J. Phys. Chem. 1982, 86, 5115-5123
experiments imply that these reactions proceed through a thermal mechanism similar to that which is operative in the pyrolysis chamber, and the effect of using a focused or unfocused laser beam and various laser powers is the same as using different temperatures. The production of TMDSCB from DMSCB and the production trimethylchlorosilane from HC1 and DMSCB or TMDSCB can be explained on the basis of the formation of a dimethylsilaethylene intermediate as proposed previously. TMDSCB is formed from the dimerization of the intermediate, and the HCl can add across the “double bond” of the intermediate in a fashion which is analogous to Markownikoff s rule of additions to alkenes. The exact mechanisms for HC1 addition to DMSCB and TMDSCB may be more complicated. It is possible that the initiation step in each case involves direct reaction of HCI with the ring compound to produce a complex which subsequently and very rapidly eliminates an appropriate segment. Such a mechanism of course would preclude the existence of a silaethylene intermediate in the case of the DMSCB/HCl reaction, and this seems unlikely in view of the previous data for the decomposition of DMSCB. Additionally, the optimum temperature for the DMSCB/HCl reaction is similar to that for the simple decomposition of DMSCB, so it is likely that silaethylene species are available to react with HC1. For TMDSCB, there has been only one previous pyrolysis study in which it was claimed that a silethylene intermediate waa observed.l’ Therefore, it is quite possible that a two-step mechanism (the secnd step of which would involve a reaction of HC1 and (CH&3i=CH2) is operative for this system. Further work is necessary in order to resolve the uncertainties in the reaction mechanism. At elevated temperatures an additional mechanism becomes important in all the reactions discussed above. The production of methane and acetylene can be a significant problem if the temperature in the pyrolysis chamber is raised above 550 OC or if the laser power density is increased. This behavior suggests that multiple pathways are available in these reactions. A thorough investigation of the various reaction pathways available for the unimolecular decomposition of cyclobutyl chloride under conditions of multiple infrared photon absorption has recently been reported by Francisco and Steinfeld.23 The lowest
5115
energy pathway was one in which 1,3-butadiene and HC1 were produced, and the next most likely pathway was one in which ethylene and vinyl chloride were formed. The latter of these is analogous to one in which the silaethylene intermediate would be produced in our reaction system, and the former suggests an interesting alternative mechanism which could account for the formation of the methane. The cleavage of a Si-C(methy1) bond can produce a methyl radical and a silacyclobutyl radical. Abstraction of a proton by each of these radicals could produce methane and 1-methyl-1-silacyclobutane. Alternatively, the silacyclobutyl radical could rearrange to form a sila-1,3-butadiene. Whether this conversion proceeds through a silacyclobutene is unclear but certainly possible. The intermediate is probably not a l-sila-l,3-butadiene since such a species should be expected to form a stable l-silacyclobut-2-ene.2 It is more likely that a 2-sila-1,3butadiene is formed and dimerizes to a vinyl-substituted 1,3-disilacyclobutane such as the one described by Bertrand et At the high temperatures of the reaction systems, it is possible that a further reaction of 1,3-dimethyl-l,3-divinyl-l,3-disilacyclobutane produces acetylene and 1,3-dimethyl-1,3-disilacyclobutane. This mechanism can account for the formation of both methane and acetylene and the appearance of Si-H-containing compounds as the temperatures of the reactions are raised beyond the optimum temperatures for the production of the “simpler” silaethylene intermediate. These results are still in preliminary stages, and further work aimed at unraveling this complex problem is currently underway.
Acknowledgment. The authors acknowledge support by the donors of the Petroleum Research Fund administered by the American Chemical Society. We are also grateful to Dr. James A. Merritt for providing access to the infrared laser and the Digilab FTS-BOB. Acknowledgment is also extended to the Mississippi Imported Fire Ant Authority for the funds to purchase the Nicolet and Spex instruments. ~~~~
~
~
(23) J. S. Francisco and J. I. Steinfeld, Int. J. Chem. Kinet., 13, 615 (1981). (24) G. Bertrand, G. Manuel, and P. Mazerolles, Tetrahedron Lett., 2149 (1978).
Fast Atom Bombardment Study of Glycerol: Mass Spectra and Radiation Chemistry F. H. Field The Rockefeller Universlty, New York, New York 10021 (Received: June 78, 1982)
The 5-keV positive Ar fast atom bombardment (FAB) spectrum of glycerol changes markedly as the irradiation proceeds. The (M + 1V ion and its association complexes disappear and new ions am formed. The irradiation produces crystalline substances from the glycerol. OH- negative chemical ionization analysis provides further evidence for the formation of products from the glycerol, which can be rationalized by a free-radicalmechanism. The radiation yield has been semiquantitativelymeasured. Approximately 100 molecules of products are formed per incident argon atom. This comprises an interesting radiation chemistry, which presumably involves a non-Franck-Condon initial excitation.
Fast atom bombardment mass spectrometry’ is a valuable new technique for obtaining spectra of involatile 0022-3654/82/2086-5115$01.25/0
and/or thermally sensitive compounds. The experimental procedure currently in use is t c dissolve the sample of 0 1982 American Chemical Society
5118
The Journal of Physical Chemistty, Vol. 86, No. 26, 1982
interest in glycerol and to apply the resulting solution to the surface of a metal probe which is inserted into the mass spectrometer. The solution is bombarded with energetic atoms produced in an appropriate gun, and the secondary ions resulting from this bombardment are mass analyzed and detected. The use of glycerol is found empirically to give enhanced sensitivity. The rationalization of the effect of the glycerol is that the solution presents a mobile, constantly renewed surface to the bombarding beam, which provides for a continuous replenishment of sample molecules to be ionized. No detailed information has been published concerning the FAB spectrum of glycerol itself and the effect on the glycerol of the irradiation involved in producing spectra. Because of the practical importance of glycerol in this technique and its widespread utilization, we have undertaken a FAB study of the compound. A meaningful amount of radiation chemical reaction occurs.
