Focus/Threshold Monitor for Mass Fragmentography with Extended Applications Stephen R. Pareles and Joseph D. Rosen Department of Food Science, Cook College, Rutgers-The State Unii{ersity, New Brunswick, NJ 08903
Mass fragmentography, first proposed by Hammar et al.
(I), has become a useful technique for highly sensitive, quantitative, and confirmatory analysis of trace compounds in complex environmental, physiological, and, recently, agricultural samples (2). The fundamental prerequisite for mass fragmentography, a t any level or sophistication, for any number of peaks, is effective drift-free or reproducible single ion focusing. Since it is generally difficult even during single ion focusing to achieve stable long term or repeatable static focus on the center of a particular m / e value, an accepted solution is to add a low amplitude oscillation voltage to the main accelerating voltage so that the m / e ratio of interest is scanned repeatedly within a narrow m / e range (3-7). The repetitive gaussian electron multiplier signal obtained, is demodulated electronically by various means, simple or complex. While this technique makes small variations in accelerating voltage or magnet current inconsequential, occasional drifts which result in m / e changes greater than 0.3 amu still need to be monitored and corrected manually with the aid of an oscilloscope, or automatically with more sophisticated controls (7). Instrument Operation. The present paper describes a simple instrument that monitors focus integrity of electron multiplier peaks produced in the foregoing manner and also serves as a threshold detector. Its output drives two blinking light-emitting-diodes (LED’s) which indicate four focus conditions as descri,bed in Table I with intermediate conditions of focus imbalance discernible. In particular, when the quantity of sample exceeds the adjustable threshold level and is in optimum focus, both LED’s blink alternately and give a balanced appearance. For routine work, this instrument renders a dedicated oscilloscope redundant. I t can also be used as a cost saving interface in a digital automatic refocusing system. Principle of Operation. Figure l a shows the 10-Hz triangular oscillation voltage used to manipulate the accelerating voltage on our Du Pont 21-490 (magnetic) Mass Spectrometer as described elsewhere ( 5 ) . Figure l b shows the resulting electron multiplier output for a peak whose focus is optimal. The pattern is symmetric in that the peak centers are equidistant. Figure I C shows the less desirable pattern of peaks asymmetrically focused toward a higher mle; Figure l d toward a lower m / e . The latter two patterns show peaks whose focus may be acceptable from the criteria of reproducible quantitation but ones which are in jeopardy of being lost in the event of very small accelerating voltage or magnet current drift. This symmetry or asymmetry is easily and effectively monitored by the present instrument by first transforming the electron multiplier output into a train of narrow digital pulses which coincide with peak centers, one for each peak. This is accomplished by a. special purpose differentiator (described forthwith) in combination with a single shot multivibrator or One Shot. Further, two equal time zones called “Right” and “Left” are defined electronically for each oscillation cycle (Figure l e ) and terminated by easily generated repetitious timing pulses of about 1-millisecond duration denoted “R” and “L” in synchronization with the imposed oscillation (Figures I f and g). The presence and relative positions of dif1214
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
ferentiator pulses, representing electron multiplier peak centers, within these time zones can then be detected and presented by an arrangement of simple R/S Flip Flops and NAND gates which output to two LED’s, one for each time zone. One indicates left or low m / e asymmetry of focus. The other indicates right or high m / e asymmetry of focus. With this arrangement, an LED can be made to turn on only during the one of two time zones with which it is associated, the beginning of which is signaled by the terminating pulse of the preceding opposite time zone. After initialization by this pulse, the LED will be turned on by the first pulse representing an electron multiplier peak center whose height exceeds an adjustable threshold voltage. I t stays on until the terminating pulse for that time zone occurs which shuts the LED off and indicates the start of the opposite time zone. The opposite side of the circuit and associated LED is initialized a t that point and functions as in the previous case. This process is repeated with regularity. The effect of this arrangement can be ascertained from inspection of Figures 1, b-e which show that during either asymmetric focus condition, one or the other time zone will contain no electron multiplier peak so that one or the other LED will not go on. The more asymmetric the focus, the shorter will be the duty period of the one LED which does blink. During the symmetric condition, the differentiator
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Table I. Interpretation of Output States of the Focus/Threshold Monitor
.+-
Output state
Focus condition
Both LED’s blink alternately Right LED blinking, a left LED off Left LED blinking,‘ right LED off Both LED’s off
Focus symmetric and optimal Focus asymmetric toward higher m / e Focus asymmetric toward lower m / e Focus extremely asymmetric or threshold not exceeded
Figure 3. Expanded schematic of rate crossing detector or one-bit differentiator a
Intermediate conditions characterized by shorter or longer
blink duty periods.
