Low Concentration Samples Research Articles

Monitoring of Technetium in Groundwater: Development of a Novel QCN Based Sensor

Technetium-99, a pure β-emitter with Emax =294 keV, is one of the most significant (t1/2 =2.1×105 y) nuclear waste isotopes, produced by the fission of U with a yield of 6%. Under normal environmental conditions, it is mostly encountered as the pertechnetate ion, TcO4 -, which is highly soluble and consequently mobile in water. For this reason, its monitoring into groundwater is a statutory requirement for every nuclear license site. Because of its relatively low concentration in environmental samples, current determination methods of 99Tc involve several steps, such as chemical separation from the matrix, purification and source preparation, prior to radiometric (e.g. liquid scintillation counting) or mass spectroscopic (e.g. inductively coupled plasma mass spectroscopy) determination. The detection of 99Tc may take up to several days, thus making these techniques inappropriate for an emergency situation. Hence, We report on the development of a sensor for the real time monitoring of 99Tc in groundwater based on the Quartz Crystal Nanobalance (QCN). The QCN is a piezoelectric resonator, which oscillates in a resonant frequency fs when an electric potential is applied across its body. It is capable of measuring very small changes in mass at its surface, through the change of the resonant frequency. We modify it to respond exclusively to the presence of TcO4 -, by application of a novel Ag-4,4bipyridine metal-organic framework thin film built layer-by-layer on a self-assembled monolayer of 4-mercaptopyridine on the gold surface. The film’s structure is characterised by several techniques such as XPS, AFM and XRD and we evaluate its response and selectivity by monitoring the changes in the resonant frequency as a function of the concentration of ReO4 -, which is a non-radioactive chemical surrogate for TcO4 -. Finally, we model the adsorption of ReO4 - and other interferences with adsorption isotherms such as Langmuir, Freundlich and Sips.

(Invited) Determining Surfactant Concentrations for Separation of (n,m) Chirality Swcnts in ATPE via Fluorescence Detection

In this talk I will present a methodology for rapid and quantitative determination of surfactant concentrations that result in top or bottom-phase partitioning of individual (n,m) SWCNTs species in an aqueous two-polymer phase extraction using near-infrared fluorescence measurements. This methodology enables quantitative evaluation of parameters affecting ATPE such as temperature, polymer molecular weight and composition, and surfactant choice without conducting an actual separation. Additional advantages of the technique include a very low sample concentration for measurement, rapid measurement, and no mass transfer derived limitations in interpreting results. Using the technique, we have investigated different combinations of surfactants to explore the effects of surfactant chemistry. In most cases, two transitions in the NIR fluorescence are observed with co-surfactant concentration for each (n,m) species, which we identify as differential separation of left and right-handed SWCNT enantiomers. Specific trends with (n,m) diameter and with variation of surfactant chemistry will be discussed.

Rapid bacteria-detection platform based on magnetophoretic concentration, dielectrophoretic separation, and impedimetric detection

Most biosensors employ small sample quantities (less than 100 μL) for bacteria detection, thereby resulting in inaccurate low-concentration measurements. Detection performed using small sample volumes with low bacteria concentration may produce false-negative results. Therefore, sample pretreatment plays a critical role in accurate bacteria detection. This paper presents an impedimetric bacteria-detection sensor integrated with bacteria concentration and separation devices for rapid bacteria detection. Post conjugation using magnetic particles (MPs), the MP-conjugated bacteria (MP/Bac) are concentrated via magnetophoresis by a factor exceeding 100. In addition, MP/Bac are separated from MPs via dielectrophoresis to prevent occurrence of signal errors caused by MPs not conjugated with bacteria. Subsequently, concentrated MP/Bac are captured on a sensor electrode, and bacteria concentration is detected by measuring signal changes caused by the impedance difference between bacteria and the medium. The performance of the proposed bacteria-detection device was evaluated using a 5-mL homogenized cabbage sample injected with Staphylococcus aureus at 30 mL/h flow rate. The observed signal change was measured for 10 min using a sample with a concentration of 5–5 × 103 CFU/mL and was found to be approximately 0.34 mV at 50 CFU/mL; the limit of detection was 36 CFU/mL. These results confirm that the proposed device can detect low bacteria concentrations in food samples.

