Background: Pulsed amperometric detection is a relatively new method for detection of ions and especially non-electrolytes such as carbohydrates in aqueous solutions. Pulsed amperometric detection relies on a redox reaction while electrochemical impedance simply measures the real and capacitive resistant of the solution. There is a correlation between the real impedance of a solution and the ionic strength of the solution. Method: This work explores measuring real impedance of pure water as a function of temperature from 25.0 to 60.0 °C to determine the relationship between impedance and temperature. Maintaining temperature at 25.0 °C, solutions of sodium chloride, potassium carbonate, sodium sulfate acetate and bicarbonate have been measured using impedance spectroscopy. Results: Regression analysis shows that measuring anions using impedance spectroscopy and simple stainless steel cylinders that detection limits at the parts per trillion (ppt) level are possible. There was no statistical difference when comparing impedance values of the same concentration of acetate and chloride in solution, showing real impedance is not dependent on ion size. However, ions with higher charge do result in lower impedance measurements. Conclusions: This work establishes the use of simple, small, robust stainless steel cylinders and impedance measurements following separation for the identification and quantification of ions in solution.

Scientists often encounter unobserved or missing measurements that are typically reported as less than a fixed detection limit. This especially occurs in the environmental sciences when detection of low exposures are not possible due to limitations of the measuring instrument, and the resulting data are often referred to as type I and II left censored data. Observations lying below this detection limit are therefore often ignored, or `guessed' because it cannot be measured accurately. However, reliable estimates of the population parameters are nevertheless required to perform statistical analysis. The problem of dealing with values below a detection limit becomes increasingly complex when a large number of observations are present below this limit. Researchers thus have interest in developing statistical robust estimation procedures for dealing with left- or right-censored data sets (SinghandNocerino2002). The aim of this study focuses on several main components regarding the problems mentioned above. The imputation of censored data below a fixed detection limit are studied, particularly using the maximum likelihood procedure of Cohen(1959), and several variants thereof, in combination with four new variations of the multiple imputation concept found in literature. Furthermore, the focus also falls strongly on estimating the density of the resulting imputed, `complete' data set by applying various kernel density estimators. It should be noted that bandwidth selection issues are not of importance in this study, and will be left for further research. In this study, however, the maximum likelihood estimation method of Cohen (1959) will be compared with several variant methods, to establish which of these maximum likelihood estimation procedures for censored data estimates the population parameters of three chosen Lognormal distribution, the most reliably in terms of well-known discrepancy measures. These methods will be implemented in combination with four new multiple imputation procedures, respectively, to assess which of these nonparametric methods are most effective with imputing the 12 censored values below the detection limit, with regards to the global discrepancy measures mentioned above. Several variations of the Parzen-Rosenblatt kernel density estimate will be fitted to the complete filled-in data sets, obtained from the previous methods, to establish which is the preferred data-driven method to estimate these densities. The primary focus of the current study will therefore be the performance of the four chosen multiple imputation methods, as well as the recommendation of methods and procedural combinations to deal with data in the presence of a detection limit. An extensive Monte Carlo simulation study was performed to compare the various methods and procedural combinations. Conclusions and recommendations regarding the best of these methods and combinations are made based on the study's results. / MSc (Statistics), North-West University, Potchefstroom Campus, 2014

