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Interested users could improvise many simple types of film holders t o photograph the cross-section of the energy beam a t other locations in a spectrophotometer or in other optical instruments. In some cases it is beneficial to devise a template (which could be as simple as a cardboard cutout) to exactly locate the photosensitive material with reference to some
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mechanical reference point in the instrument. This would enable comparison of the beam location observations taken at different times in the life of the instrument.
RECEIVED for review September 16,1977. Accepted December 27, 1977.
Evaluation of the Dynamic Performance of Selected Ion Monitoring Mass Spectrometers D. E. Matthews,’ K. B. Denson, and J. M. Hayes* Departments of Chemistry and Geology, Indiana University, Bloomington, Indiana 4 740 1
Since the first reported use of selected ion monitoring with gas chromatography-mass spectrometry ( I ) , the technique has become an important tool in biomedical research, environmental trace analysis, and other fields of research. Mass selection is commonly accomplished by control of a voltage level-rod voltage in the case of a quadrupole mass filter, or ion-accelerating voltage in the case of a magnetic deflection mass spectrometer. Such voltage changes can be carried out rapidly, and the rate of “ion beam switching” is, accordingly, quite high, with the “dwell time” (time allowed for a single ion current measurement at a single mass) in some systems being much less than 1 second. There is a trend toward the use of increasingly higher beam switching frequencies, not only because electronic technical developments allow it, but also because it then becomes possible t o follow more rapidly varying ion-current signals while retaining accuracy in ratio measurements ( 2 ) or to select a wider variety of masses for observation. As these developments proceed, it becomes increasingly important to understand the dynamic characteristics of the mass spectrometer systems. Most importantly, once the spectrometer-control system has commanded a change in the selected mass, how much time must elapse before accurate ion-current measurements can begin? Not only does knowledge of such ion-current settling characteristics allow avoidance of outright errors in measurement, it also allows optimization of the pattern and rate of mass selection, thus increasing both the accuracy and efficiency with which analytical information can be extracted from the system. I t has long been recognized (3) that the stability of an accelerating voltage power supply can be conveniently tested by monitoring the ion current at some mass setting at which the ion current is strongly dependent on the accelerating voltage. This requirement is easily met by selecting a voltage which allows about half the maximum ion current for a given mass to reach the collector, i.e., by selecting a voltage on the “side” of a mass peak. In these circumstances, any subsequent drift in the accelerating voltage modulates the ion current, and knowledge of the slope of the ion current vs. voltage curve allows quantitative evaluation of the stability of the voltage source. T o make a similar observation of ion-current settling characteristics important in beam switching, it is necessary only to use such a half maximum point as the target to which the mass spectrometer is directed by the selected ion monitoring control system. The time required to reach the target, crucial details of the final approach to the selected mass, and the required settling time can all then be deduced from a recording of detector output vs. time. Present address, Department of Medicine, Washington University School of Medicine, St. Louis, Mo. 63110.
EXPERIMENTAL Mass Spectrometers. Results have been obtained using two different instruments. The first, a Varian CH-7 single-focusing magnetic deflection instrument, has been described in detail elsewhere ( 4 ) . The accelerating voltage power supply of this instrument has a maximum output of 3000 V and can be externally programmed in the range 2700-3000 V. The second mass spectrometer is a CEC 21-llOB double-focusing instrument (Mattauch-Herzog geometry) in which both the accelerating voltage and electric sector voltage power supplies have been replaced ( 5 ) . These supplies have maximum outputs of 10 000 and 500 volts, respectively, and can be externally programmed in any arbitrarily selected voltage range. Both instruments are controlled via a Varian 620i minicomputer. The computer communicates with the power supplies by means of digital-toanalog converters having 12-bit resolution. Ion-Current Measurements. Ion counting was used for measurements on the single-focusing instrument. The amplifier-discriminator-counter system, which has been described in detail elsewhere (6),has an effective deadtime of less than 50 ns. An electron multiplier-analog amplifier system was used on the double focusing instrument, the original amplifier having been replaced by a feedback electrometer based on an Analog Devices 435 operational amplifier. The electrometer employs a lo7 R feedback resistor, and