Sample Pre-Fractionation

BioMEMS Microfluidics Nanofluidics Preparatory separation Sample pre-fractionation... 

Sample pre-fractionation Preparatory separation Microfluidic devices Microsystems... 

Fig. 29. Integral distribution of the degree of polymerization for polystyrene sample 0/2 polymerized anionically in tetra-hydropyran at 0 "C, Pw = 1225) and Baker-Williams-pre-fractionated PDC at 15 °C (O) and Baker-Williams fractionation of this sample by Bohm J9) (+). The integral distribution obtained by Bohm is somewhat narrower than that obtained from PDC, because of some different Pw-values (aP = Pw(/ Uz, Bohm had probably taken Pn instead of Pw)...

There are two basic approaches off-line and on-line. The off-line method, as discussed in the chapters on sample pretreatment, are most often used because they involve either manually or automatically collecting a fraction from a sample cleanup sorbent. The appropriate fraction is transferred and then assayed by a second chromatographic method. The manual steps are time-consuming and potentially introduce significant error to the j precision and accuracy of the method. The on-line method, when fully automated, would have the chromatography system perform sample pre- j treatment by column switching between two or more columns. j... 

It is possible to use multiple solvents to generate samples with varying solubility characteristics from the same ground plant material [sequential extraction of a single ground plant sample with solvents of increasing polarity (1) hexane, (2) EtOAc, (3) EtOAc/MeOH, (4) MeOH/H20], This sequential solvent use is a crude form of pre-fractionation of extract samples. More sophisticated protocols for fractionating extracts are often proprietary (Appleton, Buss and Butler, 2007 natural product company list personal communications Kip Guy, St. Jude Ikhlas Khan, NCNPR). 

On a sample pre-calcined at 673 K, further flash-heated under vacuum at 800 K, and finally treated with CO at 200 K, the subsequent TPD run showed that 63% of the preadsorbed CO was actually desorbed as CO2 (149). This fraction was much larger than that observed in a parallel study on the bare support. On repeating the experiment in a cyclic manner, successive but decreasing amounts of CO2 were observed. Conversely, on the catalyst reduced with H2 (Pic J0 Torr) at 773 K (5 min), prior CO adsorption, CO was practically the only desorption product The initial behaviour of the catalyst could be restored by subsequent oxygen treatment at 373 iC (149). Figure 4.11 summarizes the TPD-MS diagrams recorded after the two latter experiments. 

First the sample is to be chosen in an adequate manner. It can be a raw biological fluid, a cell extract, a pre-fractionated sample, etc. The choice of the sample is crucial, as it is strongly dependent on the type of separation to be applied. The sample has to be compatible with the complexity both in number of components and in the dynamic range the separation can handle. Methodological aspects are discussed in Section 4.2 of this Chapter. 

Given that the SGV is driven by ingestion of soil and indoor dust, protocol 1 best reflects the likely exposure to cadmium in the potentially contaminated soil. Thus, it is very important to ensure that the correct sample pre-treatment protocol is adopted and that the end-user of this data understands this protocol and how the analysis is reported. Not all laboratories clearly specify the protocol that they use. It is also essential that the fraction of the sample discarded prior to sub-sampling for analysis is recorded in the analysis certificate. Also, details of any unusual artefacts or large debris that were removed prior to the analysis pre-treatment stages should be included in the analysis certificate. 

An integrated module for on-line sample pre-analytical treatment and/or clean-up could be also included in the device. With this respect, field-flow fractionation (FFF) techniques, which can separate analytes based on their morphological characteristics (size, shape and superficial properties) can be exploited to develop pre-analytical modules for cells or macromolecules (e.g., proteins, protein complexes or adducts) fractionation, thus providing a selectively enriched fraction for the analysis. 

The necessity for cleaning and reconditioning a column depends on the sample introduced. Samples that have not been pre-fractionated often have too large an elution range and repetitive cyclic separation of compounds is not possible, except by using the gradient elution technique and reconditioning the column. 

D PAGE Volume 1 , Sample Preparation and Pre-Fractionation, edited by Anion Posch, 2008 393. 

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