Abstract:
A pumping apparatus for a high performance liquid chromatography system ( 350 ) is disclosed. The pumping apparatus comprises a piston ( 1 ) for reciprocation in a pump working chamber ( 3 ) to compress liquid in the pump working chamber ( 3 ) to a high pressure at which compressibility of the liquid becomes noticeable. At least one of the piston ( 1 ) and the pump working chamber ( 3 ) is at least partially coated with or comprised of silicon carbide.

Description:
[0001]    The present invention relates to a pumping apparatus in a high performance liquid chromatography system, wherein liquid is compressed to a high pressure at which compressibility of the liquid becomes noticeable. 
       BACKGROUND ART 
       [0002]    In high performance liquid chromatography (HPLC), a liquid has to be provided usually at very controlled flow rates (e. g. in the range of microliters to milliliters per minute) and at high pressure (typically 200-1000 bar and beyond up to currently even 2000 bar) at which compressibility of the liquid becomes noticeable. Piston- or plunger pumps usually comprise one or more pistons arranged to perform reciprocal movements in a corresponding pump working chamber, thereby compressing the liquid within the pump working chamber(s). The reciprocation is repeated thousand fold during the lifetime of the pump, thereby causing wear, abrasion and, hence, changes of the material and surface properties to the piston. 
         [0003]    A liquid chromatography pumping system is described in EP 0309596 B1 by the same applicant, Agilent Technologies, depicting a pumping apparatus comprising a dual piston pump system for delivering liquid at high pressure for solvent delivery in liquid chromatography. 
         [0004]    In HPLC applications, the pumping apparatus is exposed to more or less aggressive solvents ranging typically from water, Acetonitrile, Tetrahydrofurane, Methanol to Hexane or n-Hexane. Analytic HPLC applications usually work at flow rates of about 0.01 ml/min-10 ml/min, and applications in semi-preparative HPLC often work at flow rates of about 05-100 ml/min. Pistons of pumping apparatuses in HPLC applications are usually made of oxide ceramics (such as zirconia ZrO 2 ) or crystalline sapphire Al 2 O 3 , having proved—over decades—excellent characteristics and long life behavior for most HPLC applications. 
       DISCLOSURE 
       [0005]    It is an object of the invention to provide an improved pumping apparatus. The object is solved by the independent claims. Further embodiments are shown by the dependent claims. 
         [0006]    According to embodiments of the present invention, a pumping apparatus is described which is adapted to deliver liquids under high pressure in a high performance liquid chromatography system, in particular for analysis of chemical or biochemical compounds. The pumping apparatus is composed of one or more pistons, each of which being movably arranged in a corresponding pump working chamber. Moving a piston can be performed by a drive unit preferably having a piston holder. Each piston compresses the liquid in the respective pump working chamber to a high pressure at which compressibility of the liquid becomes noticeable. 
         [0007]    While pistons in HPLC applications are usually made of oxide ceramics or crystalline sapphire, which have proved—over decades of HPLC developments—an excellent characteristic and long life behavior, it has been found that an entire different material, silicon carbide, revealed a surprising characteristic and unexpected suitability for the quite rough and severe requirements, in particular high pressure and aggressive solvents, in HPLC. Accordingly, embodiments of the present invention use silicon carbide (SiC) as material for the piston and/or the pump working chamber, or parts thereof, wherein such components are either at least partially coated or even comprised as solid material. Preferably, the silicon carbide is used as sintered silicon carbide (SSiC) material. 
         [0008]    It has been shown that, for example, pistons made of a solid material of sintered silicon carbide exhibited a low friction coefficient, hardness of about 9.5, electrical conductivity of about 10 3  Ωm, chemical inertness even at higher temperatures up to 140° C., and a good mechanical stability for the HPLC requirements. Such SSiC pistons have even proved to be suitable for preparative HPLC applications using n-hexane as solvent, which represents one of the most severe requirements for HPLC pumping systems. 
