Ion exclusion techniques

Despite its potential importance, formic acid has proven difficult to quantify at submicromolar levels in non-saline water samples. Formidable analytical difficulties are associated with its detection in highly saline samples. Ion exclusion, anion exchange, and reversed-phase high performance liquid chromatography techniques based on the direct detection of formic acid in aqueous samples are prone to interferences (especially from inorganic salts) that ultimately limit the sensitivity of these methods. 

Klane and Blum [69] showed that inductively coupled plasma spectrometry was able to determine below 1000 ng/1 of arsenic in seawater. Ion exclusion chromatography coupled with inductively coupled plasma mass spectrometry has been used to determine several arsenic species in seawater [ 947 ]. Down to 3 ng/1 arsenic can be determined using hydride generation prior to this technique. 

Various miscellaneous column chromatographic techniques including ion pair, micelle, ion exclusion, size exclusion and gel permeation are discussed in Chapters 7-11. All of these techniques have found limited selected applications in the determination of organic compounds, cations and anions. 

The most important pentose is xylose which can be produced from hardwoods by mild acid hydrolysis. Reduction of xylose gives xylitol, an interesting sweetener because of its ability to prevent dental caries. Industrial production of xylitol from birch wood hydrolyzates started in the 1970s in Finland. An interesting process was developed in which an ion exclusion separation technique is applied for purification and separation of xylose and xylitol from other impurities. Among the corresponding reduction products of hexoses, mannitol, which is also a natural product, has found some use. It can be separated from other alditols by crystallization. Under more drastic... 

Nakazato, T., Tao, H., Taniguchi, T., Isshiki, K. Determination of arsenite, arsenate, and monomethylarsonic acid in seawater by ion-exclusion chromatography combined with inductively coupled plasma mass spectrometry using reaction cell and hydride generation techniques. Talanta 58, 121-132 (2002)... 

Organic anions. Carboxylic ions such as formate, acetate and propionate ions, etc. which would normally be difficult to analyse using conventional analytical techniques can be analysed successfully using ion chromatographic exclusion techniques. Using conductimetric detection, sub-ppm detection levels can be achieved. A major advantage of this type of analysis is that a complete series of anions can be analysed simultaneously. 

Ion chromatography is used for the separation of ionic solutes such as inorganic anions and cations, low molecular-mass water-soluble organic acids and bases as well as ionic chelates and organometallic compoimds. The separation can be based on ion-exchange, ion-pair and/or ion-exclusion effects. Special detection techniques like ion-suppressed conductivity detection or indirect UV detection have to be used because most analytes are transparent to conventional UV detection... 

Various detection systems can be used in ion-exclusion chromatography, among them ultraviolet (UV)/vis spectrophotometry, conductivity, electrochemistry, fiuorometry, refractive index (RI) measurement, are the most common techniques. Additionally, combined detection systems (e.g., UV/amperometry, UV/RI) may be used, leading to enhanced selectivity. 

At the end of the 1970s, ion chromatographic techniques were used to analyze organic ions for the first time. The requirement for a quantitative analysis of organic acids brought about an ion chromatographic method based on the ion-exclusion process, which was first described by Wheaton and Bauman [16] in 1953. 

A similar example represents the determination of borate that is usually analyzed with ion-exclusion chromatography. However, when borate is converted into tetrafluoro-borate by reacting with hydrofluoric acid [107], upon application of an anion exchanger indirect borate determination together with other inorganic anions becomes possible. Tetrafluoroborate is a polarizable anion therefore, when using the suppressor technique some p-cyanophenol is added to the carbonate/bicarbonate eluent to improve the peak shape of the ion that elutes after sulfate. 

Ion-exclusion chromatography is a useful technique for separation of organic and inorganic weak-acids (Chinnici et al., 2005 Morales, Gonzalez, and Troncoso, 1998). It is especially valuable for separating neutral and weakly acidic or basic substances from ionic compounds based on an ion-exclusion mechanism, as opposed to ion-exchange (Morales, Gonzalez, and Troncoso, 1998). 

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