DIALYSE CELL FOR SAMPLE PREPARATION FOR ION CHROMATOGRAPHY

Description

[0001] The present invention relates to a dialysis cell for sample preparation according to the preamble of claim 1, a device for sample preparation according to the preamble of claim 7 and a use of a dialysis cell according to claim 10. The present invention further relates to an ion chromatography system and / or a system for high-performance liquid chromatography comprising a dialysis cell according to the preamble of claim 1.

[0002] Many chemical analysis methods require sample preparation before the actual analysis can be performed. For example, samples for analysis using ion chromatography often contain, in addition to the ions to be analyzed, the analyte(s), matrix components that are corrosive or can lead to precipitation. These can sometimes make analysis difficult or even impossible. Heavily contaminated samples can also damage the separation column or significantly reduce its service life. Numerous analytical applications therefore require suitable sample preparation. While traditionally all sample preparation steps were performed manually, in recent years so-called inline sample preparation techniques have become established. These enable the preparation process to be fully automated.

[0003] EP 0 820 804 A1 describes an inline dialysis process, which is used particularly for ion chromatography. This dialysis process is carried out using a dialysis cell that has a donor channel and a parallel acceptor channel. A selectively permeable dialysis membrane is arranged between these two channels. The dialysis is carried out as so-called stopped-flow dialysis. In this process, a liquid sample, also called the donor solution, is continuously passed through the donor channel. A liquid flow of an acceptor solution through the acceptor channel is simultaneously stopped for a certain period. The concentration gradient between the donor and acceptor solutions causes the analytes to diffuse from the donor solution into the acceptor solution. Once a concentration equilibrium has essentially been established, the acceptor solution is subjected to ion chromatographic separation.

[0004] The time from stopping the acceptor flow to the end of dialysis, which is typically given by passing the acceptor solution out of the dialysis cell, is called dialysis time t D designated.

[0005] The recovery rate R is calculated as the ratio of the analyte concentration in the acceptor channel to the analyte concentration in the donor channel at the end of the dialysis time t D The recovery rate R typically increases with increasing dialysis time t D strictly monotonous.

[0006] As equilibration time t A is the time in which the analyte concentration in the acceptor channel has essentially reached that in the donor channel. In this context, the equilibration time t A the duration of dialysis after which the relative change in the analyte concentration in the acceptor channel is for the first time less than 2% / min.

[0007] The described technique separates not only particles from the analyte(s), but also colloids, oil components, and large molecules. Protein-containing samples, in particular, can be processed directly using ion chromatography. This eliminates time-consuming manual steps, such as protein precipitation using Carrez reagent. Even when samples are contaminated with particles and filtration cannot be used due to filter blockages, inline dialysis represents a practical sample preparation method.

[0008] Although this sample preparation technique has many advantages, it typically suffers from the problem that dialysis is the rate-limiting step in the analysis process. This is particularly disadvantageous when processing many samples in series, as the sample preparation makes the procedure time-consuming and costly. Furthermore, there is the problem that, in heavily contaminated samples, matrix contaminants can partially break through the dialysis membrane. A further disadvantage is the comparatively high sample volume required for stopped-flow dialysis.

[0009] US 4,311,789 and US 4,491,011 disclose analytical devices upstream of a dialysis unit. The dialysis cell is designed in such a way that a complex fluid medium, namely blood, is guided through a channel, wherein in this channel there is a further fluid guide, which is formed entirely or partially from a selectively permeable membrane, for example a fiber or capillary made of a selectively permeable membrane. The acceptor solution is provided in this further fluid guide. If the volume of the acceptor channel at least partially exceeds a volume VA which is smaller than a volume running parallel to it VDof the donor channel, this can have beneficial effects on the equilibration time. However, known dialysis cells operate without suitable control of the pressure difference between the donor channel and the acceptor channel. Pressure gradients in the donor and / or acceptor channel lead to undesired filtration. The formation of a filter cake, particularly in particle-laden matrices, changes the permeability of the dialysis membrane. The permeability can even change differently for individual analytes. The equilibration time t A is thereby influenced in an unpredictable way and is difficult to control.

[0010] It is therefore an object of the present invention to overcome the above-mentioned disadvantages of the prior art. In particular, it is an object of the present invention to reduce the equilibration time t Aand thus achieve an overall higher sample throughput. Furthermore, the present invention is based on the problem of achieving a better separation of the analyte(s) from the matrix in heavily contaminated samples. Furthermore, it is an object of the present invention to provide a device and a method for sample preparation by dialysis, by means of which a reliable and precise equilibration can be achieved within a controllable equilibration time. t A is made possible.

[0011] These objects are achieved by a dialysis cell for sample preparation for ion chromatography, which has the features of claim 1. The dialysis cell comprises a donor channel and an acceptor channel running parallel thereto, as well as a selectively permeable dialysis membrane. The donor channel and the acceptor channel are separated from each other by the selectively permeable dialysis membrane. As a result, in particular an analyte dissolved in a donor solution in the donor channel can pass through the dialysis membrane into the acceptor solution in the acceptor channel. The acceptor channel has, at least in sections, a volume VA which is smaller than a volume running parallel to it VD of the donor channel.

