2750
Anal. Chem. 1987, 59, 2750-2752
Adamantyl-Modified Silica as a Reversed-Phase Liquid Chromatographic Packing S. S. Yang and R. K. Gilpin* Department of Chemistry, Kent State University, Kent, Ohio 44242
A slllca surface extensively modhd with adamantane forms a completely nonpolar chromatographic datlonary phase. The rlgkl, balcllke adamantane mdecules bkck the umlertying underlvatlred surface sllanols from lnteractlng wlth solutes; hence the retention of solutes Is nearly governed by purely hydrophobic Interaction. The tailing problem of basic compounds Is largely reduced or eHmlnated even In the absence of secondary mobile-phase modllers.
Most high-performance liquid chromatographic (HPLC) separations are carried out in the reversed-phase mode with linear alkyl-bonded phases. Although the physicochemical phenomena responsible for solute retention on such surfaces is not understood fully, the retention of simple nonpolar solutes can be attributed predominantly to hydrophobic interaction (1-3). However, as the polarity and complexity of the solute increase, secondary effects become more important. A number of solutes, especially polar compounds, often show sizable deviations from what is predicted solely on the basis of the hydrophobic theory. In this latter case, Horvath et al. (4, 5 ) have proposed a dual mechanism which treats solute retention as a combination of hydrophobic interaction and silanolphilic interaction with accessibIe silanol groups. This theory is especially applicable to solutes which interact via ion exchange or hydrogen bonding. Similarly silanolphilic interactions are believed to be responsible for peak tailing. Basic compounds often suffer from severe peak asymmetry problems and hence poor column efficiency. The problem is attributable to strong interaction of the nitrogen lone pair electrons with residual silanol groups on the surface (5). It is not unusual for a significant number of silanols to remain underivatized following exhaustive silanization. Roughly, only about 1/4-1/2 of the total surface silanols react with the organosilane reagents. The remaining sites are unreactive due to (a) nonideal reaction conditions, (b) steric hindrance from groups already bonded to the surface, or (c) inaccessibility of silanols located in micropores. The presence of unreacted silanols can be verified by IR spectroscopy (6, 7), isotope exchange with tritium-labeled water (81, or the adsorption of methyl red. T o make this problem worse, the active underivatized sites have varying degrees of strength as well as steric accessibility (9). A number of studies have been carried out to characterize and minimize peak symmetry problems. One approach has been “end capping” the surface silanols by rereacting the derivatized surface with a smaller molecule, usually trimethylchlorosilane. Although improvements in peak symmetry have been obtained, this approach has not provided a total solution to tailing problems. Another approach involves the use of mobile-phase additives that reduce the concentration of accessible silanols by strongly sorbing to the surface. Various mobile-phase additives have been tested as silanol blocking or masking agents. Among the most successful are alkylammonium salts. For example, Sokolowski and Wahlund (10) have reduced peak tailing of tricyclic antidepressant amines by using N,N-dimethyloctylamine (DMOA) and other 0003-2700/87/0359-2750$01.50/0
alkylammonium additives. Likewise, Horvath and co-workers ( 5 ) have noted that bulkier alkyl amines such as N,N-dimethyldodecylamine are more effective a t lower concentrations than smaller amines because they form stronger silanol-amine complexes. They also are stabilized by hydrophobic ineraction between the stationary phase and the alkyl chains of the amine. Sadek and Carr (11)have evaluated the silanophilic blocking character of dimethyldiphenylcyclam (DMDPC) on 16 different commercially available reversedphase columns. Since DMDPC has a macrocyclic pocket slightly larger than the size of a silanol group, it is a very effective silanophilic blocking agent. A third way of reducing silanol accessibility is via steric exclusion of the solute from the surface. Hemetsberger et al. (12)have observed that the influence of residual silanols became more important as the alkyl chain length is reduced. Longer alkyl chains partially prevent solutes from reaching the underlying surface and thus themselves play an important role in blocking the silanols. If the molecular features of the immobilized group can shield the residual silanols from the solute, a more nearly nonpolar surface should be obtainable. This idea was tested via the immobilization of adamantyl moieties. The potential advantages of this particular surface have received little attention. To our knowledge, only one other account exists that uses adamantane as a liquid chromatographic phase. Rehak and Smolkova (13)have previously compared this surface to the other nonpolar branched and unbranched hydrocarboneous surfaces in terms of solute retention and selectivity for benzene methyl derivatives and dialkyl phthalates. However, these investigators failed to recognize the adamantyl surface’s potential importance in terms of column efficiency because they used only weak polar compounds. In the current study, small basic compounds were used to evaluate the surface under less than ideal mobile-phase conditions (Le., without use of additives). Excellent results in terms of peak symmetry were obtained. For comparison purposes, several commercial columns were also tested under the same experimental conditions.
