ADSORBENT FOR CHROMATOGRAPHY, METHOD OF PRODUCING THE SAME, AND COLUMN SYSTEM FOR CHROMATOGRAPHY

Abstract: In an adsorbent for use in chromatography comprised of secondary particles (B2) formed by aggregation of primary particles in the form of fine crystals of hydroxyapatite, the aggregated primary particles are sintered so as to be bonded together. The adsorbent is produced by heating the secondary particles (B2) at a temperature not lower than 300°C and not higher than the decomposition temperature, and has significantly improved repetitive using properties. In a column system for use in chromatography, there are provided a main column (K1) packed with the above described adsorbent and a precolumn (K2) packed with a second adsorbent comprised of conventional hydroxyapatite particles. This column has also a significantly improved repetitive using properties.

Description: The present invention relates to an adsorbent for use in chromatography, more particularly, such adsorbent comprised of hydroxyapatite particles and a method of producing the adsorbent. The term "hydroxyapatite" as used herein is intended to mean a chemical substance having a formula Ca₁₀(PO₄)₆(OH)₂ (unit cell of hexagonal system: a^b = 120°, a^c = b^c = 90°, |a| = |b| = 9.42 Å and |c| = 6.88 Å) and being a species of calcium phosphates among apatite compounds belonging in turn to a hexagonal system P6 3/m having a basic composition of M₁₀(ZO₄)₆X₂, wherein M = Ca, Z = P and X = OH. The basic composition may also be thought of as a monoclinic system P2 1/b which is double the first stated basic composition. This hydroxyapatite is hereafter referred to as HAp. These HAp particles exhibit superior properties in chromatographic separation or in substance separation or development when used as the adsorbent packed or charged into a column or as a stationary phase agent in the column in column chromatography. Thus it is possible with the chromatography using these particles as the adsorbent packed in the column, or HAp chromatography, to realize high acuteness and precision separation and fractionation of substances having a minute difference in structure from one another which was difficult to achieve with the use of the prior-art adsorbents such as ion exchange resins, activated alumina or calcium carbonates. These substances may include biological macromolecule materials having a molecular weight of 10⁴ to 10⁹ Dalton, such as, for example, proteins including immunoglobulin, interferon or enzymes, or nucleic acids such as RNA, DNA or plasmids or viruses. It is presently used as means indispensable for high purity separation and refining of a variety of ultimate useful substances obtained by gene recombination, cell fusion or cell culture en masse. It will be noted that, for stable chromatographic separation in column chromatography with high precision and efficiency, it is essential that, in addition to the superior properties in chromatographic separation of the adsorbent particulate material packed into the column as the stationary phase, the following basic properties are required. a) That the sample fluid and the carrier fluid introduced into and circulated through the column so as to flow through interstices among the packed adsorbent particles will have a flow pattern that is as uniform as possible at any point in the transverse section of the column; b) and that the fluid flow rate through the column per unit sectional area and unit time is as high as possible. The uniform fluid flow pattern at any point in the sectional column area as per item a) above may be realized when the size of the individual adsorbent particles packed in the column is small and the extent of the particle size fluctuations is also small, since then the extent of the fluctuations in the size of the interstices among the packed particles at each point in the sectional plane of the column is reduced. However, too small a size of the individual particles results in increased resistance presented to the fluids introduced into the column and correspondingly lowered fluid flow rate in the column. Thus, for realizing a stable chromatographic separation with high precision and efficiency, it is essential that the adsor bent particles that are packed into the column exhibit superior properties of chromatographic separation, that the individual particles be of a size large enough to procure a flow level through the column which is higher than a prescribed value, and that the particle size be uniform and subject to only small fluctuations. Thus, in HAp chromatography, the following type A or type B materials are predominantly used for packing into the chromatographic column. In addition, the following type C particles have been evolved by the KOKEN Co., Ltd., the assignee of the present invention. (1) Type A The type A material is the HAp crystal particles obtained by the Tiselius method proposed by A. Tiselius, S. Hjertén and Ö. Levin in Arch. Biochem. Biophys., 65 , 132 (1956). These HAp particles may be produced by having a suitable alkali act on an aqueous suspension of crystal particles of Brushite (CaHPO₄·2H₂O) used as the starting material. The particles of the type A are substantially monocrystal plate- or flake-like particles of a larger crystal size with a thickness of 1 to 3 µm, a mean diameter