A Highly Stable Alkyl Amide Silica-Based Column Packing for Reversed-Phase HPLC of Polar and Ionizable Compounds

A Highly Stable Alkyl Amide Silica-Based Column Packing for Reversed-Phase HPLC of Polar and Ionizable Compounds Researchers have designed a new polar-linked, reversed-phase column for separating polar, ionizable, and especially highly basic compounds with excellent column efficiency and peak shapes. The column combines an ultrapure, low-acidity porous silica support with a unique stationary phase that has a polar, embedded amide group linking the sterically protecting diisopropyl groups with a C14 n-alkyl functionality. This new column demonstrates stability in low-ph mobile phases because of its resistance to hydrolysis and stationary-phase loss. Dense triple-endcapping minimizes unwanted silanol interactions and allows excellent peak shapes and stability at intermediate ph. The column can be used with totally aqueous mobile phases without the phase collapse typical of alkyl stationary phases. J.J. Kirkland, J.W. Henderson, J.D. Martosella, B.A. Bidlingmeyer, J. Vasta-Russell*, and J.B. Adams Jr. Hewlett-Packard Co., Little Falls Analytical Division, Newport Site, 538 First State Boulevard, Newport, Delaware 19804 * Mac-Mod Analytical, 127 Commons Court, Chadds Ford, Pennsylvania 19317 Adams Research, 759 Morris Road, Hockessin, Delaware 19707 Address correspondence to J.J. Kirkland. C olumn packings with alkyl stationary phases containing polar-linked groups or embedded polar groups are becoming increasingly popular for reversedphase high performance liquid chromatography (HPLC) separations (1 4). Columns of these packings often exhibit good peak shapes and efficiencies for polar solutes, especially ionizable compounds that can be particularly troublesome in separations (5,6). Compared with hydrocarbon-only stationary phases, the stationary phases with polar groups linked in an alkyl matrix are less retentive and typically require a lower concentration of organic mobile-phase modifier (1,2,7). In some cases, chromatographers can separate certain ionizable compounds in purely aqueous mobile phases without using ion-pair reagents (8). Although column packings with polar groups provide advantages for certain separations, these materials have a potential disadvantage that has not been investigated carefully. Bonded-phase packings that have shorter chains or polar groups exhibit poorer stability than long-chain purely alkyl functionalities in low and high ph environments (9 12). Therefore, packings with shorter stationary-phase chains or more-polar functional groups can provide performance improvements for separating highly polar compounds but come with the price of shorter column life. Users wanted a better approach for increasing column stability that maintained the desirable features for separating polar compounds. We have developed a new silane-based, embedded polar group stationary phase that uses sterically protecting groups. This structure greatly enhances the stability of the bonded phase in low-ph applications by significantly decreasing the loss of stationary phase through hydrolysis (9,13,14). To enhance performance and stability in intermediate-ph applications, this stationary phase is densely triple-endcapped. This feature results in a packing material that resists dissolution of the silica support, which is the degradation process usually responsible for failure of bonded-phase columns in intermediate- and high-ph applications (11,15). This new packing material is based on Hewlett-Packard s Zorbax Rx-Sil ultrapure, low-acidity Type B porous silica support (Wilmington, Delaware), which forms the basis for a large family of widely used, high performance reversed-phase columns (16). The combination of this silica support with the new alkyl amide stationary phase results in a robust column packing that exhibits superior characteristics for separating difficult polar and ionizable compounds. EXPERIMENTAL Apparatus and reagents: We performed chromatographic tests using HP 1050, HP 1090, and HP 1100 chromatographs (Hewlett-Packard). We calculated plate numbers using the halfpeak-height method described by equation 2.8a in reference 17. Peak tailing values were determined at 5% of the peak height using the conventional approach (17). Samples were injected with a Rheodyne model 7125 sampling valve (Rohnert Park, California). The 15 cm 0.46 cm columns were prepared at the Hewlett-Packard Newport Site using a conventional slurry-packing method (18). The porous silica support used in this work is a Type B silica formed by aggregating ultrapure silica sols (16). Type B silicas are the newer highly purified, less acidic chromatographic supports that provide superior separations, especially for ionizable compounds (19). The physical and surface properties of this silica have been reported elsewhere (16). We performed the diisopropyl C14 amide (alkyl amide) polar-linked stationary-phase bonding and triple-endcapping procedures

