If you write software, you already understand most of PCR setup. A master mix is a build script: the inputs are fixed, the recipe is deterministic, and one wrong constant ruins every run that follows. The big difference is that your "compiler errors" show up two hours later as a blank gel.
This post treats the most common master mix mistakes as bugs. For each one you get the symptom, the root cause, and the fix. After that, there's one full worked build you can follow with a calculator, and a short code version for the developers in the room.
The tool I'll refer to throughout is this free PCR Master Mix Calculator. It scales a recipe to your plate, adds overage, and balances the water.
First, the "spec": what a master mix actually is
A master mix is one bulk tube holding everything that's identical across your reactions: water, buffer, dNTPs, primers, and polymerase (plus probes for qPCR). You dispense it into every well, then add the one thing that's different per well, the template DNA.
Why bother? Pipetting tiny volumes 96 separate times introduces 96 chances for error. One bulk mix means you make each measurement once, so every well gets the same recipe.
Every reagent volume comes from one equation, the conservation-of-solute dilution formula:
C₁V₁ = C₂V₂ → V₁ = (C₂ × V₂) / C₁
- C₁ = stock concentration
- C₂ = final concentration you want in the reaction
- V₂ = final reaction volume
- V₁ = how much stock to pipette
For a 20 µL reaction, the typical numbers work out like this:
| Reagent | Stock | Final | Volume per reaction |
|---|---|---|---|
| PCR buffer | 10X | 1X | 2.0 µL |
| dNTPs | 10 mM | 0.2 mM | 0.4 µL |
| Forward primer | 10 µM | 0.5 µM | 1.0 µL |
| Reverse primer | 10 µM | 0.5 µM | 1.0 µL |
| Taq polymerase | (given by volume) | n/a | ~0.2-0.5 µL |
Polymerase is the exception. It sits in viscous glycerol, so you aliquot it directly by volume (or by units) instead of diluting it with the formula.
Now, the bugs.
Bug #1: "The last wells are empty"
Symptom: You calculated the mix for exactly 96 reactions, and you run dry around well 90.
Root cause: Liquid is lost every time you pipette. Viscous glycerol
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