Concentration Techniques

Both extraction, derivatization processes often conclude with the sample being diluted in solvent and the solution will need to be concentrated. However, the concentration procedure can cause sources of error, particularly if the sample is small and has even a slight, but significant vapor pressure at the evaporation temperature. Assuming concentration without loss, then there are a number of devices available that will help concentrate solutions by evaporation. However, many of them (e.g. the Kurderna-Danish concentrators, rotary evaporators, turbo evaporators etc.) are largely used for preparative work, and are not useful for analytical or micro-analytical procedures. A common form of evaporative concentrator used in chromatographic analysis is shown in figure 25. It is a simple tube contained in a heated block and provision for passing a stream of filtered nitrogen or argon over the sample surface. The block can be thermostatted at any desired temperature and can have multiple apertures for drying tubes and a nitrogen manifold to evaporate 25 or 96 samples at a time. A gentle and safe method of solvent removal, particularly for aqueous solutions is lyophilization.

Figure 25. A Micro Evaporator

The solution is frozen and the solid diluent removed under vacuum. There is finite vapor pressure over the solid and all the solid diluent can be removed as vapor under vacuum. Due to the low vapor pressure of a solid, the process can take several hours to complete. This technique is commonly used in biotechnology, and equipment is available fitted with micro containers for use in sample preparation for chromatographic analysis.

Another sample-concentration technique suitable for labile materials is exclusion chromatography. If the solute molecules are large, and the solution is passed through a dry exclusion column, the solvent molecules preferentially enter the pores, and the large solute molecules are carried along with the solvent front. This procedure is, in effect, a drying system, but can be carried out at room temperature without incurring sample loss or decomposition. Extraction techniques that do not produce a diluted sample are to be preferred and it is this aspect of super critical fluid extraction that makes it an attractive alternative for those samples with which it can be used.