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Metastable ion characteristics
323
Internationd Journal of Mass S’ctrometry and Ion Physics EisevierPublishingCompany, Amsterdam. Printedin the Netherlands
METASTABLE
ION CHARACTERIST1ti
XX. AUTOMATED ACQUiSlTLON OF METASTABLE ION DATA USiNG AN ON-LINE COMPUTER WITH FEEDBACK CONTROL*
JOHN E. COUTABT**
Depurtmenr
AND F. W. MCLAFFERTY
of ChemZstry, Cornell University,
Ithaca, N. Y. 14850 (U.S.A.)
ABSTRACT
A fuIIy-automated system incorporating a Nier-Johnson geometry mass spectrometerand an on-line computer designed originally to collect high-resolution data has been modi5ed to acquire and reduce metastableion data produced by the Barber-Elliott-Major defocusing technique. Factors affecting system performance have been identsed, and a variety of operating modes are proposed.
INlRODUCTIOS
Recognition of the analytical usefulnessof the so-called ‘metastable peaks7 in mass spectra has grown rapidly’ -4. Such a peak resultsfrom an ion decomposition occurring after the ion has passed through at least one, but not aII, of the electrostatic and magnetic fields between the ion source and the cohector. Thus the magnitudes of the fieIds necessaryto focus the daughter ion of the decomposition are characteristic of the mass (m) and charge (Q) of both the precursor ion and the daughter ion. This extra dimension of information has been sho& to be valuable for identifying ion decomposition pathways1-4, characterizing ion struttures’, classifying reaction transition state?*‘, studying decompositions of ions of defined internal ener&&s6*8* ’ (in&ding nove1reactions that are dif&mlt to observe under ordinary conditions”), and providing analytical and st~ctural information on mixturesl’. Ions undergoing me&stable decompositions are relatively longlived, and are thus of relatively low internal energy, in comparison to ions which l
Dedicatedto Prof. A. 0. C. Nier in honor of his &
Annual ASTM
E-14 Conference &
Mass Spectrometer
‘b&day. .Prt&&$ Dalks, Ma% 1969.
ti_ &t
** Taken from the Ph.D. Thesis of J. E Coutkt, Cornell University, Jan% DOW CmningCorporati,on,
Midland, Michigan.48604.
_.
Inc. J. Mass Spectkwn.
Ion Phys.,.g
+ the 17th
1971. Now -y
at the
(197$ 323-339
324
3.
E. COUTANT,
F. W.
MCLAFFERTY
decompose in the ionization chamber. Decompositions characteristic of higher energy ions can also be studied using this technique by introducing ca. 10m4 torr of an inert gas into a &Id free region of the ion path; the resulting collisioninduced decompositions are generally much more numerous than unimolecular metastabIes and represent the more simpte cleavage reactions’2-‘4. Utilization of me&stable ion decompositions has been handicapped by experimental problems. The defocusing technique proposed by Barber and Elliott” has greatiy improved the sensitivity and reliability of precursor and daughter ion identification; measurement of metastable abundances over a dyanamic rangel6 of l/10’ is possible utilizing Major ef aZ.‘s suggestion” of offsetting the electrostatic analyzer (ESA) potential on a doubIe-focusing mass spectrometer, utilizing the first metastable decomposition regionf6-18. However, with this technique the focusing of the daughter ion of a particuIar metastabie transition requires both a specific ESA” (or ion acceIerating potential (ref. 15)) setting and magnetic fieId vaIue; thus an extra dimension of measurement cperations is required to obtain the extra dimension of information postuIated above for metastabIe ion data. This probiem is especially acute if one wishes to determine al1 of the metastable transitions which can occur. To accomplish this Kiser and co-workers recommend’ g approximately 100 complete magnetic fiefd scans at 25volt increments of the ESA setting to find the metastable peaks, foliowed by rescanning at 0.2- or 03volt increments of the local ESA potentiai to locate these peaks exactly. 51 utilization of the defocusin, Q technique in a number of ~tudies~-~~ we have found that both data collection and reduction is extremely laborious. Data coIIection is greatly eFdited by using photop!ate recording” with instruments of Mattauch-Herzog geometry, but the complex data measurement and reduction methods necessary have limited the use of this method. With Mattauch-Herzog or nor-ma3 Nier-Johnson geometry, all metastabIe transitions producing a particular daughter ion can be detected by scanning the ion accelerating voltage using appropriate tied ESA and magnetic fieIds15*20. Using an instrument in which the ESA follows the magnetic anaiyzer, al1 metastable decompositions of a particuiar precursor ion can be measured by scanning the ESA potential using appropriate fixed ion accelerating and magnetic fieIds21_ The former has substantial drawbacksxg*20, and both require numerous scans (nearly one per spectral peak), each at a carefuiIy-determined vaIue of the magnetic field. Because the great utility of the dedicated on-Iine computer for high-resolution data22*23 has made it available on many double-focusing mass spectrometers, it seemed desirable to tid ways in which such a computer might be utilized to increase the speed, efficiency, and accu&y for acquisition and reduction of metastable ion d2ta by the defocusing technique. We sought first24 to develop a method to find all possible metasrable ion products by searching at al1 possible values of the magnetic and ESA fieIds; this couid be used to find unexpected products, such ht. L.Ma.ss Spctrom. Ion Phys., 8 (1972) 323-339
AUTOMATED
ACQUISITION
GF
METASTABLE
ION
DATA
325
as might occur from multiply-charged ions, in cohision-induced dissociations”-‘4, in other regions of the instrument18*25, or from multiple decompositions in consecutive field-free regions26n27. Of equal importance, such a system should make it possible to define much more completely the experimental system for defocused metastables, and thus provide the data necessary to design more efficient systems to accomphsh more specific objectives.
THEORETICAL.
Extensive treatments of the equations governing the focusing of ion products resulting from decompositions in field-free regions of a double-focusing mass spectrometer have been given18-20*25. In the Hitachi RMH-2 instrument used in this study there are three field-free regions: (1) between the ion source and ESA, (2) between the ESA and magnetic field, and (3) in the “bright” ion source’*. For the decomposition of an ion of mass m1 and charge q1 yielding an ion of mass nz2 and charge q2 plus a neutral mass of rno, rq1
ml
-_, m2
-+mo,
(1)
the apparent mass, m*/q*, of the daughter ion is represented2’ by
where V is the total accelerating potential and VI is the accelerating potential experienced by the precursor ion, m, , before decomposition. When VI = V, which is true except for the third field-free region, and q1 = q2 = 1, eqn. (2) reduces to the familiar relationship,
m*=-.
m2
(3)
ml
The equation for the transmission of a charged particle through an electric sector (ESA) is given by the balance of electrostatic and centrifugal forces:
r,qE
llllV2 -=-
2
2
where t‘ is the velocity, rC is the ESA radius, and E is the ESA field strength. The ESA field, I$, required to transmit any normal ion (one that has not undergone a metastable transition) is 2q’re. The following equation
Ed
=
2(-33 Int. J. Mass Spectrom. Ion Phys., 8 (1972) 323--339
326
J. E. COUTANT,
F. W. MCiAFkERTY
representsthe ESA field, E,; needed to transmit the daughter ion of a metastable transition. Foltomkg IGser et aZ_lgthe ratio Ed/E; is decked as p_ then
&$
=: r
‘1
for the nsua.lcase of qa = q2 = I. Since the field strength of the electric sector is directly proportional to the potential applied to the sector, the latter value will be used for E, or Ed. The Gombinationof eqns. (3) (or (2)) and (6) allows the de6nitive identification of 772,and m, if m* and p are known. The extra dimension added to a mass spectrumby metastabledecomposition infkmati~n is ifiustrated schematically by Fig. 1. (If shown, the other dimension, ion abundance,would be perpendicularto the plane of the paper). The hypothetical metastable transitions of Fig. 1 are given in TabIe 1. Note that the section through p = I is the focused mass spectrum (normal ions and metastable ions formed in the second field-fi-eeregion)_ . nomai
100
x 0
ions
focused matostables defocused matastables
80 t
60 -
t2 %
40
20 I.
PFig, 1. Schematic diagram showing the relationship of the ESA metastable transitions of Table I. TABLE
NO.
1 2 3 4 5 6
I
Precursor 100.0 80.0 60.0 _ 50.0 100.0 100.0
IIU. J. Mass SgeEtrom.
Daughter
P
m*
50.0 50.0 50.0 25.0 80.0 25.0
0.500 0.625 0.833 0.500 0.800 0.250
-25.00 31.25 41.67 12.50 64.00 6.25
Ion Phys., 8 (1972) 323-339
(p) and mass values for the
AUTOMATED
ACQUISITION
OF
METASTASLE
ION
DATA
327
Summing the ion intensities for alI mass vaiues at each p value gives the ion
kinetic energy spectrum proposed by Major?Q-30. Note that either of thesc twodimensional displays can produce ambiguities in assigning ml and m, for a metastable transition. For example, both transition 1 and 4 have p values of O-500, and the m* value of transition 1 has the same mass value as that of a normal ion at p = 1.000. AI1 metastabIe decompositions producing m2 will fall on the straight line described by m2 = m*/p: all decompositions 1721 =
(7) of m, will be described by the function
nr*lpz.
(8)
Note that the mass of the molecular ion thus limits the possible combinations of p and m* values for metastable decompositions involving singly-charged ions; no metastables will occur in the region to the left of the dashed curve of Fig. 1. A precursor ion which decomposes in the third field-free is focused at an n2*/4* value described by eqn. (2) and an ESA value” by Ed =2[(:)(+)
i- (1-;)(3].