Ag nanodisks decorated filter paper as a SERS platform for nanomolar tetracycline detection

A simple and cheap filter paper-based SERS substrate has been developed for the sensing of tetracycline (TC) traces. In fact, antibiotics could represent a danger for environment and human health and TC is, in detail, one of the most wide-spread antibiotics entering the food chain and as water contaminant. According to this aim, Ag nanodisks (AgNDs) synthesis procedure was designed for tailoring the plasmonic peak position and, so, the SERS effect. Synthetized AgNDs were then used to simply decorate the surface of common filter paper, chosen because of its biocompatible, porosity and cheapness features. The obtained substrates based on filter paper and AgNDs allow the fast and easy analysis of aqueous solution containing first methylene blue used as standard and then TC samples. Fixed volume (20 μL) of TC solutions at different concentrations was simply dropped on the SERS support. The SERS spectrum was acquired before the complete water evaporation allowing TC detection down to the concentration of 10 −9 M in aqueous solution. The results demonstrated that the SERS substrate obtained with the proposed facile, low-cost and relatively green fabrication process and the possibility to tune the plasmon excitation can be implemented for TC low concentration samples.

A uniform sample preparation procedure for gas chromatography combustion isotope ratio mass spectrometry for all human doping control relevant anabolic steroids using online 2/3-dimensional liquid chromatography fraction collection

Androgenic anabolic steroids are the most misused substances in sports because of their performance-enhancing effects. Often synthetic analogues of endogenously present steroids are administered. To determine their endogenous or exogenous origin, Gas Chromatography Combustion Isotope Ratio Mass Spectrometry (GC-C-IRMS) is used in the field of doping control. Compounds subjected to IRMS analysis must be interference-free, with liquid chromatography fraction collection (HPLC-FC) being the crucial clean-up step. However, this clean-up is challenging, particularly for compounds present at low concentrations in samples with pronounced matrix effects. The compounds of interests for IRMS analyses in doping control are testosterone (T) and its main metabolites (androsterone, etiocholanolone, 5α-androstane-3α,17β-diol, 5β-androstane-3α,17β-diol), epitestosterone, 19-norandrosterone (19-NA), boldenone (B) and its main metabolite (BM), formestane (F) and 6αOH-androstenedione (6aOHADION). Currently, the available methods only deal with a selection of the above-mentioned compounds. Some of these compounds (e.g., 19-NA, B, BM, 6aOHADION) are present in very low concentrations, requiring an extensive and dedicated sample clean-up, and this makes it challenging to develop a universal clean-up procedure. Many of these methods require different and multiple offline HPLC-FC setups, which are labour-intensive and time-consuming. That is problematic during, e.g., large sports events, where reporting time is limited (e.g., 72 h). Therefore, in the current work, we developed a uniform online 2D/3D HPLC-FC method, capable of purifying all relevant target compounds in a single run, leading to the fastest clean-up procedure so far (i.e., 31 min for T and its main metabolites; 46 min for 19-NA, F and 6aOHADION; 48 min for B and BM).

Integrating Cycled Enzymatic DNA Amplification and Surface-Enhanced Raman Scattering for Sensitive Detection of Circulating Tumor DNA.

Circulating tumor DNA (ctDNA) represents an emerging biomarker of liquid biopsies for the development of precision cancer diagnostics and therapeutics. However, sensitive detection of ctDNA remains challenging, due to their short half-life and low concentrations in blood samples. In this study, we report a new method to address this challenge by integrating cycled enzymatic DNA amplification technique and Au nanoparticle@silicon-assisted surface-enhanced Raman scattering (SERS) technique. We have demonstrated a reproducible identification of a single-base-mutated ctDNA sequence of diffuse intrinsic pontine gliomas (DIPGs), with the limit of detection (LOD) as low as 9.1 fM in the spiked blood samples. This approach can be used to analyze trace amounts of ctDNA in translational medicine for early diagnosis, therapeutic effect monitoring, and prognosis of patients with cancer.