Scientists often encounter unobserved or missing measurements that are typically reported as less than a fixed detection limit. This especially occurs in the environmental sciences when detection of low exposures are not possible due to limitations of the measuring instrument, and the resulting data are often referred to as type I and II left censored data. Observations lying below this detection limit are therefore often ignored, or `guessed' because it cannot be measured accurately. However, reliable estimates of the population parameters are nevertheless required to perform statistical analysis. The problem of dealing with values below a detection limit becomes increasingly complex when a large number of observations are present below this limit. Researchers thus have interest in developing statistical robust estimation procedures for dealing with left- or right-censored data sets (SinghandNocerino2002). The aim of this study focuses on several main components regarding the problems mentioned above. The imputation of censored data below a fixed detection limit are studied, particularly using the maximum likelihood procedure of Cohen(1959), and several variants thereof, in combination with four new variations of the multiple imputation concept found in literature. Furthermore, the focus also falls strongly on estimating the density of the resulting imputed, `complete' data set by applying various kernel density estimators. It should be noted that bandwidth selection issues are not of importance in this study, and will be left for further research. In this study, however, the maximum likelihood estimation method of Cohen (1959) will be compared with several variant methods, to establish which of these maximum likelihood estimation procedures for censored data estimates the population parameters of three chosen Lognormal distribution, the most reliably in terms of well-known discrepancy measures. These methods will be implemented in combination with four new multiple imputation procedures, respectively, to assess which of these nonparametric methods are most effective with imputing the 12 censored values below the detection limit, with regards to the global discrepancy measures mentioned above. Several variations of the Parzen-Rosenblatt kernel density estimate will be fitted to the complete filled-in data sets, obtained from the previous methods, to establish which is the preferred data-driven method to estimate these densities. The primary focus of the current study will therefore be the performance of the four chosen multiple imputation methods, as well as the recommendation of methods and procedural combinations to deal with data in the presence of a detection limit. An extensive Monte Carlo simulation study was performed to compare the various methods and procedural combinations. Conclusions and recommendations regarding the best of these methods and combinations are made based on the study's results. / MSc (Statistics), North-West University, Potchefstroom Campus, 2014

Resorcinarenes can be functionalized at their upper and lower rims. In this work, the upper rim of a resorcinarene was functionalized with glutamic acids and the lower rim was functionalized with either methyl or undecyl alkyl groups. The cavitands were characterized by nuclear magnetic resonance (NMR), mass spectrometry (MS), UV-vis spectroscopy, dynamic light scattering (DLS) and electron microscopy. The binding of resorcinarene with amine guests was studied in DMSO by UV-vis titration. The obtained binding constants (K values) were in the range of 12,000-136000 M-1. The resorcinarenes were shown to form aggregates in a variety of solvents. The aggregates were spherical as confirmed by DLS, SEM and TEM experiments. Dynamic light scattering (DLS) experiments revealed the size of the aggregates could be controlled by cavitand concentration, pH, and temperature. The resorcinarene with undecyl alkyl group were adsorbed onto 55% cross-linked styrene-divinylbenzene resin to prepare a new stationary phases for ion chromatography (IC) columns. The new column packing material was applied in determination of uremic toxins and water contaminants. The new IC column afforded separation of the five uremic toxins : guanidinoacetic acid, guanidine, methylguanidine, creatinine, and guanidinobenzoic acid in 30 minutes. Detection and quantification of uremic toxins helps diagnose kidney problems and start patient care. Gradient elutions at ambient temperature with methanesulfonic acid (MSA) as eluent resulted in detection levels in water from 10 to 47 ppb and in synthetic urine from 28 to 180 ppb. Trace levels of creatinine (1 ppt) were detected in the urine of a healthy individual using the columns. The new IC stationary phase separated cationic pharmaceuticals including a group of guanidine compounds in surface water. Detection limits in the range of 5 - 32 µg L-1 were achieved using integrated pulsed amperometric detection (IPAD) for guanidine (G), methylguanidine (MG), 1,1-dimethylbiguanidine (DMG), agmatine (AGM), guanidinobenzoic acid (GBA) and cimetidine (CIM). Suppressed conductivity (CD) and UV-vis detection resulted in limits of detection similar to IPAD, in the range of 1.7 - 66 µg L-1, but were not able to detect all of the analytes. Three water sources, river, lake, and marsh, were analyzed and despite matrix effects, sensitivity for guanidine compounds was in the 100 µg L-1 range and apparent recoveries were 80-96 %. The peak area precision was 0.01 - 2.89% for IPAD, CD and UV-vis detection.

