(9) A. N. Freedman, Anal. Chim. Acta, 59 (1972), 19 Rev. (IO) C. Merritt, Appl. Spectrosc. Rev., 3, 263 (1970). (11) A. E. Gordon and A. Frigerio, J. Chromatogr, Chromatogr. Rev., 73, 401 (1972). (12) W. Blum and W. J. Richter, Tetrahedron Lett., 11, 835 (1973). (13) J. G. Leferink and P. A. Leclercq, J. Chromatogr., 91, 385 (1974). (14) W. H. McFadden. R. Teranishi, D. R. Black, and J. Day, J. FoodSci., 28, 316 (1963). (15) R. Teranishi, R. G. Buttery, W. H. McFadden, T. R. Mon, and Jan Wasserman, Anal. Chem., 36, 1509 (1964). (16) W. Henderson and G. Steel, Anal. Chem., 44, 2302 (1972). (17) M. S. B. Munson and F. H. Field, J. Am. Chem. Soc., 88, 2621 (1966). (18) A. L. Burlingame, R. E. Cox, and P. J. Derrick, Anal. Chem., 46, 248R (1974). (19) F. H. Field, "Ion Molecule Reactions", J. L. Franklin, Ed., Plenum Press, New York, 1972, pp 261-312. (20) M. S. B. Munson, Anal. Chem., 43 (13), 28A (1971). (21) D. F. Hunt, Progr. Anal. Chem., 6, 359 (1973). (22) G. W. A. Milne and M. J. Lacey, "Modern ionization Techniques in Mass Spectrometry", Crit. Rev. Anal. Chem., 45 (1974). (23) . , G. P. Arsenauit. J. J. Dolhun. and K. Biemann. Chem. Commun., 1542 (1970). (24) D. M. Schoengold and B. Munson, Anal. Chem., 42, 1811 (1970). (25) G. P. Arsenault, J. J. Doihun, and K. Beimann, Anal. Chem., 43, 1720 f\ l.9- 7 . 1. \I .
(26) E. 0. Oswald, L. Fishbein, B. J. Corbeth, and M. P. Waler, J. Chromatogr., 73, 43 (1972).
(27) M. G. Horning, J. Nowlin, K. Lertratanangkoon,R. N. Stillwell, W. G. Stillwell, and R. M. Hill, Clin. Chem. ( Winston-Salem, N.C.), 19, 845 (1973). (28) E. 0.Oswald. P. W. Albro, and J. S. McKinney, J. Chromatogr., 98, 363 (1974). (29) J. H. Futrell and Leonard H. Wojick, Rev. Sci. Instrum., 42, 244 (1971). (30) J. Michnowicz and B. Munson, Org. Mass Spectrom., 4, 481 (1970). (31) N. Einolf and B. Munson, lnt. J. Mass Spectrom /on fhys., 9, 141 (1972). (32) W.Kruger, N. Kuypers, and J. Michnowicz, Paper S-4, presented at 21st Conference on Mass Spectrometry, San Francisco, Calif., May 1973. (33) J. Michnowicz and B. Munson, Org. Mass Spectrom., 6, 283 (1972). (34) A. M. Hogg, Paper L3. presented at 21st Conference on Mass Spectrometry, San Francisco, Calif., May 1973. (35) D. P. Beggs, R. C. Dougherty, and W. H. Johnston, Paper J-6, presented at 19th Conference on Mass Spectrometry, May 1971. (36) J. Yinon and H. G. Boettger, Paper P-3, presented at 20th Conference on Mass Spectrometry, May 1972. (37) E. 0. Oswald, P. W. Albro, and J. D. McKinney, J. Chromatogr., 98, 363 (1974).
RECEIVED for review August 25,1976. Accepted October 15, 1976. This work was supported in part by a grant from the National Science Foundation, GP20231.
