Dynamic light scattering (DLS) is a powerful technique for the sizing of nanoparticles and characterization of their properties in the liquid phase. However, the precision of this technique remains unclear and therefore, in the current work, the precision of the DLS technique for sizing polystyrene latex suspensions and the uncertainty of the DLS data are estimated. Precise measurements of the short time correlation function at seven scattering angles and five different concentrations are performed for four kinds of polystyrene latex suspensions with diameters of 30–100 nm. The extrapolations of apparent diffusion coefficients to infinite dilution and to lower angles yield more precise values than those obtained at one angle and one concentration. The extrapolated particle size measured by DLS is compared to the size determined by a differential mobility analyzer (DMA) in air. Before the comparison, the intensity‐averaged size measured by DLS is recalculated to the number‐averaged size in the case of DMA from the particle size distribution. After the recalculation, consistent values of mean particle diameter are found to be between those obtained by DLS and DMA within the estimated uncertainties.
Uniform poly(ethylene glycol) (PEG) oligomers, with a degree of polymerization n=1-40, were separated by preparative supercritical fluid chromatography from commercial monodispersed samples. Diffusion coefficients, D, for separated uniform PEG oligomers were measured in dilute solutions of deuterium oxide (D(2)O) at 30 degrees C, using pulsed-field gradient nuclear magnetic resonance. The measured D for each molecular weight was extrapolated to infinite dilution. Diffusion coefficients obtained at infinite dilution follow the scaling behavior of Zimm-type diffusion, even in the lower molecular weight range. Molecular-dynamics simulations for PEG in H(2)O also showed this scaling behavior, and reproduced close hydrodynamic interactions between PEG and water. These findings suggest that diffusion of PEG in water is dominated by hydrodynamic interaction over a wide molecular weight range, including at low molecular weights around 1000.
Quantitativeness of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was elucidated using an equimolar mixture of uniform poly(ethylene glycol) (PEG) oligomers with no molecular weight distributions. Uniform PEG oligomers with degrees of polymerization n = 6-40 were separated from commercial PEG samples by preparative super-critical fluid chromatography. MALDI-TOF mass spectra of an equimolar mixture of the uniform PEG oligomers were recorded by adding a mixture of 2,5-dihydroxybenzoic acid as a matrix reagent and four chlorinated salts, i.e. LiCl, NaCl, KCl and RbCl. Remarkable non-quantitative effects were observed in the MALDI-TOF mass spectra in both the lower and higher molecular mass regions. At higher molecular masses greater than about 10(3), PEG oligomers with larger molecular mass yielded lower spectral intensities irrespective of the species of adduct cations and higher laser powers induced larger decreases in mass spectral intensities with the increase in their molecular masses. On the other hand, in the lower molecular mass region, less than about 10(3), the observed non-quantitative effect greatly depends on the species of adduct cations, indicating that the stability of the PEG-cation complex affects the MALDI-TOF mass spectral intensities of uniform PEG oligomers.
Mass discrimination effects in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) were quantitatively investigated using equiweight and equimolar mixtures of uniform polystyrene (PS) oligomers. Uniform PS oligomers were separated by preparative super-critical fluid chromatography (SFC) from commercial standard PS samples. The separated PS oligomers, with degrees of polymerization n = 2±25, have absolutely no molecular weight distributions. Equiweight and equimolar mixtures of uniform PS oligomers were accurately prepared by weighing by microbalance, and their spectra were recorded using a MALDI-TOF mass spectrometer. In the lower molecular weight region (less than about 103 ) the oligomers with lower molecular weights give lower mass spectral intensities, with no correlation with laser power. In contrast, higher laser powers yield a decrease of mass spectral intensities in the higher molecular weight region. These results clearly show that mass discrimination effects occur at lower and higher molecular weights depending on the laser power, and provide quantitative information about the discrimination. Using the data on equiweight and equimolar mixtures of PS oligomers, it was possible to calibrate the MALDI-TOF mass spectral data for an analysis of molecular weight distribution of a standard monodisperse PS sample with number-averaged molecular weight of 10 3 , and to compare it with the molecular weight distribution measured by analytical SFC. The result from the calibrated MALDI-TOF mass spectrum, however, does not agree perfectly with that from the SFC results, because undetectable peaks in MALDI-TOF mass spectra at lower and higher molecular weights could not be included in the calibration of peak intensities.
ABSTRACT:The Rayleigh ratio of toluene at 25°C for a wavelength of 632.8 nm was determined by static light scattering measurement of a certified polystyrene reference material PS 2400 with exactly evaluated or certified mass-average molecular mass (M w ). At first, static light scattering measurements were carried out for PS 2400 in toluene with a literature value of the Rayleigh ratio of toluene. Then, corrections of degree of depolarization, density fluctuations, and molecular mass dependence of refractive index increment were applied to the observed M w of PS 2400 to obtain apparent M w . Finally, from the ratio between the apparent M w and the certified M w , the Rayleigh ratio of toluene was reevaluated.
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