Ultra-trace Cu isotope ratio measurements via multi-collector ICP-mass spectrometry using Ga as internal standard : an approach applicable to micro-samples

The capabilities of Cu isotope ratio measurements are often restricted by the small volumes of sample available and/or their low Cu concentration. In this work, an analytical approach was developed for performing Cu isotopic analysis via multi-collector ICP-mass spectrometry (MC-ICP-MS) at ultra-trace level using Ga as an internal standard for mass bias correction. The minimum concentration of Cu required for accurate and precise isotope ratio measurements was established to be 20 mu g L-1 with wet plasma conditions and 5 mu g L-1 with dry plasma conditions. The use of Ga as an internal standard for mass bias correction provided several advantages compared to Ni, i.e. improved internal precision on delta Cu-65 values and lower blank levels. Ga can also be used at a 4- fold lower concentration level than Ni. However, in wet plasma conditions, the signals of (ArO2H+)-Ar-36-O-16-H-1 and (ArNO+)-Ar-40-N-15-O-16 interfered with the signals of Ga-69(+) and Ga-71(+), respectively, while in dry plasma conditions, realized by the use of a desolvation unit, Ga-69(+) suffered from spectral interference from (ArN2H+)-Ar-40-N-14-H-1. These interferences were resolved by using medium mass resolution. For validation purposes, the approach was applied to commercially available blood and serum samples. The delta Cu-65 values for the samples measured at a concentration level of 5 mu g L-1 Cu and 5 mu g L-1 Ga using dry plasma conditions were in good agreement with those obtained for isotope ratio measurements at the "standard" concentration level of 200 mu g L-1 Cu and 200 mu g L-1 Ni using wet plasma conditions. In addition, the delta Cu-65 values obtained for micro-samples of serum/blood (volume of 100 mu L) were in good agreement with the corresponding ones obtained using the "standard" volume for isotopic analysis (500 mu L)

Random walk on temporal networks with lasting edges

We consider random walks on dynamical networks where edges appear and disappear during finite time intervals. The process is grounded on three independent stochastic processes determining the walker's waiting-time, the up-time and down-time of edges activation. We first propose a comprehensive analytical and numerical treatment on directed acyclic graphs. Once cycles are allowed in the network, non-Markovian trajectories may emerge, remarkably even if the walker and the evolution of the network edges are governed by memoryless Poisson processes. We then introduce a general analytical framework to characterize such non-Markovian walks and validate our findings with numerical simulations.Comment: 18 pages, 18 figure

Cu isotopic signature in blood serum of liver transplant patients: a follow-up study

End-stage liver disease (ESLD) is life-threatening and liver transplantation (LTx) is the definitive treatment with good outcomes. Given the essential role of hepatocytes in Cu homeostasis, the potential of the serum Cu isotopic composition for monitoring a patient's condition post-LTx was evaluated. For this purpose, high-precision Cu isotopic analysis of blood serum of ESLD patients pre- and post-LTx was accomplished via multi-collector ICP-mass spectrometry (MC-ICP-MS). The Cu isotopic composition of the ESLD patients was fractionated in favour of the lighter isotope (by about -0.50 parts per thousand). Post-LTx, a generalized normalization of the Cu isotopic composition was observed for the patients with normal liver function, while it remained light when this condition was not reached. A strong decrease in the delta Cu-65 value a longer term post-LTx seems to indicate the recurrence of liver failure or cancer. The observed trend in favour of the heavier Cu isotopic composition post-LTx seems to be related with the restored biosynthetic capacity of the liver, the restored hepatic metabolism and/or the restored biliary secretion pathways. Thus, Cu isotopic analysis could be a valuable tool for the follow-up of liver transplant patients and for establishing the potential recurrence of liver failure

On the shear resistance of ferritic stainless steel composite slabs

Steel-concrete composite floors are commonly used in construction due to their favourable weight-to-depth ratio and erection time. Typically, concrete is poured onto a galvanised steel deck acting as formwork. However in case of floors exposed to corrosive environments, stainless steel is likely to be chosen over galvanised steel. Besides its better corrosion resistance, stainless steel also offers desirable aesthetic appearance and good mechanical properties. Composite slabs can fail in bending, vertical shear or longitudinal shear. The latter failure mode is the most common, and its prediction depends on values obtained through full-scale tests. However, for stainless steel decks, no specific treatment exists in current design standards. This paper investigates the longitudinal shear resistance of stainless steel composite slabs through an experimental study. One short and three long span slabs, made using a Cofraplus 60 ferritic EN1.4003 stainless steel corrugated deck, are tested in accordance with Eurocode 4, annex B.3 [1]. The Partial Shear Connection (PSC) method is used to assess the longitudinal shear resistance. The experimental results, together with the results provided in Task 3.3 of the “Structural Applications of Ferritic Stainless Steels (SAFSS, RFSR-CT-2010-00026)” project [2], are used to draw conclusions on the applicability of ferritic stainless steel decks in composite floors.Peer ReviewedPostprint (author's final draft

Developments on an IEEE 802.15.4-based wireless sensor network, Journal of Telekommunications and Information Technology, 2008, nr 2

