The concentration of a cytostatic drug, cyclophosphamide, in urine was determined by GC/MS/MS. The component of interest in a 5.00 mL aliquot of urine was extracted with 20 mL of diethylether. The extract was then reduced in volume, spiked with a known amount of internal standard and diluted to 10 mL in a volumetric flask. Analysis of the sample gave a peak-area ratio (Aanal/Aint std) of 0.51. A series of calibration standards containing the same amount of internal standard as the sample were prepared and analysed with the sample. The results are tabulated below:

Cyclophosphamide (µg/L); Aanal/Aint std

5; 0.20
10; 0.40
15; 0.61
20; 0.85
Sample; 0.51