EXPERIMENTAL SECTION Reagents and Columns. (Adamantylethy1)trichlorosilanewas purchased from Petrarch Systems, Inc. (Levittown, PA), and was used as received. LiChrosorb Si60 (d,, 10 pm and surface area 550 m2/g) was obtained from E. Merck (Darmstadt, West Germany). HPLC-grade methanol was purchased from Aldrich Chemical Co. (Milwaukee, WI). Water was purified by using a Milli-Q water system (Millipore Corp., El Paso, TX). The IBM 5-pm octadecyl (C-18) and octyl (C-8) columns (4.6-mm i.d. X 150 mm) were obtained from IBM Instruments, Inc. (Danbury, CT). The ALTEX 5-pm Ultrasphere octyl (C-8) and octadecyl (C-18) columns (4.6-mm i.d. X 250 mm) were obtained from Beckman Instruments, Inc. (Fullerton, CAI. The Supelco 5-pm octadecyl (C-18) columns (4.6-mm i.d. X 150 mm and 4.6-mm i.d. X 250 mm) were from Supelco, Inc. (Bellefonte,PA). The Hibar 10-pm octyl column (4.0-mm i.d. X 250 mm) was from E. Merck (Darmstadt, West Germany). The Perkin-Elmer 3-pm octadecyl column (4.6-mm i.d. X 30 mm) was obtained from Perkin-Elmer (Norwalk, N
CT). 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
2751
b
c
Y u z
U m
=
s 4
L 8
16 24 TIME ( m l n . )
I
Figure 1. Chromatogram of (a) aniline, (b) N-methylaniline, and (c) N,Ndimethyianiline. Conditions: column, adamantyl (1.8-mm i.d. X 150 mm); mobile phase, methanol-water, 50/50, v/v; flow rate, 0.2 mL/mln. a
8
16 14 TIME / m i ”
0
16 24 TIME ( m i ”
1
32
4Q
40
56
64
72
80
88
91
Flgure 2. Chromatogram of (a) aniline, (b) N-methyianiline, and (c) N,Ndimethylaniline. Conditions: column, IBM octadecyl (4.6-mm i.d. X 150 mm); mobile phase, methanol-water, 50150, v/v; flow rate, 1.3 mL/min. Preparation of Adamantyl Phase. The (adamantylethy1)trichlorosilane was chemically bonded on 10-pmLiChrosorb Si60 as previously described (14). Subsequently, this material was packed into l.%mm-i.d. X 15omm stainless steel columns by using a dynamic reservoir packing procedure (15). Column Evaluation. Prior to evaluation, all columns were conditioned with at least 100 mL of methanol followed by 100 mL of the mobile phase (50/50, v/v, methanol-water). Chromatographic experiments were carried out under ambient conditions by using a LC/9533ternary gradient liquid chromatograph (IBM Instruments, Inc.) equipped with a UV detector. All measurements were made at least in triplicate.
RESULT AND DISCUSSION All columns were evaluated by using a mixture of aniline, N-methylaniline, N,N-dimethylaniline, o-toluidine, mtoluidine, p-toluidine, and p-chloroaniline. These compounds were chosen as test solutes because they show strong interaction with free silanols and hence poor peak symmetry. The commonly used nonpolar octyl and octadecyl surfaces (commercially available) were used as references due to their similarity (i.e., polarity) with the adamantyl phase. For comparative purposes, representative chromatograms from
32
40
48
Flgure 3. Chromatogram of (a) aniline, (b) N-methylaniline, and (c) N,Ndimethylaniline. Conditions: column, IBM octyl (4.6-mm i.d. X 150 mm); mobile phase, methanol-water, 50/50, v/v; flow rate, 1.3 mL/min.
