Online sample preconcentration, for

As mentioned before, when pressure-based sample injection is employed, the maximum injection volume must be less than the effective length of the capillary. At least 10% of the effective capillary length must remain available for separation. To inject a larger sample volume, electrokinetic injection must be employed. In most online sample preconcentration techniques, the maximum amount that can be injected without loss of separation efficiency is certainly less than the capillary volume. However, with a large volume sample injection under cathodic EOF conditions in SDS MEKC, a sample devoid of the micelle can be continuously electrokinetically injected for a volume equivalent to seven times the capillary volume without significant loss of separation efficiency under favorable... 

Most target analytes in environmental analysis are low in concentration, although the amount of sample is not very limited. As a result, CE is not an ideal technique for environmental analysis. A combination of a sample pretreatment such as SPE or liquid-liquid exU"action, and online sample preconcentration is clearly necessary here. There are several review articles published on the topic [65-67] and a detailed discussion of this topic can be found in Chapter 31 by Tonin and Tavares. 

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... 

The procedure described earlier for sample preconcentration can be easily extended for the online extraction of solid samples, e.g., powdered plant materials. Horizontal conbguration of the chromatographic plate in the chamber facihtates this procedure, because the sample to be extracted is then placed on a carrier plate at the begiiming part of the adsorbent layer (or in the scrapped channel of the adsorbent layer), which should be directed upward [15,26]. The chamber is covered with a narrow plate, and the development is started with a snitable extracting solvent. In some cases, it is advantageous to put the narrow plate directly on the adsorbent layer to press the sample to be extracted. Extracted components are preconcentrated on the adsorbent layer at the end of the narrow plate, as shown in Fignre 6.26 [15]. 

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... 

Pre- and guard columns can be used for online sample cleanup in conjunction with LC, which helps maintain sample integrity while protecting expensive and fragile stationary phases. Samples may also be preconcentrated prior to elution onto separation column. 

Slllca-lmmoblllzed brown alga Pilayella littoralis Online metal preconcentration procedure in a flow-injection system for determination of Al, Co, Cu, and Fe in lake water samples by (ICP-OES) [70]... 

The majority of publications on preconcentration for FAAS and ETAAS using FI techniques are based on solid sorbent extraction [47,51]. A typical FI manifold for this type of application is shown schematically in Fig. 8. A microcolumn with 50-100 jiL of sorbent for FAAS or 15 p.L for ETAAS is mounted on the FI valve instead of the sample loop. Sample and complexing agent are mixed online and the analyte complex collected for typically 30-60 sec. The analyte is then eluted directly into the flame or a graphite tube atomizer, usually reversing the direction of flow compared to sample loading. 

Pereiro Carcla et al. [53] described an online preconcentration of aluminum from dialysis concentrates using cation and anion exchangers, Chelex 100, and a synthetic resin, followed by FAAS determination. There is no doubt that there are many more application possibilities when the wealth of chemical literature on ion exchange, chelate formation, and solvent extraction is reexplored for online FI sample handling. Online preconcentration could become even more attractive if it is combined with online sample digestion [54,55]. It was demonstrated that... 

FIG. 8. How injection manifold for online sorbent extraction preconcentration for electrothermal AAS (simplified). PI, P2 = peristaltic pumps PC = precolumn V = valve C = conical microcolumn GF = graphite tube atomizer W = waste, (a) Sample loading on column, (b) Analyte elution. Dotted lines indicate directions of flow. (From Ref. 61 by permission.)... 

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