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Fig. 5.7. Position-sensitive LDMS profile of the ~30-mm-long sublimed deposit along the temperature gradient of the fullerene-containing raw soot generated in the system in Fig. 5.6. Going from the top to the bottom spectrum, we see changes in the concentration of the higher-mass constituents of the deposit in going from colder to hotter regions of the temperature gradient [5.51].

Fig. 5.8. Comparison of the intensities of the LDMS signal for [email protected] (open squares) and [email protected] (open triangles) relative to C7() as a function of the position along the temperature gradient [5.51].

Fig. 5.8. Comparison of the intensities of the LDMS signal for [email protected] (open squares) and [email protected] (open triangles) relative to C7() as a function of the position along the temperature gradient [5.51].

4-cold and mm along gradient hot and—►

of the overall intensities of the higher-mass peaks relative to C60 and C70 is observed, without any major intensity variations among the two metallofullerene species (see §5.4 and §8.2). However, in the last 10 mm, at the hot end of the gradient, an overall steep increase of the higher-mass fullerenes is observed with interesting variations of the relative intensity distribution. Most notable is the pronounced increase of [email protected] at the hot end of the ribbon [5.51].

The STG method has not been used to produce large quantities of purified fullerenes. Its potential application appears, for the moment, to lie more in the isolation and purification of particularly insoluble fullerenes, or perhaps as a method to preconcentrate certain higher fullerenes or en-dohedral fullerenes (see §5.4 and §8.2) in advance of their purification by a final liquid chromatographic step.

5.3.3. Gas-Phase Separation and Purification

Gas-phase purification utilizing the difference in vapor pressure for the various species is an attractive approach for the purification and isolation of species that are either highly reactive, not soluble, or suffer irreversible retention on the stationary phase during conventional chromatography [5.52,53]. On this basis, some good candidates for gas-phase separation are the endohedral fullerenes and [email protected] [5.54—56], which have not been successfully isolated by chromatography.

Gas-phase purification utilizes the difference in vapor pressure between C60 and the higher fullerenes. This method has been used successfully to produce ultrahigh purity (99.97%) C60 [5.57], Raw fullerene soot or fullerene extract is introduced into one end of a distillation column lined with a series of evenly spaced baffles with circular perforations. The fullerene starting material is heated under high vacuum to ~1000 K, while a linear temperature gradient is simultaneously established along the length of the column. As the mixed fullerene vapor traverses to the cooler region of the column by effusion through the perforated baffles, it becomes enriched in the more volatile species. Because of the repeated evaporations and condensations necessary to move down the column, virtually all volatile impurities are removed and pumped away. Since the perforations in the baffles are small compared to the mean free path of the fullerenes, effusion is the dominant mass transport mechanism in the column. Using the Knudsen effusion equation [5.58], the theoretical rate of effusion dWn/dt through an aperture for each fullerene component (in g/s) is written as where pn is the equilibrium vapor pressure of the nth component in dyn/cm2, T is the temperature in degrees Kelvin, M„ is the molecular weight of the nth component, A is the effective aperture area in cm2, k is a geometrical factor used to account for the thickness of the plate, and R is the gas constant. The apparatus consists of a meter-long fractional distillation column made from stainless steel tubing, which is lined with a removable quartz tube containing the separation baffles, consisting of many stainless steel disks with circular perforations [5.57],

The mass flow down the distillation column is initiated by subliming the starting material, and the fullerenes now in the vapor phase experience a pressure gradient induced by the temperature gradient. Starting with a higher-purity fullerene source material gives a higher-purity product and a significantly higher yield of ultrahigh purity material. It is believed that the gas-phase separation approach can also be used to purify higher-mass fullerenes and endohedral fullerenes.

5.3.4. Vaporization Studies of C60

Separation of fullerenes by utilizing differences in their vapor pressure has been discussed as one approach to the purification of fullerenes (see §5.3.3). This separation method depends on knowledge of the temperature dependence of the vapor pressure of fullerenes and the associated heat of sublimation [5.26,57,59,60]. These quantities have been measured for a C60/C70 mixture obtained from an unpurified extract from arc soot [5.26] and for chromatographically purified C60 [5.59].

We summarize here the temperature dependence of the vapor pressure obtained from a purified C60 sample [5.59] employing a Knudsen cell as a source (see Fig. 5.9). A thermocouple which touched the base of the

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