Freeze-drying technique

Insensitive to impact, it decomposes, sometimes explosively, above its m.p. [1], particularly if heated rapidly [2], Although used in aqueous solutions as a preservative in pharmaceutical preparations, application of freeze-drying techniques to such solutions has led to problems arising from volatilisation of traces of hydrazoic acid from non-neutral solutions, condensation in metal lines, traps or filters, and formation of heavy metal azides in contact with lead, copper or zinc components in the drying plant [3,4],... 

Nevertheless, it may sometimes be proven extremely difficult to extract highly polar drug residues from edible animal products at any pH value. In that case, a possible solution is salting out by addition of sodium sulfate until the aqueous phase of the sample disappears (59) or use of a freeze-drying technique. After freeze-drying, tire resulting solid residue can be dry-extracted with a suitable... 

Itatani, K., Akiyama, K., Aizawa, M., Scott Howell, F., Okada, I. Sinterability of apatite-zirconia composite powder prepared by ultrasonic spray freeze-drying technique. J. Soc. Inorg. Mater. Jpn. 286,194-202, 2000... 

Heinzelmann, K., Franke, K., Velasco, J., and Marquez-Ruiz, G. (2000). Micro-encapsulation of fish oil by freeze-drying techniques and influence of process parameters on oxidative stability during storage. Fur. Food Res. Technol. 211,234 239. 

Dudek, R W., Vamdell, I M., and Polak,/. M. (1984) Combined quick-freeze and freeze-drying techniques for improved electron immunocytochemistry, in Immunolabelbngfor Electron Microscopy (Polak,/. M. and Vamdell, I M, eds.), Elsevier, Amsterdam, pp 235-248... 

In addition to the freeze drying technique to prepare protein-based molecularly imprinted materials, chemical cross-linking has also been employed to prepare... 

The freeze-drying technique is also used in the analytical laboratory. Two additional benefits make it attractive to analysts, namely (a) because the removal of water allows samples to be subsequently reconstituted in a minimal volume of liquid, a preconcentration effect is achieved and (b), even more important, because of their texture, freeze-dried materials are very easily attacked by solvents. The process can be optimized for a specific purpose by using an appropriate temperature, pH, reagent addition sequence and time. One major disadvantage of freeze-drying, however, is that some volatile compounds may be removed from the sample matrix during the process. 

A product can be freeze-dried for various purposes depending on the particular features the resulting material is required to possess. Below are discussed some of the more salient uses of the freeze-drying technique. 

Broader, more systematic studies need to be conducted with a view to unequivocally elucidating the behaviour of compound families towards the freeze-drying technique and establishing its true potential, which has so far been underexploited in analytical chemistry. 

Left - Recovered by conventional vacuum fi Itration and drying. Right - Produced by the spray-freeze/freeze dry technique. Both have surface areas of 78 m /g. 

Brown Boveri Cie and Electrolyzer Corporation achieved a main progress in the mid-1980s with respect to the performance of the alkaline monopolar cells. The first company was able to enlarge the efficiency from 2 to 3.4 kA/m2 with 500 m3/h [13], on the other hand the second company that was able to achieve only 1.74 V reached 134 mA/cm2 at 87% efficiency with nickel-boride materials and NiCo204 anodes (prepared from the freeze-dry technique) [14]. 

Apart from the high cost, the freeze-drying technique remains an excellent and almost unique method for preserving a variety of compounds, among which are valuable medicinal products sensitive to humidity. 

Ishikawa K, Ducheyne P, Radin S (1993) Determination of the Ca/P ratio in calcium-deficient hydroxylapatite using X-ray diffraction analysis. J Mater Sci Mater in Med 4 165-168 Ishikawa K, Miyamoto Y, Kon M, Nagayama M, Asaoka K (1995) Non-decay type fast-setting calcium phosphate cement composite with sodium alginate. Biomaterials 16 527-532 Itatani K, Iwafune K, Howell FS, Aizawa M (2000) Preparation of various calcium-phosphate powders by ultrasonic spray freeze-drying technique. Mater Res Bull 35 574-85 Ito M, Hidaka Y, Nakajima M, Yagasaki H, Kafrawy AH (1999) Effect of hydroxylapatite content on physical properties and coimective tissue reactions to a chitosan-hydroxylapatite composite membrane. J Biomed Mater Res 45 204-208... 

This procedure is convenient for acid-sensitive complexes and is similar to that described elsewhere. To a solution of M(S04) (1 g) dissolved in water (—10 mL), Ba(CF3S03)2 (/i equiv.) is added. ( Caution. Barium salts are extremely toxic. Avoid contact with skin.) The solution is stirred, and the precipitated BaS04 is filtered using a medium-porosity sintered-glass filter. The solvent is removed by rotary evaporation to yield a powder, which may be recryStallized as before. For heat-sensitive complexes, the solvent is removed by freeze-drying techniques. 

Itatani, K., Iwafune, K., Howell, F.S., and Aizawa, M., Preparation of various calcium-phosphate powders by ultrasonic spray-freeze-drying technique, Mater. Res. Bull., 2000, 35,575-585. 

Researchers have synthesised and characterised a highly conductive, porous and biocompatible MWNT/CHT biocomposite film by the freeze-drying technique. The process was performed by freezing a MWNT/CHT dispersion inside an aluminum mould, followed by drying. 

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