CHROMATOGRAPHIC PACKING AND PREPARATION THEREOF

Abstract: A chromatographic column packing for liquid chromatography, particularly suitable for the separation of polycyclic aromatic compounds, wherein the surface of the base material is treated with a silane coupling agent of general formula (I), wherein Aro represents an aromatic moiety; R¹ and R² represent each independently alkyl, alkenyl, aryl, lower alkoxy or halogen; and R³ represents lower alkoxy or halogen. Since the functional groups of the coupling agent serve to immobilize the aromatic moiety of the same molecule on the surface of the base material by a number of linkages, a packing with stable and excellent separation characteristics can be obtained.

Description: This invention relates to a packing material for chromatography and in particular, to an improvement of a surface modifying agent. The packing material of the invention is especially used for separation and analysis of polycyclic aromatic compounds and related compounds. Chromatography has been employed for separation and purification of various chemical components. In such chromatography, as the packing material for adsorption chromatography in which mutual separation between the sample molecules is accomplished through the dipole-dipole interaction, π electron- π electron interaction or hydrogen bond formation force between the sample molecules (or analyzed molecule) and the packing material surface, porous silica gel having macropores and micropores distributed in a network or porous polymeric material obtained by copolymerization of styrene, etc. have been employed. Particularly, packing material of a porous silica gel surface modified with a silane coupling agent differs in separation characteristics of sample molecules depending on the structure of the silane coupling agent employed, and it is therefore useful for efficient separation of sample molecules having various structures. Incidentally, as the surface modifying agent of the surface-modified silica gel to be used as the packing agent for chromatography, the octadecylsilyl derivative is most widely used, but also the butylsilyl derivative, octylsilyl derivative, etc. have been employed. However, all of these have only one functional group for silylation capable of bonding to the silica gel surface in one molecule. Therefore in a developing solvent for chromatography, functional groups which influence separation characteristics, such as the octadecyl group, butyl group or octyl group, at the interface between the filler and the solvent exist in disordered form on the silica gel surface. Further, the octadecyl group, butyl group, octyl group, etc. have carbon chains constituting the molecules that are freely rotatable, and therefore there is no homogeneity in the adsorption form when sample molecules are practically adsorbed onto the packing material surface. Such absence of homogeneity is a crucial defect in designing the surface modifying agent to obtain the most suitable stationary phase depending on the structure of sample molecules. EP-A-269,447 discloses compositions useful in chromatographic separations and comprising a substrate surface modified by a bidentate silane containing at least two silicon atoms bridged by groups so arranged that an at least seven-membered ring system is formed including two atoms on the substrate surface to which the silicon atoms are linked through oxygen atoms. The groups bridging the silicon atoms in the silane are selected from -O-, CH₂-, -CH₂CH₂-, -(CH₂) n -, -(CHZ-CHZ) and -(CH₂) n CHZ-, where Z has a variety of meanings, including aryl or substituted aryl. Thus, any aryl residue Z in the bidentate silane is attached to the bridge between the silicon atoms through only one point. The present invention has been made in view of the problem of the prior art as mentioned above, and its object is to provide a packing material for chromatography that can separate various substances, and further can efficiently be useful for separation and purification within a short time. For accomplishing the above object, the present inventors have studied intensively and consequently have found that excellent separation characteristics can be exhibited by having the organic functional groups regularly arranged on the surface of the base material for the packing material by use as the surface modifying agent of an aromatic compound having a plurality of functional groups for silylation in one molecule (hereinafter called "multi-legged silane coupling agent"), the functional groups being attached at different points on the aromatic residue of the coupling agent. According to the present invention there is provided a packing material for chromatography comprising a porous silica gel subjected to surface modification by a multi-legged silane coupling agent having functional groups capable of bonding with silanol groups on the surface of the porous silica gel via two or more bonds between the porous silica gel and the silane coupling agent, wherein the multi-legged silane coupling agent has the formula (I): wherein Aro represents an aromatic residue; R¹ and R² are independently an unsubstituted or substituted alkyl group of 1 to 20 carbon atoms, alkenyl group of 3 to 5 carbon atoms or aryl group of 6 to 12 carbon atoms, a lower alkoxy group, or a