Ultra-clean Sampling Research Articles

Development of an ultra-clean sample heating stage for thermal desorption spectroscopy

Control of surface molecular contamination (SMC) for components used in chemical vapor deposition (CVD), atomic layer deposition (ALD) and EUV photolithography is important to maintaining high yield and optimal tool operation at the latest process nodes in leading edge semiconductor manufacturing. High temperature thermal desorption spectroscopy (TDS) is a versatile tool for analyzing the cleanliness of surfaces, simulating thermal vacuum processes and studying the kinetics of desorption processes. A basic analysis of TD spectra allows for full characterization of volatile outgassing from surfaces, while detailed analysis can provide chemical information about the substrate surface.In fundamental studies, TDS is often carried out from low temperatures to room temperature or for small samples. However, for microelectronics applications, high temperature studies of large (100 mm or greater) samples are of greater interest due to direct applications for cleanliness testing and thermal vacuum simulation. A limitation for TDS sensitivity is the outgassing of sample stage materials, particularly when analyzing gases that may be present in the chamber background such as water, CO and CO2. Typical sample stages are often tested only for total pressure or at room temperature.In this study, we present a simple ultra-high vacuum (UHV) compatible sample heating stage for trace outgassing analysis of 100 mm samples at high temperatures. Simulation results are presented to support the feasibility of the concept. Experimental results verify the cleanliness of the stage via room temperature residual gas analysis (RGA) analysis and X-ray photoelectron spectroscopy (XPS) of stage components. Finally, use of this stage in a TDS analysis of a 100 mm Si witness wafer and comparison to room temperature RGA demonstrates operational capability.The sample heating stage is both shown to be clean at high temperature and capable of analyzing 100 mm wafers to higher sensitivity than room temperature RGA for all m/z at the 1 × 10−9 mbar level. Despite its high performance, the heating stage is also easily produced by any laser machining service, greatly improving the accessibility of UHV science for all researchers.

Resonant fractional conductance through a 1D Wigner chain

In recent experiments on conductance of one-dimensional (1D) channels in ultra-clean samples, a diverse set of plateaus were found at fractions of the quantum of conductance in zero magnetic field. We consider a discrete model of strongly interacting electrons in a clean 1D system where the current between weak tunneling contacts is carried by fractionally charged solutions. While in the spinless case conductance remains unaffected by the interaction, as is typical for the strongly interacting clean 1D systems, we demonstrate that in the spinful case the peak conductance takes fractional values that depend on the filling factor of the 1D channel.

Density-tuned effective metal-insulator transitions in two-dimensional semiconductor layers: Anderson localization or Wigner crystallization

Electrons (or holes) confined in 2D semiconductor layers have served as model systems for studying disorder and interaction effects for almost 50 years. In particular, strong disorder drives the metallic 2D carriers into a strongly localized Anderson insulator (AI) at low densities whereas pristine 2D electrons in the presence of no (or little) disorder should solidify into a Wigner crystal at low carrier densities. Since the disorder in 2D semiconductors is mostly Coulomb disorder arising from random charged impurities, the applicable physics is complex as the carriers interact with each other as well as with the random charged impurities through the same long-range Coulomb coupling. By critically theoretically analyzing the experimental transport data in depth using a realistic transport theory to calculate the low-temperature 2D resistivity as a function of carrier density in 11 different experimental samples covering 9 different materials, we establish, utilizing the Ioffe-Regel-Mott criterion for strong localization, a direct connection between the critical localization density for the 2D metal-insulator transition (MIT) and the sample mobility deep into the metallic state, which for clean samples could lead to a localization density low enough to make the transition appear to be a Wigner crystallization. We believe that the insulating phase is always an effective Coulomb disorder-induced localized AI, which may have short-range WC-like correlations at low carrier densities. Our theoretically calculated disorder-driven critical MIT density agrees with experimental findings in all 2D samples, even for the ultra-clean samples. In particular, the extrapolated critical density for the 2D MIT seems to vanish when the high-density mobility goes to infinity, indicating that transport probes a disorder-localized insulating ground state independent of how low the carrier density might be.

