Thermochromatographic column

We report on a number of on-line chemical procedures which were developed for the study of short-lived fission products and products from heavy-ion interactions. These techniques combine gas-jet recoil-transport systems with I) multistage solvent extraction methods using high-speed centrifuges for rapid phase separation and II) thermochromatographic columns. The formation of volatile species between recoil atoms and reactive gases is another alternative. We have also coupled a gas-jet transport system to a mass separator equipped with a hollow cathode- or a high temperature ion source. Typical applications of these methods for studies of short-lived nuclides are described. 

Fig. 5.6 Scanning electron microscopy [37] of the surface of the thermochromatographic columns, which were used in the thermochromatographic experiments at FLNR Dubna and PSI Villigen. The black bars in the left corners indicate 1 pm in shots one to three, and 100 nm in shots four to six.

Distribution of fission tracks along the thermochromatographic column (Zvara et al. 1998). 

In radiochemistry, the most popular version is TC with a stationary temperature gradient, in which the concentration of a substance and its local distribution depend on the distance or temperature, i.e., c = f x) or c = f T). This means that the volatile compounds remain in the thermochromatographic column. In the literature this method was previously referred to by various jargon terms, e.g., horizontal distillation, separation on the gradient tube, separation on the temperature gradient, etc. 

Thermochromatographic columns (TCC) are usually made of glass, quartz, porcelain, or metallic tubes, chemically resistant to reactant gases. They may be open (hollow) or filled with various substances (metals, oxides, halides, etc.). Probes, containing the radionuclides of interest, are inserted into the TCC at places of maximum temperature (Ts) in a batch mode before the experiments (off-line) or continuously (online) using gas jets with the radionuclides of interest adsorbed on microdisperse aerosol particles (KCl, M0O3, etc.). 

Element distribution in a thermochromatographic column with 9 vol% hydrogen bromide as reactive additive to the carrier gas nitrogen, together with the temperature profile along the column. Quartz-powder column, 15 min exposure time (Reproduced with permission from Radiochimica Acta, Hickmann et al. 1993)... 

Variation in the chemical form of substances injected in the starting zone of thermochromatographic columns for formation of volatile compounds... 

During the experimental studies of chemical properties of transactinides early approaches exploited the possibility to detect spontaneous fission in thermochromatographic columns by mica or quartz track detectors at temperatures below 400°C, while more modern setups employ Si detectors to register a decay or spontaneous fission at room temperature and below (liquid nitrogen). Then, the technique, however, is limited to the study of highly volatile atoms or compounds. 

The measured distribution of decay events over the thermochromatographic column is a basis for evaluating the adsorption enthalpy AH . Evaluation of AH for a single decay event is performed by the method proposed by Zvara (1985), who developed a microscopic model of TC. On its basis the migratory process of individual atoms in the column is simulated by the Monte Carlo method with allowance made for their radioactive decay. 

Fig. 1.1 Schematic of an experimental setup for thermochromatographic separations. The carrier gases Cl2 or Cl2 + CCI4 also served as chlorination agents the 90 cm long quartz column was of 0.5 cm i.d. Processing lasted several hours. Except for the initial and ending segments of the column, over some 60 cm, the temperature profile was nearly exponential.

The experimental data are shown in the bottom of Fig. 1.4. A filling in the initial section of the column served to enhance deposition of the nonvolatile chlorides of actinoid elements by disturbing the flow patterns. Success is demonstrated by the distribution of Sc activity as a marker for the nonvolatile species and by a few fission events (open circles) detected within the Sc zone. These are followed by one track over 100 cm of the isothermal section of the column. The thermochromato-graphic zone of Hf isotopes (measured by y-activity) and that of Rf fission events, which were observed in the thermochromatographic section of the column, have... 

Fig. 1.22 Thermochromatographic separation of fission products in the form of chlorides [22], Conditions quartz column, 4mm i.d., filled with quartz powder, 0.75 mm carrier gas nitrogen, 1.2L min-1, reagent hydrogen chloride, 6 percent vol.

Bachmann and co-workers interpreted their data for molecules halides in columns loaded with alkaline halides as evidence for the formation of complexes between the adsorbate and the crystalline halide. Such complexes were considered for application in radiochemical separations earlier by Zvara and co-workers in [3,13,14], who estimated the adsorption enthalpies from the rough adsorption isobar obtained in thermochromatographic experiments using Eq. 5.14. The adsorption enthalpy for NbCls on KC1 [14] is intermediate between the data of Table 5.1 for the adsorbents NaCl and CsCl, so that the results of the two groups are consistent. 

There are few works on the modification of the surface of metals by haloginating reagents, which are as detailed as the studies of silica. In the meantime, a sole work compares thermochromatographic behavior of molecular bromides in the columns made of nickel and of silica the observed deposition temperatures happened to be equal [32], This finding is very difficult to rationalize if the microscopic picture of adsorption is such as shown in Figs. 5.4 and 5.5. 

Based on a microscopic model of gas-solid thermochromatography in open columns by Zvara (1985), a Monte Carlo code has been made available (Tiirler 1996), which allows to generate thermochromatographic deposition zones as well as yield versus temperature curves as observed in isothermal chromatography. This model accommodates the influence of the carrier gas flow, the actual temperature profiles, and the different half-lives of the investigated species. For each isothermal temperature, the transport through the column is modeled for a large number of sample molecules. These calculations result in a curve of yield versus temperature for each value of the adsorption enthalpy The curve for the particular... 

Upper panel Distribution of fission tracks circles) attributed to element 105 bromides from three thermochromatographic experiments compared with the deposition zone of 14.6 h °Nb histogram) and the temperature gradient along the column. Lower panel Integral distribution of 14.6 h °Nb and of 3.3 min Hf smooth solid lines) and the integral fission-track distribution dashed step function). The shaded zone depicts the limits of the corrected" position of the step function for element 105 after consideration of the difference in the half-life of Db and the Hf and Nb isotopes (Adapted from Zvara et al. (1976))... 

Six experiments with the Cf( 0,4n) Sg reaction were performed with a total number of ions (called beam dose ) between 0.8 x 10 and 2.6 x 10 in each case. In the first two thermochromatographic runs, a quartz wool filter was present in the start zone as in most ancillary tests. No fission tracks were found in the column. In the four following experiments, this filter was omitted. In the fifth experiment, no chlorinating reagents were added and the column was kept at ambient temperature to test the behavior of the nonvolatile actinides. 18 SF events were detected within the first 15 cm of the column. The number of tracks found in runs 3, 4, and 6 are 10, 20, and 11, respectively, and these are mostly distributed along the first 20-120 cm of the column, see Fig. 20.33. 

Thermochromatographic setup and typical profile of temperature along the column (Reproduced with permission from Radiochimica Acta, Hickmann et al. 1993)... 

These theoretical considerations of the thermochromatographic process presume that the adsorption entropy and enthalpy do not depend on the temperature. It was also postulated that the adsorbent was homogeneous, its surface was not saturated with the adsorbate (monolayer or less), and the carrier (reagent) gas was unsorbable. Diffusion in the solid phase (adsorbent) and surface diffusion were ignored. Furthermore, in the theoretical considerations the effect of the carrier (reagent) gas pressure on the substance transport was not taken into account, which, however, should be considered in the case of TC at reduced reactant gas pressures and vacuum TC or with densely filled columns. 

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