Measurement prefixes
^{b} "zepto" and "yocto" were accepted as prefixes by the 19^{th} Conférence Générale des Poids et Mesures in 1990 as permissible prefixes used to modify SI units. (SI is an abbreviation for Système International d'Unités.) Zepto had already been in unofficial use for several years. Zepto was coined from septa by replacing the "s" and "a" (for consistency with other units); "seven" times the standard unit multiplicative increment, 10^{3}, yields 10^{21}. Likewise, yocto was coined from octa because "eight" times the multiplicative increment, 10^{3}, yields 10^{24}. The initial letters, "z" and "y" were selected so that future units could be named in reverse alphabetical order. A Perspective on Prefixes and Magnitude One mole of M&M candies would occupy more than 14.5 million cubic miles of space (a cube 244 miles on side) [this assumes closest possible packing by a cubedshaped candy occupying 0.1 cm^{3}, actual candies would occupy a significantly larger volume (from Chem. Eng. News 18 Feb 2002, page 120)] This same volume is sufficient to cover the entire conterminous U.S. to a height of 17,000 feet. One liter of coffee containing 1 millimole of caffeine (1 mmol/L, or 1 mM) would have nearly one SEXTILLION MOLECULES of caffeine. One liter of solution whose analyte concentration is 1 zeptomolar (zM) would contain only about 600 MOLECULES of analyte. In contrast, the highly sensitive human nose can only detect concentrations down to about 1 picomolar (pM)  or 600 billion molecules per liter. Detection of a chemical at a concentration of 1 partperbillion (ppb; e.g., 1 µg/L) is similar to looking for one family among the world's entire population. In 1 nano second (ns), light travels a distance of only 1 FOOT. One hydrogen atom has a mass of almost two yoctograms (yg). Visual
demonstration of "powers of 10" (demonstration of
traveling from the edge of the universe [starting at a distance
of 10^{23} meters from earth] down to the subatomic dimension
of 10^{16} meters — a total change of almost 40 "orders
of magnitude" in scale) How much is a part per million?
For a discussion of statistical issues related to solutions of
ultralow concentrations, see the following reference: Comprehensive Background and Information on SI FootRule.com (unit conversions) Google Conversions  help (e.g., 1000 femtograms = ? milligrams) The NIST Reference on Constants, Units, and Uncertainty The International System of Units (SI): NIST Special Publication
330 Interpretation of the SI for the United States and Federal Government
Metric Conversion Policy Guide for the Use of the International System of Units(SI)
