Equilibrium Partition Coefficient Research Articles

Equilibrium partitioning of macromolecules in confining geometries: Improved universality with a new molecular size parameter

We present a new framework for the description of macromolecules subject to confining geometries. The two main ingredients are a new computational method and the definition of a new molecular size parameter. The computational method, hereafter referred to the confinement analysis from bulk structures (CABS), allows the computation of equilibrium partition coefficients as a function of confinement size solely based on a single sampling of the configuration space of a macromolecule in bulk. Superior in computational speed to previous computational methods, CABS is capable of handling slits, channels, and box confining geometries for all molecular architectures. The new molecular size parameter, hereafter referred to the steric exclusion radius R(s), is explicitly defined and computed for a number of rigid objects and flexible polymers. We suggest that R(s) is the relevant molecular size parameter for characterization of spatial confinement effects on macromolecules. Results for the equilibrium partition coefficient in the weak confinement regime depend only on the ratio of R(s) to the confinement size regardless of molecular details.

Separation of phycocyanin from Spirulina platensis using ion exchange chromatography

This paper presents the evaluation of some important parameters for the purification of phycocyanin using ion exchange chromatography. The influences of pH and temperature on the equilibrium partition coefficient were investigated to establish the best conditions for phycocyanin adsorption. The equilibrium isotherm for the phycocyanin-resin system was also determined. The separation of phycocyanin using the Q-Sepharose ion exchange resin was evaluated in terms of the pH and elution volume that improved the increase in purity and recovery. The highest partition coefficients were obtained in the pH range from 7.5 to 8.0 at 25 degrees C. Under these conditions the equilibrium isotherm for phycocyanin adsorption was well described by the Langmuir model, attaining a Q (m) of 22.7 mg/mL and K (d) of 3.1 x 10(-2) mg/mL. The best conditions for phycocyanin purification using the ion exchange column were at pH 7.5 with an elution volume of 36 mL, obtaining 77.3% recovery and a 3.4-fold increase in purity.

MODELLING OF CU, NI, ZN, MN AND PB TRANSPORT FROM SOIL TO SEEDLINGS OF CONIFEROUS AND LEAFY TREES

Transport of trace metals (TMs) from the soil to a plant involves chemical, physical and biological processes (such as diffusion, adsorption, absorption, growth of a plant, transpiration rate, etc.) in the soil, the soil rhizosphere and in the plant itself. Because of the complexity of these processes and external factors (e.g. weather conditions, biotic factors, type of substrate, etc.) the processes are difficult to describe by mathematical formulas. Modelling of TMs transport to plants is even more complex than that of organic contaminants because, contrary to organic contaminants, TMs are essential for plant growth, metabolism, enzyme activity, etc. The octanol‐water partitioning coefficient for TMs in plants gives only limited information about their accumulation, therefore, other coefficients, such as bioconcentration and translocation coefficients, must be incorporated. The aim of this work was to simulate transport of TMs (Zn, Pb, Cu, Ni and Mn) from the soil to tree seedlings by adapting a generalized model of contaminant uptake by plants. The model applied for transport of TMs from the soil to trees was created by Hung and Muckay. When employing this model for modelling transport of TMs from the soil, amended with industrial sewage sludge, to seedlings of coniferous and leafy trees, some adjustments were made by evaluating the equilibrium partitioning coefficient of TMs between octanol and water (Kow); by introducing the equilibrium partitioning coefficient of TMs in the soil and water (K d ), which depends on the soil pH and the amount of organic matter; by introducing the coefficient (KT) of TMs solubility in water as well as by introducing corrective coefficients. When using the Hung and Muckay's model of the transport of trace metals from the soil to tree seedlings, the results of modelling differed approximately by 6 % in leaves, 5 % in the stem and 8 % in roots as compared with those of measurements.

