Light scattering accuracy

Photometry 1. Number Light scattered Gives automatic particle sizing but accuracy... 

The light-scattering objects must track the flow accurately, ensuring that their velocity represents the fluid velocity with a high accuracy. The light-scattering objects are either in the flow as a natural impurity such as dust, or are artificially introduced into the airflow at an optimum concentration ( seeding ). 

Equation (37), or (37 ), may be expected to represent the turbidity as a function of concentration over the range in which (c/r )/(c/r°)o<4 with adequate accuracy. Molecular weight heterogeneity may affect the coefficients F2 entering the osmotic and light-scattering expression somewhat differentlywith the result that the values obtained for F2 by the two methods may not necessarily be identical if unfractionated samples are used. 

Further measurements appear necessary before the molecular weights of the amylopectin component of starches can be adequately characterized, and it may well be that light-scattering is the only method which can be satisfactorily applied to these polysaccharides of very high molecular weight. Certainly, it is the only method which enables studies of very dilute solutions to be made with high accuracy, particularly in the case of aqueous solutions. 

For accuracy in light-scattering measurement the proper choice of solvent is necessary. The difference in refractive index between polymer and solvent should be as large as possible. Moreover, the solvent should itself have relatively low scattering and the polymer-solvent system must not have too high a second virial coefficient as the extrapolation to zero polymer concentration becomes less certain for high A2. Mixed solvent should be avoided unless both components have the same refractive index. 

This is a valid empirical criterion, since sensitivity and hence precision decreases as the curve flattens. However, linearity does not directly measure the performance parameter of interest precision. In practice an analyst would accept curves at high concentrations, providing the precision is still adequate and providing the method does not have accuracy problems at high concentrations e.g., because of light scattering in absorption methods. 

In the past, analyses of submicron particles have been limited to time-consuming techniques, such as electron microscopy, or, to methods such as light scattering, which require a fairly narrow size distribution for accuracy. Recently, reports of a number of studies of a new method have been published in which modifications... 

Selected entries from Methods in Enzymology [vol, page(s)] Anisotropy effects, 261, 427-430 determination by dynamic laser light scattering (quasi-elastic light scattering), 261, 432-433 determination for nucleic acids by NMR [accuracy, 261, 432-433 algorithms, 261, 11-13, 425, 430 carbon-13 relaxation, 261, 11-12, 422-426, 431, 434-435 cross-relaxation rates, 261,419-422, 435 error sources, 261, 430-432 phosphorus-31 relaxation, 261, 426-427, 431 proton relaxation, 261,51,418-422 relaxation matrix calculations, 261,12] deuterium solvent viscosity effects, 261,433 effect... 

The light scattering method is considerably more involved (theoretically and practically too) than the other methods. It can be used at extremely low concentrations of solution with sufficient accuracy. A wide range of molar mass (from a few hundreds to a few lakhs) can be determined and it is one of the most reliable methods for determining the shapes of large molecules. 

GPC is a relative method, especially for UV and refractive index (RI) detectors, and must be calibrated using polymer standards whose molecular weight has been determined using absolute methods such as intrinsic viscosity or light scattering. Consequently, the accuracy is relative to the calibration. 

In view of experimental simplicity and accuracy, viscosity measurements are extremely useful for routine relative molecular mass determinations on a particular polymer-solvent system. K and a for the system are determined by measuring the intrinsic viscosities of polymer fractions for which the relative molecular masses have been determined independently - e.g. by osmotic pressure, sedimentation or light scattering. 

Since the accuracy of the light scattering apparatus and the reliability of the inversion procedure had been proven by comparison of four methods on Dow polystyrene latex LS-1028-E (6), it was decided to examine a larger latex in more detail as a further study of the method of determining the internal structure of a latex by wide-angle light scattering. 

Wide angle light scattering experiments have been carried out with sufficient accuracy to separately identify dust or extraordinary particles, latex particles and small-molecule scattering. 

The basic limitations to the overall accuracy of the data presented here lie in the Raman measurement process - inherently weak, but possessing sufficient intensity as utilized here to produce, for example, only 5-7% standard deviations for instantaneous temperature determinations in a "calibrated" premixed laminar flame (9). Further development of this light scatter-... 

Particle size and distributions can be determined by a number of different methods. The technique described here is light scattering. Different measurement methods produce different results which can be correlated experimentally. The absence of distribution standards for light scattering particle sizing instruments precludes any determination of size accuracy. This is further complicated by particles of non-spherical shape which makes the concept of size very difficult to define. However, for particulate materials encountered in most industrial processes, the assumption that particles are spheres produces quite useful results that are repeatable and relate to important parameters of many processes. 

Data from a number of different particle size analysis instrumental methods including light scattering, field flow fractionation, hydrodynamic chromatography and microscopy were obtained for a series of polymethylmethacrylate latexes and were compared to DCP results (2). These and other comparative results have demonstrated the accuracy of the instrument and method. The reproducibility and precision of the instrument also were studied and are reported elsewhere ( 1 ). 

The major limitation of LA-ICP-MS is the need for standards that closely match the properties of the samples. In some cases it is possible to use NIST glass standard reference materials for calibration in the analysis of geological materials [67,68], Internal standardization employing MS signals from elements at known concentrations has been used to improve precision and accuracy. Other techniques, such as acoustic [69] and light scattering [70] measurements, have been used in an attempt to monitor the relative amount of material ablated. These approaches seem to work well for variations in the amount of material sampled for similar sample matrices but not for very different types of solids. Dual-sample introduction systems with either wet [71] or dry [72] aerosol introduction in addition to laser ablation have also been reported. 

The bio-activity of synthesized samples was estimated from mobility parameters of unicellular microorganisms measured by Dynamic Light Scattering (DLS). The following characteristics of cell population were measured cell concentration (units/ml) % of mobile cells mean translation velocity (pm/s), cell rotational frequency of (Hz) flagella beat frequency (Hz), distribution of beat frequencies, and kinetic energy of cell motion in a viscous environment (arbitrary units). Details of experimental equipment and measurement procedures may be found elsewhere.7,8 All parameters were measured with an accuracy of about 2%.9... 

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