Salt bridge for analytical chemistry system

Abstract: A salt bridge for a flow cell which includes a porous bridge element between fluid flow in the flow cell and in an internal reference fluid flow. The bridge element is surrounded by a body with a bore into which different tubes are located about the porous element. A cylindrical anchoring tube adjacent one end of the body is screw threaded into an aperture in the wall of the flow cell so that self-alignment of the salt bridge in the flow cell is effected. The salt bridge provides a conductivity path between the two flow paths without introducing another potential.

Description: Related Applications This invention relates to the inventions and disclosures which are the subject of EP-A-388 012, EP-A-388 013, EP-A-388 014, EP-A-388 015, and EP-A-388 018. BACKGROUND This invention relates to the field of electrodes for automatic analytical chemical instruments. More particularly, the invention is concerned with a salt bridge in flow cells of analytical instruments. Flow cell analyzers operate with a combined mixture of diluent and fluid sample flow past various electrodes for the determination of electrolytes in the fluid sample. Usually four electrolytes, namely, sodium, potassium, chloride and CO₂ are determined in the flow cell. It is often desirable to connect different flow paths together without introducing new potential to the electrodes. The salt bridge provides a non-flow or very low flow path between these different flow paths and in effect a conductivity path. In the prior art, salt bridges are relatively permanently located in a flow cell, and have drawbacks in that they are difficult to align and assemble relative to the flow cell and different flow streams. The alignment of the salt bridge is particularly critical. It is thus difficult to set the salt bridge effectively and accurately in the flow cell. Additionally, when maintenance is necessary and the salt bridge needs to be removed from the flow cell, it is necessary to disassemble the entire flow cell. Difficulties also arise in the creation of air bubbles in adjacency with elements of the salt bridge electrode so that electrolyte measurements are inaccurate. A potentiometric analysis system for rapid and accurate analysis of whole blood is proposed in EP-A-0 101 236. The system employs a restricted diffusion type liquid junction where the whole blood sample is caused to flow past a sensing electrode and liquid junction. A cellophane dialysis membrane divides the sample from KCl electrolyte solution in the reference electrode whilst allowing ionic diffusion therebetween. A replaceable electrolyte cartridge tip for attachment to a pH monitor electrode is described in US-A-4 413 628, the purpose of which is to reduce risk of breakage of its fragile glass probe supported ion-sensitive membrane and other difficulties during installation of the electrode. The cartridge includes a cavity defined by a plastic element and at least one permeable membrane, and containing an electrolyte common to the sample to be analysed to permit operation of the electrode by providing electrical continuity between the sample under test and the electrode. The present invention seeks to overcome these drawbacks and to provide a salt bridge element which is relatively simple to locate in a flow cell in self-alignment. SUMMARY The present invention provides a salt bridge for a flow cell of a chemical analyzer, where the device includes a salt bridge in a self-aligning body which overcomes the problems in the prior art. According to the present invention there is provided a salt bridge for a flow cell of a chemistry analyzer comprising a porous bridge element for location in an aperture in the flow cell, the porous bridge element having a portion for exposure to fluid flow through the flow cell and another portion for exposure to flow of a reference fluid, an inlet and exit for the reference fluid to a chamber adjacent the portion of the bridge element exposed to flow of the reference fluid, the two portions being spaced to form between them a salt bridge, means for removably securing the porous element between the flows in alignment in the aperture, the porous bridge element being surrounded by a body which is retained by a cylindrical anchoring tube with an outer surface, the outer surface being for engagement in the aperture to the flow cell thereby to secure the porous bridge element in alignment in the aperture. In a preferred form of the invention, the securing means includes a body surrounding the bridge element. Preferably the aforesaid outer surface is threaded for engagement with threads in the aperture. Also in a preferred form of the invention, the bridge element is surrounded by a selected arrangement of cylindrical sleeves. There is preferably a lead-in cylindrical sleeve provided to the body for internal reference fluid to be positively directed to the one exposed portion of the bridge element. There is preferably a low resistivity and slow diffusion rate between the two exposed portions of the bridge element. The present invention also provides a method of aligning a salt bridge for a flow cell of a chemistry analyzer in an aperture in the flow cell comprising the steps of housing a porous bridge element in a body, locating sleeve elements between the body and the porous bridge element, causing a cylindrical anchoring tube with an outer surface to engage in the aperture to the flow cell and thereby urge the porous bridge element via the body into alignment in the aperture, exposing a portion of the porous bridge element of the cell to fluid flow through the flow cell, exposing another portion to flow of a reference fluid, the two portions being spaced to form between them the salt bridge, the porous bridge element being thereby removably secured. The invention is further described in the following detailed description with reference to the accompanying drawings. DRAWINGS Figure 1 is a diagrammatic