SAMPLE INJECTOR FOR A LIQUID CHROMATOGRAPH

Abstract: A sample injector for a liquid chromatograph comprises a high pressure syringe unit (A) , a sample intake unit (B) having a needle (8) which can be introduced into a sample container (22) as well as a seat (9) , and a 6/2-way valve (C). The high pressure syringe unit is designed as a piston pump with a pump head (6) and a piston (5) which is driven by a motor (36) via a drive mechanism. Pressurized liquid from a sol­ vent delivery system (SDS) can enter the pump head (6) through an in­ let port (39) and flow along the piston (5) to an outlet port (40). When sample is to be injected into a chromatographic column, the 6/2-­ way valve is first switched into the position depicted by dotted lines, the needle (8) is introduced into the container (22) and the piston (5) is retracted by a corresponding distance. Thereafter, the valve is switched back to its original position so that solvent deliv­ ered from the solvent delivery system can transport the sample to the column. Since the solvent flushes the entire syringe unit, it is en­ sured that no sample residues remain in the syringe unit which would otherwise impair the chromatographic measuring process.

Description: The invention relates to a sample injector for a liquid chromatograph according to the preamble of claim 1. Such injectors are used for injecting the sample to be chromatographically analyzed into the chromatographic column. Known injection systems typically comprise an injection loop which can be connected to the chromatographical system, i.e., the solvent delivery system and the chromatographic column, by one or several valves, and through which solvent flows under high pressure when no sample is injected. When sample is to be injected, the injection loop is decoupled from the chromatographic system and one end of the loop is connected to a low-pressure metering syringe, and the other end is connected to a sample-intake arrangement which is moved such that it dips into a sample container. Then, the low-pressure metering syringe is operated such that a desired amount of sample is drawn from the sample container into the injection loop. Once the sample is drawn in, the injection loop is switched back into the chromatographic system by means of the above-mentioned valves. The stream of solvent from the solvent delivery system then compresses the sample and transports it to the column. With such known injection systems, air bubbles may arise in the injection system by small leakages or by degassing of the liquid in the injection system. Such air bubbles distort the accuracy of proportioning and therefore have a negative influence on the accuracy of the chromatographic measuring process. Furthermore, in case that the solvent is changed it may come to incompatibilities, e.g., mixing, reaction with the sample, which may lead to a loss of the sample or to variations of the reproducibility. Such problems can be reduced by providing additional flushing circuits, whereby, however, the complexity of the injection system is increased, leading to increased cost and less reliability. Furthermore, the addition of flushing circuits causes an increase of the dead volume leading to a deterioriation of the chromatographic measuring accuracy. An injection system having such an additional flushing circuit is known from the article "Automatic Liquid Chromatograph Injection and Sampling" , Hewlett-Packard Journal, April 1984, pages 21-24. This known injection system comprises an additional flushing valve which permits to flush the metering syringe and associated capillaries before each new injection of sample. From US-A-4 059 009 , a sample injector for a liquid chromatograph in accordance with the preamble of claim 1 is known. This known injector comprises a source of pressurized carrier liquid, conduit means for supplying the carrier liquid to a chromatography column, and a rotary switching valve positioned in the conduit means for selectively introducing a sample into the pressurized carrier liquid. The sample can be introduced by means of a syringe into a sample loop connected to the rotary switching valve, whereby the volume of the sample loop defines the amount of sample which can be injected into the carrier liquid and thus into the chromatographic column. Relative to the prior art, it is an object of the invention to provide a sample injector for a liquid chromatograph which has a simple design and which permits to perform repetitive injections in a simpler manner without suffering from the above-mentioned problems of known injectors. According to the invention, this object is solved by the features of claim 1. According to an underlying principle of the invention, the metering means for drawing in the sample, for example a syringe, is integrated in the conduit system through which liquid is pumped under high pressure to the chromatographic column, in contrast to prior art injectors which are arranged in a bypass configuration relative to said conduit system. According to the invention, it is thus ensured that the metering means is permanently flushed by solvent, except for the time intervals when sample is drawn in. Consequently, the above-mentioned problems, e.g., bubble formation or accumulation of unwanted residues, are avoided without requiring extra flushing circuits and flushing procedures. Thus, the injector according to the invention has a less complex and therefore more reliable design. Since no additional flushing procedures are required, the duration of an injection cycle is reduced as compared to known