Experimental Section The measurements were made in the Rockefeller Chemical Physics mass spectrometer,2modified for operation in the fast atom bombardment mode. This is a 12 in. radius of curvature, 60° sector single-focusing mass spectrometer. The fast atoms were produced by a Capillaritron fast atom source obtained from Phrasor Scientific Inc., Duarte, CA. In this device a discharge occurs in the nozzle of a capillary tube maintained at high voltage with respect to a ground electrode. Restricting structures outside the nozzle maintain the pressure of the gas emerging from the nozzle a t a high enough value that charge exchange of the ions formed in the discharge occurs to produce the fast atom beam. This beam is almost surely inhomogenous with respect to energy and particle identity. The atoms produced may well be in both the ground and excited states, and some unexchanged ions may exist in the beam. Since the charge exchange can occur anywhere from the discharge nozzle to the ground electrode, the atoms produced will have an energy distribution from small values up to a maximum equal to the applied voltage. In this study argon was used as the reactant gas in the fast atom gun. The voltage applied to the fast atom gun was generally 5.0 keV, and the intensity of the beam incident on the target was equivalent to 5 PA, the determination of which is described later. The sample holder consisted of a stainless steel extension which could be screwed onto the source insertion probe used in conventional mass spectrometry. The surface of the holder was formed by making a 45O cut with respect to the axis of the extension, and thus the shape of the surface was an ellipse. The surface area was 0.18 cm2. The fast atom beam was normal to the axis of the probe, and thus the angle of incidence of the beam to the surface was 45'. The shaft of the probe tip behind the 4 5 O surface was covered with insulating teflon tubing to facilitate the measurement of the incident current. The probe and its surrounding structures were maintained at the ion accelerating voltage of the mass spectrometer, which was 3.0 keV in the measurements reported here. The current output of the Capillaritron fast atom gun depends upon the applied voltage and the gas pressure. The flux of particles incident on the probe was measured with a Keithley Model 610C electrometer connected to the sample probe. Argon atoms with kilovolt energies incident (1) Barber, M.; Bordoli, R. S.; Elliot, G. J.; Sedgwick, R. D.; Tyler, A. N. Anal. Chem. 1982, 54, 645A. This review by the originators of the method contains an extensive list of references to original research papers. (2) Field, F. H. J . Am. Chem. SOC.1961,83, 1523. Beggs, D. P.; Field, F. H. Ibid. 1977, 93, 1567.
Field
upon the probe will effect the emission of secondary electrons. When the probe is biased negatively these electrons will be repelled to neighboring grounded surfaces, and the resulting negative current from the probe may be taken as a measure of the flux of particles incident on the probe. With fixed gun conditions the negative current observed was sensibly constant from a bias voltage of -135 to +67 V, and a t higher positive voltages it fell sharply. This behavior is quite understandable in that the physical configuration permits penetration of the positive electric field from the fast atom gun toward the probe, and this field will capture electrons emitted from the probe. It is assumed that the probability of electron emission is unity, i.e., one electron is emitted for each argon entity striking the surface. Little data seem to be available on this question, but Benazeth3 in his review of kinetic ion-electron emission from metal targets states that, for the same velocity of an ion, many results show that the kinetic secondary electron emission coefficient is independent of the ion charge. Presumably this independence could be extended to particles with zero charge. The emission coefficient increases with the velocity of the impacting particle, and it depends on the cleanliness of the surface being bombarded. Clean surfaces exhibit the smallest secondary emissions. Benazeth shows that for 5-keV ar ions bombarding clean Mo and Zr surfaces the secondary emission coefficient is approximately 0.5. No effort was made in this work to clean the surface of the stainless steel probe of molecular films, and consequently an emission coefficient of greater than 0.5 should probably be expected. Argon ions are present in unknown quantities in the beam from the fast atom gun, and the probe current produced by these will be at least twice the probe current produced by argon atoms. Thus the estimate that the probe current is equivalent to the flux of entities bombarding the probe is subject to uncertainties. A conservative estimate is that the probe current represents the bombarding flux with an accuracy of a factor of 2 or better. Relative values will be known with much more accuracy. An experiment was made where the fast atom gun voltage was varied between 3 (the firing voltage) and 10 keV with an argon pressure of 371 torr and a probe bias of -90 V. The probe current varied smoothly (slightly more rapidly than linearly) from 1.7 X lo4 A at 3 keV to 25.0 x lo4 A a t 10 keV. All of the measurements reported in this paper with one exception were made at a gun voltage of 5 keV and an argon pressure of 371 torr. The probe current under these conditions was 5 x lo4 A. The exception involved experiments where the gun voltage was varied as a parameter, and these will be described later. Thus the standard bombarding flux to the probe was 3 X 1013 argon particles per second (17 X 1013/cm),which is equivalent to 5 PA. The uncertainty in these figures is probably a factor of 2. The mass spectrometer was scanned repetitively with a period of approximately 35 s over the range mlz 600-12. The mass spectra were stored in a DEC 11/03 computer by use of an ADAC (Worcester, MA) analog/digital (A/D) converter and programs written in this laboratory in conjunction with an A/D program obtained from ADAC. Mass spectrometric analyses were made on some of the glycerol samples after bombardment with the beam of fast atoms, and these were made with a Biospect mass spectrometer operated in the OH- negative chemical ionization modea4 The data from this mass spectrometer were stored and manipulated by a VG data system. After fast atom (3) Benazeth, N. Nucl. Instrum. Methods 1982, 194, 415. (4) Smit, A. L. C.; Field, F. H. J. Am. Chem. SOC.1977, 99, 6471.
