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Anal. Chem. 1984, 56,2274-2277
Large Cluster Ions Desorbed from Organic Salts under Particle Bombardment D. N. Heller, Catherine Fenselau,* James Yergey, and R. J. Cotter Middle Atlantic Mass Spectrometry Laboratory, Johns Hopkins School of Medicine, Department of Pharmacology, Baltimore, Maryland 21205
David Larkin Department of Chemistry, Towson State University, Towson, Maryland 21204
The sizes and lntensltles of cluster Ions formed by fast atom bombardment from a series of organlc salts In nearly saturated solutions have been evaluated. Clusters Involving as many as 29 anions and 30 cations have been recorded for cesium perfluorohexanesulfonate. The average mass of this n = 29 cluster Is 15 561.1. Abundances of the cluster Ions are found to vary as a function of salt concentration In the llquld matrix and the number of Ion pairs In the clusters. The abundance of the organlc salt cluster Cs(CsC,F,,SO,),,+ at m / z 10 240.4 was found to be 8 times greater than the abudance of the Cs,,I,, cluster at m / z 10265.5. The relatively high abundances of the sulfonate clusters permit them to be analyzed by using unit resolutlon In the high-mass range (for example at m / z 8110.5) and suggest that they may be useful as mass-marklng reference compounds.
Cluster ions of alkali salts of alkane and aromatic sulfonates have been observed for 2-7 molecules (n = 2-7) by using a variety of ionization techniques, including field desorption (1-4), laser desorption (5, 6 ) , and thermal ionization (7, 8). In these studies the gas-phase sulfonate clusters were formed from salts in the solid state. It has been observed in this laboratory (9) and others (10) that clusters of organic salts are more readily desorbed from solutions such as those used in fast atom bombardment and liquid SIMS. Accordingly we have characterized clusters of a series of cesium perfluoroalkanesulfonates in which the chain length of the fluorocarbon group varies between one and eight carbons. Clusters from other alkali alkanesulfonates have also been examined. We have examined the complexity of clusters formed and also the relative abundances of clusters as a function of the value of n, the length of the anionic fluorocarbon chain, and concentration of the salt in the liquid matrix delivered to the FAB probe.
EXPERIMENTAL SECTION Perfluorobutanesulfonyl fluoride and perfluorohexanesulfonyl fluoride were purchased from PCR Research Chemicals, Gainesville, FL. Heptafluorooctanesulfonic acid, tetraethylammonium salt, was purchased from Fluka Chemical Corp., Hauppage, NY. Cesium hydroxide (50% in water), silver trifluoromethanesulfonate, heptanesulfonic acid, sodium salt monohydrate, tetraglyme (tetraethylene glycol dimethyl ether), and tetrahydrofuran were purchased from Aldrich Chemical Co., Milwaukee, WI. Dowex 50-W cation exchange resin was purchased from Sigma Chemical Co., St. Louis, MO. Cesium iodide was purchased from Alfa Products, Danvers, MA. Sulfonyl fluorides were readily converted to sulfonic acids by reaction with H20 and sulfonic acids to their cesium salts by titration with cesium hydroxide in H20 to pH 10. Silver and ammonium sulfonate salts were converted to the corresponding 0003-2700/84/0358-2274$01.50/0