Defocused Metastable Scanning with a Mass Spectrometer and Laboratory Data System Alexander
M. Ferguson, Stephen A.
Gwyn, Lewis K. Pannell, and Graeme J. Wright*
Chemistry Department, University of Canterbury, Christchurch, New Zealand
The use of a laboratory data system with a double focusing mass spectrometer to permit rapid determination of metastable transitions to selected daughter ions by acceleratingvoltage scanning is described. The data system controls the accelerating voltage for tuning the daughter ion and scanning. The system provides four mass ranges ( m to 8 m, 4 m, 2 m, and 1.3 m for daughter ion mass m ) and three scan rates (30,90, and 240 s). A complete scan analysis including peak detection, operator selection of basellne and peak window parameters, and printout of precursor masses requires less than 2 min. Accuracy and precislon of the system are shown to be the same as manual scanning, giving mass assignment to better than 0.4 amu, and sensitivity is about eight times better than the manual mode.
Metastable ions, arising from ion decompositions outside the mass spectrometer ion source, are used routinely in the interpretation of mass spectra, and for the elucidation of ion structures, reaction pathways, and decomposition energetics in mass spectrometry. Their use has been the subject of a large number of papers, several recent reviews ( I ) , and a major book ( 2 ) , and needs no elaboration in this paper. Different mass spectrometers have different metastable characteristics, determined mainly by the geometry of their ion optics; the work described here was designed to take advantage of one of the metastable scanning modes of an AEI MS902 instrument with Nier-Johnson geometry. In this, as in most mass spectrometers, the largest number of metastable transitions usually occur in the first field-free region between the source and the electrostatic analyzer (ESA). Decompositions which take place in this region do not appear in the normal mass spectrum recorded a t the collector. However, two metastable defocusing techniques can be used to define all the transitions leading to a particular daughter ion which occur in the first field-free 174
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
region. These are a) scanning the ion accelerating voltage at a fixed ESA voltage ( 3 ) and b) scanning the ESA voltage at fixed accelerating voltage ( 4 ) . Both techniques have advantages; we used the first because our instrument is equipped with an accelerating voltage supply which can be externally controlled, but our method would be equally applicable to the second technique. Manual metastable defocusing by accelerating voltage scanning on the MS902 and similar instruments requires that each daughter ion be focused magnetically a t a reduced accelerating voltage; Vis then ramped manually or electrically back to 8 kV. These methods use either meter or oscillographic recorder output and they are tedious and of limited sensitivity. An early paper ( 5 ) reported a semi-automatic system for metastable defocusing of single precursors, and a very sophisticated automatic system which scans all precursor masses has been reported recently (6). Our own requirements fell between these two; we wanted the speed and sensitivity of a computer-controlled system, without the need for costly hardware additions to our DEC Lab8/e data system. We also wanted flexible data manipulation and analysis facilities and visual display of the data. This paper describes a system which meets these requirements. Although designed for the MS902-LabWe pair, it could easily be adapted to most other mass spectrometer-computer combinations.
EXPERIMENTAL A schematic diagram of t h e mass spectrometer-computer system used in t h i s w o r k is shown in Figure 1. Mass Spectrometer. T h e MS902 mass spectrometer has been extensively modified, w i t h m o s t o f t h e original electronics replaced by solid state amplifiers and power supplies. T h e key changes relevant t o t h i s w o r k are a) a Spellman RHSR-1OP h i g h voltage power source for the accelerating voltage supply and b) a K e i t h l e y 301 electrometer amplifier in place o f t h e original m o n i t o r amplifier. T h e mass spectrometer accelerating voltage ( o u t p u t o f t h e Spellman RHSR power supply) can b e controlled either m a n u a l l y by means of a f r o n t p a n e l
Internal
8kVset:
External
7
tJ
t
Keithley 301
Keithley 427
7
4
Inverter
I
9
I
+kq&q
Buffer
Monitor
Figure 1. Mass spectrometer-data system block diagram
I
I 2
I VOLTAGE ( k V )
I
6
1
Figure 2. Defocused metastable scan (2-8 kV) for the m/e 231 ion of the Dieis-Alder adduct I. Peak numbers correspond to those in Table 111
