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At least 649 records · Page 36Linked to original sources
PubMed2003

Molar mass characterization of cationic methyl methacrylate-ethyl acrylate copolymers using size-exclusion chromatography with online multi-angle light scattering and refractometric detection.

Size-exclusion chromatography (SEC) combined with online multi-angle light scattering (MALS) and refractometric (RI) detection has been employed for the molar mass characterisation of water-insoluble cationic methyl methacrylate-ethyl acrylate copolymers (Eudragit RS and RL). Due to their positive charge, cationic polymers are particularly difficult to separate on a SEC column, in worst cases being completely adsorbed on the oppositely charged packing material. This work has examined how a careful addition of salt (LiCl) to the copolymer solution in ethanol decreases the electrostatic interactions, clearly seen as a decrease in elution volume from the SEC column as well as an improved recovery. At a certain level of ionic strength, typically about 50 mM, the copolymer recovery from the SEC column reached 100% and molar mass distributions corresponding to the complete sample could be obtained. The combined MALS/RI detection gives the opportunity to measure the absolute molar mass independent of recovery and retention. Thus, in this study, it turned out to be a favourable tool for tracing the changes in elution behaviour of the charged copolymer as the ionic strength was increased.

Acrylates↗
PubMed1997

Refractive-index detection by interferometric backscatter in packed-capillary high-performance liquid chromatography.

A simple, inexpensive, micro-volume refractive index detector has been applied to packed capillary high-performance liquid chromatography. Nanoliter detector volumes produced by a unique optical train, based on interferometric backscatter, allows for universal solute detection at the picogram level. System utility is demonstrated by reversed-phase separation of a test mixture containing ppm levels of NaCl, phenol, ethyl benzene and toluene.

Chromatography, High Pressure Liquid↗
PubMed1997

Characterization of antigen-antibody complexes by size-exclusion chromatography coupled with low-angle light-scattering photometry and viscometry.

In this paper, the molecular masses (M(r)s) of the complexes of monoclonal anti-BSA (antibody to bovine serum albumin) (clone: 33) and monomer BSA were determined on-line by using size-exclusion chromatography (SEC) coupled with a low-angle laser light-scattering (LALLS) detector and two concentration detectors, ultraviolet (UV) and refractive index (RI) (SEC-LALLS/UV/RI system). Also, the size and M(r)s of the complexes were evaluated by the SEC-LALLS/UV/viscometer (VISC) system. This study demonstrated that, for small size macromolecules, the combination of light scattering and viscosity detection was a suitable choice for determining their M(r)s and sizes.

Antibodies, Monoclonal↗
PubMed1998

Electrostatic effects on protein partitioning in size-exclusion chromatography and membrane ultrafiltration.

Although size-exclusion chromatography and membrane ultrafiltration are generally viewed as size-based separation processes, there is considerable evidence for the importance of electrostatic interactions. Experimental studies of size-exclusion chromatography and membrane ultrafiltration were performed in parallel using both neutral dextrans and charged proteins. Data for protein retention time and membrane sieving clearly indicate that the effective protein size increases with decreasing ionic strength due to the reduction in electrostatic shielding. These results were quantified using available theoretical models for the partitioning of charged solutes. The data clearly demonstrate the similarity of the electrostatic interactions and partitioning effects in size-exclusion chromatography and membrane ultrafiltration.

Chromatography, Gel↗
PubMed1986

Is the refractometer specific gravity a reliable index for pediatric fluid management?

Serial measurement of urinary output has been one of the mainstays to gauge adequacy of hydration of the newborn surgical patient. Urine output, however, is dependent not only on circulating blood volume but also on osmolar load and the ability of the kidney to excrete solute and water. Therefore, a second measurement is necessary for more accurate monitoring urine concentration. The purpose of this study is to determine the capacity of the kidneys of the newborn surgical patient to dilute and concentrate, and to excrete solute and water; and the accuracy of the refractometer to determine the tonicity of urine. To determine renal capacity, 24 newborn patients were studied during the first 48 hours following major surgical procedures. Analysis of 823 urine samples demonstrated a range of specific gravity between 1.002 and 1.040, and of osmolality between 67 and 582 mosm/kg. The lowest urine output was 0.1 mL/kg/hr and the highest 14.9 mL/kg/hr (mean 3.54 +/- 1.97 mL/kg/hr). The osmolar load excreted by the kidney ranged from 3.7 to 33.4 mosm/kg/24 hr (15.3 +/- 6 mosm/kg/24 hr). To determine the accuracy of the refractometer, 2,007 urine samples from 900 newborn infants were studied. The correlation coefficient for refractometer derived specific gravity and osmolality ranged from 0.7347 to 0.9061. Refractometer specific gravity measurements of 1.008 or below predict hyposmolality as defined as an osmolality below 270 mosm/kg with an accuracy of 70.6% to 95.4%. Accuracy is significantly less in the TPN and the glucose groups. A specific gravity of 1.012 to 1.020 is accurate in predicting hyperosmolality as defined as a urine osmolality above 290 mosm/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Diuresis↗
PubMed1988

Specific gravity test strips used in monitoring urine concentrations of urolithiasis patients.

