Simultaneous determination of nitrite and nitrate at nanomolar level in seawater using on-line solid phase extraction hyphenated with liquid waveguide capillary cell for spectrophotometric detection

A flow system coupled with on-line solid phase extraction (SPE) and liquid waveguide capillary cell (LWCC) spectrophotometric detection has been established to simultaneously determine nanomolar levels of nitrite and nitrate in surface seawaters. The azo-compound that formed from nitrite could be quantitatively extracted on HLB SPE cartridge, and then eluted and detected in a LWCC detector of 16 cm path length. In a separated portion of sample, nitrate was reduced to nitrite with a Cu–Cd reduction column. The initial and reduced nitrite together formed azo-compound in a parallel flow channel, and the azo-compound formed was extracted, eluted and detected via the same way as that of nitrite. Due to the hyphenation of SPE and LWCC, the matrix interferences to the performance of LWCC could be avoided. Limit of detections at the nanomolar level (0.3 nmol L⁻¹ for nitrite and 1.5 nmol L⁻¹ for nitrate) were obtained. Application to seawater analysis showed good agreement with an alternative analytical configuration.