Anal. Chem
of compounds, have revealed a number of parameters that can be used to control retention time and resolution, and have elucidated the mechanistic aspects of ionic retention ( 2 ) . The present report describes columns of XAD-2 on which these separations can be performed a t column efficiencies approaching those of the best commercially available reverse phase bonded packings, with some sacrifice in analysis time. The desirability of having high efficiency columns of XAD-2 for use in "ion pair" chromatography at high mobile phase pH has been suggested by Gloor and Johnson (30). In a recent study on lower efficiency columns, Rotsch and Pietrzyk have shown that the retention on XAD-2 of the anionic conjugate bases of a variety of carboxylic acids is enhanced when the mobile phase contains a large quaternary ammonium ion (16). Also, a study underway in this laboratory has utilized "ion pair" enhanced retention of the anionic conjugate base of a weak acid drug metabolite (pK, = 9) on a microparticle XAD-2 column for its determination in urine. The aqueous/organic mobile phase contains 7 X 10 M sodium hydroxide to ionize the metabolite and 3.5 x 10 M tetrahexylammonium bromide as an "ion pairing" agent. The high pH needed to ionize this weak acid is well above the pH range that can be tolerated by silica-based bonded packings. In another recently described innovation, microionized XAD-2 resin of nominally 12-pm particle size was used to pack preparative liquid chromatography columns which were then operated at low mobile phase linear velocities (31). The mobile phase contained several solvents, the least polar of which, according to the authors, forms a stationary liquid phase on the XAD-2 to produce a reverse-phase partition system. Similar mobile phases might prove useful with the analytical columns described herein. Finally, current work in this laboratory is aimed at obtaining narrower particle ranges, preferably of spherical particles, of XAD-2-like resins. These would increase column permeability and they would hopefully pack more regularly to produce higher column efficiencies.
19-23
19
LITERATURE CITED Wilks, A. D.; Pietrzyk, D. J. Anal. Chem. 1972, 44. 676. Cantwell, F. F.; Puon, S. Anal. Chem. 1979, 5 1 , 623. Cantwell, F. F., Anal. Chem. 1976, 48, 1854. Mohammed, H. Y.; Cantwell, F. F. Anal. Chem. 1978, 5 0 , 491. Pietrzyk, D. J.; Kroeff, E. P.; Rotsch, T. D. Anal. Chem. 1978, 50, 497. Pietrzyk, D. J.; Chu, C. H. Anal. Chem. 1977, 49, 860. Chu, C. H.; Pietrzyk, D. J. Anal. Chem. 1974, 46, 330. Grieser, M. D.; Pietrzyk, D. J. Anal. Chem. 1973, 45, 1348. Pietrzyk, D. J.; Chu, C. H. Anal. Chem. 1977, 49, 757. Kroeff, E. P.; Pietrzyk, D. J. Anal. Chem. 1976, 50, 502. Zaika, L. L. J . Chromatogr. 1970, 49, 222. Uematsu, T.; Sukadolnik, R. J. J . Chromatogr. 1976, 123, 347. Puon, S.;Cantwell, F. F. Anal. Chem. 1977. 49, 1256. "Amberlite XAD-2", Technical Bulletin, Rohm and Haas Co.: Philadephh, Pa., 1972. Lundgren, J. L.; Schilt, A. A. Anal. Chem. 1977, 49, 974. Rotsch, T. D.; Pietrzyk, D. J. submitted for publication in Anal. Chem. Scott, C. D. Anal. Biochem. 1968 2 4 , 292. Deelder, R. S.;Hendricks, P. J. H.; Kroil, M. G. F. J . Chromatogr. 1971, 5 7 , 67. Hamilton, P. B. Anal. Chem. 1958, 3 0 , 914. Tesarik, K.; Necasova, M. J . Chromatogr. 1973, 75, 1. Allen, T. "Particle Size Measurement", 2nd ed.; Chapman and Hall: London, 1975. Chamot, E. M.; Mason, C. W. "Handbook of Chemical Microscopy", Vol. 1, 3rd ed.; John Wiley: New York, 1958; Chapter 15. Keller, H. P.: Erni, F.; Linder, H. R.; Frni, R. W. Anal. Chem. 1977, 49, 1958. Linder, H. R.; Kelier. H. P.; Frei, R. W. J . Chromatogr. Sci. 1976, 14, 234. Knox, J. H. J . Chromatogr. Sci., 1977, 15, 352. Kirkland, J. J.; Stoklosa, H. J.; Dilks, J. C. H. J . Chromatogr. Sci. 1977, 15, 303. Snyder, L. R.; Kirkland, J. J. "Introduction to Modern Liquid Chromatography"; John Wiley: New York, 1974; Appendix I and 11. Vierk, A. L. Z.Anorg. Chem. 1950, 261, 283. Dallavalle, J. M. "Micromeritics", 2nd ed.;Pitman Publishing Corp.: New York, 1948; Chapter 6. Gloor, R.; Johnson, E. L. J . Chromatogr. Sci. 1977, 15, 413. Dieterle, W.; Faigle, J. W.; Mory, H. J . Chromatogr. 1979, 168, 27.
