Research Group Prof. Dr. G. Friedrichs

VSFG Spectra of Natural Marine Samples

ScreenSamplingAir-sea interaction takes place at the ocean surface microlayer, which is known to modulate physical, chemical, and biological processes. It is enriched by organic material, flooded by sunlight, inhibited by many organisms, and thus forms a unique reaction environment. In particular, surface active material such as lipids, carbohydrates, and proteinaceous material is enriched at the topmost layer (organic nanolayer), which can be investigated by VSFG spectroscopy. So far, the detailed composition, its molecular structure and seasonal trends of these quantities are largely unknown.

Seasonal changes in the characteristics of the sea surface nanolayer

The topmost part of this microlayer, located at the interface between ocean and atmosphere, is called the sea surface nanolayer. Because of its small vertical dimension and the tiny amount of substance it represents, an analytical tool is needed that is sensitive enough to detect this small amount of substance and selective enough to specifically probe the ocean surface. One tool that serves this purpose is vibrational sum frequency generation spectroscopy (VSFG). We have demonstrated the suitability of VSFG spectroscopy for this purpose [1] by taking samples of the sea surface nanolayer using the screen sampling method (see photo).

SFG Intensity














 

An example of a VSFG spectrum taken on seawater is shown in the Figure. Vibrational spectra of seawater interfaces basically show stretch vibrations of C-H and O-H bonds. The resulting spectra allow conclusions regarding composition and structure of the layer samples, notably the solubility of the surfactants (“wet” or “dry” surfactants) and the abundance of lipid-chain-carrying and saccharide compounds [2].

A time-series of seawater SFG spectra was taken at Boknis Eck Time Series Station (https://www.bokniseck.de) based on on monthly measurements. A seasonal variability has been found that indicates a peak in SFG active surfactant abundance in late summer [3]. This is significantly later than the spring algea bloom showing that nanolayer abundance is not directly related to the phytoplankton concentration. These findings suggest that biochemical and/or photochemical transformation of organic substances is an important factor in ocean nanolayer formation.

[1] Kristian Laß, Joscha Kleber, and Gernot Friedrichs. "Vibrational Sum-Frequency Generation As a Probe for Composition, Chemical Reactivity, and Film Formation Dynamics of the Sea Surface Nanolayer." Limnology and Oceanography: Methods 8 (2010): 216-228. [doi: 10.4319/lom.2010.8.216]

[2] Kristian Laß and Gernot Friedrichs. "Revealing Structural Properties of the Marine Nanolayer From Vibrational Sum Frequency Generation Spectra." Journal of Geophysical Research 116 (2011): C08042. [doi: 10.1029/2010JC006609]

[3] K. Laß, H. Bange, G. Friedrichs, "Seasonal signatures in SFG vibrational spectra of the sea surface nanolayer at Boknis Eck Time Series Station (SW Baltic Sea)" Biogeosciences10 (2013) 5325-5334; doi:10.5194/bg-10-5325-2013.
 
Contributing Researchers: G. Friedrichs, H. Bange and (formerly) K. Laß, J. Kleber