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interfering transition metals from the digest. This, however, is time consuming a n d forces considerable deviation from established procedures. The nondestructive method described not only allows recovery of the sample, it also provides a n additional check on its purity. We analyzed one compound, PhzPCH2PPhMeMo(CO),, which had given marginally acceptable CH values a n d a n excellent P value by the digest N M R method, and consistently found the phosphorus to be low by about 2.3%. An extended run on the spectrometer revealed a series of small, very broad peaks downfield stemming from impurities which would not have been detected by classical microanalytical methods.
ACKNOWLEDGMENT T h e authors express their gratitude t o the research group of Professor Sam Grim of the University of Maryland for the preparation of some of the samples analyzed.
LITERATURE CITED Kasler, F.; Tierney, M. Mlkrochimica Acta, in press. ThiauR, 0.; Mersseman, M. Org. Magn. Reson., 1975, 7 , 575-578. Thiault, B.; Mersseman, M. Org. Magn. Reson., 1976, 8 , 28-33. Brame, E. G.. Jr.; Yeager, F. W. Anal. Chem. 1976, 4 8 , 709-711. Pesek. J. Anal. Chem.-1978. 50. 767-791. Hajek, M.; Sklenar, V.; Sebor, G.; Lang, I.; Weisser, 0. Anal. Chem. 1978. 50. 773-775. Gurley, T.: Ritchey, W. Anal. Chem. 1975, 4 7 , 1444-1446. Gurley, T.; Ritchey, W. Anal. Chem. 1976, 48. 1137-1 140. O'Neill, I. K.; Pringuer, M. A. Anal. Chem. 1977, 4 9 , 588-590. Sojka, S. A.; Wolfe, R. A. Anal. Chem. 1976, 5 0 , 585-587. Freeman, R.; Hill, H.; Kaptein. R. J. Magn. Reson., 1972, 7 , 327-329. Moeller, T. (Ed.), Inorg. Synth. 1957, 5 , 130-131. Ehrenberger. F.; Gorbach, S. "Methoden der organischen Elementar-und Spurenanalyse"; Verlag Chemie: Weinheim, 1973; pp 359-389.
RECEIVED for review November 14, 1978. Accepted January 22,1979. The instrument employed was purchased with funds from an institutional grant ( G P 43155) from the National Science Foundation.
Accurate Mass Measurement at Low Resolving Power with Mass Spectrometer-Computer Systems Robert J. Weinkam" and Jay L. D'Angona Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco, California 94 143
Selected ion monitoring circuitry utilizing computer-set mass spectrometer accelerating voltages has been used to determine molecular compositions. A simplified circuit is described for computer-controiied accelerating and eiectric-sector voltage switching of double focusing mass spectrometers. Mass measurements accurate to within 10 ppm have been determined by repetitive voltage sweeps over sample and reference peaks. Up to six masses may be measured using either singie- or doubie-focusing instruments at resolving powers below 2000. The system features rapid data acquisiiion, hgh sensitivity, and visual inspection of acquired data.
Accurate mass measurements are widely used to determine molecular compositions of ions generated using a variety of mass spectrometer ion sources. These measurements are most commonly made using double focusing instruments a t high resolving power (C\M/M = I l O O O O ) . Peak matching ( I ) , photoplate detection ( 2 ) , and computer analysis of magnet field scans ( 3 , 4 ) can provide mass determinations within acceptable limits of error. High resolving power facilitates mass measurements by giving sharp, easily measured traces, although this is achieved with some loss of sensitivity and with increased instrument cost and complexity. Computer averaging of multiple determinations has made accurate mass measurements practical a t lower resolving power. Operation a t low resolving power may cause peak overlap between ions a t the same nominal mass. T h e probability of this occurring is highest between fragment ions formed by electron impact and is greatly reduced for molecular ions and ions generated under chemical ionization conditions. Determinations a t low resolving power (12000) have been made using a double-beam mass spectrometer-computer 0003-2700/79/0351-1074$01.00/0
system ( 5 ) in which reference perfluorokerosene and sample spectra are obtained on separate beams. Errors are reported to be less than 12 ppm on 85% of peaks determined using computer averaging of six spectra (6). Pulsed positive ion negative ion chemical ionization mass spectrometer computer systems may also be used to obtain mass measurements accurate within 10 ppm for 84% of determinations obtained from an average of five consecutive scans (7). With this instrument, low amounts of reference perfluorokerosene are detected as negative ions and samples are detected as positive ions, thereby limiting sample peak overlap with reference ions. In this paper, we describe the use of computer generated accelerating voltage sweeps to make rapid accurate mass measurements a t low resolving power with single- and double-focusing magnetic sector mass spectrometers. T h e circuitry involved was originally described for selected ion monitoring (8,9) using single focusing magnetic sector mass spectrometers and has been adapted to operate double focusing instruments (see Figure 1).