RECEIVED for review June 18,1979. Accepted October 9,1979. This work was supported by the National Sciences and Engineering Research Council of Canada and by the University of Alberta. This paper was presented in part at the 175th Meeting of the American Chemical Society, Anaheim, Calif., March 1978.
Photoacoustic Spectroscopy of Chemically Bonded Chromatographic Stationary Phases C. H. Lochmuller," S. F. Marshall, and D. R. Wilder Paul M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706
A qualitative study of chemically-bonded, chromatographic stationary phases was undertaken to assess the applicability of photoacoustic spectroscopy (PAS) to the characterization of chemically-modified surfaces. The ability of this technique to identify complicating reactions in the derivatization scheme and probe the free-solution behavior of the bonded moieties was investigated. It was found that polymerization on the surface of the support substrate resulting from a prederivatization step in the bonded phase synthesis could be discriminated by PAS. An estimation of the free-solution behavior of the bonded chromophores, as evidenced by absorptlon maxima shifts in solvents by differing polarity, yielded spectra that were most oflen more similar to the spectrum of the solid model compound than to the solution spectrum of the model compound in the solvent.
Photoacoustic spectroscopy (PAS) has proved to be a valuable tool for the acquisition of optical absorption data 0003-2700/80/0352-0019$01.00/0
from solids (1,2). It offers a particular advantage in its ability to cope with the problems presented by highly scattering samples, like powders, and represents one o f the few means available for obtaining molecular spectroscopic information from such sample types. Optical absorption spectra have been obtained for homogeneous bulk powders (3-6) and from molecules physisorbed on the surface of a powder matrix (7-9). Few studies to date have made use of PAS for the examination of chromophores chemisorbed on a solid matrix (8, I O ) . The chemical modification of solid surfaces has important analytical applications both in the preparation of bonded stationary phases for use in chromatography and chemically modified electrodes in electrochemistry. The characterization of these unique materials has practical significance for the description of interaction mechanisms with solutes in both disciplines. PAS would seem to provide a rapid and easy means for studying chemically modified surfaces, for it is capable of handling optically opaque condensed state samples C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
without elaborate sample preparation. A qualitative comparison of the spectral properties of free and immobilized chromophores should yield information about the nature of the derivatization of the chemically modified surface, and about the physical state and mobility of the bonded chromophore. Similar spectra for the pure chromophore and the monolayer or multilayer of the immobilized chromophore should indicate that the derivatization reaction used t o bond the chromophore to the surface has gone as planned, and complicating reactions that may alter the chromophore are absent or at least minimized. Similar spectra also assure that the bonded chromophore will interact with other molecules in the manner intended to fulfill its analytical purpose. Since charge-transfer chemically bonded stationary phases and chemically modified electrodes are inescapably dependent on the electron donor-acceptor properties of the bonded moiety, a n examination of the UV-visible absorption spectrum is eminently suited to probe perturbations in electron density or changes in transition probability brought about by anchoring the chromophore. The present study presents data obtained by PAS from the examination of chemically modified powder surfaces, as exemplified by bonded charge-transfer chromatographic stationary phases which employ organic moieties immobilized on microparticulate silica gel. Work has been done on elucidating the characteristics of immobilized molecules using conventional transmission spectroscopy on derivatized optically transparent planar supports (11). This report attempts to extend this work to more intractable samples, i.e. powders, and also serves as an example of a further application of PAS to molecular surface studies. The systems chosen for this study represent a general class of