Derivatisation techniques

Easy post-chromatographic visualisation/derivatisation techniques... 

Figure 2.1.1 shows the most common derivatisation methods for anionic surfactants reported in the literature [1]. The first method of LAS determination by GC consisted of a microdesulfonation procedure in which LASs were desulfonated in boiling phosphoric acid at high temperature [2-4] and the corresponding alkylbenzenes analysed. The microdesulfonation method was further improved by introducing additional concentration and clean-up steps [5—11], which allowed the determination of LAS in influent, effluent and river water samples at low qg L-1 levels [7,8] and sediment and sludge samples [8] at pg g-1. In addition to the desulfonation procedure, several derivatisation techniques have been used to make LAS analysis amenable to GC. 

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. 

In both these areas, chemical derivatisation has traditionally played a role and with the advent of gas chromatography an even more important role. The reasons for preparing a derivative suitable for GC analysis are many and varied and have been discussed thoroughly in a number of books and reviews (l- >). For convenience they are summarised in Table I. As can be seen, two different types of chemical derivatisation techniques are mentioned under Item 4 of Criteria, There is the chemical derivatisation of a pesticide as a pre-requisite of the method of analysis, e.g. esterification of the chlorophenoxy acids, as well as derivatisation as a method for confirmation of identity. The former must meet all the requirements associated with a practical, viable analytical procedure while for the latter the emphasis is on speed, ease of operation and reproducibility. 

The increased use of derivatisation reactions is evident from the gradual increase in the number of publications dealing with the subject. Figure 1 shows the yearly variation in numbers of publications dealing with derivatisation in pesticide analysis over the period 1963 to 1978. While the interest in derivatisation techniques in organophosphorus insecticide analysis has remained fairly constant and a low level of activity, the OC insecticides underwent an increased period of attention from 1968-1972 which has stablized over the last few years. It is in the insecticidal carbamate and herbicide areas that an overall steady increase in the use of derivatisation reactions for quantitative and con-... 

Since well-established chemical derivatisation techniques already exist for the majority of the organochlorine insecticides (5, 1) it is not surprising that recent activity in this area has centred round compounds such as Kepone, Mirex, HCB, the PCBs, etc. which co-interfere in both the identification and quantitation of pesticide residues. 

A pre-column derivatisation technique using dansyl hydrazine is used for the quantitative analyses of glucose in blood or vitreous humour in cases of people suffering from hypoglycaemia and hyperglycaemia. 

Silylation is the most widely used derivatisation technique. It involves the replacement of an acidic hydrogen on the analyte molecule with an alkylsilyl group, for example, -SiMc3. The derivatives are generally less polar, more volatile and more thermally stable. Two examples are shown below ... 

Post-column derivatisation techniques, though imposing restrictions on the reagents and solvents that can be used has the advantages that it may be readily incorporated as part of a fully automated system and thus will... 

Until recently, gas-liquid chromatography (GLC) was the most popular method for the identification and quantitation of fatty acids however, with the emergence of new derivatisation techniques and improvements in detector technology HPLC offers a viable alternative. The most frequently used HPLC mode for the analysis of fatty acids is reversed phase either with or without pre-column derivatisa-... 

Several different types of detectors have been used for the quantitation of the biogenic amines however, currently the most popular techniques are electrochemical detection and fluorescent detection, usually coupled with pre- or post-column derivatisation techniques. The specific details of these detection techniques are discussed elsewhere (Chapter 3) and in the present section only the basic principles as applied to the biogenic amines will be reviewed together with a comparison between the two techniques in terms of their respective advantages and disadvantages. 

The importance of strong specific interactions in polymer adhesion (eg., covalent bonding, acid-base interaction) is well-known. However, these interactions are difficult to study directly, particularly for multifunctional surfaces. Derivatisation techniques have been developed which allow X-ray photoelectron spectroscopy (ESCA) to quantify functional group populations and to assess the success of specific modification techniques. Derivatised surfaces can then be used to study the role of specific interactions. 

The specific reaction of SO2 with hydroperoxides has previously been described (15) and this is used, with IR, as a diagnostic tool for bulk R-OOH assay. Being a gas-phase reaction it is admirably suited to the ESCA derivatisation technique. 

The derivatisation technique as developed for the extension of ESCA analytical capacity has the added bonus that the same surfaces can be used to study specific interactions in adhesion in the spirit of Owens approach (13), but with a built-in monitor of surface composition. This overcomes the possibility that artefacts, such as contamination, loss of surface material or solvent-induced re-orientation effects could be responsible for the observed adhesion changes - artefacts that Owens experiments could not rule out. 

Challinor [611,620] has reported the use of pyrolysis derivatisation techniques, simultaneous pyrolysis methylation (SPM), cq. THM. In Py-THM-GC the sample (about 5 /xg) is typically placed in the hollow of a flattened Curie-point pyrolysis wire with approximately 0.5 /u-L tetramethyl ammonium hydroxide (TMAH) (25 wt.% aqueous solution) or tetramethylsulfonium hydroxide (TMSH). The prepared wire is then immediately located in the py-rolyser without allowing aqueous TMAH to evaporate and pyrolysis is carried out at the predetermined temperature. Special injectors for chemolysis (e.g. PTV injector) allow THM also for furnace PyGC experiments. Moldoveanu [499] has listed other common derivatisations utilised in GC analysis. 

In traditional electrophoresis, electrically charged analytes move in a conductive liquid medium under the influence of an electric field. Introduced in the 1960s, the technique of CE was designed to separate species based on their size to charge ratio in the interior of a small capillary filled with an electrolyte. The technique uses fused silica capillaries under an electrical field, and separation occurs either solely on the basis of molecular weight and charge, electrophoretic mobility, or associated with differential solubilisation in surfactant micelles. Associated with preconcentration, pre-derivatisation or incapillary derivatisation techniques in order to overcome the problem of inherent low sensitivity, this method has shown potential for excellent separation efficiency (Asthana et al. 2000 Cavallaro et al. 1995 Sun et al. 2009). 

One of the first proposals for the determination of available lysine was the derivatisation of the critical e-amino group with fiuoro dinitrobenzene (5, 4, 5, 6) which led to a worldwide application of this techniques. Later on many other derivatisation techniques for the determination of the "reactive lysine" were tested, proposed and applied (7,8),... 

The majority of species present in aqueous or solid samples will be ionic, and must be derivatised to non-polar, volatile compounds, accessible for GC analysis. Extraction and derivatisation can be accomplished simultaneously or sequentially, as described in the following sections. The derivatisation techniques most often applied are hydride generation with NaBHa, ethylation with NaBEt4 or Grignard alkylation, and were recently reviewed by Morabito et a/. ... 

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