Morphological and structural changes in a copolymer of glycolide and lactide occurring as a result of hydrolysis

The changes in the structure and morphology of a random copolymer containing 92 mol % glycolide units and 8 mol % lactide units as a function of in vitro hydrolysis have been studied. A number of techniques, such as wide-angle x-ray diffraction, small-angle x-ray scattering, density measurements, infrared spectroscopy, and viscometry were employed in this study. The results showed that the hydrolytic attack was initiated in the amorphous areas of the polymer. After 28 days in the in vitro medium most of the amorphous areas had been chemically converted to lower-molecular-weight species such as oligomers of poly(glycolide-co-lactide). These low-molecular-weight species become water solble and are removed rapidly after day 28. Thus the fiber takes on a Swiss cheese character as the amorphous matter is hydrolyzed and eventually dissolved. As hydrolysis proceeds further, the crystalline areas of the polymer are attacked and eventually removed from the fiber by solubilization. The resulting voids in the fiber are eventually filled by a “collapse” of the structure. The role of the amorphous areas of the polymer in controlling the tensile strength of the suture is discussed.     

A further link(s) with inequivalent representations in quantum field theory

In recent years it has become more apparent how useful inequivalent representations of the CCR and CAR in quantum field theory may be in describing and explaining physical phenomena, and several properties and concepts have been stated, referred to, and/or developed in the literature on these ideas. In this paper, some of these are reviewed, and some further properties and concepts are developed as further links in understanding these inequivalent representations in quantum field theory. One of these is a statement as to what actually breaks down in some field theories in the transformation between representations which are unitarily inequivalent. This is developed using the language and ideas of point quantum mechanical invariance, since this should be more familiar to a much larger number of physicists. Also, a statement on state expectation values is developed which can be used as a criterion for the occurrence of inequivalent representations of the CCR and CAR in field theories.     

Radiation boundary conditions for wave-like equations

In the numerical computation of hyperbolic equations it is not practical to use infinite domains. Instead, one truncates the domain with an artificial boundary. In this study we construct a sequence of radiating boundary conditions for wave-like equations. We prove that as the artificial boundary is moved to infinity the solution approaches the solution of the infinite domain as O(r^?m?1/2) for the m-th boundary condition. Numerical experiments with problems in jet acoustics verify the practical nature and utility of the boundary conditions.     

An inexpensive method for the determination of serum ionized calcium

The method described here may be used for the determination of serum ionized calcium. The physical conditions of the measurement, including sample volume (0.6 ml) and temperature (25 or 37 °C), are fixed. Stasis of the sample may contribute to a prolonged but not prohibitive response time. Other than the prolonged response time, the performance characteristics of the electrodes were not altered by incorporating them into the electrode chamber. Exemplifying the utility of this method were the results of protein-binding studies, in vitro and ex vivo CaCl₂ and EDTA administration and a comparison with the Orion SS-20. In consideration of the data presented, the cost and simplicity of construction, and method described should serve to make the determination of serum ionized calcium accessible to a greater number of investigators and encourage research in pharmacological and physiological processes thought to be mediated by serum ionized calcium.     

Correcting grade deflation caused by multiple-choice scoring

In a study involving three sections of 181 pre-calculus students at a four-year college, partial credit scoring on multiple-choice questions was examined over an entire semester. Questions were constructed by experienced teachers to pose carefully designed, incorrect alternatives, many of which implicitly suggested varying degrees of partial knowledge or understanding. Computing students' course grades based solely on percentage correct scoring resulted in a course-wide GPA of 1.38. This seemed inconsistent with the fact that students were screened for entry into the course. To avert apparent grade deflation due to such right/wrong scoring, student scores were recomputed based on supplementing percentage correct scores with partial credit. This resulted in a course-wide increase in GPA to 1.92. Using that same letter grade distribution, and curving percentage correct averages to achieve the 1.92 GPA, it was found that 10% of the students were given different grades from those determined by the partial credit scoring. Regarding the efficacy of awarding partial credit points, it was found that partial credit by itself measured achievement of relevant mathematical skills that were missed by percentage correct scoring. Overall, grades determined by partial credit scoring seemed more reflective of both the quantity and quality of student knowledge than grades determined by either traditional percentage correct scoring or any corrective curving.     

