Catalytically enhanced spectrophotometric determination of manganese in seawater by flow-injection analysis with a commercially available resin for on-line preconcentration

The sensitive, laboratory- and ship-based, flow-injection ( FI) method for the determination of dissolved manganese in seawater developed by Resing and Mottl ( Anal. Chem. 1992; 64: 2682-2687) has been significantly modified and improved by incorporating five significant changes. The three major changes are the use of a commercially available iminodiacetate ( IDA) resin ( Toyopearl AF-chelate 650M) in place of 8-hydroxyquinoline for on-line preconcentration of manganese and matrix removal, the addition of nitrilotriacetic acid as an activator ligand which increases sensitivity by a factor of 7 and decreases the limit of detection, and the on-line buffering of acidified samples before column loading. Two minor improvements include use of the more soluble sodium periodate in place of potassium periodate and elimination of Brij-35 surfactant. To accommodate these changes, the pH of samples was adjusted to 8.5 ( vs. 7.8), a higher acid concentration was required to elute the Mn from the IDA resin, and stronger reaction buffer was required to neutralize the acid. The accuracy of the method was evaluated with the use of NASS-4 standard seawater and by a comparison study of samples from the California coast that were analyzed by this method and an FI method coupled to inductively coupled plasma sector field mass spectrometry detection. The detection limit and precision of the method depend on the amount of sample preconcentrated onto the column. By preconcentrating 1.6 mL of sample, a detection limit of 0.03 nM ( 3 times the standard deviation of a blank) and precision of 3.2% ( as percent relative standard deviation) were obtained. The method was used on board ship to determine dissolved manganese in coastal waters off Oregon and Washington.