Sigma Detection Limit Research Articles

Exploring cosmic origins with CORE: Cluster science

We examine the cosmological constraints that can be achieved with a galaxy cluster survey with the future CORE space mission. Using realistic simulations of the millimeter sky, produced with the latest version of the Planck Sky Model, we characterize the CORE cluster catalogues as a function of the main mission performance parameters. We pay particular attention to telescope size, key to improved angular resolution, and discuss the comparison and the complementarity of CORE with ambitious future ground-based CMB experiments that could be deployed in the next decade. A possible CORE mission concept with a 150 cm diameter primary mirror can detect of the order of 50,000 clusters through the thermal Sunyaev-Zeldovich effect (SZE). The total yield increases (decreases) by 25% when increasing (decreasing) the mirror diameter by 30 cm. The 150 cm telescope configuration will detect the most massive clusters (>1014 M⊙) at redshift z>1.5 over the whole sky, although the exact number above this redshift is tied to the uncertain evolution of the cluster SZE flux-mass relation; assuming self-similar evolution, CORE will detect 0∼ 50 clusters at redshift z>1.5. This changes to 800 (200) when increasing (decreasing) the mirror size by 30 cm. CORE will be able to measure individual cluster halo masses through lensing of the cosmic microwave background anisotropies with a 1-σ sensitivity of 4×1014 M⊙, for a 120 cm aperture telescope, and 1014 M⊙ for a 180 cm one. From the ground, we estimate that, for example, a survey with about 150,000 detectors at the focus of 350 cm telescopes observing 65% of the sky would be shallower than CORE and detect about 11,000 clusters, while a survey with the same number of detectors observing 25% of sky with a 10 m telescope is expected to be deeper and to detect about 70,000 clusters. When combined with the latter, CORE would reach a limiting mass of M500 ∼ 2−3 × 1013 M⊙ and detect 220,000 clusters (5 sigma detection limit). Cosmological constraints from CORE cluster counts alone are competitive with other scheduled large scale structure surveys in the 2020's for measuring the dark energy equation-of-state parameters w0 and wa (σw0=0.28, σwa=0.31). In combination with primary CMB constraints, CORE cluster counts can further reduce these error bars on w0 and wa to 0.05 and 0.13 respectively, and constrain the sum of the neutrino masses, Σ mν, to 39 meV (1 sigma). The wide frequency coverage of CORE, 60–600 GHz, will enable measurement of the relativistic thermal SZE by stacking clusters. Contamination by dust emission from the clusters, however, makes constraining the temperature of the intracluster medium difficult. The kinetic SZE pairwise momentum will be extracted with 0S/N=7 in the foreground-cleaned CMB map. Measurements of TCMB(z) using CORE clusters will establish competitive constraints on the evolution of the CMB temperature: (1+z)1−β, with an uncertainty of σβ ≲ 2.7× 10−3 at low redshift (z ≲ 1). The wide frequency coverage also enables clean extraction of a map of the diffuse SZE signal over the sky, substantially reducing contamination by foregrounds compared to the Planck SZE map extraction. Our analysis of the one-dimensional distribution of Compton-y values in the simulated map finds an order of magnitude improvement in constraints on σ8 over the Planck result, demonstrating the potential of this cosmological probe with CORE.

Solvothermal synthesis of reduced graphene oxide/Au nanocomposite-modified electrode for the determination of inorganic mercury and electrochemical oxidation of toxic phenolic compounds

A facile, one-step solvothermal approach has been utilized for the synthesis of a reduced graphene oxide/Au nanocomposite (RGO/Au) material. Deposition of Au nanoparticles (AuNPs) on the surface of reduced graphene oxide sheets has been realized by a simple reduction of Au precursors and graphite oxide (GO) in ethylene glycol (EG), without using any additional reductants and surfactants. The morphology of the RGO/Au nanocomposite was thoroughly examined by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The morphology of the RGO/Au nanocomposite was characterized by HRTEM and 20nm was the average size of the AuNPs obtained. The performance of the RGO/Au nanocomposite has been investigated in the application of the Hg determination at the trace/ultra trace level using differential pulse anodic stripping voltammetry. The composite material was also applied for the determination of p-methoxy phenol (PMP) and, most importantly, it was observed that the material was suitable for the decomposition of PMP by the electrochemical oxidation process. The three sigma detection limits for Hg2+ and PMP were obtained as 0.25μgL−1 and 0.64μM, respectively. The applicability of the RGO/Au nanocomposite was further extended to determine Hg2+ in soil samples. Chronoamperometric tests were carried out to investigate the performance of modified and unmodified electrodes in the decomposition of PMP by the electrochemical oxidation route.