8
b
I
16 24 TIME ( m i ” . )
32
Flgure 4. Chromatogram of (a) aniline, (b) N-methylaniiine, and (c) N ,N-dimethylaniline. Conditions: column, Altex Ukrasphere octyl (4.6-mm i.d. X 250 mm); mobile phase, methanoi-water, 50150, v/v; flow rate, 1.3 mL/min.
the adamantyl and three of the eight commercial columns are shown in Figures 1-4. A direct comparison between the different surfaces (i.e., an overlay of chromatograms) is difficult because the bonded phases differ in size and structure of the attached groups, phase loading, and nature of the base silica. In the current evaluation, the use of LiChrosorb Si60 10 hm, pore size 60 A) in the preparation (irregular shape, d, of adamantyl surface gives relatively inferior conditions to those of most of the commercial columns tested, where spherical and/or smaller silica particles with larger pore size (e.g., 100 A) were used. Likewise, all but one of the commercial bonded phases are endcapped. Even with these limitations, the peak shape on the adamantyl column (Figure 1)was superior to those obtained from either the octyl or octadecyl phases (Figures 2-4 and Table I). No significant peak tailing was exhibited by the adamantyl column even under unfavorable mobile-phase conditions (i.e. absence of a secondary modifying reagent). Peak asymmetry values for each of the test solutes were calculated by using eq 1 (16),where a’and b’are the leading
-
As2 = ( b ’ / ~ ’ ) ~
(1)
and tailing half of a peak measured at 10% of the total height. This is illustrated in Figure 1. Results from these calculations
2752
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
--- -- __ ____
___I-_____
Table I. Asymmetry Factors for the Adamantyl Surface and Commercially Available Surfaces S2b
Slb
S3h
S5*
S6h ___
k'
hJa
k'
bla
k'
bla
k'
-b/a
k'
bin
k'
A B C
1.4 1.1 1.6 1.5 1.2 0.9 1.1 4.1 1.2
2.4 12.8 7.5 4.3 14.2 7.9 10.1 11.1 18.8
3.1 5.2 4.4 3.5 4.3 7.8 6.5 14.9 4.6
2.7 14.8 13.4 4.4 12.9 8.1 11.5 11.2 23.5
7.7 21.0 15.2 8.0 12.6 10.1 20.0 26.3 16.8
3.3 17.0 14 3 5.3 16.2 15.2
3.3 9.2 6.2 4.6 3.2 2.0 7.4 15.2 2.3
4.0 4.9 13.2 6.0 19.6 17.4 9.8 5.5 18.7
2.1 2.2 1.9 2.1 3.0 1.8 3.3 3.,5 2.3
2.8 9.0 8.5 5.1 5.2 8.4 5.5 9.8 21.8
25 8.3 3.8 36 4.9 2.0 112 169 34
l _ _ l
D E F
G H I
--
54b
col"
13.1
13.7 28.0
"
-
S7h
.. . .. .~ ....
h/(i ,1 0 51 11 1 41 69 11 7 31 75 20.2
-.
k'
hltr
3.0
1.9 4.9 5.9 3.4 4.1 4.1 3.5 7.5 16.1
4.6
2.4 2.5 :$.O
4.i 6.6 8.8 5.0
nColumns: A = 10-pm adamantyl (1.8-mm i.d. X 50 mm), B = IBM 5-pm octadecyl (4.6-mm d. x 150 mm), C = IBM 5.pm octyl (4.6-mm i.d. X 150 mm), D = Altex 5-pm Ultrasphere octyl (4.6-mm i d . X 250 mm), E = Altex 5-pm Ultrasphere octadecyl (4.6-mm i.d. x 250 mm), F = Supelco 5-pm octyldecyl(4.6-mm i.d. X 150 mm), G = Supelco 5-pm octadecyl (4.6-mm i.d. X 250 mm), H =: Hibar 10-pm octyl (4.0 mm i.d. X 250 mm), and I = Perkin-Elmer 3-pm octadecyl (4.6-mm i.d. x 30 mm). bSolutes: SI -- aniline, S2 = N-niet,hylaniline,S3 = NAN-dimethylaniline,S4 = p-toluidine, S5 = o-toluidine, S6 = mtoluidine, and S7 = p-chloroaniline.
