Computer-assisted ion cyclotron resonance appearance potential

Appearance Potential Measurements for C5H10 Isomers. Michael L. Gross and Charles L. Wilkins. Department of Chemistry, University of Nebraska, Lincoln...
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Computer-Assisted Ion Cyclotron Resonance Appearance Potential Measurements for C5HI,, Isomers Michael L. Gross and Charles L. Wilkins Department of’Chemistry, Uniwrsity of Nebraska, Lincoln, Neb. 68508

We have designed a simple computer-ion cyclotron resonance spectrometer interface which permits fast, accurate data acquisition as well as the rapid and precise determination of ionization and appearance potentials. In this paper we describe-the use of the new system for the measurement of ionization potentials of a variety of C5HI0 isomers and the appearance potentials for their two major fragment ions (C,H,’ and C,Hs+*). The purpose of the research described was to develop a method for the rapid, routine and accurate measurement of appearance potentials for both molecular and fragment ions. Additionally, we wished to provide on-line operator interaction during both the data acquisition and analysis stages of the experiment. The C5H1, compounds (virtually indistinguishable by conventional mass spectrometry) were chosen as a suitable test series to permit evaluation of the system. The appearance potentials thus obtained were reproducible within 0.1 eV and agreed well with previous values, in those cases where such values were available, thus verifying the achievement of our objectives.

As PART of a comprehensive program of study of ion-molecule reactions, we have designed and begun assembling an automated ion cyclotron resonance (ICR) Spectrometer. From the outset, it was clear to us that there were a number of different tasks we wished to perform with the system we designed. Accordingly, rather than build a piecemeal patchwork as the project developed, we felt it imperative to carefully analyze our problem in order that we might develop the system in an orderly fashion. First, we wished to provide computer controlled data acquisition capabilities. At the same time, it was important to maintain all of the original capabilities of the ion cyclotron resonance spectrometer. In other words, computer control could not be permitted to interfere with noncomputer controlled operation of the spectrometer. We also felt it very important to provide for operator interaction during the data acquisition stage of the experiment. A second objective was to permit rapid and immediate computer-aided analysis of the data thus acquired. It was also important to permit operator interaction in this part of the experiment, since if this were not possible, programming would be very much more complex and the results might well be less satisfactory. Finally, we wanted to provide, ultimately, for computer control of all of the instrumental operating parameters. We intend to use the system thus evolved to study a variety of properties of gaseous molecules and ions. One property which is of particular interest to chemists is the appearance potential, the amount of energy required to produce a new ion from a molecule or another ion. We have chosen to use the data acquisition system developed in the first step of our project to investigate the possibility of measuring this property accurately and rapidly by ICR spectrometry. In this paper we report our first results. A major problem in the investigation of isomeric olefins by conventional mass spectral techniques is the frequent occurrence of .skeletal rearrangements and hydrogen randomization. These problems are so severe as to preclude the use of this method as an analytical technique for the analysis of such 1624

isomers. For example the isomeric butenes have been shown to undergo extensive skeletal rearrangements and the isomeric pentenes have similarly been demonstrated to undergo 1,2hydrogen rearrangements ( I , 2). These rearrangements give rise to either a common molecular ion or a common mixture of molecular ions (arising from rearrangements) which then fragment to give identical mass spectra. Obviously, in these cases, it is impossible to use the fragmentation pattern to reflect the original structure. Determination of appearance potentials provides an alternative approach. In general, a unique set of such potentials should exist for any compound which fragments in the mass Spectrometer. This has, in fact, been suggested as a valuable technique for such analytical problems ( 3 , 4 ) . This idea, although a good one, has not been extensively exploited because of the tediousness of such measurements. At the outset, we believed it would be possible t o develop a computer-assisted technique to allow the use of ICR in a way which would circumvent these difficulties. An isomeric group of compounds (C5Hlo) was selected as an ideal test of the usefulness of our method. It was our hope that this would prove to be a worthwhile means of distinguishing isomers. Conventional mass spectrometry produces identical spectra (5)for m o 9 of the CsHIocompounds, and thus identification is difficult. Others have recently reported an on-line acquisition technique for determination of ionization efficiency data (6). According to these workers, their method was to use a conventional mass spectrometer and to take a very large number of samples of a mass spectral peak intensity while varying the ionizing voltage. Multiple scans of this type were then averaged to give the required taw data. Following this data acquisition phase, a further mathematical analysis of the data yielded values for ionization potentials which compared favorably with those determined by photoionization. The technique we have developed differs in a number of important ways from that just mentioned. First, we control the peak intensity sampling rate by means of a precise real-time clock which is part of the computer system. This allows us to vary our sampling rate until the optimum time-sample relationship is achieved. Second, as the data are collected, a real-time display is produced. This permits the experimenter to monitor the progress of the experiment as it continues and to determine at any time that it is either adequate or should be aborted. The use of an integrating analog-to-digital converter makes unnecessary the averaging process described by the (1) G. G. Meisels, J. Y . Park, and B. G. Giessner, J . Amer. Chem. SOC.,91, 1555 (1969). (2) B. J. Millard and D. F. Shaw, J . Chem. SOC.( B ) , 1966,664. (3) R. E. Winters and J. H. Collins, J . Amer. Chem. SOC.,90, 1235 (1968). (4) R. E. Winters and J. H. Collins, Org. Muss Spectrom., 2, 299 (1969). ( 5 ) J. H. Ekynon, R , A. Saunders, and A. E. Williams, “The Mass

