Mass Spectrometry of Pyrolysis Products during Thermal Decomposition of Ablator Materials
Start Date
1-3-2011 3:25 PM
Description
Mass spectrometry is a powerful technique for the characterization of pyrolysis gas composition during the thermal decomposition of ablative materials. Coupling a mass spectrometer to a thermogravimetric analyzer allows for relatively straightforward qualitative identification of chemical species and their correlation with mass loss during heating. However the quantification of species in a pyrolysis gas mixture is a much more difficult task.
In this presentation, we will review different aspects of mass spectrometry including gas sampling approaches, species ionization methods, and ion measurement techniques. We emphasize the advantages of time-of-flight (TOFMS) over quadrupole (QMS) ion separation because of its fast response time and the ability to capture an entire mass spectrum after every ionization event. The most ubiquitous ionization method utilizes 70eV electron impact (EI) ionization to generate ions, which can be used to identify molecular compounds. One complication of this technique is that molecules generally undergo significant fragmentation, which makes identification and quantification of parent species from complex and overlapping fragmentation patterns extremely difficult for complex gas mixtures. The use of single photon ionization (SPI) minimizes fragmentation and generates much 'cleaner' spectra, albeit only for species with ionization energies less than the incident photon energy.
We will describe our attempts to combine both EI-QMS and SPI-TOFMS to measure and quantify the decomposition products of characteristic ablator materials, including the development of a quantification method based on a benzene standard and a “ladder” quantification procedure to evaluate the gas-phase mole fractions of decomposition products. The results of this preliminary approach will be presented, the challenges of quantification will be discussed, and suggestions will be made for future experimental designs that utilize improved analytical instrumentation.
Mass Spectrometry of Pyrolysis Products during Thermal Decomposition of Ablator Materials
Mass spectrometry is a powerful technique for the characterization of pyrolysis gas composition during the thermal decomposition of ablative materials. Coupling a mass spectrometer to a thermogravimetric analyzer allows for relatively straightforward qualitative identification of chemical species and their correlation with mass loss during heating. However the quantification of species in a pyrolysis gas mixture is a much more difficult task.
In this presentation, we will review different aspects of mass spectrometry including gas sampling approaches, species ionization methods, and ion measurement techniques. We emphasize the advantages of time-of-flight (TOFMS) over quadrupole (QMS) ion separation because of its fast response time and the ability to capture an entire mass spectrum after every ionization event. The most ubiquitous ionization method utilizes 70eV electron impact (EI) ionization to generate ions, which can be used to identify molecular compounds. One complication of this technique is that molecules generally undergo significant fragmentation, which makes identification and quantification of parent species from complex and overlapping fragmentation patterns extremely difficult for complex gas mixtures. The use of single photon ionization (SPI) minimizes fragmentation and generates much 'cleaner' spectra, albeit only for species with ionization energies less than the incident photon energy.
We will describe our attempts to combine both EI-QMS and SPI-TOFMS to measure and quantify the decomposition products of characteristic ablator materials, including the development of a quantification method based on a benzene standard and a “ladder” quantification procedure to evaluate the gas-phase mole fractions of decomposition products. The results of this preliminary approach will be presented, the challenges of quantification will be discussed, and suggestions will be made for future experimental designs that utilize improved analytical instrumentation.