has a bandpass, taking account of the effects of stray capacitance, of better than 3 kHz. A Teledyne Philbrick 470501 voltage-to-frequency converter (0-10 V input, 0-1 MHz output) was used to interface the amplifier with the same counting system used in ion-counting measurements. Procedure. Programs have been written allowing the entire experiment to proceed under software control, with the computer both controlling the power supplies and recording the ion-current signals. In a typical measurement, argon ( m / e = 40) is introduced to provide a single, prominent resolved ion-current signal, the magnetic field is adjusted to place the peak at either the high or the low end of the accessible selected mass range, and software control is initiated. Information required for effective control and data interpretation is acquired in the first steps, which vary the power supply voltages in order to locate the half maximum point on one side of the mass peak, measure the signal at that point, and then “map” the voltage vs. ion-current function on the side of the peak by systematically varying the voltages and recording observed ion currents. Data allowing determination of the settling characteristics are obtained by stepping to some voltage well removed from the half maximum point, allowing an adequate time for the system to stabilize, and commanding a step back t o the half maximum point while recording the ion current at a function of time. This process is repeated under do-loop control for a systematically varying sequence of voltage steps in order to determine the relationship between settling time and the magnitude of the commanded voltage change. RESULTS AND DISCUSSION Figure 1 shows the ion-current settling profile for an 8.5% increase in the accelerating voltage of the single-focusing
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
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Figure 1. Ion current settling profile for an increase in the accelerating voltage of the single focusing mass spectrometer. The figure plots ion current vs. time after a step change in the accelerating voltage from 2700 to 2950 V (correspondsto an 8.5% decrease in mass) ending at the half-maximum point of m / e = 40 (expected signal level indicated by the s o l i line). The long broken lines indicate signal levels &2 standard deviations from the half-maximum signal intensity. The short broken lines indicate the signal levels at points 2 voltage increments (49 ppm) on either side of the half-maximum point
Figure 2. Ion current settling profile for a decrease in the accelerating voltage of the single focusing mass spectrometer. The figure plots ion current vs. time after a step change in the accelerating voltage from 3000 to 2740 V (correspondsto an 8.5% increase in mass) ending at the half-maximum point of m l e = 40 (expected signal level indicated by the s o l i line). The long broken lines indicate signal levels f2 standard deviations from the half-maximum signal intensity. The short broken lines indicate the signal levels at points 2 voltage increments (49 ppm) on either side of the half-maximum point
instrument. Prior to jumping to the target, the half-maximum on the high mass side of mass 40 (from argon), the computer determined the ion current a t the target location (the solid line a t 1550 counts) and at two voltage increments (k49 ppm) above and below the target (the short-broken lines at 1710 and 1270 counts). For the first 25 ms after the command to jump to the target was made, the accelerating voltage was still slewing toward the mass peak half-height and no ion current was observed. At 30 ms the signal suddenly appeared and surpassed its expected value (the solid line), indicating that the voltage had overshot the target. At 70 ms the signal dropped below the solid line, indicating that the voltage was ringing slightly as it finally stabilized after 130 ms had elapsed. Several strengths of the technique become apparent through consideration of Figure 1. First, unlike an oscilloscopic recording of the power supply output waveform, the ion-current monitoring technique provides its most detailed information in the range of greatest mass spectrometric interest, namely the small voltage range closest to the target point. Oscilloscopic testing allows convenient visualization of the gross features of the voltage vs. time output of the power supply; ion-current monitoring allows the last few ppm of voltage settling to be observed in detail. Second, while the ion-current monitoring technique undoubtedly acts primarily to test the power supply performance, the results will alert the experimenter to any other problems in the mass spectrometer system, since the measurement involves all system components from the ion source to the ion-current-readout system. Of course, for this same reason, it is not possible instantly to be sure that an observed instability is due to the accelerating or deflecting power supplies. Instabilities in ion-source-filament or detector power supplies would, however, modulate not only the signal observed a t the side of a peak but also the signal a t the top of the peak. By applying this simple test, the origins
