Quality control spikes in the same matrix (75 µg L -1 ) yielded median recoveries equaling 99% of 31 P and 57 Fe. An independently-prepared spike at 40 µg L -1 yielded a median 57 Fe recovery of 96%. Detection limits were estimated using the 3! method, with ! calculated separately for the elution step of 5 blanks. Median instantaneous detection limits were estimated at 1.7 and 0.13 µg L -1 for 31 P and 57 Fe.