hydrocarbon, the other based on perfluorokerosene (PFK). Both processors use the exact masses of the peaks involved in the calibration procedure, rather than nominal masses (3-7). This eliminates problems of calibration and reduction of spectra of compounds or mixtures containing peaks of widely differing mass defects. In addition, .the use of exact masses in CALIBRATE allows determination of accurate masses in an unknown when the PFK calibration and reduction processors are used on the same spectrum (internal calibration), which must be, of course, recorded at sufficiently >2000) to resolve the P F K peaks high resolution (MIAM from the peaks of the compound of interest. Whether used for internal or external calibration, both CALIBRATE processors operate by searching for m/e 28 (Nz+) within operator specified time limits. The limits will depend to some extent on the mass spectrometer employed. In some cases the reproducibility (for 1-2 hours) may be within a peak width, in which case N2+ can be distinguished unambiguously under high resolution operation. Should lower reproducibility be encountered (in no case, barring instrumental malfunction, has observed variation exceeded 0.1 mass unit at m/e 28), the time limits can be expanded and Nz introduced to the ion source through a suitable inlet system. The processor will choose the most intense peak within the interval. In the case of hydrocarbon calibration, m/e 57 is searched for as the largest peak above mje 28, mje 43 is found as the largest peak between mje 28 and 57 14 n , n = 1, 2, 3 . . .) and the remaining peaks (m/e 57 found by extrapolation in a stepwise fashion using the exponential mass/time function. In the case of PFK as a calibration mixture, nile 69 (CFa+) is found as the largest peak above m / e 28, nile 51 (CFZHI)is found by interpolation and the remaining peaks, supplied in a table, found by extrapolation. Both processors then transfer this calibration data to the disk for subsequent use by the REDUCE processor. The REDUCE processor first corrects for time drift in the mass spectrometer by comparing the time of m/e 28 in the calibration data with the time of m / e 28 in the spectrum in memory. All peak times are then corrected by this time difference, if any. Using the calibration data, peak times are converted to masses, at all times keeping the mass defects as calculated rather than truncating or rounding to nominal masses. The DISPLAY and PLOT processors are designed to produce graphical output for four types of data. For C/G data, bar graphs of relative ion abundance cs. time are produced. For mje data, bar graphs of relative ion abundance cs. mass are produced. For SUMMARIZE outputs, graphs of total spectrum ion current, calculated by summing all ion abundances in a spectrum, or single mje ion abundance N
+
SUMM mu-e MAX T I =
TTY CODE FUNCTION
COMMAND PROGRAM
PR
END PROGRAM
EN
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Sets mode to PROGRAM and initiates command list. Sets mode to NORMAL and terminates command list. Clears all program lists not protected by SET command. Initiates a ‘DO’ loop in program command list. Terminates a ‘DO’ loop. Executes a program list. Loads a system generation package which has the following functions. Protects the existing program file from the CLEAR command. Clears all existing program files. Adds a processor to the system. Deletes a processor from the system. Allows a processor in the system to be modified. Generates the entire M.S. system on a self-booting tape. Modifies the system identification code.
Figure 6 . System Commands us. scan number (Mass Chromatogram) (23) may be constructed. Parameters governing the size of graphs, scale factors, and labels are all under operator control It is seen from Figure 5 that magnetic tape and/or the R A D may be used for data storage. Experiment codes input under the PARAMETER processor serve as a means of recalling the stored data from a particular experiment. The SELECT TAPE processor allows the operator to input from tape any scan number or series of scan numbers from a selected experiment code, supplied to the system through the PAR AM ETER inputs. Data may be acquired under high resolution conditions with either the SCAN or ACQUISITION processor depending on the desired digitization rate (see Figure 5 for maximum rates). These data can then be reduced using the same processors employed in reduction of low resolution data, with the extra ability for determination of possible elemental compositions for each mass. The numbers and kinds of heteroatoms to be considered in this step are input via the teletype In addition to the processors described in Figure 5, the available system commands are listed in Figure 6. The functions Ere essentially self-explanatory based on the preceding discussions of the LOGOS monitor operations.
