Samples stacking

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], 

Although stack injection has been employed previously [316,582], the benefit of stacking for sample pre-concentration was only studied in detail later [346]. With the sample buffer (0.5 mM) at a 10-fold lower conductivity than the separation buffer (5 mM), simple EK stacking using the gated injection was observed. This was applied to the separation of dansylated amino acids (dansyl-lysine, didansyl-lysine, dansyl-isoleucine, and didansyl-isoleucine) [346]. 

It was found that sample stacking did not improve further if the dilution factor in the sample buffer (relative to the run buffer) was more than 350-fold. This is because of a loss of resolution due to parabolic flow formed from the difference in the EOF between the sample buffer (faster EOF) and the run buffer (slower EOF) [578], 

Owing to the EK bias in favor of faster migrating species in stack injection, the signal enhancement ranged from 31 to 8. However, stack injection produced less resolution, i.e., fewer plate numbers (N), as compared to those obtained in pinched injection. In addition, RSD (n = 6) in peak areas for stack, non-stacked, and pinched injection are 2.1%, 1.4%, and 0.75%, respectively [346]. 

Stacking of a neutral analyte can also be achieved, but only when a high-salt (NaCl) sample buffer is employed. The co-ion (CE) should be present in the sample buffer at a concentration sufficiently higher than that of the electrokinetic vector ion (cholate). This causes the formation of a pseudo-steady-state co-ion boundary that forces an increased concentration of cholate near the boundary, leading to the stacking effect [583], 

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Huang, H.Y. et al.. Analysis of food colorants by capillary electrophoresis with large-volume sample stacking, J. Chromatogr. A, 995, 29, 2003. 

On-line sample-stacking techniques " and, more recently, the use of isotacho-phoresis have added to the potential benefits of CE by permitting the concentration of analyte in a large volume by exploiting the difference in the electric field between the dilute sample and system buffer. The electric field is much stronger in the dilute buffer-sample and hence analyte ions move faster until they reach the border with the separation buffer. At this point they slow down, causing the analyte to concentrate as a sharp sample band at the interface. 

He Y. and Lee H.K., Large-volume sample stacking in acidic buffer for analysis of small organic and inorganic anions by capillary electrophoresis, Anal. Chem. 71, 995, 1999. 

QUIRINO, J.P., TERABE, S., Sample stacking of fast-moving anions in capillary zone electrophoresis, J. Chromatogr., A., 1999, 850, 339-344. 

Recent studies examining the distribution of smelter-derived elements around a copper smelter found that arsenic deposition rates in snow and concentrations in organic soils decrease gradually with distance from the smelter (Henderson et al. 2002 Knight Henderson 2006 Telmer et al. 2003 Zdanowicz etal. 2006). In order to explore the nature and behaviour of arsenic, a limited number of samples (stack and impacted soil) were characterized. 

An important pre-concentration method is the sample-stacking procedure [68,69]. When the sample is dissolved in a solvent with an electrical conductivity lower than that of the buffer electrolyte, sample stacking occurs because of the difference in electric field strength in the sample and the electrolyte medium. As the field strength in the sample zone is higher, the migration velocity in the sample zone is also higher. At the interface of the sample and the buffer electrolyte zone... 

The objectives of each theoretical approach are not only the explanation of the experimental results or failures of practice but also the prediction of new possibilities to increase the sensitivity, separation capacity and velocity of the chromatographic procedure under investigation. Numerous theoretical reviews deal with the problems of the CE separation technique. In recent years the methods to enhance the precision in CE by the modification of operational parameters [113], the theory and methodological improvements of sample stacking of cationic and anionic solutes in CE [114-116], and the results and difficulties of the application of conductivity detection in CE technologies [117] have been reviewed. 

J.P Quirino and S. Terabe, Sample stacking of cationic and anionic analytes in capillary electrophoresis. J. Chromatogr.A 902 (2000) 119-135. 

Fig. 3.146. Electropherograms of commercially available jelly and milk products determined by LVSS. (a) Grape-flavoured jelly, (b) peach-flavoured jelly, and (c) apple-flavoured milk. LVSS conditions were sample injection time 25 s (2 psi) sample stacking time 2.9 min (—5 kV), after which the current reached 95 per cent of its maximum value. Samples were heated with a polyamide column SPE and then separated by LVSS-CE. Reprinted with permission from H.-Y. Huang el cd. [189].

Since both the bubble cell and the Z-cell need high resolution in order to observe the sensitivity increase, test whether you can avoid the use by a clever injection procedure such as sample stacking or transient ITP (isotachophoresis) instead. Further information on detection approaches is provided in Chapters 3, 5, and 15. 

Sample dissolution in a low-conductivity solvent can induce sample stacking. This might also be achieved by injecting a low-conductivity solvent plug before or after the sample plug. 

Matson, M. T., Ramstad, T., and Dunn, M. J. (2005). Purity determination of alprostadil by micellar electrokinetic chromatography with signal enhancement involving field-amplified sample stacking and extended path length detection. /. Liq. Chromatogr. Relat. Technol. 28, 3181—3203. 

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. 

Pai, Y. E, Chun-Chi Lin, C. C., and Liu, C. Y. (2004). Optimization of sample stacking for the simultaneous determination of nonsteroidal anti-inflammatory drugs with a wall-coated histidine capillary column. Electrophoresis 25, 569—577. 

Chen, Y. R., Tseng, M. C., Chang, Y. Z., and Her, G. R. (2003). A low-flow CE/electrospray ionization MS interface for capillary zone electrophoresis, large-volume sample stacking, and micellar electroklnetlc chromatography. Anal. Chem. 75, 503 — 508. 

Jacobson, S. C., and Ramsey, J. M. (1995). Microchip electrophoresis with sample stacking. 

Mi) Low buffer concentrations reduce sample stacking following injection Lowering the applied electric field may reduce separation efficiency and raising the field strength may cause heating Easy to control... 

A method is described for extraction and determination of sildenafil citrate and its metabolites in human serum by micellar electrokinetic capillary chromatography (MEKC) coupled with sample stacking and polarity switching <2002JCH279>. 

As an example, Figure 4.9 shows the overall set of atomic peaks of the calibration samples stacked on the same plot. This corresponds to a study where antimony had to be determined by ETAAS in natural waters (drinking water, wells, springs, etc.) [12]. An extensive study of potential interferents [21] revealed that several ions may interfere in the atomisation process. A PLS multivariate study was carried out as an alternative to classical approaches and standard solutions were prepared with various concentration of Sb and the... 

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. 

Book ASTM Stand., Standard Method for Sampling Stacks for Particulate... 

R Szucs, J Vindevogel, P Sandra, LC Verhagen. Sample stacking effects and large injection volumes in micellar electrokinetic chromatography of ionic compounds direct determination of iso-a-acids in beer. Chromatographia 36 323-329, 1993. 

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