Reference substance

Table II, 9, A.—Reference Substances for the Calibration of Thermometers BY THE Melting Point Method...

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

If a sample substance (S) has been compared against one standard compound (A) to give but comparison with another standard (B) is required (5sb)> then this change can be effected easily if the relation between the two standards 5, 8 is known (Figure 48.4). For delta values, the order in which the suffixes appear is important. For a sample S measured against reference substance A, delta is written as 5sa- This is not the same as 6 5, as can be seen in Figure 48.4. 

Polyethylene. The crystal structure of this polymer is essentially the same as those of linear alkanes containing 20-40 carbon atoms, and the values of Tjj and AHf j are what would be expected on the basis of an extrapolation from data on the alkanes. Since there are no chain substituents or intermolecular forces other than London forces in polyethylene, we shall compare other polymers to it as a reference substance. 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

Correlation methods discussed include basic mathematical and numerical techniques, and approaches based on reference substances, empirical equations, nomographs, group contributions, linear solvation energy relationships, molecular connectivity indexes, and graph theory. Chemical data correlation foundations in classical, molecular, and statistical thermodynamics are introduced. 

Numerous other methods have been used to predict properties of gases and Hquids. These include group contribution, reference substance, approaches, and many others. However, corresponding states theory has been one of the most thoroughly investigated methods and has become an important basis for the development of correlation and property estimation techniques. The methods derived from the corresponding states theory for Hquid and gas property estimation have proved invaluable for work such as process and equipment design. 

Reference Substances. Use of a reference substance has its origins in the work of Clausius-Clapeyron and equation 73, a form of equation 7 ... 

In another type of reference plot the temperature of a property of compound A is plotted vs the temperature of the reference substance at equal vapor pressures values. 

The values for a single property of two compounds, A and B, are useless unless these values are compared at equal temperature or pressure. Then a deviation from some intermediate value can be determined. If this intermediate value is chosen to be the value of one particular substance, ie. A, the reference substance, both A and B can then be expressed as functions of the reference substance. One very simplistic example is specific gravity where the density of a compound is expressed as the actual density divided by the density of water at 4°C and water is the reference substance. 

By use of Othmer plots of reference substances, large tables of thermodynamic data can be expressed as simple correlations which are extremely accurate and easy to use. The real power of these correlations is the abiUty to interpolate and extrapolate the correlations beyond the experimental values with considerable accuracy. Mathematically stated... 

Correlations based on reference substances are limited to compounds which have experimentally determined values, but the number of data points needed to produce a correlation is relatively small. A reference substance should be as chemically and physically compatible to the chemical with which it is being compared as possible. Use of reference substances in the ideal assumes no property deviations, thus the smaller the deviations, the lower the absolute error in the correlations. 

Values for many properties can be determined using reference substances, including density, surface tension, viscosity, partition coefficient, solubihty, diffusion coefficient, vapor pressure, latent heat, critical properties, entropies of vaporization, heats of solution, coUigative properties, and activity coefficients. Table 1 Hsts the equations needed for determining these properties. 

Compared to the theory of corresponding states, the reference substance method gives highly accurate results for compounds having sparse experimental data. The corresponding states method gives moderate accuracy for numerous compounds even without actual data. 

For nonKydi ocai bon organics for which normal boiling points are unknown or expected vapor pressures are below 15 kPa, the reference substance method of Othmer and YiF as given by Eq. (2-44) is recommended. 

GENERAL BASIS FOR GRUG REFERENCE SUBSTANCES ATTESTATION IN UKRAINE... 

The pharmaceutical reference substance system, accepted in USSR, did not meet to modern requirements. For creation of a modem system in Ukraine it was required to develop criteria to uncertainty of analytical phamiaceutical procedures. 

The principle of insignificancy , enabling use of the given principle for any level of probability is substantiated. The systematic application of the given principle results in developing metrological criteria for pharmaceutical reference substance, analytical validation, evaluation of results of interlaboratory testing and suitability of the analytical equipment for the phamiaceutical analysis. 

On basis of developed criteria, the theoretical background of reference substances elaboration is developed, and also ai e developed procedures of elaboration and documentation system for the Ukrainian State Phamiacopoeia RS and for working RS of the phamiaceutical plants. These approaches ai e successfully applied to elaboration of RS for inter-laboratory testing. 

The protein polymers are highly sterospecific, the aminoacid residues always adopting the L-configuration, i.e. the same configuration as the reference substance L-malic acid. 

Quantification at surfaces is more difficult, because the Raman intensities depend not only on the surface concentration but also on the orientation of the Raman scat-terers and the, usually unknown, refractive index of the surface layer. If noticeable changes of orientation and refractive index can be excluded, the Raman intensities are roughly proportional to the surface concentration, and intensity ratios with a reference substance at the surface give quite accurate concentration data. 

Sometimes called relative density, specific gravity is the ratio of the fluid density with respect to a reference substance at a specified temperature. 

Atomic Absorption An analytical method in which the sample is converted into a vapor by passing it through a flame or other energy source and the absorbance at a particular wavelength is measured and compared with that of a reference substance. The absorbance measured is proportional to the concentration ot that substance in the sample. 

Samples and reference substances should be dissolved in the same solvents to ensure that comparable substance distribution occurs in all the starting zones. In order to keep the size of the starting zones down to a minimum (diameter TLC 2 to 4 mm, HPTLC 0.5 to 1 mm) the application volumes are normally limited to a maximum of 5 xl for TLC and 500 nl for HPTLC when the samples are applied as spots. Particularly in the case of adsorption-chromatographic systems layers with concentrating zones offer another possibility of producing small starting zones. Here the applied zones are compressed to narrow bands at the solvent front before the mobile phase reaches the active chromatographic layer. 

Tlie specific gra ity (SG) is the ratio of tlie density of a substance to tlic density of a reference substance at a specific condition. 

The specific gravity (G) of a substance is the ratio of the density (p) of the substance to the density of a reference substance at specified conditions. That is,... 

For solids and liquids, the density is a weak function of pressure and, therefore, the temperatures T and T, ., are usually stated. Also, the reference substance is commonly taken as water at 4°C at which p = 1.000 g/cm = 62.43 Ib/ft . If a single temperature is stated, it implies that both densities have been measured at that temperature. 

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