We did oil in water analysis using three different methods and we got three different methods.

1. Using HACH mass spectrophotometer: 8.5 mg/l
2. Using gravimetric 8 mg/l
3 Using a combination of GC and GC MS:

C6 - C9 Fraction 28 mg/l
C10 - C14 Fraction 26.8 mg/l
C15 - C28 Fraction 11.8 mg/l
C29 - C36 Fraction 1.85 mg/l

Are those three different methods measured different fraction of hydrocarbon?

This is for regulatory purposes which only mention oil in water should not exceed 30 mg/l. I am confused as to what result I should present to the regulator. Also, which method is normally acceptable?