Abstract:
The invention relates to a method for feeding a sample into an analysis branch of a liquid chromatography system, in particular a high-performance liquid chromatography system. A solvent or a solvent mixture from at least one solvent branch is supplied as volume flow {dot over (A)} into the analysis branch. At least one sample from at least one sample branch is fed as volume flow Ė into the analysis branch within a predetermined time interval. The volume flow {dot over (A)} is reduced to an extent during the predetermined time interval, and a volume flow Ċ resulting from the sum of the volume flows {dot over (A)} and Ė remains substantially constant in the analysis branch. The invention further relates to a sampler for carrying out a method of this kind.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit under 35 U.S.C. §119 to German Patent Application No. 10 2014 101 617.3 [Attorney Docket Number 18823DE1/NAT or 26.066 CE/Ib] by Hermann Hochgraeber for “METHOD FOR FEEDING A SAMPLE INTO AN ANALYSIS BRANCH OF A LIQUID CHROMATOGRAPHY SYSTEM, IN PARTICULAR A HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY SYSTEM” filed on Feb. 10, 2014, the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to the injection of a sample and concerns the field of liquid chromatography, in particular high-performance liquid chromatography (HPLC). 
       BACKGROUND 
       [0003]    In HPLC, a sample that is stored at atmospheric pressure is generally conveyed with the aid of a metering pump, in an automated method (autosampler), into a sample loop which is connected at both ends to what is called a shear valve. When the desired amount of the sample is located in the loop, the shear valve is switched such that the solvent stream now flows through the sample loop, and the sample can thus be conveyed at high pressure through the separating column This procedure by which the sample to be analyzed is fed or introduced into the solvent stream is called (sample) injection. 
         [0004]    However, particularly at high operating pressures of a chromatography system, shear valves can be switched only with high drive moments. The rubbing and sealing shear surfaces are therefore subject to a high degree of wear and have to be frequently replaced. Despite the high sealing normal force, the positioning of the two shear surfaces relative to each other also requires a high level of precision. The shear surfaces are also very sensitive to abrasive constituents in the solvent stream or sample stream. Upon switching of the shear valves, the flow or the volume flow through the column is interrupted for the switching time, such that the pressure upstream of the separating column drops and the pressure in the high-pressure pump rises. In particular, the pressure drops particularly strongly when the sample loop, at atmospheric pressure after suction of the sample, is introduced into the feed line to the column, and the high-pressure pump has to compress the content of the sample loop to the operating pressure upstream of the column. During this time, the direction of flow in the feed line to the separating column reverses, which reduces the useful life of the separating column and has a negative effect on the separation efficiency. 
         [0005]    DE 10 2008 006 266 B4 has already disclosed an arrangement which, with the aid of a shear valve, is able to completely eliminate the fluctuations in pressure that occur during injection according to the prior art. 
         [0006]    However, manual injections of samples, of the kind described in the patent documents U.S. Pat. No. 3,985,166 and U.S. Pat. No. 3,940,994 for example, are unsuitable for an automated method in what is called an autosampler, with different samples having to be collected from several vials or several sample reservoirs. Moreover, during such an injection, the sample is diluted in the solvent stream, which results in poor sample detection. 
       SUMMARY 
       [0007]    Therefore, the object of the present invention is to make available a method for feeding a sample into an analysis branch to a separating column in liquid chromatography, in particular in high-performance liquid chromatography, and also an automatic sampler, which method and sampler reduce wear, error susceptibility and maintenance in a simple and cost-effective manner and yet still permit high quality of the sample detection. 
         [0008]    According to the invention, the volume flow Ċ in an analysis branch to a separating column is kept constant. The analysis branch (possibly also divided into several subsidiary branches with several separating columns) is connected here to (at least) one solvent branch and (at least) one sample branch, such that the volume flow Ċ results from a sum of a volume flow {dot over (A)} in the at least one solvent branch and of a volume flow Ė in the at least one sample branch. However, the injection of a sample from the at least one sample branch does not take place by switching a complicated shear valve with several ports, but instead by reducing the volume flow {dot over (A)} in the at least one solvent branch, such that a volume flow Ė of greater than zero exists during the injection time. Such reduction/increase can preferably be obtained by corresponding control (switching off or reducing the power consumption or switching on or increasing the power consumption) of a pump responsible for this branch. However, it is of course also conceivable for the reduction of the volume flow {dot over (A)} in the solvent branch to be brought about by means of a suitably controllable valve. 