Purification of conchocelis of Neoporphyra haitanensis by the method of dominant Bacteria enrichment comprehensive treatment

Conchocelis is the sporophyte generation of Porphyra sensu lato, and free-living conchocelis is the main form used for preserving the germplasm. Avoiding microbial contamination caused by algal-associated bacteria (AAB) and ensuring the cleanliness of germplasm are important tasks for the preservation of Porphyra germplasm. In this study, we applied dominant bacteria enrichment comprehensive treatment (DBECT) to purify the conchocelis by enriching the dominant bacteria (Ruegeria scottomollicae S1) to occupy the niches of AAB and then remove them via antibiotics. The results showed that DBECT could efficiently remove AAB of Porphyra conchocelis germplasm, the abundance of which decreased 2000 times that of cultures treated with combined antibiotic cocktail methods. The diversity of AAB was reduced. In addition, adding R. scottomollicae S1 could adjust the community composition of algal microorganisms and increase the proportion of beneficial bacteria. Thus, DBECT can effectively purify Porphyra germplasm and provide ultra-clean algal samples for further research.

Mars 2020 Mission Overview

The Mars 2020 mission will seek the signs of ancient life on Mars and will identify, prepare, document, and cache a set of samples for possible return to Earth by a follow-on mission. Mars 2020 and its Perseverance rover thus link and further two long-held goals in planetary science: a deep search for evidence of life in a habitable extraterrestrial environment, and the return of martian samples to Earth for analysis in terrestrial laboratories. The Mars 2020 spacecraft is based on the design of the highly successful Mars Science Laboratory and its Curiosity rover, but outfitted with a sophisticated suite of new science instruments. Ground-penetrating radar will illuminate geologic structures in the shallow subsurface, while a multi-faceted weather station will document martian environmental conditions. Several instruments can be used individually or in tandem to map the color, texture, chemistry, and mineralogy of rocks and regolith at the meter scale and at the submillimeter scale. The science instruments will be used to interpret the geology of the landing site, to identify habitable paleoenvironments, to seek ancient textural, elemental, mineralogical and organic biosignatures, and to locate and characterize the most promising samples for Earth return. Once selected, ∼35 samples of rock and regolith weighing about 15 grams each will be drilled directly into ultraclean and sterile sample tubes. Perseverance will also collect blank sample tubes to monitor the evolving rover contamination environment. In addition to its scientific instruments, Perseverance hosts technology demonstrations designed to facilitate future Mars exploration. These include a device to generate oxygen gas by electrolytic decomposition of atmospheric carbon dioxide, and a small helicopter to assess performance of a rotorcraft in the thin martian atmosphere. Mars 2020 entry, descent, and landing (EDL) will use the same approach that successfully delivered Curiosity to the martian surface, but with several new features that enable the spacecraft to land at previously inaccessible landing sites. A suite of cameras and a microphone will for the first time capture the sights and sounds of EDL. Mars 2020’s landing site was chosen to maximize scientific return of the mission for astrobiology and sample return. Several billion years ago Jezero crater held a 40 km diameter, few hundred-meter-deep lake, with both an inflow and an outflow channel. A prominent delta, fine-grained lacustrine sediments, and carbonate-bearing rocks offer attractive targets for habitability and for biosignature preservation potential. In addition, a possible volcanic unit in the crater and impact megabreccia in the crater rim, along with fluvially-deposited clasts derived from the large and lithologically diverse headwaters terrain, contribute substantially to the science value of the sample cache for investigations of the history of Mars and the Solar System. Even greater diversity, including very ancient aqueously altered rocks, is accessible in a notional rover traverse that ascends out of Jezero crater and explores the surrounding Nili Planum. Mars 2020 is conceived as the first element of a multi-mission Mars Sample Return campaign. After Mars 2020 has cached the samples, a follow-on mission consisting of a fetch rover and a rocket could retrieve and package them, and then launch the package into orbit. A third mission could capture the orbiting package and return it to Earth. To facilitate the sample handoff, Perseverance could deposit its collection of filled sample tubes in one or more locations, called depots, on the planet’s surface. Alternatively, if Perseverance remains functional, it could carry some or all the samples directly to the retrieval spacecraft. The Mars 2020 mission and its Perseverance rover launched from the Eastern Range at Cape Canaveral Air Force Station, Florida, on July 30, 2020. Landing at Jezero Crater will occur on Feb 18, 2021 at about 12:30 PM Pacific Time.

