Water for Injection
Definition
A pharmacopoeial grade of water held to published limits on endotoxin and conductivity, supplied preservative-free. The grade describes the water, not the container, and preparations made from it carry no preservative of their own.
What this means for research sourcing
Getting this right matters when preparing lyophilized research peptides for in vitro work. Volta ships compounds as lyophilized powder for reconstitution.
Related Terms in Preparation
Reconstitution
Returning a freeze-dried peptide to solution by adding a measured volume of solvent. The volume chosen is what sets the concentration, so one vial can yield many different concentrations and the arithmetic belongs with the record of the experiment.
Concentration
How much peptide sits in a unit of liquid, written as mg/mL or mcg/mL. It is a consequence of the vial contents and the solvent volume added, and nothing else, which is why it has to be recalculated whenever either changes.
Molarity
Concentration counted in moles per litre rather than in mass per volume. Receptor and enzyme work is specified in molarity because a binding site counts molecules, and converting from mg/mL requires the molecular weight.
International Unit (IU)
A unit of biological activity rather than of mass, defined separately for each substance against an agreed reference preparation. HCG and somatropin are quoted in IU, so an IU figure cannot be turned into milligrams without the conversion specific to that compound.
Microgram (mcg)
One millionth of a gram, written mcg or ug, with 1,000 of them to the milligram. Peptide quantities move between the two scales constantly, and a factor of a thousand is the easiest arithmetic error to make when reading a vial against a published protocol.
Diluent
The liquid used to dissolve or further dilute a peptide. Buffered saline, dilute acetic acid and small fractions of organic solvent all appear in the literature, chosen for what the compound will dissolve in and what the assay will tolerate.
Buffer
A solution that resists pH change when acid or base is added. Peptide stability and solubility both track pH, so a buffered diluent keeps a preparation in a known state instead of a drifting one.
pH
The acidity of a solution on a logarithmic scale. It determines what charge a peptide carries, how soluble it is, and how fast deamidation and hydrolysis proceed, which makes it a stability variable rather than a detail.