Spectra of Organic Molecules,” Elsevier, Amsterdam, The Netherlands, 1968, p 65. (6) R. A. W. Johnstone, F. A. Mellon, and S . D. Ward, Int. J . Mass Spectrom. Ion Phys., 5, 241 (1970).

ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971

previous workers. In our system, data reduction is performed immediately after the data acquisition phase and the results are available no more than one minute after completion of data acquisition. This is very important as it allows the researcher to immediately determine whether more experiments need t o be run or whether the data just gathered are satisfactory. The fast pPper tape punch may be used as a means of storage of all data collected. This is, however, optional and may be selected as one of the possible choices of output format by the experimenter. Finally, since we were not interested so much in the fine structure of the ionization efficiency curves, but rather in a rapid technique which will allow accurate measurements of differences in appearance potentials, the less accurate semilog plot method (7) was satisfactory. Additionally, since we wanted to use a minimal computer system and attain maximum data reduction speed, the more complicated electron distribution difference procedure (8) was not indicated. EXPERIMENTAL

Apparatus. A Varian ICR-9 Ion Cyclotron Resonance Spectrometer (9) was interfaced with a 4K 620/i 16-bit word computer equipped with a real-time clock, priority interrupts, a n ASR-33 teletype, and a high-speed paper tape IjO system. In addition, a Varian IjO module containing a 14-bit 4channel multiplexed analog-to-digital converter and two 10-bit digital-to-analog converters was included. A Tektronics 611 Storage Oscilloscope was interfaced to the computer and was used for display purposes. An alternative display was provided by the X-Y recorder included as part of the ICR spectromtter system. This could also be used under computer control. Interface hardware was a precise voltage divider (for monitoring ionizing potential) and a linear amplifier (for detector observation). The ICR spectrometer was equipped with a standard three-section “flat” cell of cross section 1.1 X 2.5 cm. Single resonance spectra could be obtained in either a field-modulated or drift voltage-modulated mode. The latter was chosen for the measurements reported here because a nonderivative peak shape was desired. The instrument was also equipped with a dual inlet system which permitted rapid switching from sample to standard. Pressures, measured Torr for all measureby the ion pump, were ca. 1 X ments reported. Reagents. All the C5Hl0isomers were Phillips Petroleum Research Grade and were degassed and used without further purification. The xenon used as a standard was Matheson Research Grade and was used without further purification. Method. Owing to the need to use the minimal computer configuration available, all programming was done using the Varian 620ji assembly language. Provisions were made for storage of 250 sets of eV, peak intensity readings for both a standard (xenon in these experiments) and an unknown compound. Included in the program were a number of subroutines to permit operator interaction a t each stage of the experiment. The overall logic is diagrammed in Figure 1. The use of the program is best discussed by tracing through what the actual sequence of events is during the performance of a n experiment. The operator first loads the computer program and then introduces the standard sample into the ICR sample chamber. He then focuses o n the ion of interest (in the case of xenon, m / e 129) and sets up the spectrometer, by means of adjustments on the control console, for optimum conditions. He then initiates execution of (7) R . E. Honig, J . Chem. Phys., 16, 105 (1945). (8) R. E. Winters and J. H. Collins, ibid., 45, 1931 (1966). (9) For a description of the ICR spectrometer, see J. L. Beauchamp, L. R . Anders, and J. D. Baldeschwieler,J. Amer. Chem. SOC.,89, 4569 (1967).