potentiometer, or remotely by voltage programming (0 to -6 V in for 0 to +IO kV out). In the original MS902 configuration the output of the HV supply is set, via an 8-kV reference amplifier, by a reference voltage derived from the ESA 540-V supply; this ensures that any variations in the ESA voltage do not defocus the ion beam. We used the same reference to control the Spellman output through suitable buffer and inverter amplifiers (Figure 1).A 4S-digit digital voltmeter (Electronics International DSV4) measures the output of the HV supply through a resistor divider chain in the source supply chassis. At present this voltage is read from the DVM and entered into the data system manually; we intend to build the appropriate interface to allow the DVM to be read directly by the computer. Computer Hardware. The data system used is a DEC Lab8/e Laboratory data system with 8K core and a Tektronix 603 storage monitor in place of the DEC monitor. The multiplexer-AD converter supplied with this system gives IO-bit resolution of -1 to +I V input signals. The mass spectrometer output is 0-10 V, so the dynamic range of the data system is less than that of the spectrometer. However, the bipolar input has advantages in setting up the zero signal levels, and the reduced sensitivity has not proved a problem. The Lab8/e system uses a programmable crystal clock to control data acquisition rates, and a dual DECtape transport (TD8E) for program and data storage. The computer controls the output of the Spellman HV supply through a 12-bit D/A converter (Analog Devices DAC 12QS') installed
on a DEC Omnibus Interface Foundation Module which plugs directly into the PDP8 omnibus. This board uses only one command which loads the D/A input register with the contents of the accumulator to give an output which in turn controls the Spellman through an inverter (Figure l).'The output of the Spellman can therefore be set to any desired value, from the PDP8 accumulator, and can be ramped between any two values in steps of 2.4 V (the resolution of the D/A converter) a t a rate set by the clock under program control. Software. The software was designed to provide automatic metastable defocusing from any daughter ion tuned manually on the mass spectrometer. It gives the operator complete control over the acquisition, display and analysis of the data from each scan. The Lab8/e system operates under DEC's OS/8 (Version 11) software, and our program uses the Keyboard Monitor and Command Decoder sections of OS/8; the complete program occupies 3K of core. The software consists of the following eight separate sections, written in PAL8 (PDP8 assembler ldnguage), each referenced by a command to the command decoder. SETUP RSPEC DISPCA NOISEL ANALYS STORE YANK ERASE
Set up the mass spectrometer Run a metastable scan Display the data Smooth the data (moving window average) Analyze the metastable spectrum Store a data set on DECtape Read a stored data set from DECtape to core Remove a stored data set from DECtape
The DECtape handling sections STORE, YANK, and ERASE make up a complete set of file'handling routines with read-write error checks and provision for naming each data set. SETUP and RSPEC are described in the discussion. DISPLA is a self-contained program controlling a refreshed display which allows the user to manipulate data on the screen. The spectrum can 6e moved, expanded or contracted in the x and y axes. A marker is placed a t the center of the z axis and the ramp voltage corresponding to the data point set a t the marker can be read from the DVM. A delay routine allows the display to be recorded on an XY recorder connected in parallel with the storage mon'itor. NQISEL is a nine-point moving window averaging routine (7) which returns the smoothed data to core; original data are lost unless first store'd on tatpe. ANALYS is the data analysis routine which detects peaks, calculates peak centers and converts them to precursor masses using the ramp start and end voltages and the mass of the daughter ion. The operator enters a threshold (2') and peak window ( w ;default value 20); peaks are defined by w points above threshold and one point
tk
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
175
PBB~
1 2 3 4
n
5
m/. 17' 79.0 105.0 107.0 135.0
.. .*.
... *
.
.
I I 105.0
4 VOLTAGE ( k V )
I
I 107.0 amu
I
105.2
101.2 amu
a
Flgure 3. Defocused metastable scan (4-8 kV) for the m/e 77 ion of mmethoxybenzil (11, X = MeO)
b
I
below. A doublet is sensed by w increasing points after w decreasing points before the end of a peak. Peak centers are calculated as the average of the mass at half-height above threshold (or doublet minimum) on the upslope and downslope of each peak and the calculated centers are indicated by markers on the display (Figure 2). At any point in the analysis T and w may be changed, and the precursor masses printed out.
