Magnetic levitation separates crystal polymorphs by their density

Sep 09, 2013
Magnetic levitation separates crystal polymorphs by their density

The effectiveness of crystalline pharmaceuticals is not only influenced by molecular composition; the structure of the crystals is also important because it determines both the solubility and the rate of dissolution, which in turn affect the bioavailability. Researchers from Cambridge, Massachusetts (USA) have recently developed a method by which different crystals can be separated by their density in a magnetic field. In the journal Angewandte Chemie, they have now demonstrated the extraordinary efficiency of separation through "magnetic levitation".

Many crystallize in multiple crystal structures known as polymorphs. Drugs are not the only class of products for which this can lead to problems. Different crystal structures can lead to color variation in pigments and dyes; in explosives it can lead to changes in sensitivity.

It is not always possible to control the to obtain only the desired polymorph. Clean separation is often difficult, and occurs either by chance or through long and complex procedures. A team led by Allan S. Myerson at the Massachusetts Institute of Technology and George M. Whitesides at Harvard University has recently developed a simple method that makes it possible to separate polymorphs conveniently and reliably within minutes through . The technique is based on the fact that different crystal modifications almost always have different densities.

Their clever method works like this: Two magnets are placed one over the other at 4.5 cm apart with like poles facing. This produces a with a linear gradient and a minimum in the middle, between the two magnets. The crystals to be separated are suspended in a solution of paramagnetic ions and placed in a tube within the magnetic field. The causes the crystals to sink down to the bottom of the tube. By doing so, a crystal "displaces" its own volume of the paramagnetic fluid "upwards". Yet, this is unfavorable, because the paramagnetic fluid is attracted by the magnet—the attraction gets stronger closer to the face of the magnet. The crystal sinks as long as it reaches a distance above the magnet where the gravitational force and the magnetic attraction on the equivalent volume of the paramagnetic fluid are balanced. At this point, the crystal will "float" in the fluid. As the strength of the gravitational force depends on the density of the crystal, the "floating point" is different for different crystal modification. The solution is then removed from the tube with a cannula and divided into multiple fractions.

Through separation of different polymorphs of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophencarbonitrile, sulfathiazole, carbamazepine, and trans-cinnamic acid, the scientists have presented impressive evidence of the efficiency of their new technique, which allows for the separation of crystal forms with a difference in density as low as 0.001 g/cm3.

Explore further: Machine learning algorithm makes impossible screening of advanced materials possible

More information: Whiteside, G. Using Magnetic Levitation to Separate Mixtures of Crystal Polymorphs, Angewandte Chemie International Edition.

Related Stories

Suspend the crystals, and they grow better

Dec 22, 2012

The idea is so simple you wonder why no one thought of it before.Crystals growing near the bottom of a beaker are subject to convection,but it is much quieter near the top of the beaker.In that case, why not just let them ...

How organic magnets grow in a thin film

Mar 23, 2013

( —Development of organic single molecule magnets opens a great many of applications for magnetic materials and new memory technologies. Organic magnets are lighter, more flexible and less energy intensive in production ...

Recommended for you

A dye with tunable optical characteristics

Sep 12, 2014

Researchers from RIKEN and the University of Tokyo have developed an organic dye molecule with tunable light-absorption and color characteristics. This development promises to open the door to the creation ...

How salt causes buildings to crumble

Sep 11, 2014

Salt crystals are often responsible when buildings start to show signs of aging. Researchers from the Institute for Building Materials have studied salt damage in greater depth and can now predict weathering ...

Ridding the sea and land from toxic plastics fragments

Sep 11, 2014

Plastic products made of PVC, Polystyrene and other prominent plastics are flooding the market. They are a growing threat to the environment, as they are found in the sea or dumped in land fills. But in a ...

User comments : 0