Activation analysis and dendrochronology for estimating pollution histories

Neutron activation analyses of wood shavings from tree rings have shown that plant tissues grown through periods of varying environmental conditions keep permanent records of the trace elements they received through nutrients from their immediate surroundings. Detailed examination of several species of trees, along with dendrochronological data, suggest the possibilities of utilizing this technique for pollution monitoring as well as for developing valuable information for environmental modelling.     

Variations of trace element contents within single tree rings

X-ray fluorescence was used to measure variations in relative and absolute trace-element concentrations within single tree rings taken from a 32-year-old pine tree near a coalburning power plant. The Zn/Cu ratio was found to have a constant value, both along individual rings and between different rings. The Fe, Ca and 8 concentrations varied in a single tree ring and depended on the age of the tree. The results indicate that the variations in concentrations of elements along a single ring are caused by heterogeneous tree tissue structure.     

Instrumental neutron activation analysis of tree rings for dendrochemical studies

Instrumental neutron activation analysis (INAA) was employed to determine zinc, cadmium and potassium concentrations in the growth rings of ponderosa pine (Pinus ponderosa Dougl.) trees growing along the shores of Lake Roosevelt in Washington State, U.S.A. where mineral processing activities have resulted in high burdens of zinc and cadmium in the lake sediments. The tree growing along the contaminated waterway display elevated concentrations of zinc in its growth rings relative to a tree growing along an uncontaminated tributary of Lake Roosevelt. Cadmium concentrations in the growth rings from both sites are similar from 1988 to 1993. Water quality data indicate an increased concentration of cadmium in the lake from 1984 to 1988. The increased concentrations of cadmium in the lake water were reflected in apparent increases in concentrations of cadmium in individual rings of the tree sampled at the contaminated site. This suggests that translocation of cadmium in the sapwood of heartwood-forming species does not occur in the short term, and thus may not be a limiting factor in using trees as environmental monitors for cadmium. In addition, five-year tree ring segments were analyzed and subsequently reanalyzed as individual single-year ring segments. The analytical data obtained for the pooled individual rings are essentially the same as for the five-year segments, demonstrating the utility of NAA for dendrochemical studies.     

Calculating the number of spanning trees in a labeled planar molecular graph whose inner dual is a tree

The quantum mechanical relevance of the concept of a spanning tree extant within a given molecular graph—specifically, one that may be considered to represent the carbon-atom connectivity of a particular (planar) conjugated system—was first explicitly pointed out by Professor Roy McWeeny in his now-classic 1958 memoire entitled “Ring Currents and Proton Magnetic Resonance in Aromatic Molecules.” In a recent work, Gutman and one of the present authors proposed a scheme for calculating the number of spanning trees in the graph associated with catacondensed, benzenoid molecules which, by definition, contain rings of just the one size (six-membered); here, we present an algorithmic approach that enables the determination of the number of spanning trees in the molecular graph of any catacondensed system (which, in general, has rings of more than one size, and these may be of any size). An illustrative example is given, in which the algorithm devised is applied to a (hypothetical) pentacyclic catacondensed structure comprising a five-membered ring, a six-membered ring, a seven-membered ring, and two four-membered rings. © 1996 John Wiley & Sons, Inc.     

Is wood pre‐treatment essential for tree‐ring nitrogen concentration and isotope analysis?