A low H I column density filament in NGC 2403: signature of interaction or accretion

Observed HI accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion happens in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To contrain the presence and properties of such flows, we present deep HI observations obtained with the NRAO Green Bank Telescope of an area measuring 4 by 4 degrees around NGC 2403. These observations, with a 5 sigma detection limit of 2.4 x 10^18 cm^-2 over a 20 km/s linewidth, reveal the presence of a low-column density, extended cloud outside the main HI disk, about 17' (~16 kpc or ~2R25) to the NW of the center of the galaxy. The total HI mass of the cloud is 6.3 x 10^6 Msun, or 0.15 percent of the total HI mass of NGC 2403. The cloud is associated with an 8-kpc anomalous-velocity HI filament in the inner disk, previously observed in deep VLA observations by Fraternali et al. (2001, 2002). We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigbouring dwarf galaxy.

The first Δa observations of three globular clusters

Globular clusters are main astrophysical laboratories to test and modify evolutionary models. Thought to be rather homogeneous in their local elemental Distribution of members, results suggest a wide variety of chemical peculiarities. Besides different main sequences, believed to be caused by different helium abundances, peculiarities of blue horizontal-branch stars and on the red giant branch were found. This whole zoo of peculiar objects has to be explained in the context of stellar formation and evolution. The tool of Delta a photometry is employed in order to detect peculiar stars in the whole spectral range. This three filter narrow band system measures the flux distribution in the region from 4900 to 5600A in order to find any peculiarities around 5200A. It is highly efficient to detect classical chemically peculiar stars of the upper main sequence, Be/Ae, shell and metal-weak objects in the Milky Way and Magellanic Clouds. We present Delta a photometry of 2266 stars from 109 individual frames for three globular Clusters (NGC 104, NGC 6205, and NGC 7099). A comparison with published abundances, for three horizontal-branch stars, only, yield an excellent agreement. According to the 3 sigma detection limit of each globular cluster, about 3% of the stars lie in abnormal regions in the diagnostic diagrams. The first observations of three widely different aggregates give very promising results, which will serve as a solid basis for follow-up observations including photometric as well as spectroscopic studies.

THE SLOAN DIGITAL SKY SURVEY STRIPE 82 IMAGING DATA: DEPTH-OPTIMIZED CO-ADDS OVER 300 deg 2 IN FIVE FILTERS

We present and release co-added images of the Sloan Digital Sky Survey (SDSS) Stripe 82. Stripe 82 covers an area of 300 deg^2 on the Celestial Equator, and has been repeatedly scanned 70-90 times in the ugriz bands by the SDSS imaging survey. By making use of all available data in the SDSS archive, our co-added images are optimized for depth. Input single-epoch frames were properly processed and weighted based on seeing, sky transparency, and background noise before co-addition. The resultant products are co-added science images and their associated weight images that record relative weights at individual pixels. The depths of the co-adds, measured as the 5 sigma detection limits of the aperture (3.2 arcsec diameter) magnitudes for point sources, are roughly 23.9, 25.1, 24.6, 24.1, and 22.8 AB magnitudes in the five bands, respectively. They are 1.9-2.2 mag deeper than the best SDSS single-epoch data. The co-added images have good image quality, with an average point-spread function FWHM of ~1 arcsec in the r, i, and z bands. We also release object catalogs that were made with SExtractor. These co-added products have many potential uses for studies of galaxies, quasars, and Galactic structure. We further present and release near-IR J-band images that cover ~90 deg^2 of Stripe 82. These images were obtained using the NEWFIRM camera on the NOAO 4-m Mayall telescope, and have a depth of about 20.0--20.5 Vega magnitudes (also 5 sigma detection limits for point sources).

