The performance of a new linear light path flow cell is compared with a liquid core waveguide and the linear cell is used for spectrophotometric determination of nitrite in sea water at nanomolar concentrations

Abstract

The sensitivity of a spectrophotometric assay is enhanced either by increasing the concentration of the target molecules within the flow cell or by extending the length of the light path of the flow cell. Determination of nutrients at nanomolar concentrations in sea water has therefore been based either on the preconcentration of the analyte on a microcolumn from a large volume of sample followed by its elution into a conventional 1–2 cm flow cell, or by the use of Liquid Core Waveguide (LCW) with a light path as long as several meters. In order to evaluate the relative improvements of these different approaches to increasing sensitivity we have developed a preconcentration technique for the determination of nitrite in seawater using the Gries reaction and compare its sensitivity and precision to that of non preconcentration techniques using both LCW and Linear Light Path (LLP) cells of different lengths. In this work the performance of the LLP is investigated and compared with the performance of the coiled LCW flow cell. Next, the determination of nitrite, automated by programmable Flow Injection (pFI), was carried out by using LCW and LLP flow cells, as well as by using a 10 cm LLP flow cell together with the preconcentration step of nitrite on a microcolumn. The assay of nitrite in sub micromolar range was most efficiently performed by a combination of pFI with the LLP flow cell without the need for a preconcentration step. The determination was performed at a rate of 40 samples/hour with a Limit of Detection (LOD) = 0.6 nM N using a 50 cm long, and a LOD = 2.5 nM N using a 10 cm long, LLP flow cell. Analysis of sea water samples confirmed that salinity does not affect the sensitivity of the determination. At a much lower cost than LCW, the LLP flow cell can also be easily assembled from components usually at hand in a laboratory.