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5117
Fast Atom Bombardment Study of Glycerol
TABLE I: Glycerol Spectrum
ion
mlz
277 209 185 151 149 131 117 95 94 93 87 77 75 74 73 72 61 60 59 58 57 56 55 47 46 45 44 43 42 41 40 39 31 30 29 28 27 26 19 15 14
+ 1)+ (2M + 1)'
(3M
13C isotope (M t 1)+ (M t 1 - H 2 0 ) '
(M
+ 1 - CH,OH)+
(M t 1 - 2 H 2 0 ) +
CH,CH=O+H CH,=C=O+H
CH,=O+H CHO' (?)
co+(?) CJ3' H,O+ CH3+
abs inta
%total ionizn
214 400 8260 177 315 114 126 133 657 20300 115 102 6490 771 605 127 2410 277 199 309 6300 611 621 755 106 6020 885 3490 588 595 173 925 8660 333 5620 658 2380 292 2470 1550 134
0.3 0.5 9.7 0.2 0.4 0.1 0.1 0.2 0.8 23.8 0.1 0.1 7.6 0.9 0.7 0.1 2.8 0.3 0.2 0.4 7.4 0.7 0.7 0.9 0.1 7 .O 1.o 4.1 0.7 0.7 0.2 1.1 10.1 0.4 6.6 0.8 2.8 0.3 2.9 1.8 0.2
Arbitrary units. Total ionizationa
= 85400.
%base
0
peak 1.1 2 .o 40.6 0.9 1.5 0.6 0.6 0.7 3.2
100.0 0.6 0.5 31.9 3.8 3.0 0.6 11.8 1.4 1.o 1.5 31.0 3 .O 3.1 3.7 0.5 29.6 4.4 17.1 2.9 2.9 0.9 4.5 42.6 1.6 27.7 3.2 11.7 1.4 12.2 7.6 0.7
Number
of peaks = 42.
bombardment the glycerol sample was washed from the fast atom bombardment mass spectrometer probe with 50 p L of H,O,and 1 p L of this solution was applied to the probe of the Biospect mass spectrometer. The source temperature of the Biospect was maintained at 200 "C and spectra were taken every 5 s for the duration of the lifetime of the glycerol on the probe (about 35 scans). A simple procedure was developed to load known amounts of glycerol on the probe of the fast atom bombardment mass spectrometer. Glycerol was placed in a glass vial in such an amount that its depth in the vial was about 7 mm. A hypodermic syringe needle was inserted to the bottom of the glycerol and withdrawn. The droplet of glycerol that remained clinging to the needle was then wiped on the surface of the probe and distributed over the whole area. The actual volume delivered by this method was determined by wiping the droplet of glycerol on a tared piece of glazed paper and weighing the paper. The average of five measurements of the weight of the glycerol delivered was 3.3 f 0.4 mg. The density of the glycerol is 1.26, and thus the volume of glycerol delivered was 2.6 ML. The glycerol used was Bakers Reagent Grade, with a stated purity of 99.9%.
Results The finding in this work which is of most interest is that the FAB spectrum of glycerol changes drastically as the
0)
0
aa-n0
60 IC/
O 0 0O
0 c
0
0
L
al
$
30-
0
a
z
-
0
305
Scan number 915 1220 1525 Seconds after first scan
610
1830
2135
Flgure 1. Number of mass spectra peaks as function of scan number and time after first scan.
length of the irradation with the fast atom beam is prolonged. The spectrum obtained in this work for glycerol after the shortest period of irradiation practical with our equipment (about 1 min for various adjustments and scanning) is given in Table I. The protonated molecule ion is the most intense ion in the spectrum, followed by the (2M + 1)+association ion. A (3M + 1)+association ion is also observed with very small intensities. The spectra were scanned from an upper limit of m/z 600, but no larger association ions were observed. This is not in accordance with findings of Barber and co-workers, for they report' observing protonated glycerol association ions with as many as 15 glycerol molecules. Our measurements were made with the mass spectrometer operating at a relatively low sensitivity (electron multiplier voltage = 2200 V, electrometer amplifier gain = IO7, ion accelerating voltage = 3.0 keV, and FAB voltage = 5.0 keV), which might account for the difference in experimental observations. The major fragment ions observed all occur at mass numbers corresponding to structures or dissociation processes which are easily rationalized from the structure of glycerol and established fragmentation processes. Suggested structures or ion formation processes are included in Table I. One observes that 42 peaks are included in the spectrum comprising Table I. When the irradiation with argon fast atoms is prolonged the number of peaks found in subsequent spectra increases to a maximum at 30 scans and then decreases sharply. The behavior is depicted in Figure 1. The change in the number of peaks as the bombardment proceeds has associated with it drastic changes in both the absolute and the relative intensities of the various ions. The changes in the absolute intensities can best be represented in terms of the total ion current, and a plot of this quantity as a function of scan number and time is given in Figure 2. A strong decline sets in after approximately 20 scans, and the ion current is virtually depleted at the end of 66 scans. One type of behavior of relative intensity is depicted in Figure 3, which shows the decline in the intensity at mlz 93 with increasing scan number and time. The opposite kind of behavior is depicted in Figure 4,
5118
The Journal of Physical Chemistty, Vol. 86, No. 26, 1982
Field
180
160
00
14o
0
00 03
Q
0
o o OO O
0
oo
0
0
0
0
0
0 0 0
0
0 0 0
2 0
I
0
> 0.
0
IO
20
305
610
30
50
40
Scan number 915 1220 1525 Seconds after first scan
60
70
1830
2135
30 40 50 60 70 Scan number 0 305 610 915 1220 1525 1830 2135 Seconds after first scan Figure 4. Relative intensity of m / z 43 as function of scan number and 0
20
IO
time after first scan.
