20X Wash Buffer A is a phosphate-buffered saline concentrate with potassium and polysorbate 20 (Tween-20). Dilution to 1X gives PBS with 0.1% polysorbate 20 at near-neutral pH. The 1 L fill makes 20 L of 1X buffer.

It is commonly used to wash plates between coating, blocking, and antibody steps in ELISAs and other immunoassays. Dilute 50 mL to 1 L with purified water, and warm and mix if crystals form during storage.