halogen atom, each substituent in such substituted group being located on a chain or ring moiety of the alkyl, alkenyl or aryl group and being lower alkyl or halogen; R³ is a lower alkoxy group or halogen atom, and n is an integer from 2 to 5. Further, a method for the preparation of the packing material for chromatography according to the invention comprises reacting a multi-legged type silane coupling agent having the formula I as defined above with a porous silica gel dispersed in an aromatic hydrocarbon solvent. The invention also includes the use of the packing material for chromatography, especially for chromatographie separation of polycyclic aromatic compounds. Such packing material has an excellent stability and separation characteristics, since aromatic compound moiety constructed of the silane coupling agent is immobilized on the surface of the base material through the multi-legged type functional groups (a plurality of functional groups). BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a ¹³C-solid state high resolution NMR spectrum of the packing material for chromatography according to an example of the present invention; Fig. 2 is a ¹³C-NMR spectrum of para-bis(dimethylmethoxysilyl)benzene in deuterochloroform; Fig. 3 is a ¹³C-solid high resolution NMR spectrum of the packing material according to Example 4; Fig. 4 is a chromatogram when a:benzene, b:naphthalene, c:anthracene, d:pyrene, e:chrysene are analyzed by use of the packing material according to Example 1; Fig. 5 is a chromatogram when A:triphenylene and B:o-terphenyl are analyzed by use of packing material according to Example 2. Fig. 6, 7 and 8 are chromatograms when the same sample as Fig. 4 are analyzed by use of the packing materials according to Examples 4, 5 and 6, respectively; and Fig. 9 is a chromatogram when A:triphenylene and B:o-terphenyl corresponding to Fig. 5 are analyzed by use of packing material "Vydac 201" that is a commercially available ODS type silica gel. In the present application, the "base material of the packing material" is a porous silica gel. Such material has a large surface area, and particles bearing silanol groups that are able to couple the usual coupling agent therewith. The "multi-legged silane coupling agent" is a compound having plural reactive groups for silylation, and has specifically the formula (I): wherein Aro represents an aromatic residue, that is to say an aromatic moiety tacking an appropriate number of hydrogen atoms of carbon-hydrogen bonds of an aromatic compound in accordance with the number of n and hence having an appropriate number of bonding sites in accordance with the number of reactive groups for silylation (or silylation functional groups); R¹ and R² are independently an alkyl group of 1 to 20 carbon atoms, alkenyl group of 3 to 5 carbon atoms, aryl group of 6 to 12 carbon atoms and may be substituted by a lower alkyl or halogen, lower alkoxy group, or halogen atom; R³ is a lower alkoxy group or halogen atom, and n is an integral number of 2 to 5. The term "lower alkyl" means alkyl having 1 to 5 of carbon atoms. Therefore, said multi-legged type silane coupling agent results in a substitution reaction with, for example,the silanol groups on the surface of the porous silica gel through the functional groups thereof, and the aromatic ring can be immobilized parallel to the surface at a particular position thereof. An example of the Aro's includes, but is not limited to, the residue derived from benzene, naphthalene, anthracene, pyrene, acenaphthylene, biphenyl, 1,3,5-tris(phenyl) benzene, thiophene and pyridine, and may be substituted by a lower alkyl or halogen, e.g., fluorine, chlorine or bromine. R¹ and R² are an alkyl group having 1 to 20 carbon atoms, i.e., straight or branched chain aliphatic group from methyl to eicosyl, and are preferably a lower alkyl, such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, tert.-butyl, n-pentyl. Further, the R¹ and R² may be an alkenyl group such as allyl, 2- or 3-butene; unsubstituted or substituted aryl such as phenyl, 4-methylphenyl, 4-chlorophenyl, naphthalene-1-yl or naphthalene-2-yl; lower alkoxy such as methoxy, ethoxy, n-propoxy, iso propoxy or n-butoxy; halogen such as chlorine, bromine or iodine. Of the groups described above, the halogen and alkoxy, the same as the following R³ group, can also be reacted with the reactive groups on the surface of the base material for the packing material, e.g., silanol group on the silica gel. R³ is a lower alkoxy group such as methoxy, ethoxy, n-propoxy, iso-propoxy or n-butoxy; or halogen atom, e.g., chlorine, bromine or iodine, and is most preferably methoxy and chloro in view of the surface modification of the base material. Although R¹, R² and R³ described above may be the same group simultaneously, i.e., the same lower alkyl or halogen atom, R¹ and R² are preferably different from R³ to smoothly accomplish the surface modification. Further, R¹ and R² can be the same or different, but they are preferably the same because production of the silane coupling agent is easy. n is an integral number of 2 to 5, generally an aromatic moiety of the silane coupling agent, and is immobilized by bondings corresponding to the number of n on the surface of the base material. The silane coupling agents used in the present