RESULTS AND DISCUSSION The system described in this paper has been used to carry out metastable studies on a wide range of compounds. To demonstrate the capabilities of the method, we have chosen studies on two compounds (I, 11),together with scans on the reference compound heptacosafluorotributylamine.
i
I 107.0 amu
105.0
C
I
1
105.0
101.0 amu
d
Figure 4. Manual and computer controlled scans from the m/e 77 ion of mmethoxybenzil showing masses measured for each peak
/coocn3
The two peaks are the doublet of Figure 3 (peaks3 and 4) run under the following conditions. (a) Manual scan, resolving power 1000. (b) Fast computer scan, resolving power 1000. (c)Slow computer scan, resolving power 10000. (d) Slow computer scan, resolving power 1000.
I1
I
-
erating voltage while holding the ESA voltage constant. For the process m+ m,+ at accelerating voltage V the daughter ion ml+ will be transmitted at V I = V.m/ml, and by sweeping the accelerating voltage over a sufficiently wide range, all precursor ions leading to a selected daughter ion can be detected within the limits of sensitivity of the instrument. In the MS902 and other commercial mass spectrometers, it is not practicable to increase the accelerating voltage above its
-
Operation of t h e System. An ion decomposition, m+ ml+, occurring in the first field-free region of a double focusing mass spectrometer, gives rise to a daughter ion ml+ which has insufficient energy to be transmitted by the electrostatic analyzer ( 2 ) .The metastable defocusing technique allows these ions to be transmitted and detected by increasing the accel-
Table I. Precision of Automatic and Manual Metastable Defocusing Automatic scan Individual determinations Mean aa Peak
Manual scan Individual determinations
Mean
ua
122.0 135.0 151.2 255.3
0.23 0.20 0.14 0.30
232.0 245.0 276.1
0.20 0.16 0.20
A. Precursor ions determined for the mle 105 ion in m-nitrobenzil 1
2 3 4
122.2, 122.0, 122.0, 122.2 121.8, 122.0, 121.8, 122.4 122.0, 122.2, 122.2, 121.6 121.8, 122.0, 122.2, 122.0 nb = 12 n=8 n = 14
122.0 134.9 151.0 255.0
0.21
122.0, 122.1, 122.0, 121.9 121.8, 122.4, 122.2, 122.0 121.7, 121.9, 122.0, 122.0 122.3, 122.4,121.6,122.0
0.18
0.22 0.25
B. Precursor ions determined for the m/e 217 ion in Diels-Alder adduct I' 1 2 3
232.0, 232.0, 232.1, 232.0 232.0, 231.9, 232.0, 232.0 232.0 n = 10 n = 12
232.0 245.0 276.1
0.10 0.14 0.12
232.0, 232.1, 231.9, 232.3 232.0, 231.8, 232.0, 232.4 231.9
n = number of individual determinations for obtaining mean and a Standard deviation of the measured values from the mean. sigma. The mle 217 ion is less than 1%of the base peak in the spectrum and does not appear in Figure 5a.
176
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
Table 11. Accuracy of Automatic and Manual Metastable Defocusing
Daughter mlea
306 275
274 247
243
242
216
69
100
131
169
181
264
Precursor mle
Automatic scan Measured value
A. Scans for iohs from Diels-Alder Adduct I 334.2 334 289.6 290 302.8 303 318.8 319 334.2 334 301.8 302 333.6 334 274.8 275 291.2 291 306.2 306 334.2 334 258.6 258 275.0 275 287.0 287 302.0 302 334.0 334 259.8 260 273.8 274 287.4 287 301.2 301 231.0 231 244.6 245 260.2 260 274.8 275 305.8 306 334.0 334 Variance of deviations'
AC
0.2 0.4 0.2 0.2 0.2 0.2 0.4 0.2 0.2 0.2 0.2 0.6
Manual scan Measured value
333.8 289.5 302.9 334.0 302.1 333.5 275.2
d
0.1 0.5 0.2 0.2 0.3 0.0 0.2 0.3 0.3
0.2 0.1
0.2 0.0 0.8 0.0
0.067
0.064
B. Scans for ions from heptacosafluorotributylamine 76 77.4 0.4 95 94.6 0.4 100 99.8 0.2 114 113.8 0.2 119 118.8 0.2 131 130.6 0.4 107 107.0 0.0 109 108.8 0.2 145 144.8 0.2 150 149.8 0.2 170 170.4 0.4 181 180.6 0.4 200 199.6 0.4 219 219.2 0.2 195 194.6 0.4 219 218.8 0.2 231 230.6 0.4 326 325.6 0.4 200 199.8 0.2 0.2 214 213.8 231 230.6 0.4 269 268.8 0.2 281 280.6 0.4 300 300.0 0.0 331 330.8 0.2 314 313.4 0.6 352 351.8 0.2 414 413.6 0.4 464 463.6 0.4 502 501.8 0.2 Variance of deviations' 0.10
