Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1997-08-29
2004-04-13
Tung, T. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S402000
Reexamination Certificate
active
06719888
ABSTRACT:
FIELD OF INVENTION
This invention provides a reference electrode device of the constrained-diffusion liquid junction type useful in pH and/or ion-selective electrode (ISE) potentiometric sensors and is particularly suitable for use in a mini-integrated electrochemical analyzer.
BACKGROUND OF INVENTION
Conventional types of reference electrodes have a liquid junction where the sample meets the junction solution. The junction is typically either open or constrained. In an open junction system, the liquid junction operates by free diffusion. In a constrained-diffusion junction system, a region of porous material permeable to water and salts (a membrane, porous plug, frit, or the like) is placed at the site of the liquid junction. The porous material acts as a constraint whereby passage of large molecules (such as protein) and bulk liquid transport is generally hindered.
The liquid junction solution (also commonly referred to as the salt bridge solution) typically contains a solution saturated with a salt (such as an equitransferent salt, including KCl, KNO
3
) which functions to reduce and maintain constant the interfacial potential which develops across the liquid junction boundary, typically referred to as a liquid junction potential. The difference in liquid junction potentials between the system calibrator and the sample is referred to as the residual liquid junction potential. Typically, the residual liquid junction potential increases as the ionic strength difference between the system calibrator solution and the sample solution increases. The residual liquid junction potential is generally considered to compromise the accuracy of the associated potentiometric sensors and therefore a multi-use reference electrode is typically designed to minimize the residual liquid junction potential for as long as possible, while balancing other design constraints.
In potentiometric systems that are designed with miniaturized working or indicator electrodes (typically pH and/or ion selective electrodes), the necessity of the junction solution makes miniaturization of a reference electrode difficult. Further, for the reference electrode to have a multiple use capability, the liquid junction solution must be present in a volume and concentration sufficient to minimize the residual liquid junction potential over its useful lifetime. This requisite volume particularly complicates miniaturization of constrained diffusion liquid junctions since the volume must typically be maintained close to the constraint to minimize problems associated with excessive ion depletion through the constraint. Another drawback of this type of reference electrode is that it tends to be orientation dependent. During operation, an upright orientation generally must be maintained with the liquid junction solution on top of the constraint in order to maintain contact therebetween. Moreover, in order to minimize errors due to the residual liquid junction potential, the junction solution is generally saturated with the equitransferent salt.
Conventional reference electrodes utilize a reference contact region (also sometimes referred to as electrode elements) immersed in a stagnant portion of the junction solution which contains a constant concentration of the equitransferent salt. The reference contact region is often silver based, consisting of an electrochemically reversible redox electrode couple such as Ag/Ag
+
and Ag/AgCl. When salt solutions are used with silver based reference contact regions such as Ag/AgCl, the AgCl is susceptible to dissolution. In constrained-diffusion type liquid junctions, this dissolution is problematic because it leads to subsequent precipitation of silver salts on the region of porous material constraint, thus leading to undesirable fouling of the constraint, which in turn generally results in an erratic reference electrode performance.
Commonly, because of the above-described fouling problem associated with the use of silver based reference contact regions, barrier membranes have been used to restrict Ag
+
ion migration from the reference contact region to the porous material constraint region. The use of a barrier membrane, however, carries with it inconvenience because the first use wet-up of the reference electrode is hindered by the barrier membrane, thus requiring a long soaking time in the junction solution prior to the first use. In addition, any additional component, such as this barrier membrane, tends to complicate miniaturization of the reference electrode.
Another common problem associated with the use of saturated equitransferent salt solutions in constrained-diffusion type of liquid junction reference electrodes is that when the reference electrode is stored and/or used at sub-ambient temperatures, salt crystallization and precipitation may occur between the reference contact region and the region of the porous material at the junction, which in turn leads to erratic reference electrode potentials. An additional problem associated with the use of a saturated equitransferent salt solution is that the saturated solution may contribute to reproducibility and/or accuracy problems with blood samples because of interference caused by precipitation of protein and crenation of red blood cells present in the sample.
Some of the aforementioned drawbacks associated with constrained diffusion liquid junctions are reduced or absent in open (free diffusion) liquid junctions. In this regard, reservoirs for holding of junction solutions have been described for open junction type reference electrodes, where there is no region of porous material to act as a constraint at the junction. For example, A. K. Covington et al. (
Anal. Chim. Acta
, 1985, 169, pp. 221-229) describe a open junction where the junction solution is moved from a KCl reservoir via a syringe. In this prior art system, the liquid junction is established with each sample, but because this type of system leads to cross-contamination of the liquids upon use, the junction solution must be discarded along with the sample thus leading to increased waste. Moreover, such contamination tends to dictate placement of the reference electrode downstream in the sample path of all working electrodes. This restriction disadvantageously limits flexibility of analyzer configuration.
Moreover, open free-diffusion liquid junction reference electrodes tend to require specific orientations and geometries to provide good reproducible junctions. Reproducibility for open free-diffusion liquid junctions tends to depend on uniform, generally circular junction geometries and a constant orientation. (See, e.g., R. E. Dohner et al.
Anal. Chem
., 1986, 58, pp. 2585-2589; T. R. Harbinson et al.,
Anal. Chem
., 1987, 59, pp. 2450-2456). The orientation and geometries requirements are particularly limiting when attempting to adapt such a reference electrode to a miniaturized, modular system in which it may be desirable to locate the reference electrode within an array of working electrodes and utilize componentry and dimensions common to those working electrodes. The aforementioned orientation requirement may be particularly troublesome in the event a miniature portable analyzer is desired for use in locations where the electrode array may be utilized in various or unstable orientations, such as may be encountered in the field or in mobile applications such as on board ambulances, ships and/or aircraft.
Thus, although many different reference electrodes are known in the prior art, there is a need to discover alternative reference electrodes for electrochemical analyzers, particularly modular reference electrodes that may be easily adapted for use in a mini-integrated type of analyzer.
SUMMARY OF THE INVENTION
Many of the shortcoming associated with prior art reference electrodes have been overcome with the discovery of a reference electrode assembly of the present invention.
According to the invention, provided is a modular reference electrode assembly adapted for serial integration within an orientation independent array of working electrodes,
Chan Andy D. C.
Foos Joseph S.
Rasmussen James E.
Schulkind Richard L.
Zalenski John A.
Bayer Corporation
Tung T.
Weingarten Schurgin, Gagnebin & Lebovici LLP
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