Dissolved VOC concentrations and salt contents affecting air-sweat equilibrium partition of hydrophilic and hydrophobic VOCs

Workers have frequently disregarded long-term dermal exposure to low concentration of gaseous volatile organic compounds (VOCs). To assess dermal exposure risk to gaseous VOCs, equilibrium partitioning coefficients (pc) at the air-sweat interface on human skin surface must be examined. This study analyzed the pc values of hydrophilic iso-propanol (IPA), methyl ethyl ketone (MEK), and hydrophobic benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene (BTEXs) at the air-water and air-sweat interfaces at 27–47°C. The hydrophilic VOCs were dissolved in pure water and artificial human sweat liquors at approximately 10–125 mg/L, and hydrophobic VOCs were at approximately 0.55 mg/L. According to experimental results, the dissolved VOC concentration and salt contents simultaneously have a co-effect on pc during human dermal exposure to gaseous VOCs. The salt effect resulted in increase of pc for hydrophilic and hydrophobic VOCs, and the dissolved VOC concentration effect resulted in a reduction in pc, which is dominant for hydrophilic compounds of high concentrations of aqueous VOCs. The pc data were utilized for further assessment of risk due to dermal exposure to VOCs.

Kinetics and Thermodynamics of the Association of Dehydroergosterol with Lipid Bilayer Membranes

We have examined the detailed kinetics and thermodynamics of the association of Ergosta-5,7,9(11),22-tetraen-3 β-ol (dehydroergosterol, DHE) with lipid bilayers prepared from 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), a 1:1 binary mixture of POPC and cholesterol (Chol), and a 6:4 binary mixture of egg sphingomyelin (SpM) and Chol. Association of DHE with all three membranes was shown to be entropically driven, most so in the case of SpM-Chol bilayers. Equilibrium partition coefficients for partitioning of DHE between the lipid phase and the aqueous phase were shown to be similar for POPC and POPC-Chol bilayers between 15 and 35°C. Partitioning into the SpM-Chol bilayer is favored at higher temperatures and there is a crossover in solubility preference at ∼25°C. Insertion ( k +) and desorption ( k −) rate constants were shown to be very similar for POPC and POPC-Chol bilayer membranes, but were lower for SpM-Chol bilayers. Similar results were previously reported by us for the association of other amphiphiles with these membranes. We propose a model for the microscopic structure of a POPC-Chol (1:1) bilayer membrane that is consistent with these observations.

Intermediate‐Scale Investigation of Nonaqueous‐Phase Liquid Architecture on Partitioning Tracer Test Performance

Partitioning interwell tracer tests (PITTs) have been proposed as a tool to characterize source zones of waste sites with chemicals entrapped in the form of nonaqueous‐phase liquids (NAPLs). This technique has the potential to be applied to NAPL source characterization of both the vadose and saturated zones, but the recent applications have been primarily focused on dense NAPLs (DNAPLs) entrapped below the water table. This study presents an intermediate‐scale experiment to investigate the applicability of the technique to heterogeneous sites with complex NAPL entrapment architecture. Tracer experiments were conducted in an intermediate‐scale test tank (4.87 by 1.21 by 0.05 m) with heterogeneity defined using the geostatistical parameters of a spatially correlated random field. Tetrachloroethene (PCE), a DNAPL, was spilled into the heterogeneous packing and was allowed to distribute naturally, creating zones of both residual and pooled entrapment. The saturation distribution of the DNAPL was determined in situ using a gamma attenuation system. Once the saturation distribution was characterized, a set of tracer tests was conducted. Results of this study demonstrate how the inversion of the tracer breakthrough concentrations provided valuable information to establish the validity of the use of equilibrium partition coefficients in situations where tracer partitioning might be controlled by nonequilibrium behavior. While our previous work quantified the estimation error introduced by the assumption of equilibrium partitioning in tracer data analysis, this study demonstrated that more accurate NAPL estimates can be obtained using effective partition coefficients in combination with inverse modeling methods upscaled for more realistic heterogeneous settings.

