Preconcentration sample stabilization

Collection of air samples in stainless steel canisters whose surfaces have been passivated is another common collection technique for VOCs. (Aluminum has also been used but the stability of polar organics in them is poor Gholson et al., 1990.) Indeed, this method is used not only for sampling air but in medical applications as well, where they have been used to sample organics in a single breath (Pleil and Lindstrom, 1995). Passivation of the canisters is often carried out using a process called SUMMA and hence referred to as SUMMA canisters. The canisters also have to be thoroughly cleaned before use an example of one such procedure is described by Blake et al. (1994). The sample is then typically preconcentrated by transfer to a cold trap prior to injection onto the GC column (e.g., see Blake et al., 1994). 

The quantitation of trace and ultratrace components in complex samples of environmental, clinical, or industrial origin represents an important task of modern analytical chemistry. In the analysis of such dilute samples, it is often necessary to employ some type of preconcentration step prior to the actual quantitation. This happens when the analyte concentration is below the detection limit of the instrumental technique applied. Besides its main enrichment objective, the preconcentration step may serve to isolate the analyte from the complex matrix, and hence to improve selectivity and stability. 

Stabilized liquid membrane device (SLMD) A water-insoluble organic complexing mixture diffuses to the exterior surface of the sampler through a polymeric membrane Divalent metal ions Preconcentration, in situ sampling, determination of labile metal ions in grab samples Days to several weeks Extraction with acid 73... 

The quality of analytical results obtained in the analysis of biological and environmental materials substantially depends on the sample preparation procedures applied, particularly when low concentrations of the metals and species of low stability are determined in complex matrices. Concentrations below the detection limits (DLs) of the available instrumental techniques and interfering effects from the matrix components require separation and preconcentration procedures. Instrumental techniques applicable to direct examination of solid materials are valuable for minimizing the effect of sample preparation steps on the quality of obtained results. In the analysis of noble metal samples, in particular those of complex environmental origin, use of such techniques is strongly limited because of interference from matrix components and the heterogeneity of examined materials. 

Alkaline oxidizing fusion is an effective way of dissolution of metallic powders, particularly of metals resistant to direct wet acid treatment (Ru, Os, and Ir), but is rarely used for decomposition of complex noble metal samples because of low recoveries (e.g., 34—84 % Pt, Pd, and Au from silicate materials) [64]. Low stability of the complexes formed under dissolution (water, HCl) of the melt and difficulties with quantitative conversion of analytes into complexes of strictly defined composition (suitable for subsequent separation) limit the applicability of the alkaline fusion method. Hydroxocomplexes easily formed in the solutions can cause problems with quantitative separation and preconcentration of the metals, particularly when using ion-exchange chromatography. 

Organic ion-exchangers are used for the separation of ions and for the separation and preconcentration of traces from the sample macrocomponents (matrix). Ion-exchange processes are based on the differences in ionic charge, in stability of the complexes formed, and in the associated distribution coefficients [123,124]. 

In order to avoid problems related to slow reaction kinetics and sample storage stability, precautions are often taken in the field to minimize such effects. Furthermore, sample preparation is often required to transfer the collected isocyanate derivatives to solvents, which are compatible with the analysis method of choice, or to preconcentrate the sample to improve the sensitivity of the method. 

A new silica gel-based chelating sorbent, with thiourea as a functional group, was used for the FI online preconcentration and separation of trace levels of silver, gold, and palladium. The selected metal ions were adsorbed onto a column packed with thiourea-modified silica gel (TuSG). The sorbent exhibited excellent stability and the sorption properties of TuSG did not change after 1000 cycles of use. The selected metals in a secondary nickel alloy, an anode slim, an electrolytic solution, and three national certified ore samples were determined satisfactorily using the proposed method. ... 

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