Determination of diisopropylfluorophosphate in rat plasma and brain tissue by headspace solid-phase microextraction gas chromatography/mass spectrometry

A simple, sensitive and rapid method for the determination of diisopropylfluorophosphate (DFP) in rat plasma and brain tissue using headspace solid-phase microextraction (HS-SPME) and gas chromatography/mass spectrometry (GC/MS) is presented. A 65 microm polydimethylsiloxane/divinylbenzene (PDMS/DVB) fiber was selected for sampling. The main parameters affecting the SPME process such as extraction and desorption temperature, extraction and desorption time, salt addition, and fiber preheating time were optimized in each matrix to enhance the extraction efficiency of the method. The lower limits of quantitation for DFP in plasma and brain tissue were 1 ng/mL and 3 ng/g, respectively. The method showed good linearity over the range from 1-100 ng/mL in plasma and 3-300 ng/g in brain tissue with correlation coefficient (R(2)) values higher than 0.995. The precision and accuracy for intra-day and inter-day were less than 10%. The relative recoveries in plasma and brain for DFP were greater than 50%. Stability tests including autosampler and freeze and thaw were also investigated. This validated method was successfully applied to study the neurobehavioral effects of low-level organophosphate exposures. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Renewable energy in the United States

A review of studies of biomass potential in the United States finds a wide variation in the estimates. A number of specific policy-relevant questions about the potential of biofuels in the United States are answered. A recently published global analysis of the potential conflict between land needed for bioenergy and land needed for food is extended to the situation in the United States. A renewable energy supply scenario, capable of meeting the 2001 US energy demand, indicates that there is enough land to support a renewable energy system but that the utilization of biomass would be limited by its land requirement.     

Chelation of vanadium(V) by difluoromethylene bisphosphonate, a structural analogue of pyrophosphate

The structural and functional analogy between difluoromethylene bisphosphonate (CF2PP) and pyrophosphate (PPi) is investigated in a reaction with V(V) in the form of vanadate. The reaction of CF2PP with vanadate was investigated using 1.00 M KCl as supporting electrolyte over the ranges 3 < or = [CF2PP] < or = 60 mM and 2.06 < or = pH < or = 11.80. 51V, 19F, and 31P NMR spectroscopic studies showed that a 1:1 species was formed with an H+-dependent formation constant of 110 M-1 at pH 7.22. Results of solution experiments and ab initio calculations are consistent with CF2PP coordinating V(V) in a bidentate manner, as previously reported for PPi. Below pH 4, a minor complex forms, which is consistent with a 1:2 stoichiometry. This complex was also observed with pyrophosphate. The X-ray crystal structure of the monoprotonated difluoromethylene bisphosphonate anion (H[CF2PP]3-)-toludine complex is presented. The H[CF2PP]3- anion crystallized in the triclinic space group P with a = 12.7629(7) A, b = 13.3992(7) A, c = 17.1002(9) A, and V = 2584.4(2) A3, and Z = 2. Sheets of the layers of anions are connected through a network of H-bonds and separated by a layer of toludine cations. The structural features are investigated, and the CF2PP anion was found to be longer and wider than the corresponding PPi. Given the larger size of this anion compared to PPi, the chelation affinity upon CF2 substitution was found to be 4-5-fold reduced at neutral pH.     

Analysis of polycyclic aromatic hydrocarbons by ion trap tandem mass spectrometry

An ion-trap mass spectrometer with a wave board and tandem mass spectrometry software was used to analyze gas chromatographically separated polycyclic aromatic hydrocarbons (PAHs) by using collision-induced dissociation (CID). The nonresonant (multiple collision) mode was used to determine the conditions for CID ionization of 18 PAHs. Unlike in electron impact (EI) analysis, the relative abundances of progeny ions of isomers were statistically different (using Student’s t-test) in CID analysis, thus making isomer identification by CID possible. For comparison, CID and EI were applied to the analysis of used motor oil. CID analysis was shown to be more sensitive than EI analysis of the used motor oil. Precision at the 10-ppb level for EI and CID showed relative standard deviations of 5. 2 and 7. 7%, respectively.