[0012] It has been shown that such an asymmetric structure of the dialysis cell reduces the equilibration times t A can be significantly reduced. This leads to shorter dialysis times t Dand thus to an overall higher sample throughput, which is associated with significant time and cost savings. In addition, shorter dialysis times t D The time during which the dialysis membrane is in contact with the sample matrix is ​​reduced. This significantly reduces the breakthrough of matrix contaminants. Furthermore, dialysis can be performed with a significantly smaller sample volume.

[0013] The dialysis cell according to the invention comprises two half-cells, between which the selectively permeable dialysis membrane is arranged. The donor channel and the acceptor channel are each formed as a recess in a contact surface of one of the half-cells with the dialysis membrane. This dialysis cell design has proven to be particularly robust, cost-effective to manufacture, and user-friendly in terms of operation and maintenance.

[0014] Furthermore, with such a setup, the volumes of both half-cells are known and constant. By appropriately selecting the amount / flow rate of the added and removed donor or acceptor fluid, respectively, overpressure or underpressure in one of the half-cells can be avoided. Such overpressure or underpressure leads to filtration, which can disrupt the concentration equilibrium of the analytes in the donor and acceptor solutions established by diffusion. For example, overpressure in the donor channel can cause filtrate to flow into the acceptor channel.

[0015] Pressure-free dialysis, i.e., dialysis in the absence of a pressure gradient within the dialysis cell, proves particularly advantageous when donor fluids based on changing media or matrices are to be dialyzed. With a constant matrix, such as is the case with dialysis and / or analysis of blood serum in medical applications, the flow rate and thus the pressure can be adjusted based on empirical values ​​with appropriate effort, without unduly compromising the precision of the analysis results. With changing matrices, however, as is often the case in ion analysis, a pressure difference leads to uncontrollable and unpredictable dialysis or equilibration times.

[0016] Pressure control is further supported when dialysis is performed using the stopped-flow procedure.

[0017] In a preferred embodiment of the present invention, the acceptor channel has a volume of at least 50%, preferably at least 70%, preferably at least 90%, of its length VA which is smaller than a volume running parallel to it VD of the donor channel. In a further preferred embodiment of the present invention, the acceptor channel has a volume over its entire length VA which is smaller than a volume running parallel to it VD of the donor channel.

[0018] The acceptor channel can, at least in sections, have a volume per unit length VA / L from 0.005 mm 3< / mm to 2.0 mm 3< / mm, preferably from 0.020 mm 3< / mm to 1.5 mm 3< / mm, more preferably from 0.10 mm 3< / mm to 1.0 mm 3< / mm. The donor channel can have, at least in sections, a volume per unit length VD / Lfrom 0.25 mm 3< / mm to 3.5 mm 3< / mm, preferably from 0.30 mm 3< / mm to 3.0 mm 3< / mm, more preferably from 0.35 mm 3< / mm to 2.0 mm 3< / mm. The donor and acceptor channels usually each have a length of 10 mm to 1000 mm, preferably from 20 mm to 500 mm, more preferably from 100 mm to 300 mm.

[0019] It has been shown that such dimensioning of the donor and / or acceptor channel is particularly advantageous for applications in ion chromatography. On the one hand, it allows for significantly shorter equilibration times t Acan be achieved. On the other hand, these dimensions ensure that the amount of analyte fed to the separation column remains within the detection limits of a conventional ion chromatography system. With regard to the donor channel, these dimensions also ensure that it does not become clogged, even with heavily contaminated samples, especially with particles.

[0020] The selectively permeable dialysis membrane can have a pore size of 0.01 µm to 1.0 µm, preferably from 0.02 µm to 0.5 µm, more preferably from 0.05 µm to 0.2 µm. Since the asymmetric structure of the dialysis cell according to the invention allows a required recovery rate R to be achieved more quickly with the same dialysis membrane, a finer-pored dialysis membrane can also be used for a given analytical application without requiring a longer equilibration time. t Awould have to be accepted. This can lead to better separation of the matrix from the analyte(s), especially in heavily contaminated samples.

[0021] The selectively permeable dialysis membrane can be made of a material selected from a list consisting of cellulose acetate, cellulose nitrate, polyvinylidene fluoride, polycarbonate, cellulose mixed esters, cellulose hydrate, and regenerated cellulose. Preferably, the material is selected from a list consisting of polyvinylidene fluoride, polycarbonate, and cellulose mixed esters, or polyamide. These membrane materials have proven particularly advantageous for the separation of metal cations and inorganic anions.

[0022] The donor channel and the acceptor channel can be linear, spiral, or meander-shaped. This allows for a particularly compact design of the dialysis cell relative to the channel length.

[0023] The cross-section through the donor channel and / or the acceptor channel can be circular segment-shaped, particularly semicircular, semi-elliptical, square, or rectangular. These channel geometries have proven particularly advantageous both with regard to the production of the half-cells and their performance characteristics.

[0024] However, it is particularly preferred if the cross-section of the donor channel is semicircular. This semicircular design allows the high-molecular components, such as fats or proteins, to flow unhindered, while maximizing the exposure area of ​​the donor solution to the dialysis membrane.

