README_NAAMES_eem

nbn 08/16

These data are fluorescence excitation-emission matrix (EEM) spectra (Coble 1996, 2007; Stedmon and Nelson, 2015) of filtered seawater samples. Data reported as relative to the fluorescence of 1 ppb of quinine sulfate in 0.05N sulfuric acid solution at the excitation/emission maximum of quinine sulfate (Nelson and Coble, 2010). 

Samples are collected from Niskin bottles using silicone tubes by analysts wearing nitrile gloves. The samples are collected into combusted 60ml brown borosilicate glass EPA vials with Teflon lid liners (Nelson et al., 2007). 

To remove particles the samples are filtered through 0.2 micron pore 25mm Nuclepore polycarbonate filters that have been pre-extracted with 60ml ultrapure water (Barnstead Nanopure, 18M½ cm, low carbon filter cartridge). 

Samples are stored at 4C in the dark until analysis, generally within 6 months to 1 year. 2-year stability of samples stored in this manner has been documented by Swan et al. (2009). 

The filtered samples are equilibrated to room temperature before analysis along with blanks of ultrapure water (Barnstead Nanopure, low-organic cartridge, 18M½-cm or higher resistivity). 

Quinine sulfate in 0.05N sulfuric acid solution is prepared from pure Sigma quinine sulfate powder and premeasured Sigma 1N sulfuric acid kits. A stock solution of approximately 0.1 ppm quinine sulfate in 1N sulfuric acid is diluted 20:1 with ultrapure water in volumetric flasks to produce the ca. 5 ppb working solution. Concentration of the stock solution is determined by measuring the absorption spectrum of the stock solution. Serial measurement has shown no appreciable decline in absorption or fluorescence of the stock solutions over 1 year. 

Fluorescence spectra are measured in each sample using a Horiba Jobin Yvon Fluoromax 4 spectrofluorometer, under control of the Horiba Fluoressence software. The instrument has a single grating monochromator for the excitation lamp and a single grating monochromator for the emitted light. The excitation beam is split after the monochromator and is directed to a reference photodiode detector and the sample. Emitted light passes through the monochromator to a red-sensitive PMT detector. 

Before analysis each session excitation and emission monochromators are calibrated by confirming the peak wavelengths of the Xenon emission lines in the excitation lamp and pure water Raman emission. Emission scan of quinine sulfate solution at an approximately 5 ppb concentration is measured (along with an ultrapure water blank). Three replicate scans are collected for each sample and blank. The instrument is then set into EEM mode. 

Each blank and sample is scanned for fluorescence emission (275-600 nm, 2 nm steps, 5 nm slit) at successive excitation wavelengths from 250 to 400 nm (5 nm steps, 5 nm slit). Raw data are stored as counts (16-bit resolution) for emission data and as microamperes for excitation data. Ratio data (fluorescence detector to excitation detector ratio) are stored in real time along with sample and blank spectra. Data collection takes approximately 20 minutes per sample. The samples do not heat appreciably during analysis because of the low light fluxes involved, and also because the (metal) sample holder has a large mass that helps conduct and dissipate any heat. 

In post-processing, blank EEMs are subtracted from each sample EEM (there is usually a small positive signal in the UV). Blank-corrected fluorescence spectra are normalized to the maximum fluorescence of quinine sulfate and divided by the concentration of the blank solution (ppb). 

References:

Coble, P.G., 1996. Characterization of marine and terrestrial DOM in seawater using excitation- emission matrix spectroscopy. Mar. Chem. 51: 325-346.
Coble, P.G., 2007. Marine optical biogeochemistry: the chemistry of ocean color. Chem. Rev. 107: 402-418.
Nelson, N.B., and P.G. Coble, (2010). Optical analysis of chromophoric dissolved organic matter. In Practical Guidelines for the Analysis of Seawater, ed. O Wurl, pp. 79-96. Boca Raton, FL: CRC.
Nelson, N.B., D.A. Siegel, C.A. Carlson, C. Swan, W.M. Smethie, Jr., and S. Khatiwala, (2007). Hydrography of chromophoric dissolved organic matter in the North Atlantic. Deep-Sea Res. 54, 710-731.
Nelson, N.B., and J.M. Gauglitz (2016), Optical signatures of dissolved organic matter transformation in the global ocean. Front. Mar. Sci. 2:118. doi: 10.3389/fmars.2015.00118.
Stedmon, C.A., and N.B. Nelson (2015), The optical properties of DOM in the ocean, in: Biogeochemistry of Marine Dissolved Organic Matter, 2nd Edn, eds D. A. Hansell and C. A. Carlson (San Diego, CA, Academic Press), 481Ð508.
Swan, C.M., D.A. Siegel, N.B. Nelson, C.A. Carlson, and E. Nasir (2009) Biogeochemical and hydrographic controls on chromophoric dissolved organic matter distribution in the Pacific Ocean. Deep-Sea Res. I 56: 2175-2192.