Current therapy for urolithiasis patients includes instructions to increase water intake and 24-hour urine output. Previous studies have measured changes in the 24-hour urine volume to evaluate the efficacy of fluid therapy in each patient. We used paper test strips to monitor urine pH and specific gravity in 22 of our stone clinic patients: 10 were instructed to increase water intake just before the study (group 1) and 12 were not so instructed (group 2). Mean specific gravities of 1.0222 (1.0238 corrected for pH) for group 1 and 1.0197 (1.0220 corrected for pH) for group 2 did not differ significantly. Urine specific gravities also were compared for 3 intervals: 1 to 9 a.m., 9 a.m. to 5 p.m. and 5 p.m. to 1 a.m. Of the 22 patients 10 (3 from group 1 and 7 from group 2) had significant diurnal variations in the urine specific gravities, corrected and uncorrected, among these 3 periods. In addition, both groups had a significantly higher mean specific gravity from 1 to 9 a.m. (1.0234 uncorrected and 1.0248 corrected) than from 9 a.m. to 5 p.m. (1.0194 uncorrected and 1.0218 corrected). The 5 p.m. to 1 a.m. (mean of 1.0220 uncorrected and 1.0239 corrected) specific gravity did not differ significantly in either group. If 1.015 is the highest acceptable specific gravity of urine in stone patients, the findings suggest inadequate dilution of urine in these patients, whether or not they were instructed to increase water intake. Also, the significant diurnal variation in urine specific gravity would allow a nighttime triggering event at these hours of higher urine concentration.

Adult↗
PubMedDate not supplied

The optics of fundus examination.

It has been 140 years since the invention of the ophthalmoscope in 1851 by Herman von Helmholtz. Since then, a considerable amount of time and effort has been devoted to improving the clinician's view of the fundus both for diagnostic and therapeutic reasons. Recently, there have been some notable advances in this respect, such as the 90 diopter lens, the "2.2 pan retinal" lens, and numerous "wide angle" fundus-viewing contact lenses to which the Volk Quadraspheric fundus lens has been the most recent addition. This paper is a review of the optical principles underlying the methods of fundus examination currently available.

Fundus Oculi↗
PubMed2001

Gradient refractive index of the crystalline lens of the Black Oreo Dory (Allocyttus Niger): comparison of magnetic resonance imaging (MRI) and laser ray-trace methods.

The gradient refractive index of the crystalline lens in the Black Oreo Dory (Allocyttus Niger) was determined using two methods; an optimisation program based on finite ray-tracing and the path of laser beams through the lens, and magnetic resonance imaging (MRI) and the linear relationship between refractive index and nuclear transverse relaxation rates. The methods showed good agreement in the cortical zone of the lens, but the lack of free water in the core of the lens made MRI measurement impossible in this region. The laser-optimisation method gave mean values of 1.368 and 1.543 for the surface and core refractive indices respectively, with a radial distribution for the gradient refractive index given by n(r)=1.543-0.121r2-0.033r4-0.021r6.

Animals↗
PubMed2002

Age-related changes in refractive index distribution and power of the human lens as measured by magnetic resonance micro-imaging in vitro.

We report a new technique for non-invasively mapping the refractive index distribution through the eye lens using magnetic resonance micro-imaging. The technique is applied to map the refractive index distribution throughout the sagittal plane of 18 human eye lenses ranging in age from 14 to 82 years in vitro. The results are compared with standard models for the human eye lens. They confirm that the refractive index distribution, when plotted as a function of normalised lens radius, is a function of lens age and differs both between the equatorial and axial directions and between the anterior and posterior halves of the optical axis. The refractive index of the lens nucleus exhibits a significant reduction with age amounting to 3.4+/-0.6 x 10(-4) years(-1). The contribution of the gradient index (GRIN) to the lens power decreases by 0.286+/-0.067 D/year, accounting almost entirely for the estimated overall change in lens power with age for these lenses, which were probably in their most accommodated state. The results provide experimental verification of hypothesised changes in the GRIN that have previously been invoked as contributing to presbyopia and support the hypothesis that changes in the GRIN are sufficient to offset effects of increasing curvature of human lenses with age in their unaccommodated state.

Accommodation, Ocular↗
PubMed2002

Ocular wave-front aberration statistics in a normal young population.

