Isotherms, chromatography

Splitless injection is required for very dilute solutions. It offers high resolution but is poor for quantitative analysis because less volatile compounds can be lost during injection. It is better than split injection for compounds of moderate thermal stability because the injection temperature is lower. Splitless injection introduces sample onto the column slowly, so solvent trapping or cold trapping is required. Therefore, splitless injection cannot be used for isothermal chromatography. Samples containing less than 100 ppm of each analyte can be analyzed with a column fdm thickness < 1 p.m with splitless injection. Samples containing 100-1 000 ppm of each analyte require a column film thickness 1 p.m. 

For the experimental investigation of volatile transactinide compounds two different types of chromatographic separations have been developed, thermochromatography and isothermal chromatography. Sometimes also combinations of the two have been applied. The basic principles of thermochromatography and isothermal chromatography are explained in Figure 7. 

Fig. 7. Upper panel temperature profiles employed in thermochromatography and isothermal chromatography lower panel deposition peak and integral chromatogram resulting from thermochromatography and isothermal chromatography, respectively.

On-line isothermal chromatography is ideally suited to rapidly and continuously separate short-lived radionuclides in the form of volatile species from less volatile ones. Since volatile species rapidly emerge at the exit of the column, they can be condensed and assayed with nuclear spectroscopic methods. Less volatile species are retained much longer and the radionuclides eventually decay inside the column. 

In experiments with long-lived nuclides retention times equal the time of the experimental duration. However, for short-lived nuclides, at the temperature, where 50 % of the nuclides pass the isothermal chromatography column, the retention time equals the half-life of the species ... 

In recent years predominantly continuous isothermal chromatography has been applied in gas chemical studies of transactinides. This technique offers the possibility to combine a continuous separation of volatile species with an in-situ detection of the products on the basis of single atom counting. To reach this ambitious goal, novel devices have been developed such as the On- Line Gas chemistry Apparatus (OLGA) [10] or, in a modified version, the Heavy Element Volatility Instrument (HEVI) [11] see also Chapter 4. 

Fig. 18. Temperature vs. yield curve from isothermal chromatography of l69mRe03, 2lltPo (T1/2=3.05 m, presumably as 2l8Po02), and 214Bi (T 1/2=19.9 m, presumably as BiOOH). Figure from [53] with the permission of Oldenbourg Verlag.

In practice, for isothermal chromatography, the retention index (RI) is found by logarithmic interpolation between w-paraffins that bracket the compound. Retention index =... 

Fig. 1.6 isothermal chromatography of short-lived cyclotron produced nuclides transported by helium (cluster) jet [23]. 

Fig. 1.7 Isothermal chromatography experiments (OLGA-type equipment) with volatile compounds of a-active nuclides [29].

Later, using a similar approach, Gaggeler and co-workers [24-28] at PSI, Villi-gen built the first dedicated setup for isothermal chromatography experiments with short-lived, a- or s.f.-active isotopes of transactinoids. The essential parts of the equipment are shown in Fig. 1.7. A major concern was high efficiency at every step of processing. Several modifications of this OLGA setup have been built and employed. They differed in the parameters of the column and in the complexity, as well as quality, of the detection system. For example, the older detectors were replaced by the passivated implanted planar silicon (PIPS) detectors, which are much more resistant to elevated temperature and to chemicals. 

At this point, it seems appropriate to summarize the advantages and drawbacks of thermochromatography and isothermal chromatography as the tools for chemical identification and study of chemistry of the very heaviest elements ... 

Refs. [45, 76] provide concise surveys of works on TC separations of common elements of all groups. They concern both compounds and elementary state. The present section quotes in more detail some works on separation of mixtures of the elements which have some common chemical properties, as well as of the elements which are related by their origin, like the fission products. The separations were performed by thermochromatography or by (eventually programmed) isothermal chromatography. 

Rudolph and Bachmann [90,91] studied in detail isothermal chromatography of chlorides of several fission products. They used quartz columns filled with graphite, quartz or quartz coated with NaCl, KC1, MgCl2 or CsCl. They made careful measurements of the retention time versus column temperature, and of the peak widths. The best separation was achieved by temperature-programmed chromatography. It is illustrated by Fig. 1.23. 

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