electron-deficient aromatic molecules that may reversibly interact with environmentally significant species such as polynuclear aromatic hydrocarbons. These systems are of interest for use as chemically bonded stationary phases that may be employed in the separation and analysis of such solutes. Typically, the charge-transfer moiety is immobilized by reaction with an n-alkyl amine which has been previously attached to the silica surface through reaction with surface silanol groups. In this way the electron-deficient moiety is insulated from the silica surface by the connecting alkyl chain and presumably is allowed sufficient mobility to maximize interactions with solute molecules in the chromatographic mobile phase. A chromatographic phase incorporating a 2,4-dinitrophenyl (DNP) group was enlisted as an example of a simple, bonded aromatic charge-transfer acceptor that was amenable to examination by PAS. Additionally, a novel bonded phase, 3(2-quinazolineamino)propyldiethoxysiloxane, was included for the study of a system whose absorption spectrum (and efficacy as a charge-transfer acceptor) is altered by changes in mobile phase pH.
EXPERIMENTAL Preparation of Bonded Stationary Phases. The initial step in the derivatization scheme involved reaction between silanol groups on the surface of dried, microparticulate silica gel and 3-aminopropyltriethoxysilane to form an Si-0-Si-C linkage. Approximately 10 mL of the silane were added to 100 mL of dry toluene; 2 g of hot, dried silica were quickly introduced; and the mixture was refluxed under nitrogen for 12 h. The aminated silica gel was collected and Soxhlet-extracted under nitrogen with refluxing methanol. To synthesize the DNP phase, 2 g of aminated silica were added to 150 mL of dry toluene, and a solution of 0.7 g of l-fluoro2,4-dinitrobenzene in 20 mL of dry toluene was added dropwise over 20 min to the reaction vessel. The reaction mixture was refluxed for 3 h, followed by collection of the derivatized silica. The phase was exhaustively Soxhlet-extracted with refluxing methylene chloride. The amount of coverage on the phase was determined by redox titration with TiCI3/FeC1, reagent.
Scheme I Aminated
Silica
S,-OH
+
-
( E I O ) ~ S , ( C H ~ ) ~ N H -~ s
1-0
s ~ ( c H ~ ) ~ N H ~
DNP phase
The 2-quinazoline phase was synthesized in a manner analogous to the DNP phase preparation. In this case, 0.3 g of the 2chloroquinazoline in 30 mL of dry toluene were introduced dropwise into the reaction vessel which contained 1.5 g of aminated silica gel, 100 mL of dry toluene, and 1 mL of dry pyridine. The mixture was allowed to reflux for 12 h under dry nitrogen and the material was collected and Soxhlet-extracted with refluxing toluene. The bonded phase coverage was determined by ninhydrin analysis. A schematic for the derivatization procedure appears in Scheme I. Model compounds that would duplicate the chromophores produced on the bonded phases were synthesized in a manner exactly analogous to the bonded phase preparation. 1-Fluoro2,4-dinitrobenzene and 2-chloroquinazoline, were reacted with n-butylamine to yield the solid model chromophores. The preparation of a DNP phase utilizing a polymeric support matrix was likewise carried out following a procedure virtually identical to that described above. Materials. The silica gel used for the preparation of all bonded phases was LiChrosorb SI 60 (EM) with a mean particle diameter of 10 Wm. The 3-aminopropyldiethoxy silane and the aminopropylmethylsiloxane copolymer were obtained from Petrarch Systems and used without further purification. Toluene was dried by distillation from calcium hydride and was stored over sodium metal. The l-fluoro-2,4-dinitrobenzene was obtained from Sigma Chemical Co. The 2-chloroquinazoline was synthesized by the method of Gabriel and Posner (12, 13) with starting materials purchased from Aldrich Chemical Co. and was purified by sublimation. All solvents used to obtain spectra were Fisher Spectranalyzed grade. Photoacoustic Studies. All PAS spectra were obtained using a Princeton Applied Research model 6001 spectrometer. A high pressure xenon source and three gratings allowed acquisition of spectra from 2600 to 200 nm. The present study used a wavelength range of 200 to 500 nm with a resolution of 8 nm. The samples were contained in cylindrical stainless