J. Phys. Chem. 1984, 88, 5065-5068
5065
Oxidation-Reduction Processes Occurring in Secondary Ion Mass Spectrometry and Fast Atom Bombardment of Glycerol Solutions G. Pelzer, E. De Pauw,* Dao Viet Dung, and J. Marient Lab. Physico-chimie des Surfaces, Departement de Chimie GPnCrale et de Chimie Physique, Institut de Chimie (B6), Sart Tilman, B4000 LiZge, Belgium (Received: February 21, 1984)
The oxidation-reduction chemistry of glycerol solutions submitted to ion (SIMS) or atom (FAB) bombardment has been investigated. Various solutes (inorganic salts, organometallics,and cationic dyes) were studied. We have demonstrated that a reduction process occurs for both inorganics and organics; it results in a hydrogen atom attachment to the ion molecular complex. A lowering of the oxidation state of the cation simultaneously with the protonation of an anion takes place in the case of inorganic salts whereas the reduction of a quinoidal into a semiquinoidal form is oberved for the dyes. The extent of reduction can be explained for each system on the basis of the usual standard redox potential scale. Some previous results of the literature are also reexamined taking into account this redox process.
Introduction
Bombarding solid or liquid targets with energetic particles (atoms or ions) to produce secondary ions has now become a well-established and powerful technique in mass spectrometry. Secondary ion mass spectrometry (SIMS) and fast atom bombardment (FAB) belong to the so-called soft ionization methods1P2 which are very useful for the mass-spectrometric analysis of low-volatility and thermolabile products. The sputtering of metals and semiconductors is well described by Sigmund’s theory3 but, in the case of molecular solids and liquids, a delayed energy transfer describable as a quasi-thermal process is more suitable (thermal spikes). On the other hand, little is known of the ionization mechanism. In the case of solids, electronic interactions between the leaving particle and the sample’s surface may lead to various charge-transfer proce~ses.~With regard to liquids, such phenomena have not yet been clearly demonstrated. Furthermore, the chemical properties of the solution (acidic-basic, oxidation-reduction, presence of salts) play a notable role in the secondary ion emission p r o c e ~ s . ~ , ~ SIMS and FAB are attractive in mass spectrometry because pseudomolecular ions can be found beside a lot of fragments. Nevertheless, it may be asked to what extent the “molecular” ion reflects truly the chemical nature of the solute. As regards SIMS of solids, it is indeed well-known that several ion beam induced artifacts alter the molecular information delivered by the techniquee7-’ For instance, preferential sputtering and recoil implantation occur usually during the bombardment of compounds like oxides7,*and inorganic solid salts.9 These processes lead to the chemical transformation of the crystal and to reduction-oxidation (redox) in particular. When solutions are bombarded either in SIMS or in FAB, redox phenomena other than those relative to solid targets are likely in addition to other chemical reactions like, for instance, C-C bond cleavages and ligand exchanges. In the present work, we have investigated the redox processes which take place during the ion bombardment of various substances dissolved in glycerol, and the rules of the ion beam induced redox chemistry are pointed out. The latter govern the nature of the emitted secondary ions and thus must be taken into account in a reliable interpretation of the spectra. Besides, the correlations between the ion patterns and the redox properties of the solution could also illuminate the ion-formation mechanism itself. The chemical effects of the primary ion beam have been examined for quite different molecules of solute such as inorganic salts, organometallic complexes, and dyes. Previous results of the literature are also reexamined in terms of the oxidation-reduction concept. Experimental Section