invention especially consist of, but are not limited to, the following combination of moieties, having the formula: Representative examples of the silane coupling agents described above can be included para-bis(chlorodimethylsilyl) benzene, para-bis(dimethylmethoxysilyl)benzene, 1,3,5-tris(4-chlorodimethylsilylphenyl)benzene, para-bis(chlorodiphenylsilyl)benzene and 4,4′-bis(chlorodimethylsilyl)biphenyl. The silane coupling agents are commercially available and known per se, and may be easily prepared by a known technique. Further, the "surface modifying with such silane coupling agent" means that their functional groups, such as halogen atom or alkoxy groups, are allowed to react with the silanol groups on the surface of the silica gel to raise dehydrohalogenation or dealkoxylation, and thereby the plural silanol groups of the silica gel are substituted for an aromatic ring moiety of one molecular silane coupling agent. The surface-modified silica gel has approximately 1 to 2 aromatic rings bonded per area of 10 nm (100 Å) square, when subjected to a measurement of carbon content by elemental analysis. Since the aromatic rings are bonded to the surface through the silyl groups directly connected to two or more aromatic rings, these aromatic rings are estimated to exist parallel to the surface, just so as to cover the silica gel surface with the aromatic rings, and this estimation is supported by a solid high resolution NMR spectrum. The surface modifying described above is easily accomplished by a method for preparation of the packing material according to other inventions. For example, the surface modified packing material can be formed through reacting a silane coupling agent of the formula (I) with a porous silica gel dispersed in an aromatic hydrocarbon solvent. The solvent encloses benzene, toluene, xylene, halogenated benzenes, e.g., chlorobenzene. The reaction generally can be carried out by heating the reactive mixture under a refluxing temperature of the solvent for thirty minutes to ten hours, and if desired, an organic amine compound such as pyridine and triethylamine may be added into the reactive mixture. Resulting in the surface modified packing material being removed by filtration, washed with a suitable organic solvent, and then dried for use as a packing material for liquid chromatography. Examples The following examples are to illustrate the invention but should not be interpreted as a limitation thereon. All percentages of the reactants are by weight. Synthesis of surface-modified silica gel Example 1 Preparation of the packing material using para-bis(chlorodimethylsilyl)benzene An amount 0.75 g of para-bis(chlorodimethylsilyl)benzene is dissolved in 5 ml of dried toluene, 1 g of a porous silica gel suspended in 5 ml of toluene containing 0.45 g of pyridine is added, and the mixture was heated under reflux for 6 hours. After the silica gel is sedimented by being left to cool, the supernatant is removed with a pipette. The silica gel thus obtained is shaken with addition of 10 ml of chloroform, left to stand, followed by removal of the supernatant, of which a washing procedure is repeated three times. Next, the silica gel was separated by filtration by use of a membrane filter, and washed three times with methanol. The surface-modified silica gel according to this method can be dried and used as the packing material for liquid chromatography. The surface-modified silica gel synthesized according to this method is confirmed to contain 7.97% of carbon by elemental analysis. It is also confirmed from NMR spectrum that the multi-legged type silane coupling agent is bonded to the silica gel and benzene rings exist parallel to the surface. Fig. 1 shows the solid-state high resolution NMR spectrum (¹³C) of the surface-modified silica gel. The spectrum corresponding to the carbon of the benzene structure in the spectrum comprises two large peaks with an intensity ratio of 1:2, each being attributable to the carbon directly bonded to the dimethylsilyl substituent and the carbon directly bonded to hydrogen. This is substantially identical with the spectrum corresponding to the benzene structure in the ¹³C-NMR spectrum in deuterochloroform of para-bis(dimethylmethoxysilyl)benzene shown in Fig. 2, showing that the skelton of para-bis(dimethylsilyl)benzene is doubtlessly held on the solid. Example 2 Preparation of the packing material using para-bis(dimethylmethoxysilyl)benzene The surface-modified silica gel having the para-bis(dimethylsilyl)benzene skeleton carried on silica gel described in Example 1 can be also obtained by heating 0.71 g of para-bis(dimethylmethoxysilyl)benzene together with a porous silica gel suspended in 10 ml of toluene. In this case, for removing the methanol formed, the mixture is heated under reflux for one hour followed by removal of the solvent by distillation, and then 10 ml of toluene was freshly added and the mixture is heated under refluxing for one hour, the procedure of which is repeated three times. By carrying out the post-treatment as described in Example 1, a surface-treated silica gel can be obtained. The silica gel obtained by this method contained 3.45% of carbon. Example 3 Preparation of the packing material using tris(chlorodimethylsilyl)benzene When 1 g of tris(chlorodimethylsilyl)benzene is carried