Solvent evaporation automated evaporative concentration

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. 

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. 

Anchor plates for the preparation of multiple samples have small hydrophilic islands, typically of 100-500 pm diameter, placed on a hydrophobic surface [147]. The hydrophobic surface prevents spreading of the sample solution over a larger area, as otherwise observed for dried-droplet preparations. Instead, the hydrophilic solution contracts onto these islands, thereby concentrating the matrix and analyte onto a small defined area upon solvent evaporation. This confinement to a smaller volume is particularly useful for analytes of low concentration in combination with proportionally lowered matrix concentrations, and also facilitates automated analyses of the fixed-location samples. Anchor sample plates are also commercially available as disposable targets prespotted with matrix and calibration spots. 

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. 

Fig. 4 CHCA affinity MALDI sample preparation of peptides. This technique takes advantage of prestructured sample supports (hydrophilic sample anchors surrounded by a hydrophobic support) and the observation that microcrystalline CHCA has a high RP affinity and binding capacity for peptides. It integrates sample purification and concentration in the last step of the sample preparation. Typically 0.5-2.0 p,L of acidified sample solution (pH 1.5-2.5) is deposited onto one matrix spot measuring 400, 600, or 800 (jtm in diameter. Depending on the pimity and concentration of the samples, they are either allowed to dry at ambient temperature (option 1) or removed after a defined incubation time, e.g., 3 min (option 2). In either case, all samples are washed once or multiple times with a larger volume of acidified water (3-8 xL) before they are analyzed. AH these steps can be performed manually or automated using a pipetting robot as shown on the left. If the samples contain a lot of undesired contaminants that are difficult to completely wash away, option 2 is preferred. If their concentration is very low, the affinity purification yields benefit from longer incubation times because the samples volumes continuously shrink over time until all solvent is evaporated. Therefore, if the contaminants can easily be washed away, option 1 is recommended because it provides maximum sample concentration and is easier to perform than option 2...

For molecular weights less than about 20,000, another technique has been automated. In the so-caUed vapor pressure osmometer, there is no membrane [21]. A drop of solution and a drop of pure solvent are placed on adjacent thermistors. The difference in solvent activity brings about a distillation of solvent from the solvent bead to the solution. The temperature change that results from the differential evaporation and condensation can be calibrated in terms of the number-average molecular weight of the solute. The method is rapid, although multiple concentrations and extrapolation to infinite dilution are still required. 

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