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This further suggests that molecular associations may be solution-phase phenomena.
CONCLUSION The data presented above demonstrate that FAB-MS may be used not only to determine the presence of intermolecular complexes in solutions but also to probe the selectivity of ligand-ligand interactions. In selecting ligands that can best associate via multiple interacting sites, our split probe tip experiments have demonstrated that, at least for such ligand systems, gas-phase clustering is not favored. These results indicate that the normal FAB spectra of multiple-binding-site intermolecular associations, may reflect the solution equilibria. On the other hand, when intermolecular association requires a simpler interaction as in the crown ether-alkali metal complexes, even the wide gap separating the two halves of the split probe tip cannot suppress clustering. In a broad sense, intermolecular associations of the type described here may be influenced by structural parameters and solution conditions. Stabilization of the complex is enhanced through hydrogen bonding, R bonding, and electrostatic interactions. It is envisioned that by combining information from FAB-MS analysis with crystallographic data and molecular modeling techniques (4,5),it may be possible to evaluate the relative contributions of these parameters. As a result it should be possible to design modified ligands with enhanced selectivity and increased affinity in interactions with ligands, receptors, or enzymes. FAB-MS can provide a sensitive and rapid screening for such features.
LITERATURE CITED (1) Barber, M.; Bordoli, R. S.;Sedgwick, R. D.; Tyler, A. N. J . Chem. Soc., Chem. Commun. 1981, 325. (2) Roepstorff, P.; Hojrup, P.; Moller, J. Biomed. Mass Spectrom. 1985, 72,181. (3) Meijers, J. C. M.; Lim, C. K.; Lawson. A. M.; Peters, T. J. J . Chromatogr. 1988, 352,231. (4) Williams, D. H.; Bradley, C.; Bojesen, G.; Santikarn. S.:Taylor, L. C. E. J . Am. Chem. SOC. 1981, 703,5700. (5) Clark, J. H.; (;reen, M.; Madden, R.; Reynolds, C. D.; Dauter, 2.; Miller, J. M.; Jones, T. J . Am. Chem. SOC. 1984, 706, 4056. (6) Brown, S. J.; Miller, J. M. J . Chem. Soc., Perkin Trans. 2 1987, 1129. (7) Michaud, D. P.; Brennan, T. F.; Vouros, P. Presented at 36th Annual Meeting of the American Society for Mass Spectrometry, San Francisco, CA; 1988. (8) Caprioli, R. M. Anal. Chem. 1984, 55, 2387. (9) Johnstone. R. A. W.: Lewis, I. A. S.;Rose, M. E. Tetrahedron 1983, 39. 1597. (10) S&h,G. D.; Griffin, J. F. Science 1978, 799,1214. (11) Blundell, T. L.; Hearn, L.; Tickle, I.J.; Palmer, R. A,; Morgan, B. A,; Smith, G. D.: Griffin, J. F. Science 1979, 205, 220. (12) Ishida, T.; Kenmotsu, M.; Mino, Y.; Inoue, M.; Fujiwara, T.; Tomita, K.-I.; Kimura, T.; Sakaibara, S. Biochem. J . 1984, 278,677. Protein Res. 1983, 27,223. (13) Suresh, C. G.;Vijayan, M. Int. J . Pep* (14) Garofai, A. L.; Williams, D. A.; Brennan, T. F. Cryst. Struct. Commun. 1979, 8, 953. (15) Sunner, J. A.; Morales, A.; Kebarle, P. Anal. Chem. 1987, 59, 1378. (16) Cotton, F. A.; Wilkins, G. Advanced Inorganic Chemistry, 4th ed.: John Wiley and Sons: New York. 1980. (17) Johnstone, R. A. W.; Rose, M. E. J . Chem. SOC.,Chem. Commun. 1983, 1268. (16) CRC Handbook of Chemistry andPhysics, 66th ed.; Weast. R. C., Ed.; CRC Press, Inc.: Boca Raton, FL, 1986.
RECEIVED for review July 27, 1989. Revised manuscript received November 28, 1989. Accepted February 5, 1990.
Effect of Hydrogen Substitution on the Mass Spectrometry and Size-Exclusion Chromatography of Perfluorinated Polyether Fluids as Determined by Time-of-Flight Secondary Ion Mass Spectrometry Steven M. Hues, Jeffrey R. Wyatt, and Richard J. Colton* Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000 Bruce H. Black Ceo-Centers, Inc., 10903 Indian Head Highway, Suite 502, Ft. Washington, Maryland 20744 The secondary ion mass spectrometry (SIMS) of perfluorkrated polyether (PFPE) fkrids reveals an e x t d v e series of hlgh-mass Ions whose distribution depends on the molecular weight, structure, and fragmentation pattern of the oiigomers. Analysis by site-exclusion chromatography (SEC) showed the fluids to consist of elther single or double component systems. Allquots of the components were collected from the SEC column effluent and subsequently analyzed by time-of-flight SIMS. The oligomer distributlons of the PFPE fluids were found to dlffer both in their number average molecular weight and in the degree of hydrogen substitutlon. The observed SEC separation was dependent on the molecular weight and the effect of hydrogen substitution on the molecular volume.
