While the immediate benefit of freeze drying is obvious—the ability to store samples at ambient temperatures—freeze-drying also may afford long-term savings by eliminating the need for costly cold storage.
The process of freeze-drying relies on the balance of only three controllable variables: temperature, pressure, and time. Although the process may seem straightforward, freeze-drying can often be problematic. Having an intimate understanding of the physical and chemical properties of the samples you are working with is essential to selecting a process and equipment best suited to your needs.
Knowing your sample
Controlling the freeze-drying process is dependent on two factors: the presence of a deep vacuum and a temperature differential of 15 to 20 degrees between the sample’s eutectic temperature (freezing point) and the temperature of the collector. Choosing a freeze dryer that meets the demands of your application can be a daunting task and requires careful consideration of the samples to be prepared. Choose a system that is too small and you will overload your collector; if your system doesn’t reach the correct temperature or has insufficient vacuum, you will risk sample melt back.
For many biological samples, including urea, blood plasma, serum, and vaccinia, a standard freeze-drying system with a refrigeration system that reaches -50°C will be sufficient. In the case of HPLC samples where the freezing point of acetonitrile is much lower (approximately -42°C), however, you will require a cascade-type collector. Cascade systems have dual condensers that can reach -84°C and would be a suitable choice for these samples.
Switching to methanol
Many labs are looking at methanol as an alternative to acetonitrile, due to its general availability and affordability. However, methanol has a much lower freezing point of -97.6°C, making lyophilization difficult even when it is diluted. In response to this demand, some manufacturers are offering ultra-low temperature freeze-dry systems. Labconco’s FreeZone systems, for example, are able to achieve collector temperatures as low as -105°C, making lyophilization of dilute methanol, ethanol, and acetonitrile possible.
A matter of timing
The most common question concerning freeze-drying is likely, “How long will it take?” The answer, however, is not so direct. The time required to freeze-dry a sample is dependent on a number of factors, including sample volume, thickness, and surface area; the eutectic point and solute concentration of the sample; and the temperature of the collector and the maximum vacuum obtained. In general, samples with a large surface area will freeze-dry faster than those that are thick or dense. Additionally, thick samples require moisture to pass through the layer of dried material, increasing the chance that the sample will thaw and “collapse.” Depending on your sample, freeze-drying can be a lengthy process lasting from hours to weeks. While freeze-drying provides superior drying capacity, if time is of the essence, you may consider a vacuum concentrator or evaporator for your application.
Finally, if you are shopping for a freeze-drying system, remember to budget for a rotary vane vacuum pump with vacuum deep enough to pull down to 2 x 103 mBar, as well as any glassware or adapters that may be required.
For additional resounces on freeze dryers, including useful articles and a list of manufacturers, visit www.labmanager.com/freeze-dryers