skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Guidelines for Improving the Lower Detection Limit of Ion-Selective Electrodes: A Systematic Approach

Abstract

Zero-current membrane fluxes are the principal source of bias that has prohibited researchers from obtaining true, thermodynamic selectivity coefficients for membrane-based ion selective electrodes (ISEs). They are also responsible for the mediocre detection limits historically seen with these types of potentiometric sensors. By choosing an experimental protocol that suppresses these fluxes, it becomes possible to obtain unbiased thermodynamic selectivity coefficients that are needed to produce ISEs with greatly improved detection limits. In this work, a Cs+-selective electrode based on calix[6]arene-hexaacetic acid hexaethyl ester (Cs I) is used to systematically demonstrate how unbiased selectivity coefficients can be obtained, and how they can be used to optimize inner filling solutions for low detection limit measurements. A comparison of biased selectivity methods (e.g., classical separate solution method (SSM), fixed interference method (FIM), matched potential method (MPM)) with the unbiased modified separate solution method (MSSM) found that selectivity coefficients were underestimated in several cases by more than 4 orders of magnitude. The importance of key experimental parameters, including diffusion coefficients and diffusion layer thicknesses in the aqueous and organic phases, on the minimization of ion fluxes and the improvement of lower detection limits is also described. A dramatic reduction of membrane fluxes by themore » covalent attachment of a Ca2+-selective ionophore to a methyl methacrylate-decyl methacrylate copolymer matrix is also demonstrated. The ionophore-immobilized ISE exhibited no super-Nernstian response and yielded a detection limit of 40 ppt with an inner filling solution of 1 x 10-3 M KCl. Finally, a set of guidelines for experimental protocols leading to obtaining unbiased selectivity coefficients and producing ISEs for trace level analyses is given.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
899145
Report Number(s):
PNNL-SA-51171
TRN: US0701741
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Electroanalysis, 19(2-3):144-154
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; COPOLYMERS; DIFFUSION; ELECTRODES; ESTERS; ION-SELECTIVE ELECTRODES; MEMBRANES; METHACRYLATES; MINIMIZATION; RECOMMENDATIONS; SENSITIVITY; THERMODYNAMICS; low detectioin limit; zero-current ion fluxes; ion-selective electrode

Citation Formats

Radu, Aleksandar, Peper, Shane M., Bakker, Eric, and Diamond, Dermot. Guidelines for Improving the Lower Detection Limit of Ion-Selective Electrodes: A Systematic Approach. United States: N. p., 2007. Web. doi:10.1002/elan.200603741.
Radu, Aleksandar, Peper, Shane M., Bakker, Eric, & Diamond, Dermot. Guidelines for Improving the Lower Detection Limit of Ion-Selective Electrodes: A Systematic Approach. United States. doi:10.1002/elan.200603741.
Radu, Aleksandar, Peper, Shane M., Bakker, Eric, and Diamond, Dermot. Mon . "Guidelines for Improving the Lower Detection Limit of Ion-Selective Electrodes: A Systematic Approach". United States. doi:10.1002/elan.200603741.
@article{osti_899145,
title = {Guidelines for Improving the Lower Detection Limit of Ion-Selective Electrodes: A Systematic Approach},
author = {Radu, Aleksandar and Peper, Shane M. and Bakker, Eric and Diamond, Dermot},
abstractNote = {Zero-current membrane fluxes are the principal source of bias that has prohibited researchers from obtaining true, thermodynamic selectivity coefficients for membrane-based ion selective electrodes (ISEs). They are also responsible for the mediocre detection limits historically seen with these types of potentiometric sensors. By choosing an experimental protocol that suppresses these fluxes, it becomes possible to obtain unbiased thermodynamic selectivity coefficients that are needed to produce ISEs with greatly improved detection limits. In this work, a Cs+-selective electrode based on calix[6]arene-hexaacetic acid hexaethyl ester (Cs I) is used to systematically demonstrate how unbiased selectivity coefficients can be obtained, and how they can be used to optimize inner filling solutions for low detection limit measurements. A comparison of biased selectivity methods (e.g., classical separate solution method (SSM), fixed interference method (FIM), matched potential method (MPM)) with the unbiased modified separate solution method (MSSM) found that selectivity coefficients were underestimated in several cases by more than 4 orders of magnitude. The importance of key experimental parameters, including diffusion coefficients and diffusion layer thicknesses in the aqueous and organic phases, on the minimization of ion fluxes and the improvement of lower detection limits is also described. A dramatic reduction of membrane fluxes by the covalent attachment of a Ca2+-selective ionophore to a methyl methacrylate-decyl methacrylate copolymer matrix is also demonstrated. The ionophore-immobilized ISE exhibited no super-Nernstian response and yielded a detection limit of 40 ppt with an inner filling solution of 1 x 10-3 M KCl. Finally, a set of guidelines for experimental protocols leading to obtaining unbiased selectivity coefficients and producing ISEs for trace level analyses is given.},
doi = {10.1002/elan.200603741},
journal = {Electroanalysis, 19(2-3):144-154},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • A helical, filament-filled perfluorosulfonate membrane suppressor is utilized with high concentrations (0.1-1.0 M) of NaCl (sometimes additionally including {le} 1 mM NaOH) as regenerant to convert H{sup +} in the eluent (up to 10 mM HNO{sub 3}) to Na{sup +}. Elution of the weak acid analytes from the suppressor as their Na salts causes an increase in pH, monitored optically by the postsuppressor introduction of an acid-base indicator. Theoretical considerations involve the concentration and K{sub In} of the indicator and their effects on the background absorbance, dynamic range, and analyte response. Results are presented for 4-nitrophenol as indicator (base formmore » monitored at 400 nm) introduced permeatively through a membrane reactor. Analyte acids (pK{sub A} range {approx} 4.5-9.5) are selectively detected; maximum sensitivity is reached in the middle of this pK{sub A} range. The sensitivity for high pK{sub A} acids is poor, caused by a substochiometric suppressor reaction. Detectability for the moderate pK{sub A} acids is in the low micromolar (50 pmol) level, even with a pH 2 eluent. Capability of gradient elution and advantages of a pH 2 eluent are demonstrated. Operational considerations of the membrane suppressor, e.g., adjustment of the suppressor effluent pH by addition of NaOH to the regenerant and relative flow directionality, transmembrane loss of very weak acids in the uncharged form, etc., are discussed in terms of the results.« less
  • In cleanup operations and environmental surveillance efforts, a level of concern usually specifies a concentration limit for a particular radionuclide above which some action may be warranted. It is critical that the analytical method selected for measurements has a detection limit well below the action level. This is to guarantee that the technique used provides precise assessment at the level of concern. Sample analysis made with good precision is one of the major steps for obtaining quality data that allow a sound decision on whether the nuclide concentration is in compliance. This paper examines how the magnitude of the detectionmore » limit of an analysis method affects the precision of a measurement at the action level. With the established relationship, the detection limit that would achieve a pre-set precision for measurements at a level of concern can be quantitatively determined. The desired detection capability thus serves as a guide for selecting the appropriate measurement system. 10 refs., 2 figs.« less
  • The work of Fong and Alvarez is easily extended for counting when varied counting is not employed. It remains true in the general case that the precision of a counting method can be no less than about 30% at the LLD and so it is desirable to have decision levels at least several times larger than the LLD so that measurements have sufficient precision to make valid decisions.