External Vacuum Degassing

A simple and effective form of degassing is to hold a flask of mobile phase under a vacuum while agitating the contents in an ultrasonic bath (Fig. 3.7). The eluent is then transferred to the chromatograph for several hours of reliable operation, especially if the eluent is blanketed with an inert gas such as helium. This approach is particularly useful for clean solvents that readily absorb gases such as carbon dioxide from the atmosphere. The eluent should be degassed in the solvent storage container used for chromatography to minimize contact with the air. 

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Bulk Si/Al ratios were determined by AAS. Surface areas and pore volumes were determined by N2 absorption isotherms measured at liquid nitrogen temperature using a Micromeritics ASAP 2000M (Table 1). The zeolites were degassed under vacuum at 150°C for the as-s)mthesised and 450°C for the modified zeolites for at least 3 hours. The total surface area was derived using the BET equation [12], the micropore volume and the external surface area (ESA) were estimated by means of the t-plot method of Lippens et al [13] and the total and mesopore volumes were calculated by Barrett-Joyner-Halenda anaylsis of the desorption branch of the N2 isotherm [14]. 

Beutel et al. (2001) studied the interactions of phenol with zeolite NaX using solid-state H and Si MAS and Si CP-MAS NMR spectroscopy. The H MAS NMR spectrum of NaX degassed at 400°C included peaks at 5h= 1.24,2.05,4.05, and 5.11 ppm. The signal at 2.05 ppm is characteristic of silanol protons at the external surface of the zeolite. On NaY zeolite (Si/Al=2.4), a peak at 5h= 1.6 ppm was found after pretreatment in air and in vacuum at 400°C and assigned to traces of water adsorbed on cations. Therefore, the peak at 8h=1.24 ppm was attributed to free hydroxyl protons of water molecules strongly adsorbed on Na+ ions. The peaks at great 8h values could be attributed to water bound to different active sites. 

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