Fixed interference method selectivity

Fig. 18a. 14. Illustration of selectivity determination by the fixed interference method (FIM). aj(BG) is the activity of the interfering ion J in the background and ai(DL) is the detection limit of the primary ion in the solution containing J as background.

It is important for the analytical chemist to realize the selectivity coefficient of a particular electrode. Various methods have been suggested for determining the selectivity coefficient, including the fixed-interference method, separate solution method, and the fixed primary ion method (10,11). The most popular fixed interference method involves two solutions, one containing a... 

Selectivity coefficient — Kvf], is a measure of the contribution of an interfering ion B to the potential of an -> ion-selective electrode in a mixed solution containing the primary ion A and an interfering ion B. It is defined by the modified - Nikolskij-Eisenman equation. The smaller the value of JC °g, the better the selectivity of the electrode with respect to the primary ion A. Selectivity coefficients can be evaluated by measuring the response of an ion-selective electrode in mixed solutions of A and B (fixed interference method) or in separate solutions of A and B (separate solution method). 

The selectivity of ISEs is evaluated by calculating selectivity coefficients. Over the years, several different methods have been used to obtain these coefficients, the most common being the separate solution method, the fixed interference method [15] and the matched potential method [16]. In the separate solution method, the potential response is determined in each of the two separate solutions, one containing only the primary ion at an activity ai and the other containing only the interfering ion at an activity Uj = a. The selectivity coefficient or the relative response of the electrode to the two ions I and J, K s is calculated from the following equation ... 

The fixed interference method (FIM). The potential of an ion-selective electrode is measured in solutions of constant activity of interfering ion (a,-) and varying activity of the primary ion (a,). The selectivity coefficient, K ", is calculated from the relevant calibration graph plotted for the ion of interest, i. The intersection of the extrapolated linear portions of the response curve indicates the value of a, which is used to calculate Kp° from the Nikolsky-Eisenman equation ... 

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). 

Matched Potential Method. This is a strictly empirical variation of the fixed interference method. Numerical values of selectivity coefficients may vary with solution conditions, for example, relative concentrations of the ions. The matched potential method allows the analyst to obtain an empirical value under the experimental conditions of the analysis. Suppose you wish to know the relative interference of sodium ion in blood in the measurement of lithium ion (ca. 1 mM) in the serum of a bipolar (manic depressive) patient taking Li2C03. A reference... 

A valinomycin-based potassium ion-selective electrode is evaluated for sodium interference using the fixed interference method. A potassium calibration curve is prepared in the presence of 140 mM sodium. The straight line obtained from extrapolation of the linear portion deviates from the experimental curve by 17.4 mV at a potassium concentration corresponding to 1.5 X 10 M. If the linear slope is 57.8 mV per decade, what is ATnok for the electrode ... 

The selectivity coefficient, defines the ability of an ISE to distinguish a particular ion from others (5). According to lUPAC, can be evaluated in mixed in solutions of primary and interfering ion (Fixed Interference Method), or separate solutions (Separate Solution Method and Matched Potential Method). The smaller the value of the greater the electrode s preference for the principal ion. 

The selectivity of the molecularly-imprinted NO3 ISE is much improved versus those based on the quatemaiy ammonium salts. For example, the log for perchlorate in this case is -1.24 relative to nitrate (fixed interference method). Thiocyanate, which was also a major interferent for the quatemaiy ammonium salt-based ISE, with log of 1.7 (24 is better discriminated against by the molecularly-imprinted ISE. For this nitrate-selective ISE the most important interference is chloride. 

The selectivity coefficient can also be determined by measuring the cell potentials at different analyte concentrations in the presence of a fixed concentration of an interfering ion (Figure 7.2B the fixed interference method, FIM). In this case, the Nemstian response at high analyte activities is represented by equation (7.1.1). The constant potential at low analyte activities is determined by the interfering ion, whereas the Nemstian response to the interfering ion at the fixed activity should be confirmed in additional experiments (39). Linear extrapolations of these responses at high and low activities result in a cross point, which is defined as a detection limit. The selectivity coefficient can be obtained as... 

Recently, Shinkai et al [20] studied PVC membrane electrodes based on the calix[4]arene derivative and reported that the p-t rt-octyl-calix[4]aryl tetraethyl ester-FPNPE system electrode allowed high Na" selectivity. The Na selectivity coefficient being 3.1 [20], though its value was not evaluated by a fixed interference method (FIM). Its excellent Na selectivity compared with those of our similar derivative, p-rerr-octylcalix[4]aryl tetradecyl ester [6,21], is probably derived from the introduction of shorter alkyl groups such as the ethyl ester and tert-oct group into the upper rim moiety to produce lipophilicity, as already mentioned. 

Experimental protocols for the determination of selectivity coefficients have been laid out by lUPAC. One distinguishes two main methods, the separate solutions method (SSM) and the fixed interference method (ElM). ... 

With the fixed interference method, a calibration curve for the primary ion in a fixed interfering ion background is determined. The lower detection limit is then determined and related to the selectivity coefficient as follows ... 

One salient advantage of fixed-time kinetic methods is significantly increased selectivity. Tolerated concentrations of interfering metal ions are often reported to be one to two orders of magnitude higher than in equilibrium methods. The increased tolerance to interferences of kinetic methods may be the result of the following facts ... 

In addition to external standard and standard addition, the last principal mode of calibration is called internal standard. This mode of calibration should be used when there exists variability in sample injection volume, when there is concern about the lack of instrument stability, and when there is unavoidable sample loss. Instrumental response then becomes related to the ratio of the unknown analyte X to that for the internal standard S instead of related only to the unknown analyte. If some X is lost, one can assume that some S would be lost as well. This preserves the ratio PC]/[S]. For extraction methods, the internal standard (IS) is added to the final extract just prior to adjusting the final volume. Selecting a suitable IS is not trivial. It should possess similar physieal and chemical properties to the analyte of interest and not interfere with the elution of any of the analytes that need to be identified and quantitated. The IS should be within the same coneentration range as for the ealibration standards and at a fixed eoncen-tration. The following is a ealibration curve for the IS mode ... 

Figure 2.1 also demonstrates a common method of obtaining the selectivity coefficient, whereby the emf response is measured for solutions containing a fixed amount of interferent with varying activities of the primary ion. A, for which the electrode is designed. This is known as the mixed solution method with fixed amount of interferent and calculated from... 

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