Dual modulation Faraday rotation spectroscopy of HO_2 in a flow reactor

The technique of dual modulation Faraday rotation spectroscopy has been applied for near shot-noise limited detection of HO2 at the exit of an atmospheric pressure flow reactor. This was achieved by combining direct current modulation at 51 kHz of an external cavity quantum cascade laser system with 610 Hz modulation of the magnetic field generated by a Helmholtz coil. The DM-FRS measurement had a 1.8 times better signal-to-noise ratio than an AC-FRS measurement acquired under identical flow reactor conditions. Harmonic detection of the FRS signal also eliminated the substantial DC-offset associated with electromagnetic intereference pick-up from the Helmholtz coils that is observed in the AC-FRS spectrum. A noise equivalent angle of 4x10^(-9) rad Hz^(-1/2) was observed for the DM-FRS measurement, and this corresponds to a 3 sigma detection limit of 0.2 ppmv Hz^(-1/2).

Fabrication of electrochemical sensor based on green reduction of graphene oxide for an antimigraine drug, rizatriptan benzoate

In this paper, electrochemically reduced graphene oxide modified glassy carbon electrode (ErGO/GCE) was proposed as the electrochemical sensing platform for an antimigraine drug, rizatriptan benzoate (RZB). The synthesized graphene oxide material was characterized by XRD, SEM, FTIR, AFM and UV-spectroscopic techniques. Cyclic and differential voltammetric techniques were employed to study the electrochemical responses of RZB at ErGO/GCE and bare GCE. RZB exhibited an irreversible oxidation peak at both modified and bare GCE. Electrochemical studies indicated that the ErGO/GCE greatly enhanced the oxidation signal of RZB. The oxidation peak position was shifted towards negative direction on the modified electrode compared to that at bare GCE. The electrochemical sensor response was optimized by exploring the effect of pH, accumulation time and scan rate on voltammetric behavior of RZB. Further, the experimental results demonstrated that the oxidation peak current of RZB increased linearly in the concentration range of 0.01–40μM with three sigma detection limit of 0.0571μM. The modified electrode showed higher sensitivity, selectivity and remarkable reproducibility for electrochemical response of RZB. Finally, the proposed electrochemical sensor was successfully applied for the assay of RZB in pharmaceutical formulations and analyte fortified serum samples.

Electrochemical Determination of Melamine on Static Mercury Drop Electrode and on Gold Nano Particle Modified Carbon Paste Electrode

An electrochemical method was developed for the determination of melamine (2,4,6 triamino- 1,3,5-triazine) on static mercury drop electrode (SMDE) and on gold nano particle modified carbon paste electrode. Interaction of melamine with Cu2﹢ ion was investigated and the decrease of Cu stripping peak was measured for the determination of melamine using SMDE. Oxidation property of melamine was improved using gold nano particle modified carbon paste electrode. Direct determination of melamine was proposed from the oxidation peak with three sigma detection limit of 0.43 μML﹣1. Recovery in milk powder sample has shown good recovery of melamine using the modified carbon paste electrode.

PH-controlled solid-phase enrichment of Mn(II): confirmation of the structure of the extracted ternary Mn(II) complex by single crystal X-ray structure analysis

Trace-level (μg g−1) selective separation of Mn(II) from other associated metal ions have been carried out by impregnating equimolar mixture of pyridine-2, 6-dimethanol (PDM) and thiocyanate ion on silica. The sorbent showed maximum Mn(II) sorption capacity (1.23 mmol g–1) at pH 5.0. Mn(II) was completely eluted by 8.5 mL 1.0 mol L−1 HNO3 from 1.0g sorbent and subsequently measured with flame atomic absorption spectrometer (FAAS).The structure of the extracted Mn(II) complex was established by Fourier transform infrared red spectra and confirmed by single crystal X-ray structure analysis. Thermogravimetric analysis (TGA) shows the stability of the isolated Mn(II) complex at the extraction temperature. Three sigma detection limit (N = 10) of the method was 0.6 μg mL−1 with a relative standard deviation of 0.1% (N = 10). The method has a pre-concentration factor of 105. Marginal interference from Cu2+ has been masked with NH4SCN. The developed method has been tested for trace-level separation and estimation of Mn(II) in spiked water samples.

