In our previous article about QbD, it was mentioned that lyophilization has been considered an art, and cycles developed by trial and error.
Here the aim is to give some artistic guideline about the most challenging samples you will encounter in lyophilization cycle development.
#1: DMSO (Dimethyl Sulfoxide).
DMSO has become popular in the laboratory because of its ability to dissolve both polar and non-polar compounds. It’s miscible in a wide range of organic solvents as well as water. When performing lyophilization in samples containing DMSO there are a few things that need to be taken into consideration: DMSO quickly affects acrylic components which are used in most lyophilizers’ lids and doors.
#2: Tert Butyl. The problem of gummy consistency.
Crystallization of TBA before drying is critical to achieving adequately low residual TBA levels Annealing at a temperature above T'(g) of the system after an initial freezing step significantly reduces the level of residual TBA. Secondary drying, even at increased temperature and for extended times, is not an effective method of reducing residual TBA levels.
#3: Alcohols. Alcohols (like methanol and ethanol)
Ethanol has a freezing point of -117°C. If used in a lyophilizer, the collector temperature would need to be -134˚C. Unless you’re using a liquid nitrogen lyophilizer, freeze drying a sample in pure ethanol would be impossible. In fact, just freezing pure ethanol is difficult. If you dilute ethanol with water, you can raise the sample’s eutectic temperature to a point that it could be lyophilized using a -105˚C freeze dryer.
Salts alter the freezing point of samples due to chemical properties. In addition, exposure to salt over time will oxidize the stainless-steel surfaces of lyophilizers.
If you're going to be working with compounds containing salt, be sure to invest in a cascade-style (-84° or-105° C) lyophilizer with PTFE-coated components to avoid costly damage.
Sugars are found to form fully amorphous matrix in lyophilized samples. Furthermore, sucrose stabilizes enzymes to a great extent when it is amorphous in samples. Sugars won't change the eutectic temperature of your sample, but they could ruin your vacuum pump if they pass into the equipment and crystalize there.
You can protect your pump, by adding a HEPA filter between the lyophilizer and the pump so that any sugar that bypasses the condenser gets trapped before it can do any damage.
If you're trying to lyophilize a substance with oil, your output will never be an entirely solid powder. The oil in the sample will sublimate first and leave behind a slicker consistency, but this isn't a deal-breaker. A reduction in drying chamber pressure will shorten the lyophilization time but increase the release of volatile compounds.