Sterile 1 M HEPES Buffer, 100 mL

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• HEPES buffer in cell cultureHEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) is a zwitterionic organic buffer used in cell culture to hold medium at physiological pH without depending on incubator CO2. Its pKa is about 7.5 at 20-25 degrees C and about 7.3 at 37 degrees C, giving useful buffering across roughly pH 6.8-8.2, and it is normally added to medium at 10-25 mM from a sterile 1 M stock. Because HEPES buffering does not rely on the carbonic acid/bicarbonate equilibrium, HEPES-supplemented medium resists the fast alkaline drift that occurs when a flask leaves a 5% CO2 incubator.
• HBSS (Hank's Balanced Salt Solution)HBSS (Hank's Balanced Salt Solution) is an isotonic balanced salt solution used to wash cells, transport tissue, dilute reagents and hold cells briefly outside their growth medium. It contains 8.0 g/L sodium chloride, 1.0 g/L D-glucose, phosphate and 350 mg/L sodium bicarbonate, and is supplied either with calcium and magnesium (1.26 mM Ca, ~0.9 mM Mg total) or without them. Its low bicarbonate means it is designed for use at atmospheric CO2 or in sealed vessels, not for prolonged culture in a 5% CO2 incubator.
• Endotoxin Testing in Cell CultureEndotoxin testing measures bacterial lipopolysaccharide (LPS) in laboratory water, media, sera and reagents, reported in endotoxin units per millilitre (EU/mL), where 1 EU corresponds to roughly 0.1–0.2 ng of reference-standard E. coli endotoxin. The compendial methods are the LAL (limulus amebocyte lysate) assay in its gel-clot, turbidimetric and chromogenic forms under USP <85>, and the animal-free recombinant Factor C (rFC) assay under USP <86>; kinetic formats quantify down to about 0.001–0.005 EU/mL. Practical cell-culture limits are 0.25 EU/mL for water, ≤1 EU/mL for most cell-culture-grade reagents, and ≤10 EU/mL as the general industry standard for fetal bovine serum, with low-endotoxin grades at ≤1 EU/mL and ultra-low grades below 0.1 EU/mL. Endotoxin survives autoclaving and passes 0.2 µm filters, so it must be excluded at source rather than removed later.
• Cell culture buffersCell culture buffers hold medium in the pH 7.2-7.4 range that mammalian cells require, against the acid load cells generate as they metabolise. The default system in almost every classical medium is sodium bicarbonate working with the CO2 in the incubator atmosphere, which is why bicarbonate content must be matched to the incubator setting: about 1.5-2.2 g/L NaHCO3 for 5% CO2 and 3.7 g/L for 10% CO2. Organic buffers such as HEPES (pKa 7.48) are added at 10-25 mM to hold pH when cultures are outside a CO2 atmosphere, while phosphate-buffered solutions such as PBS and DPBS are used for washing and short-term handling rather than for growth.