along the long axis of the order of tens of µm and with a shape which is substantially the replica of the shape of the Brushite crystal particles used as the starting material. (2) Type B The type B material is the secondary particles composed of aggregated or flocculated primary particles that are in the form of fine HAp crystals. In general, the B type particles are produced by the following method. An aqueous solution or suspension of calcium salts such as Ca(OH)₂, CaCl₂, Ca(NO₃)₂ or Ca(CH₃COO)₂ are mixed under a basic condition with an aqueous solution of PO₄ salts such as NH₄H₂PO₄, (NH₄)₂HPO₄ or H₃PO₄. In such a manner, fine primary HAp crystal particles are produced with a particle size of usually not larger than 1 µm. The liquid suspension of these primary crystals are spray dried to produce substantially spherical secondary particles with a diameter of 1 to 10 µm through desiccation and aggregation of the primary HAp particles. (3) Type C The type C material is the substantially monocrystal HAp particles produced by the method disclosed in our copending Japanese Patent Application No. 18868/1986. Thus the starting Brushite crystal particles are first heated to a range of temperatures of not lower than the dehydration and phase transition temperature and not higher than the decomposition temperature so as to be turned on dehydration and phase transition into crystal particles of monetite CaHPO₄, these monetite crystal particles being then acted upon by a suitable alkali to produce substantially monocrystal HAp particles. The resulting HAp particles are produced in such a manner that the relatively large-sized plate- or flake-like starting Brushite crystal particles are crushed within the crystal plane to smaller pieces having the size of the order of several µm. Thus, these C type particles are uniform in their entirety in size and shape and are 1 to 3 µm in thickness and several µm in diameter. Thus the C type particles are subject to lesser fluctuations in size and shape than the aforementioned A-type large-sized plate- or flake-like HAp particles having the size and shape that are the replica of the starting Brushite crystal particles. Hence, the C type particles are more desirable than the A type particles when used as the adsorbent used for chromatography. Insofar as the aforementioned basic properties are concerned, that is, the uniform flow pattern in item a) and uniform flow rate in item b), the B type primary HAp particles aggregated into the secondary particles rank first in evaluation among the three types of the materials, that is, the A, B and C type HAp particles. However, the B type particles are markedly inferior to the A or B type particles with r espect to the repetitive use properties or durability of the column packed with these particles, such that column exchange or adsorbent re-packing will be necessary to perform prematurely. More specifically, while the column packed with the type A or C particles may be repetitively used more than 50 times, the column packed with the type B particles becomes unable to be used after ten times of repetitive use at most such that chromatograms presenting inaccuracies or unusualties and thus indicating that the column is nearly used up will be outputted. The reason for such short service life of the column may be summarized as follows. With the conventional B-type particles, that is, the primary particles in the form of fine HAp crystals desiccated and aggregated into secondary particles, the force binding the aggregated primary particles together is not so strong that, when the particles are packed as the stationary phase medium into the column in order to perform the chromatographic operation, the aggregated primary particles lying on the surface of the secondary particles are disengaged sequentially and with relative ease under the force of circulation of the fluid introduced into the column. Then, with repetition of the chromatographic operations, the flow interstices formed between any adjacent secondary particles packed into the column are stopped locally gradually with the sequentially disengaged minute primary particles, such that the fluid introduced into the column is caused to flow mainly along the flow interstices among the secondary particles that are not stopped as yet, thus producing a locally offset circulation pattern. The result is that the flow pattern or velocity of the introduced fluid at each point on the sectional plane of the column will become progressively non-uniform with the lapse of time, such that, beyond a certain limit of the non-uniformities, the manner of development or adsorption of the sample material on the packed adsorbent will become non-uniform so that unusualties or inaccuracies are necessarily produced in the output chromatogram. An adsorbent for use in chromatography of the kind defined by the precharacterizing features of claim 1, as well as a method of producing such an adsorbent as defined by the precharacterizing features of claim 4, are known from the EP-A-0 026 090. The secondary particles of said known adsorbent are a ceramic material obtained from fine primary hydroxyapatite chrystal particles and that ceramic material is mainly used as implant material. The moulding process used for producing the ceramic material comprises filtering