Optimizing the determination of haloacetic acids in drinking waters

Three methods are currently approved by the US Environmental Protection Agency for the compliance monitoring of haloacetic acids in drinking waters. Each derivatizes the acids to their corresponding esters using either acidic methanol or diazomethane. This study was undertaken to characterize the extent of methylation of these analytes by these methods, and to fully optimize methylation chemistries to improve analytical sensitivity, precision and accuracy. The approved methods were shown to have little to no esterification efficiencies for the brominated trihaloacetic acids (HAA3). Methylation with acidic methanol was determined to be more efficient and rugged than methylation with diazomethane. A new higher boiling solvent, tertiary-amyl methyl ether, is reported which has significantly improved methylation efficiencies for HAA3. Additional modifications to the method have been made that improve method ruggedness. The revised method, EPA Method 552.3, outperforms the currently approved methods, especially for HAA3.     

Photon correlation spectroscopy of crosslinked polystyrene networks in the swollen state: Influence of macromolecular polydispersity

We report measurements of the correlation function of light scattered from thermally excited longitudinal fluctuations in crosslinked polystyrene networks at equilibrium. In order to investigate the influence of the polydispersity of linear chain elements between successive branch points, we synthesized a network by means of anionic block copolymerization, using a mixture of two “precursor” polystyrenes of different molecular weights to initiate polymerization of a small amount of divinyl-benzene (DVB). The correlation function for fluctuations having a wave vector K, has the form of an exponential decay, exp (?Γ). The decay rate is given by Γ = D′K², where D′ represents the diffusion constant of longitudinal modes of the network. We also present measurements of the correlation function of light scattered from gels prepared by anionic block copolymerization of styrene and DVB with only one “precursor” polystyrene and by a free-radical process. In the latter case, the gels should have a wide distribution of linear chain lengths between branch points.     

Structural properties of partially charged poly (acrylic acid) gels as probed by light scattering

We report results of static light scattering for partially charged gels at different swelling degrees and different ionization degrees. We measured both the ensemble-average and the rms fluctuations of the scattered intensity by scanning through various positions in the gel. It is shown that the dynamic concentration fluctuations are, to the first order, the same as in semi-dilute solutions at the same concentration. The excess of scattering of the gel with respect to the solutions arises mainly from frozen-in scattering domains with spatial extent less than ∼ 300 Å.     

Asymptotically unbiased estimation of the second order tail parameter

We introduce a class of asymptotically unbiased estimators for the second order parameter in extreme value statistics. The estimators are constructed by means of an appropriately chosen linear combination of two simple, but biased, kernel estimators for the second order parameter. Asymptotic normality is proven under a third order condition on the tail behavior, some conditions on the kernel functions and for an intermediate number of upper order statistics. A specific member from the proposed class, obtained with power kernel functions, is derived and its finite sample behavior studied in a small simulation experiment.     

An off-axis Hartmann sensor for the measurement of absorption-induced wavefront distortion in advanced gravitational wave interferometers

We describe a novel off-axis Hartmann wavefront sensor, developed for the measurement of wavefront distortions induced in the mirrors (test masses) of advanced gravitational wave interferometers by residual absorption of the circulating laser power.     

Suppression of self-pulsing in Tm:YAlO3 lasers via current feedback

Self-pulsing of continuous-wave Tm:YAlO₃ lasers limits their use for a variety of important applications. We demonstrate for the first time that the pulsing can be suppressed via feedback to the pump diode laser, a technique that is suitable for both external resonator and monolithic lasers. We also show that the optical transfer function of the laser is that of an unstable relaxation oscillator.