Trace-element mobilization in slabs due to non steady-state fluid–rock interaction: Constraints from an eclogite-facies transport vein in blueschist (Tianshan, China)

The mafic high-pressure rocks of the Tianshan (NW China) display an interconnected network of eclogite-facies veins derived by prograde blueschist dehydration. They provide insight into fluid–rock interaction and element load during long-distance fluid flow occurring due to the major fluid release of subducting oceanic crust. This case study focuses on an eclogite-facies transport vein, its blueschist host and the reaction zone (blueschist-alteration zone), which is located in the central part of the vein. The blueschist mainly consists of glaucophane, micas, epidote, dolomite, and garnet while the vein consists of omphacite, quartz, and apatite. Within the blueschist-alteration zone glaucophane, paragonite, and dolomite have been replaced by omphacite and garnet. Rock textures indicate that the infiltration of external fluids produced the transport vein, most likely due to hydraulic embrittlement. These fluids also triggered the eclogitization of the blueschist-alteration zone. The almost twice as high Li concentration of the vein and the blueschist-alteration zone in comparison to the blueschist host supports the assumption of an external origin of the fluids. The low in trace element vein-forming fluid caused a strong mobilization of all trace elements in those parts of the host the passing fluid reacted with. 40–80% of the trace elements were scavenged which coincided with a loss of the large-ion-lithophile- and light-rare-earth-elements (LILE and LREE), almost double the loss of the heavy-rare-earth and high-field-strength-elements (HREE and HFSE). Around 75% of the total carbon was released as CO 2 into the reactive fluid. The main difference between the blueschist host and the blueschist-alteration zone is the replacement of glaucophane, dolomite, and titanite by omphacite, garnet, and minor rutile respectively, whereas garnet, epidote, rutile, and phengite occur in both zones of the rock. Therefore, the fluid-flow regime rather than the mineral assemblages and equilibrium partition coefficients controls the trace element mobility. The mobilized trace elements coincide with those needed to create the slab signature of arc magmas.

Determination of ambient gas-particle partitioning constants of non-polar and polar organic compounds using inverse gas chromatography

Gas-particle equilibrium partition coefficients for organic compounds, K i p, are traditionally determined using sample-and-extract methods, in which particles and the surrounding air are simultaneously sampled and concentrations are determined from extracts. Though these techniques are necessary for determining actual ambient concentrations, they have several shortcomings when they are used to gain insight on the sorption properties of ambient particles. Some examples are that (i) the role of relative humidity and temperature are difficult to account for due to fluctuating conditions, (ii) only compounds that are present in ambient air can be studied, and (iii) extracts themselves do not directly indicate if particle-bound compounds are in equilibrium with the air phase or not. An alternative approach that could overcome these shortcomings is inverse gas chromatography (IGC). In this work, we developed an IGC method that uses particle-loaded fiber filters as a stationary phase. The measured retention of analytes injected into the IGC system is a direct measure of the collected particle's sorption behavior. A validation study that used particles with known sorption properties indicates that this approach gives reliable K i p values for a wide variety of compounds. Further, ambient particles from a suburban and urban location were found to be stable over time and to exhibit equilibrium sorption in the IGC, showing that this method is suitable for studying ambient particles. The IGC method presented here is a promising new tool for gaining deeper insights into the gas-particle partitioning behavior of polar and non-polar organic compounds.

Effects of molecular shape on osmotic reflection coefficients

The effects of molecular shape on the osmotic reflection coefficient ( σ 0) for rigid macromolecules in porous membranes were analyzed using a hydrodynamic model. In this type of model, employed first by Anderson and Malone [1], steric exclusion of the solute from the periphery of the pore induces a concentration-dependent drop in pressure near the pore wall, which in turn causes the osmotic flow. Results were obtained for prolate spheroids (axial ratio, γ > 1) and oblate spheroids ( γ < 1) in cylindrical and slit pores. Two methods, one of which is novel, were used to compute the transverse pressure variation. Although conceptually different, they yielded very similar results; the merits of each are discussed. For a given value of a/ R, where a is the prolate minor semi-axis or oblate major semi-axis and R is the pore radius, σ 0 increased monotonically with increasing γ. When expressed as a function of a SE/ R, where a SE is the Stokes–Einstein radius, the effects of molecular shape were less pronounced, but still significant. The trends for slits were qualitatively similar to those for cylindrical pores. When σ 0 was plotted as a function of the equilibrium partition coefficient, the results for all axial ratios fell on a single curve for a given pore shape, although the curve for cylindrical pores differed from that for slits. For spheres ( γ = 1) in either pore shape, σ 0 was found to be only slightly smaller than the reflection coefficient for filtration ( σ f). That suggests that σ 0 can be used to estimate σ f for spheroids, where results are currently lacking.