[0025] It is also particularly preferred if the cross-section through the acceptor channel is, at least in sections, a rectangle which has rounded corners on the side facing away from the dialysis membrane. Rounded corners here are understood to mean the sections which connect the section of the acceptor channel opposite the dialysis membrane with the lateral boundaries of the acceptor channel adjoining the dialysis membrane. These sections are circular in cross-section and have a radius of curvature r of between 0.05 and 1 mm, preferably between 0.1 and 0.8 mm, very particularly preferably between 0.2 and 0.4 mm. Additionally or alternatively, the ratio of the side bounded by the dialysis membrane to the depth of the rectangle is preferably from 80:1 to 10:1, particularly preferably from 40:1 to 15:1 and very particularly preferably from 25:1 to 20:1.

[0026] By choosing the dimensions mentioned above, the donor fluid is counterbalanced by the smallest possible volume of acceptor fluid per cell section, while maintaining a large contact area between the acceptor fluid and the dialysis membrane. In other words, the acceptor channel has a volume VA , which is minimized compared to the parallel volume section VD of the donor channel, which increases the equilibration time t A shortened.

[0027] The rounded corners prevent contamination of the cell by the accumulation of acceptor fluid compared to non-rounded corners. Such contamination typically occurs due to the slowed flow velocity near the channel wall, which would be accentuated in non-rounded corners. The problem of the accumulation of slow-flowing acceptor fluid also arises if the radius of curvature is chosen too small. If, on the other hand, the radius of curvature is chosen too large, there is a risk that the dialysis membrane, which can swell to a thickness of up to 0.15 mm in aqueous solution, will adhere to the walls and / or the floor of the half-cell recess and thus impede flow through the half-cell. It is therefore preferred that the radius of curvature be between 0.05 and 1 mm, preferably between 0.1 and 0.8 mm, and most preferably between 0.2 and 0.4 mm.

[0028] Furthermore, it is preferred that the dialysis cell is configured such that at least one, preferably several support elements are formed in the acceptor channel, which support elements space the dialysis membrane from the side of the acceptor channel facing away from the dialysis membrane. The support elements preferably have a height corresponding to the depth of the acceptor channel. The support elements are preferably cylindrically symmetrical with a support upper edge diameter between 5 and 500 µm, preferably between 10 and 200 µm, very particularly preferably between 30 and 60 µm. The cross-sectional geometry of the supports can, however, also deviate from cylindrical symmetry. Preferably, the surface area, i.e. the cross-sectional area, of the support decreases from the membrane-side upper edge. Preferably, the support elements are distributed in a regular pattern across the width of the acceptor channel; particularly preferably, a row of support elements is arranged in the center of the acceptor channel.Preferably, the distances between any two adjacent supports and / or between a support and the directly adjacent upper edge of the channel are always the same.

[0029] It is a function of the at least one support element that the volume of the channel on the acceptor side is kept constant and is not reduced due to sagging of the dialysis membrane under gravity or pressure. In principle, support elements can also be attached to the donor side to prevent a volume reduction on the donor side, for example due to sagging of the dialysis membrane under gravity or pressure in the direction of the donor channel. Since the acceptor channel according to the invention has a smaller volume VA compared to the parallel volume VDof the donor channel, the relative effect of sagging in the acceptor channel is greater and any potentially resulting flow obstruction is more severe than on the donor side.

[0030] The shape and number of support elements are selected to ensure the stability of the device, while minimizing fluid flow through the channel and covering as little membrane surface as possible. Truncated cone shapes or concavely curved support elements on the shell side are particularly suitable for this purpose.

[0031] Preferably, the support elements are formed integrally with the half-cell. The support elements are preferably manufactured by milling, injection molding, hot stamping, or 3D printing.

[0032] The present invention also relates to a device for sample preparation for a chemical analysis method in a dialysis process, wherein the chemical analysis method is chromatography, comprising a dialysis cell as described above, characterized in that the donor circuit comprises a first pumping device that transports the donor fluid to the dialysis cell and a second pumping device that transports the donor fluid away from the dialysis cell. Additionally or alternatively, the acceptor circuit comprises a first pumping device that transports the acceptor fluid to the dialysis cell and a second pumping device that transports the acceptor fluid away from the dialysis cell.

[0033] The two pumps per fluid circuit can be set to equal flow rates per unit of time, thus ensuring constant pressure in the respective half-cell, acceptor or donor half-cell.

[0034] If only one pump is used in each donor and acceptor circuit, a shut-off element is required, such as a valve that stops the flow. This allows the stopped-flow condition for dialysis to be achieved.

[0035] Preferably, the first pumping device and the second pumping device of the donor circuit are combined in a dual-channel pump, preferably a peristaltic dual-channel pump. Additionally or alternatively, the first pumping device and the second pumping device of the acceptor circuit are combined in a dual-channel pump, preferably a peristaltic dual-channel pump.

[0036] The advantage of dual-channel pumps is that the volume of donor or acceptor fluid supplied to the dialysis cell and the volume of donor or acceptor fluid removed from the dialysis cell are always the same, since the same stroke movement pumps the donor or acceptor fluid. The supply of fluid to and removal from each half-cell are automatically synchronized.

[0037] It is particularly preferred that the first and second peristaltic dual-channel pumps are controllable to operate independently of each other. Typically, during the dialysis period t D Only the first peristaltic pump is operated so that the donor fluid flows continuously while the acceptor fluid is held in the acceptor channel. Preferably, after the equilibration time t Athe first peristaltic pump is stopped and only the second peristaltic pump is operated so that the acceptor fluid flows while the donor fluid is held in the donor channel.