TABLE I: List and Valence of the Studied Cations Ia Na’, K+, Cs+ IIa Mg2+, CaZt, Sr2+,Ba2+ IIIa IVa
AI3+
Ib IIb
CuZt, Agt ZnZt, Hg2+ Cr2+, Cr3+ FeZt, Fe3+, CoZt, Co3+ Th4+
VIb VI11 Acti
Pb2+
analyzer (TN 1710-3 preamplifier, T N 1710-30 signal averager, T N 17 10-9 data processor). Details concerning the experimental setup have been previously published; lo we just recall here that the liquid samples are bombarded with a homemade cesium ion gun. The primary ion current and energy were respectively lo4 A and 6 keV. No charging effect was observed. Each solution that we studied was prepared by dissolving about g of the analyte in 1 cm3 of glycerol. A droplet of that liquid is then spread out on the nickel grid spotwelded on the specimen holder which terminates the fast introduction rod system. If required by redox potentials, the nickel grid is replaced by a copper one. Results
Inorganic Salts and Organometallic Complexes. First, we recorded the positive SIMS spectrum of various inorganic salts which are soluble in glycerol. Chlorides, nitrates, and sulfates of several metals and one actinide (thorium) were investigated. As can be seen in Table I, cations belonging to successive columns of the periodic table were selected in order to point out any influence of the cation valence on the composition of the pseudomolecular secondary ions. In addition, salts with the same metal in various oxidation states were examined to check to what extent the valency information is kept or altered through the ion bom(1) K. L. Busch and R. G. Cooks, Science, 218, 247 (1982). (2) M. Barber, R. S.Bordoli, G. J. Elliot, R. D. Sedgwick, and A. N. Tyler, Anal. Chem., 54, 645A (1982). (3) P. Sigmund in “Sputtering by Particle Bombardment-I”, SpringerVerlag, West Berlin, 1981, Top. Appl. Phys. No. 47. (4) J. W. Rabalais, Isr. J . Chem., 22, 365 (1982). (5) E. De Pauw, Anal. Chem., 55, 2125 (1983). (6) K. L. Busch, S.E. Unger, A. Vincze, R. G. Cooks, and T. Keough, J. A m . Chem. SOC.,104, 1507 (1982). (7) S.Storp and R. Holm, J. Electron Spectrosc. Relat. Phenom., 16, 183 (1979). -, (8) G. K. Wolf in “Topics in Current Chemistry”, Vol. 85, Springer, West Berlin, 1979, pp 1-189. (9) J. Marien and E. De Pauw, Int. J . Mass Spectrom. Ion Phys., 43, 233 \ - -
The mass spectra were recorded with an Extranulcear 7-126-8 quadrupole mass filter (12-1 100 amu) and a Tracor multichannel I’Research Associate of the National Fund for Scientific Research (Belgium).
0022-3654/84/2088-5065$01.50/0
( 1982).
(10) Dao Viet Dung, J. Marien, E. De Pauw, and J. Decuyper, Org. Mass. Spectrom., in press.
0 1984 American Chemical Society
5066
The Journal oj Physical Chemistry, Vol. 88, lvo.
1
Fe SO& +GLYCEROL
Cr',
Pelzer et al.
AI, 1 ~ 8 4
-
1
N
3
I
-=i m c n
Th(NO3)& +glycerol (GI
6 KeV. 10-6amp.
9
I
Cs+,6 KeV, 10-6arnp
E 3
> 6
c
c
a
50
100
150
200 250 300 350 LOO
450
500
550
600 amu.
Figure 1. Positive SIMS spectrum of ferrous sulfate dissolved in glycerol: Cs', 6 keV, lod A-cm-2.
TABLE 11: Identification of the + Secondary Ions in the Spectrum of Fieure 1" mass, mass, amu ion structure amu ion structure 391 [Fe"(Fe"SO,)(G H)(G)]+ 57 (GH - 2Hz0)+ 397 [(Fe"S0,) 2(GH)] 15 (GH - H20)' 423 [Fe"(G4 - H)]' 93 (GH)+ 429 [ (Fe"S0,) (G3H)]+ 133 cs+ 461 (GsH)' 147 [Fe"(G - H)]' 483 [Fe"(Fe"SO,)(G - H)G2]' 185 (GzH)' 489 [(Fe11S04)z(GzH)]t 239 [Fe"(G - H)(G)]' 521 [(Fe%O4)(G4H)]+ 245 [(Fe%O,)(GH)] 543 [Fe"(Fe"S04)2(G - H)G]+ 211 (G3H)' 33 1 [Fe"(G3 - H)]' 553 (GaH)' 331 [( Fe"S04)(G2H)] 58 1 [(Fe11S04)2(G3H)]' 369 (G4H)+ ~
~~~
-
~
+
+
'
'FeS04
+ glycerol. G = glycerol.