Tree-ring nitrogen concentrations and isotope ratios (δ(15)N) are gaining in popularity for environmental research although their use is still debated because of nitrogen mobility in tree stems. Modern studies generally present results on wood that is pre-treated to remove soluble nitrogen compounds and to minimize the impact of radial translocation on tree-ring nitrogen environmental records. However, the necessity to use such pre-treatment has never been fully assessed. Here we compare the nitrogen concentrations and δ(15)N values of two wood preparation protocols applied to beech and red spruce tree rings for the removal of soluble compounds from ring pairs with non pre-treated tree rings. For both tree species, pre-treatment did not minimize the radial patterns of tree-ring nitrogen concentrations and the increasing concentration trends that are coincident with the heartwood-sapwood boundary. Therefore, even if the tree-ring nitrogen concentrations are slightly modified by pre-treatment, these concentrations are considered to reflect internal stem processes rather than environmental conditions in both species. The δ(15)N values were similar for untreated and pre-treated ring pairs, suggesting that wood pre-treatment did not substantially change the δ(15)N values and temporal trends in ring series. In addition, tree-ring δ(15)N series of untreated and pre-treated wood did not show any sign of influence of the heartwood-sapwood boundary in either tree species, indicating that nitrogen translocation did not generate significant isotopic fractionation. We therefore suggest that untreated ring δ(15)N values of beech and red spruce trees can be used for environmental research.     

Investigations of the migration of elements in tree rings by Laser-ICP-MS

The autocorrelation function of the concentration of elements in growth rings has been used to study their migration from ring to ring. The correlation time which can be calculated by the autocorrelation function gives an indication of the transport of elements. However, a large correlation time might only be taken as a necessary criterion for the migration but not for a sufficient one. Laser sampling-ICP-MS has been used for the determination of the contents of elements in the rings of an 150 years old oak - the autocorrelation functions of selected elements have been calculated. Large correlation times have been only obtained for Ca, Fe and Cr. It may be concluded that Mg, Zn, Cu, Sr, Pb, Ni, Ba, Al and Mn hardly migrate radially in the trunk of an oak.     

Investigations of the migration of elements in tree rings by Laser-ICP-MS

The autocorrelation function of the concentration of elements in growth rings has been used to study their migration from ring to ring. The correlation time which can be calculated by the autocorrelation function gives an indication of the transport of elements. However, a large correlation time might only be taken as a necessary criterion for the migration but not for a sufficient one. Laser sampling-ICP-MS has been used for the determination of the contents of elements in the rings of an 150 years old oak – the autocorrelation functions of selected elements have been calculated. Large correlation times have been only obtained for Ca, Fe and Cr. It may be concluded that Mg, Zn, Cu, Sr, Pb, Ni, Ba, Al and Mn hardly migrate radially in the trunk of an oak.     

Time variations in environmental pollution

Two programmes of work are described involving the measurement of environmental pollution over periods from two hours to many years. The long period measurements are deduced from neutron activation analysis of whole wood samples taken from individual tree rings for trees grown in various locations, and some results for cadmium, antimony and germanium levels are reported. In a second series of experiments the concentration of airborne particulate matter is being measured using a moving filter (streak sampler) technique to give a continuous record with two hour resolution. The filter samples are being analysed by proton induced X-ray emission (PIXE) and preliminary data is presented.     

Determination of trace elements in tree rings ofPinus by neutron activation analysis

The elemental composition of pine trees as a function of age was achieved by the tree-ring method. By using instrumental neutron activation analysis, trace elements were determined in individual rings ofPinus elliottii var.elliottii of 32, 14 and 9 years, from an implanted forest ofPinus sp., Buri, São Paulo, far away from industrial pollution. Different components of the system such as needles, pith, bark, soil and litter were also analyzed for 18 elements (As, Br, Ce, Co, Cs, Eu, Fe, Hf, K, La, Mn, Na, Rb, Sc, Sm, Yb, Zn, and Zr). Al, B, Ba, Ca, Fe, K, Mg, and P concentrations were determined by atomic absorption and/or emission spectrometry (AAS; ICP-AES). Some elements have showed similar radial distribution of the concentration for the three ages. Abrupt concentration changes in the pith and ring-bark boundary corresponding to the first and last growth rings were observed.     

