Anal. Chem. 1987, 59, 2637-263%
length) and L3 with a f/5,254-mm focal length achromat in system led to a factor Of approximately increase in The bulk of this improvement resulted from the camera lens and is probably due to more effective collection of marginal rays.
ACKNOWLEDGMENT
meauthors thank F, T~~~~~ Gamble of ~~~i~~~University and Prabir Dutta of Ohio State University for useful comments regarding this work. In addition, we thank Professor Dutta for the use of a Spex 1877B “triplemate”spectrometer.
LITERATURE CITED Kuwana, T.; Winograd, N. Eiectroanal. Chem. 1974, 7 , 1-74. Heineman, W. R. Anal. Chem. 1978, 50, 390A. Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980;Chapter 14. Helneman, W. R.; Hawkridge, F. M.; Blount, H. N. I n Electroanalytical Cl!emlstfy; Bard, A. J., Ed.; Marcel Dekker: New York, 1984;Vol. 13, P
1.
McCreery, R. L,, In physical Methods in Chem/sw; Rossiter, B., Ed.; Wiley: New York, 1986;Vol. 2,p 561. M ~ c R. ~L,; Robinson, ~ ~ ~R. ,S,J , ~~ectfoanal, Chem, Intertack/ Electrochem. 1985, 182, 61. Jan, C. C.; Lavine, K.; McCreery, R. L. Anal. Chem. 1985, 5 7 , 752. Van Duyne. R. P. I n Chemical and Biological Applicatlons of Lasers; Moore, L. B., Ed.; Academic: New York, 1979; Vol. 4, Chapter 4. (honey, R. P.; Mahoney, M. R.; McQuilkn, A. J. I n Advances In Infrared and Raman Spectroscopy; Clark, R, J., Hester, R, E,, Eds,; H ~ den: London, 1982;Vol. 9,Chapter 4. Fleischmann, M.; Hill, I. R. In Comprehensive Treatles of Electrochemistry; White, R. E. et al., Eds.; Plenum: New York, 1984; Vol. 8,Chapter 6. Schwab, s. D.; McCreery, R. L.; Gamble, F. T. Anal. Chem. 1988, 58, 2486. Jeanmaire, D. L.; Van Duyne, R . P. J . Electroanal. Chem. Interfac/& Electrochem. 1975, 66, 235.
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(13) Jeanmaire, D. L.; Suchanski, M. R.; Van Duyne, R. P. J. Am. Chem. SOC. 1975, 97. 1699. (14) Clarke, J. S.;Kuhn, A. T.; Orville-Thomas, W. J. J . Electronanal. Chem. Interfacial Electrochem. 1974. 54. 253. (15) Van Duyne, R. P. J. Phys., Colloq. 1977, 5,i39. (16) Van Duyne, R. P. Haushalter, J. P. J . Phys. Chem. 1984, 88, 2446. (17) Beck, S.M.; Brus. L. E. J . Chem. Phys. 1981, 75, 4934. (18) Chao, J. L. Appl. Spectrosc. 1981, 35, 281. (19) Hug, W.; Surbeck, H. J . Raman Spectrosc. 1982, 13, 38. (20) Campion, A.; Brown, J.; Grizzle, W. M. Surf. Sci. 1982, 715, L153. (21) Chang, R. R.; Long, M. B. I n Topics in Applled Physlcs; Cardona, M., Guntherodt, G., Eds.; Sprlnger-Verlag: Berlin, 1982;Vol. 50, Chapter 3. (22) D’Orazio, M.; Hirschberger, R. Opt. Eng. 1983, 22, 308. (23) Asher, S. A.; Flaugh, P. L.; Washinger, G. Spectroscopy (SpringfleM, Oregon) 1968, 1 ,-26. (24) Freeman, J. J.; et al. Appl. Spectrosc. 1981, 35, 196. (25) Schwiesow, R. L. J . Opt. SOC.Am. 1989, 59, 1285. (26) Barrett, J. J.; Adams, N. I. J. Opt. SOC.Am. 1988, 58, 311. (27) Jan, C. C.; Lavine, B. K.; McCreery, R. L. Anal. Chem. 1985, 57, 752. (28) Howard* R.; M. Spectrosc. 1g86v 1245. (29) Cheng, H. Y.; McCreery, R. L.; Sackett, P. M. J . Am. Chem. SOC. 1978. 100. 962. (30) Mayausky,.~. s.;McCreery, R. L. J . Electroanal. Chem. Interfacial Electrochem. 1983, 145, 117. (31) Dryhurst, G.;et al. Biological Electrochemistry; Academic: New York, 1982;Vol. 1, Chapter 2. (32) Hawley, M. D.; Tatawawadi, S.V.; Piekarski, S.;Adams, R. N. J . Am. Chem. SOC. 1967, 89, 447. (33) Sternson, A. W.; McCreery, R. L.; Feinberg, B.; Adams, R. N. J. Electroanal. Chem. Interfacial Electrochem. 1973, 46, 313. ~ - (34) Adams, R. N.; Hawley, M. D.; Feldberg, S. W. J . Phys, Chem, 1987, 71, 851. 401
RECEIVED for review March 31,1987. Accepted July 20,1987. This work was supported by the Chemical Analysis Division ofthe National Science Foundation and by the Dow Chemical co.
