Hello Martin, I guess you are using something like streptavidin-coated magnetic beads for separation? If this is the case you can use any kind of stronger magnet to perform the assay. Magnets for example from door seals of disused fridges will do fine. These magnets have the right hight and strength and you can cut them to fit the length of an (of course hollow) Eppendorf-tube rack. Just place the magnet on the bottom of the rack, make sure that the tubes are in direct contact with the magnet and perform the assay. To get an idea how this could look like just search for "dynal magnetic rack" and build your own according to the one you like the most. HTH, Robert

Thanks, Robert - I was hoping to get an answer like this;-)

Hello Martin and Robert,

I think is better to put the magnets on a side of the tubes. Putting the magnets on the bottom would make imposible to know if you are doing magnetic separation or "gravitational" separation.

Do not adquire big and expensive magnets, because the force that moves the particles depends on the gradient of magnetic inductance, grad(B). I'll try to explain it without equations. If you buy a big and, acording to manufacturer, very strong magnet you could expect that your magnetized DNA would suffer a bigger force, and they would separate in less time. But what you would have is a bigger magnetic inductance, smaller gradient, and slower separation. Any magnet of the same material would have more or less the same magnetic induction, but as the highest gradient is located at the corners of the magnets, the smallest the dimension of the magnet that is near your tube, the fastest separation.

I recommend NbFeB ring-shaped magnets, as shown in this video.

http://youtu.be/keA5Rnj-WPg

You can achieve fast separation easily, and cheaply.

Stronger magnetic fields will speed up separation and also maximise the concentration level. Instead of permanent magnets, you could use simple electromagnets (copper coils with a DC current flowing through them) to concentrate the material in the desired location. This will also allow you to experiment with different field strengths.

Hi David, you are absolutely right I should have written it in more detail. Of course the magnet should be in contact with the side of the tube - this is what I meant, how we did it and how those Dynal separators work. But anyway, I really like that ring-shaped magnet approach you referred to. Do you know where to buy those and an approximate price?

Hi Robert,

Yo wrote it perfectly, I only wanted to add two recomendantions based on my experience with magnetic separation.

I bought the ring magnets online, https://www.hkcm.de, but there are more web pages, for example http://www.magnetmonster.de. I have two sizes, one group of rings for eppendorf tubes, and the other group for small falcon tubes. A group of 10 for eppendorf sizes costs 10 € (without taxes and delivery).

Hi Jacob,

Definition of the strength of a magnet could be misunderstood. If you define "strength" as the valour of the magnetic inductance B, any magnet of the same material would have the same "strength", independently of the size. Nowadays NdFeB are the "strongest" magnet easily avaliable, and they are also very cheap. A NdFeB magnet could raise several times its weight. The bigger the magnetic object you want to raise, the bigger and "stronger" the magnet (second definition of strength) you will need. But for magnetic nanoparticle separation, the key parameter is the gradient of induction.

Copper coils with DC current are not electromagnets, they are solenoids, and for the same size, they are not as "strong" as a NdFeB magnet. Electromagnets are solenoids with a ferromagnetic material inside (magnets are ferromagnetic materials too), and they are "stronger" than magnets.

David, Permanent magnets come in many grades. Being able to hold many times their own weight is not a reliable measure of magnetic field strength. http://tekmags.com/Properties is a good example of the many different grades that are available. With permanent magnets like NdFeB you can achieve a field of about 1 Tesla over a small volume. With an electromagnet (configured as solenoid, helmholtz coil, or dipole), and suitable poles it is feasible to achieve a field strength of at least 2 Tesla, or say 1 Tesla over a larger volume. While permanent magnets are cheap and easy to use. Their usefulness depends on the configuration of the magnetic separator and how strongly the magnetized DNA has been magnetized. Best is to experiment, if the speed and degree of separation are sufficient, permanent magnets are obviously OK. If not, electromagnets are required. It is possible to achieve a higher effective field strength with permanent magnets by using a Halbach arrangement, but that may not be worthwhile for magnetic separation as it requires careful design and possibly some machining. Electromagnets would be much a simpler

Jacob,

If you have 1 T over the complete volume of a eppendorf, magnetized particles are going to rotate, but are not going to translate, so they are not going to concentrate. This is not intuitive, but it is how it works. Check for example http://iopscience.iop.org/0022-3727/36/13/201.

I wrote "strength" with inverted commas because "strength" is not used in magnetism. "Strength" has not defined meaning, but is used by sellers and divulgation. You used "grade" and leave a link with values of magnetic field, H, and magnetic inductance B (mentioned in my two answers, and the first hypothetycal definition I wrote) . Anyway, magnetic induction (B) or magnetic field (H) values are not the important parameter for this application. Your sentence "Stronger magnetic fields will speed up..." is mistaken. Higher gradient of magnetic inductance does speed up the separation.

Magnetized DNA is extremely light. A 10 $ prize magnet would be betteror much better than a 100-1000 $ electromagnet.