Adrian Caprita

Continuous permeation of analytes through a thin poly(dimethylsiloxane) membrane followed by sorbent trapping for their gas chromatographic monitoring

Parameters that affect the permeation of analytes across a thin (0.025 mm) flat membrane of polysiloxane in a membrane and trap interface coupled to a gas chromatograph are examined taking into account a mechanism of mass transport for porous and nonporous materials and the permeation of carrier gas in the direction opposite to that of the analytes. The best permeation rate was reached with hydrogen as carrier gas at a flow-rate between 3 and 5 ml/min, an agitation of analytes outside the membrane above 0.4 m/s and thin membrane. The parameters that affect the residual diffusion time of analytes in membrane were also examined. A practical application of the thin membrane in membrane and trap interface was demonstrated by gas chromatographic monitoring of six volatile organic compounds in air. The limit of detection is at the nmol/ml level depending on the time of trapping and the parameters that affect the permeation through the membrane.

The Influence of Milk Protein Content on the Surface Tension and Viscosity

From the organic milk components the proteins influence mostly the surface tension (σ) and the dynamic viscosity (h). The physical properties of milk are influenced by the dispersion degree of the components. Casein (diameter 0.1-0.005 μ) and albumin (diameter 0.015-0.005 μ) form colloid dispersions. Lactose (diameter 0.00067 μ) is molecular dispersed. Therefore is the viscosity and surface tension influenced by protein and not by lactose. Milk samples from 16 dairy cows, with ages between 3-10 years, in different physiological states were analysed. The experiment was repeated at an interval of two weeks, four times. For the measuring of s the dynamic stalagmometric method was used (stalagmometer Traube). The dynamic viscosity was determined with the Ostwald viscometer [1]. A positive correlation between the protein content and the two biophysical parameters was observed.

The Electric Conductivity as a Parameter for Milk Quality Appreciation

The main components of the milk are: proteins (casein, lactalbumin, lactoglobulin), lactose, fat and mineral salts. The lactose concentration depends on several factors among which the health condition of the cows and the acidifying of the milk. In our days the chlorine/lactose ratio is used as a parameter for appreciating the milk quality. Although milk is isotonic with blood, the two biological liquids are very different in mineral composition, in concentration, and also in the proportion of ionised and bound to proteins forms. The osmotic pressure of the milk remains constant, and therefore when the lactose content is lower, the electrolytes are rising. Mainly, the equilibration is due to the chloride, probably the NaCl. This explains the variation of the electric conductivity. There were analyzed milk samples from 16 cows in the first lactation period, twice a month, from March to May. The pH was determined with a digital pH-meter, the conductivity (σ) with the conductivity meter type OK-102/1 (Radelkis), the chloride ion was determined by the Mohr method and the lactose by refractometry (refractometer Abbe). The determinations were performed on fresh milk and after 24 hours. σ (in mS/cm) is in negative correlation with the lactose content (in g %) [1] and in positive correlation with the chloride ion (in g %), as presented in figure 1. We conclude that the electric conductivity may be used for the appreciation of milk quality. High values show in fresh milk a high chlorine/lactose ratio, which occurs in case of mastitis. This method may be used instead of the chlorine/lactose ratio, which is more difficult to determine. The electric conductivity increases also with the acidifying of the milk when from 1 molecule of lactose 4 molecules of lactic acid are formed due the lactic fermentation of lactose. The proportion of ionic dispersed salts increases too, when casein changes (by acidifying) from calciumcaseinat in free casein

THE CONDENSED PHOSPHATES AS INHIBITORS OF THE ZINC-ENZYME CARBONIC ANHYDRASE II

The mobilization of the zinc ion from the zinc enzyme BCA II by the condensed phosphates pyrophosphate (PP), tripolyphosphate (TPP), and higher polyphosphate (HPP) induces the inhibition of the enzymatic activity. The chelating capacity increases with the ligand concentration and with the length of the polyphosphatic chain. The activity is restored when the polyphosphate is hydrolyzed, becoming incapable of chelating the zinc ion, which is able to reform the zinc enzyme.