1218
Anal. Chem. 1986, 58, 1218-1221
Binding of First and Subsequent Glycerol Molecules to Organic and Inorganic Cations Studied by Desolvation in the Mass Spectrometer W. Bart Emary and R. Graham Cooks* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Paul C. Toren Department of Medicinal Chemistry a n d Pharmacognosy, Purdue University, West Lafayette, Indiana 47907
Daughter spectra have been measured for the adducts of simple Inorganic and organlc ions wlth one or more molecules of glycerol. When the domlnant mode of fragmentation is loss of a molecule of glycerol, the adduct may be consldered to be a simple solvated species. This Is frequently not the case for monoglycerol adducts of organlc Ions, some of which exhibit dehydratlon as their malor reactlon. Even the diglycerol adducts of some inorganic Ions behave as If both glycerol molecules are covalently bonded to the catlon. Reasons for this behavior Include Intramolecular metal-oxygen for metal-halogen bond substitution. This leads to a change In the nature of the solvating molecule(s) culmlnatlng in fragmentation by desolvation of the rearranged ion.
Solvation plays a central role in most of chemistry. One reason for investigating reactivity in mass spectrometry is that solvent-free reactions can be studied (1). Solvation can also be followed in the instrument and thermochemical data assembled on the solvation process at the molecular level. Kebarle’s elegant studies (2) in which he measured enthalpies and entropies of attachment of successive solvent molecules to simple cations and anions illustrate this capability. In related work, comparisons have been made between ionmolecule reactivities of free and the corresponding monosolvated ions (3). Previous work in which chiral solvent molecules were used to distinguish chiral analyte ions in the mass spectrometer is also of note ( 4 ) . Puzo and co-workers have reported daughter scans (fragment ions arising from a selected parent ion) of the monosolvated forms of cationized isomeric sugars, viz., [M + C]+.G where C+ indicates an alkali cation, M the analyte, and G the solvent, glycerol. The differences in the relative extents of glycerol loss vs. that of the sugar allowed distinction of up to eight stereoisomers of aldohexose (5, 6). Solute/solvent interactions (7,8) can play a significant role in desorption ionization mass spectrometry. Simple acid/base reactions occurring in solution enhance gas-phase ion yields through sputtering of preformed ions (9). Less desirable solvent/solute interactions (9) can lead to chemical reactions of the analyte. The present enquiry is further motivated by the role ascribed to desolvation in reducing internal energies of the ions emitted from the condensed phase in desorption ionization experiments (10-12). The extent of fragmentation observed in these experiments may be reduced if solvated molecules are ejected (13). In addition to its connection with matrix and mechanistic aspects of particle-induced desorption, the present study derives from earlier work on the fragmentation of solvated ions. Metal ions, solvated unsymmetrically, and typified by
C2H50H... Ag+... CBHTOH, undergo competitive desolvation upon collisional activation (14). The relative extents of formation of the two monosolvated products provide information on the relative strengths of the bonds between the central metal cation and each of the solvent molecules. Investigations by tandem mass spectrometry (MS/MS) (15, 16) covering several alkali- and transition-metal cations and alkenes, alcohols, alkylamines, and pyridines as solvents revealed simple desolvation to be invariably the dominant fragmentation mode (17). The occurrence of intense adduct ions in fast atom bombardment (FAB) using glycerol as solvent led to the present investigation of ion structures as revealed by their desolvation behavior. In addition to thermochemical studies there is other precedent for the use of desolvation to study solvated species. Shukla and Stace (18-20) have examined the metastable ion dissociations of alkali-metal cations solvated by both water and methanol to draw conclusions about solvation energies and steric effects upon them. The principal finding of the present study is that the first molecule of glycerol solvent often, but not always, binds strongly to cation substrates, whereas the second molecule is usually, but not always, more weakly bound (its loss as an intact entity is far more likely). There are two prior observations that bear on these findings. Thiamine yields an adduct with glycerol (211, which fragments to a relatively small extent by desolvation (20% relative abundance) and predominantly by a reaction (loss of 119 daltons) that has been suggested to involve thiazolium ring opening to give an adduct which is not the simple solvated cation. This result suggests that the solvation of organic cations in desorption ionization experiments might in some cases involve formation of strong new bonds. However, the behavior may be rare, and Keough (22), in examining Na+ adducts between glycerol and organic alcohols, observed desolvation with loss of the intact molecules without exception.
