Parts per million (ppm): what it means and how it converts

Parts per million (ppm) is a way of writing very small amounts. One ppm means one part of something in a million parts of the whole. It is the same idea as a percentage, which is parts per hundred, just on a much finer scale. This guide explains what ppm means, how it converts to percent and mg/L, and where it appears on a certificate of analysis (COA).

Terms are defined in the glossary.

What ppm means

A ratio unit compares the amount of one thing with the amount of the whole. The family is:

One ppm is roughly one second in eleven and a half days. One ppb is roughly one second in 32 years.

Ppm is not an SI unit, and on its own it doesn't say what is being compared. A careful report states whether it means mass per mass, volume per volume or something else.

Converting ppm

ppm and percent

• ppm to percent: divide by 10,000. So 410 ppm is 0.041%.
• Percent to ppm: multiply by 10,000. So 0.5% is 5,000 ppm.

ppm and mass

For solids, ppm is usually by mass:

• 1 ppm = 1 mg per kg = 1 µg per g.

So a 10 mg sample of a powder with 100 ppm of an impurity contains 10 mg × 100 ÷ 1,000,000 = 0.001 mg, or 1 µg, of it.

ppm and mg/L

For dilute solutions in water:

• 1 ppm ≈ 1 mg per litre.
• 1 ppb ≈ 1 µg per litre.

This works because a litre of water weighs very close to 1 kg, so mg/L and mg/kg are nearly the same. It does not hold for solvents with a different density, or for concentrated solutions. For those, convert using the density of the solution.

Mass, volume, or both: w/w, w/v and v/v

Ppm and percentages can compare mass with mass, volume with volume, or mass with volume. The label says which:

• w/w (weight per weight): mass of the substance per mass of the whole. Used for solids and impurity limits.
• v/v (volume per volume): volume of a liquid per volume of the whole. Used for mixing liquids, and for gases, where it is sometimes written ppmv.
• w/v (weight per volume): mass per volume. 1% w/v is 1 g per 100 mL, or 10 mg/mL.

The same number can mean different amounts depending on the basis. A 0.6% v/v acetic acid solution and a 0.6% w/v one are close, because acetic acid's density is close to water's, but they are not the same. For working between % w/v, % v/v, mg/mL and molarity, with worked examples, see molarity, mg/mL and dilutions.

Where ppm appears on a COA

Residual solvents

Peptide synthesis and purification use organic solvents. Small amounts can remain in the final powder. When a COA reports them, it usually does so in ppm by mass.

The ICH guideline Q3C sets limits for residual solvents in pharmaceutical materials (ICH Q3C). It sorts solvents into classes by how hazardous they are. Some examples of its limits:

Research reagents don't have to meet these limits, but they are a common reference point. Residual solvents are usually measured by gas chromatography.

Elemental impurities

Metals and other elements can come from catalysts, equipment or raw materials. They are measured at ppm or ppb levels, often by ICP-MS. The ICH guideline Q3D sets limits as permitted daily exposures, which labs convert into concentrations in µg/g, the same as ppm (ICH Q3D).

Mass accuracy in mass spectrometry

High-resolution mass spectrometry reports how close a measured mass is to the expected one in ppm:

error (ppm) = (measured mass − expected mass) ÷ expected mass × 1,000,000

For example, an expected mass of 1,500.0000 and a measured mass of 1,500.0045 gives 0.0045 ÷ 1,500 × 1,000,000 = 3 ppm. Using ppm rather than daltons lets one tolerance apply across a range of masses. See mass spectrometry and identity confirmation.

NMR chemical shift

In NMR spectra, "ppm" means something different. It describes how far a signal sits from a reference compound, as a fraction of the instrument's frequency. It is not a concentration. See NMR, FTIR and amino acid analysis.

Common mistakes

• Mixing up ppm and ppb. They differ by a factor of 1,000.
• Assuming 1 ppm = 1 mg/L for any liquid. It is only close for dilute water-based solutions.
• Leaving out the basis. "500 ppm" means little without knowing if it is w/w, v/v or w/v.
• Reading NMR ppm as a concentration. It is a position on the spectrum.
• Reporting more precision than the method supports. A residual solvent result of 412.7 ppm from a method with a spread of ±20 ppm is better written as 410 ppm. See accuracy, precision and significant figures.

Related guides

For how to read a full lot document, see what a certificate of analysis tells you. For the solvents used in synthesis, see solid-phase peptide synthesis and TFA salts. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.