Lithium selective electrodes

Individual polyethers exhibit varying specificities for cations. Some polyethers have found appHcation as components in ion-selective electrodes for use in clinical medicine or in laboratory studies involving transport studies or measurement of transmembrane electrical potential (4). The methyl ester of monensin [28636-21 -7] i2ls been incorporated into a membrane sHde assembly used for the assay of semm sodium (see Biosensors) (5). Studies directed toward the design of a lithium selective electrode resulted in the synthesis of a derivative of monensin lactone that is highly specific for lithium (6). 

Development of lithium selective electrodes (LiSE) and their application in clinical chemistry have been amply reviewed Several models of lithium ion specific electrodes are commercially available. The central problems in developing such sensing devices are their dynamic range, the accuracy and precision by which the signals are correlated to the concentration of the analyte and the selectivity towards that species, especially in relation to other alkali metal cations. Additional problems of practical importance are the times of response and recovery and the durability of the electrode in the intended service. 

Dialysis units provided highly efficient means for increasing selectivity in a dynamic system by placement in front of a lithium-selective electrode constructed by incorporating 14-crown-4 ether 3-dodecyl-3 -methyl-1,5,8,12-tetraoxacyclotetradecane into a PVC membrane that was in turn positioned in a microconduit circuit by deposition on platinum, silver or copper wires. The circuit was used to analyse undiluted blood serum samples by flow injection analysis with the aid of an on-line coupled dialysis membrane. For this purpose, a volume of 200 pL of sample was injected into a de-ionized water carrier (donor) stream and a 7 mM tetraborate buffer of pH 9.2 was... 

Nonactin-based membrane solutions (52) can be used for the measurement of ammonium ion activity. A great deal of interest exists in designing lithium-selective electrodes because Li+ is used clinically to control manic depression. As a result, micropipette Li+ electrodes have also been reported (43). 

Kataky R, Nicholson PE, Parker D, and Covington AK (1991) Comparative performance of 14-crown-4 derivatives as lithium-selective electrodes. Analyst 116 135-140. 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

Ion Selective Electrodes Technique. Ion selective (ISE) methods, based on a direct potentiometric technique (7) (see Electroanalytical techniques), are routinely used in clinical chemistry to measure pH, sodium, potassium, carbon dioxide, calcium, lithium, and chloride levels in biological fluids. 

Ion-selective electrodes are available for the electro analysis of most small anions, eg, haUdes, sulfide, carbonate, nitrate, etc, and cations, eg, lithium, sodium, potassium, hydrogen, magnesium, calcium, etc, but having varying degrees of selectivity. The most successful uses of these electrodes involve process monitoring, eg, for pH, where precision beyond the unstable reference electrode s abiUty to deUver is not generally required, and for clinical apphcations, eg, sodium, potassium, chloride, and carbonate in blood, urine, and semm. 

EXAMPLE 5-5 Calculate the error caused by sodium ion, aNa = 0.01, in the measurement of lithium, au = 0.01, using a lithium ion-selective electrode... 

It has been long believed that a lithium ion-selective electrode would render obsolete the flame photometer in the clinical laboratory. Lithium is administered to manic depressive psychiatric patients. Since the therapeutic range (0.5-1.5 mM) is quite close to the toxic range (>2 mM), it must be closely monitored. Most of the iono-phores propo d to date have not met the Li" /Na selectivity required for an interference-free assay. However, it has been reported that calibration in the presence of 140 mMNa permitted the analysis of Li in serum The errors observed are due to fluctuations in the Na concentrations in the sample. More selective ionophores would certainly improve the accuracy of this method. 

Detection of Li+ in artificial serum with a voltammetric Li-selective electrode in a flowthrough system was demonstrated [64], Lithium salts such as lithium carbonate have been extensively used for treatment of manic depressive and hyperthyroidism disorders. The therapeutic range of Li concentration is generally accepted to be 0.5-1.5mM in blood serum. The authors used normal pulse voltammetry in which a stripping potential was applied between pulses in order to renew the membrane surface and expel all of the extracted ions from the membrane, similar to galvanostatically controlled potentiometric sensors described above. Unfortunately, the insufficient selectivity... 