         [0009]    SSiC tends to be a brittle material and can usually withstand a high pressure load, but as most brittle materials it might show limitations under torsion and strain. Depending on the load either coating or solid SSiC may be of advantage. 
         [0010]    Each reciprocation cycle of the piston provides liquid compression, with the plurality of reciprocation cycles demanding an increased material resistance in particular with respect to piston wear. The piston and/or the working chamber, or parts thereof, made of (preferably sintered) silicon carbide or being coated therewith provide/s an improved wear resistance and reduced abrasion of the piston. 
         [0011]    In one embodiment, the pumping apparatus is coupled with another pumping apparatus, whereby both pumping apparatuses might be embodied in the same way but may also be different. At least one and preferably both of the pumping apparatuses are embodied in accordance with embodiments of the present invention. Providing two pumping apparatuses allows providing an essentially continuous liquid flow, as well known in the art and also explained in detail in the aforementioned EP 309596 A1. Such so called dual pump might comprise the two pumping apparatuses in either a serial or a parallel manner. 
         [0012]    In the serial manner, as disclosed in the aforementioned EP 309596 A1, the outlet of one pumping apparatus is coupled to the inlet of the other pumping apparatus. The teaching in the EP 309596 A1 with respect to the operation and embodiment of such serial dual pump shall be incorporated herein by reference. The pump volume of the first pumping apparatus might be embodied to be larger than (e.g. twice of) the pump volume of the second pumping apparatus, so that the first pumping apparatus will supply a portion of its pump volume directly into the system and the remaining portion to supply the second pumping apparatus, which will then supply the system during the intake phase of the first pumping apparatus. The ratio of the pump volume of the first pumping apparatus to the second pump apparatus is preferably 2:1, but any other meaningful ratio might be applied accordingly. 
         [0013]    In the parallel manner, the inlets and the outlets, respectively, of both pumping apparatuses are coupled together. The inputs are preferably coupled in parallel to a liquid supply, and the outputs are preferably coupled in parallel to a succeeding system receiving the liquid at the high pressure. The two pumping apparatuses might be operated e.g. with substantially 180 degree phase shift, so that only one pumping apparatus is supplying into the system while the other is intaking liquid (e.g. from the supply). However, it is clear that also both pumping apparatuses might be operated in parallel (i.e. concurrently), at least during certain transitional phases e.g. to provide a smooth(er) transition of the pumping cycles between the pumping apparatuses. 
         [0014]    In both manners, serial and parallel, operation of the two pumping apparatuses is phase shifted, usually and preferably by about 180 degrees. The phase shifting might be varied in order to compensate pulsation in the flow of liquid as resulting from the compressibility of the liquid. It is also known to use three piston pumps having about 120 degrees phase shift. 
         [0015]    Embodiments of the afore described pumping apparatus are preferably applied in a liquid separation system comprising a separating device, such as a chromatographic column, having a stationary phase for separating compounds of a sample liquid in a mobile phase. The mobile phase is driven by the pumping apparatus. Such separation system might further comprise at least one of a sampling unit for introducing the sample fluid into the mobile phase, a detector for detecting separated compounds of the sample fluid, a fractionating unit for outputting separated compounds of the sample fluid, or any other device or unit applied in such liquid separation systems. 
     