CORRESPONDENCE Fourier Transform Infrared Photoacoustic Spectroscopic Study of Surface Texture in Brush and Polymeric Bonded Phases Sir: Liquid chromatographic bonded stationary phases are generally classified into two categories, “brush” and “polymeric”. They are produced by reacting monofunctional or polyfunctional silanes with the accessible surface silanols of silica gel (1-5). The effect of textural differences between polymeric and brush phase ligands on solute selectivity is of particular interest. Sander and Wise (2)suggest that changes in selectivity for polycyclic aromatic hydrocarbon mixtures is the result of surface coverage, bonded phase type, and hydrosilylation reaction conditions. Lochmuller et al. (6) synthesized a model stationary phase to test predictions based on a lattice-model, unified molecular theory for selectivity. Their results indicate that selectivity follows the order of rodlike solutes > platelike solutes > flexible chains. Chemically modified, liquid chromatographic stationary phases appear to have an active role in solute selectivity. Consequently, the examination of derivatized silica surfaces is essential to a better understanding of how bonded phases effect retention and selectivity (7-10). In a recent paper by Hunnicutt et al. (11),monofunctional pyrenesilanes covalently bound to a microparticulate silica were examined by Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS). In that paper it was suggested that
Table I. Percent Carbon for Brush and Polymeric Propylpyrene Bonded Phases Determined by Elemental Analyses % carbon
brush polymeric
1.19 1.36
1.49 1.99
3.11
3.76
5.56 5.11
6.95
7.24
for these “brush” phases, the enhancement of weakly allowed vibrational transitions between 1350 and 1725 cm-’ could be attributed to the interaction of the pyrene moiety with surface silanols. The observed decrease in intensity of these vibrational transitions with increasing surface coverage was attributed to steric constraints which limit possible conformational changes of the bound silane and, consequently, reduce the pyrene ligand/surface silanol interaction. The work presented here indicates that, for the surface coverages studied, polymeric phases interact with surface silanols to a lesser degree than brush phases. These results support a bonded-phase model currently under development in our laboratory, which suggests that covalently bound, polymeric stationary phases are motionally more constrained than corresponding brush phases.
0003-2700/87/0359-263780 1.50/0 0 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
3.0
E.
7
& E
0
urn
5 2 5
2.5 2.0 1.5
Z $
1.0 0.5
\ 0.0
0
4
2
6
a
cmam Flgure 2. Plot of peak area (absorbance units) vs. % carbon for brush (A) and polymeric (B) bonded phases. ,-,
4000
3000
2000
1000
WAVENUMBERS (crn-1) Flgure 1. FTIR-PA spectra of (A) underivatizedPartisil-10, (B) 6.95% carbon brush phase, and (C) 7.24% carbon polymeric phase.
