A HEURISTIC DERIVATION OF THE HORWITZ CURVE

The Horwitz curve is a simple exponential relationship between the rel ative standard deviation among laboratories to concentration, C, expre ssed in mass/mass units, Examination of almost 10 000 interlaboratory data sets shows that the curve is more or less independent of analyte, matrix, method, and time of publication, over the range from pure mat erials, C = 1 (100%), to trace polychlorinated aromatic contaminants ( PCCs), C approximate to 10(-12). The functional relationship can be de rived simply by assuming that the infinitesimal fractional change in s tandard deviation is proportional to the infinitesimal fractional chan ge in C, by integrating, and by determining the constant of integratio n from empirical results, Mycotoxin and PCC data show that the limit o f measurement in the interlaboratory environment is C approximate to 1 0(-9), where results become uninterpretable because of the appearance of excessive numbers of false positive and false negative values, Lowe r values are possible only because of the extraordinary specifications for quality control for these analyses.