Abstract
The invention relates to a dialysis cell for sample preparation for a chemical analysis method, in particular for ion chromatography. The dialysis cell comprises a donor channel and an acceptor channel extending parallel thereto. The donor channel and the acceptor channel are separated from each other by a selectively permeable dialysis membrane. In particular, an analyte that is dissolved in a donor solution in the donor channel can enter through the dialysis membrane into the acceptor solution in the acceptor channel. The acceptor channel has at least in some sections a volume that is smaller than the volume of the donor channel extending parallel thereto. Acceptor and donor channels are formed from half-cells, between which the dialysis membrane is arranged, wherein the donor channel and the acceptor channel are designed in each case as a recess in a contact surface of one of the half-cells with the dialysis membrane.
Claims
1. A dialysis cell for sample preparation for a chromatographic method comprising: a donor channel; and a parallelly running acceptor channel, wherein, when used as intended, the donor channel and the acceptor channel are separated from one another by a selectively permeable dialysis membrane, wherein the acceptor channel has, on its entire length, a volume V.sub.A which is smaller than a parallelly running volume V.sub.D of the donor channel, wherein the donor channel and the acceptor channel are spiral or meandering, wherein the dialysis cell comprises two half-cells, between which the dialysis membrane is arranged, wherein the donor channel and the acceptor channel are formed as, in each case, an indentation in a contact surface of one of the half-cells with the dialysis membrane, and wherein at least one support element is formed in the acceptor channel, which support element spaces the dialysis membrane from the side of the acceptor channel that faces away from the dialysis membrane.
2. The dialysis cell as claimed in claim 1, wherein the acceptor channel has at least one length section along the length thereof having a volume per unit of length V.sub.A/L of from 0.005 mm.sup.3/mm to 2.0 mm.sup.3/mm.
3. The dialysis cell as claimed claim 1, wherein the donor channel has at least one length section along a length thereof having a volume per unit of length V.sub.D/L of from 0.25 mm.sup.3/mm to 3.5 mm.sup.3/mm.
4. The dialysis cell as claimed in claim 1, wherein the dialysis membrane has a pore size of from 0.01 μm to 1.0 μm.
5. The dialysis cell as claimed in claim 1, wherein the dialysis membrane consists of a material selected from a list consisting of cellulose acetate, cellulose nitrate, polyvinylidene fluoride, polycarbonate, mixed cellulose ester, cellulose hydrate, regenerated cellulose, and polyamide.
6. The dialysis cell as claimed in claim 1, wherein the cross section through the acceptor channel is, at least sectionally, a rectangle having rounded corners on the side facing away from the dialysis membrane, wherein the ratio of the width of the side bounded by the dialysis membrane to the depth of the cross section through the acceptor channel is from 80:1 to 10:1.
7. An analytical system comprising a dialysis cell (1) according to claim 1.
8. The dialysis cell as claimed in claim 1, wherein the cross section through the acceptor channel is, at least sectionally, a rectangle having rounded corners on the side facing away from the dialysis membrane, wherein the rounded corners have a radius of curvature of from 0.05 mm to 1 mm.
9. A device for sample preparation for a chromatographic method in a dialysis process comprising a dialysis cell, wherein at least one of the following applies: that a donor circuit has a first pump device which conveys a donor liquid to the dialysis cell and has a second pump device which conveys the donor liquid away from the dialysis cell; and that an acceptor circuit has a first pump device which conveys an acceptor liquid to the dialysis cell and has a second pump device which conveys the acceptor liquid away from the dialysis cell, wherein the device comprises a dialysis cell according to claim 1.
10. The analytical system according to claim 7, wherein the system is an ion chromatography system (IC), a system for high-performance liquid chromatography (HPLC), a capillary electrophoresis system (EC) or a mass spectrometry system (MS).
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Further advantages and individual features of the invention are apparent from the following description of an exemplary embodiment and from the drawings.
(2) Shown schematically are:
(3) FIG. 1: Exploded perspective view of a dialysis cell according to the invention;
(4) FIG. 2: Cross-sectional view of a dialysis cell from the prior art;
(5) FIG. 3: Cross-sectional view of a dialysis cell according to the invention;
(6) FIG. 4: Ion chromatography system (IC) comprising a dialysis cell according to the invention;
(7) FIG. 5: Influence of the volume V.sub.A of the acceptor channel on the recovery rate R;
(8) FIGS. 6 and 7: Comparison of a dialysis cell according to the invention with such a dialysis cell from the prior art with respect to the breakthrough of lignins.