FIGURE 1: Structure of the diisopropyl alkyl amide stationary phase. FIGURE 2: Diffuse reflectance FTIR spectra of alkyl amide packing (solid line) and starting silica (dashed line). on a 5- m d p, 80-Å, 180-m 2 /g Zorbax Rx-Sil silica support (Hewlett-Packard) by proprietary processes. The carbon values for this column packing typically were 10% for this support. The other experimental columns used in this study also were prepared at the Newport site. Columns of the endcapped alkyl amide packing are now available from Hewlett-Packard under the trade name, Zorbax Bonus-RP. We obtained HPLC-grade solvents from EM Science (Gibbstown, New Jersey). Test solutes from Sigma Chemical Co. (St. Louis, Missouri) were used as received. Stationary-phase characteristics: Figure 1 depicts the sterically protected alkyl amide stationary phase of the new column. The 10% carbon content for the alkyl amide packing represents approximately 2.1 mol/m 2 of the bonded phase, which is typical for dense coverage by sterically protected bonded silanes (20). Figure 2 shows reflectance infrared spectra of the bonded alkyl amide silica material and the starting silica of this packing. The spectrum for the starting silica (dashed line) suggests unbonded (free) silanols at 3739 cm 1, associated (bonded) silanols at 3645 cm 1, and silanols and adsorbed water at 3477 cm 1. The bonded material (solid line) shows no silanol adsorption at 3739 cm 1, which suggests that the starting unbonded silanols are fully reacted. Characteristic carbon hydrogen stretching bands for CH 2 and CH 3 groups are present at 2853 and 2925 cm 1 and 2890 and 2970 cm 1, respectively. Figure 2 also indicates an overtone band for an amide II structure at 3083 cm 1 and a secondary amide (unassociated) N H stretching band at 3450 cm 1. The broad 3314 cm 1 band likely represents N H stretching from the amide group and associated silanols and adsorbed water. The characteristic 1649-cm 1 amide I band for the alkyl amide stationary phase clearly is visible over the silica absorption in this spectrum, as are other secondary amide bands at 1542 and 1650 cm 1. In short, the infrared spectrum for the alkyl amide material is in qualitative agreement with the proposed structure of Figure 1. Column aging studies: Individual columns were purged continuously with either a low- or intermediate-ph mobile phase. Low-pH aging was performed with a 1.0-mL/min flow of 50:50 (v/v) methanol 0.1% trifluoroacetic acid at 60 C. Intermediate-pH aging studies were conducted by continuously purging the columns with a 1.5-mL/min flow of 40:60 (v/v) acetonitrile 0.025 M sodium phosphate buffer (ph 7.0) at 24 C. Periodically, we tested these columns chromatographically at 40 C using a mixture of amitriptyline, trimipramine, and toluene with a mobile phase of 60:40 (v/v) acetonitrile 0.01 M sodium phosphate buffer (ph 7.0) at a 1.5- ml/min flow rate. The mobile phase ph values refer to the ph of the buffer used in the experiments. We periodically tested the columns with solutes to determine the level of changes that occurred during use. Each column first was flushed with at least 20 volumes of chromatographic mobile phase before the chromatographic test. Other apparatus and reagents: Data for the basic drug separations were obtained with an HP model 1090 chromatograph with UV detection at 254 nm. The mobile phases were methanol 10 mm sodium citrate buffers. We chose the citrate buffer because of its wide buffering range (ph 2.1 6.4). We collected and analyzed data using HP ChemStation software (Hewlett-Packard). The captions for each figure provide the experimental details for each separation shown in this study. We measured diffuse reflectance infrared spectra using a Nicolet Avatar model 380 FTIR spectrometer (Madison, Wisconsin). We made no special attempt to maintain the dryness of the samples used in these tests. RESULTS AND DISCUSSION Low-pH studies: Previous studies with sterically protecting bulky groups on the silane silicon atom showed that stationary-phase stability at low ph is greatly enhanced by the much-decreased hydrolysis rate of the silane-attaching siloxane bond (9,13,14). This characteristic also is found for the alkyl amide packing of this study, because the diisopropyl groups of the structure (Figure 1) afford the usual steric protection against low-ph hydrolysis. Figure 3 shows that the rate of change for the alkyl amide column when continuously purged with a methanol 0.1% trifluoroacetic acid mobile phase (approximately ph 1.9) at 60 C is much less than that for a nonsterically protected double-endcapped dimethyl C18 amide column prepared with the same silica support and by a comparable reaction. In Figure 3a, the diisopropyl C14 amide column shows only a slight increase in retention factor (k) for