Detection limit of any method is affected by device (electronics, noise), evaluation, software (timing inaccuracy, rounding mistakes, errors determination of parameters using measured values by numerical methods) and by a chemical reaction (diluting errors, the reaction mixture design, chemical interferents, temperature, the accuracy of pipetting). In this work was proposed a metod for determination of detection limit for measuring of inhibitors of acetylcholine esterase by biosensor analysator of toxicity (BTA). The analysator was developed by BVT Technologies a.s. in 2004. To determine the exact detection limit a calibration of output signal was done. Presently the analysator is able to detect only presence of toxins, not their concentration. Not least are mentioned all founded errors of software, which are continuously eliminated.

No description available.

Abstract Electrochemical sensors are simple, fast, accurate, and low-cost analytical devices. They are especially important to the field of healthcare since they enable measurement of important indicators of patient health such as electrolytes and glucose in blood. Continued development and improvements in electrochemical sensors can result in more accessible, affordable, and effective diagnoses and treatment strategies. Electrochemical sensors employ electrodes, usually modified with a recognition agent specific for the analyte (the biomolecule of interest). The presence of the analyte at the electrode surface is typically measured through an electrochemical reaction that generates a signal in the form of an electric current or difference in electric potential. As an alternative, electrochemiluminescence, a phenomenon whereby an electrochemical reaction generates a product in an electronically excited state that is capable of emitting light, has great benefits due to its high sensitivity, selectivity, and extremely low background signal. Here we employ a camera equipped with a complementary metal-oxide semiconductor (CMOS) detector that is interfaced with a simple zoom lens to measure ECL generated at low-cost pencil graphite electrodes and small electrode arrays using tris(2,2′- bipyridyl) dichlororuthenium(II) hexahydrate ([Ru(bpy)3]2+) with tri-n-propylamine (TPA) as the coreactant. ECL signals produced at pencil graphite working electrodes were linear with respect to [Ru(bpy)3]2+ concentrations for 45–450μM [Ru(bpy)3]2+. The detection limit was found to be 2µM using the CMOS camera with exposure time set at 10s. This proof-of-concept work suggests the pencil graphite electrode with simple imaging system platform can be applied for ECL-based biosensing strategies.

Fluorescence spectroscopy was examined as a potential technique for identifying aerosol particles like bacterial spores. This technique was used for laboratory measurements on some common biological agent simulants. We have measured the intrinsic steady-state fluorescence emission spectra as a function of the excitation wavelength for several bacterial spores (washed and unwashed) in dry and aqueous suspensions at room temperature using excitation wavelengths from 200 to 600 nm. These measurements were compared to those of common, naturally occurring biological components like fungal spores and pollen and non spore samples like ovalbumin. The spectra of samples were combined into fluorescence profiles or fluorescence fingerprints. Different substrates were used for collection and detection of spores. Each bacterium produces a unique in vitro fluorescence profile when measured in dried and aqueous suspension and exhibits a strong maximum in its fluorescence emission spectrum near 330-340 nm. The fluorescence profiles were reproducible. The complexity of microorganisms made the interpretation of their spectral signature a difficult task. Principal components analysis (PCA) and cluster analysis were done as a data reduction technique for detection and identification from different backgrounds. PCA illustrates that linear combination of detected fluorescence intensities, which are present in different ratios in each of samples studied, can be used to discriminate biological agent simulants from other biological samples. The hydration effects, washing effects and the role of tryptophan on spore fluorescence were also investigated. The emission spectra of the dried spores showed a maximum near 330 nm, suggesting a hydrophobic environment for its tryptophan residues. The aqueous solution of tryptophan showed fluorescence shifted to 360 nm and in ethanol solution the maximum was shifted to 340 nm, suggesting a rather more polar average location of the tryptophan. To find the limit of detection we measured the quantum efficiency (QE) of a few samples. We concluded that spectroscopy techniques coupled with effective interpretation models are applicable to biological simulants agents. Index Heading: Bacteria; Spores; Identification; Fluorescence; Fluorescence Quantum Efficiency; Principal Components Analysis; Cluster Analysis.

