Dr David Griffin looks at a common laboratory problem and how to control it.
Most scientists are familiar with bumping. Perhaps in a heated beaker of liquid with a burner, when one can see bumping when the liquid at the bottom of the vessel boils suddenly.
It boils at the bottom first because that is where the liquid in the beaker is hottest (where the heat is applied), and it boils suddenly because at first there is nothing convenient to nucleate the gas bubble.
Some local superheating builds up until a bubble simply has to form; and once the bubble formation has started, the bubble continues to grow (with more liquid boiling off) until all the superheat present is used up.
Had there been something to nucleate the bubble earlier, a far smaller bubble would have formed because there would be less superheat energy stored up.
This explains why aanti-bumping granules' are used to make boiling smoother and more controlled. Nucleation sites (eg sharp corners) lower the initial barrier to creating a bubble.
Unfortunately, bumping in a centrifugal evaporator is quite different. It is also very serious, because it can cause expensive cross contamination and sample loss.
Pressure gradient
The first concept we need to understand is that, unlike the example above, a vial of a single solvent in a centrifugal evaporator does not start boiling at the bottom, even though the heat is supplied predominantly at the bottom. It boils at the top, at the surface of the liquid.
This is because of the centrifugal force, which creates a pressure gradient in the liquid. At 300g, the pressure 10mm deep in a tube of water is 300mbar.
So if, for example, the tube of water is in an evaporator at low speed (300g) with a chamber pressure of 8mbar, the pressure 10mm below the surface is 308mbar.
Because boiling point varies with pressure, the boiling point at the surface of the water will be 4oC, but 10mm down the boiling point will be 72oC. But, will the water at the bottom of a 10mm deep sample ever get to 72oC as the result of the heat coming in?
The answer is certainly not, as there is not going to be a temperature gradient that steep, as the water would rather set up a circulating convection current than sit still and conduct heat as if it was a lump of metal.
Convection currents
Convection currents require a change in density with temperature and gravity, both of which are present in the sample. So it is obvious that it is never going to boil at the bottom of the tube even if the heat is applied there.
Many people imagine that the only function of the centrifugal force in counteracting bumping is to hold the liquid down in the tube should it actually bump. But one can see from the example above that the very presence of high g forces encourages boiling to happen only at the surface in a simple single solvent situation.
After considerable investigation, Genevac scientists have concluded that there are a significant number of relevant cases where you simply cannot prevent or contain bumping without at least 500g force being applied to the sample. This means that when thinking about centrifugal evaporation, if you suspect your samples will bump, you must consider equipment capable of sustained high g force to control it.
Anti-bumping
The Genevac Series II systems have an anti-bumping feature called Dri-Pure, which ensures pressure descends at a controlled rate and the rotor speed increases to the high (500g) figure.
Either of these features alone will not be able to prevent all bumping. But with both your samples are safe.
Centrifugal evaporators other than those supplied by Genevac, are designed to subject the samples to accelerations of about 250g.
Increasing the capability of the evaporation system to provide reliable operation at accelerations of 500g requires rugged design and manufacture of not only the rotating parts, but also the drive components. This explains the more substantial construction of Genevac evaporators.
Enquiry No 17
Dr David Griffin is an applications specialist with Genevac Ltd, Ipswich, Suffolk. UK. www.genevac.com