EXPERIMENTAL SECTION Silane Synthesis. Silane synthesis and subsequent reaction of the silane with silica gel have been described (12). Methyldichlorosilane (Petrarch Systems, Inc.) was used without further purification in the synthesis of the polymeric silane. Brush and polymeric phase surface coverages ranged from 1.19% to 7.24% carbon. Table I lists the percent carbon for both the brush and polymeric phases used throughout this study. Photoacoustic Spectra. All spectra were obtained with an IBM/IR-95 FTIR spectrometer with a sample-gas-microphone photoacoustic or McClelland cell (13). The modulation frequency ranged from 141 Hz at 600 cm-' to 940 Hz at 4000 cm-l using a mirror velocity of 0.059 cm/s. Prior to data acquisition each sample was purged for approximately 120 s with helium (National Welders) to enhance the PA signal and to remove vapor phase H,O and COz within the cell. A total of 512 scans at 8-cm-' resolution were accumulated and transformed by use of the Cooley-TukeyFFT with Happ-Genzel apodization for each spectrum. The sample spectrum (S) was then ratioed to the spectrum of carbon black (R) to remove any spectral features arising from the IR source, the optics, and/or the photoacoustic cell. All spectra were collected to the same integrated absorbance (67.2 A 1.8 absorbance units at 1065 cm-') for the Si-0-Si peak (asymmetric stretch) at 1000-1200 cm-' (14). This permitted quantitative comparisons among the spectra. RESULTS AND DISCUSSION Figure 1 shows a spectrum of underivatized silica gel (A), a 6.95% carbon brush phase (B), and a 7.24% carbon polymeric phase (C). The spectrum of underivatized silica gel exhibits peaks at 811, 979, and 1000-1200 cm-' which are characteristic of the Si-0-Si symmetric stretch, Si-OH stretch, and Si-0-Si asymmetric stretch, respectively. The broad band from 3400 to 3600 cm-' is characteristic of surface adsorbed water. The absorption of this band remained relatively constant for all spectra studied, which indicates similar water content for all samples. The peaks between 1350 and 1725 cm-l can be assigned to the pyrene ring. The peak of interest, at 1697 cm-', has been assigned to the BBusymmetry arising from the first overtone of a pyrene-ring deformation mode (11). The peak area, measured for the 1697-cm-' band vs. percent carbon for both bonded-phase types is illustrated in Figure
2. In line with results reported by Hunnicutt et al. (11),the intensity of the band at 1697 cm-' (aromatic C=C stretch) decreases with increasing percent carbon for both polymeric and brush phase derivatized silica. Polymeric phases exhibit lower integrated intensities at 1697 cm-' than brush phases for similar percent carbon. This may indicate that bound polymeric pyrenesilane ligands interact with surface silanols to a lesser extent than brush phase ligands. A 1% carbon polymeric surface appears to have the same hindered interaction with surface silanols as a more motionally constrained 5% carbon brush phase. At near saturation surface coverage (7% carbon), polymer samples appear to have little or no pyrene/surface silanol interaction. The pyrene/surface silanol interaction for brush phases, however, is constant from 5% to 7% carbon. The data imply that polymer formation inhibits the motional or conformational change of the propyl chain moiety to a greater extent than brush phase derivatization.
LITERATURE CITED (1) (2) (3) (4)
Majors, R. E. J . Chromatogr. Sci. 1980, 18, 488-511. Sander, L. C.; Wise, S. A. Anal. Chem. 1984, 56, 504-510. Verzele, M.; Mussche, P. J . Chromatogr. 1983, 254, 117-122. Hernetsberger, H.; Kellerrnann, M.; Ricken, H. Chromatographia 1977, 10, 726-730. (5) Colin, H.; Guiochon, G. J . Chromatogr. 1977, 147, 289-312. (6) Lochmuller, C. H.; Hunnicutt, M. L.; Mullaney, J. F. J . f h y s . Chem. 1985, 89, 5770-5772. (7) Gilpin, R. K. J . Chromatogr. Sci. 1984, 22, 371-377. (8) Lochrnuller, C. H.; Marshall, D. 6.;Wilder, D, R. Anal. Chem. 1980, 52, 19-23. (9) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068- 1075. (10) Miller, J. D.; Ishida, H. Anal. Chem. 1985, 57, 283-288. (11) Hunhicutt, M. L.; Harris, J. M.; Lochmuller, C. H. J . fhys. Chem. 1985, 89, 5246-5250. (12) Lochrnuller, C. H.; Colborn, A. S.; Hunnicutt, M. L. Anal. Chem. 1983, 55, 1344-1348. (13) Gerson, D. J.; Wong, J. S.;Casper, J. M. Am. Lab. (Fairfield, Conn.) 1984, 1 1 , 63-71. (14) Hair, M. L. Infrared Surface Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1987; Chapter 4.
C. H. Lochmuller* M. M. Thompson M. T. Kersey Department of Chemistry Duke University Durham, North Carolina 27706
RECEIVED for review January 27,1987. Accepted July 7,1987. This research was supported by the National Science Foundation, Grant CHE85-00658 (to C.H.L.).