Partitioning of ionic species and crystallization electromotive force during the melt growth of LiNbO 3 and Li 2B 4O 7

The dependence of the crystallization electromotive force (EMF) on the Li 2O composition of the melt in the growth of LiNbO 3 (LN) and Li 2B 4O 7 (LB4) crystals was measured using a micro-pulling-down (μ-PD) method. The crystallization EMF developed during the growth of LN crystals continuously decreased with increase in Li 2O composition. The eutectic compositions in the Li 2O–Nb 2O 5 system were found to be peculiar compositions, at which the development of the crystallization EMF was prevented owing to no degree of freedom in the Gibbs phase rule. It was found that no segregation of the ionic species occurred in the LB4 growth from the melt with the stoichiometric composition, which suggested that the equilibrium partitioning coefficients for all the chemical species in the stoichiometric LB4 melt were unity.

Diffusion along interphase boundaries and its effect on retrograde zoning patterns of metamorphic minerals

In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of chemical transport along interphase boundaries (ib) and through grain (s) interiors during retrograde Fe–Mg exchange between garnet and biotite at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are frequently observed along garnet–quartz interfaces, and deviations from concentric retrograde zoning patterns start to develop when chemical transport along the interphase boundaries becomes slow during cooling. The capacities for chemical transport along an interphase boundary depend on the product D ib K ib/s a, where D ib is the diffusion coefficient of the exchangeable components within the interphase boundary medium, K ib/s is the equilibrium partitioning coefficient between the cation exchange partners and the interphase boundary medium and a is the interphase boundary width. The model is applied to analyze the retrograde zoning patterns in garnets from the Mozambique belt (SE-Kenya), which cooled from 820°C at a rate of ca. 2°C/my. It is found that non-equilibrated compositions in garnet along garnet/quartz interphase boundaries started to develop below 700°C due to insufficient rates of chemical transport along these boundaries. The transport capacities of garnet/quartz interphase boundaries was estimated to have been between about 1E-23 cm3/s (575°C) and 1E-20 cm3/s (700°C) from modeling the observed X Fe pattern in garnet close to a garnet–quartz–biotite triple junction and relying on published data on the diffusivity of Fe2+ in garnet. Similar transport capacities are obtained; when the interphase boundary is assumed to be filled with a material that has the transport properties and chemical composition of a free melt in equilibrium with garnet, biotite and quartz at the respective conditions. In contrast, if the transport properties of the interphase boundary medium are related to the diffusivities and solubility of Fe2+ and FeOH+ within a free aqueous solution, chemical transport along the interphase boundaries would be much more efficient, and exchange equilibrium would have been maintained during the entire cooling history of the rocks. The observation of systematic deviations from local equilibrium along the garnet–quartz interphase boundaries leads us to exclude the presence of an aqueous fluid along the interphase boundary at any time during cooling.

What environmental fate processes have the strongest influence on a completely persistent organic chemical's accumulation in the Arctic?

