Involatile compounds, characteristics

Principles and Characteristics In the early mass-spectrometric ionisation techniques, such as El and Cl, the sample needs to be present in the ionisation source in its gaseous phase. Volatilisation by applying heat renders more difficult the analysis of thermally labile and involatile compounds, including highly polar samples and those of very high molecular mass. Although chemical derivatisation may be used to improve volatility and thermal stability, many compounds have eluded mass-spectrometric analysis until the emergence of fast atom bombardment (FAB) [72]. 

Table 7.40 summarises the general characteristics of on-line SFC-MS. The method is potentially most useful for thermally labile and involatile compounds that are unsuitable for GC-MS. Because the MS instrument is the main source of information, the reproducibility of the retention and the separation selectivity are much less important than for other SFC applications. As a result, mass spectroscopists do not feel restrained by the limits on reproducibility, which slowed the uptake of SFC by chromatographers. Method development should not be underestimated. Practical problems are associated with interfacing and the effect of the expanding... 

For ionic and involatile compounds, data must be obtained in the solid state. This has disadvantages in that bands tend to be broader, calibration of samples presents difficulties, and only surface layers of the sample are probed. These may well not be characteristic of the bulk. Also surfaces are very sensitive to contamination, and effective sample cleaning is needed rigorous vacuum conditions must prevail during the experiment to prevent recontamination. 

Using the CFD methods one can analyse polymeric and other involatile compounds by converting them, prior to chromatographic separation, into characteristically volatile products. These volatile products can be obtained either by pyrolysis or by using more selective chemical conversions. It is customary to regard the application of pyrolysis reaction to identify and analyse quantitatively involatile samples as an independent part... 

Analytical pyrolysis is one of the most important methods in analytical chemistry, known for many years. Thermal degradation and subsequent analysis of the degradation products have long been used for the qualitative and quantitative analysis of involatile compounds and for determining their structures [1—6]. The use of GC analysis of pyrolysis products has increased the practical value of the method because only certain of the products contained in the complex mixture formed are characteristic of a particular sample. 

Pure NI3 has not been isolated, but the structure of its well-known extremely shock-sensitive adduct with NH3 has been elucidated — a feat of considerable technical virtuosity.Unlike the volatile, soluble, molecular solid NCI3, the involatile, insoluble compound [Nl3.NH3] has a polymeric structure in which tetrahedral NI4 units are comer-linked into infinite chains of -N-I-N-I- (215 and 230 pm) which in turn are linked into sheets by I-I interactions (336 pm) in the c-direction in addition, one I of each NI4 unit is also loosely attached to an NH3 (253 pm) that projects into the space between the sheets of tetra-hedra. The stmcture resembles that of the linked Si04 units in chain metasilicates (p. 349). A further interesting feature is the presence of linear or almost linear N-I-N groupings which suggest the presence of 3-centre, 4-electron bonds (pp. 63, 64) characteristic of polyhalides and xenon halides (pp. 835-8, 897). 

When less thermally labile compounds require separation prior to analysis, in-line liquid chromatography-mass spectrometry (LC-MS) can be used. Even where liquid chromatographic separation is not required LC-MS interfaces may be used as a convenient method of sample introduction, e.g. for polar, ionic, thermally labile, involatile or high-molecular-weight species. There are several different types of LC-MS interface each with its own characteristics in terms of the solvent systems and sample types that it can handle and the appropriate solvent flow rates no one interface is applicable to all solvent-solute combinations. 

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