Sample preconcentration conductivity difference

Sample preconcentration techniques are used with two purposes (1) to increase concentration in order to achieve detection and (2) to eliminate disturbances of the electrophoretic system during hydraulic or electrokinetic sample introduction when the conductivity of the sample is significantly different from that of the analysis buffer. It is important to keep sample manipulations and modifications to a minimum, and a rule of thumb is to prepare the sample so that its composition is at the same pH as the analysis buffer. It is also advantageous... 

Effects of LE (NaCl) concentration, Cle, TE (HEPES) concentration, Cte and initial concentration of sample (Alexa Fluor 488), Cs,initiai on sample preconcentration are summarized in Figure 38.18. Cle was varied from 10 mM to 1 M to study its effect on maximum focused sample concentration, Cs,finai and concentration increase. Cl (Figure 38.18a). The 5 mM HEPES TE solution contained 1 nM Alexa Fluor 488 as a sample. The focused sample concentration is nearly directly proportional to the concentration of LE, as expected from a one-dimensional nondispersive model (i.e., BCRF theory). However, the nondispersive model drastically underpredicts the proportionality constant the measured focused sample concentrations are 35(X)- to 7900-fold less than that predicted by Equation 38.43 despite all cases reaching fully-focused state. This gross difference between KRF theory and experiments is because the sample is in a smeared region of locally varying conductivity and electric field, as dictated by the effects of diffusion and Taylor dispersion. 

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... 

To achieve a preconcentration of the sample with FASS, the sample is diluted in a buffer which has a much lower conductivity (a) than the surrounding run buffer used for the separation. In FASS, the high- and low-conductivity buffers differ only in concentration (which is related to the ionic strength and thus the conductivity), to overcome problems due to unwanted electrodispersion, which could occur when two buffers with different chemical compositions are introduced. 

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. 

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