partspermillion

Parts per million (ppm) is a unit of concentration expressing the ratio of a substance to a total mixture on a one-in-a-million basis. The term is most commonly written as parts per million or abbreviated as ppm. Some datasets or informal sources may render the term as a single word like partspermillion, but this is not the standard form in scientific usage.

In practice, ppm is a dimensionless quantity that equals 1 × 10^-6. Different bases may be used, depending on context. In dilute aqueous solutions, 1 ppm often corresponds to about 1 milligram of solute per liter of solution (ppm by mass or w/w in water). For gases, concentrations are typically given as ppm by volume (ppm v/v); for ideal gases, this is effectively a mole fraction expressed on a 10^6 scale (1 ppm v/v = 1 µmol of gas per mol of mixture). In solids, ppm by mass (ppm w/w) is used.

Common relations include: 1,000,000 ppm equals 100 percent, and 1 ppm equals 0.0001 percent. ppm is widely used across fields such as environmental science, chemistry, geology, and industrial quality control to describe trace levels of contaminants, additives, or pollutants.

Measurement methods include analytical techniques like inductively coupled plasma mass spectrometry (ICP-MS) for metals, gas chromatography (GC) and GC‑MS for organics, and various spectrophotometric or sensor-based approaches. Because ppm is context-dependent, it is important to specify the basis (mass, volume, or mole) when reporting values.