THIOCYANATE-ASSISTED SOLID PHASE ENRICHMENT OF Cu(II) USING PYRIDINE DERIVATIVE WITH X-RAY CRYSTALLOGRAPHIC EVIDENCE

Equimolar mixture of pyridine-2,6-dimethanol (PDM), and thiocyanate ion immobilized on silica serves as an efficient sorbent for selective retention of Cu(II) from other associated metal ions at µg g−1 level. The maximum sorption capacity for Cu(II) was found as 2.44 mmol g−1 at pH 6.0. The sorbed Cu(II) was completely eluted with 3 mol L−1 HNO3 and measured with a flame atomic absorption spectrometer (FAAS). The structure of the extracted Cu(II) complex was confirmed by single-crystal X-ray structure analysis and Fourier transform-infrared (FT-IR) spectroscopy. Thermogravimetric analysis (TGA) of the isolated Cu(II) complex was performed to determine its thermal stability at the extraction temperature. The three sigma detection limit (N = 15) of the method is 0.6 µ g mL−1 with a relative standard deviation (RSD) of 0.1% (N = 15). Pre-concentration factor of the method is 133. Slight interference from Mn2+ ion was eliminated by prior oxidation with potassium periodate. The developed method was tested for trace level separation and estimation of Cu(II) in certified reference materials and environmental samples. [Supplementary materials are available for this article. Go to the publisher's online edition of Chemical Engineering Communications for the following free supplemental resources: two tables providing details of the bond lengths and bond angles of the Cu(II) complex.]

In situ synthesis and properties of reduced graphene oxide/Bi nanocomposites: As an electroactive material for analysis of heavy metals

An in situ modified Hummers method (without the use of any surfactants) has been used for the deposition of bismuth (Bi) nanoparticles onto the surface of reduced graphene oxide (RGO) sheets. The as-synthesized nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM), thermogravimetry (TG) and differential scanning calorimetry (DSC). The morphology of the RGO/Bi nanocomposites provides a better choice as an electrode material for detection of heavy metal ions due to its better functional properties over the Bi film electrode. Trace analysis of heavy metal ions like Cd+2, Pb+2, Cu+2 and Zn+2 in water is carried out by stripping voltammetric analysis using RGO/Bi nanocomposite as an electrode material. The sensitivity and detection limit of the electrode were quantitatively estimated from the analysis. The three sigma detection limits at different deposition potential for Cd2+, Pb2+, Zn2+ and Cu2+ were obtained as 2.8, 0.55, 17 and 26μgL−1, respectively. Copper detection using Bi-film electrode was a major challenge, which has been resolved using the RGO/Bi nanocomposite electrode.

Revealing a strongly reddened, faint active galactic nucleus population by stacking deep co-added images

More than half of the sources identified by recent radio sky surveys have not been detected by wide-field optical surveys. We present a study based on our co-added image stacking technique, in which our aim is to detect the optical emission from unresolved, isolated radio sources of the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have no identified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe 82 co-added data set. From the FIRST catalogue, 2116 such radio point sources were selected, and cut-out images, centred on the FIRST coordinates, were generated from the Stripe 82 images. The already co-added cut-outs were stacked once again to obtain images of high signal-to-noise ratio, in the hope that optical emission from the radio sources would become detectable. Multiple stacks were generated, based on the radio luminosity of the point sources. The resulting stacked images show central peaks similar to point sources. The peaks have very red colours with steep optical spectral energy distributions. We have found that the optical spectral index alpha_nu falls in the range -2.9 < alpha_nu < -2.2, depending only weakly on the radio flux. The total integration times of the stacks are between 270 and 300 h, and the corresponding 5 sigma detection limit is estimated to be about m_r = 26.6 mag. We argue that the detected light is mainly from the central regions of dust-reddened Type 1 active galactic nuclei. Dust-reddened quasars might represent an early phase of quasar evolution, and thus they can also give us an insight into the formation of massive galaxies. The data used in the paper are available on-line at http://www.vo.elte.hu/doublestacking.

IS THE OBSERVED HIGH-FREQUENCY RADIO LUMINOSITY DISTRIBUTION OF QSOs BIMODAL?

The distribution of QSO radio luminosities has long been debated in the literature. Some argue that it is a bimodal distribution, implying that there are two separate QSO populations (normally referred to as 'radio-loud' and 'radio-quiet'), while others claim it forms a more continuous distribution characteristic of a single population. We use deep observations at 20 GHz to investigate whether the distribution is bimodal at high radio frequencies. Carrying out this study at high radio frequencies has an advantage over previous studies as the radio emission comes predominantly from the core of the AGN, hence probes the most recent activity. Studies carried out at lower frequencies are dominated by the large scale lobes where the emission is built up over longer timescales (10^7-10^8 yrs), thereby confusing the sample. Our sample comprises 874 X-ray selected QSOs that were observed as part of the 6dF Galaxy Survey. Of these, 40% were detected down to a 3 sigma detection limit of 0.2-0.5 mJy. No evidence of bimodality is seen in either the 20 GHz luminosity distribution or in the distribution of the R_20 parameter: the ratio of the radio to optical luminosities traditionally used to classify objects as being either radio-loud or radio-quiet. Previous results have claimed that at low radio luminosities, star formation processes can dominate the radio emission observed in QSOs. We attempt to investigate these claims by stacking the undetected sources at 20 GHz and discuss the limitations in carrying out this analysis. However, if the radio emission was solely due to star formation processes, we calculate that this corresponds to star formation rates ranging from ~10 solar masses/yr to ~2300 solar masses/yr.