steel cells with a cavity volume of 0.10 mL. The compounds used in the study, both in the bonded and unbonded state, were strongly absorbing (having molar absorptivities between 1000 and 16 000) and this necessitated diluting both the solid compound and the dry bonded phases with a nonabsorbing diluent to prevent saturation, a condition whereby the photoacoustic signal no longer parallels optical absorbance but rather yields a signal that is essentially invariant with change in wavelength. Sodium sulfate was found to be relatively transparent through the visible and ultraviolet and was admixed with the dry samples to obtain suitable spectra. It was found that when the bonded phases were wetted with solvents, the photoacoustic signal was sufficiently attenuated so that the addition of a diluent was usually unnecessary for these samples. When dilution was necessary in the solvent entrainment studies, alumina was used. The spectral traces obtained presented photoacoustic signal as a function of wavelength and were normalized to a reference spectrum of carbon black. Solution spectra of both chromophores were obtained with a conventional UVvisible spectrophotometer, Perkin-Elmer model 756, for comparison with the PAS solution spectra recorded using the PAR instrument.
RESULTS AND DISCUSSION
DNP System. Figure 1 presents PAS spectra from 200 to 500 nm obtained for the D N P system. It displays spectra for the pure solid model compound, 2,4-dinitro-N-butyl aniline
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
21
Table I. Absorption Maxima for the 350-nni Band for the Bonded and Solution 2,4-Dinitro-Ai-alkylaniline System in Solvents of Differing Polarities chromophore state
\
\
-
'/
bordea
\
m C
-
solvent
solution
bondeda
isooctane hexane benzene ethyl acetate chloroform acetonitrile methanol
327 328 343 342 348 330 34 8
35:3 348 35:3 350 348 352 350
(neat) 350
(adsorbed o n silica) 348
0,
p u r e solidb
v) v)
a
a
Immobilized on silica gel.
2.4-Dinitr0.N-butyl-
aniline (DNBA).
a
i 30C
Wavelength
400
5 0
(nm)
Figure 1. Photoacousticspectra of the 2,4-dinitro-N-alkylaniline system. The solution spectrum of the model compound was obtained using
methanol as solvent (DNBA),the DNP bonded phase (essentially the immobilized DNBA chromophore), and a PAS solution spectrum of the model compound in methanol. An analogous UV-visible absorption spectrum of DNBA in methanol was obtained with a conventional spectrophotometer and it yielded results that were identical to those obtained with the PAR instrument. There is essentially complete qualitative correspondence between the three spectra in Figure l. It may be seen that the DNP bonded phase has a spectrum that is virtually the same as the solid model compound and this signifies a derivatization scheme which has successfully immobilized the chromophore without altering its spectral properties. The absorption maximum for the most prominent band in the spectrum (at 350 nm) is identical for the DNBA solid and DNP bonded phase, as is the shape of the absorption envelope in both systems. A different scheme for immobilization of the DNP moiety that entails reactions with a linear aminopropylsilicone polymer was examined for comparison with the bonded material on silica. It was found that the absorption maximum of the prominent band had shifted to 342 nm. In addition, the spectrum of a particularly heavily-loaded DNP bonded phase was obtained and yields an absorption maximum (Amm 344 nm) between the polymeric phase and the pure compound. I t appears that the prederivatization reaction for this phase had not proceeded under completely dry conditions and an aminopropylsiloxane polymer had been formed to some extent. I t has been shown that polymerization on the silica surface can occur when water is not strictly excluded from the reaction of triethoxy- or trichlorosilanes with silica (14). The subsequent reaction with the dinitrophenyl group had evidently produced a phase whereby the charge-transfer moiety was attached to the support material by both direct, alkyl linkage to the surface and by linkage to a siloxane polymer web. PAS, therefore, appears to offer a means for determining the existence of unwanted competing reactions in a derivatization scheme including polymerization on the support matrix as in the above example.
