attomolar

Attomolar (abbreviation aM) is a unit of concentration equal to 10^-18 moles per liter (mol/L). It is a part of the SI prefix scale used to describe molar concentrations. An attomolar solution contains an extremely small amount of solute per liter; at this level, one liter of solution contains about 6.0×10^5 molecules of solute on average, assuming ideal behavior, and a microliter contains roughly 0.6 molecules on average.

Attomolar concentrations are used in contexts requiring ultra-trace detection, such as certain nucleic acids and proteins in highly dilute samples, environmental monitoring of trace contaminants, and some advanced diagnostic assays. Because of the small numbers of molecules involved, practical measurements are subject to sampling variability and background noise.

Measuring attomolar levels typically requires highly sensitive techniques and signal amplification, such as digital PCR or single-molecule detection approaches, highly selective biosensors with amplification, and advanced mass spectrometry. Experimental design often relies on rigorous controls and calibrations, and results may be interpreted using Poisson statistics to account for the discrete nature of molecular counts in small volumes.

Etymology: a- (atto) denotes 10^-18, and molar denotes moles per liter; attomolar is smaller than femtomolar (10^-15), picomolar (10^-12), nanomolar (10^-9), etc. Common practical contexts are niche and specialized.