Figure 2. Total ion current as function of scan number and time after first scan. 07i
r
c
W
151
a
z lo
8 0 0
0 0
4 0
OlC
0
0
Ib
2'0
io
O.9,
4"o
Scan number 305 610 915 1220 Seconds after first scan
io 1525
L
20 30 40 Scan number 0 305 610 915 1220 Seconds after first scan
0 '
IO
Figure 3. Relative intensity of m l z 93 as function of scan number and time after first scan.
Flgure 5. Relative intensity of m l z 183 as function of scan number and time after first scan.
which shows a sharp rise occurring in the relative intensity of mlz 43 a t about scan 20 and passing through a maximum a t about scan 45. The behavior of the relative intensities of other major ions as a function of time is either like these two examples or intermediate between them. Of great interest is the fact that new ions are formed in the course of the fast atom bombardment; that is, ions not found in early spectra (Table I) appear after n number of scans and their intensities grow as the bombardment is prolonged. An example of this behavior is given in Figure 5, which shows the relative intensity of the ion with mlz 183 plotted against scan number and time. The intensities are low, and consequently the points scatter somewhat, but there is no question that the ion intensity increases from zero a t zero time. Similar behavior is observed for ions
with mlz 153,129,123, and 113. The ion with mlz 123 exhibits a growth to a relative intensity of 0.7% a t about 38 scans, followed by a decline to 0.3% at scan 48. A useful way of representing in detail the changes in the mass spectral pattern is to plot the ratios of the relative intensities of the several fragment ions to the relative intensity of the mlz 93 ion, which is taken as being the ion representative of glycerol. An illustration of these plots is given in Figure 6, which gives the ratios of the relative intensities of mlz 61 to mlz 93 as a function of scan number and time. One observes that the ratio remains sensibly constant until scan 25, after which it exhibits an increase by a factor of 50 over the course of the run. Similar behavior was observed for all the major fragment ions, but with different amounts of increases of the ratios.
Fast Atom Bombardment Study of Glycerol
The Journal of Physical Chemlstry, Vol. 86, No. 26, 1982 51 19
0
98-
+n
0
7-
-. 0
+ -
a 6VI
a,
-
c
VI
c
c
5-
5
L
0
4-
Marc
0
8
.-0 e
B
Figure 7. OH- NCI spectrum (scan 12) of unirradiated glycerol.
30 0
i
0
Scan number 305 610 915 1220 Seconds after first scan
Figure 8. Values of ratio I B , + l I g 3 +as function of scan number and time after first scan.
The increases observed for the several ions were ( m / z (factor of increase)) 75 (3); 57 (30);45 (25); 43 (100); and 31 (50). These results all point strongly to the conclusion that the fast atom bombardment is effecting reaction of the glycerol to produce new materials. The initial increase in the total number of peaks can be attributed to the formation of new products in the glycerol. The passage through a maximum and decrease in the number of peaks can be attributed to consumption of the material on the probe by one mechanism or another. Similarly, the diminution in the total ion current shows that the material on the probe is being consumed. A likely mechanism for such consumption is that the material is evaporating in the vacuum of the mass spectrometer, but this evaporation is doubtless accelerated by the deposition of energy from the fast atom beam. From Figures 2 and 3 one observes that the diminution in the total ion current with increasing scan number occurs more slowly than the decrease of the m/z 93 ion intensity. Since the mlz 93 intensity is taken as representing the amount of glycerol present in the sample, one concludes that the glycerol is being removed from the probe by the combination of reaction and evaporation faster than by evaporation only. An experiment on this point will be described later. The increase in the relative intensities of the fragment ions with increasing scan number (Figures 4 and 6) leads to the conclusion that glycerol is being converted to materials which tend to remain on the probe with more persistance than the original glycerol. A t the conclusion of the run the sample on the probe was examined visually. The total volume of the sample had clearly diminished, and it had also clearly changed in character from liquid glycerol with its characteristic viscosity to a semisolid mass of clear crystals immersed in a clear liquid. The mixture might be characterized as having a slushy consistency. The sample was easily soluble in
Mass
Figure 8. OH- NCI spectrum (scan 13) of unirradiated glycerol (1 1&min irradiation).