the program. By means of a simple dialogue (short messages from the computer and short responses from the experimenter via teletype) the course of the experiment is determined. Samples of both ionizing voltage and peak intensity are taken (for this experiment) at 40-millisecond intervals. As each group of four such measurements for either one or the other of these parameters is accumulated, the group is averaged and the average stored as a single point. This process continues until a total of one thousand such points have been sampled (i.e., 250 pairs of data points have been stored). At this time, the computer causes the printing of a message asking the operator if he wishes a semilog plot. He indicates his desire by simply typing a “Y” or an “N”. If he types the character “N”, another message is printed, asking him if he wishes to have the data punched on paper tape for permanent storage. If he answers “Y” to this question the paper tape containing all digital values for the run, as well as the gain settings at which they were measured, is produced. The program then recycles to the beginning. A new run may be initiated at this time. Now, let us return to the semilog plot option, which is also available to the experimenter. If after examining the data which were plotted on the storage oscilloscope, as it was collected, the researcher decides it was adequate, he may answer “Y” to the question of whether he desires a plot of the transformed data. A similar computer-operator dialogue yields instructions for the next phase of the experiment (data reduction) and permits software gain selection for the maximum peak measurement (arbitrarily chosen as the intensity at 30 eV). The software allows selection of gains of 2O through 2’. Since gain controls are also available on the control console of the spectrometer, more dynamic range was not required. The need for this scaling is, of course, introduced by the dynamic range requirements of the experiment. At 30 eV, the peak intensity may well be 10,000 times that of the intensity over the range we perform the data collection. In order to provide a simple interface, we felt the scaling for plotting purposes should be handled by the software. Data reduction and plotting follow immediately. The reduction involves taking the logarithm of the ratio of each individual measurement to this maximum and correlating that with the ionizing potential for the individual intensity. Once the gain factor has been entered, the semilogarithmic plot appears immediately upon the storage oscilloscope. Following the plotting of the transformed data, a cursor appears o n the screen. This cursor is generated using the write-through function of the oscilloscope, so that its position is not stored and consequently, when it is moved, no trace is left a t the previous position. If both a standard and unknown had now been plotted, the operator would simply move the cursor (using teletype commands) to the corresponding points on the two lines and have their coordinates printed out. The difference between the coordinates can then be easily transformed, by consulting a simple look-up table, into the difference in appearance potentials. As an additional option, the operator may, once he has performed one standard measurement, replot that standard measurement a t any time by entering the program at the appropriate place. This obviates the need for multiple redeterminations of the standard compound. Ordinarily, we perform the measurement before each unknown. We find, however, that this is unnecessary since the results of the standard run are usually invariant. The logic of the program is diagrammed in Figure 1. The program is so constructed that when a new unknown is measured, it uses the data storage area previously allocated to the last unknown. This was necessitated by the memory size limitation. Similarly, if a new measurement of the standard is performed, the new data simply replace the old. When data are output to paper tape for storage purposes, a complete set of both standard and unknown data for the particular run are recorded on the

ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971

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of the complete dialog for a run such as that we have just described. The operator responses are underlined. The computer messages are the other portions. It is worthwhile to note that we purposely made the messages minimal since lengthy messages rapidly become redundant to operators who use the program often. Some redundancy was retained so that people unfamiliar with the program may also use it effectively. The next section of the paper will describe some representative results collected using this program and interface.

