A Simple, Off-Line Mass Spectra Digitizer Jos G. Leferink and Piet A. Leclercq Laboratory of Instrumental Analysis, Eindhoven University of Technology, Eindhoven, The Netherlands
THEAUTOMATION OF DATA extraction from recordings of mass spectra has been the concern of several workers. For example, Stillwell reported on a device for measuring and drawing spectra from oscillographic charts ( I ) . He used a digital chromatograph readout system to produce data in tabular form and a potentiometer recorder to plot the spectra. Although many laboratories do not possess on-line data acquisition equipment, they often have access to a computer center. For off-line handling, data have to be presented on magnetic tape, cards, or punched paper tape. A semi-automatic photoplate readout, producing cards, was described by Desiderio and Biemann ( 2 )and Biemann (3). Thus far, there are no reports on (semi-) automatic devices for digitizing low resolution mass spectra from oscillographic charts, producing an output suitable for off-line computer handling. Therefore, an apparatus was designed, which reduces the time consuming conventional way of manually measuring, writing, and punching data from a spectral chart to a simple, one-step action. Described is a set-up, consisting mainly of instruments belonging to the standard outfit of a common analytical bdboratory. Paper tape is used as preferred over magnetic tape and cards, from a viewpoint of reliability and economics. Discussed are algorithms which assign the data obtained with this system to different arrays. System and Circuit Description. The measuring system is based on a reversed potentiometer recorder. A linear rela(1) R. N. Stillwell, ANAL.CHEM., 38, 940 (1966). (2) D. M. Desiderio and K. Biemann, 12th Annual Conference on Mass Spectrometry and Allied Topics, Montreal, Canada,
June 1964. (3) K. Biemann, Pure Appl. Chern., 9, 95 (1964).
tionship is obtained between the translation of a pointer and the output voltage of the servopotentiometer (Figure 1). After amplification, this voltage is measured with a digital voltmeter with a high resolution (I00 ppm) and converted to a BCD code. The digital signal is converted to the desired code for the paper tape drive/punch unit. For ease of data handling and programming, it is convenient to convert the three traces of the UV recorder output to a single voltage range without loss of resolution (1 :10000). This conversion is done by means of a variable gain amplifier (gain 1, 10, 100 X) and by selecting an appropriate offset voltage given by the trace on which is measured. The range of the DVM is fixed at - 19999 to 19999 mV. The positive voltage, taken from the potentiometer, corresponds with intensities. Non-intensity information can be entered as negative voltages, obtained from a voltage divider connected across the negative operational amplifier supply voltage. We arbitrarily defined the “instruction-set” as in Figure 2. To use the system, an oscillographic chart is placed in the apparatus and the bdse lines of the three traces are zeroed in the appropriate “trace selector” position (Figure 3). The nominal mass of the first peak to be measured has to be entered via the keyboard. Next the intensities of this and the following peaks at nominal masses (without skip in mass) are measured and entered via pushbutton “intensity,” with the “trace selector” in the right position. After a skip in mass, the next first mass has to be entered before its corresponding intensity. Metastable ions are entered by giving the decimal fraction of the mass only (one digit) via the keyboard, followed by the corresponding intensity. Doubly charged ions are entered analogously, but by means of a special pushbutton. An off-
+
PAPERTAPE PUNCH-DRIVE
I
-
- PAPERTAPE PUNCH
15v
*
Figure 1. Schematic diagram of mass spectra digitizer
JI resistors are 1% metalfilm. Resistors selected for exact 1:lO and 1:lOO ratio. SIJ,b,O = trace selector switch. S2 = switch coupled to the istruction-set switches. I = input pulse from all switches to cause appropriate punch command delay, given by the conversion time of the DVM. )A = Any type of low drift general purpose operational amplifier with an output voltage of minimal 10 V ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
625
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470 R
INITIATE
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V
AUXILARY CODE
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ERASE SYMBOL
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OFF-SCALE
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PEAK
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END O F SPECTRUM
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ONE D I G I T OF
470 R
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Figure 4. Flowchart of read-in subroutine
Figure 2. Instruction-set and corresponding voltages
Figure 3.
