Ionic derivatisation

Post-column on-line derivatisation is carried out in a special reactor situated between the column and detector. A feature of this technique is that the derivatisation reaction need not go to completion provided it can be made reproducible. The reaction, however, needs to be fairly rapid at moderate temperatures and there should be no detector response to any excess reagent present. Clearly an advantage of post-column derivatisation is that ideally the separation and detection processes can be optimised separately. A problem which may arise, however, is that the most suitable eluant for the chromatographic separation rarely provides an ideal reaction medium for derivatisation this is particularly true for electrochemical detectors which operate correctly only within a limited range of pH, ionic strength and aqueous solvent composition. 

Speciation involves a number of discrete analytical steps comprising the extraction (isolation) of the analytes from a solid sample, preconcentration (to gain sensitivity), and eventually derivatisation (e.g. for ionic compounds), separation and detection. Various problems can occur in any of these steps. The entire analytical procedure should be carefully controlled in such a way that decay of unstable species does not occur. For speciation analysis, there is the risk that the chemical species can convert so that a false distribution is determined. In general, the accuracy of the determinations and the trace-ability of the overall analytical process are insufficiently ensured [539]. 

Another way for covalent immobilisation is to synthesise indicator chemistry with polymerizable entities such as methacrylate groups (Figure 4). These groups can then be copolymerized with monomers such as hydrophobic methyl methacrylate or hydrophilic acryl amide to give sensor copolymers. In order to obtain self-plasticized materials, methacrylate monomers with long alkyl chains (hexyl or dodecyl methacrylate) can be used. Thus, sensor copolymers are obtained which have a Tg below room temperature. Similarly, ionophores and ionic additives (quaternary ammonium ions and borates) can be derivatised to give methacrylate derivatives. 

High polarity is one of the reasons why both the ionic and amphoteric surfactants, and especially their metabolites, are difficult to detect. This property, however, is important for the application tasks of surface-active compounds, but is also the reason for their high water solubility. Due to this fact, their extraction and concentration from the water phase, which can be carried out in a number of very different ways, is not always straightforward. Furthermore, they are often not volatile without decomposition, which thus prevents application of gas chromatographic (GC) separation techniques combined with appropriate detection. This very effective separation method in environmental analysis is thus applicable only for short-chain surfactants and their metabolites following derivatisation of the various polar groups in order to improve their volatility. 

Gas chromatography (GC) has developed into the most powerful and versatile analytical separation method for organic compounds nowadays. A large number of applications for the analysis of surfactants have emerged since the early 1960s when the first GC papers on separation of non-ionics were published. The only major drawback for application of GC to surfactants is their lack of volatility. This can be easily overcome by chemical modification (derivatisation), examples of which will be discussed extensively in the following paragraphs. This chapter focuses on surfactant types, and in addition discusses some structural aspects of alkylphenol ethoxylates (APEOs) that are important for, as well as illustrative of, aspects of separation and identification that are linked to the complexity of the mixtures of surfactants that are involved. 

Numerous applications have been shown to exist that overcome the general problems of lack of volatility and instability at higher temperatures that principally hamper direct analysis of surfactants by GC methods. Thus, a whole suite of derivatisation techniques are available for the gas chromatographist to successfully determine anionic, non-ionic and cationic surfactants in the environment. This enables the analyst to combine the high-resolution chromatography that capillary GC offers with sophisticated detection methods such as mass spectrometry. In particular, for the further elucidation of the complex mixtures, which is typical for the composition of many of the commercial surfactant formulations, the high resolving power of GC will be necessary. 

As mentioned above, the most commonly used method for the analysis of cationic surfactants has been HPLC coupled with conductometric, UV, or fluorescence detectors, the latter typically utilizing post-column ion-pair formation for enhanced sensitivity. Analysis by GC is only possible for cationic compounds after a derivatisation step [33] because of the ionic character of this compound. However, structural information might be lost. 

Blais et al. [12] has described a method using HPLC coupled with AAS for the determination of ionic alkyl lead compounds in soils. They demonstrated that previously published methods gave poor recoveries of lead and the formation of artifacts during the isolation and derivatisation procedures. An alternative procedure is described involving a series of selective extractions... 