acids on an ion-exchange column and the acids titrated to pH 10 with cesium hydroxide. The solutions were evaporated under vacuum. A variety of liquid matrices were evaluated for FAB analyses. Tetraglyme was chosen as the liquid support matrix to be used for quantitative studies of the fluorinated salts. Solution were prepared by combining weighed amounts of sulfonate salt and tetraglyme, and the resultant molar concentrations were calculated. Saturated levels in tetraglyme were determined by adding tetraglyme dropwise to weighed amounts of sulfonate salt until the salt was completely dissolved. The highest concentrations used were at least 50% of saturation for any given sample. The sodium heptanesulfonate was dissolved in a nearly saturated solution in 1:l MeOHH20 before adding an equal volume of 1:1 glycerokthioglycerol. All mass spectrometric measurements were carried out on a Kratos MS-50 mass spectrometer equipped with a 23-kg magnet, DS-55 data system, a fast atom bombardment source, and a post-acceleration detector. A software signal-averagingprogram, written in this laboratory (11)for the DS-55 data system, was used so that scans of uncentroided "raw" data could be acquired and averaged before the peaks were centroided and assigned m / z values. Xenon gas was used for fast atom bombardment, with a gun voltage of 8 kV. For the studies of cluster abundance (Figures 1-4), the accelerating voltage was 3 kV and the instrument was turned to a dynamic resolution of 2000 and scanned at a rate of 100 s/decade. Uncentroided data were collected by computer from a single scan under the above conditions. Only peaks at m/z 7100 and below were included, since above this range the magnet scan is not exponential, which can result in weighted intensity values at the high end. Cluster peaks were identified by peak matching against Cs(CsI),+ clusters and against each other. They were centroided with manually chosen parameters so that an entire isotope cluster was converted to a single value for absolute intensity. A volume of 1.5 p L was loaded on a FAB probe tip of 3-mm diameter. All instrumental conditions were held constant during each run. The multiplier gain value was selected by prior test so that no peak would be saturated in a single scan. The scans lasted approximately 110 s, which did not exceed the lifetime of any sample on the probe. Heavier clusters (Figure 5 ) were recorded by scanning the magnet at 300 s/decade through an appropriate mass range with the source and collector slits open. Peak profile data were collected and 10 scans were averaged off-line. Each spectrum was plotted after applying a cubic-quadratic smoothing algorithm (11).
In order to study the cluster formation as a function of alkyl chain length, two comparison experiments were conducted. The first comparison used each compound at high concentrations empirically chosen to provide maximum cluster formation, so that the best case for each compound could be compared. The "optimal" concentration used for the C8 sulfonate was 0.067 M, for the C6salt, 0.9 M, for the C4sulfonate, 1.1 M, and for the C, sulfonate, 0.56 M. The second comparison stipulated that molar concentrations in tetraglyme be constant for the four compounds. The concentration chosen was about 15% of the optimal concentration for perfluorooctanesulfonate (0.067 M), the least soluble in tetraglyme of the four salts. Cluster ion formation from each 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
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1 cm2 in area. The cell was not calibrated for absolute measurements so data are reported as relative conductivity. Surface tension measurements were made with the same solutions by the du Nouy ring tensiometer method. The instrument used was a product of the Central Scientific Co. (Chicago, IL). The tensiometer was calibrated with standard weights. Values for surface tension in dynes/centimeter were calculated using correction factors for 6-cm rings provided by the Central Scientific co.
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Flgure 1. Abundances of cluster Ions formed from cesium trifluoromethanesulfonate (A),cesium perfluorobutanesulfonate (W), cesium perfluorohexanesulfonate(X), and cesium pemuorooctanesulfonate(0) as a function of cluster number.
