The gas concentration can be calculated using following equations:

C(ng/sccm) = C(ppm) x K1
C(ppm) = C(ng/sccm) x K2

where

K1 = Molecular Weigh / 24.46
K2 = 24.46 / Molecular Weight
24.46 is the molar volume in liters @ 25°C, 760 mmHg