EXPERIMENTAL SECTION Fast atom bombardment was performed with a Kratos MS-50 double-focusing instrument equipped with an Iontech fast atom source. Xenon gas was used to generate a beam of fast atoms with a flux of ca. 3 X 10l2particles cm-2 s-l and an energy of 8 keV. Linked scans (23)( B field/E field, was constant) produced daughter spectra of selected parent ions. Although no collision gas was added, the conditions in the first field-free region were such that collision-induceddissociation was probably significant. Compounds were of reagent grade and used without further purification. One molar aqueous solutions of the salts were prepared, and 1WLof the aqueous solution was applied to 4 pL of glycerol on the solids probe tip. Spectra were recorded ca. 2-3 min after application of the sample to the probe tip after evacuation of the source to approximately torr. Signal to noise for various B / E linked scans ranged from 10 to 1OOO. Peaks below 5% relative abundance are not reported.
0003-2700/88/0358-12 18$01.50/0 0 ID88 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1219
Table I. Inorganic Compounds O f f scale
compd Ad'JO,
SnCI,
MgS04 SbCls FeS04 MgClz
fragment lost (70re1 abund)
parent ion
92 (100) 92 (100) (SnCl3.G2)+ 92 (loo), 36 (25) (SnC13.G)+ 36 (1001, 92 (0) [(SnC12.(G2- H)]+ 36 (loo), 92 (20) 18 (loo), 36 (50), 48 (44) [(SnC12.(G- H)]' 74 (16) (SnCl.G)+ 36 (100) [(MgSO**(Gz+ H)]' 92 (100) [(MgSO,.(G + H)]+ 80 (100) (SbC1Z.Gz)' 36 (loo), 92 (32) 36 (loo), 72 (27), 92 (5) (SbC1Z.G)' [(FeS04.(G3+ H)]' 92 (100) [(FeS04.(Gz+ H)]+ 92 (100) 80 (loo), 92 (12) [(FeSO,-(G+ H)]+ (MgCl-GZ)' 36 (100) (MgCl.G)+ 36 (100) (Ad&)+ (AgW
Table 11. Organic Compounds
fragment lost compd methionine dicyclohexylurea fructose glucose sucrose maltose
I
+
parent ion
( % re1 abund)
[(M + H).GzI+ [(M + H)-G]+ [ (M + H).Gz]+ [(M+ H)-G]+ [(M + H)*GzI+ [(M + H)*G]+ [(M + H).Gzl+ [(M + H)*G]+ [(M + H)*G,I+ [(M + H)*G]+ [(M + "%I3 [(M + H).G]+
92 (100) 92 (100) 92 (100) 92 (loo), 18 (10) 92 (loo), 18 (16)
- HCI
rnlz
Flgure 1. Daughter spectra of di- and monosolvated SbCI,' ions, in which rearrangement of the solvent is indicated by the observed loss of HCI (linked scans at 8 keV, G = glycerol).
