Detection of anions

S04 and (C6H5)4B . This simple, rapid and versatile methodology is based on the different 

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Direct determination of surfactants in complex matrices can also be carried out using ion-selective electrodes. Depending on the membranes and additives used, the detergent electrodes are optimized for the detection of anionic surfactants [81], cationic surfactants [82], and even nonionic surfactants [83]. The devices are sensitive to the respective group of surfactants but normally do not exhibit sufficient stability and reproducibility for their use in household appliances. With further optimization of membrane materials, plasticizers and measurement technology, surfactant-selective electrodes offer high potential for future applications. 

Liquid chromatography coupled to mass spectrometry (LC-MS) is the most usual technique applied for the detection of anionic [126-128], nonionic... 

Rabaioli G., Taglietti A. (2001) Fluorometric Detection of Anion Activity and Temperature Changes, in Valeur B. and Brochon J. C. (Eds), New Trends in Fluorescence Spectroscopy. Applications to Chemical and Life Sciences, Springer-Verlag, Berlin, pp. 209-27. 

The development of fluorescent probes for anion recognition has been very limited so far in comparison with those for cations. Most of the presently available methods of detection of anions based on fluorescence involve quenching, redox reactions, substitution reactions, ternary complex formation(15) and thus cannot be considered as recognition methods. For instance, the fluorescent sensors that are used for the determination of chloride anions in living cells are based on collisional quenching of a dye by halide ions 6-methoxy-iV-(sulfopropyl)quinoliniuni and... 

Johns, C., Macka, M., and Haddad, P. R. (2003). Enhancement of detection sensitivity for indirect photometric detection of anions and cations in capillary electrophoresis. Electrophoresis 24, 2150-2167. 

Nairn, A., and Pretsch, E. (1994). Potentiometric detection of anions separated by capillary electrophoresis using an ion-selective microelectrode. /. Chromatogr. A 676, 437-442. 

Simultaneous separation and detection of anions and cations in ion chromatography... 

Fig. 1. Simultaneous separation and detection of anions and cations on a latex agglomerate column. Column Dionex HPIC-CS5 cation exchange column (250X2 mm) with precolumn HPIC-CG5 (50 X 4 mm) eluent 0.5 mM copper sulfate, pH 5. 62 flow rate 0.5 ml/min sample volume 20 gl containing 0.1 m M of each ion detection two potentiomet-ric detectors equipped with different ion-selective electrodes in series. Peaks (1) chloroacetate, (2) chloride, (3) nitrite, (4) benzoate, (5) cyanate, (6) bromide, (7) nitrate, (8) sodium, (9) ammonium, (10) potassium, (11) rubidium, (12) cesium, (13) thallium. Reprinted with permission from [10].

J. Yan, X. Yang and E. Wang, Electrochemical detection of anions on an electrophoresis microchip with integrated silver electrode, Electroanalysis, 17 (2005) 1222-1226. 

In considering the differential or selective binding of one anion or another, the intrinsic properties of anions mean that we are not on a level playing field it is considerably easier to bind some anions than others. In general, in the absence of specific chemical recognition between anion and host, anion binding selectivity, particularly in solvent extraction experiments or in the detection of anions by membrane-based ion selective electrodes, follows the order of anion hydrophobicity. This order is termed the Hofmeister series, or lyotropic series and was first outlined in 1888 from experiments based on the... 

The detection of anions (usually in aqueous solution) is a vital part of our maintenance and understanding of the environment. Many industrial and agricultural processes can lead to the release of anions to the environment and if unchecked these can have devastating effects. Much research is being focussed towards finding inexpensive, reliable and simple ways of detecting anions in solution, and discussion of some of these approaches form the bulk of this review. 

The future of anion sensors is a very promising one. Greater interest and legal requirements for environmental, food and water monitoring will require the detection of anionic species, often at lower and lower concentrations. Technical problems, however, need to be overcome. These include some of the considerations below ... 

The detection of anions such as HPO - in water is a challenging task due to the competing solvation effect between water and anions. Han and Kim [63] have recently reported a colorimetric sensor that can detect HPO4" in aqueous solution at neutral pH. The dinuclear Zn(II) complex of 2,6-bis [bis(2-pyridylmethyl)amino]methyl -4-methylphenol (H-bpmp) 80 was synthesized. 

Mobile phases useful for suppressed conductivity detection of anions include sodium hydroxide, potassium hydroxide, and the sodium and potassium salts of weak acids such as boric acid. In nonsuppressed conductivity detection, the ionic components of the mobile phase are chosen so that their conductivities are as different from the conductivity of the analyte as possible. Large ions with poor mobility are often chosen, and borate-gluconate is popular. For cations, dilute solutions of a strong acid are often used for nonsuppressed conductivity detection. For more information on the application of electrochemical detection to inorganic analysis, see Ion Chromatography Principles and Applications by Haddad and Jackson,17 which provides a comprehensive listing of the sample types, analytes, sample pretreatments, columns, and mobile phases that have been used with electrochemical detection. 

