Automated evaporative concentration

To understand the requirements for an automated evaporative concentration system. 

Pre-concentration is concerned with the reduction of a larger sample into a smaller sample size. It is most commonly carried out by using solvent evaporation procedures after an extraction technique (see, for example, Chapters 7 and 8). The most common approaches for solvent evaporation are rotary evaporation, Kudema-Danish evaporative concentration, the automated evaporative concentration system (EVACS) or gas blow-down . In all cases, the evaporation method is slow, with a high risk of contamination from the solvent, glassware and blow-down gas. 

Table 3.15 summarizes the advantages and disadvantages of various extraction techniques used in the analysis of semivolatile organic analytes in solid samples. They are compared on the basis of matrix effect, equipment cost, solvent use, extraction time, sample size, automation/unattended operation, selectivity, sample throughput, applicability, filtration requirement, and the need for evaporation/concentration. The examples that follow show the differences among these techniques in real-world applications. 

Dienstbach and Bachmann [38] have determined plutonium in amounts down to 20 fCiP/ug soil in sandy soils by an automated method based on gas chromatographic separation and a-spectrometry. In this procedure, the sample is decomposed completely by hydrogen fluoride. The hydrogen fluoride is evaporated and the residue is chlorinated. Plutonium is separated from the sample by volatilisation and separation of the chlorides in the gas phase. The plutonium is deposited on a glass disk by condensation of volatilised plutonium chloride. The concentration of plutonium is then determined by a spectroscopy. 

The use of organic solvents may constitute a matrix compatible to subsequent liquid chromatography, thus not requiring any concentration or evaporation step. However, protein precipitation seems to be inappropriate for automation and thus requires a manual workflow. 

Several academic partners and Siemens Medical Solutions USA Inc. (Molecular Imaging) in Culver City, USA, made the synthesis of an [18F]fluoride-radiolabeled molecular imaging probe, 2-deoxy-2-[18F]fluoro-D-glucose in an integrated microfluidic device (see Figure 5.1) [21]. Five sequential processes were made, and they are [18F]fluoride concentration, water evaporation, radiofluorination, solvent exchange and hydrolytic deprotection. The half-life of [lsF]fluorine (t1/2 = llOmin) makes rapid synthesis of doses essential. This is one of the first examples of an automated multistep synthesis in microflow fashion. 

While evaporation is used for the concentration and removal of solvents, usually the reaction by-products are not volatile. Similarly, filtration of precipitated or crystallized solids is not likely to be applicable to all the members of a library, and furthermore the automation of these processes is not straightforward an interesting example of general precipitation of library members from an organic medium due to the presence of a basic ionizable group has been recently reported by Perrier and Labelle (87). Extraction procedures possess the desired separation properties and have been used for the purification of several solution-phase libraries we will cover this subject in more depth in this section. An excellent review (88) has recently been published in which the interested reader will find a description of available strategies for separation and purification of single compounds and arrays. 

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