OUTPUT START MESSAGE

I

COLLECT UNKNOWN

RESULTS AND DISCUSSION

YES

1

COLLECT STANDARD OATA

PAPER

OUTPUT

YES OUTPUT MESSAGE

YYES

W

Figure 1. Program block diagram for determination of appearance potentials same tape. This makes recomputation an extremely easy process, since all required data for both the particular standard and the unknown are available. We think this option is especially useful since it will allow the performance of more refined computations, should we wish to d o so, at some future time. Although it is not necessary for the successful use of the program, we intend to routinely produce such tapes for just that reason. This procedure takes 50 seconds. The data sampling portions of the program are written in such a way that automatic gain selection is made and, consequently, we may measure voltages with a full scale of 10 volts, 1.25 volts, 150 millivolts, or 20 millivolts. The correct range is selected by the program, and stored with the data, using the upper two bits of the data word to indicate the gain which was used for the particular sample. When the plotting sequence is entered, this gain information is used to provide appropriate scaling for each sample. As can be seen from this brief discussion, the operator plays an important part a t all stages of the experiment. While we could have programmed the computer to d o virtually everything the operator does, we felt the interactive approach was superior. The actual time elapsed from beginning to end of an experiment is generally under two minutes. F o r illustrative purposes, we have included in Figure 2 a copy INITIALIZE ICR BEGIN SCAN TYPE S I F STANDARD, U IF UNKNOWN ULOG PLOT? Y O R N

7 S FOR

STANDARD, U FOR UNKNOWN USET ICR TO 30 EV TYPE R WHEN READY R G ?-7 Figure 2. Complete computer/operator dialog for data acquisition and semilog plot 1626

0

ANALYTICAL CHEMISTRY, VOL. 43,

Returning to mass spectrometry as an analytical technique, as we mentioned, problems are introduced by the tendency of olefins t o undergo rearrangements in the conventional mass spectrometer. There is, however, another type of information available (besides fragmentation patterns) which can be used to advantage to distinguish isomeric compounds. That information is the appearance potential of molecular and fragment ions. There are a number of reasons why we would prefer to make this measurement with the ion cyclotron resonance spectrometer, rather than with the conventional mass spectrometer. For one thing, ions in the conventional mass spectrometer are formed in electrical “draw out” fields (of the order of a few volts/centimeter. These fields cause a spread in electron kinetic energies. Also, residence times of the ions in the source region are rather short due to large accelerating potentials. This latter effect gives rise to the phenomenon known as “kinetic shift” (IO),the excess internal energy added to raise the rate constant of fragmentation t o the point where it may occur with sufficient speed t o take place before the ion leaves the source of the spectrometer. The low residence times necessitated by the design of the conventional mass spectrometer require rather large rate constants for fragmentation, if such fragmentation is to be observed. Consequently, appearance potentials determined using this type of spectrometer and the semilogarithmic technique (7) we use, can be higher than their actual values. The differences between the measured and actual value for these quantities are the “kinetic shifts”. These “kinetic shifts” may be as high as a few electron volts (11) and seem to be common in fragmentation reactions which involve rearrangements (12). In contrast, the ion cyclotron resonance spectrometer has minimal electric drift fields (of the order of 0.2 Vjcm) present at the ion source and rather long ion residence times (on the order of milliseconds). This permits “kinetic shift” to be minimized in determinations of appearance potentials. There are a few disadvantages in using ICR for this purpose, among them being the relatively limited mass range (ca. m/e 1-200) and the need for volatility of sample compounds. However, the latter restriction applies to conventional mass spectrometry as well. If one studies a set of isomeric hydrocarbons in this way, he might expect to find differences in appearance potentials arising from a number of sources. First, the ionization potential should reflect differences in the substitution of double bonds and would also be expected to dramatically reflect the difference between olefins and isomeric saturated compounds (13, (10) W. A. Chupka, J . Chem. Phys., 30, 191 (1959).

Vestal in “Fundamental Processes in Radiation Chemistry,” P. Ausloos, Ed., Interscience, New York, N. Y . , 1968,

(1 1) M. L.

p 96. (12) P. Brown, Org. Mass Spectrom., 3, 1175 (1970) and references

cited therein. (13) R. E. Honig, J . Chem. Phys., 16, 105 (1948).