         [0009]    By reducing the volume flow {dot over (A)} in the at least one solvent branch (possibly divided into several subsidiary branches), it is possible for a sample from a sample branch to be supplied as volume flow Ė to the analysis branch, without a change in the volume flow Ċ taking place. The injection of the at least one sample or of the at least one sample plug as a result of the volume flow Ė (equals zero before injection of the sample) can already take place through a simple Y connection with two inputs (solvent branch and sample branch) and one output (analysis branch). Only a reduction of the volume flow {dot over (A)} causes the volume flow Ė to increase (from zero) to a corresponding extent. For this purpose, for example, a (metering device) pump, or the drive thereof, responsible for the at least one sample branch can be controlled (started up or its power consumption increased). In this way, the at least one sample can advantageously be introduced into the solvent stream to the separating column (analysis branch) without the complicated drive and the wear of a shear valve. Moreover, the method according to the invention and the arrangement for carrying out said method ensure that, when the sample is introduced into the analysis branch, the flow (volume flow) does not change either in direction or in quantity. 
         [0010]    It is assumed here that a flow running counter to the pump direction (reverse flow), for example design-related or caused by a unidirectional valve, is suppressed by the corresponding pumps, either controllably (for example a solenoid valve) or preferably by self-regulation (in particular a check valve). The pumps can be controlled via a control device or regulator connected correspondingly to the drives of the pumps. In addition, for monitoring purposes, pressure sensors can be provided in the solvent branch and/or in the sample branch, which pressure sensors transmit the detected parameters in each case to the control device or regulator. 
         [0011]    In a preferred embodiment, a unidirectional valve with 2 switching positions, in particular a closed position and an opened position, is additionally located in the sample branch (possibly divided into several subsidiary branches), in particular upstream of the sample, seen in the direction of flow, so as to be able to use the (respective) sample plug with particular accuracy. Instead of being designed as a valve controllable by a motor, such a valve can also be designed for example as a pressure-dependent valve, in particular a simple check valve. In this way, intermixing effects can advantageously be prevented at the front of the sample, and an exactly defined front separation surface between sample (plug) and solvent can be achieved. Moreover, with such a valve, the sample branch can also be disconnected if need be, such that an opening of the sample branch and the associated drop in pressure has no effect on the solvent branch and/or the analysis branch. Correspondingly, in the at least one sample branch, under other pressure conditions (than the high system pressure prevailing in the solvent branch and/or analysis branch), in particular the ambient pressure, another sample reservoir can be inserted or connected hereto (as is explained below, for example, with an injection needle being pressed into a needle seat). In this way, the longitudinal intermixing of the injected sample with the solvent on the way to the separating column can be advantageously minimized 
         [0012]    In a further embodiment of the invention, the volume flow {dot over (A)} comes to a complete standstill after a possible transition phase during a predefined time interval for the (sample) injection. The solvent pump can also be stopped and the metering device pump can also be started (instead of a reduction or increase in power consumption). The volume flow Ċ resulting from the sum of the volume flows {dot over (A)} and Ė then corresponds after a very short transition time (in the millisecond range or ≦one millisecond) to the volume flow Ė, such that a dilution of the sample or of the sample plug can be avoided. Before and after the sample injection, the volume flow Ċ corresponds by contrast to the volume flow Ė, such that the constant maximum of the volume flow Ċ is formed either by an, at the respective time, constant maximum of the volume flow {dot over (A)} or Ė. 
         [0013]    In a particularly preferred embodiment of the invention, the volume flow Ė (in particular for feeding the sample in from a sample loop) is stopped before a rear end, seen in the direction of flow, of the sample or transition between sample end and solvent (in the sample branch) reaches the analysis branch. In this way, a (new) rear transition with exactly defined separation surface is obtained on the sample used in the volume flow Ċ, compared to the transition already existing in the sample branch in previously used methods. Intermixing effects caused by the movements needed during the collection and release of a sample are advantageously avoided. 