EXPERIMENTAL Instrumentation. Analyses were performed on an Associated Electrical Industries MS-902 (Kraytos, San Diego, Calif.) double-focusing mass spectrometer which has been modified to operate under chemical ionization conditions (IO). Data were obtained using methane reagent gas (Liquid Carbonics) at 0.4 Torr ion chamber pressure. Perfluorotributylamine (Pierce Chemical Company, Rockford, Ill.) was used as a reference compound. The AEI MS-902 was modified to operate at less than 1 mA current load on the accelerating voltage power supply. Voltage divider resistor chain (R1 to R29), circuit meter (MU, and filter capacitors (C19-C22) were disconnected. The source-supply chassis voltage divider resistor chain (R41 to R72 and RVO, RV7, and RVB), which supplies lens voltages, was increased 10-fold. Analyses were also performed on an Associated Electrical Industries MS-12 single-focusing mass spectrometer operating 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
1075
ACCELERATING VOLTAGE
H I M VOLTAGE POWER SUPPLY
I 1
-rp ,; PROGRAMMABLE
,? Bit U D
NOVA 31 1 'I COMPUTER
I 12 Bit Old
32 K MOI MEMORY
D 300 U A L1 K 4DISKETTE CBYTES Y
0
5
IO
20 25
15
NUMBER T lElKPT M R Ol N NI lXl
IHEAKRTDB O CN O IPX"
Figure 1. Schematic diagram of computer system interfaced to a double focusing mass spectrometer
in the electron impact mode. Data were obtained at 70-eV electron energy with perfluorokerosene (Pierce Chemical Company, Rockford, Ill.) reference. The MS-902 and MS-12 mass spectrometers were interfaced to a Nova 3/12 computer (Data General Corp.) with 32K MOS memory. Computer system crmponents include dual 300K byte diskettes (Data General), a 4006 computer display terminal, and a 4610 hard copy unit (Textronix, Beaverton, Ore.). The interface contains two 12-bit D/A converters, and a 16-channel multiplexed 12-bit A/D converter with 1, 2, 5 , or 10-fold programmable gain (ADAC Corporation Model 550 DG-C-8-DI-2B). Accelerating and electric sector voltages are obtained from the 12-bit D/A converter output using two high voltage power supplies, one programmable and one fixed. The 0-5 V output of the 12-bit D/A converter is used to drive the noninverting input of a NTC-2000 power supply (Kepco Inc., Flushing, N.Y.). The NTC-2000 is limited to 1-mA current and a 0.001-mF output capacitance load. Twenty switch selectable input resistors are used to alter the power supply output range by 100-V increments from 0 to 2000 V. This allows the output range to be varied in order to more effectively utilize the D/A converter resolution. A Fluke 410B 0 to 10000 V power supply (J. B. Fluke Manufacturing, Seattle, Wash.) provides the fixed portion of the accelerating and electric sector voltages and floats at the NTC-2000 output voltage. The chassis of the 410B power supply has been internally isolated and shares a common ground with the remainder of the system. Electric sector voltage of the MS-902 is obtained from the accelerating potential. The positive voltage is taken from a voltage divider consisting of 47 ZOO-kR precision metal film resistors, and a 200-kQ variable resistor. This arrangement supplies a voltage approximately l / l5 that of the accelerating voltage to the positive plate. The bottom plate is grounded to the electric sector housing. In this configuration, ions entering the electric sector are partially retarded. They are accelerated to their initial kinetic energy when exiting. This configuration permits rapid accelerating and/or electric sector voltage switching. For example, a 15% decrease in the accelerating voltage settles to within 1 standard deviation of the final voltage in 8 ms. Performance of this electric sector configuration was compared to the instrument manufacturer configuration which supplies and -1/30 of the accelerating voltage to the top and bottom plates, respectively. Interchanging these configurations resulted in a 20% decrease in peak height which may be recovered by lens voltage adjustment. No significant difference in peak height or width was observed between these t w o configurations when adjusted to optimum peak shape or when the Fluke 410B was used to supply the total accelerating voltage. Low resolution scans are routinely made using the latter configuration. A voltage divider reduces the 0- to 100-V electron multiplier and preamplifier mass spectrometer output to 0 to 10 V. This signal is used as input to the 1-,2-, 5 - , and 10-fold programmable gain and 12-bit A/D converter.
