Sulfur fixation in wood mapped by synchrotron X-ray studies: implications for environmental archives.

There is a shortage of archives of sulfur that can be used to investigate industrial orvolcanic pollution in terrestrial catchments, but the role of S as a nutrient, coupled with sparse published evidence, suggests that trees are promising targets. We focused on two conifer species (Picea abies (L.) Karst and Abies alba Miller) from an Alpine site in NE Italy. Bulk analyses of Abies demonstrate that S concentrations were higher in the second half of the 20th century but with some high outliers possibly reflecting particulate impurities. X-ray synchrotron analyses confirmed the observed time trend, which is similar to that of a nearby stalagmite, and reflects an atmospheric pollution record mediated by storage in the soil and ecosystem. S and P were found to be localized in the inner cell wall (ca. 2 microm wide), local thickenings of which probably account for some outlying high values of S in synchrotron studies. S occurs as a mixture of oxidation states (0 to +0.5, +2, +5, and +6) which are consistent in space and time. The results indicate that wood older than a few years contains archive-quality S but that robust conclusions require multiple replicate analyses.

Behaviour of environmental aquatic nanocolloids when separated by split-flow thin-cell fractionation (SPLITT).

A split-flow thin-cell (SPLITT) system in different operating modes was used to size fractionate colloids and particles in a lake water. The unperturbed lake water sample and eluent fractions (nominally a<1 microm; b>1 microm) were analysed with atomic force microscopy (AFM) to assess the quality of the separation, in particular to quantify the contamination of the b (>1 microm) fraction with nanocolloids (defined here as material <100 nm in size). Particle size distribution (PSD) results from AFM indicated that there was substantial contamination with nanocolloids. This contamination was, most likely, from diffusive transport across flow regimes within the SPLITT and this is supported by the fact that vertical distances between laminar flow regimes within the SPLITT channel are similar in magnitude, but slightly larger than the mean distances travelled by diffusion during the residence time of particles within the channel. Nevertheless, AFM surface density data showed that the concentration of nanoparticles in the a fraction was 6-9 times higher than in the b fraction, depending on the SPLITT mode, indicating that contamination of the b fraction was limited. Fluorescence data using monodisperse, low molar mass standards confirm the AFM results, with substantial contamination of the b fraction by the fluorescent molecular probes. The increased contamination of the b fraction of the standard molecular probes compared to natural nanocolloids is likely because they are smaller and more diffusive than the average of the natural material. Due to this contamination and the likelihood that these small colloids bind a large fraction of metals, SPLITT can only be used for metal fractionation and speciation in combination with other methods capable of performing further metal speciation analysis.

Size fractionation and characterisation of fresh water colloids and particles: split-flow thin-cell and electron microscopy analyses.

Split-flow thin-cell (SPLITT) was employed in conventional mode (CSF), to size-fractionate colloids and particles from a selected freshwater. Imaging and quantification by calculations of particle size distributions (PSDs) and shape factors were performed on sample analyzed by conventional high vacuum scanning electron microscopy (SEM) and environmental SEM (ESEM), to investigate the ability of SPLITT to make accurate and nonperturbing separations. SEM and ESEM images of unperturbed and SPLITT-generated fractions were used in order to obtain qualitative and quantitative information about the properties of colloids and particles. Particle size distributions (PSDs) showed that separations were very good, agreeing with theoretical behavior. ESEM PSDs showed that up to 87-88% of the material in the a fraction (expected to be <1 microm) was in fact less than 1 microm and in the b fraction (>1 microm) 87-95% of the material was the expected size. The SEM data indicated a slightly higher contamination of the b fraction with the presence of submicron colloids. Moreover, analysis of conformations indicated significant nonsphericity in unfractionated colloids and particles, but after SPLITT fractionation, shape factors showed that particles were significantly more spherical than before separation.