and moulding steps. It is the object of the present invention to provide an adsorbent for use in chromatography of the kind defined by the precharacterizing features of claim 1 showing superior quality in chromatography, as well as to provide a method of producing an adsorbent for use in chromatography of the kind defined by the precharacterizing features of claim 4 resulting in an adsorbent showing superior quality in chromatography. Concerning the adsorbent, the object is attained by the characterizing features of claim 1 and concerning the method of producing an adsorbent, the invention is attained by the characterizing features of claim 4. The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which: FIG. 1 is a photo taken by a scanning type microscope of an adsorbent for chromatography consisting of secondary particles obtained by aggregating and thermally treating fine HAp primary crystal particles; FIG. 2 is a longitudinal section through a straight tube type test column and a comparative column that are used for testing the adsorbent; FIG. 3 is a chart showing the results of a test on repetitive using properties or durability of the test and comparative columns; FIG. 4a is a regular chromatogram for the test column; FIG. 5a is a similarly regular chromatogram for the comparative column; FIG. 4b is a chromatogram for the test column showing symptoms of the incumbent unusualties; FIG. 5b is a chromatogram showing unusualties for the comparative column; and FIG. 6 is a longitudinal section through a column unit comprised of a main column and a precolumn. The absorbent of the present invention consists of primary particles in the form of fine HAp crystals that are aggregated to form secondary particles, wherein the primary particles thus aggregated are sintered together so that the binding force of the individual particles is much stronger than that of the conventional B type particles, that is, the desiccated and aggregated secondary particles. In such a manner, when the chromatographic operation is executed with the novel secondary particle adsorbent packed into the column so as to be used as the stationary phase medium, the amount of the aggregated secondary particles that is released as time elapses from the surface of the secondary particles under the force of flow of the fluid introduced into the column is practically reduced to an extremely small value as compared to the case wherein the conventional B type particles are used. Hence, the uniform flow pattern and high flow level of the fluid at each point in the cross-section of the column packed with the aforementioned novel secondary particle adsorbent remain to be optimum for a prolonged period of time. Thus the repetitive use properties of the column is significantly improved. Furthermore, as may be seen from the following Examples, the fluid inlet pressure into the column, that is, the fluid inlet pressure necessary to cause the fluid to flow into the column at the prescribed supply rate in ml/min., may be markedly lower than in the case of using the conventional B type particles, whilst the chromatographic operation may be optimized. According to the method of the present invention, the novel secondary particle adsorbent consisting of the primary particles firmly bonded together as a whole may be produced in large quantities. In addition, it is possible with the column for chromatography according to the present invention to protract the service life of the main column packed with the secondary particle adsorbent as the stationary phase medium. Thus, when the fluid is introduced into and circulated through the column packed with the adsorbent in order to effect a chromatographic operation, a larger chemical and physical load acts on the portion of the adsorbent in the column that is disposed towards the fluid inlet to the column. While such portion of the C type or A type HAp adsorbent particles is subject to only slow deterioration as a result of such repetitive load application, the portion of the aggregated secondary particle adsorbent undergoes more prompt deterioration due to such repetitive load application. Thus, in accordance with the present invention, the main column packed with the secondary particle adsorbent as the stationary phase medium is operatively associated with a precolumn packed with the C type or A type HAp or the like particles having resistivity to both the chemical and physical repetitive load application. Such precolumn is provided as the preliminary column so as to be in communication with or disconnected from the fluid inlet to the main column such that the aforementioned physical and chemical load is sustained by the adsorbent in the precolumn having resistivity to such load. In this manner, the chemical and physical load acting on the secondary particle adsorbent portion lying towards the fluid inlet into the main column packed with such secondary adsorbent is reduced resulting in improved repetitive using properties or service life of the overall main column. Examples (1) Production of the Secondary Particles Composed of Fine HAp Crystal Particles Aggregated Together An aqueous suspension of Ca(OH)₂ with a concentration of 24.5 g/ℓ (0.33 M) and an aqueous solution of NH₄H₂PO₄ with a concentration of 23.0 g/ℓ (0.2 M) were mixed together at a ratio