bardment process. We previously performed a similar study on inorganic salt powdersg pressed into pellets and argon ion bombarded in the static or dynamic SIMS modes. The terms "static" and "dynamic" refer to the lifetime of a monolayer with respect to the time required for an experiment. That depends among other factors on the primary ion current density 0'). Practically, for most systems, "static" means j values lower than IOp9 Acm-2 whereas the dynamic mode corresponds typically to values higher than 10" A.cm-2. Besides the inorganic salts, we also studied organometallic complexes of copper, iron, cobalt, and nickel which all contain the strong acetylacetonate ligand. The positive SIMS spectrum of every one of the above solutes exhibits similar series of ion clusters. By way of example, the fingerprint of ferrous sulfate is shown in Figure 1 and the main peaks are listed in Table 11. Scrutiny of each of the spectra allows the following observations to be made: (a) The composition of the emitted cluster ions is governed by the usual valency rules. Consequently, the secondary "molecular" ions (monopositive) correspond to the general constitutional formula (M"+[G - (n - 1 - X ~ ) H ] ( " ~ - ~ ~ ) - G , , A , ~ - ) + (1) where n is the oxidation state of the metal M, m the charge of the x anions A in the cluster (x = 0, 1, 2, ...), and p the number of glycerol molecules 0, = 0, 1, 2, ...). For example, in the case of FeS04, cluster ions such as [Fe(G - H)G,]+ and [Fe(G + H)S04.G,]+ are detected. The latter ion becomes (FeCIG,)' in the ferrous chloride spectrum. Similar ion species have also been shown in the FAB work of Javanaud and Eagles." Beside glycerolate, several other anions can be found in the ionic aggregate (11) C. Javanaud and J. Eagles, Org. Mass Spectrom., 18, 93 (1983).
&UJJjL 300
350
400
L50
500
550
600
650
700
+y a.m.u.
Figure 2. Positive SIMS spectrum of thorium nitrate dissolved in gylcerol acidified by p-toluenesulfonic acid (PTSA). The nitrate anion is missing
and replaced by (PTSA - H)-.
when those are present in the glycerol solution. In fact, they compete with each other to bond with the acidic (Lewis) cation in the ion cluster according to their relative concentration, basicity, and ligand strength. For instance, thorium nitrate dissolved in glycerol exhibits rather intense characteristic peaks at 476,568 amu; these correspond respectively to [ThIV(N03)(G- H)2]+ and [ThIV(N03)(G- H),G]+. The addition of small amounts of p-toluenesulfonic acid (PTSA) to the glycerol solution induces the disappearance of the above peaks. They are indeed replaced by new mass lines in which thep-toluenesulfonate anion takes the place of NO,-; this is shown in Figure 2 where the 585 amu [ThIV(PTSA- H)(G - H)2]+ and 677 amu [ThIV(PTSA- H)(G - H)2G]+are clearly seen. The high intensity of the 665 and 757 amu peaks whose formulas are respectively [ThIV(PTSA- H),(G - H)]+ and [ThIV(PTSA- H)PTSA(G - H)# also illuminates the competition between (PTSA - H)-, glycerolate and PTSA, glycerol. Association constants for such ion clusters could be estimated by varying the relative concentrations of the various species in solution. From the examination of the ion patterns, one also notices that one glycerol molecule can lose up to three acidic protons, forming ions like, for instance [ThIV(G- 3H)]+. Obviously, when more than one glycerol are included in the ion cluster, i.e., [ThIV(G - 3H)G,],,o+, all three protons do not necessarily come from a single G molecule as the arbitrarily written formula could suggest. (b) In the case of easily reducible salts (FeC13, Co(NH3),&13, CuCI,, ...) the cluster ion mass lines appear 1 amu higher than expected if one takes into account the metal's oxidation state in the solute prior to the bombardment. In fact, the valency rule is still obeyed but one hydrogen atom has been added to the ion complex. That, in turn, corresponds to the one-electron reduction of the cation induced by the bombardment of the solution. For instance, in the particular case when x = 0 in formula 1, the redox equation can be written
+
-
(M"+[G - (n - l)H]("')-G,]+ e- + H+ (M("')'[G - (n - 2)H]("2)-Gp)+ (2) CS+
An examination at high resolution of the p = 0, p = 1 regions of the spectra relative to the above salts is shown in Figure 3, which includes the standard redox potential values of the corresponding Mn+/M("')* couple.'2 One notices a continuous increase in the relative intensity of the peaks characteristic of the reduced form of the cation in a parallel direction with the increase of the standard redox potential. (12) "CRC Handbook of Chemistry and Physics", 59th ed., CRC Press, Boca Raton, FL, 1978-1979.