Solid phase pre-concentration of cobalt(II) using pyridine–thiocyanate reagents: X-ray crystal structure of the extracted ternary cobalt(II) complex

Mixture of pyridine-2,6-dimethanol (PDM) and thiocyanate (SCN −) (1:1, mole ratio) immobilized on silica served as a very efficient sorbent for selective retention of Co(II) from other associated metal ions at trace level. The maximum sorption capacity for Co(II) was found to be 0.203 mmol g −1 at pH 9.0. Sorbed Co(II) was completely eluted by 3.5 mL of 3 mol L −1 HCl and measured using flame atomic absorption spectrometer (FAAS). The structure of the extracted Co(II) complex was confirmed by single crystal X-ray structure and Fourier transform infrared (FTIR) spectroscopy. Thermo gravimetric analysis (TGA) of the chelated Co(II) complex revealed its stability at the optimum extraction temperature (55 °C). The method was reproducible with a relative standard deviation (RSD) of 0.6% ( N = 10) with three sigma detection limit (N = 10) of 0.6 μg g −1. A pre-concentration factor, 94 was achieved. Interferences from Mn 2+ and Cu 2+ ions were eliminated by prior oxidation of Mn 2+ by KIO 4 to MnO 4 − and masking of Cu 2+ with NH 4SCN, respectively. A plausible mechanism for the selective extraction of Co(II) was attributed to the formation of a first order water insoluble inner–metallic complex as confirmed by the single crystal X-ray structure analysis. The developed method has been tested for trace level separation and estimation of cobalt in some certified reference materials. Analyses of some biological and environmental samples were performed.

Spectrophotometric Determination of U(VI) with Rifampicin in Soil Samples

AbstractA validated spectrophotometric method has been developed for the determination of uranyl ion in soil samples. The method is based on the complexation reaction between uranyl ion and rifampicin in methanol‐water medium at room temperature. The method is followed spectrophotometrically by measuring the absorbance at 375 nm. Under the optimized experimental conditions, Beer's law is obeyed in the concentration range of 1.35–20.25 μg mL‐1 with apparent molar absorptivity and Sandell's sensitivity of 8.0 × 103 L mol‐1cm‐1 and 0.042 μg/cm2/0.001 absorbance unit, respectively. The interference of a large number of anions and cations has been investigated and the optimized conditions developed have been utilized for the determination of uranium(VI) in soil samples. The three sigma detection limit (n = 9) for uranyl ion was found to be 0.20 μg mL‐1. The proposed method was successfully applied to the determination of uranyl ion in soil samples.

Use of amidoximated hydrogel for removal and recovery of U(VI) ion from water samples

Poly(acrylamidoxime-co-2-acrylamido-2-methylpropane sulfonic acid) (PAMSA) hydrogel was prepared by copolymerization of acrylonitrile and 2-acrylamido-2-methylpropane sulfonic acid as monomer, N,N′-methylenebis(acrylamide) as crosslinking agent and potassium peroxodisulfate as initiator. Amidoximated copolymer network was prepared by the reaction of copolymer network with hydroxylamine hydrochloride. A batch procedure was used for the determination of the characteristics of the U(VI) solid phase extraction from the amidoximated hydrogel. The determination of U(VI) was performed by spectrophotometric method using arsenazo-III as complexing agent. Optimal pH value for the quantitative preconcentration was 3, and full desorption was achieved with 3 mol L −1 HClO 4. The adsorption process can be well described by the pseudo-second-order kinetic model, and the equilibrium adsorption isotherm was closely fitted with the Langmuir model. A preconcentration factor of 20 and the three sigma detection limit of 2.8 μg L −1 ( n = 20) were achieved for uranium(VI) ions. The PAMSA hydrogel was used for separating and preconcentrating the uranyl ion existing in sea water samples, thermal spring water samples and the certified reference materials (TMDA 64; fortified lake water sample).