amitriptyline, a strongly basic drug (pk a 9.5). Retention values increase steeply with aging for the comparable dimethyl C18 amide column. Presumably, the k value increase is the result of a loss in silane stationary phase by hydrolysis with subsequent formation of silanol groups that bind the basic drug. Values of toluene k for the dimethyl C18 amide column show a decrease of approximately 40% for the period of testing. Figure 3b shows that the amitriptyline plate height for the diisopropyl C14 amide column is essentially unchanged (within experimental error) after purging with approximately 37,000 column volumes, when the test arbitrarily was terminated. This result equates to almost four months of 8-h/day operation. In contrast, the comparable nonsterically protected dimethyl C18 amide column showed significant degradation in column efficiency for both amitriptyline and toluene after purging with approximately 2000 column volumes under these conditions. Low-pH separations with the diisopropyl C14 amide column produce excellent peak shapes and column efficiencies for both acidic and basic compounds. Figure 4 shows the separation of a mixture of acidic fruit acids of interest to the food industry using a methanol trifluoroacetic acid mobile phase. This column also exhibits useful characteristics for basic compounds, as illustrated in Figure 5 for the separation of basic cardiac drugs using a methanol sodium dihydrogen phosphate (ph 3.0) mobile phase. In both cases, column efficiency and peak shapes generally are superior to those found for columns with simple alkyl stationary phases (for example, C18). Intermediate-pH studies: Columns with polar-linked groups often are used in the ph 4 8 range for separating polar or ionizable compounds. Under some separating conditions, particularly at intermediate ph, columns with conventional alkyl-bonded phases can show less-than-desirable peak shapes and column efficiencies for some compounds. In these instances, O Gara and co-workers (2) speculated that the superior performance of polar-linked group columns may be the result of the association of the embedded polar groups with unreacted silanol groups on the surface of the silica support. An alternative postulation is that the linked-polar group reduces the hydrophobicity of the stationary phase near the silica support surface, allowing water molecules to approach and deactivate unreacted silanol groups. These effects would minimize unwanted interactions that cause poorer column performance, and they would result in better peak shapes and column efficiency for polar and ionizable compounds. Compared with conventional alkyl-bondedphase columns, the alkyl amide column of our FIGURE 3: Plots of (a) retention factor and (b) plate height versus column volumes of mobile-phase purge for alkyl amide columns at ph 2. Columns: 150 mm 4.6 mm; aging mobile phase: 50:50 (v/v) methanol aqueous (0.1%) trifluoroacetic acid (ph 1.9); flow rate: 1.0 ml/min; temperature: 60 C. Mobile phase for the amitriptyline test: 60:40 acetonitrile 0.01 M sodium phosphate buffer (ph 7.0); flow rate: 1.5 ml/min; temperature: 40 C. Mobile phase for the toluene test: 80:20 methanol water; flow rate: 1.0 ml/min; temperature: 23 C. dimethyl C18 amide, amitriptyline; diisopropyl C14 amide, amitriptyline; dimethyl C18 amide; diisopropyl C14 amide, toluene. study usually produces superior peak shapes and column efficiencies for difficult ionizable compounds when operated with intermediateph mobile phases. Figure 6 shows the separation of a mixture of highly basic drugs on our alkyl amide column, an endcapped alkyl C18 column, and a nonendcapped alkyl C8 column, all of which were prepared with the same silica support. Solute retention is lower on the alkyl amide column, but the peak shapes and column efficiency clearly are superior under the same ph 6 conditions. Figure 7 illustrates the improved performance of the alkyl amide stationary phase compared with conventional alkyl phases. In this figure the separation of the strongly basic drug amitriptyline (pk a 9.4) is performed on three columns an endcapped alkyl amide column, an endcapped dimethyl C18 column, and a nonendcapped C8 column at ph 5. At this ph, amitriptyline should be largely protonated. All columns were prepared with the same silica support. Two important features arise from this comparison. First, the alkyl amide column is less retentive, as in Figure 6, presumably because of the reduced hydrophobicity caused by the polar group in the stationary phase. However, another reason may be the greater interaction of the solute with surface silanol groups for the two pure alkyl columns. The second feature of this comparison is the broader, tailing peaks for the alkyl columns; the nonendcapped column shows the largest effect. Clearly, the alkyl amide column provides superior performance under these separation conditions. Figure 8 shows the greatly reduced interaction of surface silanols when using the alkyl amide column for protonated or partially protonated basic compounds at intermediate ph. Peak-tailing factors for a series of basic compounds remain essentially constant throughout