of 1:1 and allowed to stand for 3 hours at 40°C. In this manner, there are produced and precipitated fine primary HAp crystal particles having a particle size usually not larger than 1 µm, herein about 0.1 µm, and being substantially free from cell defects, that is, stoichiometric. The resulting aqueous suspension of these primary HAp crystal particles was then spray-dried by using a spray drier manufactured by Yamato Kagaku K.K., DL-41, commercial code number 212708, to produce substantially spherical secondary particles having a diameter in the range of 1 to 10 µm and consisting of the primary HAp crystal particles aggregated together. The desiccated and aggregated secondary HAp particles produced by the above method are equivalent to the B type adsorbent mentioned above. A portion of these aggregated secondary HAp particles was taken as the reference or comparative sample which is designated herein as the thermally untreated particles B2. (2) Thermal Treatment (Sintering) The aggregated HAp secondary particles produced in (1) were packed into an open hearth furnace or crucible and thermally treated by an electrical heating unit at 700°C for 30 minutes such that the primary particles aggregated as the secondary particles were sintered and bonded together to form the secondary HAp particles. These particles are herein designated as the thermally treated particles B1. FIG. 1 is a photo taken by a scanning type electron microscope of these thermally treated particles B1. It is noted that, when viewed through an electron microscope, the thermally untreated particles B2 are substantially similar in appearance to these thermally treated particles B1 such that these particles B1 and B2 are not distinguishably different from each other. In general, the thermal treatment temperature of not higher than 300°C used for sintering the aggregated primary particles together to form the HAp secondary particles is not desirable since the primary particles are not sintered together substantially or at least the binding force resulting from sintering is not sufficient such that the sintering need be performed over a longer time interval. The sintering time may differ with the amount of the secondary particles that are thermally treated at one time or with the mean particle size of the secondary particles and thus need be determined appropriately th rough, for example, preliminary tests. The thermal processing for about 30 minutes or longer is usually sufficient. (3) Chromatographic Operation The thermally treated particles B1 under item (2) were packed as the stationary phase agent in a stainless steel column 1 used as the column for chromatography having an inside diameter of 6 mm and an effective length of 3 cm (Fig. 2). The numerals 2 and 3 designate a fluid inlet cap and a fluid outlet cap of the column, respectively, while the numerals 4 and 5 designate filter elements disposed at the inlet cap 2 and at the outlet cap 3, respectively. The thermally treated particles B1 as the stationary phase medium is packed in a column space defined between these filter elements 4 and 5. The column is the test column. On the other hand, a column similar to the test column was similarly packed with the thermally untreated particles B2 under item (1) as the column for chromatography, which is the comparison column. The following sequence of the chromatographic operations was repeatedly carried out for the test and comparative columns to test their repetitive use properties or durability. i) The egg white lysozyme of the hen's egg was used as the sample and the potassium phosphate buffer solution of PH ≈ 6.8 was used as the carrier liquid in the linear molar concentration gradient elution method. ii) The potassium phosphate buffer solution was set to an initial concentration of 1 mM and to a linear concentration gradient of 7 mM/ml. iii) The carrier liquid was supplied into the column at the rate of 0.5 ml per minute. iv) The temperature used was the ambient temperature. (4) Test Results In FIG. 3, the number of times of repetition of the chromatographic operations was plotted on the abscissa, while the inlet fluid pressure into the column for each chromatographic operation, or the pressure required for maintaining the supply rate setting of the carrier liquid into the column which is 0.5 ml/min., is plotted on the ordinate. The fluid inlet pressure values for the respective chromatographic operations for the test and comparative columns are indicated by marks (x) and (·) respectively. It is seen from the chart of FIG. 3 that the pressure levels for the test column may be markedly lower than those for the comparative columns throughout the respective operations and that the pressure levels are increased gradually in the case of the comparative columns as the number of times of the chromatographic operation is increased. Thus it may be seen that the test column is practically more useful than the comparative column. FIG. 4a shows the chromatogram in the regular state obtained with the test column while FIG. 5a shows the same chromatogram obtained with the comparative column. As compared with the regular chromatogram for the test column shown in FIG. 4a, the regular chromatogram for the comparative column of FIG. 5a already shows certain unusualties that the obtained