Lipids

Lipids constitute another important group of biomaterials which often have insufficient volatility to be analysed by direct chromatographic or mass spectrometric techniques. With the exception of the terpenes, which could be regarded as oligomers or polymers of isoprene, lipids usually do not possess the repeating subunits which are the hallmark of polymeric materials. Figure 8 shows the pyrolysis mass spectrum of natural rubber, a polyterpene (ois-1,4-polyisoprene). The dominant pyrolysis products appear to be the isoprene monomer at m/z 68 and the dimer at m/z 136. Most other prominent mass signals appear to be residual EI fragments in spite of the low electron energy conditions. The strong EI fragmentation tendency is explained by the aliphatic, branched hydrocarbon nature of the pyrolysis products. Natural rubber is perhaps one of the few biopolymers that show a more or less regular depolymerisation behaviour under pyrolysis conditions. In other biopolymers thermal degradation prevails over depolymerisation, as discussed in Section 3.1.

Curie-point Py-MS has had only limited success in the study of non-polymeric lipids because the rapid heating and short residence time of the products in the reaction zone results in the escape of intact or at most minimally fragmented lipid moieties, e.g. complete fatty acids, which are easily lost by condensation on the walls of the reaction chamber. Thus, lipid moieties tend to be strongly under-represented in the pyrolysis mass spectra of complex 1ipid-containing samples. By using heated reaction chamber walls and pyrolysing directly in front of the ion source, however, lipid molecules may provide intense signals, as shown by the work of Anhalt and Fenselau (ref. 18). Even then, the use of electron impact ionisation, especially at the 70 eV electron energy used by these authors, results in severe further fragmentation of the lipid products. In this respect, the use of chemical ionisation techniques for the analysis of lipid products from direct probe pyrolysis, as reported by Risby and Yergey (ref. 19, 20), may be regarded as an improvement. From the point of view of pyrolysis conditions, however, it would seem to be f

Was this article helpful?

0 0

Post a comment