Peptide blend calculator: multi-peptide vial arithmetic

A concentration is an amount per volume, and in a blend every component is distributed through the whole of the finished preparation rather than sitting in one part of it. Each component concentration is therefore its own amount divided by the total volume, and every component uses that same denominator. The temptation is to divide the second component by the diluent alone, as though it had been layered on top of the first. It has not. There is one volume, it holds both components, and both concentrations are statements about the same liquid.

Whether that denominator grows when a component is added depends on something the arithmetic cannot assume either way. A liquid component adds its whole volume to the total, so it lowers the concentration of everything already there. A solid component, in the model this page uses, contributes no volume, so it leaves the other concentrations where they are and adds only to the sum. That is why the distinction between a mass entry and a volume entry is not bookkeeping pedantry: it decides whether the components dilute each other at all. The solution mixing calculator treats every input as a volume, which is the correct model whenever any component is already a liquid.

Add the individual concentrations together and the result is the total mass of peptide divided by the total volume, which is the concentration of the mixture treated as one substance. That identity is the most useful check on this page, because it is easy to perform and it fails in exactly the way a mistake would. If the components do not sum to the total, then one of them was divided by the wrong volume, and the row that is wrong is the one whose figure does not fit the pattern.

It is equally worth being clear about what that sum is not. It is a mass density of peptide in a liquid, not a concentration of anything, because no chemical has that concentration and nothing on the vial corresponds to it. Treating the total as though it described a usable substance is how a blend page gets misread as a compound page. The arithmetic also runs usefully backwards: knowing the total volume and one component concentration gives that component mass directly, which is how you would check what actually went into a vial against what went into the calculator. The concentration after a volume change calculator covers the related case of what a concentration becomes once liquid is added or taken away.

Addition is the only operation this page performs, and it is closed under itself: masses add, volumes add, and the quotients follow. Compatibility is not an arithmetic property. Whether two substances stay dissolved together, whether one shifts the pH that the other is stable at, whether either aggregates at the concentration reached, and whether the pair has any business sharing a container at all: none of that appears anywhere in the three formulas above, and no rearrangement of them will produce it. There is no setting on this page that would change that, because the inputs it accepts are masses, volumes and a syringe, and nothing in that list describes how two substances behave together.

So the page will build a table for a pair of substances that should never share a vial, and it will do so without a murmur, because any warning it printed would be a claim about chemistry that it has no way to support. That silence is deliberate, and it is the most important limitation on the page. It is not a gap that a later version will close with a better formula. The things that would actually settle compatibility are the product documentation for each component and the judgement of someone qualified to make the call, and the site disclaimer is where this site puts its own version of that statement.

The sum of the components is only meaningful if they share a unit, and the two common mass units differ by a factor of a thousand. One component entered in milligrams and another in micrograms produces a total that is out by that same factor while still looking like a plausible mass. Converting first is slower than typing carefully and considerably faster than debugging the result afterwards. The mass unit converter and the converters page both cover that step, and the guide to mass concentration against molarity explains why the unit a concentration is reported in decides what it can legitimately be compared against.

Ratios are the durable way to describe a blend. A mass ratio between components is dimensionless, so it survives both a change of unit and a change of diluent volume, whereas a concentration is tied to one specific volume in one specific unit. If a blend is specified as a ratio and a total volume is then chosen, the concentrations follow deterministically and every later volume change leaves the ratio intact. That is a far better position than holding two concentrations and hoping they were entered in the same unit. The batch scaling calculator is the tool for resizing a specification of that shape once one exists.