injectors. The chromatographic analysis time is thus reduced. It is a further advantage of an injector according to the invention that it has only a small dead volume resulting in an increased metering accuracy. According to the invention, the metering means is designed like a piston pump with a pump head and a piston movable in an inner bore of the pump head. The dimensions of the piston and of the inner bore of the pump head are selected such that there is a ring gap in which liquid can flow from an inlet port of the metering means to an outlet port thereof. In the normal mode, when no sample is to be drawn in from a sample container, the inlet port of the metering means is connected to the solvent delivery system of the liquid chromatograph so that solvent is delivered at high pressure through the ring gap to a sample needle. In the loading mode, when sample is to be taken in, the connection to the high pressure solvent delivery system is interrupted, the sample needle dips into a container with the sample, and the piston is retracted until the desired amount of sample is drawn in. Then, in the injection mode, the connection to the solvent delivery system is reestablished so that the pressurized solvent transports the sample to the column. The establishing and interrupting of connections to the solvent delivery system can be accomplished by means of a 6/2-way valve which has the advantage that it is a comparatively simple component which is well available. The transmission of the driving force to the piston of the metering means can advantageously be accomplished via a ball which can move freely in a recess of an actuating element coupled to the rest of the drive means , e.g., a ball-screw drive, with the ball contacting a piston holder to which the piston is fixed. Since there is no rigid connection between the drive means and the piston, tilt of the piston can be avoided, resulting in an increased lifetime of the seal of the pump head. Subsequently, an embodiment of the invention is explained with reference to the drawing. Figure 1 shows an embodiment of the injector according to the invention. According to Figure 1, the injector comprises three units (indicated by blocks of dotted lines): a high pressure syringe unit A , a sample intake unit B , and a 6/2-way valve unit C. Figure 1 shows the injection system at start-up. The solvent is delivered from a solvent delivery system (SDS), typically comprising a high-pressure pump, via a capillary 1 to the 6/2-way valve. Such a 6/2 way valve is a known component in liquid chromatography and is a valve having six ports and two switching states. With the switching state of the valve shown, the solvent flows via the internal connection 2 and the capillary 3 to the high-pressure syringe unit A. The high-pressure syringe unit A comprises a pump head 6 with an inner cylindrical bore 30 and a piston 5 which can be moved inside the bore. The outer diameter of the piston 5 is smaller than the diameter of the inner bore by a certain amount such that liquid can flow in the ring gap between the piston and the inner wall of the pump head. The pump head 6 is sealed off against the outside by a high pressure-tight seal 4 which prevents that any liquid leaves the inner bore of the pump head. The piston 5 is driven via a ball-screw drive 31 . The lower end of the piston 5 is attached to a piston holder 32 and the piston holder is coupled via a ball 33 and an actuator 23 to a recirculating ball spindle 31. Also provided is a return spring 25. The ball-screw drive 31 is coupled to a toothed gear 34 which is coupled to a second gear 35 fixed to the shaft of a drive motor 36. The actuator 23 which is rigidly connected to the ball-screw drive 31 comprises a recess 37 for receiving the ball 33 . The ball 33 is free to move in the recess 37 . The top of the ball 33 contacts the piston holder 32 connected to the piston 5. A guiding element 38 is provided for guiding the piston 5. A return spring 25 is connected with one of its ends to the pump head 6 and with its other end to the piston holder 32. The return spring 25 forces the piston away from the pump head when the actuator 23 moves away from the pump head. When the actuator 23 moves towards the pump head, the ball 33 pushes against the piston holder 32 so that the piston 5 moves into the interior of the pump head. The driving of the piston 5 via the freely moving ball 33 in combination with the guiding element 38 ensures that the movement of the piston 5 is free of tilt, resulting in an increased lifetime of the seal 4 . In the embodiment shown in Figure 1, the pump head 6 and the piston 5 are shown to be arranged horizontally, but it is understood that a vertical or any other arrangement is also possible. It is furthermore understood that the drive of the piston can be of any kind and is not limited to the embodiment shown in Figure 1. As shown in Figure 1, the solvent is introduced into the high-pressure syringe unit immediately above the seal 4 so that a permanent flushing of the entire syringe unit is ensured. A capillary 7 is connected at port 40 to the pump head 6 such that liquid entering the syringe through capillary 3 and solvent inlet port 39 can leave through capillary 7. The capillary 7 is connected to the sample-intake unit B which can be of any known design. According to Figure 1, the sample-intake unit may comprise a needle 8 and a seat 9 into which the needle can be pressed. The seat is connected via capillary 10 to the 6/2-way valve unit C. The needle 8 can be raised or lowered with an arm 41 which is actuated by a motor 21. Also