PROGRAM OPERATION Programs are established to operate single- or double-focusing instruments in any ionization mode. Reference and
30 35 40 45 50 OF SCANS AVERAGED
Figure 2. Relation between mass measurement average error (absolute value i S.D.) and the number of mass determinations averaged to make the measurement
sample compounds are introduced simultaneously during data acquisition. Operation of the mass measurement program requires input of the precise mass of two reference ions, nominal mass of up to four sample ions, and the power supply fixed voltage and variable voltage range. Voltages required to focus each ion are computed starting with the highest mass at low A/D (low variable voltage) settings. The mass window (the number of A/D settings monitored a t each mass), the delay time between and the dwell time on each A / D setting, and the number of repetitive scans may be entered. T h e operator manually selects and focuses on any of the entered masses. Samples are usually introduced a t this point and data acquisition is initiated. Data acquisition consists of the stepwise high to low sweep over each mass. This cycle is repeated during each sequence. T h e time required for acquisition varies between 10 and 30 s depending on entered parameters and the number of repeated sequences. collected data are stored in binary form on the diskette. Data analysis options include a 5- or 11-point cubic smoothing routine (111, plots of all or individual masses, and table of calculated masses. Mass calculations are based upon computation of the weighted peak centroid (C'I from D / A settings (M and intensities ( I ) ,
over n most intense data points a t that mass. T h e number of data points (n)is usually selected to cover the peak width a t 2 / 3 height. T h e calculated centroid extrapolates between D / A settings so that mass measurement precision is not necessarily limited by D/A converter resolution. T h e mass of the sample ion (Ms)is calculated from the entered reference masses (MI and M,) and the observed weighted peak centroids (Cl, Cz, and C,) using Equation 1. These masses
M,/M, =
v,/v,
= (Nf + C,)/(Nf + C,)
(1)
are calculated for each sweep sequence and repeated sequences are averaged to minimize effects of magnet drift during data acquisition.
RESULTS A N D DISCUSSION Mass measurement accuracy is related to a number of system parameters. These include the dwell time on each data point, the number of repeated sweeps averaged in the mass measurement, instrument resolving power, the number of data points obtained over the width of the peak, the signal to noise ratio of t h a t peak, and the voltage range over which the instrument is operated during the determination. These parameters were individually varied to optimize data collection
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7 , JUNE 1979
Table I. Mass Measurements made at Low Resolving Power. Duplicate Measurements on Parent and Isotope Peaks Were Determined from an Average of 10 Measurements
16
g
MF Ref. 119-131'
MW
observed Res. 700
error Mass range 10%
123.0446 123.0442 0.0004 123.0449 0.0003 Ref. 119-131a Res. 1500 Mass range 10% C7H70,
1
,
0
I
I
20
10
I
30 40
L
I
1
50
60
70
Figure 3. Effect of increasing data point dwell time on the absolute value of mass measurement error 50 40 [e
e
e
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,
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e
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,
,
,
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COMPUTER I N P U T ( m v i
Figure 4. Mass measurement error measured at different sample ion
signal intensities. The system parameters were offset so that random noise would appear as 300 to 500 ppm positive error efficiency and accuracy. Figure 2 shows the reduction in mass measurement error observed by increasing the number of sweeps averaged in a mass measurement. Accumulation and averaging of repeated sweeps compensates for low signal to noise ratios, short dwell times, and the presence of few and widely spaced data points on the measured peak. It is clearly advantageous to perform as many repeated sweeps as are feasible in the course of the determination. In order to assess the importance of individual system parameters, the following d a t a were obtained as 10-sweep averages, even though this is near to the minimum number of sweeps required for acceptable (*IO ppm) mass measurements. Increasing dwell time from 2 to 64 ms (60 to 1900 A / D samples per d a t a point) had little effect on the accuracy of the mass measurement, Figure 3, although the signal to noise ratio on the peaks was greatly improved. Mass measurement accuracy was not related to signal intensity of the sample and reference ions if the signal intensity was above 10 mV (5/1 signal to noise) a t the 0- to 10-V computer interface, Figure 4. Similarly, five-point cubic smoothing of the accumulated d a t a improved signal to noise ratios and the precision of individual mass measurements, b u t had no apparent effect on the accuracy of 10 sweep averaged mass. Efficient utilization of limited data acquisition time involves the use of short dwell times,