[0038] Such controllability has the advantage that under- or overpressure in one of the half-cells can be prevented, which could lead to undesired filtration.

[0039] Preferably, a first capillary, which connects an acceptor solution container, preferably via the second pumping device, to the acceptor channel, and / or a second capillary, which connects the acceptor channel, preferably via the first pumping device, to an injection device for injection onto a chromatography column, is / are designed such that the diameter of the capillary is at most 0.5 mm, preferably at most 0.25 mm.

[0040] The selected diameter should be small to limit diffusion of the analyte(s) during the transfer process. If diffusion along the capillary is too rapid, the maximum achievable recovery rate R is reduced, which effect should be contained by using small-diameter capillaries. For the same reason, the transfer rate is preferably kept low, which is again favored by a small capillary diameter.

[0041] The present invention further relates to the use of a dialysis cell as described above for sample preparation in a chemical analysis method selected from a list consisting of ion chromatography (IC) and high performance liquid chromatography (HPLC).

[0042] A membrane is used as a dialysis membrane in a dialysis cell as previously described for sample preparation for a chemical analysis method selected from a list consisting of ion chromatography (IC) and high performance liquid chromatography (HPLC).

[0043] However, it goes without saying that coupling techniques such as IC-MS, HPLC-MS or CE-MS can also be used within the scope of the present invention.

[0044] The two uses mentioned above may include the following steps: Providing an acceptor solution; introducing a specific amount of the acceptor solution into the acceptor channel or into an acceptor circuit containing the acceptor channel; holding the specific amount of the acceptor solution in the acceptor channel or in the acceptor circuit containing the acceptor channel; providing a donor solution containing at least one analyte; passing the donor solution through the donor channel, whereby the at least one analyte contained in the donor solution passes through the dialysis membrane into the acceptor solution.

[0045] In this case, fresh donor solution is passed through the donor channel at least until the concentration of the at least one analyte in the acceptor solution is at least 90%, preferably at least 95%, more preferably at least 99%, of the concentration of the at least one analyte in the donor solution.

[0046] At the time when the concentration of the analyte in the acceptor solution reaches the predefined threshold, the following steps are then carried out: Holding the donor solution in the donor channel (2) or in a donor circuit containing the donor channel; draining the acceptor solution and supplying the acceptor solution to an analysis device, in particular selected from a list consisting of an ion chromatography device (IC), a high performance liquid chromatography device (HPLC), a capillary electrophoresis device (CE) and a mass spectrometry device (MS).

[0047] Keeping the donor solution in the donor channel (2) or in a donor circuit containing the donor channel while draining the acceptor solution has the advantage that, even during the transfer of the acceptor solution to the analysis device, no pressure differences arise between the donor and acceptor channels, for example, due to non-synchronous switching of the pumps located on the donor or acceptor circuits. This ensures that unwanted filtration, which could distort the established equilibrium, does not occur.

[0048] Fresh donor solution can be continuously passed through the donor channel. The flow rate of the donor solution through the donor channel can be in the range of 0.01 ml / min to 10.0 ml / min, preferably 0.05 ml / min to 5.0 ml / min, and more preferably 0.10 ml / min to 1.0 ml / min. These flow rates have proven to be suitable for the mass exchange rate typically observed in this type of dialysis.

[0049] The present invention further relates to an ion chromatography system (IC) and / or a system for high performance liquid chromatography (HPLC) comprising a dialysis cell as described above.

[0050] Further advantages and individual features of the invention will become apparent from the following description of an embodiment and from the drawings.

[0051] They show schematically: Figure 1: Perspective exploded view of a dialysis cell according to the invention; Figure 2: Cross-sectional view of a dialysis cell from the prior art; Figure 3: Cross-sectional view of a dialysis cell according to the invention; Figure 4: Ion chromatography system (IC) comprising a dialysis cell according to the invention; Figure 5: Influence of the volume VAthe acceptor channel on the recovery rate R; Figures 6 and 7: Comparison of a dialysis cell according to the invention with one from the prior art with regard to the breakthrough of lignins. Figures 8 and 9: Example of sample preparation using a dialysis cell according to the invention. Figures 10 and 11: Cross-sections through an acceptor channel of a dialysis cell according to the invention with and without support elements. Figure 12: Perspective view of an acceptor channel of a dialysis cell according to the invention. Figures 13 and 14: Cross-sections through two alternative arrangements of two half-cells of a dialysis cell according to the invention, each with support elements. Figure 15: Schematic representation of a device for sample preparation for a chemical analysis method in a dialysis process.