SIMS and FAB of Glycerol Solutions
1
l
Crt(G-H1+
II
JUh
,Jw
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 5067
1
'kA-., 143
Y-Y
133
225
.li 235
I
//
a.m.u.
357
.
k59
4
200
Ib
,li
Cu IG-HIG*
I/
253
I
225
250
275
300
325
>
350
375 a m u .
Figure 4. Positive SIMS spectrum of the crystal violet dye dissolved in glycerol. The intact cation ( m / r 373) is the base peak and little reduction is observed. The value of the half-wave polarographic reduction potential (Eo1,2,rd)is also indicated. METHYLENE BLUE Cf CATION
[FeIG-HI GI*
[FelG-Hr E>e+++/Fe*=
+
(CIl)+
I
977 v
Figure 3. Characteristic regions of the positive SIMS spectra of various salts (Cr(N03)3,CuCI2, Fe2(S04)J dissolved in glycerol showing the
ratio between the oxidized and reduced forms of each cation. The left is relative to the p = 0 portion, the right to the p = 1 one (see the definition of p in the text). The standard redox potential values (EO) are
also indicated for each couple. To get a reliable estimation of the extent of reduction, each spectrum was obviously recorded with the same primary ion doses. Comparing, in Figure 3, the low-mass portion (p = 0) of the Cr"' and Cu" spectra with the high-mass region (p = 1) shows that the more the glycerol molecules in the ion complex, the less reduction is observed. (c) Likewise, the complexation of the metal by a strong ligand such as, for instance, the acetylacetonate anion, (acac)-, stabilizes the high oxidation state of the cation in the ion cluster. With regard to the ferric salts dissolved in glycerol, no molecular ion containing FelI1 is emitted but, when (acac)- is present in the solution, [Fe"'(a~ac)~]+is exhibited besides the [Fe"(acac)G,]+ reduced species. The primary ion current density has a strong influence on the ratio between the two above ion clusters. An increase of the Cs+ flux results initially in an increase of the reduced ion complex; this is followed by a leveling out at about 2 X lod a-cm-2 and a final decrease. That crossing through a maximum will be tentatively explained further; it recalls the bell-shaped curves previously observed for the solid inorganic salts? Organic Dyestuffs. Under nonreducing ambient conditions, several organic dyes exhibit in fact the oxidized quinoidal form. It is thus worthwhile to compare the redox chemistry of both organic and inorganic compounds due to the ion bombardment. For that purpose, a number of cationic dyes (crystal violet, rhodamine B, methylene blue) were investigated. Each of those spectra shows the base peak in the molecular region around the cation. As an example, we show in Figure 4 the spectrum of crystal violet dye in which the intact cation mass line (C+ = 373) is predominant besides fragments corresponding to a loss of methane ( m / z 357), a loss of C6H4N(CH3)2( m l z 253), and the substitution of methyl groups by hydrogen ( m l z 359, 345, ...). Similar fragmentation processes were found for the other investigated dyes and these results agree with the SIMS work of
IC4t
A
Figure 5. Molecular regions of the SIMS spectrum of rhodamine B and methylene blue dissolved in glycerol showing the ratio between the oxidized (Ct) and reduced [(C + l)+, (C + 2)t, (C + 3)'] forms of the cation.
