Why your 3ml peptide cartridge might hold less than you

What dead volume is

Dead volume is the residual liquid a cartridge retains after withdrawal, leaving less than its stated capacity available. In a 3ml cartridge, this trapped liquid typically accounts for 0.1ml to 0.4ml of the nominal capacity.

In this article

1. What actually determines usable volume
2. Glass vs plastic: the purity question
3. Pen compatibility and what "3ml" really means
4. The mistakes that cost researchers the most
5. What actually matters when you're at the bench
6. Frequently asked questions
7. What the community gets wrong

Not all 3ml cartridges actually hold 3ml of usable liquid. That's the first thing most researchers learn the hard way, usually after they've already mixed their peptide and can't draw out what they need.

The difference is called dead volume. It's the liquid that stays trapped in the cartridge no matter how hard you try to draw it out. Every cartridge has it. The question is how much, and whether it matters for your work.

Key point: Stated cartridge capacity is nominal; always account for a 0.1ml to 0.4ml dead volume loss when calculating diluent and final peptide concentrations.

All of our reusable peptide pens (V1, V2, V3 and Gansulin) are compared on one page.

What actually determines usable volume

A standard 3ml glass cartridge doesn't give you 3ml to work with. Depending on the brand and design, you might lose 0.1ml to 0.4ml to dead volume. That sounds small until you realize that if you're working with expensive peptides, that "wasted" 0.2ml might represent $50 or more of material you paid for but can't use.

Sources of dead volume

The dead volume comes from two places:

• Internal pooling: The space below the internal needle tip where liquid pools and can't be withdrawn.
• Surface tension: The volume that stays clinging to the walls of the cartridge.

Glass cartridges with tapered or funnel-shaped internal geometry tend to perform better than those with flat-bottomed designs.

Impact on concentration math

When you're calculating how much diluent to add to your peptide vial, you need to account for this discrepancy. If you want a final concentration of 1mg/ml and you have a 5mg vial, you might think you need exactly 5ml of diluent.

However, if your cartridge loses 0.25ml to dead volume, you're actually getting 4.75ml into your vial. Your concentration is slightly off, and your math is now wrong.

Glass vs plastic: the purity question

For peptide research, glass is the standard for a reason. This is especially true when you're storing reconstituted peptide for any length of time.

Why glass remains the standard

Neither glass nor plastic is perfectly inert: glass flakes due to chemical delamination have been observed in parenteral liquid formulations after long-term storage (Jiang et al. 2013), the unpredictable nature of peptide binding to surfaces requires optimization of experimental containers (Goebel-Stengel et al. 2011), and cetrorelix adsorbed more to glass than to polypropylene (Grohganz et al. 2004).

The only downside is that glass is more fragile and costs more. But if you're working with limited peptide material and need confidence in your concentration and purity, glass is worth the extra cost.

The exception: immediate single-use

There's one exception worth knowing about. Some researchers use disposable plastic cartridges for single-use reconstitution, especially when they're going to use the entire solution immediately and won't store it.

But for any storage beyond a few hours, stick with glass.

Pen compatibility and what "3ml" really means

Here's something that causes constant confusion: "3ml cartridge" doesn't mean the cartridge is exactly the same across every brand. The outer diameter, threading pattern, and internal needle length vary between manufacturers. A cartridge that fits perfectly in one pen might not seat properly in another, or might not thread in at all.

If you're using a reusable metal pen (like the PreppinPeppers metal peptide pen), check the specifications carefully. Most metal pens accept standard 3ml cartridges with a specific threading profile.

Plastic pens are often more forgiving because they use slightly more flexible housing, but they also introduce more variables into your precision.

Mechanism precision

The pen itself matters too. Metal pens tend to have more consistent click-dial advancement, meaning each "click" of the dial delivers a more predictable volume.

Plastic pens can have more play in the mechanism, which adds up if you're doing multiple withdrawals throughout a day.

The mistakes that cost researchers the most

1. Ignoring dead volume in dilution math: Researchers mix their peptide, draw up what they think is the right amount, and then realize they're short when they need a second aliquot. Always calculate for slightly less than the cartridge's stated volume, or use a cartridge with documented low dead volume.
2. The cartridge is fine for mixing, but it is not designed for weeks or months of cold storage.
3. Assuming all bacteriostatic water is identical: Some brands of bacteriostatic water have slightly different ionic concentrations or pH, affecting how well the liquid draws up and empties. If you switch diluent brands and notice your withdrawals behaving differently, that is why.

What actually matters when you're at the bench

• Select low-dead-volume glass: Pick a glass cartridge with documented low dead volume from a known manufacturer and account for it in your math.
• Keep diluents consistent: Use bacteriostatic water from a consistent source so you avoid chasing variable draw behavior.
• Transfer for storage: If you're storing reconstituted peptide, transfer it to a proper glass vial rather than keeping it long-term in the cartridge.
• Avoid the final 0.2ml: When drawing out your solution, stop a full 0.2ml before you think you're done. That last bit is where most dead volume lives, and fighting to extract it usually just introduces air bubbles into your sample.

What the research community gets wrong about 3ml peptide cartridges

• A "3ml" cartridge is not 3ml of usable liquid. The number describes the nominal barrel size, not what you can draw back out. Some liquid always stays behind below the plunger tip and clinging to the glass wall. Do your reconstitution math against the volume you can actually recover, not the printed label.
• Glass is chosen for chemistry, not looks. In dilute solution, peptide sticking to container walls can quietly lower your working concentration, so the container material is part of your result, not just packaging.
• Chasing the last drop can cost you more than the drop. Fighting to empty the dead volume pulls air into the sample. Air and liquid interfaces (like the foam from a bubble) can push some proteins to clump together, so squeezing out that final fraction can damage more material than it saves.
• Bacteriostatic water is not the same as sterile water. It carries benzyl alcohol as a preservative (around 0.9 percent on the common label), which is why it is only meant as a diluent for reconstitution. Switching diluent brands can shift pH or draw behavior, so keep the source consistent when you compare runs.
• "3ml" does not mean interchangeable across makers. Outer diameter, threading, and internal needle length vary between manufacturers. A cartridge that seats perfectly in one pen tells you nothing about whether it will seat or thread in another.

From our bench: We want to log real dead volume by cartridge design. If you fill a 3ml glass cartridge with a known volume of water, then draw back everything you reasonably can, weigh or measure what stays trapped and note the brand and internal geometry (tapered versus flat bottom). Send us your recovered volume, the leftover, and the cartridge you used, and we will add measured numbers to this page instead of estimates.

Sources

Correction (2026-10-03): An earlier version said glass is chemically inert or does not leach, and that plastic leaches or loses more peptide than glass. Neither is perfectly inert, and which surface loses less peptide depends on the peptide; the passage now says so and cites the research.

Reminder: research and educational reference only. PreppinPeppers 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.