water, and it was quantitatively dissolved in 50 pL of water, and the solution preserved for analysis. A run of this sort carried out for quite a large number of scans (35 min of radiation) may be referred to as a run to extinction. A replicate run to extinction was made, and it gave the same general results, although these were not subjected to the extensive graphical analysis referred to above. Negative chemical ionization (NCI) spectra with OHas the reagent ion were run on the aqueous solution of the glycerol sample which was irradiated to extinction. To provide a control, 3.3 mg of glycerol was spread on the probe tip and immediatelywashed off with 50 pL of water, saving the resultant solution. These aqueous solutions (1.0 pL) were applied sequentially to the probe of the NCI mass spectrometer and the samples were completely volatilized from the probe while the mass spectrometer was repetitively scanned. Figure 7 gives the OH- NCI spectrum of unirradiated glycerol (scan 12), and Figure 8 gives an analogous spectrum for glycerol irradiated to extinction (scan 13). The main peaks in the unirradiated glycerol occur at m/z 91 and 183, which correspond to (M - 1)-and (2M - 1)-, respectively. The relative amounts of these two ions vary with the pressure of glycerol in the ionization chamber, and thus these change as the evaporation from the probe proceeds. Several minor ions appear, some of which can be rationalized as fragment ions, and others of which may be from impurities. Thus m/z 89 is well-known (M - 1 - H&-, and m / z 181 is (2M - 1 - HJ. m/z 151 is (2M - 1 - CH,OH)-, and m / z 127 is (2M - 1 - 56)-. However, it is clear from Figure 7 that most of the ionization in the spectrum exists in the form of the (M - 1)-
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The Journal of Physical Chemistry, Vol. 86, No. 26, 1982
TABLE 11: Molecular Weights of Compounds Produced from Glycerol by Ar Atom Bombardment FAB (M
+
M, 90 112 122 128 152 168 182 198 21 2 214 244
1)'
113 123 129 153
183
OH- NCI (M - 1)89 111 121 127 151 167 181 197 21 1 21 3 243
quasi-molecular ion and its association complex. This is in accordance with the behavior found previously with OHNCI of alcohol^.^ The much more elaborate spectrum produced by the irradiated glycerol sample (Figure 8) gives clear evidence of the presence in the sample of many compounds in addition to glycerol. As an approximation it may be assumed that only quasi-molecular ions are formed, and then the difference in the number of peaks found in the spectrum of Figure 8 compared with that of Figure 7 is a measure of the number of new compounds produced by the irradiation. In some cases ions present in the spectrum of Figure 7 are also present in the spectrum of Figure 8, but with greater intensity, and this increase in intensity can be attributed to the formation of new compounds by the irradiation. An illustration of this is the ion at m / z 89. The ion at m / z 127 also falls into this category, even though this is not completely apparent from a comparison of Figures 7 and 8. This m / z 127 ion maximizes later in the distillation curve, and its intensity in later scans of the irradiated sample is larger than in Figure 8. The ions which can be identified as coming from the products of the irradiation are those with m/z 89, 111,121,151, 167, 181, 197, 211, and 243. The fast atom bombardment spectra contained a number of ions which were not present in the early spectra and whose intensities grew as the irradiation continued ( m l z 113, 123, 129, 153, and 183). These were identified above as ions coming from new products produced by the irradiation. If one argues by analogy with the behavior of glycerol, the new ions produced in the fast atom bombardment may be taken as quasi-molecular (M 1)+ions, and, conversely, the new ions produced in the OH- NCI spectra may be taken as quasi-molecular (M - 1)-ions. Table I1 can then be constructed. The occurrence of (M + l)*ions corresponding to M = 112,128,152, and 182 is particularly gratifying and convincing that new compounds are being formed. The data system used in obtaining the OH- NCI spectra permits the plotting of the mass chromatograms of individual ions and the determination of the integral of the intensity of each ion over all of the scans comprising the complete distillation of the sample from the probe. These areas are proportional to the total amounts of each ion being formed from the sample, and if one makes the assumption that the cross sections for the formation of these ions from the parent compounds are all the same, these areas give the relative amounts of the parent compounds. The areas are given Table 111. The data given in Table I11 also enable one to calculate the fraction of new ions in the total number of ions. The mlz 91 and 183 ions are attributed to glycerol, and from the areas given in Table 111one calculates that 45.5% of the ions in the tabulation are new. Again if one makes the assumption that the ionization cross sections of the compounds producing these ions are all the same, this is also the figure for the per-
+
Field
TABLE 111: Relative Amounts of Ions from Irradiated Glycerol by OH-NCI % base
miz
areaa
89 91 111 121 127 151 167 181 183 197 21 1 21 3 243
28 000 108000 19 200 31 900 7 500 37 800 8 200 28 600 8 3 600 3 700 5 400 7 700 2 400
a
%total area
% new ion
7.3 29.0 5.2 8.6 2.0 11.0 2.2 7.7 22.0 1.o 1.5 2.1
15.5
1.o
area
area (new ions)
".l,l
10.6 17.7 4.2 21 .o 4.5 15.9
51.6 83.9 20.0 100 31.6
2.1 3.0 4.3 1.3
9.7 14.1 20.3 6.5
77.i
Arbitrary units.
centage of compounds produced by the irradiation of the sample. An obvious loss process for glycerol from the tip of the fast atom bombardment mass spectrometer probe is simple evaporation in the relatively high vacuum of the ion source envelope volume torr). To gain some information about the relative importance of glycerol consumption by normal evaporation and by processes promoted by the fast atom beam, an experiment was made wherein scans were taken at 3.0 min intervals, with the fast atom beam being applied only for 20 s of that period while the scan was in progress. Results similar in kind to those depicted in Figures 1-6 were obtained, but the time scale was significantly increased. Thus in Figure 2 the time required for the total ion current to decrease by a factor of two from its starting value is approximately lo00 s (17 min); whereas in the experiment with limited ion bombardment the corresponding time was approximately 66 min. The ratio of these two times is 3.9. In the irradiation to extinction experiment the fast atom beam was applied continuously, but in the limited bombarment experiment it was applied only 11% of the time. If glycerol consumption occurred only in the presence of the fast atom beam, the time scale for the ion trends in the limited bombardment experiment should have been approximately 9 times greater than that for the irradiation to extinction experiment. On the other hand, if only normal evaporation caused loss of glycerol the scales for the two experiments should be equal. The observed relative values of the two time scales is about half way between these two extremes, which indicates that normal evaporation and atom bombardment-induced glycerol consumption have about the same importance. Duplicate experiments utilizing relatively short radiation time (approximately 10 min) were made. In this way loss of both glycerol and radiation products by simple evaporation were minimized, as were further conversion of radiation products. The experiments also were expected to give information about the radiation products yields. Since this set of experiments involved the determination of the absolute amounts of glycerol and radiation products, an aqueous solution of glycerol containing 10 kg glycerol per microliter of water was prepared and six quantitating experiments were made with the OH- NCI mass spectrometer. One microliter of this glycerol solution was applied to the probe, and the glycerol was completely evaporated from the probe while the spectrometer was continuously scanned as described previously. Integrals of the sum of the intensity at mlz 91 and m / z 183 were taken to represent the amount of glycerol. Six separate l.O-& samples were measured and the average deviation from average of
Fast Atom Bombardment Study of Glycerol
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5121
TABLE IV: Ion Intensity Enhancements Effected by 20 Scan Irradiation, OH-NCI run 25
run 22 % new ion
ion
aAa
9 1 t 1 8 3 -104000b 89 16500 111 121 27600 127 5 100 151 23400 167 11 300 181 20600 197 6 200 211 7 300 21 3 9 700 243 2 600 E(89-243) 130 300
% new
TABLE V: Effect of Fast Atom Beam Intensity gun voltage, relative relative slope keV dE/dt of TIC plot
ion
AA
AAa
AA
12.7 0 21.2 3.9 18.0 8.7 15.8 4.8 5.6 7.4 2.0
- 241 900' 8900 1500 15700 2 800 14300 6 600 13400 1500 1900 12200 840 79 200
10.6 1.9 19.8 3.5 18.1 8.3 16.9 1.9 2.4 15.4 1.1
a Difference in areas of irradiated and not irradiated glycerol. A(91-183) = 300 000 i 39 OOOd (not irrad); = 1 9 6 000 k 24 OOOd (irrad). A ( 9 1 + 1 8 3 ) = 618 000 k 30 OOOd (not irrad); = 376 900 i 11000 (irrad). d Average deviation from average of replicate runs.