Quantitative analysis of free glycerol in biodiesel using solid-phase extraction and high-performance liquid chromatography

This study developed a method for separating and quantifying free glycerol in biodiesel. The free glycerol in biodiesel samples was separated using solid-phase extraction (SPE) and subsequently quantified by high-performance liquid chromatography (HPLC) using an ion-exchange stationary phase and refractive index detector (IR). The proposed method (SPE-HPLC-IR) was validated for high- and low-concentration samples (coefficient of determination, R2≥0.99): limit of detection (0.00029% m/m, high; 0.00018% m/m, low); limit of quantification (0.00089% m/m, high; 0.00054% m/m, low); precision (standard deviation < 5.0%), and accuracy (recovery of 98.93% to 106.71%). The proposed method was compared with the reference method (ASTM D6584) using the paired t-test, and no statistical differences were found between them (95% confidence). As such, the proposed method offers a simple, quick, effective method for determining free glycerol in biodiesel and a viable alternative to the reference method.

Quantitative analysis of 20 fentanyl analogues by modified QuEChERS-LC–MS/MS in health products and transdermal patches

The spreading of narcotics especially illicit novel psychoactive substances is a continuing problem in recent years. In response to reduce the morbidity and crime related to fentanyl analogues, the accurate measurement of fentanyl analogues concentrations is significantly important in the analytical laboratories for customs checks and clinical application. In this work, ethyl acetate was selected as extraction solvent, 50 mg of PSA, 100 mg of C18, and 10 mg of GCB were optimized for purification. A modified QuEChERS extraction method followed by high performance liquid chromatography-tandem mass spectrometry with the mode of multiple reaction monitoring has been developed for the simultaneous determination of 20 fentanyl analogues in collagen peptides, slimming capsules and fentanyl transdermal patches. The limits of detection (LODs) varied from 0.004 to 0.02 μg L−1 with relative standard deviations of 4.89–11.4 % and showed good linearity in the range of 0.02−10 μg L−1 and 0.01–1.00 mg L−1, respectively. The recoveries for 20 fentanyl analogues in the low (at μg L−1 level) and high (at mg L−1 level) concentration spiked samples were in the range of 77.7–114 % and 83.9–116 %, which demonstrated the application potential of the proposed method for the determination of fentanyl analogues with low and high concentration in real case samples. In addition, the matrix effect and the cross-reactivity were also proved to not interfere with quantitation of targeted fentanyl analogues. Thus, the developed method showed high sensitivity and good accuracy, which makes it suitable for the rapid detection of fentanyl analogues for customs and border service as well as pharmaceuticals.

A sensitive and label-free electrochemical microRNA biosensor based on Polyamidoamine Dendrimer functionalized Polypyrrole nanowires hybrid.

Thepotential of functionalized polypyrrole nanowires (PPyNWs) are demonstratedas a platform for lable-free miRNA detection using electrochemical impedance spectroscopy (EIS). MicroRNAs (miRNAs) detection methods and sensors are mainly challenged by very low concentrations in physiological samples and high similarity among family members. Herein, a sensitive and selective miRNA biosensor was constructed based on electrochemically synthesized PPyNWs, which were functionalized with polyamidoamine dendrimer (PAMAM) by an electro-oxidation method. The prepared PPyNWs/PAMAM hybrid combines the excellent electrical conductivity of conducting polymer PPyNWs with high surface to volume ratio of PAMAM. DNA probes were immobilized onto the PPyNWs/PAMAM hybrid for the construction of the miRNA biosensor. Using the sensitive EIS technique to monitor DNA/miRNA hybridization, the developed biosensor demonstrated excellent sensing performances, such as wide linear range (10-14M-10-8M) and low detection limit (0.34 × 10-14 M). Even more encouraging, the response sensitivity of the biosensor was 3.12 times higher than that of the bulk PPy-modified sensor, which proved that the microstructure of the PPy nanowires array can greatly improve the performance of the biosensor. An ultrasensitive and selective miRNA biosensor was constructed based on electrochemically synthesized polypyrrole nanowires array (PPyNWs), which were functionalized with polyamidoamine dendrimer (PAMAM) by an electro-oxidation method.