(9) FIGS. 8 and 9: Example of a sample treatment using a dialysis cell according to the invention.
(10) FIGS. 10 and 11: Cross sections through an acceptor channel of a dialysis cell according to the invention with and without support elements.
(11) FIG. 12: Perspective view of an acceptor channel of a dialysis cell according to the invention.
(12) FIGS. 13 and 14: Cross sections through two alternative arrangements of two half-cells of a dialysis cell according to the invention, with support elements in each case.
(13) FIG. 15: Schematic representation of a device for sample preparation for a chemical analytical method in a dialysis process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(14) As is evident from FIG. 1, 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. On their contact surfaces 8, 8′ with the dialysis membrane 4, the half-cells 6 and 7 each have a spiral indentation 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 discharge line 10, by means of which the sample can be discharged after passing through the dialysis cell 1 and can be supplied usually to a disposal point 17. The acceptor channel 3 as well is connected to a supply line 11 and a discharge line 12. The supply line 11 and the discharge line 12 of the acceptor channel 3 can be united to form an acceptor circuit. The sample solution conducted through the donor channel 2 contains at least one analyte, which is depicted here symbolically as metal ion 5, and also matrix molecules, which are depicted here symbolically as proteins 13. The metal ions 5 can cross the selectively permeable dialysis membrane 4 (cf. arrows), whereas the proteins 13 are held back. As a result, a sample containing metal ions 5 can be treated, for example for ion chromatography.
(15) FIG. 2 shows a cross section through a congeneric dialysis cell from the prior art. It can be seen that the donor channel 2 and the acceptor channel 3 have an identical width w. Moreover, the depth d.sub.D of the donor channel is also identical to the depth d.sub.A of the acceptor channel. By contrast, FIG. 3 depicts a cross section of a dialysis cell according to the invention, which cross section corresponds to FIG. 2. Both the donor channel 2 and the acceptor channel 2 still have the width w. Also, the depth of the donor channel d.sub.D is identical with respect to the example as per FIG. 2. However, the acceptor channel has a reduced depth d.sub.A. It is self-evident that the acceptor channel 3 thus has, in the region of the sectional plane, a volume per unit of length V.sub.A/L which is smaller than a corresponding volume per unit of length V.sub.D/L of the donor channel 2.
(16) FIG. 4 shows 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 here as donor solution, is provided in a sample container 15. The donor solution is pumped by means of a pump 16 through the first half-cell 6 of the dialysis cell 1 and then collected in a collection container 17 for the purpose of disposal. An acceptor solution is provided in an acceptor container 18 and is pumped by means of a further pump 16′ through the second half-cell 7 of the dialysis cell 1. Excess acceptor solution is likewise collected in a collection container 17′ for the purpose of disposal. As has already been elucidated above, a so-called stopped-flow dialysis is carried out by stopping the flow of the acceptor solution, also referred to here as acceptor flow, through the half-cell 7 while continuing the flow of the donor solution, also referred to here as donor flow, through the half-cell 6. This is maintained until the acceptor solution within the half-cell 7 has a desired minimum proportion of the concentration of the analyte in the donor solution within the half-cell 6. Such a minimum proportion can, for example, be 90%, 95% or 99%.
(17) After dialysis has been carried out, the injection valve 19 can be switched, the result being that the analyte is supplied to the chromatography column 20. While the analyte is supplied to the chromatography column 20, the donor flow is stopped. The actual chromatography part of the chromatography system 14 is depicted here in a highly simplified manner. An eluent is provided in a eluent container 21 and is pumped by means of a pump 16″″, especially a high-pressure pump, through the separation column 20 via the injection valve 19. After detection has been carried out by means of the detector 22, the sample separated by ion chromatography is likewise collected in a collection container 17″ for the purpose of disposal. However, it is self-evident that so-called tandem techniques, for example a coupling of a conductivity detector and a mass spectrometer (MS), are realizable too in the context of the present invention.