R/V Isabu-Based First Ultraclean Seawater Sampling for Ocean Trace Elements in Korea

Trace elements (Mn, Fe, Co, Ni, Cu, Zn, etc.) are essential micronutrients for marine organisms and are thus related to major issues in the ocean, such as climate change, ecosystem change, and pollution. However, given their extremely low concentrations in seawater, contamination-free clean seawater sampling for trace elements onboard is the most crucial factor in this study. Because of the absence of both clean seawater sampling techniques and related infrastructure (e.g., a research vessel equipped with a nonmetal sampling system), the trace element community in Korea has not contributed to the global community studying trace elements in the ocean, although the associated global joint study program “GEOTRACES” has been operating for 10 years. With the newly launched research vessel (R/V) Isabu of the Korea Institute of Ocean Science and Technology (KIOST), Korea is now equipped with a large comprehensive R/V complete with a contamination-free PRISTINE ultraclean seawater sampling system for trace elements. We conducted clean seawater sampling 2 research cruises in the Indian Ocean during 2017–2018 aboard the R/V Isabu, and we successfully obtained reliable data, which agree well with previously reported trace element data at these locations. Here, we present the procedure and preliminary results of clean water sampling for trace elements based on the R/V Isabu, as the first Korean open ocean trace element study. From this study, we expect that R/V Isabu will play an important role in research on oceanic trace elements and isotopes in the future.

Custom-made PTFE filters for ultra-clean size-fractionated aerosol sampling for trace metals

Atmospheric deposition of marine aerosols is one of the important sources of dissolved trace metals in the ocean. Analyses of trace metals and stable isotope ratios in size-fractionated aerosol particles using extraction experiments are useful to understand the dissolution behavior and biogeochemical cycles of trace metals in the marine environment. These analyses require sample treatments in clean environment without contamination of target metals from the sampling filter. However, the contamination of target metals from sampling filter is not low enough to determine the fractional solubility (extractive metal/total metal) and the stable isotope ratio of trace metals. In this study, we optimized the acid-washing procedure of hydrophobic PTFE filter for the Sierra-type cascade impactor. The results showed that the filter blanks of the custom-made PTFE filters used for the upper stages (= PTFE-ST) in the cascade impactor were about 10–1000 times lower than those of cellulose filters. In addition, the filter blanks of major ions and water-soluble organic carbon were also decreased by using PTFE-ST. Trace metal concentrations in size-fractionated (7 fractions) aerosol particles with very short sampling periods have been successfully determined except for the backup stage. Thus, aerosol sampling with PTFE filter with only a tiny contamination is a useful method to obtain chemical and isotopic compositions of various elements in marine aerosol particles.

Landau Level Diagram and the Continuous Rotational Symmetry Breaking in Trilayer Graphene.

The sequence of the zeroth Landau levels (LLs) between filling factors ν=-6 to 6 in ABA-stacked trilayer graphene (TLG) is unknown because it depends sensitively on the nonuniform charge distribution on the three layers of ABA-stacked TLG. Using the sensitivity of quantum Hall data on the electric field and magnetic field, in an ultraclean ABA-stacked TLG sample, we quantitatively estimate the nonuniformity of the electric field and determine the sequence of the zeroth LLs. We also observe anticrossings between some LLs differing by 3 in LL index, which result from the breaking of the continuous rotational to C_{3} symmetry by the trigonal warping.

An Ultra-clean Multilayer Apparatus for Collecting Size Fractionated Marine Plankton and Suspended Particles.

The distributions of many trace elements in the ocean are strongly associated with the growth, death, and re-mineralization of marine plankton and those of suspended/sinking particles. Here, we present an all plastic (Polypropylene and Polycarbonate), multi-layer filtration system for collection of suspended particulate matter (SPM) at sea. This ultra-clean sampling device has been designed and developed specifically for trace element studies. Meticulous selection of all non-metallic materials and utilization of an in-line flow-through procedure minimizes any possible metal contamination during sampling. This system has been successfully tested and tweaked for determining trace metals (e.g., Fe, Al, Mn, Cd, Cu, Ni) on particles of varying size in coastal and open ocean waters. Results from the South China Sea at the South East Asia Time-Series (SEATS) station indicate that diurnal variations and spatial distribution of plankton in the euphotic zone can be easily resolved and recognized. Chemical analysis of size-fractionated particles in surface waters of the Taiwan Strait suggests that the larger particles (>153 µm) were mostly biologically derived, while the smaller particles (10 - 63 µm) were mostly composed of inorganic matter. Apart from Cd, the concentrations of metals (Fe, Al, Mn, Cu, Ni) decreased with increasing size.

Dominant source of disorder in graphene: charged impurities or ripples?