external side view of a salt bridge. Figure 2 is a diagrammatic cross-sectional enlarged view of the salt bridge located in an aperture associated with the flow cell and showing the internal reference fluid inlet and exit. Figure 3 is an enlarged partial view of the salt bridge element. Figure 4 is a diagrammatic view of a different embodiment of the salt bridge in relation with inlet and exit tubes for the reference fluid. DESCRIPTION In an automatic chemistry analyzer there is a flow cell 10 which is diagrammatically illustrated, in part. The flow cell 10 is an element of an analyzer where different electrolytes, such as sodium, potassium, chloride and CO₂ are measured by the flow cell 10. The cell 10 is formed between two clamped plates 11 and 12. A flow path 13 for sample fluid and diluent to travel as indicated by arrow 14 is formed between the plates 11 and 12. An internal reference fluid enters the salt bridge 50 through an inlet tube 15 and exits through an outlet tube 16. The internal reference fluid is supplied directly or indirectly from a container in the analyzer. After passage from tube 16 the fluid is drained directly or indirectly to a suitable reservoir which may be a waste reservoir. If the fluid is indirectly fed to and from the electrode 50 there may be other electrodes to or from which the internal reference fluid is directed. The salt bridge 50 includes a bridge element 17 with the first exposed portion 18 adjacent the fluid flow 14 in the flow cell 10. A second exposed portion 19 is adjacent to fluid flow 20 through the internal reference tubes 15 and 16. A chamber 21 is formed between the inlet tube 15 for the internal reference fluid and exit tube 16 for the internal reference fluid. The bridge element 17 is constituted by a porous material, and is selectively a glass rod covered with a ceramic coating, a porous ceramic rod or a plastic porous fiber material. The material can be silk thread, ceramic tubing, fibrous porous plastic or similar porous material. There is a slow diffusion rate between the one surface exposed portion 18 and the other surface exposed portion 19. The resistivity is about 200 kohms. About the bridge element 17 there is provided a ceramic coating 22 and about that ceramic coating 22 there is formed a shrink tube 23. An epoxy material 24 is formed about the shrink tube 23 and is used to bond the assembly of shrink tube 23 and bridge element 17 into the salt bridge tip 35. The outer surface of the epoxy 24 is surrounded by a cylindrical body which form the salt bridge tip body 35. A bore in the body 35 accommodates the bridge element 17 and surrounding tubes. The body 35 includes a circular outer periphery and a shoulder section 27 about which an O-ring 28 can be located in adjacency to the aperture 29 in the wall 11 of the flow cell 10. The wall 11, which is the back wall of the flow cell 10, acts as the anchor for the salt bridge 50. The O-ring 28 facilitates positive and accurate location of the salt bridge 50 with the wall 11 and prevents leakage. The aperture 29 includes a tapered circular portion 30 against which the O-ring 28 is located. The location of the body 35 in the aperture 29 is positively established as a cylindrical anchoring tube 51 with a threaded outer surface 26 engaging the internal threaded surface 30 in the aperture 29 to the wall 11 of the flow cell 10. The cylindrical anchoring tube 51 is adjacent the end of the body 35 remote from the flow cell path 13. The tube 51 can slide longitudinally to effect anchorage with the flow cell wall 11. Extending in a leading direction from the bridge tip 35 is a lead-in cylindrical tube 31 which is affixed in a circumferential slot 32, within the trailing end of the tip 35. Within the lead portion 31 there is housed a setting material 33 through which the inlet and outlet tubes 15 and 16 pass and are cemented. The cylindrical anchoring tube 51 extends about the lead-in cylindrical tube 31 for the sliding action. The shrink tube 23 extends beyond the bridge element 17 and engages the inlet tube 15 and outlet tube 16 in a sealing relationship. The component design is one where the bridge element 17 acts as a salt bridge, and is relatively easy to insert and remove from the flow cell 10. The salt bridge is self-aligning and non-critical as the body 25 of the salt bridge 50 and lead-in portion 31 screw thread into connection with anchoring wall 11 associated with the flow cell 10. This is effected by means for the tube 51 which threadingly, through threads 26 and 30, engages aperture 29. As the tube 51 is tightened in the aperture the leading end 52 of the tip 35 is forced into tight engagement with the corresponding circumferential ring face 53 about aperture 29. The shoulder section 27 engages O-ring 28. The positive location of internal reference fluid inflow 15 and exit tube 16 avoids the trapping of air or gas bubbles in the chamber 21 adjacent to portion 19 of the salt bridge. The flow through the chamber 21 is relatively laminar and the chamber 21 surfaces are of a nature inhibiting entrapment of air or gas. Air bubbles are also avoided at the portion 18 by the characteristics of the bridge element 17 and surrounding tubes. In Figure 4, there is shown an embodiment where the inflow tube 15 and exit tube 16 are jammed up against the portion 19 of the element 17. The ends of tubes 15 and 16 are half cut away about semi-circular sections so that between the semi-circular cut out sections from each of tubes 15 and 16 there is formed a chamber 21 for fluid flow 20. Many other examples of the invention exist, each different from the other in matters of detail only. For instance, different bonding materials can be used about the porous bridging element to effect the positive location in the cylindrical body. Also, different fluids can enter the chamber about the one exposed portion of the porous bridging element. The scope of the invention is to be determined solely by the following claims.