(18) FIG. 5 shows the influence of the volume V.sub.A of the acceptor channel on the recovery rate R. The bars 23, 23′, 23″, 23′″ each depict the observed values for chloride, whereas the bars 24, 24′, 24″, 24′″ represent data for sulfate. The depicted values each show the recovery rate R after a dialysis time t.sub.D of 2 min. For all the measurements, a donor channel having a depth of 515 μm and a volume V.sub.D of 240 μl was used. Acceptor channels having volumes V.sub.A of 240 μl, 135 μl, 93 μl and 61 μl were tested. It can be seen that the recovery rate R becomes higher with decreasing volume V.sub.A of the acceptor channel while the dialysis time t.sub.D remains the same.
(19) FIGS. 6 and 7 show a comparison of a dialysis cell according to the invention with such a dialysis cell from the prior art with regard to the breakthrough of matrix molecules. In this connection, a sample having a concentration of lignin of 25 mg/l was prepared. The lignin concentration was determined using a UV detector at a wavelength of 274 nm. The graph 27 shows the breakthrough rate B of lignin as a function of the dialysis time t.sub.D for a symmetrical dialysis cell having a donor channel depth and acceptor channel depth of 515 μm, a donor channel volume V.sub.D and acceptor channel volume V.sub.A of 240 μl and a cellulose acetate membrane having a pore size of 0.2 μm. It is evident that the breakthrough rate B is above 70% after a dialysis time t.sub.D of 4 min. In contrast, as shown in graph 28, the breakthrough rate B for an asymmetrical dialysis cell having a donor channel depth of 515 μm, a donor channel volume V.sub.D of 240 μl, an acceptor channel depth of 515 μm, an acceptor channel volume V.sub.A of 90 μl and a polycarbonate membrane having a 0.1 μm pore size increases significantly more slowly over the period of dialysis and only reaches precisely a value of 10% even with a dialysis time t.sub.D of 10 min.
(20) FIG. 7 shows the recovery rate R of nitrate as a function of the dialysis time t.sub.D for the dialysis configurations shown in FIG. 6. Graphs 29 and 30 show the recovery rates R of nitrate corresponding to graphs 27 and 28, respectively, in FIG. 6. It is evident from FIG. 7 that the reduced matrix breakthrough as shown in FIG. 6 for asymmetrical dialysis with a fine-pored membrane is present even when the equilibration time t.sub.A is lower for asymmetrical dialysis with a fine-pored membrane than for symmetrical dialysis with a coarse-pored membrane.
(21) FIGS. 6 and 7 show that, through the use of an asymmetrical dialysis cell as opposed to a symmetrical dialysis cell, it is possible, by choosing suitable membranes, to reduce the equilibration time t.sub.A for the analyte and to reduce the matrix breakthrough at the same time.
(22) An application example of a dialysis cell 1 according to the invention is provided below and by FIGS. 8 and 9. Specifically, the nitrate and sulfate content of water samples from the Britzer Kirchteich (Berlin, Germany) was ascertained. What is concerned here is a surface water exhibiting signs of eutrophication and having a high content of humic substances. Altogether five water samples were collected and the concentration of said anions was determined by ion chromatography. The sample preparation was carried out by means of stopped-flow dialysis.
(23) Use was made of two different dialysis cells which both had a structure corresponding to that shown in FIG. 1. The first dialysis cell was a dialysis cell known from the prior art, in which both the donor channel and the acceptor channel had a depth of 515 μm and a volume V.sub.A and V.sub.D, respectively, of 240 μl. The second dialysis cell was a dialysis cell according to the invention, wherein the donor channel likewise had a depth of 515 μm and a volume V.sub.D of 240 μl. The acceptor channel had a depth of only 210 μm and a volume V.sub.A of 90 μl.
(24) In both cases, a membrane composed of mixed cellulose ester and having a pore diameter of 0.05 μm (Merck Millipore) was used. Because of the small pore size, said membrane is distinguished by a high level of retention with respect to potentially interfering substances, especially macromolecular substances, such as, for example, humins or lignins.
