Increase In Apparent Concentration Research Articles

Chemical crosstalk between heated gas microsensor elements operating in close proximity

Gas microsensor arrays often have closely-spaced elements, typically separated by hundreds of microns. For such devices, crosstalk between elements operated within a gaseous environment is a concern because sensing materials held at elevated temperatures have an increased probability for disrupting gas flows and activating gas-film interactions that can consume analytes or evolve reaction products. To explore such effects in a microarray, microhotplate array platforms were used to sense carbon monoxide. Carbon monoxide sensing was chosen as a model system because the oscillatory kinetics of CO oxidation on Pt films are known to exhibit sensitive gas-phase coupling. Under proper conditions, a Pt/SnO 2 microsensor was observed to oscillate between two stable CO/O 2 coverage ratios. The high CO coverage state results in higher film conductance, and the oscillation frequency is extremely sensitive to gas-phase CO concentration. Crosstalk was observed between adjacent microsensors with Pt particles supported on SnO 2 films, as evidenced by synchronization of sensor response and mixed-mode oscillations (similar to those observed in CO oxidation on Pt films). The range of crosstalk effects was studied by operating a single device in an oscillatory CO sensing mode and heating neighboring elements in an array. The operation of nearby sensors is believed to produce reactions that effectively lower the CO partial pressure at the monitored device, thereby reducing the period of oscillation. The magnitude of the effect was calculated from the frequency change using a CO concentration calibration, and the effect is greater than 10% when operating 15 neighboring sensors. The effects of heating neighboring microsensors have also been studied for hydrogen and methanol sensing on both Pt/SnO 2 and bare SnO 2 microsensor arrays. While crosstalk effects are observed for these gases, the effects we observe on Pt/SnO 2 sensors are less pronounced than in the case of CO sensing. A less than 1% effect occurs for methanol sensing and the largest effect for H 2 sensing was an apparent concentration decrease of ≈4% when heating 15 neighboring devices in 10 μmol/mol (10 ppm) of the analyte. On the bare SnO 2 devices, we observe an apparent increase in concentration of methanol and H 2 of approximately 5 and 15%, respectively. While crosstalk is only measured for the case of conductometric sensing in this work, similar phenomena are likely to occur for sensors that utilize other detection principles.

Systematic salt effects in the automated determination of nutrients in seawater

The Bran + Luebbe TRAACS continuous flow analyser, commonly used for the determination of nutrients in seawater, primarily employs standard methods that have been modified and adapted for automated use. The analyser is fitted with self aligning spectrophotometric flow-cells with parallel optical ends of uniform thickness. As the light beam strikes both ends of the flow-cell perpendicularly, there is no refraction therefore no spurious absorbance caused by changes in the refractive index of the flow-cell contents. Investigation of methods for the determination of ammonia, nitrate, nitrite, orthophosphate, soluble reactive silica and urea, using TRAACS, confirmed that this was the case. Also, for the methods assessed, the absorbance signal was not biased by either sample turbidity or wetting agents. Chemical interference from salt was assessed by analysing nutrient spiked solutions, over the 0–35 ppt salinity range, relative to working calibration solutions prepared using demineralised water. A major salt effect which caused an apparent decrease in concentration, was detected over the analytical working ranges in the ammonia and nitrate methods. The salt effect results from a chemical interference which causes the production of reaction products to vary as a function of salinity. The salt effect is totally unrelated to refraction and produces a concentration error in the ammonia and nitrate methods up to 35 and 25% respectively. The degree of salt effect was shown to be dependent on both the salinity and true nutrient concentration of the sample. Over the salinity range 8.75–35.00 ppt, corrective equations were derived for ammonia and nitrate to express the true nutrient concentration in terms of the observed concentration and the salinity of the solution. Within the range 0–8.75 ppt no correction was required for ammonia wheras the nitrate showed a 3.5% increase in apparent concentration at 8.75 ppt. The orthophosphate, soluble reactive silica, urea and nitrite methods showed no significant salt effect as the coefficient of variation (CV) of the observed nutrient concentrations over the 0–35 ppt salinity range was within the accepted CV or analytical precision of the methods. The ammonia and nitrate corrective equations were applied in assessing the true nutrient content of “depleted” or low nutrient seawater, used in the preparation of working calibration solutions, after analysing relative to working calibration solutions prepared in demineralised water.

The effect of cooking on veterinary drug residues in food: 2. levamisole.

The heat stability of levamisole was investigated. Results obtained indicated that the drug was stable in boiling water at 100 degrees C, but unstable at 260 degrees C in cooking oil, with a half-life of about 5 minutes. The effect of cooking (microwaving, boiling, roasting, grilling and frying) on levamisole residues in a range of fortified and incurred tissue was studied. No evidence of instability was obtained in any of the cooking methods investigated. Most observed net changes fell within the limits of the precision of the method once allowance for weight loss during cooking was made to counter an apparent increase in concentration. Roasting was the only method of cooking where a net loss of levamisole was observed. Insufficient juices were produced to permit analysis in this instance. The net loss of levamisole in the cooked tissue was similar to that found with other cooking methods, where the levamisole lost was found in the cooking liquid or juices. An assessment of homogeneity of the incurred tissue used in the investigation was made. The pig muscle was found not to be homogeneous with larger differences seen between different areas of the animal than within the same muscle. The findings of this investigation showed that data obtained from measurements on raw tissue are suitable for use in consumer exposure estimates and dietary intake calculations.

Simple solid-phase radioimmunoassays for total tri-iodothyronine and thyroxine in serum, and their clinical evaluation

1. 1. Simple solid-phase radioimmunoassay methods for total tri-iodothyronine and thyroxine in serum or plasma are described. By using antibodies that are covalently coupled to microcrystalline cellulose, virtually identical assay procedures can be used for the determination of both hormones. An alkaline sodium glycinate buffer provides better assay conditions than the buffers often recommended for thyroid hormone radioimmunoassay. 2. 2. Assay results are unaffected by moderate sample haemolysis. Plasma samples stored at room temperature for more than nine days often show an apparent increase in concentration of both thyroid hormones. 3. 3. Serum tri-iodothyronine and thyroxine concentrations in healthy euthyroid subjects, and in euthyroid pregnant women are reported. In a series of 100 consecutive patients referred to a thyroid clinic the tri-iodothyronine assay discriminated better than the thyroxine assay between hyperthyroid and euthyroid patients. The thyroxine assay was much better than the tri-iodothyronine assay in discriminating between hypothyroid and euthyroid patients.

Tissue thiamine changes in rats with experimental trypanosomiasis or malaria

Progressive changes of thiamine content and concentration in the tissues of rats with infections of Trypanosoma rhodesiense or Plasmodium berghei have been described. During the course of infection with P. berghei there was no change in concentration in either the liver or kidney. In the spleens of young rats thiamine concentration doubled, whereas, in older animals an increase of one third was observed. In the blood of infected animals the apparent increase in concentration was not great, because of the attendant anemia. However, an average infected red cell contained three times as much thiamine as a normal erythrocyte and each purified parasite ten times as much. With infections of T. rhodesiense an increased concentration of thiamine was noted in the livers of young animals. During the terminal portion of the infection there was a rise of concentration in the spleens of both young and old rats and a drop in the kidney. In the blood a threefold increase of thiamine concentration occurred which could be related to the rise in parasitemia. Each purified trypanosome contained about twenty times as much thiamine as a single red cell. When animals with infections of T. rhodesiense were fed thiamine deficient diets the changes in thiamine concentration were of the same order as those observed in rats on routine feeding but at a lower level.