than isomeric species possibly generated upon excitation (24). The data for (SnC4.G)' therefore suggest that the ion is not the simple solvated analogue of (AgG)'. The loss of HC1 instead implies a rearranged structure, containing a stronger glycerol-SnC13+ bond, possibly a covalent 0-Sn linkage: CI
CI CI
CI
\ /
CI
92 (100) 92 (100) 92 (loo), 18 (5) 92 (loo), 18 (50)
+ lSnC13. G)
RESULTS AND DISCUSSION
-. t-
18 (100) 92 (100) 18 (100)
Tables I and I1 summarize the fragmentation behavior of a number of inorganic and organic cations. The ions selected for examination are all formally glycerol adducts of cations (although in some cases they can also be considered as the protonated forms of the glycerol adducts of neutral molecules) generated under conventional FAB conditions. In addition to inorganic and organic species, both mono- and diglycerol adducts are represented. In examining the data we seek to distinguish between simple solvated species and more strongly bound adducts. Those ions that fragment predominantly or exclusively by loss of an intact glycerol molecule are considered to be solvated. In these cases the glycerol unit remains present as an intact unit in the adduct, and its elimination is favored over other possible reactions. On the other hand, those ions that undergo other reactions as major contributors to their fragmentation behavior clearly contain, or can rearrange to give, units which are less strongly held than the glycerol solvent (even if these units include parts of the glycerol molecule). As such, they cannot be classified as typical solvated molecules. We recognize that there may be intermediate situations between the extremes of solvated and covalently bound structures. The behavior of the silver ion adducts is representative of what might be expected for simple solvation, viz., strong intramolecular bonding in the solvent and weaker solvent-solute binding. Both (AgG)' and (AgG,)+ fragment exclusively via loss of a single molecule of glycerol (Table I). In contrast, the mono- and diglycerated forms of the SnC13+ ion show markedly different behavior. The former ion, (SnC13.G)+displays no loss of glycerol; instead HC1 is eliminated from the complex. Daughter spectra are generally accepted as reflecting the structure of the selected ion rather
-plycarol
,%+
\ /
4
,S?+ OR H
,OR
C[
H
-
+ [SnCl2(G-H).HCl
CI
CI
\ I
Sntt
+
HCI
OR
(1)
t [SnCl2(G-H)1
ROH=G=glycerol
The fragmentation of the diglycerol adduct suggests a similar rearrangement. Loss of glycerol is the major process, but significant competition by loss of HCl also occurs. This is consistent with an [SnC12(G- H)]+ structure containing strong Sn-C1 and Sn-O bonds and solvated by molecules of both HCl and glycerol. The behavior of (SnCl.G)+ provides more support for the concepts just presented. Exclusive loss of HC1 is observed, the original Sn-C1 bond having been exchanged for an Sn-0 bond in the fragment ion. The greater strength of the Sn-0 (131 kcal/mol) compared to the Sn-Cl bond (99 kcal/mol) and the possibility of forming additional dative Sn-0 bonds with one or more of the remaining oxygen atoms both argue in favor of the concepts presented here. Neither of these factors applies to the silver-glycerol ions already discussed, given the preference of silver for sp hybridization and the relative strength of Ag-Cl (81 kcal/mol) and Ag-0 (51 kcal/mol) bonds (25). We have also tested the structural proposal embodied in eq 1 by examining the behavior of the ion that contains two fewer chlorine atoms. As expected, the dominant fragmentation involves loss of HCl. The reaction should also proceed if the product species [SnC12(G- H)]+ is itself solvated by glycerol; viz., one then expects the internal oxydechlorination of the metal shown in eq 2. This rearranged ion is expected [SnCl,(G - H).G]+
[SnCl(G - H),.HCl]+
(2)
to lose HC1 as the most weakly bound solvent species, a reaction which does indeed comprise the base peak in the spectrum of the diglycerated adduct of SnC12+. Some loss of glycerol is also observed (20% relative abundance), perhaps indicative of incomplete isomerization and loss of the glycerol molecule of solvation from the original reactant in eq 2.