Antifouling Strategies for Electrochemical Biosensing: Mechanisms and Performance toward Point of Care Based Diagnostic Applications.

Although there exist numerous established laboratory-based technologies for sample diagnostics and analyte detection, many medical and forensic science applications require point of care based platforms for rapid on-the-spot sample analysis. Electrochemical biosensors provide a promising avenue for such applications due to the portability and functional simplicity of the technology. However, the ability to develop such platforms with the high sensitivity and selectivity required for analysis of low analyte concentrations in complex biological samples remains a paramount issue in the field of biosensing. Nonspecific adsorption, or fouling, at the electrode interface via the innumerable biomolecules present in these sample types (i.e., serum, urine, blood/plasma, and saliva) can drastically obstruct electrochemical performance, increasing background "noise" and diminishing both the electrochemical signal magnitude and specificity of the biosensor. Consequently, this review aims to discuss strategies and concepts used throughout the literature to prevent electrode surface fouling in biosensors and to communicate the nature of the antifouling mechanisms by which they operate. Evaluation of each antifouling strategy is focused primarily on the fabrication method, experimental technique, sample composition, and electrochemical performance of each technology highlighting the overall feasibility of the platform for point of care based diagnostic/detection applications.

One-Pot Detection of Heavy Metals and Biosynthesis of Fluorescent Nanoparticles from Metals in Groundwater Samples

Heavy metals like Pb2+ and Hg2+ are well known for their toxicity and harmful health impacts. The present study was designed to carry out on-site detection of heavy metals like Pb2+ and Hg2+ in the water samples passed through the egg shell powder column. Egg shells are washed and powdered, preheated at 110 °C for activation, and used as stationary phase in the column. Two metal solutions of varying concentrations (1 μg·L−1, 10 μg·L−1, 100 μg·L−1, and 1000 μg·L−1) were prepared in different buffers of basic pH (8–11) and then passed through the column, and plain buffer solutions were used as control. The column eluent was observed under a trans-illuminator for fluorescence and checked for absorbance in a UV-visible spectrophotometer from 200 to 600 nm. The presence of fluorescence and UV absorption indicated the presence of heavy metals in the sample. Proteins present in the egg shell, on reacting with metal ions, formed stable nanoparticles due to reduction and capping. The biosynthesized nanoparticles from egg shell proteins were confirmed by SEM analysis, where Pb2+ and Hg2+ nanoparticles were formed. The study was repeated for the detection of heavy metals present in groundwater samples, which was confirmed from fluorescence, UV-visible spectrum, EDAX, and FESEM where heavy metal nanoparticles of size range 12–23 nm were formed. Therefore, this method could be a good alternative for the detection of heavy metals at low concentrations in environmental samples and is feasible for on-site detection.

Accurate detection of Newcastle disease virus using proximity-dependent DNA aptamer ligation assays.

Detecting viral antigens at low concentrations in field samples can be crucial for early veterinary diagnostics. Proximity ligation assays (PLAs) in both solution and solid‐phase formats are widely used for high‐performance protein detection in medical research. However, the affinity reagents used, which are mainly poly‐ and monoclonal antibodies, play an important role in the performance of PLAs. Here, we have established the first homogeneous and solid‐phase proximity‐dependent DNA aptamer ligation assays for rapid and accurate detection of Newcastle disease virus (NDV). NDV is detected by a pair of extended DNA aptamers that, upon binding in proximity to proteins on the envelope of the virus, are joined by enzymatic ligation to form a unique amplicon that can be sensitively detected using real‐time PCR. The sensitivity, specificity, and reproducibility of the assays were validated using 40 farm samples. The results demonstrated that the developed homogeneous and solid‐phase PLAs, which use NDV‐selective DNA aptamers, are more sensitive than the sandwich enzymatic‐linked aptamer assay (ELAA), and have a comparable sensitivity to real‐time reverse transcription PCR (rRT‐PCR) as the gold standard detection method. In addition, the solid‐phase PLA was shown to have a greater dynamic range with improved lower limit of detection, upper‐ and lower limit of quantification, and minimal detectable dose as compared with those of ELAA and rRT‐PCR. The specificity of PLA is shown to be concordant with rRT‐PCR.

