The efficiency of the zinc column for conversion of nitrate to nitrite was investigated by passing the solutions of these ions through the column and gave complete conversion over the whole range tested. As a routine procedure, the efficiency of any new zinc column was measured by determining the ratio of the peak heights of 100 μg L−1 NO3−–N and 100 μg L−1 NO2–N, which was always greater than 95%.
The lifetime of the zinc column depends on the amount of oxidizable material passing through it. Under continuous flow operation, as described here, the effective lifetime of column was found to be approximately 9 h with a ≤10% decrease in signal being observed during that time. While this is not ideal, regular injection of a calibrating standard enabled back correction to compensate for slight decreases in sensitivity. Reactivation of the column in situ, e.g. with 0.1 mol L−1 hydrochloric acid or 0.05% EDTA solution, which is used to regenerate copperized cadmium columns, did not prove satisfactory, and installation and activation of a fresh column was usually performed after a day of use.
The analytical sensitivity is also affected by the total flow rate as shown in Table 2. At the lower combined flow rate of 2.1 mL min−1, the sensitivity of method increased, with a corresponding improvement in the detection limit, which can be attributed to either or a combination of lower hydrodynamic dispersion and increased residence time. The improved sensitivity and lower buffer consumption favours the use of combined flow rate of 2.1 mL min−1 except in situations where a higher sample throughput is required.