the ph 3 6 range for a methanol citric acid buffer mobile phase. In contrast, peak-tailing values for an endcapped C18 column and a nonendcapped C8 column generally are higher and increase with ph increases. These results suggest less interaction of the alkyl amide column for these protonated basic compounds with increasingly ionized silanol groups as the ph increases. We found that the alkyl amide column produced better peak shapes and column efficiencies for polar ionizable compounds in the intermediate ph range where ionizable solutes and unreacted silanol groups on the silica support often are at least partially ionized. Previous studies have shown that column packings that were exhaustively endcapped show superior stability in intermediate- and high-ph mobile phases (10,20). This effect apparently is the result of increased protection of the silica support surface from dissolution, the typical cause of silica-based column failure at intermediate and high ph. To test the stability of the diisopropyl C14 amide column at intermediate ph, we continuously purged a column of this material with 40:60 (v/v) acetonitrile 25 mm sodium phosphate buffer (ph 7.0) at ambient temperature. Periodically, we tested this column chromatographically, with the results shown in Figure 9. For both toluene (Figure 9a) and amitriptyline (Figure 9b), plate height values show little change for the diisopropyl C14 amide column after purging with more than 36,000 column volumes, after which the experiment was terminated arbitrarily. This result suggests that the column underwent little change after the use equivalent of more than three months of 8-h workdays. We believe that the exhaustive endcapping method used in preparing the diisopropyl C14 amide column provided superior protection for the silica support from dissolution by the ph 7 mobile phase. Retention values for the basic solute amitriptyline increased approximately 35% after the purging study, indicating the formation of more acidic silanol groups on the silica support surface caused by slow silica hydrolysis. The alkyl amide column can be operated routinely at intermediate ph for separating a wide range of polar and ionizable compounds. Figure 10 shows the separation of a mixture of benzodiazepine drugs using a ph 4.6 buffer with good peak shapes and column efficiencies for all solutes. Bromazepam (pk a 2.9, 11.0), clobazam (pk a unavailable), lorazepam (pk a 1.3, 11.5), and diazepam (pk a 3.3) compose this mixture, which has a wide range of protonated (ionized) and nonionized structures at the separation ph. Figure 11 shows the separation of cephalosporin drugs at ph 7.0, again with superior peak shapes and column efficiency. These compounds have pk a values ranging from 2.2 to FIGURE 4: Separation of fruit acids at low ph. Column: 150 mm 4.6 mm diisopropyl C14 amide; mobile phase: 70:30 (v/v) aqueous (0.1%) trifluoroacetic acid methanol; flow rate: 1.0 ml/min; temperature: 24 C; detection: UV absorbance at 254 nm; sample volume: 5 L. Peaks: 1 gallic acid, 2 protocatechuic acid, 3 syringic acid, 4 vanillic acid (N 7500, tailing factor 1.05), 5 gentisic acid. FIGURE 5: Separation of basic cardiac drugs at low ph. Mobile phase: 55:45 (v/v) methanol 0.025 M sodium phosphate buffer (ph 3.0); sample volume: 3 L. Other conditions were the same as in Figure 4. Peaks: 1 diltiazem, 2 dipyrimidole, 3 nifedipine, 4 lidoflazine, 5 flunarizine (N 4900, tailing factor 1.08). 7.3, and some compounds, such as cefalexin, have multiple pk a values. As a result of the linked polar amide group of the alkyl amide structure, this stationary phase is readily wetted by totally aqueous mobile phases. Consequently, analysts can separate compounds that require little or no organic modifiers for proper retention. Figure 12 shows an example of this effect in which a mixture of nucleic acid bases and related compounds were separated with a simple aqueous sodium acetate mobile phase. Studies showed no change in retention and column efficiency properties for this column when the mobile phase was adjusted from partially organic to totally aqueous mobile phases and back again; equilibration also is very rapid. These results indicate that this stationary phase does not collapse when used with low-organic-content or totally aqueous mobile phases.

