Drug substance assay

Drug substance or active pharmaceutical ingredient calculation will give the anhydrous FLP in each sample. This may act as an additional confirmation of the sample purity, but considering sample concentration as it is prepared by the analyst. As such, FLP purity and drug substance assay should correspond, that is, subtracting purity and assay should be within a certain allowable percentage.

First, the concentration of UV-pure material in a sample is calculated by:

where
UV_Main	Area of UV main peak (mAU*s)
c	y-intercept of least squares line (calibration curve)
m	slope of least squares line (calibration curve)
l_v	sample injection volume (µL)

Concentration (%)(w/w) is then calculated by:

where
Concentration (%)(w/w)	Concentration weight per weight
V_sample	Total volume of sample solution (mL)
C_sample	Concentration of UV-pure material in sample (mg/mL)
MSP_sample	MS Purity, expressed as decimal; determined by dividing MS Purity by UV Purity
Wt_sample	Weight of sample in mg
S	Decimal fraction of sum of water, solvents, and sodium acetate, as applicable