(9)
The resulting metastable peak will therefore fall on the energy continuum described by eqn. (7) (solid lines, Fi g. 1) between the focused daughter ion and the defocused metastable ion. Its relative position between these two points is determined by the fraction VI/V of the total accelerating potential that the ion m1 receives before decomposition to ~2,.
EXPERIblENTAL
In this system the magnetic field is scanned slowly while the ESA potential is varied rapidly over all possible values of p (see Fig. I); the speed and ffexibility possible with the reverse procedure” is limited by the ability to controi and measure the magnetic field due to hysteresis and mass-marking di.Biculties. The Hitachi RMH-2 double-fmusing, high-resolution mass spectrometer used in this system has a modified Nier-Johnson geometry resulting in a long (589 mm) fieldfree region between the ion source and ESA which enhances the sensitivity for measurement of defocused metastables16~1‘_ Normal operating conditions were an electron emission of 125 PA, an accelerating potential of 9.6 kV, ESA potential of 730 V, and a source sample pressure of 1.0 x TOW6torr. The detector system consists of a 16 stage electron muhiplier, a wide bandpass d-c. amplifier, and a logarithmic scaling amplifier (Philbrick Model 4350). The. source exit slit and collector slit were set at their maximum value of 350 microns for met&tble mea- _ ht. J.- Mass Spectrom. Ion Phys., 8 (1972)
323-339
328
3. E_ COUTANT,
Fe W. MCLAFFERTY
surements resulting in an overall resolution of approximately M/H = 2,000. The beta-slit was set at 4 mm. y The computer used in this system is a PDP-8 (Digital Equipment Corporation, Maynard., Mass.) which has a 12-bit word length, 4K core size, two 32K random-access disc units, a high speed paper-tape punch and reader, the hardware multiply and divide option, and a high-speed line printer (Shepard, Model 880). The -mass spectrometer
F8akSignal
ESA __-_----A/D
I
I
Converter
I
Fig- 2. schematic -diagramof the computer-mass spectrometerinterfacefor the metastabledata co&ction system_ Znr.J. Mau Spcctiom. ion Phys., 8 (‘,972) 323-339
AUTOMATED
ACQUISITIOX
OF
METASTABLE
ION
DATA
329
In the operatin g mode used for the survey data reported here, the ramp generator is adjusted to give a ramp risetime of 2.0 set (0 to 1000 V at the output of the operational power supply) and a fall of 0.1 sec. The upper limit of the ramp is manually set to a value slightly greater than that necessary to focus normal ions (P = I.000). The lower Iimit is controlled by the computer, and the ramp direction may be changed at any point from down to up by the computer. Data is coIIected at a 20 kHz rate during the increasing ESA potentia1 scan by the computer with calculations and bookkeeping being performed on the fast flyback. The magnetic field is scanned at a rate to make possible a total of IO ESA scans through each mass peak. In this manner the entire three-dimensional space is recorded. Software
Direct processing of al1 of the programs invoIved in the metastab!e data system would require a core of about 16K. Operation in the 4K core was achieved through the use of modular software ‘swapped’ from the disc as required under program contro12’ -’ 4_ The five separate phases used are: Pha.w I - Dafa acquisition. This program is responsible for detecting and digitizing ESA-scan peaks, controlling the electric sector potential, and storing the compressed data on the disc. Before the scan is started the values of the A/D rate, the digital threshold, and the lower limit of the ESA scan are chosen and set into the computer. (The Iatter can be calcuIated from eqn. 8). The program then determines the slope of the ESA scanning ramp. After the magnetic scan is initiated and the start signaI given, the program triggers the ramp generator to start the ESA scan. The analog signal from the electron multiplier is directed to channel 0 of the A/D converter; when the signal exceeds the threshold value, it is continuously digitized at the A/D rate until four consecutive points below threshold are digitized. At this point the channel of the A/D converted is incremented to channel 1, w’hich reads the voltage on the electric sector, and on the next A/D pulse this vaIue is digitized and the input channel returned to channel 0. On the succeeding pulse the value of a 24-bit binary clock is read. III this way a digitized peak profile of an ESA peak is stored in core along with a digital ESA potential and time value. The process is repeated for any further peaks unti1 the ramp generator reaches its maximum vaIue on the increasing scan, where it reverses itseIf and interrupts the computer. The program then sorts through the data excIuding those peaks consisting of less than 10 digital points and stores the remaining data on the disc terminated by a fIag ‘-0 signify the completion of one-electric sector scan. The ESA potexitialvalues are sampIed until the Iower Iimit is reached, when the computer issues a signal which reverses the ramp direction from down to up, and then the cycle begins over again. After the stop pulse is received, the data can be optionally dumped on the teletype, line printer, plotter, or high speed punch Fig. 3 illustrates an actual data dump from a smaI1 portion of a spectrum. Int. J. Mass
S’ctrom.
Zon P&s.,
8 (1972)
323-339.
;
; : . .
\
‘,
AUToMATED
ACQUISITION
OF METASTABLE
ION
DATA
331
Phase II - ESA peak center calculation.This phase takes the raw data stored on the disc by Phase I, calculates the time and voltage centers and the intensity of peaks, and stores this information back on the disc as a four-word group (the values req-uire two, one and one words, respectively)_ The center is calculated by taking the average vaiue between two points that are at one-half the height of the maximum value. A flag separates each ESA scan. For routine use where data dumps are not needed the Phase I and Phase II programs are combined. The calculations of Phase 11 are performed during the ESA-scan fiyback, and the resuking compressed data stored on the disc. Phase III - ESA peak sorting. Each focused and defocused peak should have produced several ESA scan peaks occurring at essentially the same value of ESA potential but at increasing time (mass) values. The Phase IIf program creates six large tabIes in core and sorts each electric sector scan peak into its correct group. As these groups are compieted, they are written on the disc for use by Phase iv. Pase N - Time centroid calculation. The grouped mass peaks from Phase III are read from the disc and inspected to see if the peak is significant. Peaks with three or less ESA scans are ignored. The centroid of the remaining time peaks is then calculated by
Y (centroid)
b f (X)X& f = a b II
f(x)
(10)
dx
in which f(x) = amplitude at position (time value) x in the peak envelope. Since dx is actually Ax, the experimentally obtained centroid is a close approximation to that represented by Eqn. (10). The lower integral gives the area of each peak. The corresponding ESA value is taken as the average of each individual ESA scan peak potential. The time value is converted to seconds, and these values are stored on the disc. Phause V - Time-to-mass comersion ad ?n.ztastabletransitionassignment. At the beginning of this phase the program asks the operator to assign the nominal masses of two focused peaks and the average ESA potential vaIue of the focused peaks (thus defining p. = 1.OOO)from an inspection of the output of Phase IV. For operator convenience whole number mass values have been used here instead of exact nuclidic masses; the error involved is small. The program then dxtrapolates to assign mass values to the remaining focused peaks. For each defocused peak, precursor and daughter ion mass are calculated by the. use of the equations discussed below and displayed on the line printer, the high-speed punch, or the teletype. After inspection of the results, the operator may repeat Phase V changing the ESA or mass vahJes of the assigned peaks or the output device. ht. J. Mass Spectrom. ion P&s.,
8 (1972) 323-339
332
J_ Em COUTANT,
F. Wa MCLAFFERTY
RESULTS AN-D DISCUSSION
Peak shape and iocation
.
The illustrated data from the 85’ +
general form and relationships of the data acquired by Phase I are in Fig. 3. This is a computer-generated three-dimensional plot of raw n-deeane, including the focused and defocused metastabIe peaks for 57’ transition and the normal fragment peak at mass 38. The ESA scans show successive profiles of each p:ak which serve to define a voIume.reIated to the abundance of the peak. In these relative dimensions each peak appears as ridge. It can be seen that the center of each successive ESA profile increases slightly with increasing mass;. this is because the observed mass values, M*, are a direct function of p by eqn. (7) for normal as well as metastable ions3’. At the beta-slit in the Nier-Johnson configuration the width of the focused ion beam (p = 1.000) is only a small fraction of the beta-slit width Changing the ESA potential at P = l.ooO &-st sweeps the focused ion beam across the beta-slit, at the same time changing the magnetic fieId required to focus the ion beam according to eqn. (7). The ion ridge thus produced (Fig. 3) should exhibit a length which is dependent on the width of the beta-slit. Further, with optimal focusing, approximateIy the same number of ions will be in focus as the ion beam is scanned electrostaticahy across the beta-slit, so that the ion ridge should exhibit a flat top in its long dimension; this is approximately true for the nz/e 38 peak (p = l.ooO) of Fig. 3 within experimental error. Note that the width of the ion beam (as defined by the source and collector slits) determines the width, not length, of the ion ridge. This width is shown in the section through the ridge from the ESA scan at constant mass, or in the normal magnetic fieId scan at a constant v&e of the ESA potential, as both sections are at an angle to the long dimension of the ion ridge. Thus a “flat-topped” metastable will exhibit an ion ridge which is Sattened in width as well as length_ ‘T&se variations were noted by observing the raw data dumps of Phase I. The system does not indicate such changes in ridge width and length in the final report, although such things couId be noted as the data are available to the system. AIthough decreasing the beta-slit width decreases the observed width of a flat-topped metastable2 3 (the ridge width at the center of its length), this does not appreciably affect the width of a normal ion ridge31. The long dimension of the ion ridge defined by eqn. (7) is also the line on which aI metastabIes wiIl fd1 producing a common daughter ion (t;ide supra). This is illustrated in Fig. 3 by the small isotopic metastable 86+ --, 57 + at m* = 37.8 andp = 0.663 which faIIs on an extension of the ridge Iine of the 85+ * 57” metastable- The dispersion of the beta-slit is 4.0 mm for an ion beam energy change of 0.85 %18; in keeping with this, overIapping of the ends of ion ridges of metastables Ieading to the same. daughter ion occurs when the precursors differ in mass by approximately 0.85 %. This rtiolution
can be improved, of course, by narrowing
Int. J. iUas.rSpectm~. IOUPhys., 8 (1972) 323-239
AUTOMATED
ACQUISITION
OF METASTABLE
ION
333
DATA
the beta-slit. Note also the focusing at the beta-slit characteristic of the NierJohnson geometry makes possible a far superior resolution of such metastable transitions than that possible with Mattauch-Herzog type instruments3 3 ; such an instrument gave only marginal resolution of the 71 i + 43+ and 72+ -+ 43+ transitions2’. TABLE
2
MJZTASTABLE TR4XSITIOXS
iwlss
OF R-DECANE InreJzsiry
P
Found
Calc.”
