Michael E. Zorn

Weighted least-squares approach to calculating limits of detection and quantification by modeling variability as a function of concentration.

The limit of detection and limit of quantification are current critical issues in environmental testing. In most laboratories, limits are currently calculated on the basis of the standard deviation of replicate analyses at a single concentration. However, since the standard deviation depends on concentration, these single-concentration techniques result in limits that are directly dependent on spiking concentration. A more rigorous approach uses a weighted least-squares regression analysis of replicates spiked at a series of concentrations [Formula: see text] a calibration design. In this work, the use of weighted tolerance intervals is introduced for estimating detection and quantification limits. In addition, models for estimating the weights used in calculating weighted prediction intervals and weighted tolerance intervals are presented. Using this method, detection and quantification limits were calculated for gas chromatographic analyses of 16 polychlorinated biphenyls. Results show that the approach developed provides improved estimates of analytical limits and that the single-concentration approaches currently in wide use are seriously flawed. Future work should reduce the data needed for the calibration design approach so that more rigorous detection and quantification limits can be routinely applied.

Photocatalytic oxidation of acetone vapor on TiO2/ZrO2 thin films

Abstract The photocatalytic oxidation of acetone vapor on TiO2/ZrO2 thin films is presented in this study. Acetone conversion data collected using a non-circulating tubular reactor are analyzed using a general power law model and a Langmuir–Hinshelwood–Hougen–Watson (LHHW) model; the LHHW model provides a slightly better fit than the 1/2 order power law model. The effects of reaction temperature and relative humidity on the rate of reaction are presented. Using either kinetic model, increasing the reaction temperature from 30 to 77°C (in a dry feed stream) significantly increases the reaction rate constant at a 95% confidence level; however, increasing the temperature from 77 to 113°C does not have a significant effect. The addition of water vapor (50% relative humidity) to the feed stream at 77°C significantly increases the reaction rate constant at a 95% confidence level. Also, at three different temperatures and a single reaction condition, the conversion of acetone is enhanced in the presence of water vapor. No byproducts are detected in the effluent stream of the photocatalytically active reactor.

Relationship concerning the nature and concentration of Fe(III) species on the surface of TiO2 particles and photocatalytic activity of the catalyst

Abstract There have been contradictory results concerning the effect of doping TiO2 with Fe(III) upon its photocatalytic activity. We believe that this is due to the method used in adding the Fe(III) to TiO2. This paper addresses this issue by using a precise adsorption process for loading stable aqueous suspensions (sols) of TiO2 doped with Fe(III). In this manner, we control the speciation of Fe(III) on the surface of TiO2 and subsequently use these loaded sols to produce stable thin-film photocatalysts. Adsorption isotherms for these systems at different pH (2.0 and 2.5) values showed that both pH and the concentration of Fe(III) in solution influence the adsorption density of Fe(III), the stability of the resulting sols, and the speciation (degree of polymerization) of Fe(III) ions on the surface of the TiO2 nano-particles. UV-Vis spectroscopy on selected systems was the technique employed to confirm these results. The pore structure of the resulting gels (xerogels) prepared from these sols, as well as thermal stability of porous materials prepared by firing the xerogels have been determined by measuring N2 adsorption as a function of firing temperature. Thermal analysis (TG–DTA) studies on xerogels obtained from some of these systems showed that the presence of Fe(III) species on the TiO2 retards the conversion of anatase into rutile and therefore increases the thermal stability of the gel microstructure. Lastly, porous films, resulting from the casting of the sols on glass rings and subsequent firing, were used in photocatalytic studies. In the presence of UV light, these supported films were capable of degrading ethanol in the gas phase. While photocatalytic activity for Fe(III) loaded TiO2 films is less than for TiO2 films without Fe(III), the speciation of Fe(III) on the surface directly influences the behavior of the intermediate—acetaldehyde—formed in this reaction. Since we have only evaluated the effect of iron speciation on the degradation of ethanol, other target species may be influenced either advantageously or deleteriously depending upon the state of Fe(III) on the surface of TiO2 photocatalysts.