(23) R. A. Hites and K . Biemann, ANAL.CHEM., 42, 855 (1970).
P66-7 8-4-69 179232
Figure 7. Total ion current cs. scan number plot from LOGOS-GCjMS analysis of a York mesh monitor from Apollo 12
030 040 O S 0 060 070 080 030 100 1 1 0 120 130 140 160 160 170
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
0
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Figure 8. CRT display of scan 87 (see Figure 7)
RESULTS
Low Resolution Applications. The following discussion will serve to illustrate the use of LOGOS in two different types of experiments performed at low resolution. The first example is a n application to GC/MS work, in this instance using the Perkin-Elmer Model 270 mass spectrometer, an instrument employing a n exponential up-scan. This instrument is equipped with a G C interfaced to the mass spectrometer through a Watson-Biemann separator. Because of the extremely short interval between scans (approximately 1 sec), LOGOS was programmed to acquire C/G data and simply store it on the disk. The magnet was scanned at a rate of 3 secldecade and covered the mass range 18-600. At the completion of the G C run, the operator interrupted the acquisition program and executed a program which read the disk, converted the C/G data to m/e data and output to magnetic tape. The SUMMARIZE processor was used to generate the total ion current us. scan number data. A plot of the summarized data was produced which very closely resembles the output of a suitable G C detector (3, 4). The plot generated from this experiment is shown in Figure 7. The operator can then selectively input from tape the desired mass spectra chosen on the basis of the summary plot, examine them on the display and plot them if desired. Figure 8 shows the CRT display and Figure 9 the resulting hard-copy plotted spectrum of scan 87, a trichlorobiphenyl found on Apollo 12 York mesh monitors (24). (24) B. R. Sirnoneit and D. A. Flory, “Apollo 11, 12, and 13 Organic Contamination Monitoring History,” NASA, Lunar and Earth Sciences Division Note MSC 04350 (May 1971). 87 X
1
+
OR-2 PSS-7 8-4-69 > 200 186= 7008
050
1802
A second application of LOGOS has been in connection with preliminary lunar sample analysis for organic matter (12, 13). In this case, the mass spectrometer used was the Hitachi Perkin-Elmer RMU6-D, operating with a decreasing exponential scan. Although a detailed description of the experimental procedures involved in this study is beyond the scope of this paper, it is perhaps instructive to briefly examine how LOGOS was used and the type of data produced. In this experiment, spectra are taken continuously as the sample is heated in an oven. The magnet scan is controlled by the computer for length of scan, both down and re-scan. Operation of LOGOS is very similar to that described above, with the exception that, because of slower scan (10 sec/spectrum) and re-scan (6 seconds) speeds, it is possible to reduce the data and output it to tape between scans. Data presentation is carried out in the same way as the G C data with some important exceptions. It was quickly determined that the major fraction of the ion current for the scans was due to ions of lesser analytical importance, for example, m/e 18 (HzOf), mje 28 (N2+ CO+), and m/e 44 (COz+). For this reason the SUMMARIZE processor was modified to provide the capability of deleting the ion currents due to a specified list of masses. This has the effect of making the resulting summarize plot much more sensitive to slight changes in other, more important masses from an organic analysis standpoint. This feature has also been used to eliminate peaks due to column bleed from the summarize results in conventional GC/MS work. Some data from a run on a rock chip from Apollo 12 are presented in Figure 10. The top summarize plot represents
100
150
200
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
260
010
0e0
SUMM )I(/€ MRX T I
=
44
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040
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060
OVCa
lt07, U l t O S ' l r
83190
070
080
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. V I , ,
~,ili~lli.,,i~,,,ilill,l,lll,lllllll,l,l,l,l,l,i,l,l,i,l,l,ll,lll,,ili,l,i,i,i,i,i,i, 010
0t0
OS0
040
OS0
060
070
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080