the integral representing glycerol was 8.5%. This result (as well as experience gathered from other experiments) shows that the precision of analysis made with this method will be on the order of 10%. To make these short radiation time experiments, we applied 3.3 mg of glycerine to the probe of the fast atom bombardment mass spectrometer and irradiated it with 20 scans, where the period of each scan was 35.4 s. Thus the total time of irradiation was 708 s (11.8 min). The probe was then immediately withdrawn from the mass spectrometer, examined visually, and the sample on the probe washed into sample vessel with 50 p L of water. The probe surface after the irradiation was covered with a film of glycerol which did not look different from its appearance before irradiation. Certainly the crystals observed in the irradiation to extinction experiment were not present. To provide a reference standard, we applied 3.3 mg of glycerol to the probe and then immediately washed it into another sample vessel with 50 p L of water. These samples were subjected to analysis in the OH- NCI mass spectrometer. Plots were made analogous to those made for the irradiation to extinction experiments (Figures 1-6), and the results in the irradiation time period common to both experiments were identical. For example, the relative intensity of the m / z 93 ion showed the same small diminution in the course of the first 20 scans as that depicted in Figure 3, and the ratios of the absolute intensities of the several fragment ions to that of mlz 93 were quite constant for the 20 scans taken. Indeed, the spectra changed little enough that one would not suspect that reaction induced by the irradiation occurred, and this is in accordance with the results of the visual observation described above. However, the OH- NCI mass spectrometric analysis of the irradiated sample showed that reaction did, in fact, occur. To provide a current and continuous calibration of the absolute sensitivity of the mass spectrometer, measurements on the irradiated sample and unirradiated glycerol were interspersed. The results of duplicate runs are given in Table IV. The relative intensities of the new ions produced in run 22 and run 25 agree acceptably, and there is also considerable body of agreement with the relative intensities in the irradiation to extinction experiment (Table 111). However, discrepancies exist in the absolute values of the intensities of the several ions, and in particular the sum
1.0 4.0 8.0
5.0 7.5 10.0 a
1.o 5.4 7.5, 8.5a
Replicate measurements.
of the intensities of the new ions in run 25 is relatively small. The calculation of the radiation yields can be made from the data of table IV as follows. One calculates from the values of A(91 183) the amount of unconverted glycerol left in the irradiated samples, and making the assumption that the molar sensitivities for glycerol and the radiation products are the same, one uses the value of C(89-243) to calculate the number of moles of product. Taking the intensity of bombarding particles as the equivalent of 5 pA, one calculates the number of argon particles hitting the sample during the period of the irradiation. The radiation yield is defined as the number of radiation product molecules formed per bombarding argon particle. The value of the yield in run 22 is 152, and that in run 25 is 44. The average is 98. Obviously, this quantitation is not very precise, and the reason is that these measurements were made as part of a mass spectrometric investigation, the conditions of which are far from ideal for quantitative radiation chemistry. Another uncertainty is that the argon particle flux was calculated by making an assumption about the secondary electron yield produced from the bare probe by bombardment with argon particles. A quantitative estimate of the accuracy of the value of 98 molecules produced per incident argon particle cannot be made, but it is believed unlikely that the true value lies outside the range 30-300. What is certainly true is that radiation products are formed relatively quickly, and the radiation yield cannot be very small. An experiment was made to determine the effect of bombarding beam intensity. Irradiations were made with 7.5 and 10.0 keV applied to the Capillaritron atom gun. As was mentioned earlier, increasing this voltage increases both the number of argon particles and the energy per particle. It was found that if the rate of energy input at 5.0 keV is taken as 1.0, the value at 7.5 keV is 4.0 and that at 10.0 keV is 8.0. It was quite apparent that the glycerol consumption processes were much faster at the higher bombarding beam intensities. Thus a t 10.0 keV a very rapid diminution was observed for the total ion current and for the absolute intensities a t mlz 185 and mlz 93. At the end of 10 scans the probe was removed and examined, and it was found that the volume of sample had very much diminished and some crystals had formed. The spectrum obtained in the fourth scan was markedly different from the standard glycerol spectrum given in Table I. The behavior at a voltage of 7.5 keV was intermediate between that at 10.0 and 5.0 keV. To provide some quantitative information about the effect of bombarding particle intensity, we plotted the total ion current values against scan numbers for three gun voltages, and the slope of the decrease in the first five scans was determined. The results obtained are given in Table V. Clearly the rate of decrease of the total ion current is proportional to the energy input to the sample, and it may be inferred that the other changes produced by the ion beam will also be proportional to this quantity.