Getting rid of rain and stars: mitigating inhibition effects in ddPCR assays, the case of the invasive crayfish Pacifastacus leniusculus in the streams of Luxembourg

ddPCR is getting more and more popular in the field of eDNA-based aquatic monitoring. Even if emulsion PCR used in ddPCR confers a partial resistance to inhibition due to the high number of reactions for the same sample (between 10K and 20K), it is not impervious to it. Inhibition impacts the fluorescence amplitude of positive droplets, affecting both their dispersion and their position relatively to the negative droplets cloud. This fluctuation could jeopardize the use of a shared threshold among several samples and thus the objective assignation of the positive droplets. This is even more critical for low concentration samples such as eDNA samples: the positive droplets are scarce and it is thus crucial to objectively discriminate if they can be counted as positive by establishing an appropriate threshold. Another issue is the artifactual generation of high fluorescence droplets that could be counted as positive with a single threshold solution. Here we propose a double threshold method to take both high fluorescence droplets and PCR inhibition impact into account allowing for an objective sorting of the positive and negative droplets in ddPCR assays.

Rapid β-human chorionic gonadotropin detection in urine with electric-double-layer gated field-effect transistor biosensors and a handheld device.

In this experimental study, a portable biosensor was developed to detect β-human chorionic gonadotropin (β-hCG), which is extensively used in pregnancy tests and serves as a biomarker for ectopic pregnancy. The sensor used is an electric-double-layer field-effect transistor biosensor with the extended-gate design. Bias voltage is applied on the sensor to measure the resulting drain current signals. Gold electrode surface is functionally activated with an anti-β-hCG antibody to capture β-hCG protein. Fluorescence imaging technique is utilized to confirm the surface functionalization. The biosensor demonstrates a dynamically wide range of molecules as detection targets at very low sample concentrations, which shows the potential to detect ectopic pregnancy in very early stages and easily keep track of its periodic changes. It can be produced en masse and does not use additional labels/reagents or pre-processing techniques for the sample. This biosensor can significantly reduce the manufacturing costs and is comparable with the currently available commercial ß-hCG assays. It is suitable for early diagnosis of ectopic pregnancy with low cost and easy operation at home with urine samples.

Efficacy of hormone stimulation on sperm production in an alpine amphibian (Anaxyrus boreas boreas) and the impact of short-term storage on sperm quality

The Southern Rocky Mountain boreal toad (Anaxyrus boreas boreas) has disappeared from much of its range in the alpine regions of Central and Western North America, and restoration efforts are compromised by limited knowledge of this species’ reproductive biology. This study aimed to establish whether assisted reproductive techniques could be used to improve breeding output in captive boreal toads by determining the most effective concentration of human chorionic gonadotropin (hCG) for induction of spermiation and viability of sperm during cold storage. Male toads (n = 21) were treated with a Low (3 IU g-1), Medium (10 IU g-1), or High (15 IU g-1) concentration of hCG and spermic urine samples were collected over 24 hrs. Treatment effectiveness was evaluated by measuring the response rate, Total Motility (TM), Forward Progressive Motility (FPM), Quality of FPM (QFPM), and concentration. For short-term cold storage, spermic urine samples (n = 13) were stored at 4 °C for 14 days and sperm TM and FPM monitored daily. All treatments induced spermiation; however, a greater number of toads produced sperm in the Medium and High treatments compared to the Low. Overall, TM, FPM, QFPM and sperm concentration were similar across all three treatments, but variation existed in the timing and duration of peak sperm production. Sperm motility was maintained for up to 14 days in cold storage, although the quality slowly decreased over time. An effective reproduction strategy for the boreal toad will provide a means to improve captive breeding efforts and increase our understanding of the reproductive physiology of alpine Bufonids.