Photocatalytic oxidation of ketones in the gas phase over TiO2 thin films: a kinetic study on the influence of water vapor

Photocatalytic oxidation of two ketones having different hydrophilicity, acetone and methyl isobutyl ketone (MIBK), over TiO2 thin films supported on glass rings was studied under different relative humidity (RH) conditions. Adsorption isotherms of both ketones in the dark under different concentrations of water vapor were also measured and modeled considering a two-site Langmuir model. As expected, an increase in RH resulted in a decrease in the surface concentration of the organic molecules, but the photocatalytic oxidation rates did not show a parallel variation. In the case of acetone, almost total mineralization was achieved, and the kinetic constants obtained from fitting the data to the Langmuir–Hinselwood–Hougen–Watson (LHHW) model, were significantly larger in the presence of water vapor. Photocatalytic elimination of methyl isobutyl ketone (MIBK) is proposed to take place simultaneously by a direct oxidation route and by a sequential mechanism. Acetone is a stable intermediate in this last reaction pathway. Photodegradation of MIBK at different levels of RH was also modeled using LHHW kinetic expressions, and the rates obtained were slightly lower than those obtained for the photooxidation of acetone. However, a carbon balance was not achieved for MIBK, very likely due to the accumulation of partially oxidized products on the surface of the photocatalyst. The influence of water vapor on the photocatalytic oxidation rate of this larger ketone is more complex, and both inhibition and promotion effects can be envisaged. In addition, experimental results show that humidity exerts a significant influence in the mineralization efficiency of MIBK.

Analysis of the Impacts of the Zebra Mussel, Dreissena polymorpha, on Nutrients, Water Clarity, and the Chlorophyll-Phosphorus Relationship in Lower Green Bay

ABSTRACT Zebra mussels, Dreissena polymorpha, invaded Green Bay, Lake Michigan in the early 1990s. In 1986, prior to zebra mussel invasion, the Green Bay Metropolitan Sewerage District initiated a long-term water quality monitoring program involving 12 stations in three distinct zones along a trophic gradient in lower Green Bay. We analyzed this data set pre and post invasion using various regression models to determine the impacts of the zebra mussel on water clarity, nutrient concentrations, and the relationship between chlorophyll and phosphorus in this system. Following zebra mussel invasion, Secchi depths did not change in all three zones. Chlorophyll a concentrations decreased post zebra mussels in all zones. These differences were attributed to the filter feeding abilities of zebra mussels. Lower Green Bay exhibits a strong trophic gradient and zebra mussel impacts on the chlorophyll-phosphorus relationship differed between the three zones. We saw no changes in the chlorophyll-phosphorus relationship in zone 1, zone 2 appeared to be a transition zone with slight changes in the chlorophyll-phosphorus relationship, and in zone 3 there was evidence of an altered chlorophyll-phosphorus relationship post zebra mussels. These results indicate that the impact of zebra mussels on water quality parameters and on chlorophyll-phosphorus dynamics may differ depending on initial trophic status and on zebra mussel densities.

Interfering contaminants in carbon dioxide solvent used in the supercritical fluid extraction of polychlorinated biphenyls

In supercritical fluid extraction (SFE), contaminants in the CO2 can also be concentrated, interfering with analyte quantitation and increasing analyte detection limits. As an example, we measured levels of electron capture-responding contaminants in three grades of CO2, using off-line GC–ECD. In all cases, the contaminant is chlorotrifluoroethylene (CTFE) grease, which significantly affects congener specific analysis of polychlorinated biphenyls (PCBs). For total PCB levels of 30–300 ng/ml, the PCB/CTFE grease ratio should be ≥0.6 to allow quantitation of the 21 largest PCB peaks to within 10–20% of the true value. Accurate quantitation of coplanar congeners or total PCB mass, both sums of many small peaks, requires PCB/CTFE≥6.

Negative chromatographic peaks with oxygen doped electron capture detection of polychlorinated biphenyls

Abstract Electron capture detector make-up gas was doped with O 2 during polychlorinated biphenyl (PCB) analysis. At 6% O 2 , PCB congeners 1, 3 and 4 produce negative peaks, with magnitude enhancement of 11 to 30-fold. For the other 14 PCB congeners studied, normal (positive) response enhancements of 3 to 30-fold are observed. Electron affinities may explain these results.