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i L 0 7 19057 C H I P DOC. PBX > 50 44= 80400
HTIL
Figure 10 (Top). Total ion current 1;s.scan number plot from analysis of an Apollo 12 rock-chip sample (background deleted) (Middle). Summary plot of m/e 44 (Bottom). Two single scan plots the results after deletion of masses 14, 17, 18, 19, 20, 24, 28, 32, 35, 37, 40, and 44. Also presented is the result of the SUMMARIZE processor, programmed in this instance to sum m/e 44 for all scans (this mass was deleted in the top plot of Figure 10). The summarize plot (Figure 10, Top) indicates that some material is being pyrolyzed from the
sample, with a maximum rate of evolution in the range of scans 35-40. Two spectra are plotted at the bottom of the Figure, scans 10 and 45. Scan 10 indicates a very small amount of volatilizable material evolving from the sample, and the height of the mje 44 peak (42,000 units) is notable. At scan 45, as indicated in the Mass Chromatogram (23), m / e
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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Table I. Scan Reproducibility Test at High Resolution Employing an External Mass Standard m/e (true)
Ti 43.055 43.054 141.164 141.152 254.297 254.270 TI = Time of calibration Tz = Time of calibration T3 = Time of calibration T4 = Time of calibration
TZ
T3
43.053 43.052 141.148 141.135 254.262 254.230 30 minutes. 50 minutes. 70 minutes. 90 minutes.
+ + + +
T4
43.051 141.137 254.233
44 has increased considerably in intensity, indicating that C o n is evolvjng from the sample as a pyrolysis product. The cause of the total ionization maximum in the summarize plot after the deletions mentioned above was determined by examination of scan 45 (and previous scans) t o be mie 64. Further experiments using the SUMMARIZE processor determined that m / e 48 increases and decreases in intensity with m/e 64, indicating that m/e 64 in probably SO?.+. Fragmentation of SO2‘+occurs with loss of a n oxygen radical t o give SO+, mie 48. Two brief examples can serve t o further illustrate the utility and flexibility of LOGOS in low resolution studies. In applications involving use of a batch inlet system, samples can be introduced into the instrument at the operator’s convenience with the computer continuously monitoring the spectra, providing a C R T display of every spectrum t o the operator. The operator can then adjust the sample flow rate and multiplier/amplifier gain as required t o obtain an optimum spectrum. When such a spectrum is obtained, the operator can then save one or more as desired on magnetic tape, discarding the rest. LOGOS can then be used t o monitor the spectra in the form of intensity us. m/e displays, with the capability of intensity multiplication in various mass ranges, as the sample is pumped off. Viewing the spectra in this form prevents possible misintepretation of the mass scale that might arise by viewing the analog data on a storage oscilloscope and allows detection of trace amounts of the previous sample within general instrument background. The operator then can easily determine when the sample is gone and introduce another sample. A second example is in applications using a direct introduction probe. In this case LOGOS can be programmed t o save all spectra as the probe is inserted into the ion source and heated. With all spectra saved, decisions on which scans t o record are not necessary as the sample is being run, but can be made later at the operator’s convenience using the C R T t o quickly review all the data and plot those spectra containing desired information. High Resolution Applications. As was described previously, the two CALIBRATE processors and the REDUCE processor use accurate rather than nominal masses in all calculations. LOGOS was designed based on our previous experience with the XDS 930 (21) and Sigma 7 on-line systems (22) such that accurate mass determination would be possible with the computer. To evaluate its performance under high resolution operation ( M / A M = 2,500 t o lO,OOO), several experiments were performed using the MS-902 mass spectrometer linked t o Sigma 2-LOGOS. These experiments were designed t o evaluate the performance of the system in detail using both external and internal methods of calibration. EXTERNAL CALIBRATION MODE. The capability of saving and examining calculated mass defects permits a n investiga1804
