FUSED-SILICA MICROBORE PACKED CHROMATOGRAPHY COLUMN WITH CHEMICALLY MODIFIED COLUMN WALL

Abstract: A packed, microbore fused-silica column (10) for use in liquid chromatography, wherein the column wall has been chemically modified, is shown. The chemical modification of the column wall is used to modify and improve the performance of the column (10). In one embodiment a stationary phase (20), different than the stationary phase bonded to the packing material (25), is bonded to the column wall. For reasons that are not yet understood column performance may be greatly enhanced.

Description: FUSED-SILICA MICROBORE PACKED CHROMATOGRAPHY COLUMN WITH CHEMICALLY MODIFIED COLUMN WALL Background of the Invention Microbore chromatography columns, i.e., columns having an inner diameter of approximately .6 mm or less, are becoming an increasing important tool in modern liquid chromatography. Fused-silica is the preferred material for fabricating such columns because its properties offer many advantages over available alternative materials. Fused-silica is flexible, strong, able to withstand high pressure, optically transparent, and has very good chemical inertness. Moreover, columns made of fused-silica have very smooth inner walls which minimizes wall effects. In contrast, stainless steel tubing, also commonly used in chromatography, has relatively rough surfaces which exacerbate wall effects. Fused-silica microbore columns may be packed, as shown for example in the inventor's prior U.S. Pat. No. 4,483,773, or left open, as shown, for example, in the articles "Open Tubular Column LC: Theory and Practice," vol.20, pp. 241 et seq. , - Journ. of Chro . Science (1982) , by the inventor; and "Contemporary Capillary Column Technology for Chromatography," May-June 1986 Chromatography Forum, pp. 38 et seq., by Jones, et al. As described in these articles and in the referenced patent, both types of columns, i.e., packed and open, may be used in either gas or liquid chromatography. When making fused-silica microbore columns for open tubular chromatography it is necessary to deactivate the inner column wall and then to chemically bond a uniform layer of a stationary phase to the surface of the wall. The stationary phase provides the partition medium for separating sample components and should be selected from a variety of substances according to the intended use of the column. Fused silica is chemically similar to the silica substrates used in traditional LC * packings and, accordingly, similar stationary phase materials may be used. The stationary phase layer will typically have a thickness of between .1 to 5 ^ . The methods for surface deactivation and chemical bonding of stationary phase materials are well known in the art and will not be further described. See, e.g., "Contemporary Capillary Column Technology for Chromatography," cited above, for an overview of these methods. As has already been noted, fused-silica columns have very smooth inner walls which tends to minimize wall effects. For packed microbore fused-silica columns it has traditionally been thought that the column walls played no significant role in the operation of the column. Accordingly, in the past, no effort has been made to deactivate or chemically modify the column wall of a packed fused-silica microbore column. Summary of the Invention The present invention comprises a packed fused- silica microbore column with a deactivated and/or chemically modified column wall. The chemical modification of the column wall can be used to modify and improve the performance of the column in a variety of ways. The stationary phase bonded to the column wall may be different than the stationary phase bonded to the packing material. When this is done column performance may be greatly enhanced for reasons which are not fully understood as yet. Brief Description of the. Drawing Figure 1 is a cross-sectional view of a portion of a microbore packed column in accordance with the present invention. Figure 2 is a chromatogram obtained using a packed microbore column having a stationary phase bonded to the inner wall in accordance with the present invention. Detailed Description of the Invention In Figure 1 a portion of a fused-silica microbore column (10) of the present invention is shown in cross-section. The column comprises a fused-silica microbore tube (15) . The term microbore is used to denote that the tube has an inner diameter less than approximately .6 mm. Column length may vary considerably depending on the application and needs of the chromatographer. Lengths ranging from 10 cm to 200 cm are typical. The manner of construction of such fused-silica microbore tubes is well known in the art and will not be described further. In the preferred embodiment of the present invention, a stationary phase (20) is chemically bonded to the inner wall of the fused-silica tube (15). Prior to bonding of the stationary phase (20), the inner wall should, preferably, be deactivated using methods well known to those skilled in the art of making open tubular fused-silica columns. A stationary phase such as methylphenylsiloxane, di ethylpolysiloxane, polyethylene glycol, dimethyl- diphenylpolysiloxane, octadecylsiloxane, octylsiloxane, aminopropylsiloxane, propylsiloxane, cyanopropyl- siloxane, cation or anion exchanger siloxanes, liquid crystals, etc., may be selected according- to the specific application need. Methods of bonding the selected stationary phase are well known to those skilled in the art of making open tubular fused-silica columns. The resulting stationary phase layer (20) may range in thickness from .1 to 5 /x m. However, as noted below, a relatively thin layer (in the 0.1-1.0 range may be preferred. The column is filled with packing material (25) comprising icroparticulate silica. A selected stationary phase (not shown) is bonded to the micro- particulate silica prior to packing. The packing should preferably consist of substantially uniform particles having a diameter in the range of 1 to 200 /m. The method of packing a microbore fused-silica column is described in the inventor's U.S. Pat. No. 4,483,773. In accordance with the present invention, it is not necessary that the stationary phase selected for the packing material be the same as the stationary phase (20) bonded to the column wall. Indeed, there may be distinct advantages to selecting a stationary phase for the wall that is different than the station¬ ary phase of the packing material. It appears experi¬ mentally that, by selecting appropriate surface modifi¬ cation chemicals one can minimize surface adsorption sites and modify physicoche ical interaction between solute molecules and the active sites on the column wall, adding one more parameter for control of chroma- tographic processes. The scope of chemical modification of the surface of the inner wall may include, for example: 1) Modification of surface hydrophilic or hydrophobic activity. It appears that better column efficiency can be obtained if solute molecules are repelled from the column wall. It is hypothesized that this minimizes the wall transcolumn zone spreading of the solute peak. 2) Masking or creating surface adsorption sites. Surface adsorption sites, e.g., SiOH, may be masked by simple chemical deactivation of the column wall by known techniques. Selective surface adsorption sites can be created by depositing a stationary phase on the wall or by chemical modification of the wall in accordance with the present invention. 3) Modification ' with chemically bonded stationary phases to obtain additional selectivity, retention, resolution and sample capacity. Selective phases, e.g., liquid crystal, can also be used to modify the surface for isomeric and other separations. Table 1 shows the chromatographiσ characteristics of relative retention between toluene and benzene and total plate count for anthracene for a four different columns. Three of the columns were prepared according to the present invention and compared to a untreated fused-silica column of the prior art. Relative retention, *. , between toluene and benzene increases for the chemically modified columns (<= ~1.52) compared to the untreated column (^ ^l^l) . This increase in relative retention suggests a significant intermolecular interaction between solute molecules and the stationary phase on the column wall. TABLE 1 EFFECT OF COLUMN WALL CHEMICAL NATURE ~ (2 ^m ODS. 50% ACN:50% H20) «=» (toluene/benzene) N(plates/m) Pure fused silica 1.41 71K 0.25 yt< m 5% phenyl- methyl siloxane 1.52 39K 1.0 r m 5% phenyl- methyl siloxane 1.53 23K 1.0 ^ Carbowax 20M 1.51 257K The plate count for the untreated column is 71,000 plates/m for anthracene. After a 5% phenylmethylsiloxane modification of the column wall, the plate count is reduced to 39,000 for a 0.25 ^m film and to 23,000 for a 1.0 ^ film. Because there is only a slight difference in relative retention ^ of the two test components between the different films, the degradation of plate count with the thicker film may result from non-equilibrium distribution due to the thick film. These results suggest that a thinner film may be preferred. With the Carbowax (polyethylene gycol) modi ication, a very significant improvement in column plate count is measured. This is also shown in Figure 2. A reduced plate height of 1.9 (257K plates/m, anthracene, k'= 36.5) was measured for the 2^m ODS, 35 cm x 0.32 mm I.D. column. This plate count is to date the highest number reported in LC using 2 ^ particles. The reasons for this result are not fully understood. The improvement in column plate count may result from the non-polar nature of anthracene molecules which thus does not have significant soluability in the highly polar polyethylene gycol stationary phase. The anthracene is thus repelled from the column wall. This would minimize both wall adsorption effects (silanol and trace metal) and would also minimize transcolumn zone spreading of the solute band. The deactivation of adsorptive active sites such as silanol groups on the glass surface is also suspected as playing a role in column efficiency. From the experimental data gathered to date, it appears that other chemical modifications of the walls of packed microbore columns may be used to significantly improve column performance. The number of possible stationary phases is quite large and the thickness of the layer varies over a significant range. The column configuration selected will largely depend on the intended application of the chromatograph. At present, the theoretical aspects are not well developed and can only provide limited guidance. Hopefully, these theoretical considerations will be explored and developed so that a systematic basis for wall modification can be applied.