         [0014]    In a further embodiment of the invention, after a sample has been collected, an injection needle for injecting the sample is pressed with a high degree of sealing (high degree of sealing in relation to the pressures customary in HPLC) into a needle seat, which is connected to the analysis branch, preferably via a valve, in particular a check valve. The injection needle can in this case not only remove samples in an automated manner from vials (i.e. different reservoirs) under motor control, preferably maintaining its vertical position, but can also be pressed sealingly into the needle seat in an automated manner. This can permit not only semi-automatic but even completely automated sample injection, including sample removal from different vials or reservoirs. Moreover, the dispensing of a defined amount of sample is also facilitated by a valve of this kind (in particular closing in one direction), since the sample branch is closed, or sealingly disconnected from the analysis branch and/or solvent branch, after the desired amount of sample has been dispensed. 
         [0015]    In a further embodiment of the invention, after the sample has been used, the injection needle is automatically cleaned by flushing before a further sample is collected. The flow off can take place, for example, via what is called a waste port (collector, optionally with a drain additionally provided). In this way, it is possible to avoid undesired contamination of the (next) sample. In particular, in order to permit multiple sampling while avoiding gassing, the injection needle can be connected to what is called a sample loop which, prior to the sampling, contains a sample with solvent. 
         [0016]    Moreover, it is also conceivable to clean or flush the sample loop and metering device by introduction of solvent. For this purpose, for example, solvent can be introduced via the needle or via another attachment of the metering device, in particular by means of a flushing pump, and can be discharged via the other attachment or the needle or additional (waste) port of the metering device. 
         [0017]    In a further embodiment of the invention, solvent or a solvent mixture can be fed into the analysis branch after or before a sample injection by means of the metering device and/or an additional pump (for example flushing pump). In this way, for example, the analysis branch (including the column) can be flushed or cleaned. Moreover, it is also conceivable in this way, for example by injection of a solvent fraction, to form the gradient in a desired manner (in particular in the direction of a higher elution). 
         [0018]    According to the invention, as a result of the constant volume flow in the analysis branch (not only during a sample run but in particular during a sample injection), it is advantageously possible to ensure a desired, distinct separation (in particular exact separation surfaces) between sample or sample plug and solvent, both at the front and also at the rear as seen in the direction of flow. 
         [0019]    By contrast, the effects of intermixing (diffusion, etc.), which occur at the separation surfaces on account of fluctuations in volume flow in the analysis branch, for example in sample injection in U.S. Pat. No. 3,940,994, can advantageously be reduced or even avoided, and the analysis precision can be greatly increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention is explained in more detail below on the basis of an illustrative embodiment depicted in the drawings. 
           [0021]      FIG. 1  shows a schematic view of a sampler according to the invention. 
           [0022]      FIG. 2  shows a schematic view of a detail from  FIG. 1 . 
           [0023]      FIG. 3  shows a diagram of the volume flows in a sampler according to  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0024]    The arrangement in  FIG. 1  shows an automatic sampler, or autosampler, and its integration into a chromatography system. The autosampler comprises an injection needle  1  which, in a manner not shown in detail in the drawing, can be moved under the control of a motor in the x-y-z direction (x horizontal, z vertical in the plane of the drawing, and y perpendicular to the plane of the drawing), and also a sample loop connected thereto, and a metering device  10 . 
         [0025]    By way of the injection needle  1 , a sample from a vial  8  is drawn into a sample loop  2  by means of the metering device  10  (for example, as shown in the drawing, in the form of a piston pump). Thereafter, the injection needle  1  is pressed sealingly into a needle seat  6 , to which a check valve  3  is attached. In normal operation, a solvent or mobile phase is aspirated (solvent branch  19 ) via a pump  20  (HPLC pump) and is guided via the check valve  3  to the column  9  (analysis branch  17 ). In this state, the port on the valve leading to the needle seat  6  is closed by a closure means, for example a ball  5 , optionally pretensioned by a restoring element, for example a spring  4 . 
         [0026]    If the sample is now to be fed in or introduced, the pressure that the metering device  10  has to apply must be greater than the prevailing system pressure, so that the check valve  3  opens. Moreover, the solvent pump  20  has to stop its flow during the sample injection time, so as to prevent a pressure increase in the connected solvent branch  19  and analysis branch  17 . 
         [0027]    The sample is now pumped by the metering device  10  out of the sample branch  18  (content of the pressed-in needle  1  as far as the connected port of the check valve  3 ) in the direction of the column  9 . After a desired amount of the sample located in the needle  1  has been fed in and the metering device  10  has stopped the supply, the check valve  3  closes automatically and the solvent pump  20  starts supplying again. 
