3 ml glass cartridges explained: the 300-unit standard, seals and storage

The 3 ml glass cartridge is the main supply item used with cartridge pens. It is a small tube made from borosilicate glass (a tough, heat-resistant type of glass common in lab equipment). One end has a squeezed-on metal cap with a rubber disc inside. The other end has a sliding rubber stopper.

The "3 ml" tells you how much liquid the cartridge holds. On a U-100 scale (a common measuring scale where 100 units equals 1 ml of liquid), 3 ml equals 300 units. That works out because 300 units times 0.01 ml equals 3 ml.

Why the format is standard

The outside size of these cartridges is standardized, meaning it is the same across many brands. Because of that, one cartridge format fits a wide range of cartridge pens, including the Gansulin metal pen.

Think of it like a standard AA battery that works in many different devices. There are no special or exclusive supplies and no lock-in: you can reorder standard 3 ml cartridges without having to match them to one specific brand of pen.

Key point: Standardized dimensions eliminate brand lock-in, allowing you to use a single cartridge format across a wide range of pens.

The two seals

• The septum at the capped end is a self-sealing rubber disc held in place by the metal crimp. A pen needle passes through it to reach the liquid inside, and the rubber seals itself back up afterward.
• The plunger at the other end is a rubber stopper that slides down the tube as liquid is drawn out. It keeps the contents sealed from behind at all times.

Both seals work together to hold a stable volume of liquid inside. A cartridge with a cracked glass body, a damaged crimp, or a torn pouch should not be used.

Individually pouch-sealed

Quality cartridges come individually pouch-sealed, meaning each one is wrapped in its own protective pouch. The pouch acts as a barrier that keeps each cartridge clean and dry until you are ready to use it. Keep cartridges in their pouches until you need them.

Storing sealed cartridges

• Store in a clean, dry place at a comfortable room temperature, ideally below 75°F (24°C).
• Keep humidity moderate. High moisture in the air can weaken the pouch paper and the seal over time.
• Keep cartridges out of direct sunlight and away from heat sources.
• Do not use a cartridge whose pouch is torn, wet, or damaged.

Planning Your Needs

To figure out how many cartridges a project needs, the peptide calculator works it out from your draw size and schedule.

Preppin Peppers sells laboratory hardware and consumables for research, educational, and demonstration use only. This article is general educational information about equipment and does not describe, recommend, or instruct the use of any peptide or other substance in humans or animals. Nothing here is medical advice.

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

How many units does a 3 ml glass cartridge hold on a U-100 scale?

On the U-100 scale, where 100 units equals 1 ml, a 3 ml cartridge holds 300 units. This is derived from 300 units × 0.01 ml per unit = 3 ml.

Are standard 3 ml cartridges compatible with multiple cartridge pens?

Yes. The outer dimensions of 3 ml cartridges are standardized across brands, so one cartridge format fits a wide range of pens, including the Gansulin metal pen, eliminating brand lock-in.

How should sealed 3 ml glass cartridges be stored?

Store below 75°F (24°C) in a clean, dry place away from direct sunlight and heat sources. Keep cartridges in their individual pouches until needed; discard any whose pouch is torn, wet, or damaged.

More in our 3 ml glass cartridges collection.

What the research community gets wrong about 3 ml glass cartridges

The 3 ml glass cartridge looks simple, so a few wrong ideas get repeated around the bench. Here is what the numbers and the glass actually tell you.

• "3 ml" is a liquid volume, not a count of anything. On a U-100 scale the tube reads 300 units, because 300 times 0.01 ml equals 3 ml. The marks describe how much liquid the cartridge holds, not what is dissolved in it or how many draws you get.
• Not all clear glass is the same glass. Type I borosilicate is chosen for high hydrolytic resistance (it stands up to water attack) and better thermal shock resistance than ordinary glass. USP Chapter 660 now grades glass by how it performs in tests, not only by its recipe, so "looks like glass" is not the same as "meets Type I." The format also comes in amber borosilicate, which adds UV filtering for light-sensitive contents.
• Standardized does not mean identical. Outer dimensions follow the recommendations in ISO 13926-1, but each pen still has a working range for length and crimp width. Confirm a cartridge seats all the way before assuming any pen fits any cartridge.
• The septum is not endless. Each needle pass through the rubber disc is a small mechanical event, and repeated punctures can shed tiny rubber fragments (coring). Fewer, cleaner passes keep a sample cleaner.
• Colder is not automatically better. Freezing can crack the glass body or unseat the plunger. General storage guidance is a cool, dry room below 75 F (24 C), away from direct sun and heat, not a freezer.

From our bench: If you handle 3 ml cartridges from more than one supplier, we would like your calipers. Measure the outer diameter, overall length, and crimp width of each empty cartridge, note which pen model each one seats fully in, and send us the raw numbers. Real cross-brand measurements show how tightly the standard dimensions actually hold in practice, and we will not publish any figure we did not measure.

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. SCHOTT Pharma , Pharmaceutical Cartridges (FIOLAX Type I borosilicate glass, ISO-standardized 1.5/3/5/10/20 ml, pen and auto-injector compatible)
5. West Pharmaceutical Services , USP <660> Containers, Glass revision (Type I glass and hydrolytic resistance, performance-based classification)
6. EndoLab , ISO 13926-1 (dimensions, material, leakage, and hydrolytic resistance for glass cylinders used in pen injectors)