Fiber optic spectrometers for in-situ measurements in the oceans: the ZAPS Probe

The fiber optic spectrometers developed in this laboratory combine the power of a xenon flash source and the sensitivity of a photomultiplier detector with the flexibility of bifurcated fused silica lightguides. In the ZAPS (zero angle photon spectrometer) instrument optical interference filters are used to select wavelengths with source and detector fibers terminating in the same polished surface. ZAPS is a versatile light-processing instrument with a 6000 m capability. This report describes ZAPS deployments as a UV fluorometer and chemical sensor. As a fluorometer ZAPS can excite and detect violet fluorescence (FLU) from dissolved organic matter with the flash lamp and PMT filtered at 320 and 420 nm. A profile of FLU generated in situ with the ZAPS Probe is nutrient-like, increasing with depth below the euphotic zone. These results are consistent with previous measurements using shore-based instruments. ZAPS can be deployed as a chemical sensor. A technique developed for ZAPS makes it possible to determine dissolved Mn in the oceans in situ and in real time. The chemical method relies on the fact that reduced Mn in the presence of periodate has a catalytic effect on the oxidative coupling of fluorescent N,N′-diethylaniline (DEA) molecules producing the non-fluorescent product N,N,N′,N′-tetraethylbenzidine (TEBOx). In the ZAPS technique a pump pulls seawater through 2 chemical cartridges arranged in tandem with a flow-cell. The first cartridge contains retained periodate and the second bound DEA. The sample stream leaves the cartridges with micromolar levels of these reagents and is irradiated with UV light (250 nm) in the flow-cell. The level of fluorescence detected in the flow-cell is indirectly related to the concentration of Mn. The fluorescent response to Mn is linear over at least three orders-of-magnitude in concentration and the detection limit less than 0.1 nM. Seawater Mn profiles developed with this sensor are presented for an open-ocean station in the North Pacific and a hydrothermal vent site on the Gorda Ridge.