Drying Down

Once we have obtained a number of solutions containing groups of molecules extracted from our red algae, it is time to dry down those solutions.  This involves evaporating off the solvent so that solid compounds remain.  To do this we use a rotary evaporator.

A rotary evaporator works by spinning the solution in a round-bottomed flask. The solution is spread around the inside surface of the flask and this increases the surface area from which the solvent can evaporate.

The rotary evaporator is attached to a pump which removes some of the air from inside the evaporator and therefore creates a partial vacuum. As the gas pressure above the surface of the solution is decreased, the solvent will evaporate off more quickly than it would under normal pressure.

The rotary evaporator to the left has a cold finger. This is the cone shaped object you can see inside the top part of the evaporator.  This is made very cold by filling it with dry ice (by removing the yellow lid at the top).  Solvent gas that has evaporated from the solution condenses into liquid when it comes into contact with the cold finger.  The liquid then drips into the flask at the bottom of the condenser.

As the solvent evaporates from the flask, the flask begins to get very cold and ice may even begin to form on the outside of it.  This is because when liquids evaporate they take heat from their surroundings.  If the flask gets too cold it will slow down the rate at which the solvent evaporates.  To get around this, the flask can be lowered into a water bath which prevents the flask from getting too cold.

On of the tricky things about drying down solutions is that they sometimes become unstable (particularly if they contain a mixture of water and acetone).  This can result in them bumping and spurting material up towards the condenser.  This would be a problem as we would be losing precious material and also contaminating the rotary evaporator.  For this reason, there is a glass trap above the round-bottomed flask. (It is between the red and green clips in the picture.)  Material that spurts gets caught in the trap and can be retrieved from there.


Once the solution has been dried down, you are left with a mixture of solid compounds attached to the the inside of the flask.  As the flask would be bulky to store (and we want to free it up so we can use it again), the next step is to transfer the compounds into a small glass vial. Before doing this we use a balance to find the mass of the vial so that we can later calculate the mass of material in the vial.


To transfer the material we need to redissolve it.  In the lab we use a solvent referred to as Magic Brew.  It is called this as it will dissolve most molecular compounds.  Magic Brew is a mixture of methanol and dichloromethane.  A small amount of Magic Brew is added to the flask in order to redissolve the solid compounds.  A glass pipette is then used to transfer the solution into the glass vial. The solution then needs to be dried down again on the rotary evaporator.  

The final step is to remove any traces of water that may not have evaporated on the rotary evaporator (as water has a higher enthalpy of vaporisation than most of the organic solvents we are using).  Any water needs to first be frozen and this is done by rolling the flask in liquid nitrogen.  The flask is then attached to a freeze dryer. As with the rotary evaporator, a freeze dryer creates a partial vacuum. This causes the frozen water to sublime (turn straight into  gas).  The water vapour is continually removed from the system by the freezer component of the freeze dryer.

Finally, the vial is reweighed and then stored in the fridge until we next want to work on the sample.