Preconcentration Subject

Supercritical fluid extraction (SFE) is generally used for the extraction of selected analytes from solid sample matrices, but applications have been reported for aqueous samples. In one study, recoveries of 87-100% were obtained for simazine, propazine, and trietazine at the 0.05 ug mL concentration level using methanol-modified CO2 (10%, v/v) to extract the analytes, previously preconcentrated on a C-18 Empore extraction disk. The analysis was performed using LC/UV detection. Freeze-dried water samples were subjected to SFE for atrazine and simazine, and the optimum recoveries were obtained using the mildest conditions studied (50 °C, 20 MPa, and 30 mL of CO2). In some cases when using LEE and LC analysis, co-extracted humic substances created interference for the more polar metabolites when compared with SFE for the preparation of the same water sample. ... 

Berman et al. [735] have shown that if a seawater sample is subjected to 20-fold preconcentration by one of the above techniques, then reliable analysis can be performed by ICP-AES (i.e., concentration of the element in seawater is more than five times the detection limit of the method) for iron, manganese, zinc, copper, and nickel. Lead, cobalt, cadmium, chromium, and arsenic are below the detection limit and cannot be determined reliably by ICP-AES. These latter elements would need at least a hundredfold preconcentration before they could be reliably determined. 

Solvent extraction is a very widely used and simple preconcentration technique. After the sample is extracted with a suitable solvent (such as methylene chloride), the extract is concentrated by evaporation and subjected to analysis. One important requirement is extremely clean solvents fortunately these are now commercially available. Because of the evaporation step, solvent extraction cannot be used for the analysis of very volatile compounds. Depending on sample size, sensitivities of 0.1 ppb can easily be achieved. 

Relative error values for the elements ranged from zero to a high of 18% with most values being 5% or less. The 18% relative error was obtained for indium and is attributable to the low concentration of this element in the solution analyzed. Moreover, the indium values were obtained on the lower end of the working curve where the sensitivity is greatly reduced. Standard deviations and coefficients of variation for the elements of interest are at acceptable levels (less than 1% standard deviation and around 5% coefficient of variation) for this technique. Again it should be pointed out that the original purpose of the subject method was to develop a rapid routine analysis for the major and minor constituents in coal ash and related materials without the necessity of several preconcentration steps, solvent extraction techniques, or pH adjustments. 

In an attempt to determine low levels of As in beers by ET-AAS, Cervera et al. [117] introduced a preconcentration step that enhanced the sensitivity by a factor of 2.5, reaching an LoD of 0.7 ng of As per gram of beer. Samples were degassed and subjected to a preliminary dry ashing using Mg(N03)2 and then evaporated until total dryness. A slurry was prepared by adding 100 jjlL of HNO3... 

In this method preconcentration is achieved by evaporating the sample to low volume under a heat lamp or on a hot plate. An obvious disadvantage of this technique is that in addition to the analyte being concentrated, other species which might interfere in later analysis will be concentrated with it. The method is also subject to contamination throughout the evaporation period and furthermore, losses of volatile elements may occur. Notwithstanding, the technique has an application in water analysis where waters are relatively clean with a low total dissolved solids content. 

The concentration of microelements from seawater on sorbents is widely used as a preconcentration step for their analytical determination by different instrumental methods. Several recent publications [293-298] and reviews [299-301] are recommended to those who are further interested in this subject. 

As can be seen from Fig. 6.9, dynamic pressurized hot solvent extraction (DPHSE) has evolved similarly to ASE however, as noted earlier, DPHSE has been the subject of many fewer reports, primarily as a result of the lack of commercially available equipment for implementation. In any case, the relatively scant reported applications of DPHSE are of especial interest as regards automation of the analytical process in fact, the dynamic nature of the system facilitates its coupling to other dynamic systems with a view to accomplishing preconcentration [39,42,45,145], filtration [42,45], chromatographic separation [145,146], derivatization [46,57] and detection [44,147], among others, and the partial or total automation of the analytical process. 

Figure 7.2.4. GC/MS traces for extracted single ion chromatograms corresponding to masses 202, 216,228, 252 in a cellulose pyrolysate subjected to clean-up and preconcentration for PAH analysis [21],...

The possibility of determining trace phenolic pollutants in water by TLC was investigated. The analytes were preconcentrated by SPE and subjected to both classical and multiple gradient development TLC. In situ quantation was performed by UV absorption or by visible light absorption after treatment with Wuster s reagent (115). LOD was... 

This is where the LC pump is stopped, and the sample in the NMR cell collected and subjected to static NMR measurements. This enrichment step improves sensitivity (S/N ratio) since the eluted component can be repeatedly scanned or a number of fractions can be combined or preconcentrated to increase sample size/concentration. 

PUF has also been used as a solid-phase to sorb PAHs from water samples. PUF sorbents were batch-equilibrated with PAH standard solutions, removed, squeezed with a filter paper, air-dried, and subjected to solid-matrix luminescence (SML). PAH was converted to 3,4 benzopyrene for luminescence analysis. " A level of 8ngmU PAH was detected in sewage water from a petroleum refinery. More recently, cotton was applied as a solid-phase sorbent in the online investigation of tap water, sewage treatment plant effluent, and river water in Beijing, China. Water samples, of 100-ml volume and 10% 2-propanol, to prevent sorption on container walls, were preconcentrated using a precolumn attached to an HPLC-FLD system. Recoveries between 92 and 119% were reported at ngU spike levels for FLU, PHN, FLA, and B[k]F in real water samples. 

Sample preparation is an extremely critical step in the course of a speciation analysis, as the original sample has to be transformed into a form which can be subjected to analysis, while the original distribution of an element over its various chemical forms may not be altered. In general, determination of Hg species involves the following steps (1) sample collection/pretreatment/ preservation/storage (2) extraction of Hg from the matrix/cleanup/preconcentration (3) separation of Hg species of interest (4) detection. 

The attractive features of splitless injection techniques are that they allow the analysis of dilute samples without preconcentration (trace analysis) and the analysis of dirty samples, since the injector is easily dismantled for cleaning. Success with individual samples, however, depends on the selection of experimental variables of which the most important sample size, sample solvent, syringe position, sampling time, initial column temperature, injection temperature and carrier gas flow rate, often must be optimized by trial and error. These conditions, once established, are not necessarily transferable to another splitless injector of a different design. Also, the absolute accuracy of retention times in splitless injection is generally less than that found for split injection. For splitless injection the reproducibility of retention times depends not only on chromatographic interactions but also on the reproducibility of the sampling period and the evaporation time of the solvent in the column inlet, if solvent effects (section 3.5.6.2) are employed. The choice of solvent, volume injected and the constancy of thermal zones will all influence retention time precision beyond those for split injection. For quantitative analysis the precision of repeated sample injections is normally acceptable but the method is subject to numerous systematic errors that may... 

Here the model is subjected to a more general treatment to include FI preconcentration systems using other flow-through detectors. 

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