Cooks13on solid dyestuffs. Scrutiny of the spectra shows that, in the molecular region, peaks appear also at 1, 2, and 3 amu higher than the m / z value relative to the intact quinoidal cation. This observation suggests again that reduction of the dye occurs during bombardment. The (C l)', (C 2)+, (C 3)+ peaks are less intense than C+ for rhodamine B and crystal violet but prominent in the case of methylene blue. Taking into account charthe half-wave polarographic reduction potentials E1/2,r414 acteristic of each dye (see Figures 4 and 9,one can see that the the less intense is the remore negative is the value of E1,2,red, duction process. Comparison of the spectra in Figures 4 and 5 shows this tendency.
+
+
+
(13) S. M. Scheifers, S.Verma, and R. G. Cooks, Anal. Chem., 55, 2260
(1983). (14) H. Berg, Chem. Tech. (Berlin), 6, 585 (1954).
5068 The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 Discussion and Conclusion The above investigations and some results of the literature show indubitably that reduction processes occur during the SIMS analysis of various solutes dissolved in glycerol. This is valid for either inorganics or organics. Besides, reduction takes place no matter whether the energetic primary particle is an ion (SIMS) or an atom (FAB). In preliminary work15 we have shown indeed that experimental data quite similar to those reported here are also obtained if glycerol is bombarded by energetic argon or xenon atoms instead of cesium ions. It is also noteworthy that the reduction processes observed here are inherent in the fact that the substance is dissolved in glycerol. They are completely different from what is observed when the sample is in the solid state. For instance, in a previous studyg we have shown that sodium sulfate is partially reduced to sodium sulfite after a low-dose of bombardment. Such a reduction can be interpreted in terms of preferential sputtering of oxygen. Contrarywise, we do not observe here a reduction of the anion but rather the capture of one hydrogen atom by the ionic complex. One proton is used to neutralize a glycerolate anion and one electron reduces the oxidation state of the cation in the cluster. With regard to organic dyes and organometallics, reduction results also from the capture of one or more hydrogen atoms. Furthermore, for easily reducible dyes such as methylene blue, we observe an intense ion beam induced reduction [(C + 1)’ > C’] whereas there is little if any reduction when SIMS is performed on the solid dyestuff. For instance, the SIMS spectrum of methylene blue spread on a silver foil has been recorded by Cooks, l3 it shows an intense intact cation C+. In view of all these observations, it is likely that the reduction process occurring in FAB-SIMS of glycerol solutions could be governed by a simple redox equilibrium between hydrogen atoms produced in glycerol by bombardment and the oxidized species present in the solution. In a recent paper concerning the radiation chemistry of glycerol, Field16 suggested that its fast atom bombardment liberates hydrogen atoms besides other radicals. If such a model is valid, it would ben then possible to predict from a scale of redox potentials which cations or organics will be heavily reduced. Such a scale is not available in glycerol but, in a first approximation, we can assume the validity of the usual standard redox potential scale. Moreover, such an assumption seems to be reasonable in the case of simple alcohol solvents.20 It is worth noting that all the above experimental results can be interpreted qualitatively on the basis of the usual redox scale. For instance, the fact that the redox potential value of Fe3+/FeZ+is positive against the normal hydrogen electrode whereas the Cr3+/Cr2+one is negative explains well why the ferric salts are so strongly reduced compared to the chromium(II1) salts. To go beyond the explanation of the reduction trends is presently out of the question because knowledge of the specific interactions with the glycerol solvent and the association constants of the various ion clusters would then be required. In other terms, the apparent redox potentials in glycerol have to be known. This is illustrated by the fact that gylcerol molecules and strong ligands (acac-, for instance) stabilize the high oxidation state of the cations. As mentioned above in the case of ferric acetylacetonate, the ratio between the reduced and oxidized ionic forms is closely related to the primary ion density and crosses through a maximum. Up to now, we have had no satisfactory explanation for this phenomenon. Although there is much evidence that the reduction process is performed in the liquid phase, one cannot however (1 5 ) E. De Pauw, unpublished work performed at the University of Bonn
(F.R.G.) during a A. Von Humboldt research grant. (1 6) F. H. Field, J . Phys. Chem., 86, 5 115 (1 982). (17) F. Lohmann, 2.Naturforsch. A , 22, 813 (1967). (18) K. Burger, “Organic Reagents in Metal Analysis”, Pergamon Press, Elmsford, NY, 1972. (19) K. D. Cook and K. W. S. Chan, In?.J . Mass Spectrom. Ion Phys. 54, 135 (1983). (20) J. O’M Bockris and A. K. N. Reddy in “Modern Electrochemistry”, Vol. 11, Plenum Press New York, 1970, p 1425.
