Solid adsorbents aqueous samples

Solid-phase microextraction (SPME) consists of dipping a fiber into an aqueous sample to adsorb the analytes followed by thermal desorption into the carrier stream for GC, or, if the analytes are thermally labile, they can be desorbed into the mobile phase for LC. Examples of commercially available fibers include 100-qm PDMS, 65-qm Carbowax-divinylbenzene (CW-DVB), 75-qm Carboxen-polydimethylsiloxane (CX-PDMS), and 85-qm polyacrylate, the last being more suitable for the determination of triazines. The LCDs can be as low as 0.1 qgL Since the quantity of analyte adsorbed on the fiber is based on equilibrium rather than extraction, procedural recovery cannot be assessed on the basis of percentage extraction. The robustness and sensitivity of the technique were demonstrated in an inter-laboratory validation study for several parent triazines and DEA and DIA. A 65-qm CW-DVB fiber was employed for analyte adsorption followed by desorption into the injection port (split/splitless) of a gas chromatograph. The sample was adjusted to neutral pH, and sodium chloride was added to obtain a concentration of 0.3 g During continuous... 

The aqueous sample is adsorbed and distributed into a thin film over the solid support. 

Analytical methods used to determine dinitrophenols in environmental samples are given in Table 6-2. For aqueous samples, continuous liquid-liquid extraction with organic solvent(s) provided higher recovery of 2,4-DNP in solvent extract than discontinuous (e.g., shaking in separatory funnel) extraction (Levsen et al. 1993). Dinitrophenols can also be extracted from aqueous samples by solid-phase extraction. The adsorbed 2,4-DNP is eluted from the column by a suitable solvent. The recovery of 2,4-DNP was >90% with membrane disks impregnated with acetyl-polystyrene-divinyl benzene (Schmidt et al. 1993) and 89% with a C-18 adsorbent (Levsen et al. 1993). 

The experimental detection and quantification of surface species on in situ soil particles and other natural colloids is a difficult area of research because of the sample heterogeneity, low surface concentrations, and the necessity to investigate the solid adsorbents in the presence of water. Unambiguous information can be obtamed only with in situ surface spectroscopy, such as x-ray photoelectron (XPS), extended x-ray absorption hne structure (EXFAS), x-ray absorption near-edge structure (XANES), melastic electron tunneling (lETS), and electron energy loss (EELS) spectroscopies. Recent advances in the development of nonevasive, in situ spectroscopic scarmed-probe and microscopic techniques have been applied successfully to study mineral particles in aqueous suspensions (Hawthorne, 1988 Hochella and White, 1990). 

The newly developed solid-phase microextraction (SPME) technique, first reported by Pawliszyn in 1989, is increasingly used for the gas chromatographic determination of a wide variety of volatile and semivolatile organic compounds in water or aqueous extracts of different substrates. Basically, it involves the extraction of specific organic analytes directly from aqueous samples or from the headspace of these samples in closed vials. The extraction is achieved onto a fused-silica fiber coated with a polymeric liquid phase. After equilibration, the fiber containing the absorbed or adsorbed analyte is removed and thermally desorbed in the hot injector port of a gas chromatograph or in an appropriate interface of a liquid chromatograph. ... 

The most important feature of the adsorbent is the surface. The phenomenon of adsorption onto solids involves van der Waals forces, which bind the sorbate to a solid surface. It is not possible to predict accurately just which materials will be adsorbed well by a given adsorbent. Generally, hydrophobic or nonpolar molecules or sections of molecules are attracted to hydrophobic surfaces and hydrophilic or polar materials to hydrophilic or polar surfaces. Hydrophobic adsorbents, such as Amberlite XAD2 and XAD4, attract the hydrophobic end of molecules from aqueous samples. X AD7, an adsorbent of intermediate polarity or hydrophobicity containing acrylic ester groups (RCOO), can attract either hydrophobic or hydrophilic ends of molecules and functions well in aqueous and nonaqueous systems. 

In SPE, choice of an appropriate solid phase is based on their possible molecular interactions with the targeted compounds. For historical reasons, reversed-phase type adsorbents have been largely utilized to extract compounds from aqueous samples, given the fact that many of them have been developed already for use in reversed-phase chromatography applications and are commercially available. Ion exchange adsorbents have been used to retain counter ions based on electrostatic interactions. Solid phase adsorbents combining hydrophobic and ionic interactions are also present, for example graphitized carbon black (GCB) obtained by... 

A major part of all ion-pair separations is made in the liquid-solid mode with a hydrophobic adsorbent and an aqueous mobile phase. These systems are compatible with aqueous samples and are highly flexible, which makes them applicable to both hydrophilic and hydro-phobic compounds. The solid stationary phase, most often alkyl-bonded silica, has a limited binding capacity, which is of importance for retention and selectivity [22, 23]. Models for the retention can be based on the following statement. 

---