Micro preconcentration

There is considerable work being carried out to make micro preconcentrators and micro noses (e.g. see recent review from Sandia National Laboratories [19]), but this is still fundamentally a research program and micro nose systems are expensive to fabricate. 

METHODOLOGICAL ASPECTS OP CLOUD POINT PRECONCENTRATING OF MICRO-ADMIXTURES BY PHASES OF NON-IONIC SURFACTANTS Doroschuk V.O. 

Complex matrixes typically cannot be analysed directly to obtain the selectivity and sensitivity required for most trace analysis applications. To circumvent this problem, solid-phase micro extraction techniques were used to preconcentrate analytes selectively prior to gas chromatography/ion trap mass spectrometry analysis. 

Leoni [366] observed that in the extraction preconcentration of organochlo-rine insecticides and PCB s from surface and coastal waters in the presence of other pollutants such as oil, surface active substances, etc., the results obtained with an absorption column of Tenax-Celite are equivalent to those obtained with the continuous liquid-liquid extraction technique. For non-saline waters that contain solids in suspension that absorb pesticides, it may be necessary to filter the water before extraction with Tenax and then to extract the suspended solids separately. Analyses of river and estuarine sea waters, filtered before extraction, showed the effectiveness of Tenax, and the extracts obtained for pesticide analysis prove to be much less contaminated by interfering substances than corresponding extracts obtained by the liquid-liquid technique. Leoni et al. [365] showed that for the extraction of organic micro pollutants such as pesticides and aromatic polycyclic hydrocarbons from waters, the recoveries of these substances from unpolluted waters (mineral and potable waters) when added at the level of 1 xg/l averaged 90%. 

For analysis of solutions, ICP-mass spectrometry (ICP-MS) is very promising (Houk et al., 1980 Houk, 1986 Bacon et al., 1990). Recent advances in separation and preconcentration techniques are discussed by Horvath et al. (1991). Bacon et al. (1990) report that although ICP-MS is a multi-element technique, recent papers tend to concentrate on a small number of target elements. With isotope dilution mass spectrometry (IDMS), detection limits are further reduced (Heumann, 1988) IDMS is also suitable for accurate speciation in very low concentration levels of elements (Heumann, 1990). For the direct analysis of solid samples, glow discharge mass spectrometry (GD-MS) (Harrison etal., 1986) is of interest. Tolg (1988) has suggested that a substantial improvement in the absolute detection power of GD-MS, as applied to micro analysis, can be expected, at least in comparison with the ICP as ion source. 

The earliest applications of acoustic levitation in analytical chemistry were concerned with the development of various steps of the analytical process. Thus, Welter and Neidhart [72] studied the preconcentration of n-hexanol in methanol by solvent evaporation and the liquid-liquid extraction of n-hexanol from water to toluene in a levitated droplet, which they found to be efficient when using GC-FID with n-pentanol as internal standard. Solvent exchange of fluorescein from methylisobutyl ketone to aqueous sodium hydroxide was also accomplished. Sample concentration in an acoustically levitated droplet prior to injection into a CE equipped with an LIF or UV detector has also been accomplished [73,118]. The target analytes (namely, dansylated amino acids) were concentrated in the levitated drop and a limit of detection of 15 nM — much lower than the 2.5 pM achieved by hydrodynamic injection without preconcentration — was achieved following CE separation and quantification. For this purpose, 36000 sample droplets 2.3 pi in volume each were sequentially positioned in the acoustic Ievitator and evaporated. This example illustrates the potential of acoustic levitation for coupling to any type of detector for micro- or nanotrace analyses. 

Preconcentration of Micro-organisms into Tiny Liquid Volume 113... 

J.-H. Wang, E.H. Hansen, Interfacing sequential injection on-line preconcentration using a renewable micro-column incorporated in a Tab-on-valve system with direct injection nebulization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 16 (2001) 1349. 

F.-Q. Nie, M. Macka, B. Pauli, Micro-flow injection analysis system on-chip sample preconcentration, injection and delivery using coupled monolithic electroosmotic pumps, Lab Chip 7 (2007) 1597. 

S.L. Lin, C.S. Zheng, G.Z. Yun, Determination of palladium by flame atomic absorption spectrometry combined on-line with flow injection preconcentration using a micro-column packed with activated carbon fibre, Talanta 42 (1995) 921. 

The high sensitivity of this MGA is due to the large number of stages (20 0) and short response time of the preconcentrator (left-hand side of Eig. 9.3.2a), consisting of an array of rapidly heatable micro-elements that can adsorb gas components and later rapidly and precisely desorb them via a thermal pulse into the sample stream,... 

Following cell lysis, DNA extraction, purification, and preconcentration are usually achieved by microsolid phase extraction (micro-SPE) devices. This step is essential in order to purify and isolate the genomic materials from other cellular components, contaminants, and chemicals introduced in the cell lysis step that might potentially interfere with downstream enzymatic reactions. In addition, the nucleic acids may be enriched in this phase of the processing strategy to preconcentrate the targets to a level that is amenable for further downstream processing. 