[0052] As from Figure 1As can be seen, one embodiment of a dialysis cell 1 according to the invention consists of two half-cells 6 and 7, between which a selectively permeable dialysis membrane 4 is arranged. The half-cells 6 and 7 each have a spiral-shaped depression at their contact surfaces 8, 8' with the dialysis membrane 4, which forms the donor channel 2 and the acceptor channel 3, respectively. The donor channel 2 is connected to a supply line 9, via which a sample can be supplied. Furthermore, the donor channel 2 is connected to a drain line 10, with which the sample can be drained off after passing through the dialysis cell 1 and usually fed to a disposal line 17. The acceptor channel 3 is also connected to a supply line 11 and a drain line 12. The supply line 11 and the drain line 12 of the acceptor channel 3 can be combined to form an acceptor circuit.The sample solution passed through the donor channel 2 contains at least one analyte, symbolically represented here as metal ion 5, as well as matrix molecules, symbolically represented here as proteins 13. The metal ions 5 can pass through the selectively permeable dialysis membrane 4 (see arrows), while the proteins 13 are retained. This allows a sample containing metal ions 5 to be prepared, for example, for ion chromatography.

[0053] The Figure 2 shows a cross-section through a prior art dialysis cell of this type. It can be seen that the donor channel 2 and the acceptor channel 3 have an identical width w. Furthermore, the depth d D of the donor channel equal to the depth d A of the acceptor channel. In the Figure 3 is, however, one of the Figure 2A corresponding cross-section of a dialysis cell according to the invention is shown. Both the donor channel 2 and the acceptor channel 2 still have the width w. The depth of the donor channel is also d D compared to the example according to Figure 2 identical. However, the acceptor channel has a reduced depth d A It goes without saying that the acceptor channel 3 has a volume per unit length in the area of ​​the cutting plane VA / L which is smaller than a corresponding volume per unit length VD / L of donor channel 2.

[0054] The Figure 4shows a chromatography system 14 comprising a dialysis cell 1 according to the invention. In said system 14, a sample solution, which is also referred to herein as a donor solution, is provided in a sample container 15. The donor solution is pumped through the first half-cell 6 of the dialysis cell 1 by means of a pump 16 and then collected in a collection container 17 for disposal. An acceptor solution is provided in an acceptor container 18 and is pumped through the second half-cell 7 of the dialysis cell 1 by means of another pump 16'. Excess acceptor solution is also collected in a collection container 17' for disposal. As already explained above, in order to carry out a so-called stopped-flow dialysis, the flow of the acceptor solution, also referred to herein as the acceptor stream, is stopped by the half-cell 7, while the flow of the donor solution, also referred to herein as the donor stream, is continued by the half-cell 6.This is maintained until the acceptor solution within half-cell 7 has a desired minimum concentration of the analyte in the donor solution within half-cell 6. Such a minimum concentration can be, for example, 90%, 95%, or 99%.

[0055] After dialysis, the injection valve 19 can be switched, whereby the analyte is fed to the chromatography column 20. While the analyte is being fed to the chromatography column 20, the donor flow is stopped. The actual chromatography part of the chromatography system 14 is shown here in a highly simplified manner. An eluent is provided in an eluent container 21 and is pumped through the separation column 20 by a pump 16', in particular a high-pressure pump, via the injection valve 19. After detection by the detector 22, the sample separated by ion chromatography is also collected in a collection container 17' for disposal. It goes without saying, however, that so-called tandem techniques, for example, a coupling of a conductivity detector and a mass spectrometer (MS), are also feasible within the scope of the present invention.

[0056] The Figure 5 shows the influence of volume VA of the acceptor channel on the recovery rate R. Bars 23, 23`, 23'', 23‴ represent the observed values ​​for chloride, while bars 24, 24`, 24'', 24‴ represent data for sulfate. The values ​​shown show the recovery rate R after 2 min of dialysis. t D For all measurements, a donor channel with a depth of 515 µm and a volume VD of 240 µl. Acceptor channels with volumes VA of 240 µl, 135 µl, 93 µl and 61 µl. It can be seen that the recovery rate R with decreasing volume VA of the acceptor channel with constant dialysis time t D gets higher.

[0057] In the Figures 6 and 7A comparison of a dialysis cell according to the invention with one from the prior art is shown with regard to the breakthrough of matrix molecules. In this context, a sample with a lignin concentration of 25 mg / l was prepared. The lignin concentration was determined using a UV detector at a wavelength of 274 nm. Graph 27 shows the breakthrough rate B of lignin as a function of dialysis time. t D for a symmetrical dialysis cell with a donor or acceptor channel depth of 515 µm, a donor or acceptor channel volume VD or VA of 240µl and a cellulose acetate membrane with a pore size of 0.2 µm. It can be seen that the breakthrough rate B after a dialysis time t D of 4 min is above 70%. In contrast, as shown in Graph 28, the breakthrough rate B for an asymmetric dialysis cell with a donor channel depth of 515µm, a donor channel volume VDof 240µl, an acceptor channel depth of 515µm, an acceptor channel volume VA of 90µl and a polycarbonate membrane with 0.1µm pore size increases significantly slower over the duration of dialysis and even with 10 min dialysis time t D only a value of 10%.

[0058] The Figure 7 shows the recovery rate R of nitrate as a function of dialysis time t D for the Figure 6 Dialysis configurations shown. Graphs 29 and 30 show the recovery rates R of nitrate corresponding to graphs 27 and 28, respectively, in Figure 6 . Out of Figure 7 it is evident that the Figure 6 Reduced matrix breakthrough shown in asymmetric dialysis with fine-pore membrane is present even in the case that the equilibration time t A is lower in asymmetric dialysis with a fine-pored membrane than in symmetric dialysis with a coarse-pored membrane.