Experimentally produced graphene sheets exhibit a wide range of mobility values. Both extrinsic charged impurities and intrinsic ripples (corrugations) have been suggested to induce long-range disorder in graphene and could be a candidate for the dominant source of disorder. Here, using large-scale molecular dynamics and quantum transport simulations, we find that the hopping disorder and the gauge and scalar potentials induced by the ripples are short-ranged, in strong contrast with predictions by continuous models, and the transport fingerprints of the ripple disorder are very different from those of charged impurities. We conclude that charged impurities are the dominant source of disorder in most graphene samples, whereas scattering by ripples is mainly relevant in the high carrier density limit of ultraclean graphene samples (with a charged impurity concentration less than about 10 ppm) at room and higher temperatures. Our finding is valuable to theoretical modelling of transport properties of not only graphene, but also other two-dimensional materials, as the thermal ripples are universal.

Trace metals in the dissolved fraction (

Water samples were collected on the Athabasca River (AR), upstream and downstream from bitumen mines and upgrading facilities, to identify changes in water quality due to industrial activities in this region of northern Alberta, Canada. Starting upstream of Fort McMurray and proceeding downstream ca. 100km, waters were collected in duplicate at 13 locations on the main stem of the river, as well as 5 tributary streams, using ultraclean sampling protocols developed for polar snow and ice. To estimate potential bioaccessibility, trace elements of concern (Ag, Cd, Pb, Sb, Tl) were determined in the dissolved fraction (<0.45μm) along with metals known for their enrichments in bitumen (V, Ni, Mo, Re) and those found mainly in ionic (Li, Sr) or colloidal forms (Al, Co, Cr, Fe, Ga, Mn, Th, Y). Analyses were performed in the metal-free, ultraclean SWAMP lab using quadrupole and sector-field ICP-MS. Concentrations of Ag, Cd, Pb, Sb and Tl were extremely low, not significantly more abundant downstream of industry and probably reflect "background" values. In contrast, V, Ni, Mo and Re concentrations were all significantly (p<0.05) greater downstream of industry. However, chloride also increased downstream, due to natural inputs of saline groundwaters and it is unclear whether the increases in V, Ni, Mo and Re are due to natural or anthropogenic inputs to the river. Although it had been claimed that the industrial development of the Athabasca Bituminous Sands (ABS) is a significant source of Ag, Cd, Pb, Sb and Tl to the river, our study failed to find any evidence to support this. Here we provide a first, robust (accurate and precise) description of baseline values for these trace elements in the AR, and suggest that V, Ni, Mo and Re are more valuable tracers for environmental monitoring and source assessment.

Not just a phase

In some two-dimensional materials, there's a puzzling intermediate metallic phase between superconducting and insulating states. Experiments on ultraclean crystalline samples suggest this metallic phase could be bosonic.

“PRISTINE”, a new high volume sampler for ultraclean sampling of trace metals and isotopes

Many trace elements like Mn, Fe, Co, Ni, Cu and Zn are essential for marine life, some trace elements are of concern as pollutants, e.g. Pb and Hg, while others, together with a diverse array of isotopes, are used to assess modern-ocean processes and the role of the ocean in past climate change. GEOTRACES is an international program that aims to measure the distribution of trace elements and isotopes throughout the world oceans to improve our understanding of their marine biogeochemical cycles. To contribute to GEOTRACES a new sampler system was developed at NIOZ allowing efficient sampling of large volumes of seawater under ultraclean conditions. The 24 “PRISTINE” samplers each with a volume of 24.4L are made of a high-purity polymer Polyvinylidene Fluoride (PVDF) and are opened and closed using a butterfly-valve closing mechanism. The samplers are mounted on an all-titanium frame and deployed using a poly-aramide hydrowire (Super Aram) with internal power/signal conductors. Upon recovery the complete frame is immediately placed in its own clean-air laboratory unit. Samplers are (i) always closed when onboard, (ii) always mounted on the frame without the need for hand-carrying heavy samplers, and (iii) can be deployed again with minimal (manual) preparation. The PRISTINE ultraclean sampling system was used for the first time during the GA02 GEOTRACES cruises in the West Atlantic Ocean (2010–2012). During 60 full ocean depth stations all 24 samplers closed with a 100% success rate. Sampling proved to be much faster, less labor intensive, and ultraclean. A comparison of salinity, temperature, nutrient and oxygen data collected with the rectangular titanium frame with PRISTINE samplers and a traditional CTD frame with Niskin samplers showed that the CTD systems functioned equally well, that the PRISTINE samplers took discrete seawater samples without any inward leakage of seawater during the up-cast, and that no atmospheric oxygen contaminated the seawater samples in the PRISTINE samplers after return on deck. The excellent agreement between 13 trace elements sampled with PRISTINE and sampled during the cross over occupation of US-GEOTRACES at the Bermuda BATS site (32°N, 64°W) shows its suitability for ultraclean trace element and isotope sampling (see accompanying paper).