Found
21.83 21.83 24.44 25.20 26.09 28.76 29.49 29.47 29.98 33.00 32.99 36.90 36.90 37.08 37.35 38.19 38.22 39.12 40.21 40.64 43.18 44.63 44.72 56.67 67.64 90.17
21.75 21.75 24.36 25.20 26.04 28.75 29.49 29.49 30.02 32.52 32.82 36.91a 36.87b 37.10 37.33 38.3P 35.22 39.09 43.02 40.65” 43.21 44.61 444.61 56.68 67.63 89.92
1.oooo 0.5047 0.5933 0.5990 0.6043 0.5029 1.0000 0.7187
0.7317 1.moo 0.5746 0.8566 0.8986 0.9510 0.6658 0.9822 0.6702 0.9551 0.9812 0.9917 0.7859 O.G276 1.0000 0.8214 0.6897 0.7937
Ca Ic.
0.5059 0.5942 0.6000
0.6056 0.5044 0.7193 0.7321 0.5758
0.9512 0.6667 0.6706 0.9535
0.7857 0.6283 0.8214 0.6901 0.7958
Calc. for drift region lst, 3rd
28 149 18 28 340 109 79 334 47 53 301 34 67 110 101 20 134 232 197 20 42 184 46 73 41 32
43.26 41.20 42.07 43.17 57.18
1st
85.72 69.43 70.24 71.45 113.69
41.00
57.05
40.96
55.98 1
57.41 43.08 41.07 38.98 56.09 38.88 57.02 40.95 40.97 40.98 54.94 71.11
99.90 45.70 40*99 84.23 39.59 85.07 42.88 41.76 41.32 69.91 113.29
68.99 98.08 113.60
83.98 142.20 143.12
50.29
3rd
-
71.08 57.02
40.90
-!43.21 41.95
-
Assigned c0lue.s
43 43 41 42 43 57 41 41 41 57 57 43 41 39 56 39 57 41 41 41 55 71 71 69 98 113
85 85 69 70 71 113 57 57 56 99 99 71 57 41 84 41 85 43 42’ 42 70 113 113 84 142 142
a Based on assigned values of ml and ml. b Based on decomposition in the third field-free drift region. E Apparently contains some contribution from the 43 + 41 transition occurring in the third field-free region.
The metastabie ions, both focused and defocused, found for n&cane using the described experimenta conditions are shown in Table 2, with the experimentahy determined mass, p value and an absohrte intensity vahre along with the calcuIated theoretical m* and p values for the observed transitions. In a total time of approximately two hours the system was able to detect and identify 18 metastable decompositions occurring in the first field-free drift region, plus each in the Int. J. Mass Specrrom. Ion Phys., 8 (1972) 323-339
-
J. E. COUTANT,
334
: : : :
Int_ J_ Maps Spectmrn:Ion
Phyx,
8 (1972) 323-339
F. W.
: ; :.
.
MCLAFFERTY
AUTOMATED
ACQUISITION
OF METASTABLE
ION
335
DATA
second and third regions; 103 transitions have been reported previously. A portion of those found here are shown as a computer-drawn three-dimensional display in Fig. 4. As further e_xamplesthe metastable transitions found by the system for 2hexanone-1,1,1,3,3-d and p-bromophenetole are shown in Tables 3 and 4. Table 4 also shows a collision-induced decomposition found for acetone. TABLE
3
raTm~i_~
IRU~SITIO~S
MaS.5
OF
Z-HEX,WO?CZ-1 , 1, 19 3, 3-d,
Intensity
P
Found
Calc.’
Found
Calc.
28.49 29.89 31.33 32.10 33.55 37.12 39.09 40.03
28.49
0.6939
0.6949
29.90 31.34 32.14 33.59 37.10 39.09 40.01
0.7103 0.7277 0.7131 0.7289 0.9516 0.9526 0.6782
0.7119 0.7281 0.7143 0.7302 0.9512 0.9535 0.6782
Assigned ualues
Calc.
12 151 61 68 159 23 20 108
mz
ml
m2
ml
41.05 42.07 43.05 45.02 46.03 39.01 41.03 59.02
59.15 59.23 59-15 63.12 63.14 40.99 43.07 87.03
41 42 43 45 46 39 41 59
59 59 59 63 63 41 43 87
a pBromopheneto1e. TABLE
4
M3ZT/,.Sl%BLE TRANSITIONS
OF Jl-BROMOlWEbETOLE
AND
ACETONE:
Assigned calues
IntenMass
Calc.
sity
P
Found
Calc.”
Found
Calc.
147.83’ 149.87’ 149.73= 31.92b
147.92 149.88 149.88 31.89
0.8590 l.OOXl 0.8602 0.7406
0.8600 0.8614 0.7414
229 24 227 123
mz
ml
m2
ml
172.09
200.34
172
200
174.06 43.09
[email protected] 58.18
174 43
202 58
a pBromop henetole. b Acetone with 2 x X0-+ torr argon in the 1st field-free drift region; ao rnetastable transition ions detected at 1 x lo-’ torr.
There are a number of reasons for the low proportion of themetastables of n-decane detected by the system_ A higher proportion is found with longer scan rimes, but the two hours required here is hardly practical in most cases. A data dump after Phase I of the program showed that a large number of the metastable transitions had been detected but were eliminated by the requirement of a minimum of 10 digital points in an ESA peak proGle plus a minimum of 4 ESA scans:over the me&stable ion ridge. These minima were patterned after those used in the highresoiution system’ 3; our present~experience shows that some requirements for the metastable system are quite different. Int. L
Mass
Spectrom.
Ion Phys.,
8 (1972) 323-339
3. E.
336
COUTANT,
F. W.
MCLAFFERTY
A critica difference is in the dynamic range of the signal intensities that it is desirable to record. The present system is scanning the normal as well as the me&stable peaks; the intensity of the largest normal peak is approximately 10’ times as large as the smallest cietectabie metastable peak16. Even with the logarithmic amplifier the 12-bit A/D converter provides a dynamic recording range of only 2 x I@. Somewhat lower abundance metastables were recorded in the data of TabIe 2 by increasing the gain to the point that large peaks saturated the ampiifier; their abundance values are thus subject to error. Sensitivity and abundance accuracy were aIso hampered by the necessity of setting a high threshold level to overcome the Iow signal/noise ratio of the commercial amplifier (actually that of the smailer Hitachi RMU-7) in the system. A 15bit A/D converter and/or an autoranging ampli6er3S are planned to aheviate this problem; the computer can also be used to set the multiplier gain through feedback control of the muhiplier high voltage supply (for exampie, the gain couid be reduced at p = 1.0). Since completion of this work an improvement of CQ. x 30 in sigual/noise level has been achieved by installation of a Kiethley 427 amplifier3’. A system for software thresholding also appears promising for improving the detection of small peaks. The data of Table 3 demonstrate the system’s ability to distinguish metastables from a gammon precursor ion_ In the spectrum of 2-hexanone-l,l,f,3,3-d, the m/e 59 and 63 pe&s should correspond to the ions CH&H2CH2CD,i and CD&(-O*H)CD,, respec\ively. The metastabIe transitions of the former thus represent mainly the losses of CHzD2, CH3D, and CH,, while those of the latter represent mainly the losses of CD,- and CHD,-36. The higher mass decompositions of p-bromophenetoIe (Table 4) were correctly identified. In the case of acetone (Table 4), no metastable was detected under unimolecular decomposition conditions; _=ising the pressure in the first field-free drift region to 2 x 10m5 torr with argon gave the decomposition corresponding to the loss of CH3 - . More extensive testing of this system3’ showed that nz, and m2 for the transitions are generally assigned with high reliability in the low mass region, and indicates the parameters which must be controlled to increase the reliability of all mass assignments. Note that the accuracy for the determination of m, will in general be lower than that for m,, as calculation of these involve p2 and p, respeotively (eqns. (7) and (8)). In general the accuracy of p value determinations was at ieast l/lo3 with careful assignment of the p = 1.000 value of the ESA potential. This is gratifying, as the linearity of the scale is assumed The accuracy of the determination of m* was generally within several parts per thousand. The most serious errors usuaI:y arose in mass regions with few normal ion peaks, which serve to calibrate the mass scale, or when the program mistook a normal metastable peak (p = LQOO) for a calibrant no-al ion (for example, Table 2, the 99+ + 57* transition, m* = 32.82). The need for a much larger number of calibrating peaks than that necessary in high-resolution measurements is due to the extremely slow- magnetic scan. In 43u_ 1. MizsrSpecfrom.
