Sample preconcentration techniques types

CE presents potential advantages in forensic science to carry out the analysis of opium alkaloids, as it can be in the different applications being published. Nevertheless, the lack of the sensitivity required for this type of analysis is always a great problem. One of the most basic approach for sensitivity enhancement is based on increasing analyte mass loading via online sample preconcentration techniques. The most widely employed in the analysis of major alkaloids is FASI that basically consists in a mismatch between the electric conductivity of the sample and that of the miming buffer. It is achieved by injecting the sample diluted in a solvent of lower conductivity than that of the carrier electrolyte. Upon the application of the... 

ESI, triazines) [537]. Recoveries observed were > 80% except for carbendazim, bu-tocarboxim, aldicarb and molinate, all better than 67% [500]. An aoTOF-MS interfaced by ESI was used to screen and identify unknown compounds and pesticides in water samples by MS and MS/MS. Structures for compounds observed besides pesticides were proposed [538]. Traces of the phenylurea pesticides Hnuron and monolinuron in water were determined quantitatively. Calibration graphs obtained after Supelclean ENVI-18 SEE were Hnear with detection limits < 25 pg [511]. Large numbers of phenylurea herbicide analyses led to the elaboration of on-line preconcentration techniques coupled to ESI-LC-MS. The procedure was demonstrated and validated with several pesticides using 10 ml of sample, resulting in detection Hmits of about 10 ng [539]. ESI-LC-MS and MS/MS were applied to quantify and to confirm 16 different herbicides of sulfonylurea [527] type in surface water samples. Surface water samples were extracted by SPE (Spe-ed RP-102). As confirmation criteria RT, molecular ion and two fragment ions besides ion abundance ratios were defined. Quantitation at 0.1 and 1.0 ppb level was demonstrated [540]. 

The preconcentration techniques that have to date been most used in microfluidic devices have been presented in the preceding pages. Most of the techniques were originally developed for conventional CE or liquid chromatography and have (more or less simply) been adapted to microchips. FASS, ITP, and lEF are examples of these types of techniques. A few new techniques have been developed for the preconcentration of samples on microfluidic devices. TGF and nanofluidic filtering are both examples of techniques that have been developed solely for microfluidic chips. High enrichment factors can be achieved with both techniques, on the order of 10,000 and 1,000,000, respectively. Table 50.1 summarizes the top performers in each device category with respect to preconcentration factor and time. 

In water sampling, close attention must be given to the type of sample, the sampling equipment, sample container, holding times, and proper preconcentration techniques. In the case of radioactive material, it is very important that they are present generally in extremely low concentrations in water, so that sorption and volatilization can occur. It has been recommended that preservatives be added at the time of sampling unless suspended and dissolved fractions are to be separated. Concentrated hydrochloric or nitric acid may be used to adjust the pH (usually to <2). Acidified samples should be held for at least... 

Analysis to determine the rare-earth content of materials can have many different objectives. Successful separations require a judicious combination of appropriate group separation/preconcentration, separation of individual members of the series, and the proper detection technique. Recent reviews that are readily available in the chemical literature offer compilations of cookbook methods for conducting analyses of samples of different types. In the following sections, we will offer a brief summary of preferred methods for specific types of analyses and provide appropriate literature references for the reader to pursue for details beyond those offered herein. The emphasis of this section will he on the literature covering the period 1990-1997, which has not been discussed in previous English-language reviews of the subject. Some earlier reports describing important fundamental advances in lanthanide separations will be included. 

This is followed, in Chapter 3, by the description of the analytical procedures used for monitoring the presence of uranium in the environment air, water, soil, and plants. As there is a large variety of sample types, there is no universal procedure for all environmental samples. Therefore, we present a myriad of sample treatment procedures and preparation methods as well as some separation and preconcentration techniques. Several examples of analytical procedures, based on the sample preparation methods, are described to demonstrate the diversity required for characterizing environmental samples. 