INTRODUCTION Perfluorinated polyether (PFPE) fluids are finding widespread application as high-temperature lubricants ( I , 2 ) because of their thermal stability, good lubricity, viscosity index,
and general chemical inertness. However, the reaction of these fluids with ferric fluoride, aluminum chloride, and other Lewis acids and with iron and titanium alloys a t elevated temperatures has been noted (3, 4). The mechanism of fluid degradation or reactivity may be associated with the presence of a small number of substitutional hydrogen species ( 5 ) ,due to incomplete fluorination. The fluids used in this study are homopolymers of trifluoro(trifluoromethy1)oxirane derived from two different sources: Krytox (E. I. du Pont de Nemours & Co.) and Aflunox (PCR, Inc.). The structure of the polymer is CF,CF,(OCF,CF(CF,)),F, and its IUPAC name is a-perfluoroethyl-w-fluoropoly(oxyperfluoro-2-methylethylene). The specific perfluorinated fluids used in this study are commonly known as Krytox 143 AZ and Aflunox 606. These fluids are low vacuum grade P F P E s of relatively low viscosity and low molecular weight. Krytox 143 AZ and Aflunox 606 have average molecular weights of approximately 1850 (determined by NMR) and 2100 amu, respectively (6, 7). The third fluid used in this study, Krytox TLF-7076, was specially hydrogenated by Du Pont to give the molecular
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structure of CF,CFH(OCF&F(CF,)),F, where fluorine on the second carbon atom in the a-terminal group is replaced by hydrogen (as confirmed by NMR (8)). The IUPAC name for the polymer is a-1,2,2,2-tetrafluoroethyl-w-fluoropoly(oxyperfluoro-2-methylethylene), and the polymer is said to be “hydrogen-endcapped”. The measured hydrogen content of the hydrogenated fluid is 0.86 mg/mL, compared to 0.02 mg/mL for Krytox 143 AZ (8)and C0.005 mg/mL for Aflunox 606. The physical properties of these PFPE fluids are determined by the molecular weight distribution and structure of their oligomers (6, 7). A common method to determine the oligomer distribution of polymers is size-exclusion chromatography (SEC) (9). In SEC, the polymer is dissolved in a suitable solvent and passed through a porous gel column. Those polymer molecules having a molecular volume that is greater than the pore diameter will be excluded from the pore permeation. As a result, these molecules have a retention volume only slightly greater than the void volume of the column. As the molecular volume of the oligomer decreases, pore permeation increases with a concomitant increase in retention volume. Retention volume, i.e., the volume of mobile phase necessary to elute a particular component from the column, is, therefore, a function of the component’s molecular volume. If the structural characteristics of a given molecule remain constant, then the molecular weight of a given fraction may be derived directly from its retention volume. However, if effects occur that can alter the tertiary structure of the molecule, then its effective molecular volume, and retention volume, will be commensurately affected. Time-of-flight secondary ion mass spectrometry (TOFSIMS) was used to measure both the oligomer distribution and structure of the eluted PFPE fractions prepared by SEC. These PFPE have been studied previously in this laboratory by TOF-SIMS (8). In general, the negative secondary ion mass spectra of the PFPE’s consist of series of alkoxy ions formed predominately by cleavage of the ether linkages in the polymer backbone. Each ion in a series is separated in mass by the polymer repeat unit, which is 166 amu. The most intense alkoxy ion series (see below) does not, however, provide information about the a-terminal group (which contains the substitutional hydrogen), because this end of the polymer is lost during fragmentation. Determining the presence of the hydrogen impurity involves a more detailed analysis of the polymer fragmentation pattern (10, 11) or, as was found in the previous study of these fluids (B), the correlation of Hsecondary ion intensity with hydrogen content. This correlation could not be applied with certainty to this study, however, because thick films of bulk fluids are required to mask the signal due to surface contaminants. The present study uses very thin films deposited on acid-etched Ag substrates where the contributions by surface contaminants to the H-ion signal intensity are variable. The positive secondary ion mass spectra of the PFPE’s consist of series of fragment ions formed by cleavage of C-C or C-0 bonds in the polymer backbone giving rise to carbonium-type ions. Each ion in a series is separated in mass by the polymer repeat unit. Again, the polymer fragmentation pattern can be used to deduce the structure of the polymer including the terminal groups (10, 11). In addition, with a thin film of the fluid deposited onto an acid-etched Ag foil, the Ag cationized oligomer distribution of the PFPE fluids can be measured from the TOF-SIMS spectra (10, 11).