FIGURE 7: Peak characteristics of a basic drug at ph 5 for different columns. Same conditions as for Figure 6, except mobile phase: citrate buffer (ph 5.0); solute: amitriptyline. Traces: 1 endcapped alkyl amide (N 6500, tailing factor 1.04), 2 endcapped C18 (N 4800, tailing factor 2.32), 3 nonendcapped C8 (N 3100, tailing factor 4.05). FIGURE 6: Separation of basic drugs at ph 6 for (a) endcapped diisopropyl C14 amide, (b) endcapped C18, and (c) nonendcapped C8 columns. Column dimensions: 150 mm 4.6 mm; mobile phase: 62:38 (v/v) methanol 0.01 M citrate buffer (ph 6.0); flow rate: 1.0 ml/min; temperature: 30 C; detection: UV absorbance at 254 nm. Peaks: 1 uracil, 2 propranolol, 3 nortriptyline, 4 doxepin, 5 amitriptyline, 6 trimipramine. CONCLUSION A silica-based column with a sterically protected, triple-endcapped C14 stationary phase with a linked polar amide group showed significant promise for separating a wide range of polar and ionizable compounds with good peak shapes and column efficiency. Because of the presence of protecting diisopropyl groups on the silane silicon atom, this column demonstrates outstanding stability at low ph. The column also exhibits excellent stability with intermediate-ph mobile phases because of the exhaustive endcapping that inhibits degradation of the column, typically caused by dissolution of the silica support. Based on previous experiences, monofunctional attachment of the silanebased stationary phase should provide good preparation reproducibility. The excellent kinetic properties (column efficiency and peak shape) we found for the bidentate stationary phase is typical of those of monofunctional structures. ACKNOWLEDGMENTS We thank C.H. Dilks Jr. of the Hewlett-Packard Co. (Newport, Delaware) for experimental data from column aging studies. REFERENCES (1) T. Czajkowska and M. Jaroniec, J. Chromatogr. A 762, 147 158 (1997). FIGURE 8: Effect of ph on peak tailing factors for basic drugs with (a) alkyl amide, (b) endcapped C18, and (c) nonendcapped C8 columns. Conditions were the same as in Figure 6 except for citrate buffers with various ph values. Analytes: propranolol, nortriptyline, doxepin, amitriptyline, trimipramine. FIGURE 9: Plots showing the aging of an alkyl amide column at ph 7.0 with (a) toluene and (b) amitriptyline. Column: 150 mm 4.6 mm diisopropyl C14 amide; mobile phase: 40:60 (v/v) acetonitrile 0.025 M sodium phosphate buffer (ph 7.0); flow rate: 1.5 ml/min; temperature: 24 C. FIGURE 10: Separation of benzodiazepine drugs at intermediate ph. Mobile phase: 55:45 (v/v) methanol 0.05 M sodium acetate buffer (ph 4.6); sample volume: 1 L. Other conditions were the same as in Figure 4. Peaks: 1 bromozepam, 2 clobazam, 3 lorazepam, 4 diazepam (N 7700, tailing factor 0.99).