tion of the stability of the mass spectrometer scan and the ability for the time correction procedure of the REDUCE processor t o correct for peak time drift in the spectrometer. The following experiment was performed t o evaluate these factors under external calibration. With the MS-902 operating at a resolution of 2500, calibration with P F K was performed. Thirty minutes later, a sample of n-Cls hydrocarbon, octadecane, was introduced into the spectrometer and scans were recorded at a scan rate of 16 secondsidecade at regular intervals for one hour. Table I presents the results for three peaks in the spectrum of the hydrocarbon, one at low mass, m / e 43, one at intermediate mass, m/e 141, and the molecular ion, m/e 254. The data in Table I indicate that a slight compression of the mass/time curve has taken place over the 90-minute duration of the experiment. This compression is due entirely t o long term variations in the magnet hysteresis curve and/or associated magnet scan circuitry. A brief experiment of this type immediately provides important information about the scanning precision of a given mass spectrometer. This evaluation is difficult, if not impossible, on other computer-mass spectrometer systems described in the literature (3-7) due t o their nominal M U S S calibration orientation. The maximum mass displacement observed at m/e 254 is about 0.07 amu (280 ppm) 90 minutes after calibration, a trivial variation when only nominal masses are desired from the system. Similar experiments utilizing the Perkin-Elmer 270 showed maximum variations of 300 ppm over a 24-hour period (25). INTERNAL CALIBRATION MODE. To test mass measurement accuracy using PFK as an internal standard, three experiments were performed under different operating conditions using the MS-902 mass spectrometer. Table I1 summarizes these experimental parameters. As in the external calibration experiment, the sample was octadecane. The lowest resolution tested, M/AM = 2500, is sufficient to separate the calibration peaks of PFK from the hydrocarbon fragment peaks arising from the fragmentation of octadecane. In these experiments 10 scans were acquired, calibrated, and reduced automatically a t each of the three instrument operating resolutions. Data presented in Table 111 cover the mass range from mie 40 to 254 and a dynamic range of about 500:l. Examination of the results presented in Table I11 reveals two important features. First, the accuracy of mass measurement under LOGOS for single and multiscan averaged spectra is comparable to that obtained previously using a n XDS Sigma 7 computer (22, 25-27). However, a systematic negative error is obtained in the Sigma 2 which is due to the inherent precision of the mathematical routines in the existing single precision mode. The magnitude of this error is between 0 and a maximum of 2 ppm in mass and results from truncation of intermediate mass calculations of 24-bit binary precision. Notwithstanding, such high quality mass mea(25) A. L. Burlingame, D. H. Smith, F. C. Walls, and R. W. Olsen, Proc. 17th Annual Conference Mass Spectrorn. Allied Topics, Dallas, Texas, May 18-23, 1969, p 28. (26) A. L. Burlingame, “Developments and Applications of Real-
Time in High Resolution Mass Spectrometry,” in “Recent Developments in Mass Spectroscopy,” K. Ogata and T. Hayakawa, Ed., University Park Press, Tokyo, pp 104-115 (1970). (27) A. L. Burlingame, “Impact of Computer-coupled High Resolution Mass Spectrometry on Molecular Structure Studies,” Proc. International Chromato-Mass Spectrometry Symposium, Moscow, May 21-28, 1968; U. D. Sitnianskey, Ed., Academy of Sciences of the U.S.S.R., Chemical Physics, Commission on Mass Spectrometry, Moscow, 1969, pp 248-266.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
Table 11. Experimental Parameters for Mass Measurement Accuracy Evaluation Digitization Data ptstpeak Resolution Peak width, psec rate, KHz Expt No.& Scan rate 1 16 secldecade 8 2,500 2800 22 20 5,000 1400 28 2 16 sectdecade 10,000 700 14 16 secidecade 20 3 Sample flow rate for the above experiments was -5 ng/sec. Total sample consumed for each experiment was -1.75 pug.
