Stopped flow luminol chemiluminescence determination of Fe(II) and reducible iron in seawater at subnanomolar levels

A highly sensitive stopped flow chemiluminescence method has been developed for the analysis of Fe(II) at subnanomolar levels in seawater. Oxidation of Fe(II) by O2 in the absence of H2O2 is used to catalyze luminol chemiluminescence. The sample and luminol reagent are pneumatically injected into a 1 cm flow cell positioned in front of a photomultiplier tube. The luminescence intensity is measured with a picoammeter and recorded on a personal computer. Fe(II) can be determined directly in open ocean and coastal waters with 5 ml of sample. Interference studies were conducted with Cr(III), Fe(III), Cu(II), Mn(II), Zn(II), Co(II), and Ni(II). Ni(II), Cr(III), and Fe(III) did not exhibit a significant interference over the concentration range 5 to 500 nmol/kg. Mn(II) and Cu(II) caused a reduction in signal at all levels greater than 5 nmol/kg. Zn(II) and Co(II) produced an interference only when their concentrations were 100 times or more that of Fe(II), resulting in an enhanced signal. Dissolved organic matter reduced the sensitivity of the method in coastal waters relative to open ocean waters. The detection limit for Fe(II) with a 200 μl sample injection volume is 0.06 nmol/kg in open ocean waters and 0.15 nmol/kg in coastal waters. The relative standard deviation of three replicates is ±8%. Direct application to seawater is accomplished with a minimum of post-sampling manipulations or chemical treatments. In addition the reducible iron concentration can be determined by reduction with sulfite. The instrumentation required is easily adapted for field use.