+
Discussion The mass spectrometric implication of these results is that the bombarding beam used in fast atom bombardment mass spectrometry can effect chemical changes in
5122
The Journal of Physical Chemistry, Vol. 86, No. 26, 7982
the sample. This can result in spectra which are not really characteristic of the sample under investigation. In current FAB technology glycerol is used only as a solvent, and its spectrum is not of much interest in itself. However, one immediately wonders whether radiation damage such as that found for glycerol will also occur for solutes dissolved in the glycerol, which could lead to erroneous spectra. Obviously, this matter deserves investigation. Barber and co-workers state in one place in their paper' that FAB sample lifetimes are long, typically 20 min and in another place they state that the lifetimes of the spectra could be extended to hours. Furthermore, the bombarding intensity used in FAB analysis is oftentimes much higher than that used in this work. For example, Barber and co-workers report the use of a beam of argon atoms of 6 keV and a beam current of 200 pA,5 and in another case a 7-keV argon beam with a current of 400 pAee Glycerol does not appear to have been used as a solvent in these experiments. In another experiment Barber and co-workers report' that bradykinin (in the absence of glycerol) was bombarded by 6-keV argon atoms at a beam current of 200 pA for 10 min with no measurable effect on the bradykinin biological activity. Williams and co-workers8report using a 4-6-keV beam of argon atoms a t a current of approximately 40 pA with the sample dissolved in glycerol. In all of these references it is not clear whether the beam current specified is that actually striking the target or a current associated with the ion gun.It is probably premature to decide whether a conflict exists between our findings on glycerol and the inferences to be drawn from these statements made in the literature by other workers. However, the practice in this laboratory is to gather spectra as rapidly as possible with the minimum amount of bombardment. The radiation chemistry aspect of the observed phenomena involves interesting questions about the reaction mechanisms. The molecular weights of compounds formed by the irradiation are listed in Table 11,and the reactions producing four of these compounds can be postulated by using simple free-radical mechanistic concepts. Thus we write reactions 1-7 for the formation of the substances with molecular weight 122, 152, 182, and 90. The question of interest is that of the detailed mechanisms involved in the initial free-radical formation reactions d and 3. In conventional radiation chemistry the incident radiation is ionizing radiation such as y rays, X rays, fast electrons, a particles, etc. This type of radiation produces ions and possibly excited electronic states as the primary process. Speaking more generally, the primary processes involve some kind of Franck-Condon transitions in the irradiated materials. The ions and/or excited states initially produced can undergo decompositions and reactions to produce products and/or free radicals, and these in turn can produce products. By contrast, argon atoms with an energy of several kiloelectronvolts cannot be considered as ionizing radiation. Wolfgangghas reviewed hot atom chemistry, and he discusses the application of the adiabatic principle to the question of the nature of the transition effected by an incident particle. The adiabatic principle states that the probability of an electronic transition on collision is at a (5) Barber, M.; Bordoli, R. S.; Segwick, R. D.; Tyler, A. N.; Whalley, E. T. Biomed. Mass Spectrom. 1981, 8, 337. (6) Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. Biomed. Mass Spectrom. 1981, 8, 492. (7) Barber, M.; Bordoli, R. S.; Sedgwick,R. D.; Tyler, A. N.; Whalley, E. T. Biomed. Mass Spectrom. 1981, 8, 337. (8) Williams, D. H.; Bradley, C.; Bojesen, G.; Santikam, S.;Taylor, L. C. E. J. Am. Chem. SOC.1981,103, 5700. (9) Wolfgang, R. Prog. React. Kinet. 1965, 3, 97.
Field CH20H SL
+
*C-OH I
H
I I
CH20H
I H
+
J+
GL
CH20H
I
! L
GL
H2
+
*CH20H
*CHOH
I11
I1
CH20H
I
+
IIi
- HO--C-CH,OHII
(4)
CH20H
M , = 122
I
A
I1
-
CH20H hO-C-CHOH
I CH,OH I
(5)
HO-CH,
M, = 152 HO-CH2
I + I
4
CHZOH
I I HO-C-C-OH I Ch,OH / HO-CH2
(6)
M,= 182 CIOH
I + I -
I/ C-OH
CH,OY
I + CHOH
CH20H
I CH20H
M,= 90
GL
(7)
maximum when the time of collision corresponds to the frequency ( E / h )of the transition. The relative velocity, V-, for maximum crom section of a process characterized by an internal energy change AE is given by Vln, = IAEla/h where a is a distance on the order of atomic dimensions. cm. For the The value suggested by Wolfgang is 7 X ionization of glycerol, if we assume an ionization potential of 10 eV, the energy of the bombarding particle which will produce the maximum ionization cross section is 6.0 X lo5 eV, and the inference to be drawn is that the probability of glycerol ionization by 5-keV argon atoms in a FranckCondon type of transition will be very small. If we take the weakest bond in the glycerol molecule to be -4 eV, the argon energy which would give a maximum cross section for an electron excitation to a state at approximately the bond dissociation limit would be 2.4 X lo5 eV, and the cross section for this process would also be rather small. These considerations make it unlikely that the initial processes involved in fast atom bombardment radiation chemistry are the same as those in conventional ionizing radiation chemistry. Rather it appears that fast atom bombardment radiation chemistry is a kind of hot atom ~hemistry.~ However, conventional hot atom chemistry involves incorporation of the bombarding entity into the molecule being transformed, and that certainly is not the case here. The passage of an energetic particle such as a 5-keV argon atom in fast atom bombardment mass spectrometry produces a localized high temperature region. This temperature has been estimated to be on the order of lo4deg.1°
J. Phys. Chem. 1982, 86,5123-5127
A temperature of this magnitude would certainly thermally pyrolyze glycerol, but the lifetime of this high temperature is very short,1° which raises questions as to whether indeed thermal formation of free radicals is involved. A characteristic of a fast argon atom which may be of interest in this problem is that it has a high momentum as well as a high energy. Thus non-Franck-Condon processes can be effected, and it is suggested that free radicals may be produced by momentum and energy transferring collisions which would bring about the direct fragmentation of glycerol molecules. This comprises a novel and interesting kind of radiation chemistry. It would be of interest to compare the products obtained in this work with those obtained by the use of ionizing radiation in neat glycerol, but no information on this point has been found. Note Added in Proof. A further experiment was made wherein glycerol was bombarded with a beam consisting (10)McNeal, C. J. Anal. Chem. 1982,54,43A.
