Refractive index sample problem

External reflection. This is not as well developed a technique as internal reflection the physics of reflection of light from surfaces is less accommodating to the infrared spectroscopist. Smooth or shiny surfaces are particular problems. Specular reflection from the surface itself is governed by Fresnel s equations—the reflectance depends on a complicated combination of refractive index, sample absorbance and polarisation. Consequently, samples where the reflectance is mainly from the surface give rise to spectra which bear little relation to conventional transmission spectra. A transformation known as the Kramers-Kronig transformation does exist which attempts to convert a specular reflectance spectrum into a conventional-looking one. It is not 100% successful, and also very computer-intensive. For these reasons, specular reflectance is not commonly used by the analyst. 

The common contrast modes include polarized light, phase contrast, differential interference contrast, and Hoffman modulation contrast [5]. Depending on the nature of the polymer, such as refraction index, sample thickness, and optical anisotropies in the materials, different modes of transmission optical microscopy can be employed by mounting special accessories in a classic optical microscope to overcome different problems. For example, a polarizer and analyzer can be mounted before and after the sample to construct a polarized light microscope, commonly used for semicrystalline polymers a phase plate and phase ring can be added to construct a phase contrast optical microscopy, which is common for studying a noncrystafline multiphase polymer system. 

A more difficult criterion to meet with flow markers is that the polymer samples not contain interferents that coelute with or very near the flow marker and either affect its retention time or the ability of the analyst to reproducibly identify the retention time of the peak. Water is a ubiquitous problem in nonaqueous GPC and, when using a refractive index detector, it can cause a variable magnitude, negative area peak that may coelute with certain choices of totally permeated flow markers. This variable area negative peak may alter the apparent position of the flow marker when the flow rate has actually been invariant, thereby causing the user to falsely adjust data to compensate for the flow error. Similar problems can occur with the elution of positive peaks that are not exactly identical in elution to the totally permeated flow marker. Species that often contribute to these problems are residual monomer, reactants, surfactants, by-products, or buffers from the synthesis of the polymer. 

A further benefit of the low absorbtivity of most samples in the NIR is that measurements involving scattering effects (both diffuse transmission and diffuse reflectance) are possible. The penetration depth of the sampling beam in diffuse reflectance measurements of powders in the NIR can be on the scale of millimeters, despite the numerous refractive index interfaces and extended optical path involved. Thus relatively large volumes of material can be interrogated, avoiding problems of surface contamination and sample nonhomogeneity. 

Analytical Method Development for TRIS. The detection of brominated compounds of very low volatility such as TRIS posed special analytical problems. Since TRIS has no recognizable chromophore, the detection systems which are commonly used with high performance liquid chromatography (hplc), such as refractive index or short wavelength (<220 nm) uv detectors, are too non-specific to be of much practical use for the analysis of environmental samples. Furthermore, the sensitivities available with these detection methods are generally inadequate. 

Refractive-index detection is seldom used for many of the same reasons just mentioned for the spectroscopic detection of amino acids in their native forms. In fact, these problems are even more severe. Refractive-index detection has almost no selectivity whatsoever. Nearly every sample component passing the detector will register a signal. Also, this makes refractive index entirely incompatible with gradient elution. Even for isocratic separation, and detection of only a select few amino acids, refractive index can be very troublesome because of the detector s tendency to drift due to temperature changes in the laboratory (perhaps newer models have fixed this problem ). Finally, detection limits tend to be very poor for refractive-index detection. 

Another point of interest was the time required to equilibrate the system after changes were made in solvent composition. While the ChromSpher Lipids column had a column volume of ca. 3 ml, an increase in ACN concentration was not noted until the introduction of 7-8 ml of solvent (determined with refractive index detector). The problem of ACN-silver ion interaction and subsequent ACN retention is not new and may be noted in all forms of chromatography employing silver ions in the stationary phase. In the isocratic system, the column was equilibrated with the appropriate solvent mix for at least 0.5 h before sample injection. Since ACN dissolves very slowly into hexane, the ACN-hexane solvent mix was thoroughly stirred for 5 min before use. To obtain reproducible retention times, thorough mixing of the ACN and hexane is essential. 

One major problem of all these techniques is the sensitivity in the parameter selected to detect the presence of inhomogeneities. With visible light for example, inhomogeneous samples can appear transparent if the difference in the refractive index between the phases is less than 0.01. Staining (in the case of transmission electron microscopy, TEM), or chemical etching (in the case of scanning electron microscopy, SEM), can be helpful in revealing the structure. 

Another approach, which is used in this experiment, is to develop the analytical separation on the high-surface-area packing and increase the amount injected into the column to determine the loading level for preparative work. A common problem in preparative LC is detector saturation. Detector saturation occurs when the concentration of sample eluting from the column is so high that the detection system is electronically overloaded. The result of detector saturation is loss of the ability to observe the peaks. This is demonstrated in this experiment when the spectrophotometer is saturated, and the refractive index detector is not. 

Where R is the reflectivity and d is the thickness. Very accurate values of R and T are needed when the absorptance, (id, is small. The technique of photothermal deflection spectroscopy (PDS) overcomes this problem by measuring the heat absorbed in the film, which is proportional to ad when ad 1. A laser beam passing just above the surface is deflected by the thermal change in refractive index of a liquid in which the sample is immersed. Another sensitive measurement of ad is from the speetral dependence of the photoconductivity. The constant photocurrent method (CPM) uses a background illumination to ensure that the recombination lifetime does not depend on the photon energy and intensity of the illumination. Both techniques are capable of measuring ad down to values of about 10 and provide a very sensitive measure of the absorption coefficient of thin films. 

The same information is obtained if consecutive decisions are reached between choices with different values of m, either as alternatives or as multiple choice questions (Fig. 3.3-11). This allows to calculate how much information a certain analytical procedure may supply or how much information is needed to solve a particular analytical problem. An analytical measure, such as a melting point or a refractive index with 3 significant digits, may supply Id 999 = 9.96 = 10 bit. In order to identify one compound in a spectral collection of N different samples, Id N bit are required to identify one out of 100 000 spectra, at least 16.61 bit are needed. 

Air dissolved in the sample will usually be eluted close to the dead volume and will modify the refractive index of the mobile phase. The change in refractive index will produce a detector disturbance when the refractive index detector or any light absorption detector is being used. The problem can be easily eliminated by degassing the sample with helium, provided the sample does not contain any volatile components. 

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