High-Speed Zone Refining

Zone refining is a process in which organic and inorganic compounds are purified.1 Like distillation and gas and liquid chromatography, zone refining is dependent on the distribution of material between two phases. However, the distribution in zone refining is between the solid and liquid phases, while the distribution in distillation and gas chromatography is between the liquid and vapor phases.

When a homogeneous liquid mixture is cooled, the composition of the solid that crystallizes out is usually different from that of the liquid: the purification of a substance by zone refining relies on this difference. Early Americans made use of this when they left a jug of fermented apple juice (hard cider) outside on cold nights. The water would freeze out of the mixture, leaving the alcohol and other impurities that could be poured off the ice the next morning. Repeating this process several nights would increase the concentration of alcohol to high levels.

 Figure 1 – MiniZone high-speed zone refiner.

Zone refining was developed in 1952 by William G. Pfann at the Metallurgical Research Department of Bell Telephone Laboratories to purify materials used in the semiconductor industry. In the manufacture of certain electronic components such as integrated circuits and transistors, extraordinarily pure specimens of silicon or germanium are required. Pfann found that by passing a molten zone down a rod of silicon or germanium, he could cause the impurities to move in the direction of the molten zone. Repeating this process multiple times reduced the impurities in the main part of the rod to a few parts per billion, and then concentrated them at the end of the rod.

High-speed zone refining

While used successfully to purify organic chemicals, zone refining is time consuming—purification takes days. The MiniZone high-speed zone refiner from Design Scientific (Holland, Mich.) (Figure 1) reduces that time to hours. It concentrates impurities in a sample or purifies chemicals, with purity of 99.999% (5N) or higher.

The compact instrument can purify 1–20 g of sample at a time. Controls include melt temperature, freeze temperature and zone migration rate. Any thermally stable sample that is a solid between –10 °C and + 300 °C can be purified.

Purification process

 Figure 2 – Zone-refining process.

In zone refining, a short molten zone travels slowly through an elongated sample of a crystalline material. A molten zone traversing a solid sample has two liquid–solid interfaces—a melting interface and a freezing interface. At the melting interface, the composition does not change; at the freezing interface, the impurities favor the liquid phase because the crystal structure tends to exclude molecules that do not fit into the crystal lattice. Passing the molten zone(s) through the sample many times concentrates the impurities at one end, leaving the opposite end highly purified (Figure 2).

 Figure 3 – MiniZone sample refining process.

Figure 3 shows a sample during the refining process in the MiniZone. One or two disposable, thin-walled PFTE sample tubes filled with the sample are clamped on each end in the tube transport mechanism (the metal blocks at the top and bottom of the photo). The transport mechanism slowly moves the tubes downward, passing them through the four heating and four cooling zones of the temperatureregulated metal blocks. After the tubes move downward the distance of one hot zone and one cold zone (2 cm), the transport mechanism is reversed and the tubes are quickly moved upward to the original position. The result of this motion is that the molten and crystalline zones migrate slowly and continuously upward.

With a typical zone rate of 2 cm/hr and four active zones, each portion of the sample is subjected to one purifying phase transition per hour. The sample is usually purified within 18 hours of automatic operation (18 recrystallizations). After the sample tube is cooled, it is removed from the MiniZone. The refined sample is easily recovered by cutting the tube into sections. Most impurities are concentrated at one end of the tube.

Applications

Applications of zone refining include metals, semiconductors and inorganic and organic compounds. The MiniZone is especially useful for scientists involved in preparing or using organic chemical standards for GC, HPLC, FTIR, absorbance or fluorescence spectroscopy or any analytical method that requires the highest purity for standardizing or calibrating instrumentation.

Another useful application is in the concentration of heat-labile biological materials in aqueous solutions, with the objective of utilizing the power of crystallization to concentrate an enzyme, peptide, antibiotic or any other thermally unstable substance.

Refer to Tables 1 and 2 for lists of compounds that have been purified using zone melting.

Table 1 – Compounds that have been purified using zone melting, with details available in the MiniZone users’ manual
Table 2 – More compounds that have been purified using zone melting (Design Scientific does not have details of these compounds available)

Reference

  1. http://courses.washington.edu/bioe555/Pfann.pdf

Julie DeCook is marketing communications director and office manager, Design Scientific, 590 East 32nd St., Holland, Mich. 49423, U.S.A.; tel.: 616-582-5225; e-mail: [email protected]www.designscientific.com

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