out on a porous silica gel according to the method shown in Example 1, a surface-modified silica gel containing 7.46% of carbon is obtained. Also in this case, it is confirmed by a solid-state high resolution NMR spectrum that the silane coupling agent is bonded in multi-legged type to the silica gel surface. Example 4 Preparation of the packing material using 1,3,5-tris(4-chlorodimethylsilylphenyl)benzene This example is a repeated procedure as described in example 1 except that it uses 1,3,5-tris(4-chlorodimethylsilylphenyl)benzene (0.66 g) instead of para- bis(chlorodimethyl)benzene, and produces a surface-modified silica gel containing 4.48% of carbon. The resulting silica gel from this method is confirmed by a solid-state high resolution NMR spectrum that the silane coupling agent is bonded in multiple to the silica gel surface (see Fig. 3). Example 5 Preparation of the packing material using para-bis(chlorodimethylsilyl)benzene This example is a repeated procedure as described in Example 1 except that it uses para-bis(chlorodiphenylsilyl)benzene (0.73 g) instead of para-bis(chlorodimethylsilyl)benzene, and procedures a surface-modified silica gel containing 9.08% of carbon. Example 6 Preparation of the packing material using 4,4′-bis(chlorodimethylsilyl)biphenyl This example is a repeated procedure as described in Example 1 except that it uses 4,4′-bis(chlorodimethylsilyl)biphenyl instead of para-bis(chlorodimethylsilyl)benzene, and produces a surface-modified silica gel containing 13.84% of carbon. Application to liquid chromatography Example 7 Into a fused-silica tube with an inner diameter of 0.53 mm and a length of 15 cm was filled according to the slurry method the silica gel surface modified with tris(chlorodimethylsilyl)benzene shown in previous Example 3 to prepare a separation column. The present column was connected to a high performance liquid chromatograph, methanol:water (70:30) is permitted to flow as the mobile phase at 2 µl/min., and benzene, naphthalene, anthracene, pyrene, chrysene are chosen as representatives of aromatic compounds and injected. By detection of the effluent at 254 nm by use of a UV detector, a chromatogram is obtained. This is shown in Fig. 4. As shown in this Figure, these substances are well separated. Example 8 By use of the column obtained in Example 7, a mixture of triphenylene and o-terphenyl was analyzed. As the mobile phase, methanol:water (60:40) is permitted to flow at a flow rate of 2 µl/min. and the effluent detected at 254 pm by use of a UV-detector. The two compounds are both aromatic compounds comprising three benzenes, and deemed to be analogous as the chromatographic behaviors governed by the electrical properties. However, the shapes of molecules are greatly different, the former having a planar structure, while the latter a non-planar structure. The chromatogram is shown in Fig. 5. Both are well separated and the retention time of triphenylene is shorter than o-terphenyl. Example 9 This example is a repeated procedure as described in Example 7 except that it uses the surface-modified silica gel according to Example 4, and methanol:water (60:40) as mobile phase. As a result, the chromatogram according to this procedure is shown in Fig. 6, each aromatic compound is well separated, respectively. Example 10 This example is a repeated procedure as described in Example 7 except that it uses the surface-modified silica gel according to Example 5. As a result, the chromatogram obtained on this procedure is shown in Fig. 7, and it is shown that each aromatic compound is well separated, respectively. Example 11 This example is a repeated procedure as described in Example 9 except that it uses the surface-modified silica gel according to Example 6. As a result, the chromatogram obtained in this procedure is shown in Fig. 8, and it is shown that each aromatic compounds is well separated, respectively. Comparative Example 1 By use of the column filled with a commercially available silica gel (Vydac 201), the analytical results under the same conditions with the use of methanol:water (70:30) as the mobile phase are shown in Fig. 9. In this case, as different from the silica gel surface modified with the multiple type silane coupling agent as described in Example 2, the retention time of o-terphenyl is shorter than triphenylene. As shown in Examples as described above, not only the packing material of the present invention has good separation characteristics as shown in Fig. 4 to 8, but it can also be understood from a comparison between Fig. 5 and Fig. 9 that it has clearly the specific separation characteristics different from those of the silica gel of the prior art. According to the packing material for chromatography as described above, particularly in separation of a polycyclic aromatic compound (PAH) which is a carcinogeneic substance, through its π electron - π electron interaction, separation of molecules, which could be separated with difficulty in the prior art are rendered possible to make the column efficiently good. INDUSTRIAL APPLICATION The present packing material, which is a novel packing material for a liquid chromatograph column, is useful in separating various organic compounds, in particular it is useful as a packing material for separation of polycyclic aromatic compounds.