Multielemental analyses of tree rings by inductively coupled plasma mass spectrometry

Inductively coupled plasma mass spectrometry (ICP-MS) was evaluated for major, minor, trace, and ultra-trace elemental analyses of individual tree rings. The samples were obtained from an old-growth Douglas fir (Pseudotsuga menziesii) growing 15 km northeast of Mount St. Helens volcano, Washington, USA and from trees at various other North American sites. Samples were brought into solution by microwave digestion in sealed Teflon vessels. Eightly percent of elements from Li to U had detection limits in the solid (wood) below 8.0 ng g^–1 (parts per billion, ppb). When selected element concentrations in the Mount St. Helens samples are plotted against time, two anomalous peaks occur at A. D. 1478 and 1490 that closely correlate with past eruptions of the volcano. These preliminary results show that ICP-MS is a rapid and sensitive analytical method for multielemental analyses of individual tree rings.     

A new method for the determination of the interstitial site occupancy of carbon atoms at trace levels in the MO-VPE Ga1−x Al x As lattice

The uptake and the long term behavior of Cs-137, Cs-134 and K-40 in the annual tree rings of spruce were examined. The youngest tree rings which are most active in water transport have higher activity concentrations, of K-40 and of radiocesium than the older ones. The activity concentration of Cs-137 in a water transporting tree ring can be well described as a function of the activity concentration of K-40. Furthermore a depth profile of the soil was taken and gives information, about the depth distribution of radiocesium and K-40.     

Cation-exchange properties of pumice: Taking a sip from a volcanic cocktail

The search for the telltale signs of volcanic eruptions in easily dateable (biological) material like tree rings or speleothems has been going on for a very long time. Even though the reactions of tree rings to cataclysmic events are fairly well understood, there has not been a satisfactory method to distinguish volcanic from other climatically active events. Recent findings of differing Ba-contents in soil and the unexpected mobility of Ba during volcanic eruptions strongly suggest that tephra changes the concentrations of trace elements in the surrounding topsoil. To better understand the mechanics of this enrichment, leaching experiments with volcanic ashes have been performed that show that pumice acts as a cation-exchanger. Neutron activation analysis (NAA) with its reasonably simple sample preparation and the possibility to distinguish volcanic ashes from contamination of solvents by using activated ashes offers a hands on approach to the problem above. Several samples of known composition of volcanic ashes have been activated and leached with different solvents and solutions, aiming to both recreate a realistic situation comparable to natural leaching due to rain fall and obtain quantifiable results.     

Importance of tensor asymmetry for the analysis of 2H NMR spectra from deuterated aromatic rings

We have used ab initio calculations to compute all of the tensor elements of the electric field gradient for each carbon-deuterium bond in the ring of deuterated 3-methyl-indole. Previous analyses have ignored the smaller tensor elements perpendicular to principal component Vzz which is aligned with the C-2H bond (local bond z-axis). At each ring position, the smallest element Vxx is in the molecular plane and Vyy is normal to the plane of the ring. The asymmetry parameter = (Vyy - Vxx)/Vzz ranges from 0.07 at C4 to 0.11 at C2. We used the perpendicular (off-bond) tensor elements, in concert with an improved understanding of the indole ring geometry, to analyze prototype 2H NMR spectra from well-oriented, hydrated peptide/lipid samples. For each of the four tryptophans of membrane-spanning gramicidin A (gA) channels, the inclusion of the perpendicular elements changes the deduced ring tilt by nearly 10 and increases the ring principal order parameter Szz for overall "wobble" with respect to the membrane normal (molecular z-axis). With the improved analysis, the magnitude of Szz for the outermost indole rings of Trp13 and Trp15 is indistinguishable from that observed previously for backbone atoms (0.93 +/- 0.03). For the Trp9 and Trp11 rings, which are slightly more buried within the membrane, Szz is slightly lower (0.86 +/- 0.03). The results show that the perpendicular elements are important for the detailed analysis of 2H NMR spectra from aromatic ring systems.     

F-对沸石分子筛双四元环影响的量子化学计算

采用PM3量子化学计算方法研究了沸石分子筛及无机微孔晶体结构中双四元环内含F^-及不含F^-的8种模型簇的几何结构、电子结构以及稳定性,考察了F^-对双四元环结构和性质的影响,发现F^-的存在依次增强了纯硅、磷铝和磷镓双四元环的稳定性,而降低了硅铝的稳定性,F^-在双四元环中所处位置的计算值与实验晶体数据基本吻合.     

