Paraformaldehyde, 4% in PBS, pH 7.4 is a ready-to-use fixation solution for cells or tissues. It is electron microscopy-grade and methanol-free. UV light and oxygen are known to cause formaldehyde degradation over long-term storage. Biotium's unique packaging method ensures the high quality of the formaldehyde by using amber glass vials packaged under argon gas and tightly sealed with pharmaceutical grade enclosures for safer and more convenient handling than paraformaldehyde solid. Unlike 16% paraformaldehyde in water that is sold in score-break glass ampoules, our fixative is supplied in easy-to-open, re-sealable 20 ml amber glass bottles that can be stored for at least a month at 4̊°C, protected from light. Unopened bottles can be stored at room temperature for at least 5 years.