Unravelling the effect of impurities on the methanol-to-olefins process in waste-derived zeolites ZSM-5

One of the main challenges in the next decades for the chemical industry is the valorization of low-grade industrial waste streams into valuable materials, as their increasing amount can have a major environmental impact. For example, in copper mining industry, waste flotation tailings can be valorized into solid acid catalysts, such as zeolites. Numerous studies have shown that modification of zeolites by metal ions has a positive effect in their catalytic performance. However, the presence of impurities in waste-derived materials is usually disregarded. In this work, the influence of impurities in waste-derived zeolites ZSM-5 on the physicochemical properties and the catalytic performance in methanol-to-hydrocarbons (MTH) reaction was investigated. Flotation tailings from copper industry, mainly consist of SiO2 as quartz, next to Ca, Mg and Fe species. By altering the pH in the SiO2 precipitation step, the impurities content in the waste-derived zeolites ZSM-5 could be controlled. Chemical analysis confirmed the presence of Fe, Ca and Mg in low concentrations in the waste-derived samples. UV–Vis Diffuse reflectance spectroscopy (DRS) and electron paramagnetic resonance (EPR) showed the presence of different Fe species in the waste-derived samples in the form of isolated Fe3+, oligomeric FexOy species and some extended Fe2O3-like clusters. Temperature programmed desorption (TPD) of NH3 and infrared (IR) spectroscopy measurements after CO and pyridine adsorption were used in order to probe the effect of the cations present in the acidity of the waste-derived materials. It was found that the zeolite material with a higher amount of metal impurities contained less Brønsted acid sites and more Lewis acid sites. Catalytic testing showed that the metal impurities had a positive influence on the overall catalyst stability and the yields towards ethylene and propylene were respectively 8% and 6% up. Meanwhile, operando UV–Vis DRS showed the influence of impurities on the catalytic mechanism, as its increase led to the formation of less hydrocarbon pool species. This study not only show that waste can be transformed into value-added materials, namely a zeolite-based catalyst material, but also that the impurities present in the waste can further improve the catalytic performance of the solid catalyst.

Thermal performance analysis of a 1–5 TEMA E shell and coil heat exchanger operating with SWCNT–water nanofluid with varied nanoparticle concentration

Single-walled carbon nanotube–water nanofluids were tested in a 1–5 TEMA E shell and coil heat exchanger. Cold nanofluid, flowing inside the coil, was heated by hot water flowing in the shell side. Volumetric fraction of nanoparticles, inlet temperature of nanofluid, and mass flow rate of nanofluids ranged from 0 to 0.21%, 2.3 to 23.4 °C, and 40 to 90 g/s, respectively. For a given Reynolds number, at the coil side, pure base fluid (φ = 0%) performed better than low-concentration nanofluid samples (φ = 0.035% and 0.053%) and was nearly equivalent to the nanofluid of highest concentration, φ = 0.21%. The thermal conductivity enhancement factor of the nanofluid ranged from 0 to 0.2 and to 0.45, at inlet temperatures of 30 °C and 50 °C, respectively. It is believed to work in favor of a better performance of the nanofluid samples. On the other hand, the unusual (literature-wise) low temperature of the nanofluid further amplified the enhancement of the nanofluid viscosity, with a reduction effect on the Reynolds number. Besides, other thermal resistances of the heat exchanger work toward an attenuation of the enhancement effect that nanoparticles may have in the heat exchanger performance.

Evaluation of fast nucleic acid detection system in severe acute respiratory syndrome coronavirus 2