1220
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
The behavior of [SnC12(G - H)]', the ion corresponding to the proposed product of reaction 1, was also examined by generating this species from SnC14in glycerol. The daughter spectrum showed a number of fragment ions, including one due to dehydration. This behavior is consistent with a covalently bound structure, and the low abundance of the fragment ions relative to the cases already discussed supports the argument that this ion contains only strongly bound groups. Turning from tin- to antimony-containing ions, one observes parallel behavior (Figure 1). Note particularly the fact that HC1 loss dominates glycerol loss, even from the formally disolvated ion, suggesting that both glycerol molecules are covalently bound to the metal. The monosolvated ion displays a peak due to the loss of two molecules of HC1, no doubt due to intramolecular oxygen-for-chlorine substitution by two oxygen atoms of a single glycerol molecule (eq 3). CI
\ $ H ,Sb,,,O CI OH
OH
C!H 0
C!H + ..S bt- 0
c';
. / +,.a"\)
CIH
OH
(3)
0
lizing factor. For example, the fragment ion generated from reaction 4 could have a tricoordinate structure, although the 18-electron rule is satisfied by the structure given. Glycerol adducts of a small number of protonated organic molecules have also been examined (Table 11). In all but two cases, loss of glycerol gives the most abundant fragment ion in the daughter spectrum. The exceptions, protonated glucose and fructose, show exclusive loss of water. Another monosaccharide, maltose, shows loss of both water and glycerol, while the sucrose adduct displays only glycerol loss. These results confirm the sensitivity of the fragmentation behavior of sugar/glycerol adducts to the particular structure being examined as suggested by Prome and co-workers ( 4 , 5 ) . It seems very likely that the detailed explanation of this behavior will turn on reorganization of solvent to yield the most stable structure possible. For example, the loss of HzO from the glycerol adduct of protonated dicyclohexylurea may proceed from a rearranged form of solvated ion in which glycerol takes the place of water because it is better able to stabilize a charge, both by its greater polarizability and its coordinating power (eq 5). Ho\ +OH
= glycerol
15)
(0 H
Moving from the main group and transition metals to an alkaline-earth chloride, magnesium chloride adducts were studied and found to exhibit similar behavior to that described above. Specifically, the daughter ion spectra of both (MgCl.G)+ and (MgC1.Gz)+exhibit only ions corresponding to the loss of HC1 from the selected parent ion. The initial adduct (MgCl-G)+probably rearranges to [Mg(G - H).HCl]+ in which a glycerol oxygen atom has been substituted-for the chloride anion in binding the metal cation. Subsequent fragmentation through the loss of HC1 can then occur to yield an ion described as [Mg(G - H)]+. Two sulfates, magnesium, and iron(I1) were mixed with glycerol, subjected to fast atom bombardment, and examined by tandem mass spectrometry. Both of the diglycerated ions selected for examination show exclusive desolvation (loss of glycerol, mass 92) in their daughter spectra. In comparison to the disolvated adducts, the monosolvated ions, [MgSO,.(G + H)]+ and [FeSO,.(G + H)]+, behave distinctly different. These ions display as their major fragments, products resulting from the loss of the species SO3 (mass 80). These reactions probably proceed similarly to those already discussed. Specifically, the initial ion may be an adduct between FeS04 and protonated glycerol or a FeS04H+solvated by neutral glycerol. The key rearrangement appears to be that which gives a covlently bonded iron-glycerol adduct solvated by HzSO,. The final form of the reactant can probably be best represented as the dicoordinate SO3 solvated ion shown in reaction 4. Desolvation is then the only low-energy mode of fragmentation available.
+
+ -e, [LFeS04H).G]
[FeSO4.lGtH)1
-e, Fi,..S04H2
I
0-OH OH (OH
=
L OH
-
- so3
+ /Fe-OH
0
4
G = glycerol
2
14)
Q03
/Fe-oH2
HO , 0
In the preceding. case the possibility that free hydroxyl groups of covalently bound glycerol (G - H) are coordinated to the central metal must be allowed as an additional stabi-
R OH (OH
= G=glycerol
/O\/NHR
HO
C
II
N+
HO-0, -H20
+
,NHR
C // N -t
H' k
.
H20
H" "R
The results encountered here illuminate the structural aspects of solvation. They also reveal how large a contribution solvation plays in the types of fragment ions generated in the desorption ionization techniques using a liquid matrix. The ability to probe solvent reorganizations of the type encountered here is a valuable attribute of tandem mass spectrometry.
