How to use a freeze-drying machine correctly
1. Before using the freeze dryer, place the sample in a low-temperature refrigerator or liquid nitrogen to pre-freeze it into ice. This step must completely freeze the sample into ice, because freeze drying is sublimation drying, and the moisture contained in the sample directly sublimates from the solid state to the gaseous state without going through any liquefaction process. If the sample is not completely prefrozen, sublimation cannot be completed and drying will fail.
2. Before placing the sample into the freeze-drying machine, the refrigeration function of the freeze-drying machine should be turned on in advance, usually about 30 minutes in advance. The purpose of this is to ensure that the cold trap of the freeze dryer drops to the lowest temperature for capturing moisture in advance, so that the sublimated moisture is not pumped into the vacuum pump.
3. The thickness of the sample preparation table should not be too thick. If it is a liquid sample, it is recommended that the thickness should not exceed 1 cm. If the sample is a block, it is best to cut it into thin slices. The principle to follow is that the thinner the sample, the better the drying effect. If the samples are stacked too thickly, the surface layer of the sample will be dried, while the deeper layers of the sample will be delayed in drying.
4. Drying time: Generally, a drying cycle takes about 24 hours, but this time is not fixed. The length of drying time also depends on factors such as sample thickness and ambient temperature. Under the same conditions, the thinner the glass, the faster it dries. Under the same conditions, the higher the ambient temperature, the faster the drying speed. Sublimation absorbs heat, and the glass cover solves the absolute vacuum state after vacuuming. The internal thermal conductivity is poor, and a slightly higher ambient temperature can supplement the heat required for sublimation.
5. After drying is completed, first open the air inlet valve of the freeze dryer, slowly introduce gas, and then close the vacuum pump. Then, turn off the refrigerator, and after the internal pressure returns to normal, open the glass cover and take out the sample. It should be noted that the vacuum pump must be turned off after gas enters. If the vacuum pump is turned off first, the pump oil in the vacuum pump may be sucked back into the freeze-dryer due to air pressure problems due to the vacuum inside.