Claims: Claims What is Claimed is: 1. _ A microbore column for use in liquid chroma¬ tography comprising, a packed tubular fused silica column having a diameter of less than approximately 600 ύats. wherein the inner wall of said column has been deactivated and chemically bonded with a stationary phase. 2. The microbore column of claim 1 wherein said stationary phase is different than the stationary phase bonded to the packing material. 3. The microbore column of claim 2 wherein said stationary phase is chosen to mask surface adsorp¬ tion sites on said inner wall. 4. The microbore column of claim 2 wherein said stationary phase is chosen to create surface adsorption sites on said inner wall. 5. The microbore column of claim 2 wherein said stationary phase is chosen to enhance hydrophilic activity on the surface of said inner wall. 6. The microbore column of claim 2 wherein said stationary phase is chosen to enhance hydrophobic activity on the surface of said inner wall. 7. The microbore column of claim 2 wherein said staionary phase is chosen to enhance selectivity, retention, resolution, or sample capacity. 8. The microbore column of claim 2 wherein said stationary phase is an organosiloxane. 9. The microbore column of claim 8 wherein said stationary phase is chosen from the group consisting of methylphenyliloxane, octylsiloxane, dimethylpolysiloxane, polyethylene glycol, dimethyldiphenylpolysiloxane, octadecylsiloxane, and cyanopropylsiloxane. 10. A process of making a microbore column for use in liquid chromatography comprising the steps of: selecting a fused-silica tubular column of a desired' length and with a diameter less than approximately 600 ~^ m , deactivating the inner wall of said column, chemically bonding a stationary phase to said inner wall, and uniformly packing said column. 11. The process of claim 9 wherein said stationary phase is different than the stationary phase bonded to the packing material. 12. The process of claim 10 wherein said stationary phase is an organosiloxane. 13. The process of claim 12 wherein said stationary phase is chosen from the group consisting of methylphenylsiloxane, octylsiloxane, dimethylpolysiloxane, polyethylene glycol, dimethyl-diphenylpolysiloxane, octadecylsiloxane, aminopropylsiloxane, and cyanopropylsiloxane.

Inventors: YANG FRANK J, YANG, FRANK, J.
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1987
Main classification: B01D15
IPC codes: B01D15, B01J20, G01N30