LITERATURE CITED (1) Comlta, P. B.; Brauman, J. I. Science (Washhgton, D . C . )1985, 227, 863. (2) Kebarle, P. Am. Rev. Phys. Chem. 1972, 28, 445. (3) Bartmess, J. E. J. Am. Chem. SOC. 1981, 103, 1338. (4) Madhusudanan, K. P.; Mittal, S.; Durani, S.; Kapil, R. S. Org. Mass Spectrom. 1985, 20, 323. (5) Puzo, G.; Fournie, J. J.; Prome, J. C. Anal. Chem. 1985, 5 7 , 892. (6) Puzo, G.; Prome, J. C. Org. Mass Spectrom. 1985, 2 0 , 288. (7) Barofsky, D. F.; Giessmann, U. Int. J. Mass Spectrom. Ion Phys. 1983, 46, 359. (8) Tondeur, Y.; Clifford, A. J.; DeLuca, L. M. Org. Mass Spectrom. 1985, 2 0 , 159. (9) Busch, K. L.; Hsu, B. H.; Xie, Y. X.; Cooks, R. G. Anal. Chem. 1983, 55, 1157. (10) Orth, R. G.; Jonkman, H. T.; Michl. J. Int. J. Mass Spectrom. Ion Phys. 1982, 43, 41. (11) Michl, J. Int. J. Mass Spectrom. Ion Phys. 1983, 5 3 , 255. (12) . . Stulik. D.: Orth. R. G.;Jonkman, H. T.; Michl. J. Int. J. Mass Spectrom. Ion Phys. 1983, 5 3 , 341. (13) Busch, K. L.; Hsu, B. H.; Xie, Y. X.; Cooks, R. G. Anal. Chem. 1983, 5 5 , 1157. (14) McLuckey, S. A.; Schoen, A. E.; Cooks, R. G. J. Am. Chem. SOC. 1982. 104. 848. (15) McLafferty, F. W. Tandem Mass Spectrometry; Why-Interscience: New York, Chichester, Brisbane, Toronto, Singapore, 1983. (16) Cooks, R. G.; Beynon, J. H.; Caprloli, R. M.; Lester, G. R. Metastable Ions; Elsevier: Amsterdam, 1973. (17) McLuckey, S. A. Ph.D. Thesis, Purdue University, West Lafayette, IN, 1982. (18) Stace, A. J.; Shukla, A. K. Int. J. Mass Spectrom. Ion Phys. 1980, 36, 119. (19) Stace, A. J.; Shukla, A. K. J. Am. Chem. Soc. 1982, 104, 5314. (20) Stace, A. J.; Shukla, A. K. J. Phys. Chem. 1983, 87, 2286.
Anal. Chem. 1986, 58, 1221-1225 (21) (22) (23) (24) (25)
Glish, G. L.; Todd, P.
J.; Busch, K. L.; Cooks, R. G. Int. J . Mass Spectrom. Ion Phys 1084, 56, 177. Keough, T. Anal. Chem. 1985, 57, 2027. Jennlngs, K. R.; Mason, R. S. In Tandem Mass SePctrometrK Mcbfferty, F. W., Ed.; Wiley-Interscience: New York, 1983. Levsen, K.; Schwarz, H. Mass Spectrom. Rev. lg83, 2 . 77. CRC Handbook of Chemistry and Phys/cs; Weast, R. C., Ed.; CRC
1221
Press: Cleveland, OH, 1979.
RECEIVED for review September 9, 1985. Accepted January 13, 1986. This work was supported in part by the National Science Foundation (Grant CHE84-08258).
Instrumental Conditions of Secondary Ion Mass Spectrometry That Affect Sensitivity for Observation of Very High Masses William Aberth Mass Spectrometry Facility, Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, S a n Francisco, California 94143-0446
I t is demonstrated by comparlng the mass spectra of cesium iodlde cluster Ions obtalned under different Operating parameters that such factors as acceleration voltage, system pressure, and primary beam energy affect the high-mass signal current to a greater extent than that of the low mass. The measurements Imply that consideration should be given to these parameters for more effectlve analysis of highmass compounds.
