There are challenges for small volumes, a big problem can be that of keeping the sample frozen. Vacuum pressure in the freeze drying equipment may not be enough, particularly if you have a small vessel for a small sample, then a vacuum micro climate may develop and you can then see melt back in the product. Formulation is always important, but more so in a small sample where "excess" cryo or lyo protectant in a large sample may just make a freeze drying cycle longer, in a small sample it may prevent freezing altogether.

I suggest you look at the cannon on freeze drying diagnostics, where small samples are typically processed. There is a good webinar here:

https://www.gotostage.com/channel/9a646dc31402467d8f3e217b0f31a186/recording/2e4d40259512423b97849c45f455f56d/watch?source=CHANNEL

There are many webinars from that supplier:

https://www.gotostage.com/channel/splyouniversity

This is a good basic guide to freeze drying, but not specifically focussed on small volumes:

https://www.scientificproducts.com/resources/articles-tech-notes/basic-principles-of-freeze-drying

Many thanks for taking the time to answer and for all these precious information. Best regards,

Laurent