Spectra from suspensions of the compounds in PBS buffer, pH 7.
b The three most intense peaks are underlined. It should be noted that the fragments marked with * must have been formed dimeric units of the polypeptide chain, or else represent products of recombination reactions.
1.7. NUCLEOTIDE AND NUCLEIC ACID SPECTRA (GROUP C)
In view of the complete loss of signals derived from the base moieties in regular Curie-point Py-MS, as discussed in Part I, Section 3.3, it is worth mentioning that special methods can be used to obtain mass spectra of the tarry residue condensed on the wall of the glass reaction chamber during the Curie-point pyrolysis procedure. One approach is taking the used wire out of the reaction chamber after pyrolysis, heating the h.f. coil area up to 225°C (by means of a tungsten wire heater) and reinserting the reaction chamber into the heated h.f. coil zone (ref. 204). Another method involves the application of a thin ferromagnetic metal foil surrounding the sample wire and heated simultaneously with the wire by the h.f. field (ref. 205). The spectra produced are qualitatively similar to those obtained by direct probe Py-MS techniques (refs. 15-17) and by laser pyrolysis-MS (see Part 1, Figure 5) and show relatively large purine and pyrimidine base signals. In contrast to direct probe and laser Py-MS techniques, however, the actual pyrolysis step is performed under well-defined Curie-point Py-MS conditions.
1.8. LIPID SPECTRA (GROUP D)
Because of the relatively high volatility of many of the lipid compounds analysed, pyrolysis was mostly carried out in ferromagnetic tubes. Attempts to pyrolyse lipids on ferromagnetic wires often fail to produce peaks of sufficient intensity because the large pyrolysis products formed are lost by condensation on the relatively cold walls of the reaction chamber. Alternatively, the whole lipid may escape from the pyrolysis wire intact and subsequently condense on the reaction chamber wall.
Such condensation losses may be prevented by (a) oven pyrolysis, which degrades the compound into smaller, more volatile products or (b) special pre-heating or post-heating of the reaction chamber, such as used in the analysis of nucleic acids (ref. 205). An important question is, of course, whether relatively volatile compounds such as lipids should be analysed by pyrolysis mass spectrometry when many of these compounds can directly be analysed by other MS techniques. However, the mass spectra of many hydrocarbons, especially aliphatic compounds, differ very little whereas pyrolysis techniques are capable of greater distinction between these compound series. Therefore it was decided to include a selection of pyrolysis mass spectra of lipids. Different homologous ion series, as presented in Section 1.4. of this Introduction, have been marked. In many cases, these series will represent hydrocarbon molecular ions with different degrees of unsaturation, i.e. the series:
CnH2n' CnH2n-2' CnH2n-4 and CnH2n-6'
1.9. SPECTRA OF HUMIC MATERIALS AND GE0P0LYMERS (GROUP F)
Compared with most other groups of compounds analysed for this Atlas, humic materials and geopolymers are usually ill-defined with regard to chemical composition and structure. Therefore, when examining the pyrolysis mass spectra three important facts should be kept in mind:
1. The high degree of heterogeneity even in apparently closely related samples, e.g. coals of the same rank, implies that the spectra shown are not necessarily representative for other samples of the same type.
2. A given ion series does not necessarily represent one class of molecules only, e.g. the series at m/z 78, 92, etc. may represent benzenes as well as benzthiophenes (refs. 73, 196).
3. The yield of pyrolysis products obtained with the Curie-point Py-MS techniques used decreases rapidly with increasing degree of geochemical transformation. For instance, whereas lignites may produce pyrolysis yields of 50% or more, anthracites may yield as little as 10% or less. Thus conclusions drawn about the chemical composition of the sample on the basis of pyrolysis mass spectra pertain only to the relative composition of those pyrolysis products produced and detected by the technique used.
1.10. SPECTRA OF OTHER BIOCHEMICALLY IMPORTANT COMPOUNDS (GROUP G)
This group includes a number of relatively small molecules, mostly drugs, which are difficult or impossible to analyse by conventional MS techniques because of a lack of adequate volatility. A second reason for including this group of spectra in the Atlas is the fact that drugs and their metabolites sometimes show up in complex biological materials such as tissues and body fluids. In fact the Py-MS pattern of some drugs, e.g. penicillin, in human urine (ref. 159) is so easily recognizable that Py-MS techniques may offer a potentially new and fast approach to drug assays in urine.
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