Objective: To evaluate the performance and application of a fast nucleic acid detection system for testing severe acute respiratory syndrome virus 2 (SARS-COV-2). Methods: Clinical samples were collected from February to July 2020 from Beijing Center for Diseases Prevention and Control and the Laboratory Department of China-Japan Friendship Hospital, to evaluate the sensitivity, specificity, anti-interference ability, precision and clinical sample coincidence rate of fast nucleic acid detection system for SARS-CoV-2. The analytical sensitivity was determined by a dilution series of 20 replications for each concentration. Analytical specificity study was performed by testing organisms whose infection produces symptoms similar to those observed at the onset of corona virus disease 2019 (COVID-19), and of the normal or pathogenic microflora that may be present in specimens collected. Potential interference substances were evaluated with different concentration in the interference study. Precision study was conducted by estimating intra-and inter-batch variability. Clinical evaluation was performed by testing 230 oropharyngeal swab specimens and 95 sputum specimens in fast nucleic acid detection system, comparing with conventional real-time fluorescent quantitative PCR (RT-qPCR) and clinical diagnostic results. Results: The analytical sensitivity of SARS-CoV-2 using fast nucleic acid detection system was 400 copies/ml. The result is negative for testing with the organisms that may likely in the circulating area or causing similar symptoms with SARS-CoV-2 and human nucleic acid, indicating that no cross reactivity with organisms. The results of precision test showed that the Coefficient of variation of Ct value of high, medium and low concentration samples was 1.90%-3.92%, and all of them were less than 5% in intra-and inter-batch testing. The results of the samples were still positive after adding the potential interfering substances, indicating that the possible interfering substances in the samples had no effect on the results. 98.46% and 97.85% diagnosis results of fast nucleic acid detection system were consistent with RT-qPCR and clinical diagnostic results, respectively. Conclusion: The fast nucleic acid detection system based on molecular parallel reaction can be used as a selection method for SARS-CoV-2 testing.

Impact of doping on the performance of p-type Be-doped Al0.29 Ga0.71As Schottky diodes

The effects of changing the acceptors concentration on the electrical characteristics of Au/Ti on Be-doped Al0.29Ga0.71As Schottky contact have been investigated in the temperature range of 100—400 K. Using three devices with three different doping levels, the barrier height (ΦB), ideality factor (n) and series resistance (RS) for each diode were evaluated using both thermionic emission (TE) theory and Cheung's method. Our experimental results showed that the sample with a moderate doping concentration of 3 · 1016 cm-3 has the best performance, including ideality factor of 1.25 and rectification ratio of 2.24 · 103 at room temperature. All samples showed an abnormal behavior of reducing ΦB and increasing n with increase of temperature. This behavior was attributed, in case of low concentration samples, to barrier inhomogeneity and was explained by assuming a Gaussian distribution of barrier heights at the interface. While for the heavily doped sample, such non-ideal manner was ascribed with tunneling through the field emission (FE) mechanism.

Evaluation of iodine speciation and 129I/127I ratios at low concentrations in environmental samples using IC-ICP-MS

Subterranean radioiodine contamination at the Hanford Site in Washington State is believed to be present as iodide, iodate, and organo-I species, with iodate being the predominant form. Because these species have different sediment-sorption characteristics, understanding their distribution is important for developing an accurate understanding of iodine migration in the subsurface. Herein, we report a novel, rapid technique for simultaneous iodine speciation (iodide/iodate) and isotopic ratio (129I/127I) measurements using ion chromatography (IC) joined with collision/reaction cell inductively coupled plasma mass spectrometry (ICP-MS), collectively referred to as IC-ICP-MS. This approach employs online dynamically regenerated eluent suppression post chromatographic separation of the samples and collision cell technology, with pure oxygen as a collision gas for the active suppression of 129Xe (which naturally exists in the argon supplied to the ICP source) to rapidly (< 15 min) achieve precise and reproducible results. Speciated standard reference materials yielded detection limits for 127I of approximately 23.8 ng/L for iodate and 24.3 ng/L for iodide, and for 129I of approximately 1.81 ng/L for iodate and 2.62 ng/L for iodide. The method was demonstrated by analyzing groundwater samples from six wells from 129I-contaminated regions of the Hanford Site; iodate was the primary species for both 127I and 129I. Small quantities of 127I-iodide were also detected in most of the samples, but all 129I-iodide results were below the detection limit. An interference from molybdenum prevented the estimation of organo-iodine concentrations but did not affect the iodate and iodide results. This new analytical capability will enable rapid, simultaneous characterization of speciated inorganic iodine in vadose zone sediments and groundwater samples at levels below the US federal drinking water standard for 129I of 1 pCi/L (~ 5.6 ng/L).