Nitrogen is an essential element for phytoplankton growth in aquatic environments. Nitrate is the main nutrient supporting this production in the open ocean, and the supply of nitrate often controls primary production in oligotrophic areas. Excluding the high-latitude and eastern equatorial regions of the Pacific, the surface waters of the North and South Pacific Ocean from about 40˚ N to 40˚ S are relatively depleted in nitrate. In the North Pacific subtropical gyre, nitrate concentrations are typically below 10 nmol/l.1 To understand the nitrogen cycles in the open ocean ecosystem, a sensitive analytical method to determine the nanomolar levels of nitrate in seawater is essential.The standard method of nitrate determination in seawater is based on a flow-injection technique with spectrophotometric detection. [2][3][4] In this method, nitrate is first reduced to nitrite in a copperized cadmium column.Nitrite is coupled with sulfanilamide to form a diazonium cation. This diazonium reacts with N-1-naphtylethylenediamine dihydrochloride, forming a purple azo dye whose absorbance is measured at 550 nm. Next, the nitrite is measured independently according to the same method without reduction. Nitrate is then calculated from the difference in the concentration between [nitrate + nitrite] and nitrite. This method can be conventionally applied to the determination of nitrate and nitrite on board; its detection limit is about 50 nmol/l as [nitrate + nitrite].A more sensitive method of nitrate analysis has been developed by utilizing the chemiluminescence of nitrogen monoxide. [5][6][7] In this method, both nitrate and nitrite are reduced to nitrous oxide in a highly acidic solution of sulfuric acid containing iron(II) ammonium sulfate and ammonium molybdate. Nitrous oxide in the solution is carried to a drying tube and reacts with ozone, resulting in chemiluminescence. The detection limit is around 2 nmol/l. This measurement method also requires nitrate reduction to nitrite. By this method, Cavender-Bares et al. 8 measured 2 to 10 nmol/l concentrations of [nitrate + nitrite] in surface seawater that were below the detection limit of the standard method using azo dye formation. They revealed the vertical gradients of [nitrate + nitrite] in the North Atlantic Ocean.As a simple method, direct UV determination 2,4,9 has also been examined, but often encounters interference by bromide or organic compounds, such as humic substances, which also have UV absorbance in a wavelength range similar to that of nitrate. Capillary electrophoresis 10,11 is very useful because it gives high efficiency in the separation of analytes, and enables rapid analysis and the direct injection of seawater. However, the small injection volume and short path length of the detector results in a poor detection limit for nitrate and nitrite.Ion chromatography (IC) 12 is advantageous in the determination of nitrite and nitrate because these anions can be separated from other ions, such as bromide and organic substances. However, with highly ionic mat...