Abstract: Objective To estimate the inevitable uncertainty of determining heroin by GC. Methods The source of the uncertainty was confirmed from the determining procedure. Each component′s uncertainty was calculated. The combined uncertainty was then obtained by synthesizing the uncertainties of various components variables, and the expand uncertainty was finally obtained. Results The uncertainty of repeated measurement was greater than other uncertainty components introduced by balance,content vessels and instruments in determining heroin. Conclusion The errors of repeated measurement and GC instrumental were confirmed as the major sources of uncertainty in determining heroin by GC.

Key words: gas chromatography, heroin, uncertainty