Solid Phase Extraction of Uranium by Naphthalene‐Methyltrioctylammonium Chloride and Arsenazo(III) Adsorbent and Subsequent Spectrophotometric Determination

AbstractA simple and effective method has been presented for the preconcentration of uranium by solid phase extraction. For this purpose arsenazo(III) supported on naphthalene‐methyltrioctylammonium chloride was used as an adsorbent and uranium solution at pH 3.5 with flow rate of 1 mL·min−1 was passed through the column. Therefore, uranium‐arsenazo(III) complex was formed onto column. Uranium was quantitatively eluted with 5 mL of a 0.1% ammonium tetraphenylborate and determined by spectrophotometric method at 652 nm. Several parameters such as pH, amount of reagents, sample volume, etc. were investigated. The effect of diverse ions on the preconcentration has also been studied and the optimized conditions developed have been utilized for the trace determination of uranium. A preconcentration factor of 100 was achieved. The relative standard deviation (N=8) was 0.5% for 3 ng· mL−1 of uranium. The three sigma detection limit (36) was 0.045 ng·mL−1

PH dependent separation of uranium by chelation chromatography using pyridine 2,6-dimethanol as a chelator: Single crystal X-ray structural confirmation of the chelated uranium complex

A very effective sorbent material which exhibits exceptional capture of hexavalent uranium from other actinides and lanthanides at μg g −1 level, features pyridine 2,6-dimethanol immobilized onto alumina. The maximum sorption capacity for dioxouranium(VI) was found as 1.96 mmol g −1 at pH 3.0. The adsorbed uranium complex was eluted completely by 0.05 mol L −1 (NH 4) 2CO 3 solution and the concentration of hexavalent dioxouranium ion was monitored spectrophotometrically using Arsenazo III. The structure of the chelated uranium complex has been confirmed by single crystal X-ray structure analysis and Fourier transform infrared red (FTIR) spectroscopy. Thermo gravimetric analysis/differential thermo gravimetric analysis (TGA/DTG) of the chelated uranium complex was performed. The method is reproducible with a relative standard deviation (RSD) of 1.2% ( N = 10) and the three sigma detection limits ( N = 15) 1.2 μg mL −1 respectively. A pre-concentration factor, almost 500, for uranium was achieved. Interferences from Th 4+, Mn 2+, Ni 2+ and Cu 2+ ions were masked with ethylene diamine tetra acetic acid (di-sodium salt) (Na 2EDTA) in the aqueous phase. The developed method has been tested for uranium recovery and estimation in some certified reference materials and environmental samples.

Direct detection of galaxy stellar halos: NGC 3957 as a test case

We present a direct detection of the stellar halo of the edge-on S0 galaxy NGC 3957, using ultra-deep VLT/VIMOS V and R images. This is achieved with a sky subtraction strategy based on infrared techniques. These observations allow us to reach unprecedented high signal-to-noise ratios up to 15 kpc away from the galaxy center, rendering photon-noise negligible. The 1 sigma detection limits are R = 30.6 mag/arcsec^2 and V = 31.4 mag/arcsec^2. We conduct a thorough analysis of the possible sources of systematic errors that could affect the data: flat-fielding, differences in CCD responses, scaling of the sky background, the extended halo itself, and PSF wings. We conclude that the V-R colour of the NGC 3957 halo, calculated between 5 and 8 kpc above the disc plane where the systematic errors are modest, is consistent with an old and preferentially metal-poor normal stellar population, like that revealed in nearby galaxy halos from studies of their resolved stellar content. We do not find support for the extremely red colours found in earlier studies of diffuse halo emission, which we suggest might have been due to residual systematic errors.

Voltammetric Determination of Trace Levels of Hg in Ayurvedic Medicine and in Cobalt-Containing Samples Using a Carbon Paste Electrode

An analytical method has been described for the determination of mercury using a carbon paste electrode. Conditions for the preparation of the carbon paste electrode were optimized for low background current in order to use it for the determination of ultra trace levels of mercury. The carbon paste electrode was found to be a good electrode material for the determination of mercury. The optimized pH for the determination was in the range of 2.5 to 3.5. The three sigma detection limit of mercury was obtained as 0.095 µg L(-1). Method has been modified for determination of mercury in cobalt sulfate samples.