Fate and transport models can be used to identify and classify chemicals that have the potential to undergo long-range transport and to accumulate in remote environments. For example, the Arctic contamination potential (ACP), calculated with the help of the zonally averaged global transport model Globo-POP, is a numerical indicator of an organic chemical's potential to be transported to polar latitudes and to accumulate in the Arctic ecosystem. It is important to evaluate how robust such model predictions are and in particular to appreciate to what extent they may depend on a specific choice of environmental model input parameters. Here, we employ a recently developed graphical method based on partitioning maps to comprehensively explore the sensitivity of ACP estimates to variations in environmental parameters. Specifically, the changes in the ACP of persistent organic contaminants to changes in each environmental input parameter are plotted as a function of the two-dimensional hypothetical “chemical space” defined by two of the three equilibrium partition coefficients between air, water and octanol. Based on the patterns obtained, this chemical space is then segmented into areas of similar parameter sensitivities and superimposed with areas of high default ACP and elevated environmental bioaccumulation potential within the Arctic. Sea ice cover, latitudinal temperature gradient, and macro-diffusive atmospheric transport coefficients, and to a lesser extent precipitation rate, display the largest influence on ACP-values for persistent organic contaminants, including those that may bioaccumulate within the polar marine ecosystems. These environmental characteristics are expected to be significantly impacted by global climate change processes, highlighting the need to explore more explicitly how climate change may affect the long-range transport and accumulation behavior of persistent organic pollutants.

Influence of an axial high magnetic field on the liquid–solid transformation in Al–Cu hypoeutectic alloys and on the microstructure of the solid

The influence of an axial high magnetic field (up to 10 T) on the liquid–solid interface morphology and microstructure of the solid has been investigated experimentally during Bridgman growth of Al–Cu hypoeutectic alloys. It is found that the field causes the interface to become destabilized and irregular, and promotes planar–cellular and cellular–dendritic transformation. The field has a great influence on the cellular and dendrite array morphology. Indeed, the field causes severe distortion in the cellular array and enhances cell branching. The field makes the morphology of the dendrite array more complex and, with the increase of the magnetic field intensities and decrease of the growth velocities, the dendrites become broken and orientate with the 〈1 1 1〉-direction along the solidification direction instead of the 〈1 0 0〉-direction. Furthermore, the field also enlarges the primary dendrite spacing and promotes the branching of the dendrites to form higher-order arms. The above phenomena may be attributed to the change of the equilibrium partition coefficient k and the liquidus slope m L caused by the field, the magnetic anisotropy of the α-Al crystal and the flow created by the thermoelectromagnetic convection.

Equilibrium Evaporation Behavior of Polonium and Its Homologue Tellurium in Liquid Lead-Bismuth Eutectic

Experimental study using the transpiration method investigates equilibrium evaporation behavior of radionuclide polonium (210Po) generated and accumulated in liquid lead-bismuth eutectic (LBE) cooled nuclear systems. The experiment consists of two series of tests: preliminary evaporation tests for homologue element tellurium (Te) in LBE, and evaporation tests for 210Po-accumulated LBE in which test specimens are prepared by neutron irradiation. The evaporation tests of Te in LBE provide the suggestion that Te exists in a chemical form of PbTe as well as the information for confirming the validity of technique and conditions of Po test. From the evaporation tests of 210Po in LBE, we obtain fundamental data and empirical equations such as 210Po vapor concentration in the gas phase, 210Po partial vapor pressure, thermodynamic activity coefficients, and gas-liquid equilibrium partition coefficients of 210Po in LBE in the temperature range from 450 to 750°C. Additionally, radioactivity concentration of 210Po and 210mBi vapor in a cover gas region of a typical LBE-cooled nuclear system is specifically estimated based on the obtained experimental results, and the importance of 210Po evaporation behavior is quantitatively demonstrated.

Selectivity of electrospray response in small polymer analysis by mass spectrometry

The selectivity of electrospray was explored for a small poly(ethylene)glycol by comparing the oligomer response obtained from direct polymer introduction in flow injection analysis with the signal recorded in high-performance liquid chromatography/mass spectrometry (HPLC/MS). When the oligomer mixture was ionized, a suppression effect was measured for all but the more hydrophobic congeners for which the response was enhanced. This result would reflect the influence of electrospray droplet chemical composition on the equilibrium partitioning coefficient in Enke's model. On average, the electrospray selectivity observed for the studied poly(ethylene)glycol did not affect the molecular weight distribution parameters as response for the most concentrated oligomers was suppressed to the same extent while over-expressed largest congeners had a low contribution to the total polymer sample.