NO. 12, OCTOBER 1971

Table I. Appearance and Ionization Potentials for C5Hlo Isomers. Compound

IP

A(42+' )

A(55+)

10.91 i 0.07 (10.53)b 11.36 f 0.08 (12.1)d 11.74f 0.07(11.4)d Cyclopentane 9.82 f 0.06(9.50)b 11.35 f 0.07 11.61 f 0.08 1-Pentene 11.54 f 0.10 9.60 i 0.03 (9.51)b 11.15 f 0.12 >Methyl- 1-butene 11.34 f 0.07 11.66 f 0.06 2-Methyl-1-butene 9.35 f 0.08(9.12)b 9.32 0.03 (9.06)c 11.35 f 0.03 11.73 f 0.11 rrans-2-Pentene 9.23 f 0.02(9.11)c 11.24 f 0.02 11.54 f 0.02 cis-2-Pentene 8.83 f 0.11 (8.67)* 11.33 f 0.12 11.70 f 0.11 2-Methyl-2-butene The deviations are standard deviations of triplicate determinations, each measured on a different day. K. Watanabe, T. Nakayama, and J. Mottl, J . Quant. Spectros. Radiat. Transfer, 2, 369 (1962) as cited in J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Draxl, and F. H. Field, Nut. Sfand. Ref. Data Ser., Nar. Bur. Srand., 26, 55 (1969). c J. Collin and F. P. Lossing, J. Amer. Chem. Soc., 81, 2064 (1959) as cited in Nut. Sraird. Ref. Data Ser., Nut. Bur. Stand., 26, 55 (1969). d J. Hissel, Bull. SOC.Roy. Sci. Liege, 21, 457 (1952).

*

0

14). Additionally, it should be possible to distinguish cis and trans isomers o n the basis of their ionization potentials. Intuitively, one might predict the cis isomers would possess lower ionization potentials. The reason is that the heat of formation of a neutral cis olefin will, in general, be higher than the heat of formation of the corresponding trans isomer. If a common molecular ion is now formed, it will be formed more readily from the cis neutral. A large difference would not be expected for cis- and trans-2-pentene, since their neutral heats of formation differ only by 0.9 Kcal/ mole (15). Let us now turn to a consideration of the data we have collected t o see how they bear out these original assumptions. Those data are tabulated in Table I. Considering first the molecular ions, we find that cyclopentane, as expected, has a much higher appearance potential than any of the olefinic substances. Furthermore, there is a rather clear-cut trend among the olefins we studied. The less substituted the olefin, the higher its appearance potential. So, the monosubstituted olefins (1-pentene and 3-methyl-lbutene) show appearance potentials between 9.6 and 9.8 eV. The three disubstituted olefins (2-methyl-l-butene, cis-, and trans-2-pentene) show intermediate ionization potentials between 9.2 and 9.4 eV, and the one trisubstituted olefin (2methyl-2-butene) shows the lowest ionization potential of the entire series. Finally, with respect t o the question of cisjtrans isomers, we d o note a difference (albeit it small) between the 2-pentene pair. Turning now t o the fragment ions (m/e 5 5 , C4H7+and m/e 42, c3&+') a consideration of Figure 3 and Equations 1-3 will facilitate the discussion. As Meyerson in a recent review noted (16), when fragmentation of isomers leads to a common ion, it is expected that the differences in appearance potentials for formation of that ion will simply reflect the differences in the heats of formation of the respective neutral species. This appears t o be the case in our study as well as in a number of those cited by Meyerson. An initial ionization, to form the molecular ion, takes place and subsequently that ion rearranges to some common isomeric molecular ion (of unknown structure) which then fragments. The fragment ions found at mje 55 and mje 42 are then common regardless of the source. If this picture of ionization and fragmentation is correct, it then becomes possible to define quantitatively certain relationships between heats of formation and appearance

n

potentials. Equations 1 and 2 show the relationship between the appearance potentials of the C3H6 and C4H7 ions and the heats of formation of relevant species. As may readily be seen, if the same CsH6and C4H7ions are formed in all cases, the heat A(CsHe+')

=

AH/(C3He+') f AH/(C2H4)

(1)

A(CdH,+)

=

AH/(CaH?+)

+ AH/(CH3') - AHXNeutral) (2)

of formation for this ion will be constant. The heats of formation of neutral ethylene and methyl radical are, of course, constant. In both of these equations, therefore, we see the only quantity which might be expected t o vary from isomer to isomer is the heat of formation of the neutral species and, the more negative the heat of formation of the neutral species, the higher the appearance potential. In general, the expected order of appearance potentials within any series can be predicted provided two conditions are met. First, members must fragment to yield common products and second, the heat of formation of the appropriate neutrals must be known. The general relationship is expressed by Equation 3. A(R+) = ZAH/(Fragments)

(14) J. Collin and F. P. Lossing, J. Amer. Chem. Soc., 81, 2064 (1959). (15) C . C. Browne and F. D. Rossini, J . Phys. Chem., 64, 1685 ( 1960). (16) S. Meyerson, Org. Mass Specfrom., 1, 659 (1968).

