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 deve loped for the analysis of Fe(II) at subnanomolar levels in seawater. O xidation of Fe(II) by O-2 in the absence of H2O2 is used to catalyze l uminol chemiluminescence. The sample and luminol reagent are pneumatic ally injected into a 1 cm flow cell positioned in front of a photomult iplier tube. The luminescence intensity is measured with a picoammeter and recorded on a personal computer. Fe(II) can be determined directl y in open ocean and coastal waters with 5 ml of sample. Interference s tudies were conducted with Cr(III), Fe(III), Cu(II), Mn(II), Zn(II), C o(II), and Ni(II). Ni(II), Cr(III), and Fe(III) did not exhibit a sign ificant interference over the concentration range 5 to 500 nmol/kg. Mn (II) and Cu(II) caused a reduction in signal at all levels greater tha n 5 nmol/kg. Zn(II) and Co(II) produced an interference only when thei r concentrations were 100 times or more that of Fe(II), resulting in a n enhanced signal. Dissolved organic matter reduced the sensitivity of the method in coastal waters relative to open ocean waters. The detec tion limit for Fe(II) with a 200 mu l sample injection volume is 0.06 nmol/kg in open ocean waters and 0.15 nmol/kg in coastal waters. The r elative standard deviation of three replicates is +/- 8%. Direct appli cation to seawater is accomplished with a minimum of post-sampling man ipulations or chemical treatments. In addition the reducible iron conc entration can be determined by reduction with sulfite. The instrumenta tion required is easily adapted for field use.