0.0
d
0.0
0.0
0.1
d
306.2
0.4 0.2 0.2 0.2
0.2 0.2 0.4 0.2
0.2 0.5
d
258.3 275.0 d 302.2 334.3 259.7 274.2 d 300.9 231.2 245.0 259.2 275.0 d
0.0 0.0 0.0 0.0
AC
d d 100.0
114.1 119.3 131.4
0.0 0.1 0.3 0.4
d d
145.1 150.2 d 180.9 200.1 219.2
0.1 0.2 0.1
0.1 0.2
d
218.5 230.9 326.8 200.0 d 231.0 269.0
0.5 0.1
281.2
0.2 0.7 0.3
300.7 331.3 314.1 351.6 413.9 464.5 502.1
0.8
0.0 0.0 0.0
0.1
0.4 0.1 0.5
0.7 0.13
Main beam ion in focus at V,. Known precursor ion. A = I (known precursor mass-measured mass)/ for complete set of measurements. d Intensity too low to measure. ' Variance of deviations = A2/n - l for n observations. It should be noted that the software analysis routines round the calculated mass to 0.2 amu in the automatic measurements.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
177
a
4
Table 111. Analysis of Automatic Metastable Scan from m/e 231 Ion of the Diels-Alder Adduct I (Figure 2). Effect of Peak Window Size Peak
334
A M
lC0-r
135
1 2 3 4 5 6 7 8
231.0 246.0 259.4
not detected > i
* m'I
WE
b a
Figure 5. (a)Mass spectrum of Diels-Alder adduct I. (b) Mass spectrum of m-methoxybenzil (11, X = MeO)
normal operating value and metastable defocusing is carried out by tuning the main ion beam a t reduced accelerating and ESA voltages and then increasing the accelerating voltage. The reduced accelerating voltage V, is chosen for V / V , 3 m/ml so that the mass range ml to m is spanned. The selected daughter ion ml is then focused at the collector and the accelerating voltage increased to defocus the main beam and transmit precursor ions formed by metastable decomposition. Our automatic system still requires that the operator tune the mass spectrometer a t reduced accelerating voltage and focus the selected daughter ion, but once this is done the scan and data acquisition proceed rapidly and automatically. With the program running in the PDP8 and the accelerating voltage switched to computer control, the software section SETUP allows the operator to specify the scan rate, set the reduced accelerating voltage, and trim this set value to optimize the focusing of the main ion beam. Three scan rates (L,M , or H ) corresponding to scan times of about 30, 90, or 240 s are available. One of four starting voltages may be used; 1, 2, 4, or 6 kV, so that the scans can range from the daughter ion to masses of 8 X , 4X, 2X, or 1.33X the daughter ion mass. (The mass spectrometer ESA voltage must be preset to match the selected accelerating voltage with the front panel switch). The program then lowers the accelerating voltage 40 V below the preset value, ramps the voltage over about 80 V to sweep the main ion beam across the total ion current collector, and displays the total ion current (TIC) on the display monitor. Using the display controls available with the DISPLA software, the accelerating voltage can be set to the TIC plateau center to give optimum ion beam focusing. RSPEC then asks the operator to set the digitized ion beam zero which is displayed on the computer front panel lights and tune the selected daughter ion on the mass spectrometer with the magnet controls. The computer then drops the accelerating voltage about 20 V below the preset value so that the daughter ion is included in the scan, increments the accelerating voltage in 2.4-V steps, samples and sums the ion beam 64 times at each step, and divides the resulting intensity by 8 to give the ion current used in the analysis. (All three sampling rates are out of phase with the 50-Hz mains supply and this averaging technique effectively removes 5 0 - H ~noise from the signal.) The data are 178
Precursor m/ea Peak window 20 Peak window 7
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
275.0 290.4 306.0 334.2
231.0 246.0 259.4 263.6 275.0 290.4 306.0 334.4
mle of daughter ion.
