Ionophore detection

The majoiity of the various analyte measurements made in automated clinical chemistry analyzers involve optical techniques such as absorbance, reflectance, luminescence, and turbidimetric and nephelometric detection means. Some of these ate illustrated in Figure 3. The measurement of electrolytes such as sodium and potassium have generally been accomphshed by flame photometry or ion-selective electrode sensors (qv). However, the development of chromogenic ionophores permits these measurements to be done by absorbance photometry also. 

Fig. 9. Representation of the use of the modification of an ionophore s potentiometric response in order to detect antibody binding. A constant ion activity (in this case K ) must be maintained in the sample solution...

A composite polymer membrane has also been used as an effective amperometric detector for ion exchange chromatography [42] and showed detection limits similar to those obtained with a conductivity detector. An advantage of the amperometric detector based on micro-ITIES over the conductometric detector is that selectively can be tailored by proper choice of the ionophore. For instance, the selectivity of the membrane toward ammonium in the presence of an excess of sodium could be substantially increased by introducing an ammonium-selective ionophore (such as valinomycin) in the gel membrane [42]. 

The SHG measuring system is shown in Fig. 1. The dependence of the SHG intensity, /(2a,), of a membrane incorporated with ionophore 2 (membrane 2, see Fig. 2) in contact with 0.18 M aqueous KCl on the power of the irradiation light beam, /(a,), is shown in Fig. 3. A linear relationship is seen between the input power, /(a,), and the square root of the SHG intensity, I(2co)- This relation satisfies the principle of SHG [see Eq. (1)] and thus confirms that the light detected in this system is indeed arising from SHG. 

We recently synthesized several reasonably surface-active crown-ether-based ionophores. This type of ionophore in fact gave Nernstian slopes for corresponding primary ions with its ionophore of one order or less concentrations than the lowest allowable concentrations for Nernstian slopes with conventional counterpart ionophores. Furthermore, the detection limit was relatively improved with increased offset potentials due to the efficient and increased primary ion uptake into the vicinity of the membrane interface by surfactant ionophores selectively located there. These results were again well explained by the derived model essentially based on the Gouy-Chapman theory. Just like other interfacial phenomena, the surface and bulk phase of the ionophore incorporated liquid membrane may naturally be speculated to be more or less different. The SHG results presented here is one of strong evidence indicating that this is in fact true rather than speculation. 

Potentiometry is the measurement of the potential at an electrode or membrane electrode, so the detector response is in units of volts. The potentio-metric response tends to be slow, so potentiometry is used infrequently in analysis.47 One example is the use of a polymeric membrane impregnated with ionophores for the selective detection of potassium, sodium, ammonium, and calcium 48 In process chromatography, potentiometry may be used to monitor selected ions or pH as these values change over the course of the gradient. 

Theoretical insight into the interfacial charge transfer at ITIES and detection mechanism of this type of sensor were considered [61-63], In case of ionophore assisted transport for a cation I the formation of ion-ionophore complexes in the organic (membrane) phase is expected, which can be described with the appropriate complex formation constant, /3ILnI. 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Besides, potentiometric sensors with ion-selective ionophores in modified poly(vinyl chloride) (PVC) have been used to detect analytes from human serum [128], Cellular respiration and acidification due to the activity of the cells has been measured with CMOS ISFETS [129], Some potentiometric methods employ gas-sensing electrodes for NH3 (for deaminase reactions) and C02 (for decarboxylase reactions). Ion-selective electrodes have also been used to quantitate penicillin, since the penicillinase reaction may be mediated with I or GST. 

It is to be hoped that this chapter will have given readers a better understanding of the basic principles of ion recognition detected by changes in photophysical properties of a fluorophore coupled to an ionophore. Examples have been chosen to illustrate the immense variety of structures that have been already designed and the inexhaustible possibilities of creating new systems. 

