The Horwitz curve is a simple exponential relationship between the relative standard deviation among laboratories to concentration, C, expressed 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 materials, C = 1 (100%), to trace polychlorinated aromatic contaminants (PCCs), C approximate to 10(-12). The functional relationship can be derived simply by assuming that the infinitesimal fractional change in standard deviation is proportional to the infinitesimal fractional change in C, by integrating, and by determining the constant of integration from empirical results, Mycotoxin and PCC data show that the limit of measurement in the interlaboratory environment is C approximate to 10(-9), where results become uninterpretable because of the appearance of excessive numbers of false positive and false negative values, Lower values are possible only because of the extraordinary specifications for quality control for these analyses.