[0059] Figures 6 and 7show that by using an asymmetric dialysis cell compared to a symmetric dialysis cell by choosing suitable membranes, the equilibration time t A for the analyte and at the same time reduce matrix breakthrough.

[0060] Below and based on the Figures 8 and 9 An application example of a dialysis cell 1 according to the invention is given. Specifically, the nitrate and sulfate content of water samples from the Britzer Kirchteich (Berlin, Germany) was determined. This is a surface water body that shows signs of eutrophication and a high content of humic substances. A total of five water samples were collected, and the concentrations of the anions in question were determined by ion chromatography. Sample preparation was performed using stopped-flow dialysis.

[0061] Two different dialysis cells were used, each with a structure similar to that used in the Figure 1The first dialysis cell was a state-of-the-art one in which both the donor and acceptor channels had a depth of 515 µm and a volume V A or V D of 240 µl. The second dialysis cell was one according to the invention, with the donor channel also having a depth of 515 µm and a volume V D of 240 µl. The acceptor channel had a depth of only 210 µm and a volume V A of 90 µl.

[0062] In both cases, a cellulose mixed ester membrane with a pore diameter of 0.05 µm (Merck Millipore) was used. Due to its small pore size, this membrane is characterized by strong retention of potentially interfering substances, especially macromolecular substances such as humins or lignins.

[0063] As shown by the Figures 8 and 9As shown, the dialysis time required for both nitrate and sulfate t D as a preliminary test. For this purpose, standard solutions with a concentration of 5 mg / l were prepared and dialyzed for each ion. Figure 8 shows the results for the anion nitrate. Graph 25 represents the recovery rate R for an acceptor channel with a volume V A of 240 µl. Graph 26 represents the corresponding recovery rate R for an acceptor channel with a volume V A of 90 µl. It can be seen that with the asymmetric dialysis cell for nitrate, a dialysis time of t D of 2 min, a recovery rate R of over 90% is achieved. In the symmetrical dialysis cell, however, this value is only reached after a dialysis time t D of over 6 minutes. The Figure 9shows an analogous representation for the anion sulfate. Here it can be seen that a recovery rate R of 90% was achieved with the asymmetric dialysis cell with a dialysis time of approximately 5 minutes. t D is reached, while a corresponding value in the symmetrical cell structure only after more than 15 minutes of dialysis time t D can be achieved. The following table summarizes the results of these preliminary tests for nitrate and sulfate, as well as for other anions. Anion Dialysis time t D [min] Recovery rate R [%] Sample consumption [ml] Dialysezeit t D [min] Recovery rate R [%] Sample consumption [ml] 240 µl acceptor channel (515 µm depth) 90 µl acceptor channel (210 µm depth) F -< 15 97.2 11.6 6 98.3 5.4 Cl -< 9 97.1 7.5 4 99.0 4.1 NO. 2 -< 11 98.2 8.8 4 98.5 4.1 Br -< 9 98.4 7.5 3 98.3 3.4 NO. 3 -< 9 97.1 7.5 3 98.8 3.4 SO 4 2-< 24 94.8 16.3 8 96.4 6.8

[0064] As can be seen from the table above, shorter dialysis times were achieved for all tested anions using the asymmetric dialysis cell 1 t D The recovery rates achieved Rwere in the same range as with a symmetrical dialysis cell, or often even higher. Furthermore, an asymmetrical dialysis cell design generally resulted in lower sample consumption.

[0065] The following table summarizes the nitrate and sulfate levels for the surface water analysis mentioned above. Sample number NO. 3 -< SO 4 2-< Concentration [mg / l] RSD [%] Concentration [mg / l] RSD [%] 1 0.317 4.7 3.74 1.1 2 0.099 4.6 3.80 0.5 3 0.132 4.4 3.84 0.6 4 0.068 6.5 3.85 1.3 5 0.074 1.7 3.83 0.2

[0066] In addition to the concentration value for each individual sample, the relative standard deviation (RSD) of the concentration is also given.

[0067] In summary, it can be stated that with a dialysis cell 1 according to the invention with an asymmetric structure, significantly shorter dialysis times t Dand thus a higher sample throughput can be achieved. Furthermore, it has been determined that with such a dialysis cell 1, the amount of sample required can be reduced by at least a factor of 2. The shorter time during which the matrix is ​​in contact with the dialysis membrane 4 also reduces the undesirable breakthrough of matrix components and the associated negative effects on the ion chromatography system.

[0068] Figure 10shows a cross section through an acceptor half-cell 7 according to the invention with an acceptor channel 3. The cross section through the acceptor channel 7 is designed as a rectangle, which has rounded corners 29`, 29" on the side m facing away from the membrane. The ratio of the width w of the side delimited by the membrane 4 to the depth d A of the rectangle shown in the cross section is not shown to scale. The same applies to the radius of curvature r. According to the invention, the ratio of the width w of the side delimited by the membrane to the depth d A of the rectangle shown in the cross section is from 80:1 to 10:1, particularly preferably from 40:1 to 15:1 and most preferably from 25:1 to 20:1. The radius of curvature of the circular arc-shaped section, which connects the section m of the acceptor channel 3 opposite the dialysis membrane with the lateral boundaries of the Acceptor channel 3 is between 0.05 and 1 mm, preferably between 0.1 and 0.8 mm, most preferably between 0.2 and 0.4 mm. The channel design shown minimizes the ratio of fluid volume per section to the contact area with the membrane 4 in the same section. Furthermore, it prevents the acceptor solution from accumulating in the corners 29', 29" of the cross-section facing away from the membrane 4 as the fluid flows.