The correspondence of the solid and bonded phase spectra in Figure 1 also suggests that interactions (of the bonded chromophore with residual silanols and unreacted n-propylamine functions on the silica gel surface do not measurably affect the electronic transition probability (and so, the electron density) in the bonded molecule. There does not appear to exist, then, a strong charge-transfer complex between the bonded compound and these surface species which might adversely affect the ability of the bonded phase to interact reversibly with solute molecules in the chromatographic mobile phase. A solution of DNBA in chloroform was used to treat native and aminated silica gel and the chloroform was allowed to evaporate. In this way, a layer of physisorbed DNBA on the surface of these substrates was obtained. The PAS spectra were acquired and it was found that there was neither a perceptible shift in the A,, for the 350-nm band nor the appearance of a separate charge transfer band of sufficient magnitude to significantly alter the shape of the absorption profile. This appears to further substantiate the absence of a strong interaction between the chromophore and silica surface species. An assessment of the free-solution behavior that the bonded chromophores are capable of exhibiting was attempted by evaluating the ability of the immobilized molecules to respond to changes in solvent polarity. Solvent polarity differences may significantly alter optical absorption spectra by preferentially stabilizing the ground or excited state in an electronic transition. The chromophores used in this study exhibit H T* transitions and such transitions are shifted hypsochromically with a decrease in solvent polarity (15). Table I presents the effects of solvent polarity changes on the absorption maximum of the 350-nm (solid-state) band for the DNP phase. It is seen that solutions of the model compound exhibit a significant shift into the blue when nonpolar solvents such as isooctane and hexane are employed. The bonded chromophore, however, does not parallel this change, but rather it basically retains the, A of the solid model compound and seems unaffected by nonpolar solvents. This behavior was thought to be a consequence of poor wetting of the chemically modified surface by the hydrocarbon solvents. The bonded phase was subjected to sonification in the nonpolar solvents to break apart aggragates of particles and was allowed to remain in the hydrocarbons with gentle heating for two days in the hope of attaining solvation of the bonded groups. In addition, DNP phases linked to the silica surface by longer alkyl chains, hexyl and dodecyl, were employed to further promote terminal group solvation by permitting the solvent to intercalate between the longer, alkyl amine bonded phase linkages. I t was found that the bonded phases have the same spectrum as the pure solid material, regardless of pretreatment or length of alkyl linkage, when poor solvents are employed.