Pelzer et al. completely discard the fact that some reduction is taking place in the gas phase according to the following dissociation mechanism: [(oxid)G,]+
-
[(oxid
+ H)G,,]+. + (G - H).
(3)
In such a case, the energy liberated by the redox process (electroaffinity of the oxidized species and heat of protonation) must overcome the energy needed to break the weakest bond of glycerol (Le., about 4 eV according to ref 16) and the bond of the complex. Taking into account a conversion factor of 4.5 eV” between the standard redox potentials and the electronaffinity, assuming the limit value of 0.5 eV for the energy of formation of the complex, (estimation based upon the stability constants of acetylacetone complexes18)and 0.6 eV for the heat of protonation of glycerolate (estimation from the pK, value of glycerol), the energy balance should be for the ferric and chromium(II1) salts Fe“’ 0.11
+ 4.5 + 0.6
4 + 0.5 eV
(4)
+ 4.5 + 0.6 6 4 + 0.5 eV
(5)
Cr”’
-0.41
The above relations suggest that intense reducing dissociation would be expected in the gas phase for ferric salts but not for the chromium ones. Again, relations 4 and 5 can only indicate trends, the values not being known accurately and not specific for the gas phase. The reduction process emphasized here must be taken into account for a reliable interpretation of the SIMS-FAB spectra and in the analytical use of those techniques. The above results show that the reduction depends on glycerol as solvent. Other common solvents like diethanolamine and tetraglyme have also been examined in a cursory fashion. The reduction process is less severe in those solvents. This can be tentatively explained by the increased difficulty of producing hydrogen radicals in molecules containing no secondary carbon atom. Moreover, several results of the literature can be reinterpreted in terms of redox processes. For instance, the fragmentation by charge separation of diquaternary ammonium salts, as observed in SIMS-FAB, can be explained otherwise than Cook and Chanlg did. After comparison of the electrohydrodynamic ionization (EHMS) and fast atom bombardment mass spectra of diquaternary ammonium salts, these authors postulate that the extensive fragmentation observed in FAB and not in electrohydrodynamic ionization is due to the increase in internal energy of the ions produced by bombardment. An alternative interpretation could be that in E H M S no hydrogen is produced; no reduction may thus occur and this leads to the emission of intact dications. Contrarywise in FAB, reduction takes place. The more the two positive charges are neighboring, the more positive will be the redox potential in order to lower the Coulombic repulsion. A one-electron reduction of the diquaternary thus proceeds; this, in turn, induces the loss of one carbon chain on nitrogen (dequaternization). If, as in the case of dyes, a simiquinoidal form exists, a (C + 1)’ ion will be detected. It is noteworthy that reduction can also occur in solid SIMS when the sample is simply deposited on a metallic surface. Indeed, energy buffering by the matrix does not then take place. This leads to appreciable fragmentation, hydrogen release, and feasibility of subsequent reduction.
Acknowledgment. We are grateful to the National Fund for Scientific Research (Belgium) for its financial support. G.P. thanks also the I.R.S.I.A. for the awarding of a grant. The skillful technical assistance of G. Quoilin and M. Fedyniak is also acknowledged. Registry No. FeS04, 7720-78-7; Cr(N0,)3, 13548-38-4; CuCI,, 7447-39-4; Fe2(S0&, 10028-22-5; thorium nitrate, 13823-29-5; glycerol, 56-81-5; crystal violet, 548-62-9; rhodamine B, 81-88-9; methylene blue, 61-73-4.
