Scattering qualities

Quality control involves several subjects that must be discussed in connection with dry chemistry. It is the aim of all quality control methods that all results supplied by the clinical laboratory representing a useful estimation of actual data should approximate these actual data as closely as possible and with a low scatter. Quality assurance is subdivided into internal and external quality control. Internal quality control should accompany each series of examinations even if there is only one sample to be examined. In addition, external quality control serves to examine the precision and accuracy of a laboratory as compared to other laboratories. Any external quality control without internal quality control is meaningless. 

For optical transmission, tire parameters of greatest importance are attenuation (i.e. loss) and material dispersion. In effect tliey define tire limits of tire optical communication system. Loss, due to absorjDtion and scattering, limits tire lengtlis between tire transmission nodes. In transmission quality fibre, tire loss is in units of decibels per kilometre. 

Accuracy Under normal conditions relative errors of 1-5% are easily obtained with UV/Vis absorption. Accuracy is usually limited by the quality of the blank. Examples of the type of problems that may be encountered include the presence of particulates in a sample that scatter radiation and interferents that react with analytical reagents. In the latter case the interferant may react to form an absorbing species, giving rise to a positive determinate error. Interferents also may prevent the analyte from reacting, leading to a negative determinate error. With care, it maybe possible to improve the accuracy of an analysis by as much as an order of magnitude. 

Quartz also has modest but important uses in optical appHcations, primarily as prisms. Its dispersion makes it useful in monochromators for spectrophotometers in the region of 0.16—3.5 m. Specially prepared optical-quality synthetic quartz is requited because ordinary synthetic quartz is usually not of good enough quality for such uses, mainly owing to scattering and absorption at 2.6 p.m associated with hydroxide in the lattice. 

Suspended particles are the most important factor in visibility reduction. In most instances, the visual quality of air is controlled by partide scattering and is characterized by the extinction coeffident The size of particles plays a crucial role in their interaction with light. Other factors are the refractive index and shape of the particles, although their effect is harder to measure and is less well understood. If we could establish these properties, we could calculate the amount of light scattering and absorption. Alternatively, the extinction coeffident associated with an aerosol can be measured directly. 

Fig. 10-7. Scattering and absorption cross-section per unit volume as a function of particle diameter. Source Charlson, R. J., Waggoner, A. P., and Thielke, H. F., "Visibility Protection for Class I Areas. The Technical Basis," Report to the Council on Environmental Quality, Washington, DC, 1978.

Fig. 10-8. Single particle scattering to mass ratio for particles of four different compositions. Carbon particles are also very efficient absorbers of light. Source U.S. Environmental Protection Agency, "Protecting Visibility," EPA-450/5-79-008, Office of Air Quality Planning Standards, Research Triangle Park, NC, 1979.

Optical measurements permit the quantification of visibility degradation under different conditions. Several instruments are capable of measuring visual air quality, e.g., cameras, photometers, telephotometers, transmis-someters, and scattering instruments. 

Other measurements important to visual air quality are pollutant related, i.e., the size distribution, mass concentration, and number concentration of airborne particles and their chemical composition. From the size distribution, the Mie theory of light scattering can be used to calculate the scattering coefficient (20). Table 14-2 summarizes the different types of visual monitoring methods (21). 

The use of various statistical techniques has been discussed (46) for two situations. For standard air quality networks with an extensive period of record, analysis of residuals, visual inspection of scatter diagrams, and comparison of cumulative frequency distributions are quite useful techniques for assessing model performance. For tracer studies the spatial coverage is better, so that identification of meiximum measured concentrations during each test is more feasible. However, temporal coverage is more limited with a specific number of tests not continuous in time. 

Usually, particle size has relatively little effect on Raman line shapes unless the particles are extremely small, less than 100 nm. For this reason, high-quality Raman spectra can be obtained from powders and from polycrystalline bulk specimens like ceramics and rocks by simply reflecting the laser beam from the specimen surface. Solid samples can be measured in the 90° scattering geometry by mounting a slab of the solid sample, or a pressed pellet of a powder sample so that the beam reflects from the surface but not into the entrance slit (Figure 3). 

In order to determine the quality (or the validity) of fit of a particular function to the data points given, a comparison of the deviation of the curve from the data to the size of the experimental error can be made. The deviations (i.e., the scatter off the curve) should be of the same order of magnitude as the experimental error, so that the quantity chi-squared is defined as... 

The energy threshold appears to be dependent on the excitation spot size at constant pump intensity, which indicates that amplification occurs over the whole illuminated area. It should be stated that, despite the large domains, the optical quality within the domains is lower than that of the annealed films. This gives rise to additional scattering losses which decrease the magnitude of the amplification. 

It can be seen that k in Eq. (10) replaces the system-describing parameters L and Ah in Eq. (1). A direct test of the hypothesis is therefore to plot (j> against k for fixed values of P, G, and d, with L and Ah varying. For the hypothesis to be correct, the data points must all lie on a smooth curve. Experience shows, however, that plotting (f> against k often produces an undue amount of scatter which may obscure and distort any true relationship existing. This enhanced scatter is caused by the cumulative effect of experimental errors in the various terms in the heat-balance equation from which the quality k is derived. 

Thus, for the investigation of buried polymer interfaces, several techniques with molecular resolution are also available. Recently NMR spin diffusion experiments [92] have also been applied to the analysis of a transition zone in polymer blends or crystals and even the diffusion and mobility of chains within this layer may be analyzed. There are still several other techniques used, such as radioactive tracer detection, forced Rayleigh scattering or fluorescence quenching, which also yield valuable information on specific aspects of buried interfaces. They all depend very critically on sample preparation and quality, and we will discuss this important aspect in the next section. 

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