Ion Phys., 8 (1972) 323-339
AUTOMATED
ACQUISITION
OF METASTABLE
ION
DATA
337
addition, the length-wise flat-topped nature of the ion ridge reduces the accuracy of the centroid determination. We are presently developing software so that the metastable peaks will be located in a much faster magnetic scan; the scan speed will be set so that the length-wise flat region of the ion ridge will be crossed only once or twice. (This would reduce the scan time for the tlata of Table 2 and Fig. 3 to 15 to 20 minutes even without the other proposed impi-ovements.) The beta slit would then be reduced to eliminate the length-wise flat-top of the ion ridge without reducing the maximum height of the ridge at its center, and the magnetic field would be scanned again under computer control to find accurate p and m* values by rapid ESA scanning over the areas of the specific ridge locations identified in the preliminary scan. Despite the poor sensitivity of the present system, a few metastable decompositions occurring in the second and third field-free regions were detected. These can provide valuable evidence of the variation of the ion decomposition rate constants with internal energy ‘_ All of the defocused metastables occurring in the third field-free region should obey eqn. (7) (vide :,upra), so that the system will calculate the correct mass vaIue, m, , of the daughtr;r ion for these transitions. However the value for ml found by the system using eqn. (8) will be incorrect, as can be seen by the examples in Table 2. Eqn. (9) must be used to calculate m, ; this reduces to 2 m2 “11
= m*3 [
_
(“Zz--z*d
(11)
1 ’
R
%
in which R = VJV and m*3 is the apparent mass of the third field-free region metastable. Values of m, so calculated (Table 2) generally show comparable accuracy to m, values from first drift region metastables. In most cases it is possible to distinguish between metastabies formed in the first or third drift regions by inspection of the m, values calculated by eqns. (8) and (1 I); further, for each metastable formed in the third region the same transition usuaIIy produces a more abundant metastable in the first fieid-free drift region. Ambiguities can arise, however, from accidental overlapping of me&stables from these two regions. The poor agreement found for the 42+ --* 41’ transition in Table 2 may be due to interference of 43+ + 41+ metastables formed in the third field-free region. Alremative modes of operation A number of possible improvements to increase the scan speed, sensitivity, and accuracy are under investigation. Improvements discussed above include increased dynamic recording range, software threshold+, and an initial fast scan for ion ridge detection foliowed by a scan with narrowed beta slit under program control for exact measurement of m* and p values. For many applications only Int. 3. Mass
Spectrom.
ion Phys., 8 (1972) 323-339
338
J. E. COUTANT,
F. W. MCLAFFERTY
particular me’%tabIes might be of interest_ All o;‘ the decompositions of a particular ion can be found by scanning through the combination of m* and p vahres predicted by eqn. (8); similarly ail transitions leading to m2 are predicted by eqn. (7). Collision-induced decompositions in the field-free drift region have been proposed as a powerful tool for determining the amino acid sequences of mixtures of oligopeptides14*38; in this application during a magnetic scan the computer would direct the ESA scanning at only those combinations of m* and p values which correspond to transitions between sequence peaks (those arising by direct fragmentation of the peptide backbone) arising from the possxble sequences found by the computer from the previously-scanned high-resohrtion data. Mass data from such a preliminary scan should also provide a substantial improvement in the accuracy and convenience of calibrating the mass scaIe for any metastable data acquisition by the system_ We conclude that the same on-fine computer system which is used for highresolution mass spectral data can be applied for the acquisition and reduction of metastable ion data obtained from the same instrument by the defocusing technique_ The main modifications are ones necessary to improve sensitivity because of the unusuaI dynamic range of the metastable ion abundances so produced_ This and our reIated development3’ comprise the frrst examples of utilizing an on-line computer for feedback control of a magnetic mass spectrometer.
ACKNOWLEDGXENT
Drs. R Venkataraghavan and R. J. Klimowski provided substantial help on both the hardware and software aspects of this research. We are indebted to the National Institutes of Health, Grant GM-16609, for generous Gna&al support.
REFERENCES 1 J. H. BEYNOX, rUass Spectrometry a& its Appiication to Organ-c Chemistry, Eisevier, Hamsterc!an& I960. 2 F. W. MCLAFFERTY, rnterprerption of Mass Spectra, Benjamin, New York, 1966. 3 5. H. BEYSON, in E. %ZNDRICK (Editor), Adcances in Mass Spectrometry, Vol. 4, bs.i#e of ?etroIeum, London and Elsevier, Arnsterdaxn, 1968. 4 F. W. MC~SERTY, R. A. K~XFEW, D. 3. McADoo, 33. D. R. SCX-ITJDDEMAGEAKD 3. S. SXRH, in R OGATA AND T. &~YAKXWA (Editors), Recent Deuelopmenis in Mass Spectroscop_V, University Fark Press, Baltimore, 1970, p_ 70. 5 T- W. SRASTON AN~D F- W- MCLSFISTY, J_ Amer. Gem. Sot., SF (1966) 5021. 6 F- W. McLA~RTY &ND R. B. F.XIFLWGUISR, J. Amer. Chem. Sot., 90 (1968) 5915. 7 F. W. M~z-A-Y, T. WKHS, C_ LIFSHITZ,G. INNORTA AND P. IRVING, J- Amer. Chem. Sot., 92 (1970) 6867. 8 I.HowEANDF.W-Mw IT, J. Amer. Chem- Sot., 92 (1970) 3997. 9 L H0w-E A?? F. w- MCLAFFER~, .r- Amer. Gem- Sot-, 93 (1971) 99. IO F. W. MCLAFFEXIY, D. J- McADoo ATZD J_ S. Sm, .T. Amer. Chem. Sot-, 91 (1969) 5400. Int. J. Mass Spectrom. fan Phys., 8 (1972) 323-339
AUTOMATED 11 12 13 14 I5 16 I7 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
ACQUISITION
OF METASTABLE
ION
DATA
339
F. W. MCLAFFERlY AND T. BOYCE, C’henr.Commrm. (1967) 1215. W. F. HADD~N AND F. W. Mctim-ry, J. Amer. Chem. Sot_, 90 (1968) 4745. F. W. MCLAFFLRTY tim H. 23. R. SCHXJDDLK~GE, J. Amer. Chem. Sot., 91 (1969) 1866. R. A. Kom, Ph.D. The&, Cornell University, January 1971. M. BARBERxw R. M. ELLIO~-~, 12th Annzml ASTM E-I4 Conference on Mass Spectrometry, Montreal, June 1964. F. W. MC-=, J. O~i~wofo, H. TSUYAMA, Y. NAKXJI~+IA,T. NODA AND H. W. MAJOR, Org. Mass Spectrom., 2 (1969) 751. M. L. GRR. B. FAZRWW~, W. F. HADDON, F. W. MCLAFFERTY hw H. W. MAJOR, 16th ANIuaI ASTM E-14 Conference OR Mass Spectrometry, Pittsburgh, Pa_, May 1968. J. H. B=xox_ W. E. BAITINGER, J. W. I\My A?? T. Ko~~u, Inr. J. Mass Specrrom. ion Physw, 3 (1969) 4??. R. W. Kmx, R. C. SULLIVAN ANI, M. S. LAPIN, Anal. Chem., 41 (1969) 4985. T. W. SOON, T. E. MEAD, C. G. WARNER AE~D F. W. McLmm, Anal. Chem., 39 (1967) 1748. K. H. MA-, C. BRIJNNEE,G. -PUS, K. -F_x~T, U. SCHRGDX ~~13 P. SCKULZE, 19th Anmd ASTM E-14 Conference on Mass Spectrometry, Atlanta, Ga., May 1971. R. VEXK~T_ARAGHAVAX, R. J. I(LLhfows~~ AND F. W. MCLAFFERTY, Accounts Chem. Res-, 3 (1970) 158. R. J. G~?.fows~r, R. VEXKATARAGHAVEN,F. W. McL -x-r .4m E. B. DELANY, Org. Mass Spectrom., 4 (1970) 17. R. VENICATARAGHAVAX,R. 3. KLLHOWSI~I, J. E. Cou-rtir~ ez F. W. MC LAmRru, 17th Annual ASTM E-14 Conference on Mass Specrrometry, Dallas, May 1969. P. SCHULZE AZ Ai L. B~RL.NGA~IE, J. Chem. Phys., 49 (1968) 4870. K. R. JENNINGS. Chem. Commun., (1966) 283. J. H. BEYNON, W. ET BAITINGER, I. W. A.. AND R. M. CAPRIOLI, Inf. J- Mass Specfrom. Ion Phys-, 3 (1969) 309. J. A. HIPPLE, R E. Fox AND E. V. C~NDON, Phys. ReD., 69 (1946) 347. H. W. MAJOR, Docket D-IISS, U.S. Patent Office, Washington, D. C. J. H. Bmox, AMI. Chem., 42 (January, 1970) 97A. F. W_ McLAIIERTy, R. VENKATX~GHAVAN, J. E. Covr~xr AND B. G. GIESSNER,Anal. Chem, 43 (1971) 967. M. B~~snr, K. R. JENNINGS AND R. RHODES, 2. Natur/orsch., 22a (1967) 15. T. W_ S-ON, F. W. MCLAFEER~ m C. R. MC-Y, Chem. Commun., (1966) 478. G. S. W~rsox drip L. lRAw=r, And Chem-, 42 (1970) 611. R. VEXKXTAR~GHAVAN, P. ROGERSON~LYDF. W_ McLi\m~ N, unpublished work. F. W. MCLAFFERTY, D. J. MC-, J. S. Sxrm APIP R. KORNEELD, J. Amer. Chem. Sot-, 93 (1971) 3720. 3. E. Corrr~m, Ph. D.Thesis, Cornell Ursversity, January 1971. F. W. MC-~, R. VEN~T~GHAVAN AND P. IRVING, Biochem. Biophys- Res. Commun., 39 (1970) 274. Int. J. Mass Specfrom. Ion Phys., 8 (1972) 323-339
Internationd Journal of Mass S’ctrometry and Ion Physics EisevierPublishingCompany, Amsterdam. Printedin the Netherlands
METASTABLE
ION CHARACTERIST1ti
XX. AUTOMATED ACQUiSlTLON OF METASTABLE ION DATA USiNG AN ON-LINE COMPUTER WITH FEEDBACK CONTROL*
JOHN E. COUTABT**
Depurtmenr
AND F. W. MCLAFFERTY
of ChemZstry, Cornell University,
Ithaca, N. Y. 14850 (U.S.A.)