-______
__--__
H.*H H H Figure 5. Structure of an adamantane molecule.
are listed in Table I. Asymmetry factors from the adamantyl column are smaller than any of the commercial columns tested. The structural and physical properties of the adamantane molecule can be used to explain the current results. Adamantane is a symmetrical, three-dimensional molecule in which four chair forms of cyclohexane are present and all bridgehead hydrogen atoms are equatorial with respect to each of the rings (Figure 5). Hence adamantane is a nonpolar molecule with an extremely rigid conformation, which corresponds to a building block in the structure of the diamond. In a crude sense the current adamantyl surface can be pictured as a hydrocarbon ball attached to the surface by a short hydrocarbon spacer arm. Theoretically, on the basis of the molecular size of adamantane (-7.5 A) and a close pack structure, the surface density can reach a maximum of 1.8-1.9 molecules/nm2. For the current study, the surface density of attached adamantane molecules calculated from carbon analysis of 12.2% and the manufacturer's listed total surface area is 1.1 molecules/nm2, which is less than the theoretical maximum. However, the effective bonding area of microporous silica is often significantly smaller than its total surface area. This is especially true for silica with pore sizes near those used in the current because a fraction of the surface area is contained study in micropores, which is sterically unavailable for reaction. After this adjustment, the actual density of immobilized adamantyl chains is probably much closer to complete coverage. Hence the surface that is sterically available is covered by ball-like adamantyl molecules in a more or less close pack arrangement. As a result, a rather rigid surface structure is formed, which blocks underlying silanols. Although some small spaces between bonded ligands and micropores might still exist, most solutes (e.g., benzene 7A), except for extremely small molecules, will be blocked from reaching the
(In,
-
__-____
. . .. .__-.....I
..
.
.. .
.~
surface silanols. The superior properties of the adamantylmodified surface are due to its structural rigidity compared to other surfaces modified with linear or slightly branched chains, which exhibit conformationally more dynamic structures and permit easier access to silanols. For this nonpolar stationary phase, the retention of solute is more nearly governed by purely hydrophobic interactions, even in the absence of secondary mobile-phase modifiers.
CONCLUSION Since the adamantyl-modified surface appears to be influenced less by residual silanols, the retention of solutes should be more predictable and hence separations more easily optimized. Since the adamantyl derivatized surface was performed under less than ideal conditions, better performance should be obtainable when particle geometry is optimized. It, should also be possible to obtain a variety of selectivities by modifying silica with different adamantyl derivatives that contain various funct,ional groups. We are now in the process of trying to synthesize several different types of adamantyl materials and to evaluate their chromatographic performance.
LITERATURE CITED (1) Horvath, C. High-Performance Liquid Chromatography; Academic: New York, 1980; Vol. 2. (2) Horvath, C.; Melander, W.; Molnar. I . J. Chromatogr. 1976 125, 129. (3) Horvath, C.; Melander, W.; Nahum, A. J. Chromatogr. 1979, 186, 371. (4) Nahum, A.; Horvath, C. J Chromatogr. 1981, 203,53. (5) Bij, K. E.;Melander, W. R.;Nahum, A.; Horvath, C. J. Chrornatogr. 1981. 203.65. (6) Major, R. k.; Hopper, M. J. J. Chromatogr. Sci. 1974, 72. 767. (7) Little, L. H.; Infrared Spectra of Adsorbed Species ; Academic: London, 1966; 428 pp. (8) Unger, K.; Gallel, E. Kolloid 2.2 . Polym. 1970, 237, 358. (9) Snyder, L. R. Principles of Adsorption Chromatography; Marcel Dekker: New York, 1968. (IO) Sokolowski, A.; Wahlund, K. G. J. Chromafogr. 1980, 789, 299, (11) Sadek, P. C.;Carr, P. W. J. Chromatogr. Sci. 1983, 21,314. (12) Hemetsberger, H.; Maasfeld. W.; Ricken, H.; Chromatographia 1976, 7,303. (13) Rehak, V.; Smolkova, E. J . Chromafogr. 1980, 191, 71. (14) Gilpln, R. K.: Squires, J. A. J. Chromafogr. Sci. 1981, 79, 195. (15) Gilpln. R. K.: Slsco, W. R. J . Chromatogr. 1980, 794, 285. (16) Johnson, E.; Stevenson, R. Basic Liquid Chromatography: Varian: Palo Alto, CA, 1978. (17) Gilpln, R. K.;Burke, M. F. Anal. Chem. 1973. 45, 1383.
RECEIVED for review January 16, 1987. Resubmitted July 7 , 1987. Accepted August 7 , 1987.