[0069] Figure 11shows a cross-section through an acceptor half-cell 7 according to the invention with an acceptor channel 3, in which a plurality of support elements 30', 30" are mounted, which space the membrane 4 from the side m of the acceptor channel 3 facing away from the membrane. The support elements 30', 30" have a height d A that corresponds to the depth of the acceptor channel 3. The support elements are designed here as waisted cylinders in order to impede the fluid flow as little as possible and at the same time to offer the most solid support possible for the membrane 4. By widening the circumference towards the membrane, the risk is reduced that the membrane 4 could be perforated by a conically tapered cylinder end or a conical design of the support element 30', 30".

[0070] Figure 12shows a perspective view of an acceptor half-cell 7 according to the invention with an acceptor channel 3. Here, the support elements are conically concavely curved. The support elements 30', 30" are formed integrally with the acceptor channel 3. The support elements 30', 30" are arranged centrally in the width m of the acceptor channel 3. The distance between any two adjacent support elements is constant over the channel length. Also shown is a supply line 11 for an acceptor solution. The support element 30' is formed close to the supply line 11 in order to prevent the membrane 4 (not shown) from resting on the walls and / or the floor of the acceptor channel 3, especially at the supply line opening 11.

[0071] Figures 13 and 14 each show a cross section through an arrangement of two half-cells of a dialysis cell according to the invention in the assembled state, each with support elements 30`, 30". In Figure 13The donor half-cell 6 comprising the donor channel 2 is designed such that the cross-section shows a semicircle. The acceptor cell 7 comprising the acceptor channel 3 is designed as already described for Fig. 10 to 12 described. In Fig. 14 Support elements 30', 30" are formed in both the donor channel 2 and the acceptor channel 3. Although this is not preferred, it is nevertheless part of the invention.

[0072] Figure 15 shows a schematic representation of a device for sample preparation for a chemical analysis procedure in a dialysis process. As already described in Figure 4, the system comprises a dialysis cell 1 according to the invention. The donor solution in the sample container 15 is pumped through the first half-cell of the dialysis cell 1 by means of a pump 16 and then passed to a collection container 17 by the pump 16". The acceptor solution in the acceptor container 18 is pumped through the second half-cell of the dialysis cell 1 by means of a further pump 16'. Excess acceptor solution is also collected in a collection container 17' for disposal. The forwarding of the acceptor solution to the collection container 17' is additionally driven by the pump 16'.

[0073] The stopped-flow method corresponds to the Figure 4 described method. In addition, the system is configured so that while either excess acceptor solution is being supplied to the collection container 17' or the acceptor solution containing the predetermined analyte concentration is being supplied to an analysis method, the flow in the donor circuit is stopped.

[0074] This ensures that at most one of the two half-cells of the dialysis cell 1 has a flow. In other words, the pumps 16, 16" on the one hand and the pumps 16' and 16‴ on the other hand are operated alternately. During the dialysis time t D Only the pumps of the donor circuit are operated and after the dialysis time t D only the pumps of the acceptor circuit are operated.

[0075] Whether the acceptor liquid is fed to the collection container 17' or the chromatography column 20 is determined by switching the injection valve 19. The actual chromatography part of the chromatography system may comprise an eluent degasser 33 in addition to the eluent container 21 and the high-pressure pump 16'. Before detection in the detector 22, the chromatographically separated sample may pass through at least one suppressor module 32.

[0076] Although not shown, it is also part of the invention that the pump 16 and the pump 16" are combined in a two-channel pump, preferably a peristaltic two-channel pump. This ensures that the inflow and outflow of the donor liquid into / out of the donor half-cell are determined by the same stroke movement and are thus synchronized. The pumps 16' and 16‴ of the acceptor circuit can additionally or alternatively be designed as a two-channel pump, in particular as a peristaltic two-channel pump, so that the inflow and outflow of the acceptor liquid into / out of the acceptor half-cell are determined by the same stroke movement and are thus synchronized. Such an embodiment prevents pressure fluctuations in the respective half-cells.

[0077] Furthermore, it is part of the invention that the capillary 31 which connects the acceptor solution container 18 via the pump 16' to the acceptor half-cell, as well as the second capillary 31' which connects the acceptor half-cell to the injection valve 19, are designed such that the diameter of the capillary has a maximum of 0.5 mm.

Claims

1. A dialysis cell (1) for sample preparation for ion chromatography, comprising a donor channel (2), an acceptor channel (3) running parallel thereto and a selectively permeable dialysis membrane (4), wherein the donor channel (2) and the acceptor channel (3) are separated from one another by the selectively permeable dialysis membrane (4) and in particular an analyte (5) dissolved in a donor solution in the donor channel (2) can pass through the dialysis membrane (4) into an acceptor solution in the acceptor channel (3), wherein the acceptor channel (3) has, at least sectionally, a volume VA, , which is smaller than a volume VD of the donor channel (2) running parallel thereto; characterized in that the dialysis cell comprises two half cells (6, 7) between which the dialysis membrane (4) is arranged, the donor channel (2) and the acceptor channel (3) each being formed as an indentation in a contact surface (8, 8') of one of the half cells (6, 7) with the dialysis membrane (4).