EXPERIMENTAL SECTION SEC separation was achieved with a 50-nm pore size, 10-pm particle size, 300 mm X 7 mm i.d. high-resolution size-exclusion column (Beckman Corp., Microsperogel). The mobile phase was
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1,1,2-trichloro-1,2,2-trifluoroethane and the flow rate was 0.5 mL/min. Samples were introduced onto the column by a sample injector valve (Rheodyne, Model 7125) equipped with a 20-pL sample loop. Effluent detection was performed with a differential refractometer detector (Waters Corp., Model 401). The TOF-SIMS instrument used in this experiment was recently constructed at the Naval Research Laboratory and is discussed in detail elsewhere (8, 12). This instrument utilizes a pulsed ion gun, containing a thermionic emitter source, which produces 2-5-11s pulses of 13.0-keV Cs’ ions. These primary ions bombard the sample, which is held at the secondary ion accelerating potential of *5.0 keV resulting in a primary ion impact energy of 18.0 keV and 8.0 keV for negative and positive secondary ions, respectively. The mass analyzer consists of a 60-cm flight path into a reflectron electrostatic mirror analyzer where the secondary ions are energy focused and reflected back along the flight path to a dual channel plate annular detector. The mass spectra shown are the sum of 3 X lo6 spectra collected at a rate of approximately 5 kHz (Le., data acquisition time of 600 s) with a flight time resolution of f l ns and a mass resolution ( m / A m ) of approximately 1500. The primary ion beam had an average continuous current of 0.2 pA and bombarded a sample area of approximately 3 mm2. This corresponds to a primary ion current density of 6 pA/cm2 and a total Cs’ ion fluence density of 4 nC/cm2, which places the measurements well within the static SIMS limit. The samples were prepared by dissolving 20-25 pL of the PFPE (J. T. fraction in 0.5 mL of 1,1,2-trichloro-1,2,2-trifluoroethane Baker Chemical Co.). Five microliters of this solution was then deposited on Ag foil, which had been freshly etched in 20% nitric acid and triply rinsed in distilled water. The Ag foil was then air-dried for 5 min and placed directly into the TOF-SIMS instrument.
RESULTS AND DISCUSSION A. SIMS Analysis of Krytox 143 AZ, Aflunox 606, and Krytox TLF-7076. A.1. Negative Secondary Ion Mass Spectra. The secondary ion mass spectra of PFPE’s are characterized by an extraordinarily high negative ion abundance stemming undoubtedly from the high stability of the perfluorinated negative ion. The negative secondary ion mass spectra of Krytox 143 AZ, Aflunox 606, and Krytox TLF-7076 are compared in Figure 1. The figure insets show an expanded region of each spectrum giving details about the fragmentation pattern. The mass spectra are composed of several series of alkoxy-type ions where each ion in a particular series is separated by the polymer repeat unit of 166 amu. The most intense ion series corresponds to alkoxy ions of the form [ (OC3F6),,F]-where fragmentation of the polymer backbone occurs at any of the perfluorinated methine C-0 linkages (the oxygen is retained by the ion). The ion series extends to n = 18 ( m / z 3007) for Krytox 143 AZ, to n = 24 ( m / z 4003) for Aflunox 606, and to n = 27 ( m / z 4501) for Krytox TLF-7076. The extent of the ion distribution gives a relative ranking to the average molecular weight order of these fluids, e.g., Krytox 143 AZ < Aflunox 606 < Krytox TLF-7076, and can be used qualitatively to fingerprint the fluids and to look for lot-to-lot variations in the same fluid. The loss of the a-terminal group during fragmentation makes it impossible to use this fragment ion series to distinguish between perfluorinated or hydrogen-endcapped fluids. The minor fragment ion series (see insets in Figure 1) contain quantitative information about the structure of the polymer including the terminal and side groups. The mass peak at m/z 633 (and 799,965, ...) labeled a,for example, has been identified by Bletsos et al. (10) as an oxonium ion of the form [CF3CF2(OC3F6),0]-, which contains the a-terminal group. We agree with this assignment and can further substantiate it by showing how this oxonium ion changes in intensity as a function of the hydrogen content of the fluid. For example, in Krytox 143 AZ and Aflunox 606 where the hydrogen content is low, the mass peak at m / z 633 is relatively
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I ‘ i
&
i
Table I. Relationship between Negative Ion Peak Intensity Ratio ( m / z 615 to m / z 633) and the Hydrogen Content of the Fluids
integrated peak intensity, counts fluid
m / z 615
m / z 633
ratio
H concn,b mg/mL
Aflunox 606 Krytox 143 AZ Krytox TLF-7076