A variety of well-established macroscale SPE methods for nucleic acid extraction have been successfully transferred to microscale devices [10, 31-57]. Although the physical principles of these methods may be different (e.g., chaotropic interactions, electrostatic interactions, affinity interactions, etc.), micro-SPE protocols typically consist of three steps (1) selective adsorption of nucleic acids onto a solid phase (2) removal of contaminants by a washing step and (3) elution of the preconcentrated nucleic acids from the solid support using water or a low salt buffer [31]. Like their macroscale counterparts, micro-SPE devices possess a loading level of target material that is dependent upon the available surface area within the extraction bed and, thus, are manufactured either by packing the solid phase... 

Solid phase micro extraction (SPME) fibres have improved in quality over the last few years and are now commonly used for introducing analytes into a gas chromatograph [1,2], The technique utilises a one cm length of fused silica coated with an adsorbent or absorbent phase. This allows preconcentration of organic compounds from a volatile or liquid sample that are adsorbed or absorbed in the fibre. The fibre is inserted into a heated injector port of a gas chromatograph where the analytes are thermally desorbed directly onto the column. The fibres are made up of a variety of coatings, of which the most common ones are shown in Table 1. 

An example of one of TSA/TSL s R D funded MEMS based project is the Sandia National Laboratories (SNL) MicroHound project. This is based on the SNL Micro Chem Lab on a Chip , illustrated in Figure 1. The original prototype system from SNL was developed for high vapour pressure, chemical weapons (CW) detection, which utilized a MEMS GC separator, with miniature surface acoustic wave (SAW s) based sensors. The system included an inlet, coated pre-concentrators, detectors, and pumps. To make this useful for trace explosives detection, the addition of an alternate front-end sample collection/macro-preconcentrator and MEMS based coated-preconcentrator is necessary, along with the option to utilize or exclude the MEMS GC separator followed by detection by either, or both, SAW s and miniaturized IMS detectors. 

The second ORNL microcantilever project, funded by the ATF and technically co-directed hy TSA, involves polymer-coated micro cantilevers. In this case, the explosive molecule would be adsorbed on the surface of the polymer, and the swelling or change in physical properties would be measured indicating the presence of an explosive. Several polymers are currently being developed to provide the necessary selectivity for the different types of explosives signatures. The sensitivity is similar to the non-coated cantilever estimated limit of detection of low parts-per-billion (with proper collection and preconcentration system, LOD should approach low parts-per-trillion). Figure 6 illustrates two modes of detection for the coated cantilevers beam stress response and beam resonant frequency response. 

Chapters 17 and 18 look at the importance of sampling technologies and the design of the microfluidic systems. In particular, the use of preconcentrators and solid phase micro extractors to boost the vapour concentration before it is introduced to the chemical sensor or electronic nose. 

Eight micro liter of octane extraction solvent was placed inside a porous, polypropylene fiber following an 8 min analyte preconcentration step, 4 fil of extract was injected into a gas chromatograph Static subcritical water extraction was coupled with styrene-DVB (SDB-XC) extraction discs soil, water, and the SDB-XC disc are placed in a sealed extraction cell, heated to 250°C for 15 to 60 min, cooled, and the PAHs recovered from the disc with acetone/methylene chloride With a Soxhlet apparatus (18 h), by PLE (50 min at 100°C), SEE (1 h at 150°C with pure CO2), and subcritical water (1 h at 250°C, or 30 min at 300°C)... 

Fig.4.11 a-c FI manifold and operation sequence of sorbent extraction preconcentration for ETAAS. P], P2, peristaltic pumps C, micro-conical column (15 pi packed with Ci8 sorbent) V, multifunctional valve L, eluate collector (75 /xl, 0.35 mm i.d.) W, waste and GF, graphite furnace [10]. 

Sperling et al.[4] determined arsenic(lll) and total arsenic in sea and lake waters by ETAAS, following on-line reduction and preconcentration of the DDC complex using a micro-colunm packed with C g sorbent, using a manifold modified on the basis of that shown in Fig. 4.11. A mixture of sodium sulphite, hydrochloric acid, sodium thio-sulphate and potassium iodide was used to achieve fast reduction of arsenic(V) to arsenic(IIl), and ethanol was used to elute the sorbed arsenic-DDC complex. The detection limits for As(ni) and As(ID+V) were 0.11 and 0.15 pg 1 respectively. 

Porta et al.(ll] silso developed an on-line column preconcentration method for the determination of cadmium and other trace metals (Pb, Cu, Ni, and Fe) in Antarctic sea water using ETAAS. A detection limit of 0.(KX)4 /ig 1 was achieved for cadmium. The APDC complexes of the trace metals were collected on a micro column packed with C 8< XAD-7 or XAD-2 sorbent and acetonitrile was used for elution. According to the authors, the main drawback of the method is at least two successive 80 //I elutions (with an intermediate evaporation in the furnace) are necessary to recover all the analytes from the column. 

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