-
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
pH 6
h
300
400
300
400
-
300
400
W a v e l e n g t h (nm)
Figure 3. The change in the photoacoustic spectra of the 2-quinazoline system with change in solvent pH. The solvent system is a 60/40(v/v) methanol-water mixture with Uw pH adjusted using H3P04and phosphate salts
2
I
300
400
500
ti
Wavelength (nm)
ti
Figure 4. Scheme for protonation and covalent hydration of the 2-
Flgure 2. Photoacoustic spectra of the 2-quinazoline system. The solvent for the solution was methanol
Similar findings were reported in an ESR study of silylated surfaces (16). Unfortunately, those solvents which probably readily solvate the bonded chromophore did not effect a substantial shift in the free solution spectra of DNBA. The unresponsiveness of the bonded phase to changes in solvent polarity appears to be a consequence of an inability of the poorer, low polarity solvents to overcome the attraction of adjacent chromophores for each other and for the support surface. This attraction may be exaggerated in the bonded state because of the reduction in translational motion that is an inevitable result of anchoring the chromophore. The accompanying greater collisional frequency could result in an artificially high incidence of interactions of the bonded chromophore with adjacent groups and with the surface. An inordinate number of persistent interactions would essentially mimic precipitation of the chromophore and could yield a solid-like spectrum. The unresponsiveness to solvent polarity does not appear to be an inherent result of anchoring the chromophore to an effectively infinite mass because the DNP phase synthesized from the high molecular weight (MW 4000) linear silicone polymer exhibited shifts in A, that essentially mirrored those of the free solution chromophore. 2-Quinazoline System. The other charge-transfer acceptor used in the study, 2-quinazoline, was examined using PAS and the resulting spectra are seen in Figure 2. Again there is good qualitative correspondence between the solidstate, bonded, and solution spectra, implying no gross changes
-
quinazoline system in acidic aqueous media
of the chromophore after immobilization and no inordinately strong interaction with the support matrix surface. Samples of the 2-quinazoline bonded phase were treated with three 60/40 (v/v) methanol-water solvent mixtures whose pH had been adjusted to approximately pH 2,4, and 6 using phosphate buffers. These solvent mixtures simulate those intended for use as mobile phases when the 2-quinazoline bonded phase is used as an LC stationary phase. Based on the known solution behavior, these solvents should cause a change in the charge-transfer related chromatographic capacity. Photoacoustic spectrometry seemed a convenient method for ascertaining the tendency of the bonded phase to react like the free model compound to changes in mobile phase pH. The PAS spectra of the bonded quinazoline in the three solvents are shown in Figure 3 accompanied by the solution spectra in these same solvents. It can be seen that while there is good correspondence between the bonded phase and the PAS solution spectra, the bonded phase systems appear to lag the solution systems in their response to a particular pH regime. The change observed in the absorption spectrum of the quinazoline system with lowered pH is a consequence of an alteration of the chromophore through protonation and, to some extent, covalent hydration (17), see Figure 4. These changes are evidently produced to some extent in the bonded system as well, but the effect appears to be less extensive for the bonded phase compared to the free chromophore. This is evidenced by an incomplete absorption maximum shift
ANALYTICAL CHEMISTRY, VOL 52, NO 1, JANIJARY 1980
(which results from protonation of the neutral chromophore) in the bonded phase, e.g., 353 nm for the solution vs. 358 nm for the bonded phase, in the pH 2 solvent. In addition, the growth of a new absorption band at 285 nm that is a consequence of covalent hydration of the protonated chromophore appears to be suppressed in the bonded phase spectra. These results possibly point out the fact that the equilibrium ratio of hydrated and protonated chromophore to the neutral species may be artificially shifted as a consequence of the existence of some chromophore that is (at any given moment) inaccessible to solvent. The apparent pK, of the bonded system may be altered, then, because of an extra term in the equilibrium expression which accounts for inaccessible chromophore, (see Equation 1). The effect of this extra term in the denominator would be to yield a spectrum for the bonded phase which resembles a solution spectrum in a higher p H solvent. In apparent agreement with the observations made in the DNP system, complete solvation of the bonded chromophore is not in evidence, even in comparatively good solvents. The exact nature of the unavailable chromophore in the bonded phase is still to be determined, however. One might suggest that some bonded chromophore has been made inaccessible by its residence in small pores in the silica support which have been blocked in the derivatization of the phase. Alternatively, a portion of chromophore may become unavailable simply as a consequence of the greater probability for the formation of competitive associations with species on the support surface other than solvent that is an inescapable result of its immobilization. In this way, a dynamic equilibrium may be produced between chromophore that is associated with solvent and chromophore that is associated with surface species.
