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In pharmaceutical and chemical synthesis laboratories, high performance liquid chromatography (HPLC) is probably the most frequently used analytical separation technique. A widespread and growing application for HPLC is for product purification following chemical synthesis. Common HPLC solvent mixtures used to separate the products of synthesis include water and acetonitrile (ACN) and also water and methanol.
To identify the structure of the newly synthesised compounds the HPLC fractions typically have to be dried before being prepared for analysis by techniques such as mass spectrometry (MS), nuclear magnetic resonance (NMR) or infrared (IR) spectroscopy. Centrifugal evaporation is an ideal technique for drying HPLC fractions as (with the right equipment) it can be both fast and controllable. There are, however, certain techniques to be aware of without which problems may be encountered and drying times will be needlessly long.
The driving force for centrifugal evaporation
Chemists used to synthesize compounds individually and remove solvent using a rotary evaporator. Demand for more efficient methods means many compounds are now generated by parallel synthesis. Inevitably some form of concentration is required to dry down the final purified products and this is where a centrifugal evaporator (Figure 1) becomes the equipment of choice.
The boiling point of a solvent reduces as the surrounding pressure is lowered. Thus for a given unit heat, the rate at which the solvent boils off will increase as pressure decreases: less energy is required to give complete evaporation. The rapid rotation of the centrifuge ensures that the dried products stay in the bottom of the vessels.
A phenomenon familiar to many scientists who have heated solutions in static vessels is known as ‘bumping’, which results in samples being lost or cross-contaminated. Bumping is a result of large bubbles of vapour forming suddenly at the bottom of the vessel where the heat is being applied. If the bubbles are formed too violently then liquid can be ejected from the evaporation vessel. Solvent bumping is caused by a variety of factors and is unpredictable. Usually it is with mixtures of solvents that the worst bumping occurs. However, bumping may be avoided by using appropriate precautions.
Tags: chemical synthesis, fractionation