<p>This study firstly reports the development, characterisation, and application of thick-film acetylcholinesterase (AChE) biosensors based on a gold electrode modified with a mercaptobenzothiazole (MBT) self-assembled monolayer and either poly(omethoxyaniline) (POMA) or poly(2,5-dimethoxyaniline) (PDMA) in the presence of polystyrene(4-sulphonic acid) (PSSA). The Au/MBT/POMA-PSSA/AChE and Au/MBT/PDMA-PSSA/AChE biosensors were then applied to successfully detect standard organophosphorous and carbamate pesticides in a 0.1 M phosphate buffer, 0.1 M KCl (pH 7.2) solution. Secondly, it reports the construction of the Au/MBT/PANI/AChE/PVAc thick-film biosensor for the determination of certain organophosphate and carbamate pesticide solutions in selected aqueous organic solvent solutions.</p>

Orogenic gold deposits occur within metamorphic belts throughout the world and have through time represented the source for over 25% of the world’s gold production. Although orogenic gold deposits are of great economic importance, controversies exist on the subject of fluid and metal sources and there have been few studies of gold´s distribution and mobility outside of large economic deposits. Research made by Pitcairn et al. (2006), on the Mesozoic Otago and Alpine schists of New Zealand, observed systematic depletion of Au and a suite of 6 associated elements with increasing metamorphic grade. This depletion was identical to the suite of elements enriched in the Otago gold deposits and provided strong evidence that orogenic gold deposits form due to metamorphic processes. The mobilization of metals was attributed to the recrystallization of sulfide minerals during prograde metamorphism causing dehydration and release of metal-rich metamorphic fluids.  This thesis is part of a larger project aimed at testing the “Otago model” in a classic metamorphic terrain: The Dalradian metamorphic belt of Scotland. Rocks in the study are from the southern higlands group and the Appin and Argyll group which range in metamorphic grade from chlorite zone greenschist facies to sillimanite zone amphibolite facies. Three main aspects, which supplement earlier research, are addressed in this study: 1) Investigation of the sulfide paragenesis at Loch Lomond and Stonehaven was carried out to map the evolution of sulfides with metamorphic grade and the possible relations to the distribution of gold. Using SEM scanning to quantify the abundance of different sulfide minerals together with previous data on the Glen Esk region, a complex sulfide evolution pattern for the Dalradian Supergroup is identified. The sulfide evolution describes the same changes in texture and chemistry as observed in the Otago Schists but is made complex by the difference in geological evolution for the different regions. 2) Reinvestigation of the higher grade zones of Glen Esk (staurolite to sillimanite) was carried out as samples from the previous study were very weathered. Results from ultralow detection limit methods (HG-AFS and a gold detection method developed by Pitcairn et al. 2006) showed significant systematic depletion of Au and As with metamorphic grade. From chlorite to sillimanite zone average values of Au and As were showed to decrease by 65% and 88% respectively. Furthermore, a suite of 10 major and 12 trace elements were analyzed using ICP methods showing no trends of systematic depletion with increased metamorphic grade.  3) Investigation of Pb-Ag Veining and vein samples from each of the metamorphic index mineral zones in the Glen Esk area was carried out to identify fluid composition and ore mineralogy. Using microthermometry and Raman laser spectroscopy two distinct fluids were identified. The first type is a H2O-CO2-N2-salt fluid of low salinity (0-15 weight percent NaCl equivalent) and medium temperature (150 to 250 °C) locally containing minor amounts of CH4. It is found in the veins from the mineral index zones of Glen Esk and was formed in the ductile regime most likely related to late stage metamorphic devolatilization released during Caledonian uplift of the Dalradian. Pb-Ag veins from the locality of Hardhill host the second fluid type which was formed in the brittle regime  accompanied by brecciation as a high salinity (15 to 20 weight percent NaCl equivalent) low temperature (70-140°C) H2O-salt fluid with calcic composition was precipitated. This fluid bears much resemblance to Carboniferous calcic brines responsible for economic base-metal precipitation with widespread occurrence in southwest Scotland and Northern Ireland. Results of this thesis show many similarities with the Otago study, with a connection between metal mobility and metamorphic grade, providing support for the dehydration model as a viable mechanism for the generation of orogenic gold deposits.