I am computing limit of detection of zinc of an electrolyte. Using standard additions method, I have obtained positive x intercepts for 6/8 samples all very close to zero. Positive x-intercepts imply final concentration values to be negative. if x-intercept taken to be zero, the concentration is zero likewise. I have read that one can just take the modulus as the concentration value determined, but I am sure that this is only on the assumption that the x-intercept is negative. Is my problem then that the amount of zinc in my electrolyte to begin with is less than the smallest quantifiable amount? Even so, I am not sure how to tackle this y = mx + c ; y = signal (nA), x = concentration (ppm); Attempt has been made taking modulus of everything, but as I say positive this isn't right thing to do. Only other possibility can foresee is taking intercepts to be zero, giving a zero concentration in the electrolyte to begin with.