    
     
       DETAILED DESCRIPTION 
         [0016]    Other objects and many of the attendant advantages of embodiments of the present invention will be readily appreciated and become better understood by reference to the following more detailed description of embodiments in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to by the same reference signs. 
           [0017]      FIG. 1  schematically shows a pumping apparatus comprising a coated piston. 
           [0018]      FIG. 2  shows a dual serial and  FIG. 3  a dual parallel pumping apparatus. 
           [0019]      FIG. 4  shows a liquid separation system  500 . 
       
    
    
       [0020]    Pumping apparatuses for delivering liquid at a high pressure shall first be described in more general terms. The pressure applied by the piston provides a noticeable compression of the liquid. The piston of the pumping apparatus is reciprocated in the pump working chamber containing the respective liquid. The pump working chamber may be coupled to one or more valves in order to permit liquid flow unidirectional only. Driving the piston may be performed by a drive unit which permits pressurizing of the liquid in the pump working chamber to high pressure. Advantageously, silicon carbide (preferably sintered) is used as material for the piston and/or the pump working chamber, or parts thereof. Such components might be at least partially coated by the silicon carbide or even be comprised as solid material parts of silicon carbide. 
         [0021]      FIG. 1  depicts an embodiment of a pumping apparatus comprising a piston  1  reciprocating in a pump working chamber  9  formed by a cylindrical inner bore of a pump cylinder body  3 . The pump working chamber  9  has an inlet port  4 ′ and an outlet port  5 ′. A capillary  5  having an inner bore  4  is coupled to the inlet port  4 ′ and also couples an inlet valve  13  with the pump working chamber  9  to permit liquid flow only unidirectional into the pump working chamber  9 . The reciprocating movements are driven by a drive unit (not shown herein—e.g. as disclosed in the aforementioned EP 309596 A1), which operates the piston  1  in a spindle drive manner via an actuator  7  coupled e.g. via a ball  8  (embedded in a recess  10 ) and a piston holder  6 . 
         [0022]    A seal  11  is provided for sealing off the pump working chamber  9  at an opening in the pump cylinder body  3  where the piston  1  moves into the pump working chamber  9 . Thus, unwanted liquid flow-out (towards the drive) can be prevented. Guiding of the piston  1  into the pumping chamber  9  can be supported by a guiding element  12 . 
         [0023]    The liquid in the pump working chamber  9  is compressed to a high pressure before being delivered via the outlet port  5 ′ and the capillary  5  (having an inner bore  15 ) into a liquid receiving device (not shown in  FIG. 1 ). 
         [0024]    Generally, wear and abrasion are well known phenomena causing material destruction in driving units, pumps and other devices. The piston  1  performs the reciprocating movement manifold during its lifetime and is subjected to abrasion due to friction loading, accordingly risking to be damaged from wear. 
         [0025]    Further, the working chamber as well as the piston are exposed to more or less aggressive solvents as the mobile phase to be compressed by the pumping apparatus. Accordingly, the piston  1  and/or the pump working chamber  9 , or parts thereof, are made of silicon carbide, preferably SSiC, and/or at least partly coated with. In the embodiment of  FIG. 1 , the piston  1  is a solid material body of SSiC. 
         [0026]    In another embodiment, the piston  1  has a solid material body made of a material such as sapphire, ceramics, tungsten carbide, or metals (such as steel), and is (at least partly) coated with silicon carbide. In embodiments, the SiC coating has a thickness ranging from 0.1 to 10 micrometer, a preferred range of thickness is 0.2 to 5 micrometer, depending e.g. on the piston base material and typical application of the piston. 
         [0027]    Typical solvents, as used in the pumping apparatus as shown in  FIG. 1 , can be water, Acetonitril, Tetrahydrofurane, Methanol, Hexane or any other solvents used in HPLC. 
         [0028]    In the serial dual pump of  FIG. 2 , a first pumping apparatus  200 A is coupled at its input to a liquid supply  205 , and its output is coupled to the input of a second pumping apparatus  200 B. At least one and preferably both of the pumping apparatuses  200 A and  200 B are embodied in accordance with the aforementioned embodiments. In order to provide a continuous flow of liquid, the pump volume of the first pumping apparatus  200 A might be embodied larger than the pump volume of the second pumping apparatus  200 B, so that the first pumping apparatus  200 A will supply a portion of its pump volume directly into a system  210  and the remaining portion to supply the second pumping apparatus  200 B, which will then supply the system during the intake phase of the first pumping apparatus  200 A. The ratio of the pump volume of the first pumping apparatus  200 A to the second pump apparatus  200 B is preferably 2:1, but any other meaningful ratio might be applied accordingly. Further details of the operation mode of such dual serial pump are disclosed in the aforementioned EP 309596 A1 and shall be incorporated herein by reference. 
         [0029]    In the parallel dual pump of  FIG. 3 , the inputs of a first pumping apparatus  300  and a second pumping apparatus  310  are coupled in parallel to the liquid supply  205 , and the outputs of the two pumping apparatuses  200 C and  200 D are coupled in parallel to the system  210  receiving the liquid at high pressure. The two pumping apparatuses  300  and  310  are operated usually with substantially 180 degree phase shift, so that only one pumping apparatus is supplying into the system while the other is intaking liquid from the supply  205 . However, it is clear that also both pumping apparatuses  300  and  310  might be operated in parallel (i.e. concurrently), at least during certain transitional phases e.g. to provide a smooth(er) transition of the pumping cycles between the pumping apparatuses. 
         [0030]      FIG. 4  shows a liquid separation system  350 . A pump  400 , which might be embodied as illustrated in  FIGS. 1-3 , drives a mobile phase through a separating device  510  (such as a chromatographic column) comprising a stationary phase. A sampling unit  520  is provided between the pump  400  and the separating device  510  in order to introduce a sample fluid to the mobile phase. The stationary phase of the separating device  510  is adapted for separating compounds of the sample liquid. A detector  530  is provided for detecting separated compounds of the sample fluid. A fractionating unit  540  can be provided for outputting separated compounds of sample fluid. 
         [0031]    Further details of such liquid separation system  500  are disclosed with respect to the Agilent 1200 Series Rapid Resolution LC system or the Agilent 1100 HPLC series, both provided by the applicant Agilent Technologies, under www.agilent.com which shall be in cooperated herein by reference.