Alternatively, the use of ionophores may enhance the selectivity of the matrix. Several ionophores have been used for the detection of metals. Simon reported a neutral ionophore with a Nile Blue derivative and an azo compound for the detection ofpotassium.(86)Table 7.4 shows the selectivity coefficients of several ionophores. 

Several papers have been devoted to the detection of toxic metal ions. Some of the probes developed for toxic metals include a selective neutral ionophore for lead... 

M. Lerchi, E. Bakker, B. Rusterholz, and W. Simon, Lead-selective bulk optodes based on neutral ionophores with subnanomolar detection limits, Anal. Chem. 64, 1534-1540 (1992). 

Therefore, the ISE potential depends on the CO2 partial pressure with Nernstian slope. Contemporary microporous hydrophobic membranes permitted the construction of a number of gas probes, developed mainly by the Orion Research Company (for a survey see [143]. The most important among these sensors is the ammonia electrode, in which ammonia diffusing through the membrane affects the pH at a glass electrode. Other electrodes based on similar principles respond to SO2, HCN, H2S (with an internal S ISE), etc. The ammonia probe has a better detection limit than the ammonium ion ISE based on the non-actin ionophore. The response time of gas probes depends mostly on the rate of diffusion of the test gas through the microporous medium [77,143]. 

FSIS laboratories also use chemical techniques and instrumentation to identify select antibiotic residues. The tetracyclines of interest are identified by thin layer chromatography. Sulfonamides are detected and quantified by fluorescence thin lay chromatography and confirmed by gas chromatography/mass spectrometry. Amoxicillin and gentamycin are identified and/or quantified by high pressure liquid chromatography. Similar techniques are used to identify ionophores and other antimicrobials of interest. 

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

Monitoring for ionophore residues in eggs produced in North Ireland was also carried out in 1994 (19). Narasin, monensin, and salinomycin were detected in 1, 6, and 2 eggs, respectively, of the 161 eggs totally surveyed. In all cases, the concentrations detected were less than 2.5 ppb. 

Capillary SFC is particularly useful for compounds that are difficult to detect using LC and too unstable for GC. Nevertheless, at present there have not been enough applications to justify an investment (74, 75). In contrast, packed-column SFC has undergone a renaissance thanks to evaporative light-scattering detection (ELSD). Packed-column SFC-ELSD is a suitable method for analyzing various compounds, with or without chromophores, and with diverse polarities, as found in drug, steroid, and ionophore complex mixtures (76-79). 

Lower detection limits for Ca21. Cd24. Ag. K+, Na, I. and OO4 ion-selective electrodes were demonstrated when concentrations were reduced in the internal filling solutions 26 A future improvement will come when the ionophore is dissolved in a conductive polymer in direct electrical contact with a metal conductor.27 This electrode entirely omits the inner filling solution. 

CT Elliott, DG Kennedy, WJ McCaughey. Methods for the detection of polyether ionophore residues in poultry. Analyst 123 45R-56R, 1998. 

It has been widely accepted for many years that the LOD of an ISE in an unbuffered solution is at micromolar level. Interestingly, if a com-plexing agent is added into the sample and the concentration of the free primary ions is significantly lowered, the LOD is reduced sometimes to subnanomolar levels [35]. In addition, if halide ions are added to samples in which a silver-selective electrode is immersed, the electrode shows a decrease in potential indicating lowering of the activity of a silver at the sample/membrane phase boundary [36]. Moreover, ionophore-based optodes showed picomolar detection limits [37], even... 

The ultimate detection limit as described by the Nikolskii-Eisenman equation (Eq. (2.2)) is defined by the displacement of a fraction of the primary ions with interfering ions at the sample/membrane phase boundary, which amounts to 50% in the case of monovalent primary and interfering ions. Equation (2.8) (the part in parenthesis) implies two obvious solutions (1) avoiding the bias introduced by the inner solution and (2) reducing the amount of the primary ion-ionophore complex at the sample/membrane phase boundary and hence reducing the absolute amount of released primary ions due to the ion exchange. 

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