Monochromatic ocular aberrations in 108 eyes of a normal young population (n=59) were studied. The wave-front aberration were obtained under natural conditions using a near-infrared Shack-Hartmann wave-front sensor. For this population and a 5 mm pupil, more than 99% of the root-mean square wave-front error is contained in the first four orders of a Zernike expansion and about 91% corresponds only to the second order. Comparison of wave-fronts aberrations from right and left eye in 35 subjects, showed a good correlation between most of the second- and third-order terms and a slight (but not clear) tendency for mirror symmetry between eyes.

Adaptation, Ocular↗
PubMed1997

"Lens mitochondria" in the retinal cones of the tree-shrew Tupaia belangeri.

In all mammals, the mitochondria of the cones of the retina are concentrated in the inner segment. Uniquely in tree-shrews (Tupaia, Scandentia, Mammalia), a "megamitochondrion" exhibiting highly specialized systems of densely packed cristae and a very electron dense matrix, is located apically in the inner segment. The ellipsoid is a solid body containing several megamitochondria and, towards its base, a large number of smaller mitochondria. The refractive index of isolated, but not oriented, inner segments of Tupaia belangeri is higher (XA = 1.405) than in any other mammal studied so far. The consistent geometrical pattern of the multilamellar crista-matrix systems, oriented longitudinally towards the outer segment, suggests an additional optical function of the megamitochondria.

Animals↗
PubMed1999

A new approach to the study of ocular chromatic aberrations.

We measured the ocular wavefront aberration at six different visible wavelengths (between 450 and 650 nm) in three subjects, using a spatially resolved refractometer. In this technique, the angular deviation of light rays entering the pupil at different locations is measured with respect to a target viewed through a centered pupil. Fits of the data at each wavelength to Zernike polynomials were used to estimate the change of defocus with wavelength (longitudinal chromatic aberration, LCA) and the wavelength-dependence of the ocular aberrations. Measured LCA was in good agreement with the literature. In most cases the wavefront aberration increased slightly with wavelength. The angular deviations from the reference stimulus measured using a magenta filter allowed us to estimate the achromatic axis and both optical and perceived transverse chromatic aberration (TCA), (including the effect of aberrations and Stiles-Crawford effect). The amount of TCA varied markedly across subjects, and between eyes of the same subject. Finally, we used the results from these experiments to compute the image quality of the eye in polychromatic light.

Adult↗
PubMed1998

Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime.

We calculate the forces of single-beam gradient radiation pressure laser traps, also called "optical tweezers," on micron-sized dielectric spheres in the ray optics regime. This serves as a simple model system for describing laser trapping and manipulation of living cells and organelles within cells. The gradient and scattering forces are defined for beams of complex shape in the ray-optics limit. Forces are calculated over the entire cross-section of the sphere using TEM00 and TEM*00 mode input intensity profiles and spheres of varying index of refraction. Strong uniform traps are possible with force variations less than a factor of 2 over the sphere cross-section. For a laser power of 10 mW and a relative index of refraction of 1.2, we compute trapping forces as high as approximately 1.2 x 10(-6) dynes in the weakest (backward) direction of the gradient trap. It is shown that good trapping requires high convergence beams from a high numerical aperture objective. A comparison is given of traps made using bright field or differential interference contrast optics and phase contrast optics.

Lasers↗
PubMed2001

Amylopectin aggregation as a function of starch phosphate content studied by size exclusion chromatography and on-line refractive index and light scattering.

Starches with a natural 65-fold span in covalently bound phosphate content were prepared from five different crops including sorghum, cassava, three potato varieties and an exotic ginger plant, Curcuma zedoaria, with extreme starch phosphate content. These starches were subjected to size exclusion chromatography with refractive index detection (SEC/RI). A simple and rapid method for starch solubilisation was used. The conditions during solubilisation (2 M NaOH) and separation (10 mM NaOH, 50 degrees C) were such as enabling > 94% recovery of the starch without detectable degradation. The aggregation properties of the starch was investigated using on line refractive index/multi angle laser light scattering (RI/MALLS) detection. Three major regions in the SEC profile were identified, consisting of large amylopectin aggregates, amylopectin particles with radius of gyration (Rg) of approx 200 nm (400 nm blocklets) and amylose. A procedure for correction of light scattering signals spread over the SEC profile as a result of aggregate tailing was developed. The significance of the relative amounts of these three molecular species on standard starch pasting parameters, as measured by a Rapid Visco Analyzer (RVA), was investigated. Starches with a high amount of amylopectin aggregates showed high peak viscosities. Moreover, very high amounts of starch bound phosphate or amylose appears to suppress the content of large aggregates resulting in low viscosity.

Amylopectin↗