Cesium iodide in many ways is an ideal sample material for investigating instrumental effects on the quality of mass spectra. The sample spectrum remains stable and produces constant-amplitude peaks for several hours thus allowing comparison of spectra obtained over a long period. Also, since the elements cesium and iodine occur as single isotopes and are relatively heavy, cluster peaks of (CsI),Cs+ or (CsI),I- can be obtained for very high mass. Positive cluster ions with n = 122 (m = 31853 daltons) ( I ) , and more recently n = 155 (m = 40433 daltons) (2),have been reported. These factors permit a sensitive analysis of the effects of various instrumental variables on the cluster ion spectrum as a €unction of mass (cluster size). The results of this investigation show the probable nature of factors that would limit observation of very high mass (>3000 daltons) bioorganic compounds and further suggest methods for overcoming these limitations.
EXPERIMENTAL SECTION Mass analysis was performed on a recently constructed single-stage Wien EXB mass spectrometer ( 3 , 4 ) .The Wein spectrometer is especially suited to the analyses of very high mass compounds because it is capable of analyzing these compounds at high acceleration voltage. The present instrument design (see Figure 1)is based partly on a previous machine design (5) and utilizes a commercial 63-cm-long EXB mass analyzer (Colutron Research Corp., Boulder, CO,Model 300-6).An acceleration voltage of up to 40 kV was used for this work. The high acceleration voltage yields high transmission efficiency (5, 6) and permits good mass resolution (7) with a single-stage analyzer. The ion source is an immersion lens type (8)that utilizes a cesium for the ion gun (Antek, Palo Alto, CA, Model Cs-160-250B) primary ion beam (9). An overall vacuum of about torr was maintained in the instrument. The ion signal was amplified by means of a postacceleration detector that utilized an aluminum collision electrode at -10 kV and a ceramic continuous dynode electron multiplier operating at -4 kV.
RESULTS AND DISCUSSION Figure 2 is a plot of (CSI)~CS+ cluster amplitudes obtained with a Wein acceleration voltage of 40 kV compared with similar spectra obtained by Campana et al. ( I O ) , Ens et al. ( 1 1 ) , and Baldwin et al. (12). All spectra are normalized to the n = 1 cluster peak. Differences in peak amplitudes for n > 1therefore reflect instrument conditions that affect the observation of the higher mass ions differently from the lower mass ions. All spectra from which the data of Figure 2 were derived were obtained by sputter ionization from a solid target of cesium iodide. The primary beams used were Cs' (Aberth and Ens et al.), Xe+ (Campana et al.), and Xeo (Baldwin et al.). Although Baldwin and co-workers had more complete data using an Aro primary beam, only the four data points published using Xeo were plotted in Figure 2. This is because the mass of the primary beam particles has been shown to have a strong influence on secondary ion efficiency (13). The similarity in mass xenon and cesium would tend to reduce the effects caused by primary particle mass difference and thus yielded a more meaningful comparison. A general observation about the data shown in Figure 2 is that the curves differ in amplitude from each other by increasing amounts as the cluster size increases. The cluster spectrum obtained with the Wein mass spectrometer lies significantly above the other curves a t the higher masses. Some possible instrumental factors that could yield such differences between the high- and low-mass relative signal amplitude are (1)pressure differences, (2) mass-dependent differences in detection efficiency, (3) differences in primary beam energy, (4) differences in flight time of analyzed ions (secondary ion acceleration voltage and drift distances), and (5) differences in ion source geometry. The relative importance of these instrumental factors will now be explored. Pressure Effects. Figure 3 shows the effect of increasing the source chamber pressure by introducing N2 gas. The source chamber (see Figure 1)includes the ion source as well as the main Einzel lens and encompasses an 80-cm path length of the secondary ion beam. The restricted pumping through a beam-defining irls, positioned between the Wein separator and the source chamber, reduces the pressure spillover into the separately pumped detector region to about 1/20 of the change in source chamber pressure. The acceleration voltage for the measurements shown in Figure 3 was 30 keV. The amplitude of the n = 1 cluster ion was unchanged for all source pressures except for the highest value of 3 X lo4 torr where
0003-2700/86/0358-1221$01.50/00 1986 American Chemical Soclety