displayed during and after the scan. The program asks for the DVM reading at the start and end of the scan and these values are used in the analysis routines to convert the time base of the scan to a voltage scale. At the completion of the scan, the data can be inspected on the screen, and the display expanded horizontally and vertically to inspect sections of the data and individual peaks. At any time, the raw data can be stored on tape and recalled a t a later time for analysis. In use the system proved to be very fast and convenient. The typical scans shown in Figures 2 and 3 were each acquired with 90s scans and analyzed in about 2 min. The corresponding manual measurements would require at least an hour and usually much longer. The gain in speed and convenience is so great that we routinely carry out metastable defocusing on samples submitted for analysis. Accuracy and Precision of the Analyses. Apart from the measurement of ion beam intensities, which is critical in this work only as far as it affects peak shapes, the system must ramp the accelerating voltage so that the voltage at each step in the ramp is known, and then correctly assign peak center voltages from the scan data. Sources of error in these processes are: (i) errors in the A D conversions of the mass spectrometer ion current, (ii) errors in the D/A conversion of the contents of the accumulator to the control voltage for the HV supply, (iii) inability of the HV supply to follow the control voltage, (iv) errors in assigning the start and end of scan voltages via the DVM, and (v) the software peak detection and arithmetic routines. Of these, i, ii, and iv can be estimated from the specifications of the devices used, and tested by measurement. In our system, the use of a 12-bit D/A converter limits the resolution of the scan to 0.024% or 0.12 amu at 500 amu. This is more than adequate to allow unambiguous assignment of precursor masses. The DVM allows the ramp start and end voltages to be read to better than 0.05%. The 9-bit A/D conversion of ion current gives 0.2% resolution which, coupled with the 64 samples taken at each voltage step and the fact that each peak spans about 200 steps, gives a more than adequate peak envelope (8). The arithmetic routines were designed to retain accuracy to 0.1 amu and to round to 0.2 amu on printout. Factor iii, the behavior of the HV supply during ramping, is more difficult to determine. Oscilloscope measurements indicated that the power supply output was capable of settling after a 1-kV step in less than 100 ms. We therefore first wrote the software to allow a 150-ms delay after each voltage increment before sampling the ion current, and then reduced the delay and compared the scans with those made a t a 150-ms delay. Even in the final software, which increments the HV and samples after one clock overflow, no discernible loss in accuracy or resolution occurred until the clock rate was increased to 6 kHz (fast scan). Some of these scans are compared with a manual scan in Figure 4; the fast scan shows a slight (0.2 amu) shift in calculated peak centers.