Passive Partitioning of Polychlorinated Biphenyls between Seawater and Zooplankton, a Study Comparing Observed Field Distributions to Equilibrium Sorption Experiments

From previous studies, it remains unclear whether polychlorinated biphenyls (PCBs) are biomagnified in zooplankton or if concentrations are simply governed by passive partitioning. In this study, in the Gullmar Fjord on the Swedish west coast, field-determined lipid-normalized partition coefficients (log Klip) were compared to equilibrium partition coefficients from laboratory sorption experiments with dead and preserved zooplankton. There was no significant difference between the linear regressions of log Klip-log Kow (analysis of covariance [ANCOVA], p < 0.05) for field and laboratory-determined partition coefficients, supporting passive partitioning being the dominant uptake pathway for PCBs in the Gullmar Fjord zooplankton. The field-observed partition coefficients were also suggestive of passive partitioning, as all field-log Klip-log Kow regressions were significant (p < 0.05, r2 = 0.74-0.95) and apparently linear. Further, there was generally no positive correlation between apparent biomagnification factors (BMF; concentration in zooplankton [pg/kgoc]/concentration in phytoplankton [pg/kgoc]) and trophic level (on the basis of delta15N). The in-situ organic carbon (-oc)-normalized concentrations in zooplankton (>200 microm) were not statistically different from oc-normalized concentrations in phytoplankton (0.7-50 microm), which supports the absence of significant biomagnification.

Serotonin 1A receptor availability in patients with schizophrenia and schizo-affective disorder: a positron emission tomography imaging study with [11C]WAY 100635

Postmortem and positron emission tomography (PET) studies have reported several alterations in serotonin 1A receptor (5-HT(1A)) binding parameters in patients with schizophrenia. This study examines 5-HT(1A) availability in vivo in individuals with schizophrenia and schizo-affective disorder. Twenty-two medication-free individuals with schizophrenia or schizo-affective disorder and 18 healthy subjects underwent PET scans with [(11)C]WAY 100635. Regional distribution volumes (V(T), in milliliters per gram) were derived using a two-tissue compartment kinetic model. Outcome measures for 5-HT(1A) availability included binding potential (BP) and the specific to nonspecific equilibrium partition coefficient (V(3)''). Eleven brain regions with high density of 5-HT(1A) were included in the analysis. No significant differences were observed in regional BP or V(3)'' between patients and controls. No significant relationships were observed between regional 5-HT(1A) availability and symptom severity. The postmortem literature reports increased 5-HT(1A) binding in the prefrontal cortex in schizophrenia. This study did not detect differences in 5-HT(1A) binding. Whereas in two recently published PET studies, one reports increased binding in the temporal lobe while the other reports decreased binding in the amygdala. These inconsistencies suggest that the alterations demonstrated in postmortem studies cannot be reliably detected at the resolution achieved with PET. This raises the question as to whether major changes in the level of expression of the 5-HT(1A) receptor play a role in the pathophysiology of schizophrenia.

In vivo quantification of serotonin transporters using [(11)C]DASB and positron emission tomography in humans: modeling considerations.

