Caffeine solution

Caffeine is extracted from beverages by a solid-phase microextraction using an uncoated fused silica fiber. The fiber is suspended in the sample for 5 min and the sample stirred to assist the mass transfer of analyte to the fiber. Immediately after removing the fiber from the sample it is transferred to the gas chromatograph s injection port where the analyte is thermally desorbed. Quantitation is accomplished by using a C3 caffeine solution as an internal standard. 

Fig. 38.18. Score plot (PC2 v. PCI) based on non-centered PCA of Tl-curves from 9 panellists for a bitter (caffeine) solution.

In the other method, PLS-1, the absorption spectra of caffeine solutions were recorded between 240-320 nm and the absorbance values recorded every 5 nm. 

FIGURE 16 The chromatogram of an injection of a caffeine solution without the column showing the instrumental bandwidth of a Waters Alliance HPLC system with a 966 PDA detector with a standard flow cell. 

Experiment 4 is intended to check the detector s wavelength accuracy at 205nm and 273nm. Using the same caffeine solution as for the OQ test, this test is performed and calculated as in the procedure described earlier under subsection (i) of Detector in section (j) of the OQ guidelines. 

FIGURE 23 Comparison of absorbance spectra for baseline (top) and second-derivative (bottom) scattering corrections for a pure caffeine standard solution and a turbid caffeine solution. 

One of the approaches found most suitable to explain the sensorial properties of sweet, bitter, and sweet-bitter substances proves to be the physico-chemical approach especially as concerns hydration and surface properties (DeSimone and Fleck, 1980 Funasaki et al., 1996 Fimasaki et al., 1999 Mathlouthi and Hutteau, 1999). Thus, solution properties of sweet and bitter molecules were found informative on their type of hydration (hydrophobic or hydrophilic) and on the extent of the hydration layer (Fiutteau et al., 2003). Physico-chemical properties (intrinsic viscosity, apparent specific volume, and surface tension) and NMR relaxation rates of the aqueous solutions of sucrose, caffeine, and sucrose-caffeine mixtures were used in the interpretation of the taste modalities of these molecules and to explain the inhibition of caffeine bitterness by sucrose (Aroulmoji et al., 2001). Caffeine molecules were found to form an adsorption layer whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine gradually increases with concentration and is delayed when sucrose is added in the caffeine solution (Aroulmoji et al., 2004). 

One of the commercial methods for decaffeinating coffee is the direct-contact method. The unroasted coffee beans are first softened with steam and then brought in direct contact with a decaffeinating agent, such as dichloromethane (most often called methylene chloride in this context), CH2CI2. The caffeine dissolves in the dichloromethane, after which the dichloromethane/caffeine solution is removed from the beans. 

Once the polymer membrane was prepared, the photosensitive dithiocarba-mate group was used to initiate the polymerization of methylenebis(acrylamide) and acrylic acid in the presence of the template. These membranes selectively permeated caffeine in the filtration of a theophylline/caffeine solution, although the membrane substrates typically required up to twenty 24 h to prepare. 

Table 3.2 Calibration data for caffeine solutions (cf. SAQ 3.3). From Stuart, B., Biological Applications of Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1997. University of Greenwich, and reproduced by permission of the University of Greenwich...

It is also worth noting that for caffeine solutions a thermodynamic parameter such as the enthalpy of dimerization of caffeine in water can be determined comparatively by dissolution calorimetry and by high resolution NMR [118,119]. 

H Bothe, HK Cammenga. Calorimetric investigation of aqueous caffeine solutions and molecular association of caffeine. Thermochim Acta 69 235-252 (1983). 

After 24-h exposure to caffeine, the LC50 for Daphnia magna was recorded as 683.7 mg/L (Guilhermino et al., 2000) and an EC50 (immobilization) as 161.18 mg/L (Lilius et al., 1995). Freshwater invertebrate species, such as Ceriodaphnia dubia, Pimephales promelas, and Chironomus dilutus, commonly used in water monitoring tests, were exposed to aqueous caffeine solutions under static exposure for 48 h and... 

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