The precision and accuracy of the automatic system both proved to be virtually identical to the measurements made manually. Table I shows the statistical analysis of results obtained from measurements made on the mle 105 ion of m-nitrobenzil and the mle 217 ion of the Diels-Alder adduct (I);a typical set of individual determinations is shown for one ion in each set. Table I1 shows the results of measurements made on over 50 different transitions; the variance of the deviations between known precursor mass and measured mass are the same for the automatic and manual measurements within the limits of measurement. The automatic metastable defocusing system described clearly meets our requirement of making unambiguous assignments of precursor masses to better than 1amu. Sensitivity of the System. The data system proved to be significantly more sensitive than manual scanning. This superior sensitivity undoubtedly results from the improved signallnoise ratio obtained by sampling the ion beam 64 times at each voltage step. The low resolution spectrum of the Diels-Alder adduct I is shown in Figure 5a; the daughter ion at mle 231 is about 5% of the base peak at mle 274. Figure 2 shows the 2-8 kV metastable scan (ESA 130V) from this mle 231 ion. This ion is off scale in the metastable scan, but peak 8 was shown to be about 0.2% of the intensity of m/e 231. Peaks 3 and 4, clearly and reproducibly resolved by the data system, could not be recognized as a doublet in manual scans. The mass spectrum of m-methoxybenzil (11, X = OMe) is shown in Figure 5b, and a metastable scan of its m/e 77 ion is shown in Figure 3. Peak 5 in the metastable scan is less than 0.01% of the intensity of mle 77, and could not be detected in manual scans. Table 11,also, includes several transitions which could not be detected manually. Although we have not made quantitative comparisons of the sensitivities of the two methods, the automatic method appears to show about the expected eightfold increase in sensitivity. The simple peak center calculation used in our software proved more effective than centroid calculations. There are two reasons for this. First, the daughter ion peak almost invariably overloads the mass spectrometer detector in these studies so that the peak produced by the daughter ion tails badly on the high voltage side, and the centroid calculation therefore shifts the peak center towards higher voltage. Since all the subsequent calculations use this daughter ion position, the calculated masses of the precursors are displaced. Second, the MS902 used in this work does not have variable B slits and its energy resolution is not high enough to resolve precursor peaks from their isotopic satellites. Thus all peaks are slightly distorted on the high mass side, and the peak centroid is not the best measure of the peak position. The peak center calculation, on the other hand, uses only peak data above half-
height to define the peak position and thus ignores the small distortions of peak bases. Because the metastable spectrum is an energy spectrum, its resolution is not improved by increasing the mass resolution of the spectrometer (2).Figure 4 shows that the resolution of the metastable spectrum is virtually the same at a mass resolving power of 10 000 as it is at 1000. A mass spectrometer with /3 slits, or other method of increasing the energy resolution, could provide a metastable spectrum with higher resolution. Larger overlapping peaks present a problem however the peaks are analyzed. With a well-resolved doublet, the data system calculated the corresponding masses within 0.4 amu (e.g., Figures 3 and 4). With less well-resolved doublets, as in Figure 2, where peak 4 is a shoulder on peak 3, the shoulder was ignored at high values of the peak window w (20, Table 111),but detected a t lower values (e.g., 7, Table 111).However, in cases like this, the center calculation is much less reliable; the mass assigned to peak 4, 263.6, is almost certainly 263 corresponding to loss of 32. We have not attempted to use deconvolution methods to improve these mass assignments. Peak shapes are determined directly from the display screen or plotter output. Peak widths can be determined from the display by setting the DISPLA marker to the desired position on each side of a peak and reading the corresponding voltages from the DVM. We make only occasional use of this facility, but a simple modification to the software would allow peak widths to be printed out for each analysis. Further details of the hardware, and listings of the software, are available from G.J.W. LITERATURE CITED T. W. Bentley in "Mass Spectrometry, Vol. 3", Specialist Periodical Reports, The Chemical Society, London, 1975, p 59; I. Howe in "Mass Spectrometry, Vol. 2", Specialist Periodical Reports, The Chemical Society, London, 1973,
p 58. R. G. Cooks, J. H. Beynon, R. M. Caprioli, and R. G. Lester, "Metastable Ions", Elsevier, Amsterdam, 1973. M. Barber and R . M. Elliot, 12th Annual ASTM E-I4 Conference on Mass Spectrometry, Montreal, Canada, 1964. K. H. Maurer, C. Brunnee, G. Kappus, K. Habfast, U. Schroder, and P. Schulze, 19th Annual ASTM E-I4 Conference on Mass Spectrometry, Atlanta, Ga., 1971. M. Barber, W. A. Wostenholme. and K. R . Jennings, Nature (London),214, 664 (1967). J. E. Coutant and F. W. McLafferty, Int. J. Mass Spectrom. /on fhys., 8,323 (1973). G. Dulaney, Anal. Chern., 47, 24A (1975). J. S. Halliday, "Advances in Mass Spectrometry", Vol. 4, Institute of Petroleum, London, 1968.
RECEIVEDfor review May 13, 1976. Accepted October 13, 1976. We thank the Research Committee of the New Zealand Universities Grants Committee for financial support, and for the award of a Postgraduate Scholarship to L.K.P.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
179