True mass 41.0391 42.0469 43.0548 55.0548 56.0626 57.0704 69.0704 70.0782 71.0861 83.0861 84.0939 85.1017 97.1017 98.1095 99.1174 111.1174 112. I252 113.1330 124. I252 125.1330 126.1408 127.1487 139.1487 140.1565 141.1643 154.1721 155.1800 168.1878 169.1956 183.2112 197.2269 211.2426 254 2973
Mean relative intensity 26.76 7.84 70.62 19.90 13.48 100.00 10.35 10.71 65.44 6.83 6.45 43.58 4.97 4.45 11.64 2.17 3.29 7.42 0.21 0.96 2.43 5.18 0.27 1.98 3.75 1.52 2.88 1.26 2.39 2.12 1.83 1.oo 3.83
Table 111. Mass Resolution 10,000 Single scan Obsd mass, Error, average error, 10 scans ppm Ppm 41 0392 +2.4 +2.4 -7.1 +2.4 42.0466 43 0549 +2.3 0 -3.6 -3.6 55 0546 -3.6 -5.4 56 0624 -5.3 57.0703 -1.8 69.0703 -1.4 -2.9 70.0780 -2.9 -1.4 71.0860 -1.4 -1.4 83.0861 0 +6.0 +1.2 -1.2 84.0938 85. IO19 +5.9 +2.4 97. I015 -3.1 -2.1 98.1091 -8.2 -4.1 99.1172 -3.0 -2.0 111.1175 $4.5 $0.9 -2.7 -2.7 112.1249 -0.9 -0.9 113.1329 -8.1 124.1242 -4.0 -4.8 -4.0 125.1325 -5.6 126. I402 -4.8 -5.5 -6.3 127.1480 139. I489 +1.4 -7.2 140. I567 0 +I .4 -0.7 141.1642 0 -1.3 154.1719 -7.8 -3.9 -8.4 155.1794 168,1875 -1.8 -3.0 -1.8 -3.6 169.1953 -5.5 183.2106 -3.3 -3.0 197.2264 -2.5 +2.8 21 1.2420 -2.8 -1.2 254.2966 -2.8 I
I
I
I
Measurement Accuracy Data Resolution 5,000 Std dev, ppm
8.8 9.0 4.2 3.6 2.9 2.5
1.3 1.6 1.7 3.4 2.6 2.8 2.4 1.9 0.8 2.3 1.6 1.7 8.6 2.4 2.1 2.1 6.3 1.8 2.7 3.0 3.2 2.3 1.5 1.6 2.5 2.3 2.4
surement is sufficient t o permit the assignment of elemental compositions for routine problems in organic mass spectrometry. Second, for isobaric peaks in a real-time mass spectrum, the accuracy of mass measurement is a function of the product of resolution and sensitivity and does not depend upon resolution per se t o any important extent in the range m / A M 2500 + 30,000 (22). In the course of this effort we have tried to describe in as much detail as possible the performance of each part of a somewhat complex system t o isolate sources of error and determine the effects of variations in scanning rates, digitization rates, mass spectrometer resolutions, and programming techniques. The detailed evaluation of the overall quality of mass spectral data obtained from a real-time system, such as LOGOS, is necessary to assess the full power of a computerized system over conventional modes of mass spectrometer utilization. Reports of a small computer system dedicated to high
Obsd mass, average 10 scans 41.0395 42.0467 43.0550 55.0548 56.0622 57.0703 69.0703 70.0779 71.0860 83,0861 84.0937 85.1018 97 I015 98 1090 99.1172 111.1173 112. I248 113.1328 124.1244 125.1325 126.1402 127.1480 139.1485 140.1566 141.1641 154.1719 155.1795 168.1875 169.1952 183.2107 197.2264 21 1.2420 254.2962 I
I
Error, ppm +9.8 -4.8 +4.7 0
-7.1 -1.8 -1.4 -4.3 -1.4
Resolution 2,500 Std dev, PPm
6.4 5.0
2.3 6.2 5.9 6.3 1.5
-2.4 $1.2 -2.1 -5.1 -2.0
1.3 2.0 2.4 2.9 2.7 2.5 1.6 1.3
-0.9
2.3
-3.8 -1.8 -6.5 -4.0 -4.8 -5.5 -1.4 +0.7 -1.4 -1.3 -3.2 -1.8 -2.4 -2.7 -2.5
1.9 1.4 8.0 2.2 2.2
0
-2.8
-4.3
2.1
5.3 1.6 0.9 2.9 2.8 1.7 1.5 1.9 2.0 2.3 1.7
3bsd mass, average 10 scans 41.0393 42.0466 43.0547 55.0546 56.0623 57.0702 69.0703 70.0779 71.0859 83.0855 84.0931 85.1011 97.1015 98.1090 99.1170 111.1170 112.1245 113.1324 124.1246 125.1329 126.1407 127.1482 139.1486 140.1563 141.1637 154.1717 155.1793 168. I878 169.1953 183.2110 197.2263 21 1.2421 254,2957