Nonlinear magnetotransport theory and Hall induced resistance oscillations in graphene

The quantum oscillations of nonlinear magnetoresistance in graphene that occur in response to a dc current bias are investigated. We present a theoretical model for the nonlinear magnetotransport of graphene carriers. The model is based on the exact solution of the effective Dirac equation in crossed electric and magnetic fields, while the effects of randomly distributed impurities are perturbatively added. To compute the nonlinear current effects, we develop a covariant formulation of the migration center theory. The current is calculated for short- and large-range scatterers. The analysis of the differential resistivity in the large magnetic field region, shows that the extrema of the Shubnikov de Hass oscillations invert when the dc currents exceed a threshold value. These results are in good agreement with experimental observations. In the small magnetic field regime, corresponding to large filling factors, the existence of Hall induced resistance oscillations are predicted for ultra clean graphene samples. These oscillations originate from Landau–Zener tunneling between Landau levels, that are tilted by the strong electric Hall field.

Invited Presentation: Prolonged Spontaneous Emission and Dephasing of Excitons in Air-Bridged Swcnts

Carbon nanotubes have recently gained tremendous interest as a nanomaterial for next generation optoelectronics and quantum photonic devices. To date, the majority of experiments reveal however low quantum efficiencies and strong dependencies on the environment and chemical processing. This is very detrimental for optoelectronic device applications dominant non-radiative optical recombination processes lead to strongly reduced optical emission rates of nanoscale light sources. Likewise, non-radiative recombination leads to reduced light-to-current conversion efficiencies in photodetectors and solar cells.The question arises to what extent previous experiments have so far probed for the true intrinsic properties of the exciton dynamics. In particular, theorists predicted about a decade ago spontaneous emission (SE) lifetimes of excitons of about 10-100 ns while most measurements found orders of magnitude faster lifetimes varying from 20 to 200 ps. In earlier work we have demonstrated how polymer capping of SWCNTs can be used to strongly reduce detrimental environmental interaction effects such as spectral diffusion and blinking [1]. Although these samples showed higher exciton emission efficiencies, the corresponding SE lifetime remained with 30 ps much faster than theoretical predictions, implying that they are still dominated by nonradiative exciton recombination.To overcome these limitations we have combined sophisticated growth of ultra-clean SWCNTs bridging an air gap over pillar posts with time-resolved photoluminescence measurements of individual SWCNTs over 14 orders of magnitude. In this talk we will discuss our recent finding of the onset of a remarkable new regime of intrinsic exciton photophysics with prolonged SE times up to 18 ns, about two orders of magnitude longer than reported before [2]. While these intrinsic optical lifetimes had been predicted by theorists about a decade ago, they had never been reported due to the masking by extrinsic effects of defects and unintentional doping.Measurements of exciton emission linewidth versus pump power demonstrated that air-bridged SWCNTs have significantly narrower linewidths than polymer-embedded SWNTs. The later display a rather large linewidth up to 10 meV, while the exciton linewidth of SWCNTs bridging an air gap reaches down to 220 μeV at low pump powers, if they are grown for short times of only 2 minutes. It appears that our best air-bridged samples are only at the onset of this intrinsic photophysics regime. Nanosecond long SE lifetimes have very recently also independently confirmed by Hofmann et al. [3], who also report even narrower spectral linewidth down to 40 meV. Future work has thus much room for improvements to reach deeper into the intrinsic regime and explore the intrinsic limits.With these supposedly ultraclean air-bridged samples we also carried out first experiments on exciton dephasing times for individual SWCNTs in the time-domain. The interferometric measurements demonstrated exciton dephasing times up to 2 ps, about four-fold longer than in previous ensemble studies (500 fs). The ultra-narrow linewidth and prolonged exciton dephasing times suggest that the acoustic phonon coupling giving rise to exciton dephasing is significantly weaker than previously believed, which is promising news for device applications, and raises the question about the ultimate limit for SE lifetimes and exciton dephasing in the intrinsic regime.