Tubulin rings: curved filaments with limited flexibility and two modes of association

Tubulin rings have been previously identified as composed of linear polymers of tubulin subunits, equivalent to a protofilament in the microtubule wall but in a curved rather than a straight conformation. We have examined and measured a number of different ring structures obtained under different conditions. The preferred curvature is indicated by a single ring of 380 A outside diameter. Radially double rings consist of two coplanar rings of 460 A and 350 A outside diameter, held together by a pattern of eight identical contacts between the 40 A subunits in the inner and outer rings. In some circumstances a larger ring, 570 A diameter, can be added to the outside, or a smaller ring, 240 A diameter, may be added to the inside of the radially double ring, in both cases repeating the pattern of eight radial contacts. The distortion of the filament from its relaxed 380 A diameter curvature apparently can be made without disrupting the longitudinal bond between subunits in the filament, but must be stabilized by the energy of the radial contact. All of these rings (single and radially double and triple) are observed to associate axially to form pairs or in some cases larger stacks. The radially double rings or an axially associated pair of these (quadruple ring) may also associate to form crystals. These are thin plates, up to 100 micrometers in extent and several micrometers thick which have been of limited use so far in diffraction studies because of irregularities in the packing of adjacent rings.     

Determination of trace elements by INAA in urban air particulate matter and transplanted lichens

Summary Lichens as biomonitors and neutron activation analysis as analytical technique have been employed to evaluate the trace element atmospheric pollution in the metropolitan area of the city of Pavia (Northern Italy). Transplanted lichens (Parmelia sulcata and Usnea gr. hirta) and air particulate matter have been monthly collected and analyzed during the winter 2001-2002. INAA and ET-AAS have been used for the determination of 28 elements in air particulate matter and 25 elements in lichens. Trace metals concentrations as well as the corresponding enrichment factors were evaluated and compared.     

The scaffold tree--visualization of the scaffold universe by hierarchical scaffold classification

A hierarchical classification of chemical scaffolds (molecular framework, which is obtained by pruning all terminal side chains) has been introduced. The molecular frameworks form the leaf nodes in the hierarchy trees. By an iterative removal of rings, scaffolds forming the higher levels in the hierarchy tree are obtained. Prioritization rules ensure that less characteristic, peripheral rings are removed first. All scaffolds in the hierarchy tree are well-defined chemical entities making the classification chemically intuitive. The classification is deterministic, data-set-independent, and scales linearly with the number of compounds included in the data set. The application of the classification is demonstrated on two data sets extracted from the PubChem database, namely, pyruvate kinase binders and a collection of pesticides. The examples shown demonstrate that the classification procedure handles robustly synthetic structures and natural products.     

Formation and dynamics of capillary-flow-induced colloidal rings

We have directly observed the ring formation of colloidal particles of 1 μm diameter at the contact lines of air, water, and oil using a laser scanning confocal microscope. Colloidal rings form and grow through the transport of particles induced by capillary flow due to water evaporation. In addition, we observe the sudden "jump in" of particles into the ring and the "depletion" of particles in the ring. Particle-tracking experiment shows that the particles within the ring exhibit 1D-like motion along the circular ring geometry, and the pair correlation function of the ring configuration suggests an equilibrium interparticle distance of approximately 2.8 μm. It is also found that the structure and formation speed of the colloidal rings can be controlled by accelerating water evaporation by the addition of methanol as a cosolvent.     

Fused ring aziridines as a facile entry into triazole fused tricyclic and bicyclic heterocycles

The intramolecular dipolar cycloaddition of an azide with an alkyne has provided a useful entry into triazole fused tricyclic heterocycles containing both the triazole ring and the oxazolidin-2-one ring system. The requisite azido-alkynes have been prepared via a two-step sequence from fused ring aziridines. A series of 6-12 membered rings containing both the oxazolidinone and triazole rings have been prepared. These ring systems have been designed as conformationally restrained analogs of RNA-binding oxazolidinones.