- ANXNeutral)

- AH/(Neutral)

(3)

We have utilized the appearance potentials measured in this study t o calculate the heats of formation of the gaseous product ions. If the conditions are satisfied, we should obtain the same value for these ions in all cases. We find good agree-

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* 1627

Table 11. Heats of Formation AH/(C,H,+)' AHXCaHs+)b Compound Kcal/rnole Kcal/rnole Cyclopentane 21 1 240 1-Pentene 224 25 1 3-Methyl-1-butene 217 247 trans-2-Pentene 22 1 249 cis-2-Pentene 219 247 2-Methyl-1-butene 219 246 2-Methyl-2-butene 217 247 Average 218 f 4 247 f 3 Lit. = n-C4Hi+, 203 kcal/mola; c-C4H7+, 213 kcal/molb; cC3H4CHa+, 221 kcal/molr. Lit. = C3Hs+ (propene) 229 kcallmold; C3&+ (cyclopropane) 245 kcal/moL. F. H. Field and J. L. Franklin, "Electron Impact Phenomena and the Properties of Gaseous Ions," Academic Press, New York, N. Y.,1958. * R. F. Pottie, A. G . Harrison, and F. P. Losing, J . Amer. Chem. SOC.,83, 3204 (1961). c P. Natalis and J. Laune, Bull. SOC. Cliim. Beiges, 73, 944 (1964). d J. A. R. Samson, F. F. Marmo, and K. Watanabe, J . Chem. Pkys., 36, 783 (1962). e K. Watanabe, T. Nakayama, and J. Mottl, J . Quant. Spectros. Radiaf. Transfer, 2, 369 (1962).

lar ion or the C4H7ion. In such cases, an extrapolated differences technique for reducing the data might be more appropriate. In any case, the present effect arises from the fact that the C3H6 ion is ca. '13 X the abundance of the C4H7ion. Second, the formation of the C3Hs ion is in competition with the energetically more facile formation of C4H7+. Thus, fragmentation to form C3H6+' migh: !-e required to take place from higher energy states of the molecular ion. Whatever the objections, however, the data in Table I show that in most of the cases a unique set of appearance potentials is obtained for the isomeric CsHl0's. If an unknown C6Hlo isomer were subjected to this kind of analysis, the list of possibilities could rapidly be reduced to no more than two isomers for correct identification.

Q

ment with the general equation and the values for the heats of' formation of the product ions are within our experimental error (Table 11). A final comment is in order, with respect to the apparent agreement between the heats of formation of the C3H6 fragment ion and the heat of formation of the cyclopropane molecular ion. A recent ICR study of the structure of C3H6+' showed that the ions produced from cyclopentane and l-pentene do not possess the structure of ionized cyclopropane (17). The discrepancy between that study and the present one disappears with the realization that the heat of formation of the CaH6 fragment ion is probably too high for a number of reasons. First, in the case of the olefins here studied, the semilog plot of the ionization efficiency curves for C3H6+' were not strictly parallel with the parallel curves for xenon, the molecu(17) M. L. Gross and F. W. McLafferty, J . Amer. Chem. SOC., 93, 1267 (1971).

1628

CONCLUSIONS

We believe these results clearly demonstrate the achievement of our objective of fabricating a fast reliable system for the ICR measurement of reasonably accurate appearance potentials. Certainly a similar approach should be workable with the conventional mass spectrometer as well. Such measurements can now be used in a routine fashion, since it is no longer the tedious time consuming task it once was. Further, it has been demonstrated that information of this type is a valuable adjunct to the interpretation of conventional mass spectral data, especially in those cases where differences are small and results puzzling. ACKNOWLEDGMENT

The authors thank Joseph Norbeck for valuable technical assistance. RECEIVED for review April 2, 1971. Accepted July 1, 1971. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 3, 1971. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this research (Grants No. 1314-G1 and 1550-G). We thank E. I. DuPont de Nemours and Company for donation of the storage oscilloscope and the National Science Foundation for providing funds for purchase of the ion cyclotron resonance spectrometer (Grant GU-2054) and the computer system (Grant GP-18383).

ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971

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