(
Error, ppm +4.9 -7.1 -2.3 -3.6 -5.4 -3.5 -1.4 -4.3 -2.8 -7.2 -9.5 -7.1 -2.1
-5.1 -4.0 -3.6 -6.3 -5.3 -4.8 -1.3 -1.3 -3.9 -0.7 -1.4 -4.3 -2.6 -4.5
Std dev, ppm 4.4 3.3 4.0 2.9 2.5 2.5 1.7 1.9 2.8 2.0 2.4 2.7 1.0 1.2 1.3 0.8 1.6 1.0 6,2
2.5 2.1 2.1 3.3 2.0 1.7 2.1 1.7
0.0
1.8
-1.8 -1.1 -3.0 -2.4 - 6.3
1.5 2.2 2.3 2.7 3.5
resolution mass spectrometry have recently appeared (14, 28). These reports reach the conclusion that increased mass spectrometer resolving power yields increased mass measurement accuracy, in marked contrast to our findings published previously and in this paper. We cannot accept this conclusion when it is based on mass measurements with a standard deviation of 59.1 ppm (2000 resolving power) or 38.5 ppm (10,000 resolving power) (14). (Compare to Table 111, this work.) Our data were acquired with a sample flow rate of -5 ng/sec. Comparable data from other studies (14, 28) are not available. In addition, although we agree, and have shown (22)that multiscan averaging techniques result in mass measurement accuracy which is improved by &, where n is the number of scans averaged, indicating that the computer can act as a powerful signal-averaging device, we cannot accept that our results are “similar” t o those reported in (28) R. Venkataraghavan, R. J. Klimowski, and F. W. McLafferty, A c c o m s Cliem. Res., 3, 158 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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reference 28 because of the large disparity (w factor of ten) in mass measurement accuracies achieved, indicating that these two approaches cannot be readily compared. Theoretical treatment of data such as described (14, 28) cannot permit meaningful conclusions about computer systems and tradeoffs in sensitivity, resolution, and so forth if standard deviations of mass measurement are 40-60 ppm. Only when a system has been studied and understood in detail and standard deviations of mass measurement in the 5-ppm range are routinely obtained, particularly above m/e 80 to 100 where high accuracy is required to distinguish among elemental compositions with very similar exact masses, can the worth of a computer system and mass spectrometer operating tradeoffs involved be assessed. ACKNOWLEDGMENT
The authors acknowledge the efforts of Mr. Rylee McPherron, Mr. Jan Hauser, and Mr. John McConnell for innovative
programming, and Mr. Robert E. Furey, Mr. Galen Collins, and Mr. Michael Issel for hardware interfacing. The authors also acknowledge the assistance of Dr. Donald A. Flory, NASA Manned Spacecraft Center, for technical and administrative assistance during this development program. RECEIVED for review April 12, 1971. Accepted July 23, 1971. This research was supported by the National Aeronautics and Space Administration, Manned Spacecraft Center, Contracts NAS9-9593 and NAS9-7889. The Sigma 2 computers were provided under NASA, MSC contract NAS9-7381; Principal Investigator A. L. Burlingame. The MS-902 mass spectrometer was provided by NASA NsG 243 Suppl. 5. The special version of the Hitachi Perkin-Elmer RMU6-D mass spectrometer was provided by Prof. K. Biemann, MIT, and coworkers R. Murphy, N. Mancuso, and R. Hebert, under separate NASA contract.