ABSTRACT
A fuIIy-automated system incorporating a Nier-Johnson geometry mass spectrometerand an on-line computer designed originally to collect high-resolution data has been modi5ed to acquire and reduce metastableion data produced by the Barber-Elliott-Major defocusing technique. Factors affecting system performance have been identsed, and a variety of operating modes are proposed.
INlRODUCTIOS
Recognition of the analytical usefulnessof the so-called ‘metastable peaks7 in mass spectra has grown rapidly’ -4. Such a peak resultsfrom an ion decomposition occurring after the ion has passed through at least one, but not aII, of the electrostatic and magnetic fields between the ion source and the cohector. Thus the magnitudes of the fieIds necessaryto focus the daughter ion of the decomposition are characteristic of the mass (m) and charge (Q) of both the precursor ion and the daughter ion. This extra dimension of information has been sho& to be valuable for identifying ion decomposition pathways1-4, characterizing ion struttures’, classifying reaction transition state?*‘, studying decompositions of ions of defined internal ener&&s6*8* ’ (in&ding nove1reactions that are dif&mlt to observe under ordinary conditions”), and providing analytical and st~ctural information on mixturesl’. Ions undergoing me&stable decompositions are relatively longlived, and are thus of relatively low internal energy, in comparison to ions which l
Dedicatedto Prof. A. 0. C. Nier in honor of his &
Annual ASTM
E-14 Conference &
Mass Spectrometer
‘b&day. .Prt&&$ Dalks, Ma% 1969.
ti_ &t
** Taken from the Ph.D. Thesis of J. E Coutkt, Cornell University, Jan% DOW CmningCorporati,on,
Midland, Michigan.48604.
_.
Inc. J. Mass Spectkwn.
Ion Phys.,.g
+ the 17th
1971. Now -y
at the
(197$ 323-339
324
3.
E. COUTANT,
F. W.
MCLAFFERTY
decompose in the ionization chamber. Decompositions characteristic of higher energy ions can also be studied using this technique by introducing ca. 10m4 torr of an inert gas into a &Id free region of the ion path; the resulting collisioninduced decompositions are generally much more numerous than unimolecular metastabIes and represent the more simpte cleavage reactions’2-‘4. Utilization of me&stable ion decompositions has been handicapped by experimental problems. The defocusing technique proposed by Barber and Elliott” has greatiy improved the sensitivity and reliability of precursor and daughter ion identification; measurement of metastable abundances over a dyanamic rangel6 of l/10’ is possible utilizing Major ef aZ.‘s suggestion” of offsetting the electrostatic analyzer (ESA) potential on a doubIe-focusing mass spectrometer, utilizing the first metastable decomposition regionf6-18. However, with this technique the focusing of the daughter ion of a particuIar metastabie transition requires both a specific ESA” (or ion acceIerating potential (ref. 15)) setting and magnetic fieId vaIue; thus an extra dimension of measurement cperations is required to obtain the extra dimension of information postuIated above for metastabIe ion data. This probiem is especially acute if one wishes to determine al1 of the metastable transitions which can occur. To accomplish this Kiser and co-workers recommend’ g approximately 100 complete magnetic fiefd scans at 25volt increments of the ESA setting to find the metastable peaks, foliowed by rescanning at 0.2- or 03volt increments of the local ESA potentiai to locate these peaks exactly. 51 utilization of the defocusin, Q technique in a number of ~tudies~-~~ we have found that both data collection and reduction is extremely laborious. Data coIIection is greatly eFdited by using photop!ate recording” with instruments of Mattauch-Herzog geometry, but the complex data measurement and reduction methods necessary have limited the use of this method. With Mattauch-Herzog or nor-ma3 Nier-Johnson geometry, all metastabIe transitions producing a particular daughter ion can be detected by scanning the ion accelerating voltage using appropriate tied ESA and magnetic fieIds15*20. Using an instrument in which the ESA follows the magnetic anaiyzer, al1 metastable decompositions of a particuiar precursor ion can be measured by scanning the ESA potential using appropriate fixed ion accelerating and magnetic fieIds21_ The former has substantial drawbacksxg*20, and both require numerous scans (nearly one per spectral peak), each at a carefuiIy-determined vaIue of the magnetic field. Because the great utility of the dedicated on-Iine computer for high-resolution data22*23 has made it available on many double-focusing mass spectrometers, it seemed desirable to tid ways in which such a computer might be utilized to increase the speed, efficiency, and accu&y for acquisition and reduction of metastable ion d2ta by the defocusing technique. We sought first24 to develop a method to find all possible metasrable ion products by searching at al1 possible values of the magnetic and ESA fieIds; this couid be used to find unexpected products, such ht. L.Ma.ss Spctrom. Ion Phys., 8 (1972) 323-339
AUTOMATED
ACQUISITION
GF
METASTABLE
ION
DATA
325
as might occur from multiply-charged ions, in cohision-induced dissociations”-‘4, in other regions of the instrument18*25, or from multiple decompositions in consecutive field-free regions26n27. Of equal importance, such a system should make it possible to define much more completely the experimental system for defocused metastables, and thus provide the data necessary to design more efficient systems to accomphsh more specific objectives.
THEORETICAL.
Extensive treatments of the equations governing the focusing of ion products resulting from decompositions in field-free regions of a double-focusing mass spectrometer have been given18-20*25. In the Hitachi RMH-2 instrument used in this study there are three field-free regions: (1) between the ion source and ESA, (2) between the ESA and magnetic field, and (3) in the “bright” ion source’*. For the decomposition of an ion of mass m1 and charge q1 yielding an ion of mass nz2 and charge q2 plus a neutral mass of rno, rq1
ml
-_, m2
-+mo,
(1)
the apparent mass, m*/q*, of the daughter ion is represented2’ by
where V is the total accelerating potential and VI is the accelerating potential experienced by the precursor ion, m, , before decomposition. When VI = V, which is true except for the third field-free region, and q1 = q2 = 1, eqn. (2) reduces to the familiar relationship,
m*=-.
m2
(3)
ml
The equation for the transmission of a charged particle through an electric sector (ESA) is given by the balance of electrostatic and centrifugal forces:
r,qE
llllV2 -=-
2
2
where t‘ is the velocity, rC is the ESA radius, and E is the ESA field strength. The ESA field, I$, required to transmit any normal ion (one that has not undergone a metastable transition) is 2q’re. The following equation
Ed
=
2(-33 Int. J. Mass Spectrom. Ion Phys., 8 (1972) 323--339
326
J. E. COUTANT,
F. W. MCiAFkERTY
representsthe ESA field, E,; needed to transmit the daughter ion of a metastable transition. Foltomkg IGser et aZ_lgthe ratio Ed/E; is decked as p_ then
&$
=: r
‘1
for the nsua.lcase of qa = q2 = I. Since the field strength of the electric sector is directly proportional to the potential applied to the sector, the latter value will be used for E, or Ed. The Gombinationof eqns. (3) (or (2)) and (6) allows the de6nitive identification of 772,and m, if m* and p are known. The extra dimension added to a mass spectrumby metastabledecomposition infkmati~n is ifiustrated schematically by Fig. 1. (If shown, the other dimension, ion abundance,would be perpendicularto the plane of the paper). The hypothetical metastable transitions of Fig. 1 are given in TabIe 1. Note that the section through p = I is the focused mass spectrum (normal ions and metastable ions formed in the second field-fi-eeregion)_ . nomai
100
x 0
ions
focused matostables defocused matastables
80 t
60 -
t2 %
40
20 I.
PFig, 1. Schematic diagram showing the relationship of the ESA metastable transitions of Table I. TABLE
NO.
1 2 3 4 5 6
I
Precursor 100.0 80.0 60.0 _ 50.0 100.0 100.0
IIU. J. Mass SgeEtrom.
Daughter
P
m*
50.0 50.0 50.0 25.0 80.0 25.0
0.500 0.625 0.833 0.500 0.800 0.250
-25.00 31.25 41.67 12.50 64.00 6.25
Ion Phys., 8 (1972) 323-339
(p) and mass values for the
AUTOMATED
ACQUISITION
OF
METASTASLE
ION
DATA
327
Summing the ion intensities for alI mass vaiues at each p value gives the ion
kinetic energy spectrum proposed by Major?Q-30. Note that either of thesc twodimensional displays can produce ambiguities in assigning ml and m, for a metastable transition. For example, both transition 1 and 4 have p values of O-500, and the m* value of transition 1 has the same mass value as that of a normal ion at p = 1.000. AI1 metastabIe decompositions producing m2 will fall on the straight line described by m2 = m*/p: all decompositions 1721 =
(7) of m, will be described by the function
nr*lpz.
(8)
Note that the mass of the molecular ion thus limits the possible combinations of p and m* values for metastable decompositions involving singly-charged ions; no metastables will occur in the region to the left of the dashed curve of Fig. 1. A precursor ion which decomposes in the third field-free is focused at an n2*/4* value described by eqn. (2) and an ESA value” by Ed =2[(:)(+)
i- (1-;)(3].