         [0028]    In order to avoid increases or decreases in flow (volume flow Ċ in the analysis branch) and in pressure in this injection process, data communication takes place (for example by means of a control device  40 ) between autosampler (metering device  10 , sample loop  2  and needle  1 ) and solvent pump  20 , which data communication coordinates the flow responsibility between metering device  10  and solvent pump  20 . 
         [0029]    In this way, a desired and definable amount of a sample is fed directly into the solvent stream to the separating column  9 , without the sample being diluted with the solvent stream. Moreover, the low sample dispersion and the low longitudinal intermixing not only increase the precision of the analysis (in the schematically depicted detector  30 ) but also prolong the useful life of the column  9 . 
         [0030]    As can be seen from  FIG. 1 , the sample loop  2  and injection needle can be flushed and cleaned via the metering device  10  by means of a flushing pump  14 , which is connected to a flushing agent reservoir  16 , after which the sample loop  2  and the inside of the needle  1  are filled by means of pump  15  which, like the pump  20 , is also connected to a reservoir for the solvent  25  (fluid). Of course, it is also conceivable to use the solvent  25  also for cleaning purposes, in which case the additional pump  14  and flushing agent  16  can be dispensed with. The sample loop is filled with solvent all the way to the tip of the injection needle  1 , in order to avoid gassing of a sample generally degassed in HPLC and to avoid undesired mixing of the sample. To hold the corresponding solvent column in the autosampler, it is possible, as shown schematically in the drawing, to provide a corresponding check valve  13 , in order to prevent reverse flow and also forward ejection from the needle  1 . 
         [0031]    Since the needle  1  is sterile before use and is contaminated during a sampling procedure in which a septum of a vial  8  is usually pierced, an aforementioned cleaning procedure according to the invention takes place after an injection of sample but before a renewed collection of sample, such that a sterile state is ensured once again after such cleaning. 
         [0032]    As is shown schematically in  FIG. 1  by the four vials  8 , it is possible according to the invention to collect samples successively from several different vials  8  (as is customary in HPLC) and then inject them. The vials  8  can be arranged in an autosampler, for example in the form of a tray or a (micro)titer plate (well plates). 
         [0033]    To permit flushing directly after the injection of a sample, the needle seat  6  has, on its upper face, an overflow container  7  which extends around the needle seat and is open toward the top, such that any emerging flushing liquid and/or solvent can collect in this overflow container and can flow off and be discarded as indicated by the bent arrow. In this way, a large number of samples can be collected and injected using the sampler according to the injection, wherein the flushing after the injection and before renewed collection of a sample prevents contamination of the subsequent sample and therefore of the next sample run. 
         [0034]    To control the aforementioned procedures and, in particular, to keep the volume flow in the analysis branch  17  constant, it is possible for the pump  20  and the metering device  10 , and the drive motor  12  thereof, to be suitably controlled via the control device. By contrast, the corresponding control of the pumps  14  and/or  15  takes place exclusively, as has been explained above, for cleaning purposes and for keeping solvent in the sampler. To permit a particularly high level of precision of the control, it is possible, as is shown schematically in  FIG. 1 , to additionally provide a pressure sensor  21  in the solvent branch  19  and a pressure sensor  11  in the sample branch  18 , said pressure sensors transmitting actual states to the control device, which actual states are evaluated there. Pressure sensors  21  and  11  of this kind can also be integrated in the pump  20  and the metering device  10 . 
         [0035]    The enlarged detail of the needle seat  6  in four positions in  FIG. 2  shows how, in a first position, the valve  3  in the blocked state closes the port to the needle seat  6  by means of ball  5 , and the volume flow Ċ thus corresponds to the volume flow {dot over (A)}. In this valve position, the needle  1  can still be located outside the needle seat  6 , without solvent emerging through the closed port of the check valve to the needle seat. Thereafter, the needle tip  1  is pressed sealingly into the needle seat  6  (as is shown in the second partial figure from the left in  FIG. 2 ), such that the port of the valve  3  to the needle seat  6  is sealingly closed and, in this position, the port is also closed by the ball  5 . 