of the four sulfonates was also studied at a variety of concentrations. Conductivity of standard solutions was measured on a Wescan conductivity meter with a range of 102.105 pE1. The conductivity cell used was 5 mL in total volume with parallel platinum plates
RESULTS AND DISCUSSION These studies were undertaken to discover cluster ions suitable for high-mass reference applications in fast atom bombardment mass spectrometry. The first work with sulfonates involved the sodium salt of heptanesulfonic acid. Increasing the sample concentration in glycerol led to formation of larger clusters. To construct higher mass ions, the obvious choice was cesium for the cation, and perfluorohexanesulfonate was chosen as the anion. Solubility was a problem for this compound: of glycerol, tetraglyme, thioglycerol, sulfolane, poly(ethy1ene glycol 200),and diethanolamine, only tetraglyme would accept the high sample concentrations necessary for formation of large cluster ions in FAB-MS. At high concentrations in tetraglyme, large cluster ions were observed. Consequently the study was extended to evaluate other heavy anions and to elucidate the influence of sample concentration in tetraglyme. The clusters described here have the form Cs(RS03Cs),+. In Figure 1relative intensities are recorded of cluster ion peaks formed from the four different cesium perfluoroalkanesulfonates at concentrations eelected to optimize cluster ion size and abundance. Heavy cluster ions are readily detected. The log of the abundances of these cluster ions decreases approximately linearly with the cluster number (n)within each series. A significant dip in intensity is observed for n = 4 cluster ions of all four salts in Figures 1-3. There is no correlation between cluster ion abundance and the length of the fluorocarbon chain in the anion. The cluster ions formed from the C1, C4, and c6 compounds have similar abundances in Figure 1, while those from the c8 salt are noticeably less abundant. When this study was repeated with concentrations of all four salts adjusted to the optimal concentration for the
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Figure 2. Abundances of the cluster ions shown In Figure 1 plotted as a function of cluster mass.
6000
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
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CS(C~F~~SO$S)~~+ AVERAGE NASS 15561.1
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fluorohexanesulfonateat a variety of concentrations as a function of cluster number. Insert shows the slope of relative intensity vs. n as a function of molarity for cesium trifluoromethanesulate (A),cesium perfluorobutanesuifonate 0, cesium perfiuorohexanesulfonate(X), and cesium perfluorooctanesulfonate (0).
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Figure 5. Cesium perfluorohexanesulfonatecluster, n
Figure 3. Abundances of the cluster Ions formed from cesium per-
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Surface tension ( 0 )and conductivity (0)plotted as log functions of the concentration of cesium perfluorohexanesulfonate. The slope of cluster intensities vs. cluster number (X) Is also presented as a function of the concentrationof cesium perfluorohexanesulfonate. Figure 4.
CBcompound used in Figure 1,linear correlations were still observed between n values and intensities in all cases. However, cluster ions from the C6 salt were distinctly more abundant than the others. Because of widespread interest in the mass spectrometry of heavy ions and the need for reference ions at higher m / z values, these relative intensities presented in Figure 1 are replotted in Figure 2 as a function of m/z. It can be seen that peak intensity decreases as a function of cluster number for
each of the organic salts and not as a function of the mass of each cluster. Conversely, higher mass clusters are more readily constructed by increasing the sizes of the ions rather than by increasing the n value. Concentration is a major determinant of cluster ion size and intensity. In general, the concentration of each cesium salt at which significant clustering (n 2 4) can be detected is inversely proportional to the length of the fluorocarbon sulfonate chain. In Figure 3 relative intensity is plotted as a function of cluster number a t six different concentrations of cesium perfluorohexanesulfonatein tetraglyme. At concentrations less than about 0.01 M, this type of cluster formation (n I 4) is negligible. A discontinuity exists at about 0.05 M. Increases in concentration affect cluster intensity differently on either side of this point: below about 0.05 M, increased concentration enhances higher clusters more than smaller ones; above 0.05 M, all cluster intensities are increased by similar but small amounts. This discontinuity is most easily perceived by examining the effect of concentration on the slopes of the lines correlating cluster ion abundance and the n value as fitted by linear