provided is an arrangement (not shown) for positioning a sample vial 22 with the sample below the needle 8 when the needle is lifted. Such an arrangement can be, for example, a rotatable platform carrying one or several sample vials. After the desired sample vial has been placed below the needle 8, the needle can be lowered into the sample vial such that sample can be drawn in. As an alternative to the embodiment shown in Figure 1, the raising and lowering of the needle 8 could also be accomplished pneumatically. For example, the sample-intake unit could be the sampling unit which is described in Hewlett-Packard Journal, April 1984, pages 21-24 and which is used in the Hewlett-Packard model HP 1090A liquid chromatograph. The seat 9 of the sample-intake unit is connected via a capillary 10 to port 18 of the 6/2-way valve C and via the internal connection 11 in the valve to port 17 which is connected via a capillary 12 to the chromatographic column. Also connected to the 6/2-way valve is a capillary 13 which leads to a waste container 20. The 6/2-way valve can be the valve which is known from Hewlett-Packard Journal, April 1984, pages 21-24 and which is used in the Hewlett-Packard model HP 1090A liquid chromatograph. By the arrangement of the syringe unit A, the sample intake unit B, and the valve unit C as shown in Figure 1, it is ensured that all parts which come into contact with sample liquid are permanently flushed. It is understood that the high-pressure syringe unit A, the sample-intake unit B and the valve unit C have associated drive and control circuits (not shown) which are connected to a common controller for automatically controlling the operation of these units in timed relation. In the following, the operation of the injector according to Figure 1 is described in more detail. When an injection is initiated by an operator, the 6/2-way valve is switched from the position depicted with solid lines in Figure 1 to its second position indicated by dashed lines. In that position, port 14 of the valve is connected to port 15 , port 13 is connected to port 18 , and port 16 is connected to port 17. With these connections, solvent flows directly from the solvent delivery system SDS via capillary 1 , ports 16 and 17, and capillary 12 to the chromatographic column. The metering branch of the injection system, i.e., the syringe unit A and the sample-intake unit B, is thus decoupled from the high pressure part of the injection system. Immediately after the switching of the valve into the state indicated by the dashed lines, the pressurized solvent in the metering branch relaxes into the capillaries 10 and 19 and is collected in the waste container 20. Then, the piston 5 moves to its top dead centre, i.e., to the left in Figure 1 , so that the total volume of the syringe is available for drawing in sample. The piston 5 is moved to its top dead centre before each injection in order to provide a defined starting position for the drawing in, since the position of the piston after the previous sample intake step is not always the same, but depends on the volume of the aspirated sample. If, for example, the total volume of the syringe is 50 microliters and if 50 microliters have been injected in the previous injection step, the piston has to eject 50 microliters of liquid before new sample is drawn in. Once this ejection step is completed, the needle 8 is lifted from the seat 9 by means of the motor 21, and the sample container 22 is positioned below the needle. Then, the needle is lowered into the sample container 22 and into the sample liquid. In order to aspirate the desired volume of sample liquid, the piston 5 is retracted a corresponding distance from its top dead centre, that means it is moved to the right in Figure 1. The movement of the piston is accomplished by a corresponding rotation of the motor 36 which is transferred via gears 35 , 34 and the ball-screw drive 31 into a linear motion of the actuator 23. Since the return spring 25 exerts a constant force against the piston holder 32, the piston 5 moves to the right when the actuator 23 moves to the right. When the desired amount of sample has been drawn in and after a short holding time which serves for pressure compensation, the needle 8 is removed from the sample container 22. Thereafter, the sample container is removed and the needle 8 is pressed again into the seat 9 to establish a high-pressure tight connection. Then, the valve unit C is switched back into its first position illustrated in Figure 1 by solid lines. The solvent delivered from the solvent delivery system now compresses the previously decompressed liquid in the metering branch and transports the sample through the capillary 7 and the needle 8 , through the seat 9, capillary 10, port 18, path 11 in the valve unit , and port 17, to the chromatographic column where the sample is chromatographically separated. Thereafter, a new injection can take place in the above described manner. It is a further advantage of the invention that it permits to inject sample volumes which are larger than the volume of the high pressure syringe A. This can be accomplished by repetitively drawing in sample through the needle 8, putting the needle back to the seat 9, and intermediately storing the sample in the capillary 10, before the accumulated volume of sample is transferred to the column. According to a further advantage, the maximum volume which can be aspirated by the syringe can easily be increased by replacing the piston 5, or the seal 4, or the capillary 7, by elements of correspondingly different dimensions.

