I was making a right turn the other day and almost rammed my car into a shiny Tesla driving past me. As I pulled up behind the electric car, thanking my lucky stars I am not 100,000€ poorer, I started thinking about how we fuel our cars. For the longest time, gasoline engines have been the most popular option for automobiles. But rising oil prices and environmental concerns have made other methods to power our cars, from electricity to ethanol to fuel cells, gain in popularity. The nature of the human mind to re-invent is blatantly apparent when it comes to the automotive engine.(人类心智重新发明的本质在于汽车引擎时显而易见).
The same could be said about freeze-dryers. Initially, chemists used laboratory freeze dryers only with water as a solvent. Soon, they realized they need more options. They wanted to lyophilize water-insoluble products. Or even more interesting to me as a lover of chromatography, they wanted to directly freeze dry chromatography products that were not always dissolved in water. So naturally, chemists turned to using organic solvents for laboratory freeze drying.
Using organic solvents can be challenging at first, but it can certainly pay off. The following tips aim to help you successfully adopt organic solvents in your lyophilization workflow.
How to determine if a solvent can be freeze driedMany organic solvents have a low freezing point, often well below that of the condenser surface temperature. Four key questions can help determine if and how a solvent can be freeze-dried:
How can the solvent be frozen?
Find the most appropriate method that can reach a low enough temperature so that the sample is fully frozen.
At which concentration can the solvent be frozen?
Determine if the solvent must be diluted prior to freezing.
Can the condenser collect the solvent?
The temperature of the condenser must be 15-20°C lower than the freezing temperature of the solvent
Will the sample remain in a frozen phase during the process?
Evaluate if you can maintain a pressure that is low enough to keep the sample frozen.
In contrast to aqueous samples, organic solvents are tricky to freeze and require dilution prior to freezing. Chemists might need to use liquid nitrogen for the solvents to reach a low enough temperature. Moreover, due to the low freezing point of organic solvents (see table above), the condenser temperature might not be low enough to completely capture the solvents. Even -105°C condensers can fail at trapping these types of solvents. Instead, the organic solvents liquify in the condenser or leave the system through the pump as vapor. Because of this:A scroll pump is recommended for all freeze-drying applications involving organic solvents.
The low freezing temperature and triple point of the organic solvents makes it difficult to evacuate the system fast enough and to maintain a low enough pressure to avoid solvent melting, even at ultimate vacuum. For diluted solutions, it is rather common to see solvents melting and evaporating, while water remains frozen. Whether this is problematic for you depends on your sample requirements.
If the solvent amount is too high, the system will not be able to maintain the required pressure and everything will melt and evaporate. The process must be terminated at this point.
Tips on how to handle organic solvents in laboratory freeze dryers
- Eliminate as much of solvent as possible before freeze-drying, using a rotary evaporator or other instrument
- Be prepared for periodic replacement of parts when etching （蚀刻）of the equipment becomes problematic
- Carefully cleanse freeze-dryer after each cycle
- Do not allow condensate to sit in the condenser. Immediately conduct defrosting step of the instrument with the drain valve open. Wash out the condenser with water and ensure it is clean and dry.
- Use a dry pump when handling solvents other than water. Make sure the exhaust port of the pump is in a fume hood to avoid solvent exposure
- Use of ultimate vacuum pump is recommended due to the low pressures (lower than 0.05 mbar) required to keep the solvents in solid form
- Dilute solvents with water whenever possible to influence solvent concentration in your favor