regression analysis. In Figure 3B slopes are plotted as log functions at varying molarities for all four of the perfluoroalkylsulfonates. A similar discontinuity is reached for the C4 case at about 0.2 M. Inflections were not defined by the concentrations studied for the C8and C1 salts. For the C4and CBsulfonates, the critical concentration is lower for the anion with the longer fluorocarbon chain length. One explanation for the behavioral discontinuities is that they are related to a change in the relative abundance of different sized aggregates of the perfluoroalkanesulfonates in either the gas-liquid interface (22) or in the bulk solution. Both surface tension and electrical conductivity were measured in tetraglyme solutions of cesium perfluorohexanesulfonate through the concentration range between 0.0008 and 0.42 M. Electrical conductivity is found to increase smoothly through this concentration range, implying no sudden change in the size of aggregates in solution. Surface tension measurement shows a sharp decline which occurs at the same concentration as the discontinuity in the slope of cluster abundance (Figure 4). Unfortunately, the behavior of ionic surfactants in nonaqueous systems is not well understood. A comprehensive
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
review of this topic is given in ref 13. Organo-fluorinated compounds, however, appear to be more effective in lowering the surface tension of some organic solvents than their nonfluorinated counterparts, indicating an enhanced surface activity (14). Tetraglyme should be classed as a nonpolar solvent, with a dielectric constant around 7.5, compared to 78.4 for water and about 2 for cyclohexane (15). In nonpolar solvents micelle formation is not initiated as a sharply defied process; there is no critical micelle concentration but rather accretion of small aggregates beginning at relatively low concentrations (13) and increasing gradually as solute concentration increases. A multiple equilibrium model for micelle formation in nonaqueous solvents (14) suggests that the average molecular weight of an aggregate is 4 times the monomer molecular weight at concentrations of 0.1 M solute. More recently (16)similar conclusions have been reached concerning the size of aggregates of ionic surfactants in solvents of low dielectric constant. Figure 4 supports the interpretation that the relative (though not the absolute) abundances of clusters in the gas phase become fixed when the surface is saturated. The log dependence of the slope of relative abundances could reflect a pseudo-fit-order rate of dworption (impinging xenon atoms held a t a constant flux) which changes to zeroth order as the surface is saturated. Although the multiple determinants of relative abundances of gas-phase clusters are not yet defined, something can be said about the anomalously low intensities of the n = 4 cluster ions of all four salts illustrated in Figures 1-3. In studies of CsI clusters by secondary ion mass spectrometry, such dips in gas-phase intensity curves have been attributed to particularly unstable geometries associated with aggregates containing one more molecule than aggregates with particularly stable geometries (17, 18). If the present curves are interpreted in this way, n = 3 is found to be favored, irrespective of the fluorocarbon chain length. This suggests that the polar sulfonate end is the critical moiety involved in forming the clusters. Consistent with characterization of solvent interactions as nonpolar, a stable grouping of three molecules plus a cation could be envisioned to take a planar form, lipophilic tails out, apparently destabilized by addition of a fourth molecule. Additionally, a planar form would enable stacking of trimers to take place to form larger clusters. Indeed, the C4 and CBsulfonate cluster ions exhibit low intensities when n = 7, consistent with this suggestion. Perforce, the intensities studied by mass spectrometry are of cluster ions in the gas phase, and their relationship to aggregates in solution is undetermined as yet. Nonetheless, it is of interest that a model for aggregate structures in aqueous solutions has been proposed recently (19) which invokes small substructures of specific size and that the example developed contains three monomers per subunit. Qualitative studies were also conducted to evaluate the occurrence of yet heavier cluster ions than those quantified in Figures 1-3. Cluster ions as high as n = 29 were recorded for cesium perfluorohexanesulfonate. This peak is accompanied by satellites separated by 19 amu, as shown in Figure 5. The average mass (9) for the cluster is 15561.1. Although the cesium fluorocarbon sulfonates provided ions with the heaviest masses, many of the other salta studied clustered with similarly high n values. A series of cluster ions of sodium heptanesulfonate have been recorded with n values as high as 25 (average mass for Na(C7H16S03Na)25, 5079.2).