In this paper a summary is given of the ongoing research at the Belgian Royal Military Academy in the field of mobile ad hoc networks in general and wireless sensor networks (WSNs) in particular. In this study, all wireless sensor networks are based on the physical and the medium access layer of the IEEE 802.15.4 low rate wireless personal area networks standard. The paper gives a short overview of the IEEE 802.15.4 standard in the beaconless mode together with a description of the sensor nodes and the software used throughout this work. The paper also reports on the development of a packet sniffer for IEEE 802.15.4 integrated in wireshark. This packet sniffer turns out to be indispensable for debugging purposes. In view of future applications on the wireless network, we made a theoretical study of the effective data capacity and compared this with measurements performed on a real sensor network. The differences between measurements and theory are explained. In case of geograph- ically meaningful sensor data, it is important to have a knowledge of the relative position of each node. In the last part of the paper we present some experimental results of positioning based on the received signal strength indicators (RSSI). As one could expect, the accuracy of such a method is poor, even in a well controlled environment. But the method has some potential

Theory of Turing Patterns on Time Varying Networks

The process of pattern formation for a multi-species model anchored on a time varying network is studied. A non homogeneous perturbation superposed to an homogeneous stable fixed point can amplify, as follows a novel mechanism of instability, reminiscent of the Turing type, instigated by the network dynamics. By properly tuning the frequency of the imposed network evolution, one can make the examined system behave as its averaged counterpart, over a finite time window. This is the key observation to derive a closed analytical prediction for the onset of the instability in the time dependent framework. Continuously and piecewise constant periodic time varying networks will be analysed, to set the ground for the proposed approach. The extension to non periodic settings will also be discussed.Comment: 16 pages, 5 figure

Stochastic Hybrid Simulation with Applications to Wired and Wireless Queueing Networks (Stochastische hybride simulatie toegepast op vaste en draadloze wachtrijnetwerken)

Evaluating the performance characteristics of telecommunication networks is a challenging task. Measurements of relevant data on an actual running system are often impossible. Therefor, analytical models are proposed in literature. However, even after simplifying assumptions and decompositions, the resulting analytical model is usually mathematically intractable. The only alternative for predicting the performance of complex telecommunication networks is a simulation.A packet-switched communication mean can be modeled by a queueing model. In queueing theory, a queue consists of a buffer, i.e. a waiting area,and a service zone. The queue itself corresponds to a switching node ora router having both a packet buffer and an associated outgoing interface with a limited transmission capacity. A telecommunication network canbe represented by interconnecting the queues. An event simulator generally speaking mimics the behavior of the queueing network by tracing the customers, i.e. the packets, using events.In a simulation experiment, samples of many thousands of observations are often required to estimate some performance measure with sufficient confidence. In view of computer run time, it is clearly important that one should attempt to reduce the volume of sampling in order to reduce therun time of the simulation. Unfortunately, it has been a common practice to apply simulation techniques uncritically with little thought to theefficiency of simulation runs. Hybrid simulation is a way to reduce thesimulation run time without compromising the precision of the estimatesby combining analytical solutions to the simulation experiment with event-driven simulations having a reduced number of events. The splitting of the model in an analytical part and a simulation part is a crucial element of a hybrid simulation. Considering a packet base telecommunicationnetwork, the packetized traffic can be split in two parts: the foreground packets, directly related to the performance measures of interest, and the background stream, having only an indirect relation with the performance measures of interest. In the novel stochastic hybrid simulation scheme detailed in this thesis, the splitting of the traffic into foreground and background traffic is combined with the general idea of using a stochastic technique to evaluate the impact of the background traffic onthe foreground packets. At the queue level, a stochastic fluid-flow models the background packets and the queueing delay of the foreground packets is estimated by exploiting the ASTA property, i.e. when the lack of bias assumption is respected, the waiting time of a packet from a foreground source has the same distribution as the virtual waiting time of theaggregation of all traffic streams at the queueing system.In a second part of the thesis, the stochastic hybrid simulation is extended to the simulation of queueing networks of wireless nodes sharing acommon channel. As soon as shared channels are dealt within a packet-switching mode, conflicts arise when more than one packet is simultaneously transmitted using the same channel. Whenever a portion of one user's transmission overlaps with another user's transmission at the receiver's side, the two interfere and destroy each other. The problem to be solved is how to control the access to the channel in a way which produces, under the physical constraints of simplicity and hardware implementation, an acceptable level of performance. The difficulty in controlling a channel which must carry its own control information gives rise to the so-called random-access modes. The MAC protocol together with the transmission capacity on the channel and the distribution of the source-destination pairs determines the saturation throughput of the system transmitter-channel-receiver. In reality not every transmitter has always a packet ready to be transmitted and some nodes can hold more than one packet in apacket queue waiting to be transmitted. The notion of capacity region based on the saturation throughput is needed to extend the stochastic hybrid simulation scheme for wireless packet radio networks. When a foreground packet arrives at a queue during the simulation, not only a sampled value of the queueing delay for the packet in the queue is provided but also the associated instantaneous transmission capacity as seen by the node.The evaluation of the saturation throughput in a realistic multi-hop scenario is a non-trivial issue. Two semi-analytical models for the saturation throughput of existing wireless IEEE Std 802.11 and IEEE Std 802.15.4 standards are proposed. Both models are supported by detailed simulations and the latter is also validated on a testbed of wireless motes.nrpages: 298status: publishe