Filament Pyms

The third line of Py-MS still actively pursued today is the filament pyrolysis approach. In 1970, Simon's group in Zurich published the results of preliminary Py-MS studies on fatty acid salts, pigment dyes and substituted benzoic acids (ref. 40) using a Curie-point pyrolyser connected to a magnetic sector mass spectrometer system through an empty capillary column and a molecular separator. Curie-point pyrolysis, as first described by Giacobbo and Simon (ref. 41), belongs to the broad group of filament pyrolysis techniques but differs from the other members of the group in that the filament is inductively heated by a high-frequency coil rather than heated by a galvanic current, and the equilibrium temperature of the filament is determined by the Curie-point of the ferromagnetic filament rather than by a servo-controlled power supply. The induction heating principle allows for batch processing of samples since the filaments are completely interchangeable, provided that the filament dimensions and the ferromagnetic alloy are kept constant. In fact, because of the low cost, Curie-point filaments are disposable. Moreover, the contactless heating principle allows for easy automation of sample exchange procedures, as demonstrated by the successful construction of fully automated Py-GC (ref. 42) and Py-MS (ref. 43) systems. Apart from these technical differences, however,

Curie-point techniques are fully comparable to other filament pyrolysis techniques employing similar heating rates and equilibrium temperatures.

In 1973, Meuzelaar and Kistemaker at the F.O.M. Institute for Atomic and Molecular Physics in Amsterdam reported the development of a Curie-point Py-MS technique for fast differentiation of bacterial strains (ref. 44). The Py-MS system differed from Simon's original system in that the pyrolysis reactor was enclosed in the vacuum system and was connected to the open electron impact ioniser of the quadrupole mass filter through a heated, gold-coated expansion chamber. A liquid nitrogen-cooled screen surrounded the expansion chamber and a signal averager allowed recording of fast repetitive mass scans.

During the next few years, the F.O.M. group reported further technical developments, such as the use of high speed ion counting (ref. 43), full automation (ref. 43) and computerised data processing techniques (ref. 45). Also, new applications to biomaterials were reported (refs. 46 - 49), including characterisation of humic materials (ref. 50) and whole soils (ref. 51), as well as geochemical applications (ref. 52) and medical applications (described in Chapter 7). The analysis of biomaterials by filament pyrolysis techniques in direct combination with mass spectrometry was also described by other groups, e.g. Schulten and Görtz (ref. 53), employing Curie-point pyrolysis in the ion source of a high-resolution field ionisation mass spectrometer for the analysis of glycogen, and by Hileman (ref. 54) using a Pyroprobe (Chemical Data Systems, Inc.) filament pyrolyser in the reagent gas inlet of a chemical ionisation mass spectrometer for the characterisation of muscle tissue samples.

Recently, the distinction between filament and oven pyrolysis techniques has become less marked through the introduction of a new Curie-point pyrolysis technique in Py-MS (refs. 55, 56). Instead of employing a ferromagnetic filament, this technique uses small, hollow ferromagnetic cylinders in which the sample is placed. The heating characteristics of these cylinders are similar to the heating profiles of the wires. The pyrolysis process differs from filament pyrolysis, however, in that the pyrolysis products experience multiple collisions with the hot cylinder wall. In this respect, the technique resembles direct probe and other oven pyrolysis techniques. It differs from the latter, however, in that the heating rates are orders of magnitude higher than in classical oven pyrolysers. Applications of this technique in Py-MS include the study of pyrolysis mechanisms in relatively volatile model compounds (refs. 55, 57 - 59), pigment dyes (refs. 55, 60) and antibiotics (ref. 61). Apart from its application to the pyrolysis of relatively volatile compounds, however, it remains to be determined whether this new approach offers any significant advantage over filament pyrolysis in the analysis of biomaterials. Because of the transient high pressures in the oven during pyrolysis secondary reactions may occur in the gas phase, e.g. formation of dibenzyl upon pyrolysis of phenylbutanoic acid (ref. 57). These reactions can be minimised by using submicrogram amounts of sample (ref. 56). At these sample levels, however, it becomes increasingly difficult to avoid significant contributions from instrument background due to residual contaminations of oven and/or vacuum system. In our experience, the above-mentioned problems have an adverse effect on repeatability and reproducibility.

Finally, a highly specialised Py-MS technique reported by Schulten et al. (ref. 62) should be mentioned. In this experiment, a DNA sample was pyrolysed directly on the heated emitter of an FD source and the ions were analysed by a high resolution mass spectrometer equipped with a photoplate detector. This Py-FD/MS technique allowed the detection of unusually large pyrolysis fragments of DNA, including some intact dinucleotide ions.

Chapter 2

0 0

Post a comment