Sample preparation unstable species

Preparation of matrix-isolated samples generally follows one of two routes. Either the short-lived species is formed in the gas phase (by photolysis, pyrolysis, or the action of a discharge) then condensed with a large excess (typically thousandfold) of the matrix material, or a stable precursor is trapped within the matrix and the unstable species are generated in situ by photolysis or radiolysis. 

Experimental limitations on the sources of primary information are usually chemical rather than instrumental. Thus chemically unstable species may be hard to prepare even in sufficient transient optical density or emitting concentration to yield a spectrum. More seriously, to obtain spectra of isotopic species requires usually the preparation of much larger samples than would be needed, e.g. in nricrowave spectroscopy, and in dominating concentration rather than as a minor constituent of a nrixtore or even in natural abundance. Thus in molecules with niunerous geometric parameters to be determined, the technique of isotopic substitution has, with the exception of deuteration, been used only relatively rarely (see e.g. s-tetrazine). There are therefore in the literature maity cases of nrolecules not listed here for which one or several rotational constants are known in excited states. 

Method development for spedation involves answering several questions. Is the sample photoredudble or photosensitive. Are any analyte species volatile, thermally unstable, or unstable when exposed to air. Could redox conditions change because of sampling, sample preservation, or sample preparation. The loss or conversion of nonanalyte sample components may also affect the balance of analyte spedes. The reader is advised to consider the chemistry closely and follow established spedation methods to prevent incorrect measurements of the various elemental spedes. 

It may be surprising to find the most extensive application of collinear laser fast-beam spectroscopy in a field that a priori has little connection with the special features of this technique. Neither the Doppler shift nor the accessibility of ionic spectra plays a decisive role for the on-line experiments on radioactive isotopes from nuclear reactions. However, most of the problems encountered in the preparation of a sample of free atoms (cf. Part B, Chapter 17 by H.-J. Kluge) are solved by a combination of the fast-beam technique with the well-established concept of on-line isotope separation. The isotope separators (with ISOLDE at CERN as an outstanding example) provide the unstable species in the form of ion beams whose phase-space volume is well matched to the requirements of collinear spectroscopy. 

The general topic of sample purity, which is so important in any structural study, has already been addressed in Section 1.3. Here we are concerned with the preparation and handling of non-standard samples. This includes compounds that may exist for only a fraction of a second, and so will require us to design our experiments with due care so that data can be collected in a very short window of time. We consider three general themes that are applicable to a range of structural chemistry techniques. Note that the very nature of mass spectrometry measurements involves the generation of many unstable species from a stable sample, but our discussion of these technique-specific methods is restricted to Section 11.2.1. We then discuss some other specialized sample preparation techniques, including the treatment of non-volatile samples, which have to be vaporized, and ways in which to control the external parameters of temperature and pressure. 

After loading titanium onto Na-Y at 400°C, a third titanium spedes was identified by a UV-Vis absorption maximum at 205 nm in a freshly prepared sample [137,218,219]. Such an absorption maximvun is also detected in the spectrum of TS-1 which possesses Ti(IV) tetrahedrally coordinated in the framework. A similar species may be obtained by multifold coordination of TiCl4 to zeolite Na-Y that is possible at defect sites, . e.,at hydroxyl nests [221]. These sites originate from a removal of aluminum atoms from framework positions by the HCl formed during the reaction of TiCl4 with the OH groups of the zeohte [137]. The tetrahedral Ti(IV)Ox species in Na-Y, however, is unstable under ambient conditions after a period of several months, the UV-Vis spectra show a red-shift, indicating an increase in the coordination number of TiO [ 137,218]. 

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