IV.l SCHEME OF CLASSIFICATION The methods available for the detection of anions are not as systematic as those which have been described in the previous chapter for cations. No really satisfactory scheme has yet been proposed which permits of the separation of the common anions into major groups, and the subsequent unequivocal separation of each group into its independent constituents. It must, however, be mentioned that it is possible to separate the anions into major groups dependent upon the solubilities of their silver salts, of their calcium or barium salts, and of their zinc salts these however, can only be regarded as useful in giving an indication of the limitations of the method and for the confirmation of the results obtained by the simpler procedures to be described below. 

Warnkea, M.M. et al. The Evaluation and Comparison of Trigonal and Linear Tricationic Ion-Pairing Reagents for the Detection of Anions in Positive Mode ESI-MS J. Am. Soc. Mass Spectrom. 2009, 20, 529-538. 

Capillary electrophoresis Capillary electrophoresis (CE) is used to analyze sodium, potassium, calcium, and magnesium in water samples. The detection is conducted by reverse absorbance measurements. Sufficient separation of the four cations is established with an electrolyte solution of 5 mM imidazole/6.5 mM a-hydroxyisobutyric acid/2 mM 18-crown-6 ether of pEI 4.1 [42]. CE with a contactless conductometric detector is used to determine small anions and cations in water samples from different sources. 2-(N-Morpholino)ethanesulfonic acid/histidine-based (Mes/Elis) electrolytes are used for direct conductivity detection of anions and cations, while ammonium acetate is used for indirect conductivity determination of alkylammonium salts. Eor the simultaneous separation procedure, involving dual-opposite end injection, an electrolyte consisting of 20 mM Mes/EIis, 1.5 mM 18-crown-6 and 20 mM cetyltri-methylammonium bromide provides baseline separation of 13 anions and cations in less than 6 min [43]. Also CE with a capacitively coupled... 

Fig. 2 Conductivity detection of anions in nonsuppressed (single column) ion chromatography using an eluent of (a) low background conductance (direct detection) and (b) high background conductance (indirect detection). The direction of the arrow indicates the increase of conductivity.

Figure 13.6 Indirect detection of anions and cations. Stationary phase //-Bondapak-Phenyl. Mobile phase naphthalene-2-sulfonate 4 x 10 M in 0.05 M phosphoric acid. Sample (1) butyl sulfate, (2) pentyl amine, (3) hexane sulfonate, (4) hep-tylamine (5) octane sulfonate (6) octyl sulfate (S) system peak. Reproduced with permission from J. Crommen, G. Schill, D. West-erlund and L. Hackzell, Chromatographia, 24 (1987) 252 (Fig. 1).

Direct detection of anions is also possible, providing a detector is available that responds to some property of the sample ions. For example, anions that absorb in the UV spectral region can be detected spectrophotometrically. In this case, an eluent anion is selected that does not absorb (or absorbs very little). 

G. P. Ayers and R. W. Gillett, Sensitive detection of anions in ion chromatography using UV detection at wavelengths less than 200 nm,/ Chromatogr., 284,510,1984. 

Potentiometric detection of anions is feasible when an electrode is available that responds quickly, reversibly and reproducibly to the concentration (or more precisely to the activity) of sample ions. It is often possible to detect a given ion or class of ions with excellent selectivity. For example, solid-state or crystalline ion selective electrodes have been used in IC to detect halide anions. The fluoride ion-selcclivc electrode is particularly selective [20,21]. A copper wire electrode has been used to detect anions such as iodate, bromide and oxalate [22]. 

K. Tanaka, K. Ohta, J. S. Fritz, S. Matsushita and A. Miyanaga, Simultaneous ion-exclusion chromatography-cation exchange chromatography with conductimetric detection of anions and cations in acid rain waters,/ Chromatogr. A, 671,239,1994. 

BGE gives a peak in the direction of reduced absorbance when a sample ion passes through the detector. The absorbing reagent, which is sometimes called a visualization reagent, should have a mobility that matches those of the sample ions as closely as possible. Chromate is often used for the indirect detection of anions and a proton-ated amine cation, such as benzylamine, for detection of cations. 

In addition to the tripodal structural motifs mentioned above, one obvious choice to imbue a ligand with threefold symmetry is to use 1,3,5-trisubstituted benzene as a base. The formation of the alternating conformation for substituents on a hexasubstituted benzene is a well-known phenomenon an early example where this was used in a supramolecular context was by Harshom and Steel in 1996 [4], Subsequently the principle has been used widely particularly in the detection of anions [5-8]. The parent 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene is fairly complex to prepare [9], however, the corresponding mesitylene... 

Figure 5.20. Schematic representation of the operation of a membrane suppressor for suppressed conductivity detection of anions and cations. (From ref. [168] Elsevier).

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