Comparison and evaluation of laboratory performance on a method for the determination of perchlorate in fertilizers

This report details the interlaboratory validation of a method for the determination of perchlorate in fertilizers. In this method (EPA/600/R-01/026), a solid sample of fertilizer is ground. Subsequently, the ground material is either leached with deionized water to dissolve any perchlorate salt in the case of minimally soluble fertilizers (e.g. supertriplephosphate or timed-release products), or simply dissolved in the case of highly soluble fertilizers (e.g. urea, NaNO3 or KCl). The resulting aqueous solution is then subjected to ion chromatography with suppressed conductivity detection. Four laboratories applied the method to field samples of 48 different products (commodity chemicals) and to seven quality control samples prepared by the Environmental Protection Agency (EPA). Additional tests were conducted by three other laboratories on specific materials. Recovery (81-111%) was demonstrated on spikes of known concentration, and a preliminary assured reporting level was determined for each fertilizer matrix by each laboratory. Injection-to-injection precision was satisfactory: generally less than 15% difference, and always less than 25%. All of the laboratories used Dionex IonPac AG16 guard and AS16 separation columns with NaOH or KOH as eluent. Detection was by suppressed conductivity. The method was shown to be sufficiently robust for the screening of fertilizers for perchlorate, but performance was low on a mixture of siliceous minerals (kaolinite and bentonite). Both laboratory performance and method performance are validated.