Threshold dilution

Methods have been developed to characterise odours according to strength (1) and offensiveness (2) using experienced panellists. Measures of offensiveness are necessarily subjective and odour strength (threshold dilution) will depend upon the odour threshold value as well as the concentration. For routine assessment these methods incur the cost of panel time and the problems inherent in the transport and storage of samples. 

In order to determine the statistical distribution of amniotic fluid samples taken at different gestation weeks, two relations are studied rupture of foam films (VP = 0) and development of RDS, and formation of a bilayer foam film (W = 1) and normal respiratory status of neonates. These correlations allowed to develop a new diagnostic method for estimation of lung maturity [20]. The function of the threshold dilution of various amniotic fluid samples (corresponding to C,) on the gestation age and the clinical results (i.e. yes/no RDS in neonates) is given in Fig. 11.5. The respiratory status of the neonates is studied with the screening system of Masson et al. [26], modified by Hobel et al. [29]. 

The linear dependence between the threshold dilution and the initial total phospholipid concentration (respectively, DPPC) found allows to determine the threshold dilution for a 100% probability for formation of NBF instead of Ct. Fig. 11.5 shows that if a sample dilution of 3.1 times is applied, then it is possible to detect almost all cases with a developed RDS. Therefore, the threshold dilution of 3.1 times allows to distinguish the mature from immature AF samples which gives a good reason to employ it in diagnosing of RDS, and respectively, to estimate the lung surfactant deficiency. Hence, the formation of black foam films from AF samples taken at different gestation weeks and diluted 3.1 times, indicates that there is no risk of RDS, while film rupture predicts an eventual RDS development. 

Fig. 11.5. Dependence of the threshold dilution d, of amniotic fluid on its gestation age clinical results...

More details about the black foam film diagnostic method can be found in [20,24,30,31] and the latest results in [32]. The clinical data accumulated during the last years indicate a decrease in the threshold dilution down about 2.5. 

The phase state of AF foam bilayers has been studied from the effect which the temperature exerts on the threshold dilution (i.e. threshold concentration) for formation of foam bilayers (NBFs) from AF [38]. Samples of AF were collected during the 39th gestation week. Microscopic foam films were formed from diluted AF in the temperature range from 10°C to 30°C. The values of d, were found from the dependence probability W for observation of foam bilayer on dilution d at different temperatures. Such a dependence is shown in Fig. 11.6,a for one of the samples at 15°C. A sharp drop in W value at d, is seen in the figure. 

The values of the threshold dilution d, for different temperatures are presented in Fig. 11.7 in Arrhenius co-ordinates for the five samples investigated. As it is seen, linear dependences of similar slope were obtained within the temperature range from 10°C to 30°C. The temperature dependences of d, were determined as concentrations higher than the critical electrolyte concentration Ceixr for formation of foam bilayers from amniotic fluid and it was found that d, is not a function of the electrolyte concentration. 

Fig. 11.7. Arrhenius dependence of the threshold dilution d, for AF foam bilayers (sample 1 to 5) ...

This extreme stability of AF foam bilayers allowed to assume that C, = Ce and to use the data for the temperature dependence of threshold dilution (shown in Fig. 11.7) for determination of Q (binding energy) for each sample of amniotic fluid. For this reason Eq. (11.2) resulting from Eq. (11.1) and Eq. (3.115) can be used... 

The above mentioned threshold dilution d, and critical concentration for formation of a bilayer are used as measures for bilayer stability [19] being determined by the first neighbour lateral and normal interactions in the foam bilayer. This is the difference of the parameter d, from the change in the free surface energy which is usually used as a measure of the surface activity. Thus, the parameters d, and C, are proposed as new characteristics of the surface activity of an amphiphile molecule, evaluated with high accuracy from the sharp W(d) and W(C) dependences, respectively. 

The flavor portion of a flavor compound gives it its name, acceptabiUty, and palatabiUty, and provides character fixation of the flavor, ie, relatively high boiling point soflds, usually in combination, are used at concentrations above thek threshold values at use level so that upon dilution the levels remain above threshold value and the perception of the flavor does not change. 

Water Quality Standards. Water quaUty standards are usually based on one of two primary criteria, stream standards or effluent standards. Stream standards are based on dilution requirements for the receiving water quaUty based on a threshold value of specific pollutants or a beneficial use of the water. Effluent standards are based on the concentration of pollutants that can be discharged or on the degree of treatment required. 

Taste and Odor. The measurement of taste and odor is somewhat subjective and depends on the personal judgements of individuals. Panels of not less than five observers, and preferably more than ten, are used. The sample is diluted with odor-free water until a ratio at which the odor is just perceptible is determined this ratio is called the threshold odor number (TON). A similar method is used to detect a distinct taste in water (see Flavor characterization). ... 

The first element, dynamic dilution, provides a reproducible sample for each panelist. The system must minimize the loss of the odorant to the walls of the delivery apparatus, provide clean dilution air of odor-free quality, maintain a constant dilution ratio for the duration of a given test, and have no memory effect when going from high to low concentrations or switching between odorants of different character. The type of mask or port and the delivery flow rate have been found to influence the response of panelists in determining odor threshold and intensity. 

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