(9)
The resulting metastable peak will therefore fall on the energy continuum described by eqn. (7) (solid lines, Fi g. 1) between the focused daughter ion and the defocused metastable ion. Its relative position between these two points is determined by the fraction VI/V of the total accelerating potential that the ion m1 receives before decomposition to ~2,.
EXPERIblENTAL
In this system the magnetic field is scanned slowly while the ESA potential is varied rapidly over all possible values of p (see Fig. I); the speed and ffexibility possible with the reverse procedure” is limited by the ability to controi and measure the magnetic field due to hysteresis and mass-marking di.Biculties. The Hitachi RMH-2 double-fmusing, high-resolution mass spectrometer used in this system has a modified Nier-Johnson geometry resulting in a long (589 mm) fieldfree region between the ion source and ESA which enhances the sensitivity for measurement of defocused metastables16~1‘_ Normal operating conditions were an electron emission of 125 PA, an accelerating potential of 9.6 kV, ESA potential of 730 V, and a source sample pressure of 1.0 x TOW6torr. The detector system consists of a 16 stage electron muhiplier, a wide bandpass d-c. amplifier, and a logarithmic scaling amplifier (Philbrick Model 4350). The. source exit slit and collector slit were set at their maximum value of 350 microns for met&tble mea- _ ht. J.- Mass Spectrom. Ion Phys., 8 (1972)
323-339
328
3. E_ COUTANT,
Fe W. MCLAFFERTY
surements resulting in an overall resolution of approximately M/H = 2,000. The beta-slit was set at 4 mm. y The computer used in this system is a PDP-8 (Digital Equipment Corporation, Maynard., Mass.) which has a 12-bit word length, 4K core size, two 32K random-access disc units, a high speed paper-tape punch and reader, the hardware multiply and divide option, and a high-speed line printer (Shepard, Model 880). The -mass spectrometer
F8akSignal
ESA __-_----A/D
I
I
Converter
I
Fig- 2. schematic -diagramof the computer-mass spectrometerinterfacefor the metastabledata co&ction system_ Znr.J. Mau Spcctiom. ion Phys., 8 (‘,972) 323-339
AUTOMATED
ACQUISITIOX
OF
METASTABLE
ION
DATA
329
In the operatin g mode used for the survey data reported here, the ramp generator is adjusted to give a ramp risetime of 2.0 set (0 to 1000 V at the output of the operational power supply) and a fall of 0.1 sec. The upper limit of the ramp is manually set to a value slightly greater than that necessary to focus normal ions (P = I.000). The lower Iimit is controlled by the computer, and the ramp direction may be changed at any point from down to up by the computer. Data is coIIected at a 20 kHz rate during the increasing ESA potentia1 scan by the computer with calculations and bookkeeping being performed on the fast flyback. The magnetic field is scanned at a rate to make possible a total of IO ESA scans through each mass peak. In this manner the entire three-dimensional space is recorded. Software
Direct processing of al1 of the programs invoIved in the metastab!e data system would require a core of about 16K. Operation in the 4K core was achieved through the use of modular software ‘swapped’ from the disc as required under program contro12’ -’ 4_ The five separate phases used are: Pha.w I - Dafa acquisition. This program is responsible for detecting and digitizing ESA-scan peaks, controlling the electric sector potential, and storing the compressed data on the disc. Before the scan is started the values of the A/D rate, the digital threshold, and the lower limit of the ESA scan are chosen and set into the computer. (The Iatter can be calcuIated from eqn. 8). The program then determines the slope of the ESA scanning ramp. After the magnetic scan is initiated and the start signaI given, the program triggers the ramp generator to start the ESA scan. The analog signal from the electron multiplier is directed to channel 0 of the A/D converter; when the signal exceeds the threshold value, it is continuously digitized at the A/D rate until four consecutive points below threshold are digitized. At this point the channel of the A/D converted is incremented to channel 1, w’hich reads the voltage on the electric sector, and on the next A/D pulse this vaIue is digitized and the input channel returned to channel 0. On the succeeding pulse the value of a 24-bit binary clock is read. III this way a digitized peak profile of an ESA peak is stored in core along with a digital ESA potential and time value. The process is repeated for any further peaks unti1 the ramp generator reaches its maximum vaIue on the increasing scan, where it reverses itseIf and interrupts the computer. The program then sorts through the data excIuding those peaks consisting of less than 10 digital points and stores the remaining data on the disc terminated by a fIag ‘-0 signify the completion of one-electric sector scan. The ESA potexitialvalues are sampIed until the Iower Iimit is reached, when the computer issues a signal which reverses the ramp direction from down to up, and then the cycle begins over again. After the stop pulse is received, the data can be optionally dumped on the teletype, line printer, plotter, or high speed punch Fig. 3 illustrates an actual data dump from a smaI1 portion of a spectrum. Int. J. Mass
S’ctrom.
Zon P&s.,
8 (1972)
323-339.
;
; : . .
\
‘,
AUToMATED
ACQUISITION
OF METASTABLE
ION
DATA
331
Phase II - ESA peak center calculation.This phase takes the raw data stored on the disc by Phase I, calculates the time and voltage centers and the intensity of peaks, and stores this information back on the disc as a four-word group (the values req-uire two, one and one words, respectively)_ The center is calculated by taking the average vaiue between two points that are at one-half the height of the maximum value. A flag separates each ESA scan. For routine use where data dumps are not needed the Phase I and Phase II programs are combined. The calculations of Phase 11 are performed during the ESA-scan fiyback, and the resuking compressed data stored on the disc. Phase III - ESA peak sorting. Each focused and defocused peak should have produced several ESA scan peaks occurring at essentially the same value of ESA potential but at increasing time (mass) values. The Phase IIf program creates six large tabIes in core and sorts each electric sector scan peak into its correct group. As these groups are compieted, they are written on the disc for use by Phase iv. Pase N - Time centroid calculation. The grouped mass peaks from Phase III are read from the disc and inspected to see if the peak is significant. Peaks with three or less ESA scans are ignored. The centroid of the remaining time peaks is then calculated by
Y (centroid)
b f (X)X& f = a b II
f(x)
(10)
dx
in which f(x) = amplitude at position (time value) x in the peak envelope. Since dx is actually Ax, the experimentally obtained centroid is a close approximation to that represented by Eqn. (10). The lower integral gives the area of each peak. The corresponding ESA value is taken as the average of each individual ESA scan peak potential. The time value is converted to seconds, and these values are stored on the disc. Phause V - Time-to-mass comersion ad ?n.ztastabletransitionassignment. At the beginning of this phase the program asks the operator to assign the nominal masses of two focused peaks and the average ESA potential vaIue of the focused peaks (thus defining p. = 1.OOO)from an inspection of the output of Phase IV. For operator convenience whole number mass values have been used here instead of exact nuclidic masses; the error involved is small. The program then dxtrapolates to assign mass values to the remaining focused peaks. For each defocused peak, precursor and daughter ion mass are calculated by the. use of the equations discussed below and displayed on the line printer, the high-speed punch, or the teletype. After inspection of the results, the operator may repeat Phase V changing the ESA or mass vahJes of the assigned peaks or the output device. ht. J. Mass Spectrom. ion P&s.,
8 (1972) 323-339
332
J_ Em COUTANT,
F. Wa MCLAFFERTY
RESULTS AN-D DISCUSSION
Peak shape and iocation
.
The illustrated data from the 85’ +
general form and relationships of the data acquired by Phase I are in Fig. 3. This is a computer-generated three-dimensional plot of raw n-deeane, including the focused and defocused metastabIe peaks for 57’ transition and the normal fragment peak at mass 38. The ESA scans show successive profiles of each p:ak which serve to define a voIume.reIated to the abundance of the peak. In these relative dimensions each peak appears as ridge. It can be seen that the center of each successive ESA profile increases slightly with increasing mass;. this is because the observed mass values, M*, are a direct function of p by eqn. (7) for normal as well as metastable ions3’. At the beta-slit in the Nier-Johnson configuration the width of the focused ion beam (p = 1.000) is only a small fraction of the beta-slit width Changing the ESA potential at P = l.ooO &-st sweeps the focused ion beam across the beta-slit, at the same time changing the magnetic fieId required to focus the ion beam according to eqn. (7). The ion ridge thus produced (Fig. 3) should exhibit a length which is dependent on the width of the beta-slit. Further, with optimal focusing, approximateIy the same number of ions will be in focus as the ion beam is scanned electrostaticahy across the beta-slit, so that the ion ridge should exhibit a flat top in its long dimension; this is approximately true for the nz/e 38 peak (p = l.ooO) of Fig. 3 within experimental error. Note that the width of the ion beam (as defined by the source and collector slits) determines the width, not length, of the ion ridge. This width is shown in the section through the ridge from the ESA scan at constant mass, or in the normal magnetic fieId scan at a constant v&e of the ESA potential, as both sections are at an angle to the long dimension of the ion ridge. Thus a “flat-topped” metastable will exhibit an ion ridge which is Sattened in width as well as length_ ‘T&se variations were noted by observing the raw data dumps of Phase I. The system does not indicate such changes in ridge width and length in the final report, although such things couId be noted as the data are available to the system. AIthough decreasing the beta-slit width decreases the observed width of a flat-topped metastable2 3 (the ridge width at the center of its length), this does not appreciably affect the width of a normal ion ridge31. The long dimension of the ion ridge defined by eqn. (7) is also the line on which aI metastabIes wiIl fd1 producing a common daughter ion (t;ide supra). This is illustrated in Fig. 3 by the small isotopic metastable 86+ --, 57 + at m* = 37.8 andp = 0.663 which faIIs on an extension of the ridge Iine of the 85+ * 57” metastable- The dispersion of the beta-slit is 4.0 mm for an ion beam energy change of 0.85 %18; in keeping with this, overIapping of the ends of ion ridges of metastables Ieading to the same. daughter ion occurs when the precursors differ in mass by approximately 0.85 %. This rtiolution
can be improved, of course, by narrowing
Int. J. iUas.rSpectm~. IOUPhys., 8 (1972) 323-239
AUTOMATED
ACQUISITION
OF METASTABLE
ION
333
DATA
the beta-slit. Note also the focusing at the beta-slit characteristic of the NierJohnson geometry makes possible a far superior resolution of such metastable transitions than that possible with Mattauch-Herzog type instruments3 3 ; such an instrument gave only marginal resolution of the 71 i + 43+ and 72+ -+ 43+ transitions2’. TABLE
2
MJZTASTABLE TR4XSITIOXS
iwlss
OF R-DECANE InreJzsiry
P
Found
Calc.”
