Solvent extraction procedures sensitivity

Although widely used, solvent extraction procedures have been demonstrated as sensitive to such variables as the content of humic matter and moisture within samples. Supercritical fluid extraction appears to be a more robust procedure. Thermal extraction procedures are sensitive to the size of the soil sample in some cases since the technique can result in cracking higher-molecular-weight... 

In spite of the sensitivity of the determination, because of the low concentrations of cadmium in most environmental samples, the element is still often preconcentrated. For example, discrete nebulization flame AAS has been used to measure foliar cadmium after extraction of the APDC complex into chloroform.15 Cobalt was extracted at the same time. Many other solvent extraction procedures have been described.1 Alternatively resins such as a chelating polydithiocarbamate resin have been employed to concentrate cadmium prior to determination.16 Extractions onto solid phase materials for preconcentration may be made more convenient by automation, for example using flow injection methodology.17... 

Spectrophotometric batch procedures based on the solvent extraction of an ion associate formed between an anionic surfactant and a cationic dye are often used for the determination of anionic surfactants. Such procedures are usually tedious to operate. The method described here, based on an optimized on-line solvent extraction procedure for the determination of anionic surfactants reported by Motomizu et al.[27], could demonstrate the efficiency and sensitivity which can be achieved by a FIA procedure. 

Theory. Conventional anion and cation exchange resins appear to be of limited use for concentrating trace metals from saline solutions such as sea water. The introduction of chelating resins, particularly those based on iminodiacetic acid, makes it possible to concentrate trace metals from brine solutions and separate them from the major components of the solution. Thus the elements cadmium, copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100 and can be recovered subsequently for determination by atomic absorption spectrophotometry.45 To enhance the sensitivity of the AAS procedure the eluate is evaporated to dryness and the residue dissolved in 90 per cent aqueous acetone. The use of the chelating resin offers the advantage over concentration by solvent extraction that, in principle, there is no limit to the volume of sample which can be used. 

Chromatographic procedures applied to the identification of proteinaceous paint binders tend to be rather detailed consisting of multiple analytical steps ranging from solvent extractions, chromatography clean up, hydrolysis, derivatisation reactions, and measurement to data analysis. Knowledge of the error introduced at each step is necessary to minimise cumulative uncertainty. Reliable results are consequently obtained when laboratory and field blanks are carefully characterised. Additionally, due to the small amounts of analyte and the high sensitivity of the analysis, the instrument itself must be routinely calibrated with amino acid standards along with measurements of certified reference proteins. All of these factors must be taken into account because many times there is only one chance to take the measurement. 

Automation CL measurements can be performed very rapidly and with minimal human participation if the flow system is carefully designed. Several reagents can be added to the sample if multiple flow lines are used and at predetermined times to fulfill the best condition for maximum CL sensitivity. Introduction of special devices into the flow line, which allow procedures otherwise time-consuming such as solvent extraction or ion exchange, improve substantially the sensitivity and selectivity of the technique. 

Radiotracers are uniquely well suited to such studies. The sensitivity of detection means that only very small amounts of tracer need be added to follow the chemical pathway of the relevant species. Furthermore, it matters little what the physical or chemical state of the tracer is, for measurements may be made on liquids, solids or gases. Chromatography, solvent extraction and precipitation are amongst separation methods widely studied by means of radiotracers. In the individual separation steps the distribution of the species may be studied by simple radioactivity measurements, and subsequently the tracer will serve as a yield indicator for the overall procedure. 

Earlier atomic absorption methods [164-167] from the determination of molybdenum in soils employed a preliminary solvent extraction step to improve sensitivity in view of the low concentrations of molybdenum occurring in most soils. Baucells et al. [5] developed a graphite furnace atomic absorption procedure which was capable of determining down to 8.4 pg of molybdenum in a soil matrix solution with a precision of 4% for 100 pg/1 molybdenum. These workers showed that a char temperature of 1500 °C and an atomisation tem-... 

Tin is a problem element in terms of its routine determination in environmental samples by conventional flame spectrometry. Partly this is because of the poor sensitivity (the detection limits for all three techniques are around 100 ng ml"1 or higher), and partly it is because of the low natural abundance of the element in most environmental samples. Moreover, it is an element that is not well suited to pre-concentration by solvent extraction techniques, although recently a procedure has been described for soil acetate extracts in which tin was extracted with crown ether into chloroform after treatment of the extract with picric acid.51... 

The analysis of food contaminants, in particular any toxic or biologically active residue, is important for public health or quality control reasons.19 Examples are mycotoxins (aflatoxins) and pesticide and drug residues. Sample preparation is typically elaborate and might involve deproteinization, solvent extraction, and clean-up via solid-phase extraction (SPE).The use of highly sensitive and specific LC/MS/MS is increasing and has simplified some of the sample preparation procedures. 

Immunoassays, analytical procedures based on the specific binding of animal-derived antibodies to a target molecule (10), have been developed as rapid methods for detection of pesticides in soil and water samples. Field samples can be analyzed on site after water or solvent extraction using a visualization method, which generally involves reaction of the antibody-pesticide complex with an enzymatically derived colored product. The most sensitive and quantitative method for monitoring the color reaction is spectrophotometry. 

---