S. Sawada, H. Torii, T. Osakai, and T. Kimoto, Pulse amperometric detection of lithium in artificial serum using a flow injection system with a liquid/liquid-type ion-selective electrode. Anal. Chem. 70, 4286-4290 (1998). 

Elemental composition Li 26.75%, F 73.25%. An aqueous solution prepared by dissolving 100 mg in a liter of water may be analyzed for lithium by AA or ICP spectrophotometry and for fluoride by ion chromatography or by using a fluoride ion selective electrode. 

Work has therefore been devoted by a number of developers to improving the cyclability of the lithium metal electrode. Since passivation of lithium is an unavoidable phenomenon, one approach has been directed to the promotion of uniform and smooth surface passivation layers, for example by selecting the most appropriate combination of solvents and electrolyte salts. An example is the inclusion of 2-methyltetrahydrofuran (2-Me-THF), since the presence of the methyl group slows down the reactivity towards the lithium metal. The selection of fluorine-based elec-... 

This section summarizes some of the most significant electrochemical results obtained to date for selected electrodes cleaned and characterized under UHV in PEO-lithium-based solutions, and include nonalloy (Ni)- and alloy-forming metals (Ag and Al), a noninteracting substrate (boron-doped diamond, BDD) and a material capable of intercalating Li+ (graphite). It is expected that the information herein contained will serve to illustrate the power of this methodology for the study of highly reactive interfaces. 

The lithium and thallium (I) salts of i satin-3-oxime (isatin oximates) were employed in the development of ion-selective electrodes for these cations. Transition metal complexes of isatin derivatives can also be employed as catalysts for the oxidative self-coupling of alkylphenol s639,640. 

Ageing of ion-selective electrodes has been reported to give inaccurately high serum lithium concentration. In one case, a concentration of 0.4 mmol/1 was reported in a patient who was not taking lithium (701). The possibility that other substances could interfere with ion-selective electrode lithium analysis has been briefly reviewed (702). 

Lithium ion selective electrodes (ISEs) have been developed (67) and may he used to provide rapid serum lithium estimations at the time of contact with the patient, for example, in the lithium clinic, as well as in clinical laboratories (68). The therapeutic advantage of this procedure is that patient compliance with medication may be improved if the results of blood monitoring are obtained on the spot. Instant results also reduce the number of times a patient needs to visit the hospital or clinic, thus saving the patient inconvenience. The psychiatrist needs fewer appointments for feedback and dosage regulation (69). 

Table 1 Inter-assay variation, CVmeas % of the three ion-selective electrode (ISE) applications during method validation and at the 6-month checkpoint for the lower and higher therapeutic levels and the toxic level of serum lithium (S-Li)...

The concentration of lithium in serum, plasma, urine, or other body fluids has been determined by flame emission photometry, atomic absorption spectrometry, or electro-chemically using an ion-selective electrode. Serum analysis, the most useful specimen for lithium monitoring, is most commonly quantified by automated spectrophotometric assay. 

A lithium ion-selective electrode gave the potentials given in the table for the following standard solutions of LiCl and three samples of unknown concentration. 

Zhukov, A. F., Erne, D., Ammann, D., Guggi, M., Pretsch, E., and Simon, W., Improved lithium ion-selective electrode based on a lipophilic diamide as neutral carrier. Anal. Chim. Acta 131, 117-122 (1981). 

Mixed Solution Method. There are various measurement methods using mixed solutions of the two ions. The. fixed interference method is commonly used. Consider, for example, the testing of a lithium ion-selective electrode in the presence of sodium ion. A lithium calibration curve is prepared in the presence of a fixed concentration of sodium, for example, 140 mM as found in blood. A plot such as that given in Figure 13.16 results. In the upper portion of the curve, the electrode responds in a Nemstian manner to the lithium ion. As the lithium concentration decreases, the electrode potential is increasingly affected by the constant background of sodium ions, and in the lower portion of the curve the electrode exhibits a mixed response to both the lithium and the sodium. When the lithium concentration is very small, the response is due solely to sodium (the baseline potential). 

---