. . . . . . . . . . "Dimensionless unit for concentration. Recommended practice is to use specific units such as \\(ug/l\\)."^^ . "Parts per trillion"@en . "[pptr]"^^ . . "PPTR" . . "http://aurora.regenstrief.org/~ucum/ucum.html#section-Derived-Unit-Atoms"^^ . "1e-12"^^ .