Precise half-life measurements for 38Ca and 39Ca

The half-lives of 38Ca and 39Ca have been measured at ISOLDE of CERN. The REXTRAP facility was used to prepare ultra-clean samples of radioactive nuclei for precision decay spectroscopy. 38Ca is one of the T z = - 1 , 0+ $ \rightarrow$ 0+ $ \beta$ -emitting nuclides used to determine the vector coupling constant of the weak interaction and the Vud quark-mixing matrix element. The result obtained, T 1/2 = 443.8(19) ms, is four times more precise than the average of previous measurements. For 39Ca , a half-life of T 1/2 = 860.7(10) ms is obtained, a result in agreement with the average value from the literature.

Mercury wet deposition and coal-fired power station contributions: An Australian study

Regions that have large coal-fired power station regions may be prone to elevated mercury (Hg) deposition fluxes. Total mercury (THg) in daily rainfall samples at a near-field sampling site (Hunter Valley) and a far-field station (Sydney) has been monitored in this study employing ultra-clean sampling techniques and Cold Vapour Atomic Fluorescence Spectrometry (CVAFS) analysis. Measurements of THg range from 0.9 to 16.5 and from 1.2 to 18.9 ng/L for the far-field and near-field sites respectively. Average daily THg wet deposition fluxes were 49.9 and 79.8 ng/m 2 for the far and near-field sites respectively. A two-sample t-test reveals the near-field site to have higher rainfall mercury (significant at the 95% level) and deposition fluxes. At the far-field site, lower Hg deposition occurs during the cooler months, while the Hunter Valley site has higher winter deposition. It is believed that the higher winter values near-field may be due to the combined impact of prevailing wind direction and high pollutant build up brought on by lower mixing height. Based on this study, the first in the Southern Hemisphere, we conclude that rainfall THg concentrations and Hg wet deposition fluxes at the two sites are comparable to rural sites in North America.

Two-band parallel conductivity at terahertz frequencies in the superconducting state ofMgB2

The optical response of the two-band superconductor $\mathrm{Mg}{\mathrm{B}}_{2}$ has been studied in the $0.7--4\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ range on films with very low impurity level. The effect of the high-energy $\ensuremath{\sigma}$ gap is observed in the ratio ${R}_{S}∕{R}_{N}$ between the normal and superconducting state reflectance, while in a neutron irradiated film with a slightly higher impurity level, mainly the effect of the $\ensuremath{\pi}$ gap is evident, as reported in previous experiments. At terahertz frequencies, the electrodynamics of $\mathrm{Mg}{\mathrm{B}}_{2}$ can be well described by the two-band parallel conductivity model and is dominated by the $\ensuremath{\pi}$ bands when the impurity level is only slightly higher than that of an ultraclean sample.

Self-doping instability of the Wigner-Mott insulator

We present a theory describing the mechanism for the two-dimensional (2D) metal-insulator transition (MIT) in the absence of disorder. A two-band Hubbard model is introduced, describing vacancy-interstitial pair excitations within the Wigner crystal. Kinetic energy gained by delocalizing such excitations is found to lead to an instability of the insulator to self-doping above a critical carrier concentration $n={n}_{c}$, mapping the problem to a density-driven Mott MIT. This mechanism provides a natural explanation of several puzzling experimental features, including the large effective mass enhancement, the large resistivity drop, and the large positive magnetoresistance on the metallic side of the transition. We also present a global phase diagram for the clean 2D electron gas as a function of $n$ and parallel magnetic field ${B}_{\ensuremath{\parallel}}$, which agrees well with experimental findings in ultraclean samples.

Horizontal distribution and levels of heavy metals in the biggest snowstorm in a century in Shenyang, China

The horizontal distribution and levels of heavy metals in the biggest snowstorm in Shenyang since 1904 were investigated by analyzing 4 metals (As, Cd, Pb, and Cu) in a series of ultraclean samples collected from 17 sites distributed in different regions of the Shenyang area, China. The results showed that the concentrations of all the 4 heavy metals in snow from the industrial regions were high, up to 7.3 (As), 2.2 (Cd), 850.0 (Pb), and 0.197-20.2 (Cu) microg/kg, respectively. In the suburb, in contrast, their concentrations were not detected, except for As. Because of the long-term application of arsenical pesticides and herbicides, As was detected in the snow samples which collected on the farm. As, Cd, and Pb were also detected in the snow samples collected from the parks, the residential areas, and the commercial districts mainly by reason of human activities. In a sense, long-term industrial activities, traffic activities, coal combustion, and agricultural activities affected the horizontal distribution and levels of these heavy metals in snow differently. The data relating to the horizontal distribution and concentrations of heavy metals in the snow under extreme climatic conditions can provide with a unique snapshot of environmental pollution situation and behaviors in urban areas.