FIGURE 11: Separation of basic cephalosporins at ph 7.0. Mobile phase: 20:80 acetonitrile 0.025 M sodium phosphate buffer (ph 7.0). Other conditions were the same as in Figure 4. Peaks: 1 cefalexin, 2 cefaclor, 3 cefoxitin (N 7000, tailing factor 1.07). (2) J.E. O Gara, B.A. Alden, T.H. Walter, J.S. Petersen, C.L. Niederlaender, and U.D. Neue, Anal. Chem. 67, 3908 3913 (1995). (3) T.L. Ascah and B. Feibush, J. Chromatogr. 506, 357 (1990). (4) T.L. Ascah, K.M.R. Kallury, C.A. Szafranski, S.D. Scott, and F. Liu, J. Liq. Chromatogr. Relat. Technol. 19, 3049 3073 (1996). (5) T. Czajkowska, I. Hrabovsky, B. Buszewski, R.K. Gilpin, and M. Jaroniec, J. Chromatogr. A 691, 217 224 (1995). (6) N. Fukunaga and T. Kaneko, Chromatography 18, 78 81 (1997) (Japanese). (7) D.V. McCalley, J. Chromatogr. A 738, 169 179 (1996). (8) M. Przybyciel and M.A. Santangelo, The Development of Hydrocarbon-Based Stationary Phase for HPLC with Highly Aqueous Mobile Phases, poster presented at the Chromatography Forum of the Delaware Valley, Media, Pennsylvania, June 1997. (9) J.J. Kirkland, J.L. Glajch, and R.D. Farlee, Anal. FIGURE 12: Separation of nucleic acid bases with a totally aqueous mobile phase. Column: 150 mm 4.6 mm alkyl amide; mobile phase: 0.050 M sodium acetate (ph 4.6); flow rate: 1.0 ml/min; temperature: 24 C; detection: UV absorbance at 254 nm. Peaks: 1 cytosine, 2 5-fluorocytosine, 3 uracil, 4 5-fluorouracil, 5 thymine (N 10,700, tailing factor 1.07), 6 adenine. Chem. 61, 2 11 (1989). (10) J.J. Kirkland, J.W. Henderson, J.J. DeStefano, M.A. van Straten, and H.A. Claessens, J. Chromatogr. A 762, 97 112 (1997). (11) J.J. Kirkland, M.A. van Straten, and H.A. Claessens, J. Chromatogr. A 691, 3 19 (1995). (12) J.J. Kirkland, M.A. van Straten, and H.A. Claessens, J. Chromatogr. A 797, 111 120 (1998). (13) J.L. Glajch and J.J. Kirkland, LC GC 8(2), 140 145 (1990). (14) L.R. Snyder, J.J. Kirkland, and J.L. Glajch, Practical HPLC Method Development (John Wiley & Sons, New York, 2nd ed., 1997), pp. 196 198. (15) H.A. Claessens, M.A. van Straten, and J.J. Kirkland, J. Chromatogr. A 728, 259 270 (1996). (16) J.J. Kirkland, C.H. Dilks Jr., and J.J. DeStefano, J. Chromatogr. 635, 19 30 (1993). (17) L.R. Snyder, J.J. Kirkland, and J.L. Glajch, Practical HPLC Method Development (John Wiley & Sons, New York, 2nd ed., 1997), p. 42. (18) L.R. Snyder and J.J. Kirkland, Introduction to Modern Liquid Chromatography (John Wiley & Sons, New York, 2nd ed., 1979), pp. 212 217. (19) J. Köhler and J.J. Kirkland, J. Chromatogr. 385, 125 150 (1987). (20) J.J. Kirkland and J.W. Henderson, J. Chromatogr. Sci. 32, 473 480 (1994).. Copyright Notice Copyright by Advanstar Communications Inc. Advanstar Communications Inc. retains all rights to this article. This article may only be viewed or printed (1) for personal use. User may not actively save any text or graphics/photos to local hard drives or duplicate this article in whole or in part, in any medium. Advanstar Communications Inc. home page is located at http://www.advanstar.com.