pulses, representing electron multiplier peak centers, coincide and just exceed each terminating pulse (Figure 1,f and g) in which case both LED’s blink at maximum duty periods. The length of time differentiator pulses exceed the terminating pulse is user-adjusted to set the selectivity of the focus monitor. The easy to interpret output states detailed in Table I are thus produced with intermediate focus conditions associated with greater or lesser asymmetry appearing as brighter or dimmer blinking LED’s from the optical effect of shorter or longer LED duty periods. The interested reader will note that “L” pulses in the present instrument exactly correspond to pulses originating on the Focus Guide control line in our previously described Dynamic Single or Multiple Ion Detection System ( 5 ) . With our mass spectrometer, there is a 50-msec phase difference between excitation by the imposed oscillation and response of the accelerating voltage power supply and, there-
fore, the electron multiplier output. This delay was considered in synchronizing the “L” and “R” timing marks with the imposed oscillation. Circuit Description. A schematic of the circuit is shown in Figure 2. All amplifiers are low cost 741 type. All gates and flip flops are made from 7400 type NAND T T L integrated logic circuits. The One Shot is a 74121 type: A2 and B are left open, pin 9 is connected to pin 14; a 0.47-mF capacitor, or smaller, for greater focus selectivity, is connected between pins 11 (positive) and 10. The One Bit Differentiator, also called a Rate Crossing Detector, is adapted from Smith (8) and is detailed in Figure 3. Electrolytic capacitance, paralleled with the existing disk capacitance, decreases the threshold of the device. Discussion. The circuit performs reliably in focus moniANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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toring when, for the mle of interest, the ratio of the signal caused by ions of a compound eluting from the GC into the source of the mass spectrometer, to the signal caused by ions from GC column bleed, exceeds (after filtering) about 1O:l. This factor determines the lowest practical sensitivity to which the threshold should be set by selection of the differentiating capacitor. The dynamic range of the device appears to be greater than two orders of magnitude of sample ion concentration with the timing and component values specified. These values were selected to optimize the analysis of compounds being analyzed in nanogram and picogram amounts. Larger sample sizes can be accommodated by lowering the gain of the inverting amplifier. For reliable operation, it is desirable that the amplitude of the triangle wave imposed on the accelerating voltage (Figure l a ) be large enough to allow a mass resolution or span of 4 u along the base a t the gaussian pattern a t optimum focus (Figure I b ) .
LITERATURE CITED (1) C-G. Harnrnar, B. Holrnstedt. and R. Ryhage, Anal. Biochem., 25, 532 (1968). (2) J. D. Rosen and S.R. Pareles, J. Agric. FoodChem., 22, 1024 (1974). (3) P. D. Klein, J. R. Haurnann. and W. J. Eisler, Anal. Chem., 44, 490 (1972). (4) W. F. Holmes, W. H. Holland, B. L. Shore, D. M.Bier. and W. R. Sherman, Anal. Chem., 45, 2063 (1973). (5) S.R. Parelesand J. D. Rosen, Anal. Chem., 46, 2056(1974). (6) C-G. Hamrnar and R. Hessling, Anal. Chem., 43, 298 (1971). (7) J. F. Holland, C. C.Swelley, R . E. Thrush, R . E. Teets. and M. A. Bieber. Anal. Chem., 45, 308 (1973). (8)J. I. Smith, "Modern Operational Circuit Design", Wiley-lnterscience, New York, NY, 1971, p 179.
RECEIVEDfor review October 4, 1974. Accepted January 6, 1975. This work was supported in part by Regional Project NE-83, United States Department of Agriculture and by the Charles and Johanna Busch Memorial Fund, Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgers-The State University of New Jersey, New Brunswick, NJ 08903.
Use of Sodium Chloride Windows in the Infrared Spectral Analyses of Irradiated Thin Insoluble Polymer Films W. F. Oberbeck, Jr., and K. G. Mayhan Graduate Center for Materials Research and Department of Chemical Engineering, University of Missouri-Rolla,
There are several methods for obtaining infrared spectra of thin insoluble polymer films which have been exposed to radiation. The simplest technique is to obtain a free film of sufficient integrity to withstand mounting and handling. Otherwise, KBr pellets containing ground fragments of the polymer films will give useful information. During a recent investigation, we measured the spectral changes of insoluble thin polymer films (2-5 p ) exposed to radiation and to other environments. These films were produced by reacting different monomers in the presence of an Rf plasma. Since our primary interest was to study time dependent changes of adherent films, it was necessary to find a window material which would accept the polymer films under the conditions of reaction and which would not change its absorptionhansmittance characteristics in the infrared wavelength region after being exposed to radiation. Further, the substrate window material had to withstand prolonged exposure to Rf fields without undergoing any changes.
Activation analyses and other evaluations of available cell window materials showed that sodium chloride maintained its integrity and could be safely handled within one week after irradiation. No special equipment or devices were required to handle or transport the radiated samples. I t should be noted that trace impurities in some sodium chloride windows or in the polymer films could produce active species as the result of irradiation which made halflives longer than those for 38Cl or 24Na.Therefore, caution should be exerted in the handling of freshly radiated samples of unknown composition. The infrared spectral quality of the sodium chloride windows was not affected by a mixed flux of gamma radiation in the presence of fast and thermal neutrons. Figure 1 shows a typical base-line trace of one of the windows before and after an exposure dose of 108-109 rads. The base line is unchanged over the transmission limits expected for sodium chloride. The irradiated salt, however, underwent visu-
Figure 1. Base-line traces of sodium chloride windows before and after radiation 1216
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
Rolla, MO 6540 7