Positron emission tomography (PET) studies of the serotonin transporter (5-HTT) in the human brain are increasingly using the radioligand [(11)C]N, N-dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine. A variety of models have been applied to such data in several published articles; however to date, these models have not been validated with test-retest data. We recruited 11 healthy subjects and conducted two identical scans on each subject on the same day. We considered four different models (one- and two-tissue compartment kinetic models, likelihood estimation in graphical analysis (LEGA; a bias-free alternative to the graphical method), and basis pursuit) along with fast noniterative approximations to the kinetic models. We considered four different outcome measures (total volume of distribution (V(T)), binding potential with (BP) and without (BP(1)), free-fraction adjustment, and specific-to-nonspecific equilibrium partition coefficient (BP(2))). To assess the performance of each model, we compared results using six different metrics (percent difference (PD) and within-subject mean sum of squares for reproducibility, interclass coefficient for reliability, variance across subjects, identifiability based on bootstrap resampling of residuals for each method, and time stability analysis to determine minimal required scanning time). We considered analysis of both at the voxel level and at the region of interest (ROI) level and compared results from these two approaches to assess agreement. We determined that 100 mins of scanning time is adequate and that for ROI-level analysis, LEGA gives best results. Average PD is 5.51 for V(T), 20.7 for BP, 17.2 for BP(1), and 16.5 for BP(2) across all regions. For voxel-level analysis we determined that the one-tissue compartment noniterative model is best.

Topography of cerebral monoamine transporter availability in families with SCA2 mutations: a voxel-wise [123I]β-CIT SPECT analysis

The purpose of this study was to investigate the monoamine transporter status of dopamine, serotonin and norepinephrine throughout the brain in spinocerebellar ataxia type 2 (SCA2). To this end, nine patients were studied with [(123)I]beta-CIT SPECT. Data were compared with ten age-matched healthy control subjects and ten patients with young-onset Parkinson's disease (YOPD), matched for age. Parametric SPECT images of the specific-to-non-displaceable equilibrium partition coefficient (V (3)''), which is proportional to the receptor density (B (max)), were generated. In order to objectively localise focal changes in beta-CIT uptake throughout the brain volume without having to make an a priori hypothesis as to their location, statistical parametric mapping (SPM) was applied to SPECT images. Data clusters revealed by SPM, showing significant differences in V (3)'' values between groups, were transformed onto the individual V (3)'' image to obtain mean regional uptake values. Both SCA2 and YOPD patients showed significant decreases in striatal [(123)I]beta-CIT SPECT uptake when compared with controls. However, in SCA2 patients, additional reductions in caudate/anterior putamen, midbrain and pons [(123)I]beta-CIT uptake were localised with SPM. Voxel-wise analysis of [(123)I]beta-CIT SPECT revealed more widespread decline of monoamine transporter availability in SCA2 than in YOPD, reflecting differences in the underlying pathology. We suggest that the quantification of midbrain and pons [(123)I]beta-CIT signal is likely to improve the diagnostic accuracy in patients presenting with clinical features of both SCA2 and YOPD at initial investigation.

Influence of dense nonaqueous phase liquid pool morphology on the performance of partitioning tracer tests: Evaluation of the equilibrium assumption

Partitioning tracer techniques, initially developed in the petroleum industry, are promoted as a way to determine entrapped dense nonaqueous phase liquid (DNAPL) mass in source zones. A suite of chemical tracers, some of which partition into the DNAPL phase, is injected into the source zone to estimate the entrapped saturation of contaminant. Current application of the technique can determine DNAPL at residual saturation but does not provide reliable mass estimates of DNAPL in high‐saturation areas (e.g., pools) that may occur in some source zones. To evaluate this technique's limitations in characterizing source zones with complex entrapment architectures that may contain pools, we investigated estimation errors associated with well‐characterized single‐isolated DNAPL pools. X‐ray attenuation was used to determine the spatial distribution of saturation within the pool under different total DNAPL mass entrapment while the DNAPL was subjected to tracer tests. Model analysis used showed that the partitioning tracer breakthrough simulations based on equilibrium partition coefficient (Kp, from equilibrium batch tests) fit the experimental data poorly, suggesting nonequilibrium partitioning that is controlled by the water flow velocity through the NAPL entrapment zone. In order to relax the requirement of partitioning to be at equilibrium, we investigated the use of “effective partition coefficients” (Kpe). A comparison of the saturation estimates using method of moments and inverse modeling with both equilibrium and effective partition coefficients was conducted. Results demonstrated that the incorporation of nonequilibrium behavior in the analysis can improve DNAPL mass estimation by tracers in high‐saturation zones.