of observed instabilities can usually be adequately defined. Third, the use of a computer-based ion-current-recording system is superior to oscilloscopic recording of the ion current. The system becomes "self-testing,'' and many uncertainties associated with other diagnostic procedures are removed. There can be no fear that the observed results are due to some inadequacy in the test equipment, cables, or connections. There can be no uncertainty about the relationship between triggering of the test device and the command to the power supply. The procedure is much more convenient, and the plotter output displays excellent resolution and is highly readable, with the signal output scale being automatically marked in terms of the expected target signal and related levels. The practical utility and quantitative interpretation of test results depends largely on the experimental context. In the present case, for example, the spectrometer was being operated with ion source and collector slits which produced peaks with flat tops more than 975 ppm in width in the mass range of interest. Because the voltage targets during actual analytical measurements lie a t the centers of these broad, flat tops, it is evident that complete, or perfect, settling to the target voltage is not required, and that collection of data could begin within 40-50 ms of any mass selection command like that shown in Figure 1. If, on the other hand, narrow peaks with widths of approximately 50 ppm were being monitored, data collection would have to be delayed for 100 ms after any mass selection command as large as that employed in Figure 1. Figure 2 shows the ion-current settling profile for an 8.5% decrease in the accelerating voltage of the single-focusing instrument. Prior to commanding mass selection, the computer supervised determination of the expected signal level a t the target location (the solid line a t 1400 counts) and a t points 49 ppm above and below the target voltage (the short
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to a doubling of the selected mass. Given the relatively wide 2o o mass range and the higher resolution of this instrument, the division of the accessible mass range into 4096 increments
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results in peaks only a few "voltage bits" in width (each bit corresponds to 122 ppm of the maximum voltage). Thus, the solid line represents the expected signal a t the target point near the peak of m/e 40, but the short dashed lines representing signal levels a t voltages two increments above and below the target are absent because no signal is present at these points. Analysis of the ion current recording indicates that the voltages settle within 100 ppm of the target in less than 20 ms, and that overshoot and ringing are absent. Further analysis of the recording beyond 50 ms reveals two residual noise sources not recognized in the preceding figures. Two small amplitude oscillations, one with a period of 400 ms (2.5 Hz), and the other with a period of 17 ms (60 Hz) can be seen. The 2 . 5 - H ~noise is apparent from visual inspection of the record, but the 60-Hz noise can be noted only after the data have been processed by the 17-point Savitsky-Golay smoothing routine (7) used to generate the unbroken curve in Figure 3. When the collector slit was opened in order to generate a flat-topped peak and allow a stable measurement of resolved beam current, (a) the 60-Hz modulation vanished, indicating that it was due to an instability in beam location rather than signal level (this oscillation was traced to an electric motor placed near the magnetic sector); and (b) the 2.5-Hz modulation was not found to be constantly present in either the top or the side of the peak, indicating that it was somehow associated with the voltage jumping process itself. Installing a new filament and clewing the ion source removed this feature, leading us to suggest that it represented an interaction between leakage currents, sudden ion source voltage changes, and the time constant of the emission current regulator circuit. It should be apparent that this very simple technique provides an extremely useful tool for examining the dynamic performance of selected ion monitoring mass spectrometers, and that the data can have great diagnostic value.
ACKNOWLEDGMENT The ion current measurement system used on the double-focusing mass spectrometer was constructed by D. W. Peterson. LITERATURE CITED C . C. Sweeley. W. H. Elliott, I. Fries, and R. Ryhage, Anal. Cbem., 38, 1549 (1966). D. E. Matthews and J. M. Hayes, Anal. Chem., 48, 1375 (1976). A. 0. C. Nier in "Mass Spectrometry in Physics Research", Natl. Bur. Stand. ( U . S . ) C r . , 522, 29-37 (1953). D. A. Schoeller and J. M. Hayes, Anal. Cbem., 47, 408 (1975). K. 6.Denson, S. P. Taylor, and J. M. Hayes, Proc. 24th Ann. Conf. Mass Spectrom. Allied Topics, 548 (1976). J. M. Hayes, D. E. Matthews, and D. A. Schoeller, Anal. Chem., 50, 25 (1978). A. Savitsky and M. J. E. Golay, Anal. Cbem., 36, 1627 (1964).
RECEIVED for review October 24,1977. Accepted December 23, 1977. We appreciate the support of the National Aeronautics and Space Administration (NGR 15-003-118)and The National Institutes of Health (GM-18979).