Claims: A packing material for chromatography comprising a porous silica gel subjected to surface modification by a multi-legged silane coupling agent having functional groups capable of bonding with silanol groups on the surface of the porous silica gel via two or more bonds between the porous silica gel and the silane coupling agent, wherein the multi-legged silane coupling agent has the formula (I): wherein Aro represents an aromatic residue; R¹ and R² are independently an unsubstituted or substituted alkyl group of 1 to 20 carbon atoms, alkenyl group of 3 to 5 carbon atoms or aryl group of 6 to 12 carbon atoms, a lower alkoxy group, or a halogen atom, each substituent in such substitued group being located on a chain or ring moiety of the alkyl, alkenyl or aryl group and being lower alkyl or halogen; R³ is a lower alkoxy group or halogen atom, and n is an integer from 2 to 5. A packing material according to claim 1, wherein in the silane coupling agent the aromatic residue Aro is selected from residues of benzene, naphthalene, anthracene, pyrene, acenaphthylene, biphenyl, 1,3,5-tris(phenyl)benzene thiophene and pyridine; R¹ and R² are selected from methyl, ethyl, n-propyl, iso-propyl, n-butyl, tert.-butyl, n-pentyl, allyl, 2-butene, 3-butene, phenyl, 4-methylphenyl, 4-chlorophenyl, naphthalene-1-yl, naphthalene-2-yl, methoxy, ethoxy, n-propoxy, iso-propoxy, n-butoxy, chlorine, bromine and iodine; R³ is selected from methoxy, ethoxy, n-propoxy, iso-propoxy, n-butoxy, chlorine, bromine and iodine; amd n is 2 or 3. A packing material according to claim 2, wherein the silane coupling agent is selected from para-bis(chlorodimethylsilyl)benzene, para-bis(dimethylmethoxysilyl)benzene, tris(chlorodimethylsilyl)-benzene, 1,3,5-tris(4-chlorodimethylsilylphenyl)benzene, parabis(chlorodiphenylsilyl)benzene and 4,4′-bis(chlorodimethylsilyl)biphenyl. A method for the preparation of the packing material according to claim 1, which comprises reacting a porous silica gel dispersed in an aromatic hydrocarbon solvent with a silane coupling agent which has the formula (I) as defined in claim 1. A method according to claim 4, wherein the silane coupling agent is as defined in claim 2 or 3. The use of a packing material according to claim 1, 2 or 3 for chromatography. The use of a packing material according to claim 1, 2 or 3 for chromatographic separation of polycyclic aromatic compounds.

Inventors: ITOH KENJI, JINNO KIYOKATSU, NAGASHIMA HIDEO, OHTSU YUTAKA SHISEIDO LABORATO, ITOH, KENJI, JINNO, KIYOKATSU, NAGASHIMA, HIDEO, OHTSU, YUTAKA, SHISEIDO LABORATORIES
Applicants: SHISEIDO CO LTD, SHISEIDO COMPANY LIMITED
Application year: 1991
Main classification: B01J20
IPC codes: B01J20