Peptide Research Glossary

A plain-language glossary of common peptide research terms, including reconstitution, lyophilized, concentration, U-100, bacteriostatic water, aliquot, diluent, and the units mcg, mg, and IU.

This glossary explains words that appear on research-supply labels, in lab notes, and in reconstitution math. Each entry is a definition only. The entries cover chemistry, units, storage, and equipment, and nothing else.

For the calculations that connect several of these terms, see the peptide calculator.

Physical Form and Preparation

Lyophilization and Reconstitution

Lyophilized. This word means freeze-dried. Lyophilization (the process behind it) removes water from a frozen material inside a vacuum chamber (a sealed space with the air pumped out). What is left behind is a dry powder or solid cake.

Many research peptides are supplied in this dry form because dry material stays more stable during shipping and long-term cold storage than a liquid would.

Reconstitution. This means adding liquid to a dry substance to turn it back into a solution. Think of it like dissolving a bouillon cube in water. The cube is the dry material, and the water is what you add to bring it back to liquid form.

Adding liquid does not change how much peptide is in the vial. It only spreads that fixed amount through whatever volume of liquid you choose.

Key point: Reconstitution changes only the liquid volume and concentration, not the fixed amount of peptide in the vial.

Diluents

Diluent. A diluent (pronounced DIL-yoo-ent) is the liquid added to a dry substance during reconstitution. It can also be added to an existing liquid solution to make that solution less concentrated. The diluent is just the carrier, not the active material.

Common diluents referenced in peptide research include:

• Sterile water
• Sterile water for injection
• Bacteriostatic water
• Phosphate-buffered saline
• Sterile saline

Water and Storage Terms

Preservation and Portioning

Bacteriostatic water. This is sterile water with a small amount of preservative added. The preservative is usually 0.9 percent benzyl alcohol. The word "bacteriostatic" means the additive slows or stops bacteria from growing inside the vial. It does not kill all microbes outright. It mainly prevents new bacterial growth from starting.

Because of the preservative, a vial reconstituted with bacteriostatic water is generally described as having a longer usable window under refrigeration than one made with plain sterile water. Plain sterile water contains no preservative at all.

Aliquot. An aliquot (AL-ih-kwot) is a measured portion taken from a larger amount. To aliquot a solution means to split it into several smaller containers that are stored separately. Think of pouring a large jug into multiple small bottles so you only need to open one at a time.

Researchers describe aliquoting as a way to reduce how often a single main container is opened. Each smaller portion can be handled on its own without disturbing the rest.

Amount Units: mcg, mg, and IU

Measuring Mass and Activity

mg (milligram). A unit of mass equal to one thousandth of a gram. Peptide amounts in a vial are commonly labeled in milligrams, for example 5 mg or 10 mg.

mcg (microgram). A unit of mass equal to one thousandth of a milligram. So 1 mg equals 1,000 mcg, and 0.5 mg equals 500 mcg. A milligram is already a tiny amount, and a microgram is one thousand times smaller than that.

The symbol mcg is used in place of the Greek version (µg) to avoid confusion in handwriting.

IU (International Unit). A unit based on how biologically active a substance is, not on how much it weighs. For substances measured in IU, a conversion number ties the unit back to milligrams. That conversion number is specific to each substance.

Because the conversion factor differs from compound to compound, IU and mg are not interchangeable without knowing which substance you are working with.

Concentration and Syringe Terms

Calculating Concentration

Concentration. This is how much of a substance is present in a given volume of liquid. It is usually written as mass per volume, such as mg per mL or mcg per mL.

Concentration is set at the moment of reconstitution by pairing a fixed peptide amount with a chosen diluent volume. A larger volume of liquid gives a lower concentration for the same amount of peptide. Think of a pinch of salt dissolved in a small glass of water versus a large pot of water. The amount of salt is the same, but it is much more spread out in the larger volume.

The relationship is amount divided by volume, and the peptide calculator handles this arithmetic.