chromatographic pattern for the lysozyme is lacking in the minute peak a (Fig. 4) that should appear immediately before the main peak. In addition, in the case of the comparative column, there is outputted, at the tenth chromatographic operation, an unusual chromatogram in which, as shown in FIG. 5b, the chromatographic peak shown by the double-dotted chain line in FIG. 5b and which should appear only once is divided into two or more separate peaks, such that it becomes impossible to carry out the chromatographic operation in the usual manner. In the test column, only small peak separation was seen to occur only at the 21st chromatographic operation, as shown in FIG. 4b. Such peak separation is still within the allowable range as to precision such that the column may still be used on end. Thus it is seen that the column for chromato graphy packed with the thermally treated particles B1 as the stationary phase medium may be used repeatedly over a larger number of times than the column packed with the thermally untreated particles B2. FIG. 6 shows an example of a column arrangement comprised of a main column such as that shown in FIG. 2 and packed with the thermally treated particles B1 as the stationary phase medium and a precolumn K2 packed with the aforementioned C type or A type HAp monocrystal particles C(A) and connected to the fluid inlet to the main column K1 so as to be in fluid communication with or disconnected from the main column. The operation and result are as described above. In the drawing, the numerals 11 to 15 denote a main body of the column, a fluid inlet cap, a fluid outlet cap, an upper filter element and a lower filter element, respectively. The numeral 16 designates a coupling for removably connecting the main column K1 and the precolumn K2. From the foregoing it is seen that the present invention provides an adsorbent for chromatography consisting of the secondary particles formed by aggregation of the fine HAp primary crystal particles and having significantly improved repetitive use properties, and a column for chromatography that is superior in repetitive use properties. While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims.

Claims: An adsorbent for use in chromatography comprising secondary particles (B2) formed on aggregation of fine primary hydroxyapatite chrystal particles, characterized in that said secondary particles (B2) are substantially spherical particles and sintered so that said primary particles form sintering bond to each other. An adsorbent for use in chromatography in accordance with claim 1, characterized in that said primary particles forming on aggregation said secondary particles (B2) of hydroxyapatite are the particles not more than 1 µm in diameter, said secondary particles (B2) thus formed on aggregation having the diameter of the order of 1 to 10 µm. An adsorbent for use in chromatography in accordance with claim 1, characterized in that said primary particles forming on aggregation said secondary particles (B2) of hydroxyapatite are of substantially stoichiometric chemical composition. A method of producing an adsorbent for use in chromatography comprising the step of aggregating fine primary hydroxyapatite crystal particles to form secondary particles (B2), characterized in that the step of aggregating said fine primary hydroxyapatite crystal particles includes a substep of forming a suspension of said primary particles and a substep of spray-drying said suspension of the primary particles to form spherical secondary particles, and said method further comprises the step of heating said secondary particles (B2) at a temperature not lower than about 300° C and not higher than the decomposition temperture thereof. A method of producing an adsorbent for use in chromatography in accordance with claim 4, characterized in that the substep of forming the suspension of said primary particles of hydroxyapatite includes mixing an aqueous solution or suspension of calcium salts such as Ca(OH)₂, CaCl₂, Ca(NO₃)₂ or Ca(CH₃COO)₂ with an aqueous solution of PO₄ salts such as NH₄H₂PO₄, (NH₄)₂HPO₄ or H₃PO₄ under basic conditions. A column system for use in chromatography comprising: a main column (K1) packed with an adsorbent composed of secondary particles (B2) formed by aggregation of fine primary hydroxyapatite crystal particles, and a precolumn (K2) provided at the fluid inlet into said main column (K1) so as to be connected in communication with or disconnected from said main column (K1), characterized in that said secondary particles packed into said main column (K1) are substantially spherical particles and previously sintered so that said primary particles form sintering bond to each other, and said precolumn (K2) is packed with a second adsorbent produced by the steps of heating a starting material of brushite crystal particles to a range of temperature not lower than the dehydration transition temperature and not higher than the decomposition temperature to cause dehydration and phase transition of the brushite crystal particles, thus producing monetite crystal particles and acting an alkali on the thus processed monetite crystal particles.

Inventors: KAWASAKI, TSUTOMU, KOBAYASHI, WATARU
Applicants: KOKEN CO. LTD.
Application year: 1987
Main classification: B01J20
IPC codes: B01J20, B01D15, G01N30