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The abundances of the sulfonate cluster ions are quite high, allowing facile recording of heavy ions at unit resolution. An example has been published (9) of the n = 15 cluster ion of cesium perfluorohexanesulfonate, with monoisotopic mass 8110.5, recorded at a resolution of 8000. A comparison was made in this study of the abundance of the n = 19 cluster ion of cesium perfluorohexanesulfonate (average mass 10 240.4) with the n = 39 cluster ion of cesium iodide (average mass 10 265.5). With all experimental parameters held constant except for the use of tetraglyme to support the sulfonate salt and no solvent for CsI, the ratio of peak intensities was about 81. Although these two clusters have similar masses, the cluster numbers are quite different, and this probably accounts for the difference in sensitivity.
CONCLUSIONS Large clusters of varius alkali alkanesulfonates can be desorbed from highly concentrated solutions by fast atom bombardment. In the present study clusters with n values as high as 29 were observed and average masses as high as 15 500. Strong peak intensities can be achieved, which make these heavy cluster ions suitable for use as mass markers and reference compounds for mass spectrometry. Peak intensities fall off linearly with the n value, rather than the overall mass of the cluster ion. The relationships between cluster ion abundances and concentration show discontinuities which correspond to a discontinuity in surface tension, not in conductivity of the solution. ACKNOWLEDGMENT We thank Kelsey Cook for hepful discussions. Registry No. Cesium trifluoromethanesulfonate, 41524-04-3; cesium perfluorobutanesulfonate, 92011-16-0; cesium perfluorohexanesulfonate, 92011-17-1; cesium perfluorooctanesulfonate, 92011-18-2. LITERATURE CITED Large, R.; Knof, H. J . Chem. Soc., Chem. Commun. 1974, 935. Schulten, H. R.; Kummler, D. Fresenius’ 2.Anal. Chem. 1979, 278, 13. Mathias, R.; Willlams, A. E.; Games, D. E.; Jackson, A. H. Org. Mass Spectrom. 1978, 11, 286. Roberts, G. D.; White, E. Blomed. Mass Spectrom. 1984, 11, 273. Vastola, F. J.; Mumma, R. 0.:Plrone. A. J. O w . Mass Sbctrom. 1970, 3 , 101. Mumma, R. 0.; Vastola, F. J. Org. Mass Spectrom. 1972, 6 , 1373. Kosugi, Y.; Matsumoto, K. Fresenlus’ Z . Anal. Chem. 1982, 312, 917
&hade, U.; Stoll, R.; Roilgen, F. W. Int. J . Mass Spectrom. Ion Phys. 1983, 4 6 , 337. Fenseiau, C. C.; Yergey, J. A.; Helier, D. N. Int. J . Mass Spectrom. Ion Phys . 1983, 53 1. Lyon, P. A.; Stebblngs, W. L.; Crow, F. W.; Tomer, K. B.; Lippstreu, D. L.; Gross, M. L. Anal. Chem. 1984, 5 6 , 8. Yergey, J. A., unpubilshed work, Middle Atlantlc Mass Spectrometry Facility, 1983. Barber, M.; Bordoll, R. S.;Elliott, G. J.; Sedgwick, R. D.; Tyler, A. N. J . Chem. Soc., Faraday Trans. 11983, 1, 1249. Kentes, A. L.; Gutmann, H. Surf. Colloid Sci. 1978, 8 , 1. Mulier, N. J . fhys. Chem. 1975, 79, 287. Cook, Kelsey, Unlverslty of Illinois at Urbana, prlvate communication, 1983. Mulier, N. J . Co//o/dInterface Sei. 1978, 6 3 , 383. Barlak, T. M.; Wyatt, J. R.; Colton, R. J.; DeCorpo. J.J.; Campana, J. E. J . Am. Chem. Soc. 1982, 104, 1212. Ens, W.; Beavls, R.; Standlng, K. E. Phys. Rev. Len. 1983, 5 0 , 27. Fromherz, P. Chem. Phys. Len. 1981, 7 7 , 480. I
RECEIVED for review April 10, 1984. Accepted July 5, 1984. This work was supported in part by National Science Foundation Grant PCM8209954.
