parts per million (ppm) and Parts Per Billion (ppb) - These terms give
scientists a way to describe how much of a substance is contained in a sample:
parts of analyte per million parts of sample, for instance. In atmospheric
chemistry these become volume parts of analyte per volume parts of atmosphere:
ppmv, ppbv, etc. At low analyte gas phase concentrations the analyte is assumed
to act as an ideal gas. For instance, a 1 ppmv concentration of formaldehyde
would represent 1 liter of formaldehyde per every 1,000,000 liters of air; also
equivalent to 1 microliter of H2CO per 1 L air. In gas phase
concentrations these units are also called gas phase mixing ratios. The reason
is because they are just that, ratios of analyte volume to sample volume; the
volume of the sample doesn't matter.
[Ecology A to Z; P. Fleisher; p.150-161;
1994; Dillon Press; New York] [Occupational Environment; S. DiNardi; p.1320;
1997; AIHA Press; Fair Fax, Virginia]
- Parts (molecules) of a substance contained in a million parts of another
substance (or water).