Energy retention - measurement

The net retention volume and the specific retention volume, defined in Table 1.1, are important parameters for determining physicochemical constants from gas chromatographic data [9,10,32]. The free energy, enthalpy, and. entropy of nixing or solution, and the infinite dilution solute activity coefficients can be determined from retention measurements. Measurements are usually made at infinite dilution (Henry s law region) in which the value of the activity coefficient (also the gas-liquid partition coefficient) can be assumed to have a constant value. At infinite dilution the solute molecules are not sufficiently close to exert any mutual attractions, and the environment of each may be considered to consist entirely of solvent molecules. The activity... 

A simplified version of equation (18), which does not contain the pressure-dependent exponential term, can be used to describe the temperature dependence of retention measurements. In this case retention volumes are measured at different temperatures and an infinite dilution of the sample. This simplified equation expressed in terms of the adsorption energy has the following form [144] ... 

Animal calorimetry methods of measuring heat production and energy retention... 

ANIMAL CALORIMETRY METHODS FOR MEASURING HEAT PRODUCTION AND ENERGY RETENTION... 

Calorimetry means the measurement of heat. The partition of food energy presented in Fig. 11.2 shows that if the ME intake of an animal is known, then the measurement of its total heat production will allow its energy retention to be calculated by difference (likewise, the measurement of energy retention will allow heat production to be calculated). In practice, measurement of either heat production or energy retention is used to establish the NE value of a food. 

The methods used to measure heat production and energy retention in animals can be quite complicated, both in principle and in practice. In the past, the complexity and cost of the apparatus required for animal calorimetry limited its use to a small number of nutritional research establishments. Improved fimding of research has gradually removed this restriction, but even so, animal calorimetry remains a specialised topic and few nutritionists become involved in it. Nevertheless, the study of animal calorimetry on paper (as in this book) is valuable to all students of nutrition, because it reinforces their knowledge of the principles of energy metabolism. In the pages that follow, the principles of the methods used in animal calorimetry are explained within the main body of the text, and the apparatus employed is described in Boxes 11.3,11.4 and 11.5. 

The heat production of animals can be measured physically using a procedure known as direct calorimetry. Alternatively, heat production can be estimated from the respiratory exchange of the animal. For this, a respiration chamber is normally used and the approach is one of indirect calorimetry. Respiration chambers can also be used to estimate energy retention rather than heat production, by a procedure known as the carbon and nitrogen balance technique. 

Measurement of energy retention by the carbon and nitrogen balance technique... 

The advantages of the carbon and nitrogen balance technique are that no measure of oxygen consumption (or RQ) is required and that energy retention is subdivided into that stored as protein and that stored as fat. 

Energy retention may be estimated indirectly as ME intake minus heat production, or directly from the animal s carbon and nitrogen retention in a respiration chamber. It may also be measured using the comparative slaughter technique, where a known amount of ME is given and body composition is measured at the beginning and end of the experiment. 

The energy retention of the sheep used in Question 1 was measured by placing it in a respiration chamber. Whilst in the chamber, the sheep consumed 536 1 oxygen and excreted 429 1 of carbon dioxide, 45.8 1 of methane and 19.0 g of urinary nitrogen. Using the Brouwer equation, calculate its heat production and energy retention. 

Scott and Beesley [2] measured the corrected retention volumes of the enantiomers of 4-benzyl-2-oxazolidinone employing hexane/ethanol mixtures as the mobile phase and correlated the corrected retention volume of each isomer to the reciprocal of the volume fraction of ethanol. The results they obtained at 25°C are shown in Figure 8. It is seen that the correlation is excellent and was equally so for four other temperatures that were examined. From the same experiments carried out at different absolute temperatures (T) and at different volume fractions of ethanol (c), the effect of temperature and mobile composition was identified using the equation for the free energy of distribution and the reciprocal relationship between the solvent composition and retention. 

Fig. 17. A schematic of the alkane line obtained by inverse gas chromatography (IGC) measurements. The relative retention volume of carrier gas required to elute a series of alkane probe gases is plotted against the molar area of the probe times the. square root of its surface tension. The slope of the plot is yielding the dispersion component of the surface energy of...

Valko et al. [37] developed a fast-gradient RP-HPLC method for the determination of a chromatographic hydrophobicity index (CHI). An octadecylsilane (ODS) column and 50 mM aqueous ammonium acetate (pH 7.4) mobile phase with acetonitrile as an organic modifier (0-100%) were used. The system calibration and quality control were performed periodically by measuring retention for 10 standards unionized at pH 7.4. The CHI could then be used as an independent measure of hydrophobicity. In addition, its correlation with linear free-energy parameters explained some molecular descriptors, including H-bond basicity/ acidity and dipolarity/polarizability. It is noted [27] that there are significant differences between CHI values and octanol-water log D values. 

The HcReynolds abroach, which was based on earlier theoretical considerations proposed by Rohrschneider, is formulated on the assumption that intermolecular forces are additive and their Individual contributions to retention can be evaluated from differences between the retention index values for a series of test solutes measured on the liquid phase to be characterized and squalane at a fixed temperature of 120 C. The test solutes. Table 2.12, were selected to express dominant Intermolecular interactions. HcReynolds suggested that ten solutes were needed for this purpose. This included the original five test solutes proposed by Rohrschneider or higher molecular weight homologs of those test solutes to improve the accuracy of the retention index measurements. The number of test solutes required to adequately characterize the solvent properties of a stationary phase has remained controversial but in conventional practice the first five solutes in Table 2.12, identified by symbols x through s have been the most widely used [6). It was further assumed that for each type of intermolecular interaction, the interaction energy is proportional to a value a, b, c, d, or e, etc., characteristic of each test solute and proportional to its susceptibility for a particular interaction, and to a value x, X, Z, U, s, etc., characteristic of the capacity of the liquid phase... 

Retention in HIC can be described in terms of the solvophobic theory, in which the change in free energy on protein binding to the stationary phase with the salt concentration in the mobile phase is determined mainly by the contact surface area between the protein and stationary phase and the nature of the salt as measured by its propensity to increase the surface tension of aqueous solutions [331,333-338]. In simple terms the solvopbobic theory predicts that the log u ithn of the capacity factor should be linearly dependent on the surface tension of the mobile phase, which in turn, is a llne2u function of the salt concentration. At sufficiently high salt concentration the electrostatic contribution to retention can be considered constant, and in the absence of specific salt-protein interactions, log k should depend linearly on salt concentration as described by equation (4.21)... 

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