Syringe Calibration

U-100. A labeling standard for insulin syringes. U-100 means the syringe scale is set so that 100 units equals 1 mL of liquid.

On that scale, 1 unit equals 0.01 mL, and 50 units equals 0.5 mL.

The unit markings on a U-100 syringe measure volume (how far the liquid sits along the barrel), not the mass of anything dissolved in the solution. Reading that volume from the scale is its own measurement task, separate from what the solution contains.

Related reading

• What Is a Peptide?
• What Does Lyophilized Mean?
• How Reconstitution Works
• How Long Does a Reconstituted Vial Last?
• Storage Guide

Tools and supplies

• peptide calculator
• Bacteriostatic Water 30 ml
• Gansulin Metal Reusable Pen
• 3 ml Glass Cartridges (10-pack)
• Complete Starter Kit

Reminder: research and educational reference only. Preppin Peppers sells hardware and materials, not peptides. Not medical, dosing, or health advice, not evaluated by the FDA, and not intended for human or animal use.

Frequently asked questions

What does lyophilized mean in peptide research?

Lyophilized means freeze-dried. Water is removed from a frozen material inside a vacuum chamber, leaving a dry powder or cake that is more stable for shipping and long-term cold storage than a liquid solution.

What is bacteriostatic water and how is it different from sterile water?

Bacteriostatic water is sterile water containing roughly 0.9% benzyl alcohol as a preservative. The additive slows bacterial growth inside the vial, giving reconstituted solutions a longer usable window under refrigeration than plain sterile water.

What is the difference between mg and mcg in peptide labeling?

A milligram (mg) is one thousandth of a gram. A microgram (mcg) is one thousandth of a milligram, 1,000 times smaller. So 1 mg equals 1,000 mcg, and 0.5 mg equals 500 mcg.

More in our bacteriostatic water and diluents collection.

What the research community gets wrong about peptide research terminology

These glossary terms show up on labels and in reconstitution math, and a few of them are read the wrong way at the bench. Here are the mix-ups worth catching before they reach your worksheet.

• Treating mcg and mg as almost the same. A microgram is one thousandth of a milligram, so the two differ by a factor of 1,000. Swapping the units on a calculation sheet does not cause a small rounding error, it shifts your concentration figure by three decimal places.
• Assuming IU converts to mg with one fixed number. An International Unit measures activity, not mass, and the number that ties IU back to milligrams is specific to each compound. There is no universal conversion, so an IU value from one material does not carry over to another.
• Thinking bacteriostatic water sterilizes the vial. The benzyl alcohol in it slows or stops new bacterial growth, it does not kill everything already present, and it is not the same as plain sterile water. Reading it as a sterilizer overstates what the preservative actually does inside the vial.
• Reading U-100 syringe markings as an amount of peptide. Those unit marks measure volume along the barrel (100 units equals 1 mL), not the mass of anything dissolved in the liquid. The same 50-unit mark is 0.5 mL whether the vial holds a little material or a lot.
• Believing more diluent means less peptide. Adding liquid during reconstitution spreads the same fixed amount through a larger volume, lowering concentration. The total amount in the vial does not change, only how concentrated the solution reads.

From our bench: If you keep lab notes on reconstituted vials, we would like to compare records. Pick one compound, reconstitute a matched pair with bacteriostatic water and with plain sterile water, store both under the same refrigeration, and log the date each solution first shows visible cloudiness or particulates. Send us the two clarity timelines and the storage temperature you held, and we will line them up against other readers' observations (real logged dates only, no estimates).

Sources

1. Bacteriostatic Water for Injection, USP , FDA/DailyMed label (0.9% benzyl alcohol)
2. Duerkop et al., Biotechnol J 2018 , Impact of Cavitation, High Shear Stress and Air/Liquid Interfaces on Protein Aggregation
3. Sigma-Aldrich (Merck) , Handling and Storage Guidelines for Peptides and Proteins
4. NIST, Metric (SI) Prefixes , definitions of milli (10^-3) and micro (10^-6)