2. The dialysis cell (1) according to claim 1, wherein the acceptor channel (3) has on at least 50%, preferably at least 70%, more preferably at least 90%, of its length a volume VA which is smaller than a volume VD of the donor channel (2) running parallel thereto.

3. The dialysis cell (1) according to claim 2, wherein the acceptor channel (3) has a volume VA over its entire length which is smaller than a volume VD of the donor channel (2) running parallel thereto.

4. The dialysis cell (1) according to one of claims 1 to 3, wherein the dialysis membrane (4) has a pore size of from 0.01 µm to 1.0 µm, preferably from 0.02 µm to 0.5 µm, more preferably from 0.05 µm to 0.2 µm.

5. The dialysis cell (1) according to one of claims 1 to 4, wherein the cross-section through the acceptor channel (3) is, at least sectionally, a rectangle which has rounded corners (29', 29'') on the side (m) facing away from the dialysis membrane (4), wherein the ratio of the width (w) of the side bounded by the dialysis membrane (4) to the depth (dA) of the cross-section through the acceptor channel (3) is from 80:1 to 10:1, preferably from 40:1 to 15:1 and most preferably from 25:1 to 20:1; and / or wherein the rounded corners (29', 29'') have a radius of curvature of from 0.05 to 1 mm, preferably from 0.1 to 0.8 mm, most preferably from 0.2 to 0.4 mm.

6. The dialysis cell (1) according to one of claims 1 to 5, wherein at least one, preferably several support elements (30', 30'') are formed in the acceptor channel (3), which support elements (30', 30'') space the dialysis membrane (4) from the side (m) of the acceptor channel (3) facing away from the dialysis membrane.

7. A device for sample preparation for a chemical analysis method in a dialysis process, wherein the chemical analysis method is chromatography, with a dialysis cell according to one of claims 1 to 6, characterized in that the donor circuit has a first pumping device (16) which conveys the donor liquid to the dialysis cell (1), and a second pumping device (16'') which conveys the donor liquid away from the dialysis cell (1), and / or the acceptor circuit has a first pumping device (16') which conveys the acceptor liquid to the dialysis cell (1), and a second pumping device (16‴) which conveys the acceptor liquid away from the dialysis cell.

8. The device according to claim 7, wherein the first pumping device (16) and the second pumping device (16'') of the donor circuit are combined in a first two-channel pump, preferably a first peristaltic two-channel pump, and / or the first pumping device (16') and the second pumping device (16‴) of the acceptor circuit are combined in a second two-channel pump, preferably a second peristaltic two-channel pump.

9. The device according to one of claims 7 or 8, wherein a first capillary (31), which connects an acceptor solution container (18) to an acceptor channel (3), and / or a second capillary (31'), which connects the acceptor channel (3) to the injector (19) of a chromatography device, has / have a diameter of at most 0.5 mm, preferably at most 0.25 mm.

10. Use of a dialysis cell (1) according to any one of claims 1 to 6 and / or a device according to any one of claims 7 to 9 for sample preparation in a chemical analysis method selected from a list consisting of ion chromatography (IC) and high performance liquid chromatography (HPLC).

11. Use according to claim 10, comprising the following steps: - providing of an acceptor solution; - introducing of a certain quantity of the acceptor solution into the acceptor channel (3) or into an acceptor circuit containing the acceptor channel (3); - holding the specified quantity of the acceptor solution in the acceptor channel (3) or in the acceptor circuit containing the acceptor channel (3); - providing a donor solution containing at least one analyte (5); - passing the donor solution through the donor channel (2), whereby the at least one analyte (5) contained in the donor solution passes through the dialysis membrane (4) into the acceptor solution, wherein fresh donor solution is passed through the donor channel (2) at least until the concentration of the at least one analyte (5) in the acceptor solution is at least 90%, preferably at least 95%, more preferably at least 99%, of the concentration of the at least one analyte (5) in the donor solution.

12. Use according to claim 11, characterized in that at the point in time in which the concentration of the at least one analyte (5) in the acceptor solution is at least 90%, preferably at least 95%, more preferably at least 99%, of the concentration of the at least one analyte (5) in the donor solution, the following steps are carried out: - holding the donor solution in the donor channel (2) or in a donor circuit containing the donor channel; - discharging the acceptor solution from the acceptor channel (3) and supplying the acceptor solution to the analytical method of an analytical device selected from a list consisting of an ion chromatography device (IC) and a high performance liquid chromatography (HPLC) device.

13. Use according to any one of claims 11 or 12, wherein the fresh donor solution is continuously passed through the donor channel (2) until, at the point in time, the concentration of the at least one analyte (5) in the acceptor solution is at least 90%, preferably at least 95%, more preferably at least 99%, of the concentration of the at least one analyte (5) in the donor solution.

14. Ion chromatography system (IC) and / or system for High Performance Liquid Chromatography (HPLC), comprising a dialysis cell (1) according to any one of claims 1 to 6.