(25) As shown by FIGS. 8 and 9, a dialysis time t.sub.D required both for nitrate and for sulfate was determined as a preliminary experiment. For this purpose, standard solutions having a concentration of 5 mg/l were prepared and dialyzed for each ion. FIG. 8 shows the results for the anion nitrate. The graph 25 represents the recovery rate R for an acceptor channel having a volume V.sub.A of 240 μl. The graph 26 represents the corresponding recovery rate R for an acceptor channel having a volume V.sub.A of 90 μl. It can be seen that, in the case of the asymmetrical dialysis cell, there is a recovery rate R of over 90% for nitrate even with a dialysis time t.sub.D of 2 min. By contrast, in the case of the symmetrical dialysis cell, this value is achievable only with a dialysis time t.sub.D of over 6 min. FIG. 9 shows an analogous picture for the anion sulfate. It can be seen here that a recovery rate R of 90% is achieved with the asymmetrical dialysis cell with a dialysis time t.sub.D of approx. 5 min, whereas a corresponding value in the case of the symmetrical cell structure can be achieved only with a dialysis time t.sub.D of over 15 min. The following table combines the results of these preliminary experiments for nitrate and sulfate and for further anions.
(26) TABLE-US-00001 Dialysis Recovery Sample Dialysis Recovery Sample time t.sub.D rate R consump- time t.sub.D rate R consump- [min] [%] tion [ml] [min] [%] tion [ml] 240 μl acceptor channel 90 μl acceptor channel (515 μm depth) (210 μm depth) F.sup.− 15 97.2 11.6 6 98.3 5.4 Cl.sup.− 9 97.1 7.5 4 99.0 4.1 NO.sub.2.sup.− 11 98.2 8.8 4 98.5 4.1 Br.sup.− 9 98.4 7.5 3 98.3 3.4 NO.sub.3.sup.− 9 97.1 7.5 3 98.8 3.4 SO.sub.4.sup.2− 24 94.8 16.3 8 96.4 6.8
(27) As can be gathered from the table above, it was possible to achieve shorter dialysis times CD with the asymmetrical dialysis cell 1 for all the anions tested. The recovery rates R achieved were in the same range as for a symmetrical dialysis cell or were often even higher. Furthermore, it was fundamentally possible to achieve a lower sample consumption with an asymmetrical structure of the dialysis cell.
(28) The following table combines the nitrate and sulfate contents for the abovementioned surface water analysis.
(29) TABLE-US-00002 NO.sub.3.sup.− SO.sub.4.sup.2− Sample Concentration RSD Concentration RSD number [mg/l] [%] [mg/l] [%] 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
(30) Besides the concentration value for each individual sample, the relative standard deviation (RSD) of the concentration is additionally reported.
(31) In summary, it can be stated that distinctly shorter dialysis times t.sub.D and thus a higher sample throughput can be achieved with a dialysis cell 1 according to the invention having an asymmetrical structure. Furthermore, it was established that the amount of sample required can be reduced by at least a factor of 2 with such a dialysis cell 1. Furthermore, the shorter time during which the matrix is in contact with the dialysis membrane 4 reduces the undesired breakthrough of matrix constituents and associated adverse effects on the ion chromatography system.
(32) FIG. 10 shows a cross section through an acceptor half-cell 7 according to the invention, containing an acceptor channel 3. The cross section through the acceptor channel 7 is formed as a rectangle having rounded corners 29′, 29″ on the side m that faces away from the membrane. The ratio of the width w of the side bounded by the membrane 4 to the depth d.sub.A of the rectangle shown in cross section is not shown true to scale. The same applies to the radius of curvature r. According to the invention, the ratio of the width w of the side bounded by the membrane to the depth d.sub.A of the rectangle shown in cross section is from 80:1 to 10:1, particularly preferably from 40:1 to 15:1 and very particularly preferably from 25:1 to 20:1. The radius of curvature of the arc-shaped section, which connects the section m of the acceptor channel 3 that is opposite the dialysis membrane to the lateral boundaries of the acceptor channel 3 that are attached to the dialysis membrane, is between 0.05 and 1 mm, preferably between 0.1 and 0.8 mm and particularly preferably between 0.2 and 0.4 mm. The channel configuration shown minimizes the ratio of the fluid volume per section to the contact surface with the membrane 4 in the same section. Moreover, what is prevented during fluid flow is the deposition of acceptor solution in the corners 29′, 29″ of the cross section that face away from the membrane 4.
