Seven laboratories participated in an interlaboratory evaluation of a liquid chromatography/mass spectrometry (LC/MS) method for the analysis of 10 chlorinated phenoxy acid herbicides. The focus of the evaluation was to test the intercomparability of LC/MS data obtained from two types of LC/MS interfaces (i.e., thermospray (TS) and particle beam (PB)). Eight simulated sample extracts were sent to each laboratory for LC/MS analysis. There were statistically significant differences between interfaces in the quantitative data for all analytes except 2-(2,4,5-trichlorophenoxy) propanoic acid (silvex). Particle beam exhibited a high positive bias and a low relative standard deviation at the highest sample extract concentration range, 500 micrograms/mL, while TS showed a low bias and a low relative standard deviation at the lowest sample extract concentration range, 5 micrograms/mL. Another factor of the study was to look for any performance differences between interfaces of the same type, but differing manufacturers. A statistical difference was observed, between PB interfaces, for 2-(1-methylpropyl)-4,6-dinitrophenol (dinoseb). (Copyright (c) 1991 by the American Chemical Society.)