CONCLUSION PAS represents a valuable addition to the limited number of methods capable of spectroscopically probing chemically modified surfaces. This study has demonstrated the potential of the method for the qualitative characterization of chromatographic charge-transfer bonded phases, but it seems likely that the technique might also be applied successfully in the study of modified catalytic surfaces and nonoptically transparent chemically modified electrodes. The present report has not undertaken to quantify coverages on chemically modified surfaces, and, indeed, an absolute determination of PAS signal response as a function of chromophore concentration is more difficult (18-20) than the analogous BeerLambert relation of optical absorption spectroscopy. Nevertheless, a semiquantitative estimate of surface coverage or
23
a verification of results obtained by conventional methods may be accessible by the construction of standard cixves employing known weights of pure solid chromophore admixed with a suitable transparent solid diluent (ideally the support substrate). The method may also be amenable i o the quantification of charge-transfer effects in bonded phiises of the type described in this report with a judicious choice of solute molecules, Le., the solute should produce a charge-transfer band sufficiently removed from the bonded phase spectrum to allow an accurate determination of the absorption maximum of the new band. In this way, the strength of the charge-transfer interaction of the solute with the bonded phase may be evaluated and correlated with chromatographic determinations obtained through capacity factor measurements, and so allow an estimation of the magnitude of the role that charge-transfer plays in solute retention in these systems.
ACKNOWLEDGMENT The authors gratefully acknowledge assistance received from Jon Howell in acquisition and interpretation of the photoacoustic records and from James Fisk who prepared the DNP silicone stationary phase.
LITERATURE CITED Rosencwaig, A. Opt. Commun. 1973, 7, 305. Rosencwaig, A. Anal. Chem. 1975, 47, 592A. Adarns, M. J.; Beadle, B. C.; King, A . A,: Kirkbright, G. F. Analyst (London) 1976, 101, 553. Monahan, E. M., Jr.; Nolle, A. W. J . Appl. Phys 1977, 48, 3519. Bard, A. J. Analusis 1978, 6, 277. Fernelius, N. C. Appl. Spectrosc. 1978, 3 2 , 554. Rosencwaig, A.; Hall, S. S. Anal. Chem. 1975, 47,548. Low, M. J. D.; Parodi, G. A. Spectrosc. Lett. 1978, 7 1 , 581. Nordal, P.; Kanstad, S. 0. Int. J . Ouantum Chem. 1977, Il(suppl 2 ) , 115. Leyden, D. E.; Steele, M. L.; Jablonski, B. 8.; Somoano, R. B. Anal. Chim. Acta 1978, 100, 545. Mimrns, L. T.; McKnight, M. A.; Murray, R. W. Anal. Chim. Acta 1977, 89. 355. Gabriel, S.; Posner, T. Berichte 1895, 2 8 , 1029. Gabriel, S.; Posner, T. Berlchte 1896, 2 9 , 1310. Aue, W. A.; Hastings, C. R. J . Chromatogr. 1969, 42, 319. Lambert, J. B.; Schurvell, H. F.; Verbit, L.; Cooks, R. G.; Stout, G. H. "Organic Structural Analysis"; Macmillan: New York, 1976. Sillescu, H. Ber. Bunsenges. Phys. Chem. Sistovaris, N.; Riede, W. 0.; 1975, 79, 882. Katritzky, A. R. "Advances in Heterocyclic Chemistry' , Vol. 4; Academic Press: New York, 1965. Wetsell, G. C., Jr.; McDonald, F. A. Appl. Phys. Lett. 1977, 30, 252. Roark, J. C.; Palmer, R. A,; Hutchinson, J. S. Chem. Phys. Len. 1978, 60. 112. Rosencwaig, A. J . Appl. Phys. 1978, 49, 2905
RECEIVED for review August 27, 1979. Accepted October 18, 1979. This work was supported, in part, by a grant (to C.H.L.) from the National Science Foundation, CHE 781807.