         [0036]    The third partial figure in  FIG. 2  shows how the volume flow {dot over (A)} has already been stopped, the ball  5  comes loose, upon pressure equality with the pressure present in the system or solvent branch  19  and the analysis branch  17  connected thereto, and frees the port to the needle seat, and the sample is injected into the analysis branch  17  through the volume flow Ė. The view on the right in  FIG. 2  shows once again how the sample injection has been stopped preferably after introduction of a predefined amount of volume flow Ė, smaller than the amount of sample contained in the needle  1  and sample loop  2 , and the volume flow Ċ is once again fed in from the volume flow {dot over (A)} of the solvent. Although, in a preferred embodiment of the method, not all of the sample amount contained in the sample loop  2  and needle  1  is fed into the volume flow Ċ, and part of the contained sample is thereby lost, this method can be advantageous since in this way intermixing effects at the rear separation surface can be avoided. Such intermixing effects otherwise occur, since the rear separation surface has been moved rearward in the sampling procedure and been moved forward again during the sample injection, until it is present as separation surface in the volume flow Ċ. However, through these movements, the separation surface is unclear because of intermixing effects, and this has a disadvantageous effect on the accuracy of the analysis in the detector  30 . 
         [0037]    The diagram of volume flow per time in  FIG. 3  shows how a volume flow Ċ is composed as resultant of the sum of volume flows {dot over (A)} and Ė. In valve position closed (partial figures on the left, second from the left, and on the right in  FIG. 2 ), the volume flow {dot over (A)} is constant and continues as constant volume flow Ċ. A volume flow Ė does not exist at this time (before T 1  and after T 4 ). 
         [0038]    In the opened position, as shown in the third view from the left in  FIG. 2 , the volume flow {dot over (A)} has been stopped by stopping the pump  20 , such that in the area between T 2  and T 3  the volume flow Ė continues to the volume flow Ċ. In the transition phases T 1 -T 2  and T 3 -T 4  which, compared to the sample injection duration of for example 10 ms to 1 min, are short (for example 1 ms or less, but at least under a few ms), the volume flow Ė rises (or falls) to the extent that the volume flow {dot over (A)} falls (or rises). Although the respective flanks are shown schematically in  FIG. 3  as straight lines, the corresponding rise and fall can of course also take place according to suitable curves. In a preferred embodiment of the invention, however, suitable control of the pump  20  and of the metering device  10 , even during opening of the valve  3  in the transition phase T 1 -T 2  and during closure of the valve  3  in the transition phase T 3 -T 4 , ensures that at each time the sum of the volume flows reaches the same level as the preceding volume flow {dot over (A)} and the succeeding volume flow Ė (transition closed to open) or the preceding volume flow Ė and the succeeding volume flow {dot over (A)} (at the transition open to closed). By maintaining a constant volume flow not only before, during and after an injection of sample but also in the transitions (T 1 -T 2  and T 3 -T 4 ), a sample or a sample plug is fed in while maintaining clear separation surfaces, and, as has already been explained above, it is possible to prevent reverse intermixing, seen in the direction of flow, on account of movements in needle  1  and sample loop  2  by a separation of a predefined amount of sample from the entire contained sample. 
         [0039]    As is customary in HPLC, a sample run takes place at very high pressure, for example in excess of 500 bar or even in excess of 1000 bar, such that, with the required narrow cross sections in the analysis branch (10 μm-max. 1 mm), it is possible to generate volume flows of several μl-10 ml per minute, preferably under 100 ml per minute, in particular under 300 μl per minute, but at the respectively desired level with a constancy of a deviation of under 25%, for example under 10% or under 5%, in particular ≦1%, in order to protect the separating column from disadvantageous and excessively high fluctuations in flow. A whole sample run can in this way last from a few minutes or so to one hour, until the substance and sample pass through the column  9  to the detector  30 . 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           1  injection needle 
           2  sample loop 
           3  check valve 
           4  spring 
           5  ball 
           6  needle seat 
           7  overflow container 
           8  vials 
           9  separating column 
           10  metering device (piston pump) 
           11  pressure sensor 
           12  drive for metering device  10   
           13  check valve 
           14  flushing pump 
           15  flushing pump 
           16  flushing agent 
           17  analysis branch 
           18  sample branch 
           19  solvent branch 
           20  HPLC pump 
           21  pressure sensor 
           25  solvent (fluid) 
           30  detector 
           40  control device 
         {dot over (A)} volume flow for supplying solvent  25  from a solvent branch 
         Ė volume flow for feeding the sample from the sample branch into the analysis branch 
         Ċ volume flow in the analysis branch to the separating column  9 , which volume flow results from the sum of volume flows {dot over (A)} and Ė