5123
of only fast argon atoms. This was obtained by applying a deflection voltage to the mixed beam of argon atoms and ions emerging from the Capillaritron gun. The behavior of the argon spectra was the same as that depicted in Figures 1-6.
Acknowledgment. This work was one of the activities of the Rockefeller University Mass SpectrometricResearch Resource, which is supported in part by the Division of Research Resources, NIH. Many of the experiments reported here and most of the computer programming were made by Louis Grace. Initial modifications of the mass spectrometer for FAB service were made by Dennis Underwood. The OH- NCI measurements were made by Aladar Bencsath and Camy Ng. Useful discussions were held with Brian Chait. I am grateful to them for their help. A computer search of the glycerol radiation chemistry literature was kindly provided by the Radiation Chemistry Data Center, Radiation Laboratory, University of Notre Dame.
X-ray Photoelectron Spectroscopy of Cadmlum Arachidate Monolayers on Various Metal Surfaces F. C. Burnst and J. D. Swalen' IBM Research Laboratory, San Jose, Cellfornkr 95193 (Recelwed: June 30, 1982)
X-ray photoelectron spectra were measured for a single cadmium arachidate (cadmium eicosonate) monolayer bonded via the carboxylate group to various metal and metal oxide surfaces. Two well-defined peaks were observed in the C 1s spectra of these systems: an intense peak due to ionizations from the CH2 and CH3carbons and a less intense peak due to ionizations from the CO; carbon. Differences between the binding energies of these two carbon peaks were found to vary with the surface to which arachidate was bonded and these differences are interpreted as arising from differences in the charge transfer between the various surfaces and the arachidate. In addition the carbon initially present on the metal/metal oxide surfaces and the effect of submerging the surfaces in the monolayer tank were studied and it was found that this source of carbon had a negligible effect on the positions of the arachidate carbon peaks.
Introduction When Langmuir monolayer assemblies of fatty acids and their salts are transferred to solid surfaces by the Blodgett technique, highly ordered organic layers coating the solid surfaces are created.' These monolayer assemblies are of interest because they offer a means of studying intermolecular interactions and energy transfers and because of their potential applications as protective overcoatings and in lithography. For the preceding reasons there has been vigorous activity directed toward understanding the interaction between Langmuir-Blodgett monolayers of fatty acid salts and various types of surfaces.2 Techniques as varied as X-ray photoelectron spectroscopy (XPS),= attenuated total reflectance (ATR) s p e c t r o ~ c o p yRaman ,~~~ spectroscopy; and reflection Fourier transform infrared spectroscopy (FT IR)6J0J1have been utilized successfully in studying monolayer systems. Anderson and Swalen3 have reported the C 1s XPS spectra for one, three, and five monolayers of cadmium arachidate (Cd(CH3(CH2),,C02),),henceforth abbreviated t Present address: I B M Development Laboratory, Endicott, NY 13760.
Cd(AA)2,on various oxide surfaces. These authors measured the difference between the carbon l s binding energies due to the aliphatic carbons and that due to the carboxylate carbon. Changes in the binding energy difference in going from one to several monolayers were interpreted in terms of charge transfer between the oxidized (1)G. L.Gaines, Jr., 'Insoluble Monolayers at LiquidGas Interfaces", Wiley-Interscience, New York, 1966,Chapter 8. (2)H. Kuhn, D.Mobius, and H. Biicher in "Physical Methods for Chemistry", Vol. I, Part IIIB, A. Weissberger and B. W. Rossiter, Eds., Wiley-Interscience, New York, 1972,Chapter VIII. (3) H. R. Anderson, Jr., and J. D. Swalen, J. Adhes., 9,197-211 (1978). (4)C. R. Brundle, H. Hopster, and J. D. Swalen, J. Chem. Phys., 70, 5190-6 (1979). (5) D. T. Clark, Y. C. T. Fok, and G. G. Roberts, J. Electron Spectrosc. Relat. Phenom., 72,173 (1981). (6)T. Ohnishi, A. Ishitani, H. Ishida, N. Yamamoto, and H. Tsubomura, J . Phys. Chem., 82, 1989-91 (1978). (7)J. G. Gordon, 11, and J. D. Swalen, Opt. Commun., 22,375 (1977). (8) I. Pockrand, J. D. Swalen, J. G. Gordon, 11, and M. R. Philpott, Surf. Sei., 74,237 (1978). (9)W. Knoll, M. R. Philpott, J. D. Swalen, and A. Girlando, J . Chem. Phys., 77,2254 (1982). (10)D.Allara and J. D. Swalen, J. Phys. Chem., 86, 2700 (1982). (11)J. F.Rabolt, F. C. Burns, N. E. Schlotter, and J. D. Swalen, J . Chem. Phys., in press.
0022-3654/82/2086-5123$01.25/00 1982 American Chemical Society