A comprehensive overview of preconcentration techniques for uranium (VI) and thorium (IV) prior to analysis was published (Prasada Rao et al. 2006). The multitude of off-line techniques that were reviewed includes liquid-liquid extraction, liquid membranes, ion exchange, extraction chromatography, flotation, absorptive electrochemical accumulation, solid-phase extraction (SPE), and ion imprinting polymers. In addition, online preconcentration methods for uranium, thorium, and mixtures of the two are also briefly surveyed. This overview includes over 100 references and is a good source for finding a suitable preconcentration technique with regard to the enrichment factor, retention and sorption capacity, method validation, and types of real samples. The review article focused on samples in which the uranium was already in solution so that digestion procedures for solid samples were not discussed (Prasada Rao et al. 2006). 

During these preliminary preparation steps, additional modification of the sample such as preconcentration of the analyte or separation from potentially interfering components in the sample can be achieved. Therefore, the sample introduction technique of choice can play an important role in the design of an optimum strategy for analysis. A summary of proven sample introduction techniques for use with ICP-MS is given in Table 5.1. This table shows the types of techniques that are useful for the determination of analyte elements in gas, liquid, or solid samples and mixtures thereof. 

Liquid-liquid extraction is by far the most popular separation method for the cleanup and preconcentration of samples because it is simple, reproducible, and versatile. There are several ways to achieve these objectives, from the original discontinuous ( batch ) and nonautomatic techniques to continuous separation techniques incorporated with automated methods of analysis. The methodologies can be classified into two general types ... 

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. 

The sensitivity of the technique depends mainly on the value of the partition coefficient of the analytes partitioned between the sample and the liber stationary phase. The efficiency of preconcentration depends on both the type of liber used and its thickness (amount). This type of liber affects the amount and character of the sorbed species.51 The general rule like dissolves like applies here, that is, polar compounds are sorbed on polar libers, and nonpolar compounds on nonpolar libers. A broad range of standard libers is commercially available. 

Tungsten-coil atomizers have to some extent been used in connection with atomic techniques in general and ETA-AAS in particular, and their popularity continues to rise. Their high simplicity and low cost make them attractive alternatives to graphite furnaces for many applications. However, they are much more interference-prone than their graphite counterparts. The interferences experienced by W-coil atomizers have been overcome in various ways, however. Thus, Barbosa et al. [25] avoid the interference from a salt matrix and implement a preconcentration step by electrochemically reducing Pb onto the coil surface. They use a flow injection system to deliver the sample through an anode inserted in the tip of the autosampler and the W-coil itself as cathode. The use of a W-coil as a platform inside a Massmann-type furnace provides no appreciable improvement over wall atomization [26]. 

As the analytes to be assayed are nearly always present in minute amounts, preconcentration steps are almost always necessary. The techniques used for this purpose have been mostly adopted from analytical procedures using gas or liquid chromatography as the separation step [5]. This fact is emphasized here because the appropriate adjustment of the sample preparation is necessary because the original procedures do not respect the fact that the sample volume injected into a CE system represents a few nanoliters only and requires a relatively high concentration of analytes assayed. However, in selected types of analysis, direct sample application is also possible (for a review, see Ref. 3). 

CZE-UVD, measuring at 210 nm, was applied for separation of a mixture of HA A of the types found in Table 2.B-C, extracted from cooked protein food. After cleanup and preconcentration by the technique described in Section III.A.5, samples were run in solution at pH 2.20 for optimal resolution (better than 1.7). The order of emergence of five such analytes was MelQ (25b), Trp-P-2 (28a), Glu-P-1 (29b), MelQx (26a) and PhIP (23a). Best overall results were obtained for hydrodynamic injection, with LOD from 35 to 50 ppb, linearity from 0.06 to 10 ppm and good reproducibility214. 

The sample solution is aspirated into a toroidal shaped plasma, where atomization and excitation occurs. The emitted radiation is either measured simultaneously at previously selected atom lines, or one line after the other is sequentially scanned. The actual sensitivity depends on many parameters of the apparatus used, such as the type of nebulizer, optics, power, gas-flows, and observation height (for general review, see Broekaert and Tdig, 1987). If solvent extraction is applied as a technique for separation and preconcentration, however, it should be kept in mind that the plasma is easily distorted and extinguished by organic solvents. For thallium, there is some choice between different lines, but sensitivity is rather poor in any case, so that the direct application to decomposition solutions of rocks, soils, and biological samples is limited. 

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