Gas hold-up time

The adjusted retention time provides a measure of the strength of intermolecular interaction between the analyte and the stationary phase, with stronger interactions giving a longer time. The gas hold-up time is derived from the flow rate and the column dimensions and is often measured by injecting a non-retained compound. The retention factor, which represents a ratio of the mass of analyte dissolved in the stationary phase to the mass in the mobile phase, can be calculated from the adjusted retention time and the gas hold-up time. 

The average volumetric flow rate (ml/min) is calculated from the gas hold-up time, the column length (cm) and the column radius (cm). 

Systems with electronic pneumatic control use pressure transducers at the inlet and outlet, the column dimensions and physical properties of the carrier gas to determine the gas hold-up time and the flow rate. 

The carrier gas viscosity is given as r, L the column length, pQ the outlet pressure, P the ratio of inlet pressure to outlet pressure and dc the column diameter. Analysts should take care to be sure that the methods used for determining the flow rate are consistent from in-strument-to-instrument and from method-to-method. Otherwise it will be difficult to compare any data that have the flow rate, gas hold-up time or linear carrier gas velocity as a component. 

Using the retention data and the chromatogram shown in Fig. 14.8, tabulate the following for each peak retention time ( r), adjusted retention time (t K), retention factor (k), partition coefficient (Kc) and number of theoretical plates (N). The column phase ratio was 250 and the gas hold up time ( m) was 0.995 min. 

One of the basic requirements is to synthesize the desirable compounds and to rapidly transport them to the equipment for chemical experiments. The time spent to accomplish such processes and its probability distribution can be properly determined only when a radioisotope of the element under study can be produced in a quantity that can be easily and accurately measured. It has never been the case for the transactinoid elements. Rough estimation of the chlorination time of Zr and Mo was done in the model experiments described in the above Section (see also Fig. 1.2). The ampoule was filled with pure inert gas, then closed and bombarded for some time with neutrons at ambient temperature. Thus, the thermalized fission products were accumulated on the walls. Then the ampoule was heated and flushed for a short time with the gas containing a reagent. Most of the activity got transferred into the nap in 30 seconds or so it involved a mean gas hold-up time of 15 seconds, so that the actual upper limit of the chlorination time could be set as 15 seconds. 

Figure 3.6 displays graphs of the pertinent formulae for the laminar flow regime from Sect. 2.2. They describe the profile of the deposit density q>v(z ) and that of penetration Fp(zv) - the fraction of the adsorbable particles still staying in gas at the exit of the channel. Notice that the reduced distance also equals the ratio of the doubled gas hold-up time fg in a tube of the length z to the average time needed by the particles to diffuse across the channel diameter dc ... 

However, Vs is affected by the size and shape of the adsorbates in such a way that the retention volume of 2,2 DMB in all the samples is negligible (very close to the gas hold-up time) which means that the limiting aperture size of the pores is 0.62 nm. Moreover, Vs for benzene and cyclohexane (Figure 1) being very similar are such that the separation ratio is close to 1 for the sample with a low degree of activation (700°C) and it increases slowly... 

This hypothesis is supported by additional experimental data. Therefore, when sample SI300 (obtained at 1300°C) is mildly gasified up to 1% burn-off the benzene/cyclohexane ratio clearly decreases (142.6). Moreover, when the gasification is increased up to a medium burn- off (6%) the molecular sieve effect disappears (benzene/cyclohexane ratio, 0.93), and Vs for 2,2 DMB is far from the gas hold-up time and can be measured (0.75 cm /m ). This means that this treatment produced carbon removal which enlarged the pore constrictions. This fact is consistent with Vs values of linear hydrocarbons rising with increasing burn-off... 

The behaviour of these samples measured by IGC shows a reduction of the adsorption capacity for both linear and cyclic hydrocarbons (Table 5). Moreover, Vs for the hydrocarbon 2,2 DMB on the oxidized carbons is quite close to the gas hold up time, similar to that on the original samples, and the separation ratio for the couple benzene/cyclohexane is similar in both cases. These results show that the oxygen surface complexes fixed on the surface produce constrictions at the entrance of the pores, but do not result in the production of carbon materials with improved molecular sieve properties. One reason for this could be that the size of the oxygen complexes is not large enough. Therefore, if the size of the chemical complexes is increased the molecular sieve character for the couple benzene/cyclohexane would be expected to be developed. For this purpose, the S900 carbon was treated to introduce sulphur complexes on the surface [30]. Also a commercial acti-... 

The average flow velocity of gas (u) The ratio of the column length, L, to the non-retained carrier gas hold-up time in the column, tM is the average flow velocity of gas ... 

The test is performed under optimized conditions of carrier gas flow and temperature program rates, which are adjusted for column length and carrier gas viscosity. Table 2.16. To obtain a correct value for the gas hold-up time for thick-film columns (df > 0.7 p,m) it should be measured at 100°C (methane is considerably retained at... 

The RF-GC, being a flow perturbation technique, is the change in the direction of flow of the carrier gas and this is done by using the four- or six-port valve. The carrier gas turns to flow in the opposite direction for a short time period f (10-60 sec), smaller than the gas hold up time in the sections I and / of the sampling column. Then, it is restored to its original direction of flow. Two questions arise now why we change the direction of flow of carrier gas and what would we observe by this change ... 

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