Physicochemical techniques for

The continuous methods combine sample collection and the measurement technique in one automated process. The measurement methods used for continuous analyzers include conductometric, colorimetric, coulometric, and amperometric techniques for the determination of SO2 collected in a liquid medium (7). Other continuous methods utilize physicochemical techniques for detection of SO2 in a gas stream. These include flame photometric detection (described earlier) and fluorescence spectroscopy (8). Instruments based on all of these principles are available which meet standard performance specifications. 

TABLE 2 Physicochemical Techniques for the Characterization of Lipid Bilayers... 

In spite of the development of physicochemical techniques for surface analysis, spectroscopic methods applicable to the study of bonding between adsorbed metal ion species and substrate are limited, especially those applicable to in situ measurement at interfaces between solid and aqueous phases (1,2). In previous papers, we showed that emission Mossbauer measurement is useful in clarifying the chemical bonding environment of dilute metal ions adsorbed on magnetic metal oxide surfaces (3,1 ) ... 

Florence, T.M., Lumsden, B.G. and Fardy, 3.3., 1983. Evaluation of some physicochemical techniques for the determination of the fraction of dissolved copper toxic to the marine diatom Nitzschia closterium. Anal. Chim. Acta, 151 281-295. 

Dickinson, ed. New Physicochemical Techniques for the Characterization of Complex Food Systems. Chapman and Hall, London, 1995. 

It can be seen from these examples that cells and tissues are very sensitive to the composition of the materials surface. Several physicochemical techniques for analysing the composition of materials surface have been described. However, attention is drawn to the fact that the surface analysed by most of these methods is not the surface analysed by living cells, as cells recognise only the outermost layer of a hydrated material. Analysing this ultimate hydrated layer by a physicochemical method is a real challenge. Indeed, the most surface-sensitive methods such as electron spectroscopy for chemical analysis (ESCA also known as X-ray photoelectron spectroscopy, XPS) and secondary-ion mass spectrometry (SIMS) can analyse respectively a few layers at once or one layer after the other, but in strictly dry conditions. Performing an ESCA analysis at a very low temperature in order to keep water frozen has been described, but this is currently far from a routine method. Conversely, analysis of hydrated surfaces by ATR-IR is usual, but this method determines the composition of many layers in addition to the ultimate layer, as it analyses a depth of more than 1 pm. 

Chemical remediation refers to the application of various minerals or chemicals to adsorb, bind, precipitate or co-precipitate trace elements and heavy metals in soils and waters thereby reducing their bioavailability, toxicity, and mobility. In situ immobilization refers to the treatment of contaminants in place without having to excavate the soils or waste, often resulting in substantial cost savings. However, in situ immobilization or extraction by these physicochemical techniques can be expensive and are often only appropriate for small areas where rapid and complete decontamination is required. 

Note The method of standard addition is particularly useful to physicochemical techniques of analysis, for instance spectrophotometry, turbidimetry. 

The importance of surface analysis for evaluating the environmental effects of toxic substances is becoming more apparent as the result of recent work in this field. Chapter 9 describes ESCA, Auger, Ion Microprobe, and SIMS surface analysis techniques for atmospheric particulates. These techniques overcome the obvious limitations of bulk analysis, that is, the wide variability in the physicochemical characteristics of different particles. 

The main advances in analysis of organolithium compounds are related to their structural characterization by instrumental methods. These rely heavily on NMR spectroscopy and, when possible, on crystallographic methods, although other spectroscopic and physicochemical techniques are occasionally employed. A modern approach to the solution of complex analytical problems involves, in addition to the evidence afforded by these experimental techniques, consideration of quantum mechanical calculations for certain structures. The results of such calculations support or deny hypothetical assumptions on structural features of a molecule or possible results of a synthetic path. The following two examples illustrate these proceedings. 

Physicochemical tests are required in essential oil monographs published in standards, pharmacopoeias and codices. Chromatospectral techniques are modern methods used to assess the quality of essential oils. The most important technique for the analysis of essential oils is GC. Several detectors may... 

Drug residues that do not sufficiently absorb or fluoresce are usually submitted, prior to their analysis by physicochemical techniques, to some type of derivatization aimed primarily at enhancing analyte detectability. Furthermore, formation of relatively nonpolar derivatives offers advantages for a successful extraction and cleanup. 

Application of microbiological or immunochemical techniques offers the advantage of screening drug residues in foods with little or no previous sample preparation. Application, on the other hand, of physicochemical techniques, allows quantification and more tentative identification of residues in those samples found positive. The problem of analyzing for drug residues is complicated by the fact that it is not known whether residues exist, and if they exist, the type and quantity are not known. 

A literature survey shows that a great variety of physicochemical techniques are applied for analyzing drug residues in food. Unfortunately, no presently available physicochemical technique is as versatile or as universal as might be desired, nor is it likely that any such technique will be developed. Which type of detection principle should be used for a particular application depends on the physicochemical properties of the analyte, the sensitivity and selectivity required, and the convenience and versatility requested. 

Microbiological or immunochemical techniques offer the advantage to screen rapidly and at low cost a large number of food samples for potential drug residues but cannot provide definitive information on the identity of the specific drug residues found. Unlike microbiological and immunochemical techniques, physicochemical techniques actually aim at the identification, quantification, and/or confirmation of the presence of violative residues in suspected samples. 

TLC-bioautography has been used in Canada since 1984 for the confirmation of tetracycline-positive in plant tests (65). However, TLC-bioautography is not quantitative and only gives direction to the analyst as to what confirmatory method of analysis should be used. Therefore, presumptive positives must be confirmed by physicochemical techniques that have been validated in terms of detection limit, precision, and accuracy. 

---