Claims: A salt bridge (50) for a flow cell (10) of a chemistry analyzer comprising a porous bridge element (17) for location in an aperture (29) in the flow cell, the porous bridge element having a portion (18) for exposure to fluid flow through the flow cell and another portion (19) for exposure to flow of a reference fluid, an inlet (15) and exit (16) for the reference fluid to a chamber (21) adjacent the portion of the bridge element exposed to flow of the reference fluid, the two portions being spaced to form between them a salt bridge, means for removably securing the porous element between the flows in alignment in the aperture, the porous bridge element being surrounded by a body (35) which is retained by a cylindrical anchoring tube (51) with an outer surface, the outer surface being for engagement in the aperture to the flow cell thereby to secure the porous bridge element in alignment in the aperture. A salt bridge as claimed in claim 1 wherein the outer surface is threaded (26) for engagement with threads (30) in the aperture. A salt bridge as claimed in claim 1 wherein between the said body and said porous bridge element there are sleeve elements being selectively a ceramic tube (22) surrounded by a shrink tube (23) and an epoxy bond (24), the body having a bore for accommodating the tubes. A salt bridge as claimed in either claim 1 or 2 including a lead-in cylindrical tube (31) for location remotely of the flow cell path, the lead-in cylindrical tube having said means (15, 16) for said reference fluid to enter and exit from the portion exposed to the reference fluid. A salt bridge as claimed in any one of claims 1 to 4 wherein the inlet and exit includes tubes (15, 16) for the reference fluid, the tubes being anchored in the lead-in cylinder. A salt bridge as claimed in any one of claims 1 to 5 including a circular outer periphery to the body, the circular outer periphery including a shoulder for locating an O-ring (28) adjacent a wall (11) of the flow cell. A salt bridge as claimed in any one of claims 1 to 6 wherein the porous bridge element is selectively a ceramic-coated glass rod, silk or cotton thread, ceramic tubing, fibrous plastic or porous plastic. A salt bridge as claimed in any one of claims 1 to 7 wherein the porous bridge element provides for a relatively slow diffusion rate between the two portions. A method of aligning a salt bridge (50) for a flow cell (10) of a chemistry analyzer in an aperture (29) in the flow cell comprising the steps of housing a porous bridge element in a body (35), locating sleeve elements (23) between the body and the porous bridge element, causing a cylindrical anchoring tube (51) with an outer surface to engage in the aperture to the flow cell and thereby urge the porous bridge element (17) via the body (35) into alignment in the aperture, exposing a portion (18) of the porous bridge element (17) of the cell (10) to fluid flow through the flow cell, exposing another portion (19) to flow of a reference fluid, the two portions being spaced to form between them the salt bridge, the porous bridge element being thereby removably secured. A method as claimed in claim 9, wherein said causing step is enabled by said outer surface (26) being threaded for engagement with threads (30) in the aperture. A method as claimed in either claim 9 or 10 including urging a circular outer periphery of the body (35) to engage an O-ring seal (28) adjacent a wall (11) of the flow cell.

Inventors: JACKSON DELBERT D, MCNEAL JACK D, NICHOLS THEODORE R, JACKSON, DELBERT D., MCNEAL, JACK D., NICHOLS, THEODORE R.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1990
Main classification: G01N27
IPC codes: G01N27