Found
21.83 21.83 24.44 25.20 26.09 28.76 29.49 29.47 29.98 33.00 32.99 36.90 36.90 37.08 37.35 38.19 38.22 39.12 40.21 40.64 43.18 44.63 44.72 56.67 67.64 90.17
21.75 21.75 24.36 25.20 26.04 28.75 29.49 29.49 30.02 32.52 32.82 36.91a 36.87b 37.10 37.33 38.3P 35.22 39.09 43.02 40.65” 43.21 44.61 444.61 56.68 67.63 89.92
1.oooo 0.5047 0.5933 0.5990 0.6043 0.5029 1.0000 0.7187
0.7317 1.moo 0.5746 0.8566 0.8986 0.9510 0.6658 0.9822 0.6702 0.9551 0.9812 0.9917 0.7859 O.G276 1.0000 0.8214 0.6897 0.7937
Ca Ic.
0.5059 0.5942 0.6000
0.6056 0.5044 0.7193 0.7321 0.5758
0.9512 0.6667 0.6706 0.9535
0.7857 0.6283 0.8214 0.6901 0.7958
Calc. for drift region lst, 3rd
28 149 18 28 340 109 79 334 47 53 301 34 67 110 101 20 134 232 197 20 42 184 46 73 41 32
43.26 41.20 42.07 43.17 57.18
1st
85.72 69.43 70.24 71.45 113.69
41.00
57.05
40.96
55.98 1
57.41 43.08 41.07 38.98 56.09 38.88 57.02 40.95 40.97 40.98 54.94 71.11
99.90 45.70 40*99 84.23 39.59 85.07 42.88 41.76 41.32 69.91 113.29
68.99 98.08 113.60
83.98 142.20 143.12
50.29
3rd
-
71.08 57.02
40.90
-!43.21 41.95
-
Assigned c0lue.s
43 43 41 42 43 57 41 41 41 57 57 43 41 39 56 39 57 41 41 41 55 71 71 69 98 113
85 85 69 70 71 113 57 57 56 99 99 71 57 41 84 41 85 43 42’ 42 70 113 113 84 142 142
a Based on assigned values of ml and ml. b Based on decomposition in the third field-free drift region. E Apparently contains some contribution from the 43 + 41 transition occurring in the third field-free region.
The metastabie ions, both focused and defocused, found for n&cane using the described experimenta conditions are shown in Table 2, with the experimentahy determined mass, p value and an absohrte intensity vahre along with the calcuIated theoretical m* and p values for the observed transitions. In a total time of approximately two hours the system was able to detect and identify 18 metastable decompositions occurring in the first field-free drift region, plus each in the Int. J. Mass Specrrom. Ion Phys., 8 (1972) 323-339
-
J. E. COUTANT,
334
: : : :
Int_ J_ Maps Spectmrn:Ion
Phyx,
8 (1972) 323-339
F. W.
: ; :.
.
MCLAFFERTY
AUTOMATED
ACQUISITION
OF METASTABLE
ION
335
DATA
second and third regions; 103 transitions have been reported previously. A portion of those found here are shown as a computer-drawn three-dimensional display in Fig. 4. As further e_xamplesthe metastable transitions found by the system for 2hexanone-1,1,1,3,3-d and p-bromophenetole are shown in Tables 3 and 4. Table 4 also shows a collision-induced decomposition found for acetone. TABLE
3
raTm~i_~
IRU~SITIO~S
MaS.5
OF
Z-HEX,WO?CZ-1 , 1, 19 3, 3-d,
Intensity
P
Found
Calc.’
Found
Calc.
28.49 29.89 31.33 32.10 33.55 37.12 39.09 40.03
28.49
0.6939
0.6949
29.90 31.34 32.14 33.59 37.10 39.09 40.01
0.7103 0.7277 0.7131 0.7289 0.9516 0.9526 0.6782
0.7119 0.7281 0.7143 0.7302 0.9512 0.9535 0.6782
Assigned ualues
Calc.
12 151 61 68 159 23 20 108
mz
ml
m2
ml
41.05 42.07 43.05 45.02 46.03 39.01 41.03 59.02
59.15 59.23 59-15 63.12 63.14 40.99 43.07 87.03
41 42 43 45 46 39 41 59
59 59 59 63 63 41 43 87
a pBromopheneto1e. TABLE
4
M3ZT/,.Sl%BLE TRANSITIONS
OF Jl-BROMOlWEbETOLE
AND
ACETONE:
Assigned calues
IntenMass
Calc.
sity
P
Found
Calc.”
Found
Calc.
147.83’ 149.87’ 149.73= 31.92b
147.92 149.88 149.88 31.89
0.8590 l.OOXl 0.8602 0.7406
0.8600 0.8614 0.7414
229 24 227 123
mz
ml
m2
ml
172.09
200.34
172
200
174.06 43.09
[email protected] 58.18
174 43
202 58
a pBromop henetole. b Acetone with 2 x X0-+ torr argon in the 1st field-free drift region; ao rnetastable transition ions detected at 1 x lo-’ torr.
There are a number of reasons for the low proportion of themetastables of n-decane detected by the system_ A higher proportion is found with longer scan rimes, but the two hours required here is hardly practical in most cases. A data dump after Phase I of the program showed that a large number of the metastable transitions had been detected but were eliminated by the requirement of a minimum of 10 digital points in an ESA peak proGle plus a minimum of 4 ESA scans:over the me&stable ion ridge. These minima were patterned after those used in the highresoiution system’ 3; our present~experience shows that some requirements for the metastable system are quite different. Int. L
Mass
Spectrom.
Ion Phys.,
8 (1972) 323-339
3. E.
336
COUTANT,
F. W.
MCLAFFERTY
A critica difference is in the dynamic range of the signal intensities that it is desirable to record. The present system is scanning the normal as well as the me&stable peaks; the intensity of the largest normal peak is approximately 10’ times as large as the smallest cietectabie metastable peak16. Even with the logarithmic amplifier the 12-bit A/D converter provides a dynamic recording range of only 2 x I@. Somewhat lower abundance metastables were recorded in the data of TabIe 2 by increasing the gain to the point that large peaks saturated the ampiifier; their abundance values are thus subject to error. Sensitivity and abundance accuracy were aIso hampered by the necessity of setting a high threshold level to overcome the Iow signal/noise ratio of the commercial amplifier (actually that of the smailer Hitachi RMU-7) in the system. A 15bit A/D converter and/or an autoranging ampli6er3S are planned to aheviate this problem; the computer can also be used to set the multiplier gain through feedback control of the muhiplier high voltage supply (for exampie, the gain couid be reduced at p = 1.0). Since completion of this work an improvement of CQ. x 30 in sigual/noise level has been achieved by installation of a Kiethley 427 amplifier3’. A system for software thresholding also appears promising for improving the detection of small peaks. The data of Table 3 demonstrate the system’s ability to distinguish metastables from a gammon precursor ion_ In the spectrum of 2-hexanone-l,l,f,3,3-d, the m/e 59 and 63 pe&s should correspond to the ions CH&H2CH2CD,i and CD&(-O*H)CD,, respec\ively. The metastabIe transitions of the former thus represent mainly the losses of CHzD2, CH3D, and CH,, while those of the latter represent mainly the losses of CD,- and CHD,-36. The higher mass decompositions of p-bromophenetoIe (Table 4) were correctly identified. In the case of acetone (Table 4), no metastable was detected under unimolecular decomposition conditions; _=ising the pressure in the first field-free drift region to 2 x 10m5 torr with argon gave the decomposition corresponding to the loss of CH3 - . More extensive testing of this system3’ showed that nz, and m2 for the transitions are generally assigned with high reliability in the low mass region, and indicates the parameters which must be controlled to increase the reliability of all mass assignments. Note that the accuracy for the determination of m, will in general be lower than that for m,, as calculation of these involve p2 and p, respeotively (eqns. (7) and (8)). In general the accuracy of p value determinations was at ieast l/lo3 with careful assignment of the p = 1.000 value of the ESA potential. This is gratifying, as the linearity of the scale is assumed The accuracy of the determination of m* was generally within several parts per thousand. The most serious errors usuaI:y arose in mass regions with few normal ion peaks, which serve to calibrate the mass scale, or when the program mistook a normal metastable peak (p = LQOO) for a calibrant no-al ion (for example, Table 2, the 99+ + 57* transition, m* = 32.82). The need for a much larger number of calibrating peaks than that necessary in high-resolution measurements is due to the extremely slow- magnetic scan. In 43u_ 1. MizsrSpecfrom.