Drawing of the mass spectra digitizer
*
PC = punch command connection. = end of spectrum code. TS = trace selector. ZA = zero-adjust. ER, OS, DC, and INT = pushbuttons for erase, off-scale, doubly charged ion, and intensity command
scale peak is indicated by actuating the pushbutton “off-scale’’ after full scale intensity has been entered. Actuating the “erase” switch will result in erasing the previous number (see under software). Software Description. An Algol-60 program was written, to handle the data obtained from the “semi-automatic spectrareader.” The procedure (subroutine) that takes care of the classification of the data will be discussed here. The program distinguishes between positive numbers (k., intensities) and negative numbers (non-intensity information). From the negative numbers (-00000 to -19999 mV), only the two 626
most significant digits are considered, thus eliminating the need for accurate voltage settings. First, the data are read in (Figure 4). If a number in between -13000 and -13999 is encountered, the program will discard this “erase symbol” itself and the preceding number. If the latter was a negative number, the next preceding number will be discarded too, if it is negative. This is repeated until a positive number is encountered. Next, the data are assigned to mass and intensity arrays for singly and doubly charged and metastable ions (Figure 5 ) . Non-intensity information is classified according to the number M of successive negative numbers preceding a positive number, as detected for each value of I . If M = 0, the mass of the previous peak handled plus one amu is assigned to the corresponding intensity. If M > 1, the mass of the first peak of a series of successive peaks is detected. The nominal mass value is calculated as explained in Figure 5. M = 1 indicates off-scale information or a metastable or a doubly charged ion peak. Exclusion is made in this case by checking the value of the only negative number detected against the defined values. Classification is done as shown in Figure 5 .
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
DISCUSSION
By using the cheap and simple equipment described here, a saving of 70% in time can be achieved easily over manual methods for digitizing and punching mass spectral data. Because of the general design, the system is very versatile The instruction-set can be changed and extended ; only the software has to be adapted. If a card puncher is available, i can be substituted for the paper tape puncher. ACKNOWLEDGMENT
The authors thank A. H. Lensen and H. E. van Leuken fo their assistance in constructing the device. RECEIVED for review April 24, 1972. Accepted September 1 1972.
tV
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1
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-
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Figure 5. Flowchart of classification subroutine
Metastable Ions in a MS-30 Double Beam Mass Spectrometer J. M. Miller1, J. Ross, J. Rustenburg, and G . L. Wilson Department of Chemistry, Brock University, S t . Catharines, Ontario, Canada
THE IMPORTANCE of “normal” metastable ions has long been recognized in the interpretation of mass spectra ( I ) ; n double focusing instruments of Nier Johnson geometry hese arise in the second field free region between the electric md magnetic sectors, where the parent ion ml, decomposes o give a daughter ion m2,and a neutral fragment, and where he metastables are observed at the apparent mass m* = n22/ml. More recently several metastable “defocusing” echniques have been introduced, which give enhanced metatable ion sensitivity by removing the normal ions. The Address correspondence to this author. ~
1 ) J. H. Beynon, ANAL.CHEM., 42 (l), 97A (1970).
first of these, the Barber and Elliott technique and its refinements (2-4), permits metastable ions formed in the first field free region between the source and ESA to be focused on the collector. After tuning the magnet to a particular ion, the accelerating voltage is increased while the ESA voltage is held constant, and the collector output is recorded as a function of accelerating voltage. By this means the precursors to the ion tuned in by the magnet are observed, Le., m2Jm1 (2) M. Barber and R. M. Elliott, ASTM E-14 Conference on Mass Spectrometry, Montreal, 1964. ( 3 ) M. Barber, W. A. Wolstenholme, and K. R. Jennings, Nature, 214,664 (1967). (4) T. W. Shannon, T. E. Mead, C. G. Warner, and F. W. McLafferty, ANAL.CHEM., 39,1748 (1967). ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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