It is also relatively easy to functionalise imidazolium cations at the 2-position.[88] For example, the phosphine derivatised salts shown in Figure 2.7 illustrate such a substitution pattern and they are easily prepare by virtue of the acidity of the 2-proton.[74] Substitution of the 2-proton tends to yield relatively high melting salts instead of ionic liquids. Despite this limitation the imidazolium-phosphine compounds are good ligands for catalysis improving the immobilisation potential of complexes dissolved in ionic liquids. 

Ionic liquids have also been used as solvents in other cellulose derivatisation reactions including etherification [166], carboxymethylation [160], phthalation [167, 168], sulfation [169], sulfonation [169], carbanilation [159, 162], silylation [170], succinylation [171], tritylation [172] and tosylation [173],... 

It has recently been found that salts which melt at or near room temperature, so-called ionic liquids, can form physical solutions of cellulose and starch. l-A -Butyl-3-methylimidazolium chloride dissolved plant and bacterial cellulose with no apparent loss of DP, and cellulose in the resulting solutions was much more readily derivatised to various esters than in the solid (Figure 4.34d). The same applied to l-A -allyl-3-methylimidazolium chloride in both solvents, NMR indicated that the cellulose chains were disordered in solution.Studies... 

The application of GC-MS to PAH-analysis will allow considerable simplification of the work-up procedure. As far as solubility and dynamic range considerations will permit, the major PAH in airborne particulate matter samples can be accurately measured in the electron impact single or multiple ion monitoring mode, using the molecular ions of the respective PAH as specific ions, in the presence of much larger amounts of aliphatic hydrocarbons and carboxylic acids. Polar and ionic material is first removed from the combined benzene-methanol extract by liquid-liquid partition in water-diethylether. After drying, the addition of diazomethane results in derivatisation of the acidic components and the sample can be injected onto the column (Van Vaeck and Van Cauwenberghe (30)). 

Due to the ionic nature of chlormequat, ion exchange chromatography using an SCX column has been used successfully to analyse this compound. No chromophores are present in the molecule therefore, a UV/Vis detector would not be suitable. Derivatising agents have been used to form colou-rimetric complexes, which can be measured using visible spectrophotometry. Mass spectrometry has also been used successfully to determine this... 

Optimisation of an HPLC system for a specific analysis can be a complex task and many additional factors must be considered when using EC detection. These can be divided into system dependent variables and analyte dependent variables. System dependent variables include the nature of the working, reference and auxiliary electrodes and associated electronics, and the applied potential as discussed in Chapters 2 and 3. In addition, there may be influences from the eluent (solvent composition, pH, ionic strength), temperature and flow-rate. Analyte-dependent variables are specifically the presence of one or more electroactive moieties or functional groups on the compound(s) of interest. With compounds which are not inherently electroactive it may be possible to derivatise them, or to chemically modify them in some other way, to impart electrochemical properties. 

Polyion complexes between acid derivatised polythio-phene (poly(thiophene-3-acetic acid)), PTAA (Figure 14.39), and a stable surface active cation have been used to construct conductive LB films. In this case, the formation of a polyion complex renders PTAA surface active and provides additional control over the molecular orientation of the polymer within the monolayer. The monolayer assembly thus obtained has a structure composed of a well ordered condensed monolayer of the surface active molecule onto which is a single monolayer of PTAA ionically bound. For example, the sodium salt of PTAA can be readily absorbed onto a monolayer of dimethyldioctadecylam-monium bromide. The monolayer assembly of the polyion complex was successfully transferred into LB film in a Z-type manner [294]. 

For speciation analysis using GC coupled to ICP-MS, it is imperative that the species in question is volatile or can be volatilised without degradation or destruction. Only a few species are directly accessible for GC-ICP-MS analysis, e.g. peralkylated organometallic compounds, element hydrides or carbonyls. Many of the species of interest are partly alkylated molecules, present in ionic form either in the water phase or in soU, sediments and biological materials. These compounds must be derivatised prior to GC-ICP-MS analysis, which usually necessitates an extraction step from the matrix, with subsequent derivalisation to the volatile compound and a final extraction into an adequate medium for GC injection. 

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