Beginning with centrifuge experiment

This is the first time I do a centrifuge experiment with my own centrifuge machine, it's just simple made by a rotor that can rotate the tube at high speed. As I know a centrifuge machine can be using for separate biology samples like bacteria, cells, molecules ... But I need some advice to understanding more how to working with it.

As the suggestion from Alan Boyd, I'm planning to separate Lactobacillus from diluted yogurt. How do I know where the bacteria will be located in the tube after centrifugation (is it usually in bottom ?) and how do I calculate the RPM that is suitable ? And any other interesting diy experiment to do with it ?

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2 Answers

It would be interesting to see a picture of your centrifuge, which you appear to have built in a few days.

Centrifugation simply accentuates the effects of gravity, and since bacteria are denser than aqueous solutions they will move to the bottom of the sample to form a fairly tight pellet.

The conditions for your centrifugation will depend very much on the design of the centrifuge since as well as the rotational velocity, the radius of rotation is also important.

I found a suggestion on the internet to use a relative centrifugal force (RCF) of 12000 g for 2 minutes (I must admit that from memory I always spun slower and for longer than this suggests). Anyway...

using RCF = 1.12 * 10-5 * r *N2

r = radius of rotation
N is revolutions per minute

and plugging 12000 g into that, for a rotational radius of 10 cm, I get 3300 rpm.

Don't know if you can manage that with your machine, but as I say, a lower RCF with a longer spin will probably be ok as long as you can dilute the yoghurt sufficiently to reduce viscous drag. Of course if you have access to any microbiological growth media you could grow a standard liquid culture by inoculating some sort of broth with a small amount of yoghurt.

Try a Google image search for hand powered centrifuge to get an idea of what has been used in the past.

after comment from Mad Scientist

see here for a typical large table top centrifuge maximum RCF is 3120 g at 4600 rpm. You can certainly pellet bacteria in something like this in tens of minutes.

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12.000g sounds rather high to me for a tabletop centrifuge. For bacteria I'd use 4000-6000g for 15-30 minutes. – Mad Scientist Jul 10 '13 at 14:46
@Mad Scientist - agreed, I have supplemented the answer, which was only intended to get the questioner thinking about his own centrifugal circumstances. – Alan Boyd Jul 10 '13 at 15:33
@MadScientist depending on his design, it could be closer to a microfuge, in which 12,000g is quite reasonable. I'm more worried about balance and shielding in case the thing goes off kilter (that's a technical term, by the way :) – MattDMo Jul 10 '13 at 16:27
Ok, i'm fairly new to the "g" unit because i never using it before and familiar to the "rpm" unit. But i will research more about it now, thank you so much. – DucFabulous Jul 11 '13 at 2:06
@AlanBoyd thank you so much, your information is very detailed. – DucFabulous Jul 11 '13 at 2:08

I certainly don't want to discourage experimentation and self-learning, but please be extremely careful. A centrifuge has to be perfectly balanced at high speed to stay stable. As an example, one of the table-top centrifuges I use has a steel jacket that's perhaps 1 cm thick or more surrounding the rotor, just in case there is a balance issue or the rotor breaks from mechanical stress. A Google image search for centrifuge accident shows what mechanical stress can do when an improper rotor was used on a high-speed centrifuge for just a few hours. Keep in mind that angular momentum increases both with the mass of the object being rotated, and with the moment of inertia, which increases as the radius of the object being rotated increases. In other words, if you tie a ball to a short piece of string (say 0.5m) and swing it around your head, you can feel a certain amount of force. Take that same ball and tie it to a 2m piece of string, and at the same number of rotations per minute the force will increase substantially. All this means is that as the radius of the rotation increases, the amount of g force increases as well, but the stress on the machine also increases, especially if the balance is off.

So, good luck with your experiments, just remember that safety always comes first. You can replace samples, and you can replace equipment, but you can't replace you. Even simple precautions like always wearing goggles or protective safety glasses should always be followed. Don't work alone, or in a situation where there is no one to call for help if you get hurt. If something doesn't feel right, don't do it.

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thank you so much for your advice, i deciding to add a chip for detect vibration - automatic stop, and using a secure box to cover the motor. – DucFabulous Jul 11 '13 at 2:01