Online sample concentration

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... 

Achiral-chiral multidimensional chromatography remains one of the best ways to separate chiral analytes from interfering matrix components or other compounds. The flexibility offered by different operation modes, stationary and mobile phases, and configurations allows analysis methods to be tailored to the analytical problem. By offering possible configurations for both online sample cleanup and concentration, achiral/chiral LC/LC reduces manual sample preparation. The ability to be coupled to... 

In our laboratories, a cycle time of 90 sec can be achieved with a dilution factor of 1 25 for a given sample concentration, allowing the purity and identity control of two and a half 384-well microtiter plates per day. The online dilution eliminated an external step in the workflow and reduced the risks of decomposition of samples in the solvent mixture (weakly acidic aqueous solvent) required for analysis. Mao et al.23 described an example in which parallel sample preparation reduced steps in the workflow. They described a 2-min cycle time for the analysis of nefazodone and its metabolites for pharmacokinetic studies. The cycle time included complete solid phase extraction of neat samples, chromatographic separation, and LC/MS/MS analysis. The method was fully validated and proved rugged for high-throughput analysis of more than 5000 human plasma samples. Many papers published about this topic describe different methods of sample preparation. Hyotylainen24 has written a recent review. 

Typical instrument set-ups for online sample preparation, for example, solid phase extraction and sample pre-concentration, require the control of a six-port valve with an additional special column and a second pump. Ideally, this system will be fully controlled by the data acquisition software (Figure 3.12 (A)). 

FIGURE 14.12 Fully automated nano HPLC-MS with autosampler injection of large sample volume using two parallel online sample trapping, concentrating, desalting, and filtering columns. Pumps 1 and 2 transfer sample from loop of the autosampler into trap column 1 while pumps 3 and 4 elute sample in trap column 2 into the analytical column then to MS. 

A very recent series of publications by Locascio and others [54-56] demonstrated successful applications of various plastic materials, such as polydi-methylsiloxane (PDMS), polymethylmethacrylate (PMMA), copolyester, and their combinations, for assembling integrated fluidic structures to perform online sample pretreatment by affinity dialysis and concentration for fast and sensi-... 

Other types of sample concentration enhancement approaches applicable to CE include ITP " and those involving concentration on an online solid phase. This latter method shows much promise for both small and large molecules and is covered in detail in the review by Wettstein and Strausbauch. ... 

There are many books, book chapters, or review articles on fundamental MEKC [4-11], In particular, online sample preconcentration techniques are becoming popular in CZE to improve concentration detection sensitivity and so are in MEKC, which will be introduced rather in detail. Although it is generally stated that CE requires minimal amounts of sample (certainly true), it should be mentioned that the volume of sample solution required is much greater than that actually introduced into the capillary because the injection end of the capillary must be dipped into the sample solution— this is typically at least a few microliters. 

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. 

A major advantage of NMR spectroscopy is that no calibration is needed for quantification in most cases, and the method features a high linearity between absolute signal area and sample concentration, also for boundary areas in concentration. Furthermore, online NMR spectroscopy allows investigations under elevated pressures, for example, to prevent the solutions from boiling, or for studies under process conditions. 

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