Sample handling aqueous samples

The flow analyser is well suited for handling aqueous samples, which are directly inserted into the manifold by taking advantage of loop-based or time-based sample introduction. Due to the portability, ruggedness, low power requirement and low maintenance requirement inherent to the unsegmented flow systems, specific approaches for in situ sample collection and handling have been proposed. 

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction with a sorbent interface). All of these techniques use a nonporous membrane, involving partitioning of the analytes [499]. SLM is a sample handling technique which can be used for selective extraction of a particular class of compounds from complex (aqueous) matrices [500]. Membrane extraction with a sorbent interface (MESI) is suitable for VOC analysis (e.g. in a MESI- xGC-TCD configuration) [501,502]. 

BioEPR samples are generally (frozen) aqueous solutions since water is the only solvent compatible with terrestrial life. The high-frequency dielectric constant of ice is circa 30 times less than that of water. As a consequence liquid-phase EPR is experimentally rather different from frozen-solution EPR. We start with a discussion of sample handling for low-temperature experiments. 

In the past two decades quite a few new techniques have emerged for the treatment of aqueous samples prior to organic analysis. Perhaps the most important development is that of solid-phase extraction (SPE), which has successfully replaced many off-line steps. This technique can be considered to have introduced a genuine new era in sample handling [1]. The many varieties in which the technique is available and can be applied have made it the key step in handling of aqueous samples. Among the successful varieties are solid-phase microextraction (SPME), matrix solid-phase dispersion, disk extraction and immunosorbent extraction. Several reviews covering these topics have appeared in the literature in the past decade (see e.g. Refs. [2,3] for nonylphenol... 

Modern methods of sample handling for determination of surfactants in aqueous samples are practically all based on SPE and modifications thereof. Substantial reductions in analysis time, solvent consumption, sample volume required, and number of off-line steps have thus been achieved. This has not only increased the analysts capacity and analysis price per sample, but also decreased the risk of both analyte loss and contamination during sample handling. Whether or not this has indeed resulted in an increased quality of analytical results still needs to be validated through, e.g. intercalibration exercises. This aspect is discussed in more detail in Chapter 4. 

Aqueous trip blanks sometimes accompany soil samples collected into metal liners or glass jars. In this capacity they do not provide any meaningful information. Soil samples do not have the same contamination pathway as water samples because they are not collected in 40-milliliter (ml) VOA vials with PTFE-lined septum caps. In addition, soil does not have the same VOC transport mechanism as water does (adsorption in soil versus dissolution in water). There are other differences that do not permit this comparison different sample handling in the laboratory different analytical techniques used for soil and water analysis and the differences in soil and water MDLs. That is why the comparison of low-level VOC concentrations in water to VOC concentrations in soil is never conclusive. 

Guenu, S. Heimion, M.C. On-hne sample handling of water-soluhle organic pollutants in aqueous samples using porous graphitic carhon. J. Chromatogr., A 1994, 665 (2), 243 -251. 

Once on the chip, the next step is usually some sort of treatment of the sample such that it is made suitable for analysis and detection. Even aqueous samples should be filtered prior to assay. Typically, sample pretreatment takes the form of various fluid handling steps such as concentration flltration ", extractions s , ion exchanges and desaltingS. Combinations and permutations of these steps are also reported. Even degassing can be achieved on chip using ultrasonic-induced cavitations. ... 

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