(33) FIG. 11 shows a cross section through an acceptor half-cell 7 according to the invention, containing an acceptor channel 3 in which multiple support elements 30′, 30″ are mounted, which support elements space the membrane 4 from the side m of the acceptor channel 3 that faces away from the membrane. The support elements 30′, 30″ have the height d.sub.A, which corresponds to the depth of the acceptor channel 3. The support elements are designed here as waisted cylinders in order to obstruct the fluid flow as little as possible and, at same time, to provide the membrane 4 with support that is as solid as possible. The broadening of size toward the membrane reduces the risk that the membrane 4 might be perforated by a conically tapering cylinder end or a conical design of the support element 30′, 30″.
(34) FIG. 12 shows a perspective view of an acceptor half-cell 7 according to the invention, containing an acceptor channel 3. Here, the support elements are curved concavely in the shape of a cone. The support elements 30′, 30″ are integrally formed 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 every two adjacent support elements is constant over the channel length. A supply line 11 for an acceptor solution is also shown. The support element 30′ is near the supply line 11 in order to prevent the membrane 4 (not shown) from resting on the walls and/or the base of the acceptor channel 3 even and specifically at the supply line opening 11.
(35) FIGS. 13 and 14 each show a cross section through an arrangements of, in each case, two half-cells of a dialysis cell according to the invention in the assembled state, with support elements 30′, 30″ in each case. In FIG. 13, the 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 FIGS. 10 to 12. In FIG. 14, support elements 30′, 30″ are formed both in the donor channel 2 and in the acceptor channel 3. Although this is not preferred, it is nevertheless part of the invention.
(36) FIG. 15 shows a schematic representation of a device for sample preparation for a chemical analytical method in a dialysis process. As in FIG. 4, the system comprises a dialysis cell 1 according to the invention. The donor solution in the sample container 15 is pumped by means of a pump 16 through the first half-cell of the dialysis cell 1 and then conducted through the pump 16″ to a collection container 17. The acceptor solution in the acceptor container 18 is pumped by means of a further pump 16′ through the second half-cell of the dialysis cell 1. Excess acceptor solution is likewise collected in a collection container 17′ for the purpose of disposal. The forwarding of the acceptor solution to the collection container 17′ is additionally driven by the pump 16′″.
(37) The stopped-flow method corresponds to the method described in relation to FIG. 4. In addition, the system is configured such that the flow in the donor circuit is stopped while either excess acceptor solution is supplied to the collection container 17′ or the acceptor solution containing the predetermined analyte concentration is supplied to an analytical method.
(38) This ensures that always at most one of the two half-cells of the dialysis cell 1 exhibits a flow. In other words, the pumps 16, 16″, on the one hand, and the pumps 16′ and 16′″, on the other, are operated in an alternating manner. During the dialysis time t.sub.D, only the pumps of the donor circuit are operated and, after expiration of the dialysis time t.sub.D, only the pumps of the acceptor circuit are operated.
(39) Switching of the injection valve 19 determines whether the acceptor liquid is supplied to the collection container 17′ or to the chromatography column 20. The actual chromatography part of the chromatography system can comprise an eluent degasser 33 in addition to the eluent container 21 and the high-pressure pump 16″″. Before the detection in the detector 22, the chromatographically separated sample can pass through at least one suppressor module 32.
(40) Although not shown, what is also part of the invention is that the pump 16 and the pump 16″ are combined in a two-channel pump, preferably a peristaltic two-channel pump. This ensures that inflow and outflow of the donor liquid into/out of the donor half-cell are determined by the same stroke movement and synchronized as a result. Additionally or alternatively, the pumps 16′ and 16′″ of the acceptor circuit can be designed as a two-channel pump, especially as a peristaltic two-channel pump, with the result that inflow and outflow of the acceptor liquid into/out of the acceptor half-cell are determined by the same stroke movement and synchronized as a result. Such an embodiment prevents pressure fluctuations in the respective half-cells.
(41) What is further part of the invention is that the capillary 31 connecting the acceptor solution container 18 to the acceptor half-cell via the pump 16′ and the second capillary 31′ connecting the acceptor half-cell to the injection valve 19 are designed such that the diameter of the capillary comprises at most 0.5 mm.
