Sample preconcentration stacking

TABLE 5.2 Sample Preconcentration Using Different Stacking Methods... 

Sample stacking, which is based on a lower electrical conductivity in the sample buffer relative to the mn buffer, has been achieved on-chip for sample preconcentration. [578-581]. A 10-fold enhancement in detection signal was reported for the samples prepared in water, as compared to those prepared in the mn buffer [560],... 

Lichtenberg, J., Verpoorte, E., de Rooij, N.F., Sample preconcentration by field amplification stacking for microchip-based capillary electrophoresis. Electrophoresis 2001, 22(2), 258-271. 

Preconcentration is another commonly encountered sample pretreatment method that has been successfully integrated onto a CE chip. Ramsey and coworkers incorporated a porous membrane structure into a microfabricated injection valve, enabling electrokinetic concentration of DNA samples using homogeneous buffer conditions [5]. Sample preconcentration in nonhomogeneous buffer systems — a technique known as sample stacking —has also been achieved on-chip [6]. 

Online Sample Concentration. Another technique used in CE systems to increase their limit of detection is to preconcentrate the sample. One of the simplest methods for sample preconcentration is to induce a stacking effect with the sample components, something easily accomplished by exploiting the ionic strength differences between the sample matrix and separation buffer. This results from the fact that sample ions have decreased electrophoretic mobility in a... 

Many investigations have been carried ont of procednres for improving the analytical qnality of GC methods by changing the matrix, increasing the concentration of the pertinent analytes and redncing the interference of other componnds present in the sample. Preconcentration by LLE, before or after derivatization, is most freqnently apphed in GC trace analysis however, other techniqnes, snch as SPE, sample stacking (see Section V.A.l) and some of their modifications, snch as simnltaneons distillation and extraction (SDE) and SPME, are also mentioned. Application of microwave-assisted processes (MAP) dnring sample preparation seems to improve recoveries. 

Whether the stacking technique is called field-amplified stacking, transient isotachophoresis, or sample self-stacking, the driving force in on-column sample preconcentration for charges species is the need to satisfy the initial KRF conditions. Under separation and sample matrix conditions where the KRF is difficult to determine, simply calculations such as flcut and a offer a rapid evaluation of conditions for optimal transient ITP. 

In the previous sections, several methods of sample preconcentration were described. Regardless of the mechanism, every example of sample stacking requires a change in the velocity of the analyte as determined by the drive to maintain the KRF value. Unfortunately, neutral analytes do not contribute to the KRF and a different perspective on sample stacking must be taken. There are a number of stacking methods available when using MEKC. This section will describe the fundamental mechanism and conditions by which sample preconcentration of neutral molecules occurs. 

Gong, M. J., Wehmeyer, K. R., Limbach, R A., Arias, R, and Heineman, W. R., On-line sample preconcentration using field-amplified stacking injection in microchip capillary electrophoresis, Analytical Chemistry, 78, 3130-5131, 2006. 

Quirino, J. P., Dulay, M. T, and Zare, R. N. (2001). On-line preconcentration in capillary electrochromatography using porous monolith together with solvent gradient and sample stacking. Anal. Chem. 73, 5557—5563. 

Carbon nanotubes could be also used in a format of disc. A comparison smdy showed that the double-disk system (comprising two stacked disks with 60 mg of CNTs) exhibited extraction capabilities that were comparable to those of a commercial Cig disk with 500 mg sorbent for nonpolar or moderately polar compounds. Moreover, the former system was more powerful than the latter for extracting polar analytes. The triple-layered CNTs disk system showed good extraction efficiency when the sample volume was up to 3,000 mL. Katsumata et al. [136] obtained very high enrichment factor for preconcentration of atrazine and simazine (3,900 and 4,000, respectively, for 200 mL of sample solution when only 30 mg of MWCNTs was used in the format of disk. 

Figure 26-26 Lower trace Sample injected electrokinetically for 2 s without stacking is limited in volume to prevent band broadening. Upper trace With stacking. 15 times more sample could be injected (for 30 s). so the signal is 15 times stronger with no increase in bandwidth. [From V Zhao and C. f. Lurie, pH-Mediated Field Amplification OmCotumn Preconcentration of Anions in Physiological Samples for Capillary Electrophoresis, Anal. Chem. 1999, 71,3985.]...

J. P. Quirino, M. T. Duylay, and R. N. Zare, On-Line Preconcentration in Capillary Electrochromatography Using a Porous Monolith Together with Solvent Gradient and Sample Stacking, Anal. Chem. 2001, 73, 5557. 

The detection limits and the analyte-to-matrix ratios for inorganic and organic anion impurities in boric acid, obtained using hydrostatic or electro-kinetic injection and with sample stacking (the capillary is filled with the sample up to the detector, a voltage is applied to preconcentrate the sample anions at the sample-buffer interface, a reversed EOF is used to remove the matrix components and... 

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