1. Place a 400 ml Pyrex beaker inside a clean, dried ice bucket. Fill the beaker with ~150 ml liquid nitrogen; filled the ice bucket one third to halfway with liquid nitrogen.
2. Add the solution, one drop at a time, into the beaker. Do not hold the pipet too close to the surface of liquid nitrogen or the solution will freeze at the tip of the pipet. Let the ball sink to the bottom of the beaker before adding the next drop of solution.
3. Place Nunc tubes in an ice bucket. Wear gloves. Pick up frozen balls with a pair of clean, long forceps and collect them inside Nunc tubes.
4. Transfer Nunc tubes to the liquid nitrogen freezer.
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