Display Oriented Mass Spectrometer-Computer System William F. Holmes Biomedical Computer Laboratory, Washington University School of Medicine, 700 South Euclid Avenue, S t . Louis, Mo. 63110
William H. Holland Department of Psychiatry, Washington Unioersity School of Medicine, 4940 Audubon Aoenue, St. Louis, Mo. 63110
John A. Parker Biomedical Computer Laboratory, Washington University School of Medicine, 700 South Euclid Avenue, St. Louis, Mo. 63110 A display oriented computer system for the LKB-9000 gas chromatograph-mass spectrometer has been de. veloped using the PDP-12 computer. The interface is very simple. Spectra may be obtained intermittently or with continuous scanning at a rate selected by the operator. Each spectrum is reduced and displayed on the computer oscilloscope as soon as the scan is done. Tape storage is optional so that the operator can monitor an entire GC run without storing unnecessary data. A description, clock time, and scan number are stored with the data. The spectrum display has expandable scales so that small regions can be highly magnified. Storage format is variable in length. The largest spectrum allowed contains 850 peaks in the range 0-1000 mass units. Over 220 spectra with 200 peaks in the range 20-500 mass units can be stored. Each spectrum contains calibration information which can be displayed as a deviation graph. This is used for calibration adjustments which are done about once a week. Calibration is routinely maintained to 850 mass units. Spectra can be plotted with a wide variety of options.
USEOF COMBINED gas chromatography-mass spectrometry requires automated data reduction to make full use of the information available. Several computer-mass spectrometer systems have been described that are capable of reducing data from continuously scanned gas chromatograph effluent, presenting the results on paper with a digital plotter or high speed line printer (2-5). A display oriented system is ( I ) R. A. Hites and K. Biemann, ANAL.CHEM., 40, 1217 (1968). (2) B. Hedfjall, P.-A. Jansson, Y. Marde, R. Ryhage, and S. Wikstrom, J . Sci. Imrrurn. Ser. 2, 1031 (1969). (3) W. E. Reynolds, V. A. Bacon, J. C. Bridges, T. C. Coburn, B. Halpern, J. Lederberg, E. C. Levinthal, E. Steed, and R. B. 42, 1122 (1970). Tucker, ANAL.CHEM., (4) C. C. Sweeley, B. D. Ray, W. I. Wood, J. F. Holland, and M. I. Krichevsky, ibid.,p 1505. (5) J. R. Plattner and S. P. Markey, Org. Mass Spectrom., 5, 463 (1971).
1806
0
described below which allows the user close control and considerable flexibility in acquiring and analyzing mass spectra. The system uses an LKB-9000 gas chromatograph-mass spectrometer interfaced with a PDP-12 computer. This computer was selected because it contains a large built-in oscilloscope, and two small LINC tape units which serve as a convenient and inexpensive medium for data storage. Each LINC tape can store over 225,000 twelve-bit words. During data acquisition, spectra are reduced and displayed as soon as each scan is finished, with optional storage on tape; thus, the operator can monitor an entire gas chromatograph (GC) run by continuous scanning without storing any more data than are necessary. A noise display provides optimal threshold adjustment for measuring peaks with low abundance. The scan repetition rate and scan speed can be changed at any time during a GC run, or scanning may be stopped and restarted intermittently. Whenever a scan is not in progress, the computer displays the spectrum of the last scan. A programmable clock keeps track of the time, which is stored as part of each spectrum for comparison with G C retention times. Calibration is routinely maintained to 850 mass units by use of a calibration deviation display. Since calibration information is included with the reduced data, this display is available for every spectrum. The mass spectrum display is also used to examine data after it has been stored on tape. The mass abundance and mass unit scales can be greatly expanded for detailed observation of small peaks. Any spectrum on display can be plotted as a bar graph, whenever a record on paper is needed. Computer and Interface. The Digital Equipment Corporation PDP-12 is specifically designed as a laboratory instrument computer, The PDP-I:! is a typical small computer (in fact, a PDP-8 with an extra LINC instruction set), but contains in addition a number of input/output devices as
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