Ion Phys., 8 (1972) 323-339
AUTOMATED
ACQUISITION
OF METASTABLE
ION
DATA
337
addition, the length-wise flat-topped nature of the ion ridge reduces the accuracy of the centroid determination. We are presently developing software so that the metastable peaks will be located in a much faster magnetic scan; the scan speed will be set so that the length-wise flat region of the ion ridge will be crossed only once or twice. (This would reduce the scan time for the tlata of Table 2 and Fig. 3 to 15 to 20 minutes even without the other proposed impi-ovements.) The beta slit would then be reduced to eliminate the length-wise flat-top of the ion ridge without reducing the maximum height of the ridge at its center, and the magnetic field would be scanned again under computer control to find accurate p and m* values by rapid ESA scanning over the areas of the specific ridge locations identified in the preliminary scan. Despite the poor sensitivity of the present system, a few metastable decompositions occurring in the second and third field-free regions were detected. These can provide valuable evidence of the variation of the ion decomposition rate constants with internal energy ‘_ All of the defocused metastables occurring in the third field-free region should obey eqn. (7) (vide :,upra), so that the system will calculate the correct mass vaIue, m, , of the daughtr;r ion for these transitions. However the value for ml found by the system using eqn. (8) will be incorrect, as can be seen by the examples in Table 2. Eqn. (9) must be used to calculate m, ; this reduces to 2 m2 “11
= m*3 [
_
(“Zz--z*d
(11)
1 ’
R
%
in which R = VJV and m*3 is the apparent mass of the third field-free region metastable. Values of m, so calculated (Table 2) generally show comparable accuracy to m, values from first drift region metastables. In most cases it is possible to distinguish between metastabies formed in the first or third drift regions by inspection of the m, values calculated by eqns. (8) and (1 I); further, for each metastable formed in the third region the same transition usuaIIy produces a more abundant metastable in the first fieid-free drift region. Ambiguities can arise, however, from accidental overlapping of me&stables from these two regions. The poor agreement found for the 42+ --* 41’ transition in Table 2 may be due to interference of 43+ + 41+ metastables formed in the third field-free region. Alremative modes of operation A number of possible improvements to increase the scan speed, sensitivity, and accuracy are under investigation. Improvements discussed above include increased dynamic recording range, software threshold+, and an initial fast scan for ion ridge detection foliowed by a scan with narrowed beta slit under program control for exact measurement of m* and p values. For many applications only Int. 3. Mass
Spectrom.
ion Phys., 8 (1972) 323-339
338
J. E. COUTANT,
F. W. MCLAFFERTY
particular me’%tabIes might be of interest_ All o;‘ the decompositions of a particular ion can be found by scanning through the combination of m* and p vahres predicted by eqn. (8); similarly ail transitions leading to m2 are predicted by eqn. (7). Collision-induced decompositions in the field-free drift region have been proposed as a powerful tool for determining the amino acid sequences of mixtures of oligopeptides14*38; in this application during a magnetic scan the computer would direct the ESA scanning at only those combinations of m* and p values which correspond to transitions between sequence peaks (those arising by direct fragmentation of the peptide backbone) arising from the possxble sequences found by the computer from the previously-scanned high-resohrtion data. Mass data from such a preliminary scan should also provide a substantial improvement in the accuracy and convenience of calibrating the mass scaIe for any metastable data acquisition by the system_ We conclude that the same on-fine computer system which is used for highresolution mass spectral data can be applied for the acquisition and reduction of metastable ion data obtained from the same instrument by the defocusing technique_ The main modifications are ones necessary to improve sensitivity because of the unusuaI dynamic range of the metastable ion abundances so produced_ This and our reIated development3’ comprise the frrst examples of utilizing an on-line computer for feedback control of a magnetic mass spectrometer.
ACKNOWLEDGXENT
Drs. R Venkataraghavan and R. J. Klimowski provided substantial help on both the hardware and software aspects of this research. We are indebted to the National Institutes of Health, Grant GM-16609, for generous Gna&al support.
REFERENCES 1 J. H. BEYNOX, rUass Spectrometry a& its Appiication to Organ-c Chemistry, Eisevier, Hamsterc!an& I960. 2 F. W. MCLAFFERTY, rnterprerption of Mass Spectra, Benjamin, New York, 1966. 3 5. H. BEYSON, in E. %ZNDRICK (Editor), Adcances in Mass Spectrometry, Vol. 4, bs.i#e of ?etroIeum, London and Elsevier, Arnsterdaxn, 1968. 4 F. W. MC~SERTY, R. A. K~XFEW, D. 3. McADoo, 33. D. R. SCX-ITJDDEMAGEAKD 3. S. SXRH, in R OGATA AND T. &~YAKXWA (Editors), Recent Deuelopmenis in Mass Spectroscop_V, University Fark Press, Baltimore, 1970, p_ 70. 5 T- W. SRASTON AN~D F- W- MCLSFISTY, J_ Amer. Gem. Sot., SF (1966) 5021. 6 F- W. McLA~RTY &ND R. B. F.XIFLWGUISR, J. Amer. Chem. Sot., 90 (1968) 5915. 7 F. W. M~z-A-Y, T. WKHS, C_ LIFSHITZ,G. INNORTA AND P. IRVING, J- Amer. Chem. Sot., 92 (1970) 6867. 8 I.HowEANDF.W-Mw IT, J. Amer. Chem- Sot., 92 (1970) 3997. 9 L H0w-E A?? F. w- MCLAFFER~, .r- Amer. Gem- Sot-, 93 (1971) 99. IO F. W. MCLAFFEXIY, D. J- McADoo ATZD J_ S. Sm, .T. Amer. Chem. Sot-, 91 (1969) 5400. Int. J. Mass Spectrom. fan Phys., 8 (1972) 323-339
AUTOMATED 11 12 13 14 I5 16 I7 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
ACQUISITION
OF METASTABLE
ION
DATA
339
F. W. MCLAFFERlY AND T. BOYCE, C’henr.Commrm. (1967) 1215. W. F. HADD~N AND F. W. Mctim-ry, J. Amer. Chem. Sot_, 90 (1968) 4745. F. W. MCLAFFLRTY tim H. 23. R. SCHXJDDLK~GE, J. Amer. Chem. Sot., 91 (1969) 1866. R. A. Kom, Ph.D. The&, Cornell University, January 1971. M. BARBERxw R. M. ELLIO~-~, 12th Annzml ASTM E-I4 Conference on Mass Spectrometry, Montreal, June 1964. F. W. MC-=, J. O~i~wofo, H. TSUYAMA, Y. NAKXJI~+IA,T. NODA AND H. W. MAJOR, Org. Mass Spectrom., 2 (1969) 751. M. L. GRR. B. FAZRWW~, W. F. HADDON, F. W. MCLAFFERTY hw H. W. MAJOR, 16th ANIuaI ASTM E-14 Conference OR Mass Spectrometry, Pittsburgh, Pa_, May 1968. J. H. B=xox_ W. E. BAITINGER, J. W. I\My A?? T. Ko~~u, Inr. J. Mass Specrrom. ion Physw, 3 (1969) 4??. R. W. Kmx, R. C. SULLIVAN ANI, M. S. LAPIN, Anal. Chem., 41 (1969) 4985. T. W. SOON, T. E. MEAD, C. G. WARNER AE~D F. W. McLmm, Anal. Chem., 39 (1967) 1748. K. H. MA-, C. BRIJNNEE,G. -PUS, K. -F_x~T, U. SCHRGDX ~~13 P. SCKULZE, 19th Anmd ASTM E-14 Conference on Mass Spectrometry, Atlanta, Ga., May 1971. R. VEXK~T_ARAGHAVAX, R. J. I(LLhfows~~ AND F. W. MCLAFFERTY, Accounts Chem. Res-, 3 (1970) 158. R. J. G~?.fows~r, R. VEXKATARAGHAVEN,F. W. McL -x-r .4m E. B. DELANY, Org. Mass Spectrom., 4 (1970) 17. R. VENICATARAGHAVAX,R. 3. KLLHOWSI~I, J. E. Cou-rtir~ ez F. W. MC LAmRru, 17th Annual ASTM E-14 Conference on Mass Specrrometry, Dallas, May 1969. P. SCHULZE AZ Ai L. B~RL.NGA~IE, J. Chem. Phys., 49 (1968) 4870. K. R. JENNINGS. Chem. Commun., (1966) 283. J. H. BEYNON, W. ET BAITINGER, I. W. A.. AND R. M. CAPRIOLI, Inf. J- Mass Specfrom. Ion Phys-, 3 (1969) 309. J. A. HIPPLE, R E. Fox AND E. V. C~NDON, Phys. ReD., 69 (1946) 347. H. W. MAJOR, Docket D-IISS, U.S. Patent Office, Washington, D. C. J. H. Bmox, AMI. Chem., 42 (January, 1970) 97A. F. W_ McLAIIERTy, R. VENKATX~GHAVAN, J. E. Covr~xr AND B. G. GIESSNER,Anal. Chem, 43 (1971) 967. M. B~~snr, K. R. JENNINGS AND R. RHODES, 2. Natur/orsch., 22a (1967) 15. T. W_ S-ON, F. W. MCLAFEER~ m C. R. MC-Y, Chem. Commun., (1966) 478. G. S. W~rsox drip L. lRAw=r, And Chem-, 42 (1970) 611. R. VEXKXTAR~GHAVAN, P. ROGERSON~LYDF. W_ McLi\m~ N, unpublished work. F. W. MCLAFFERTY, D. J. MC-, J. S. Sxrm APIP R. KORNEELD, J. Amer. Chem. Sot-, 93 (1971) 3720. 3. E. Corrr~m, Ph. D.Thesis, Cornell Ursversity, January 1971. F. W. MC-~, R. VEN~T~GHAVAN AND P. IRVING, Biochem. Biophys- Res. Commun., 39 (1970) 274. Int. J. Mass Specfrom. Ion Phys., 8 (1972) 323-339