At low concentrations this takes a very long time to happen.

She had good bones in her face, he decided after making an effort at concentration.

The effect is visible already at concentrations of 0.1% in the paint system.

But that was caused by his sudden effort at concentration.

The effect of the latter compound is present only at low concentrations.

It is used at concentrations of 0.0025% (25 parts per million).

But at higher concentrations, the true activities of the ions must be used.

Bliss lifted his head and made an effort at concentration.

He crinkled up his face in an effort at concentration.

Generally they are found in consumer products at concentrations ranging from 3-20%.