Claims: A sample injector for a liquid chromatograph, comprising: a sample needle (8) through which a sample which is to be chromatographically analyzed can be drawn in by action of a pump, a conduit system (1,2,3,7,8,10,11,12) for transporting solvent delivered by a solvent delivery system at high pressure to a chromatographic column and for transferring sample to the chromatographic column, metering means (5,6) for receiving a metered amount of sample from the sample needle (8) and for releasing that amount of sample when the metering means (5,6) are connected to the solvent delivery system , characterized in that the metering means comprise : a pump head (6) with an inner bore (30) and a piston (5) which can be moved back and forth in the inner bore (30), an intake/outlet port (40) connected to the sample needle (8) through which sample can be aspirated by actuation of the piston (5), and a solvent inlet port (39) connectable to the solvent delivery system (SDS) such that sample which has been aspirated is transported to the column and the metering means (5,6) is flushed by solvent when said solvent inlet port (39) is connected to the solvent delivery system. Sample injector according to claim 1, characterized in that the intake/outlet port (40) is located near the top dead centre of the piston (5), the solvent inlet port (39) is located near the bottom dead centre of the piston (5), and the diameter of the inner bore (30) of the pump head (6) is larger than the outer diameter of the piston (5) such that liquid can flow in the gap region from the solvent inlet port (39) to the intake/outlet port (40). Sample injector according to claim 2, characterized in that a high pressure tight seal (4) for sealing off the inner bore (30) of the pump head is arranged close to the solvent inlet port (39) . Sample injector according to claim 2 or 3, characterized in that the piston (5) is driven by drive means which comprise a motor (36) power transmission means (35, 34, 31) coupled to the motor (36), an actuator (23) coupled with its first end to the power transmission means, the actuator having at its second end a recess (37) for receiving a ball (33), a ball (33) having such a diameter that it projects above the recess (37) but can freely move inside the recess , a piston holder (32) to which the piston (5) is mounted having a surface for contacting the ball (33) , and a return spring (25) for exerting a force on the piston (5) in a direction away from the pump head (6). Sample injector according to claim 4, characterized in that the power transmission means comprise a ball-screw drive (31). Sample injector according to any of the preceding claims, characterized by a seat (9) in which the tip of the needle (8) is positioned when no sample is drawn in , lifting means (21,41) for raising the sample needle (8) from the seat (9) and for lowering it again after the intake of sample. Sample injector according to any of the preceding claims, characterized by a 6/2 way valve having a first port (16) connected to the solvent delivery system (SDS), a second port (15) connected to the solvent inlet port (39) of the metering means , a third port (13) connected to a waste container (20) , a fourth port (18) connected to the seat (9), and a fifth port (17) connected to the chromatographic column.

Inventors: STROHMEIER, FRED
Applicants: HEWLETT-PACKARD GMBH
Application year: 1988
Main classification: G01N30
IPC codes: G01N30, G01N1