The same solution can be described as 2.94 mM, 1 mg/mL, 0.1% w/v or 1000 ppm. All four can be correct. They are different units for the same thing, and converting between them is a common source of errors.
Two families of units
- Amount concentration (molar): moles per litre: M, mM, µM, nM. These count molecules, which is what matters for binding, stoichiometry and comparing compounds.
- Mass concentration: mass per volume: g/L, mg/mL, µg/mL, ng/mL. These follow directly from weighing, and they are how concentrations often appear on labels and certificates.
Within a family, conversion is just a power of ten. Between the families you need the molar mass:
cmass (g/L) = c (mol/L) × M (g/mol)
Useful identities
| This | Equals |
|---|---|
1 mg/mL |
1 g/L = 1000 µg/mL |
1 µg/mL |
1 mg/L = 1000 ng/mL |
1% w/v |
1 g/100 mL = 10 mg/mL |
1 ppm (dilute aqueous) |
≈ 1 mg/L = 1 µg/mL |
1 mM |
M mg/L, e.g. 340.38 mg/L for M = 340.38 g/mol |
Worked example: 1 mg/mL of GHK in molar units
1 mg/mL = 1 g/L. Dividing by 340.38 g/mol gives 0.002938 mol/L = 2.938 mM = 2938 µM.
Going the other way, 10 µM × 340.38 g/mol = 3.404 × 10−3 g/L = 3.404 µg/mL.
Percentages: say which one
“Percent” is ambiguous unless qualified:
- % w/v: grams of solute per 100 mL of solution. This is the common lab meaning for solids dissolved in liquids.
- % v/v: millilitres of solute per 100 mL of solution, for liquids such as ethanol or DMSO.
- % w/w: grams per 100 g of solution, used for concentrated reagents such as acids.
The converter uses % w/v.
Parts per million
ppm is strictly a ratio, most often by mass (mg per kg). For dilute aqueous solutions, 1 L weighs about 1 kg, so 1 ppm ≈ 1 mg/L. The approximation fails for organic solvents, concentrated solutions, and ppm by volume in gases. The converter treats ppm as mg/L and labels it that way.
Peptide-specific cautions
- Salt forms and net content. A label of “1 mg/mL” usually means 1 mg of material per mL. If the powder is 80% peptide by mass, the peptide concentration is 0.8 mg/mL. Convert using the net content before switching to molar units.
- Which molar mass. Use the same molar mass basis (free peptide or salt form) throughout a calculation.