Field Ionization Mass Spectrometric Analysis

Field ionization mass spectrometry (FIMS) has prove to be an invaluable technique for analyzing complex mixtures, particularly fossil fuels. Field ionization consists of passing the vaporized material of interest through a very high electric field. The intense field polarizes the molecules and guides them to regions of greater field strength, ultimately resulting in their ionization. Mass analysis is then performed by conventional magnetic sector or quadrupole techniques. For most compounds, field ionization produces only the molecular ions. Calibration to determine the relative sensitivities of the components of the mixture can be used to improve the accuracy of the quantitative estimations. However, because these sensitivity factors are not very different for the different classes of compounds, even without this correction, FIMS gives a reasonably accurate molecular weight profile. Moreover, the profile so obtained is not complicated by the size, shape, or functional groups of the molecule, nor is it affected by hydrogen-bonds. Thus, for the analysis of fossil fuels, FIMS is the method of choice.

The FIMS instrument at SRI consists of a foil-type field-ionization source, a 60^o magnetic sector mass analyzer, and a computer to control the instrument and generate reports. To achieve the needed high field strengths to cause ionization of the vapors, the foils are activated by growing carbon dendrites. Activation of foils is somewhat of an art. We prepare them under high temperatures and high electric fields so that the carbon dendrites adhere strongly to the metal substrate and are very resilient. In our experience an activated foil can last for over a month of trouble-free operation.

The resolution of our magnetic sector mass analyzer is about 1300 and the most common mass range is 100 to 800 Da. However, the instrument can scan up to m/z 3000 with less than unit mass resolution. The instrument is equipped with both batch-inlet and heatable direct insertion probes. When using the heatable probe, data are acquired continuously starting at probe temperatures as low as -78^oC, with gradual heating up to about 500^oC. We also have a quartz probe that allows us to heat the samples to above 800^oC.

We have used FIMS for a variety of applications:

• Molecular Weights and Component Analysis. Molecular weight determinations by FIMS are not affected by intermolecular associations or molecular fragmentation. Consequently, FIMS gives more accurate molecular weight profiles than any other technique. From the nominal mass data, it is often possible to obtain quantitative information regarding individual components.

• Estimation of Nitrogen Content and Distribution. Examination of the 13C-corrected mass spectra of coal liquefaction products obtained with and without tetrahydroquinoline(THQ) as a component in the solvent showed unequivocally that the loss of THQ into the product was not due to H-bonding, but through covalent bonding with the coal-derived products over the entire MW-range.

• Coking of Liquefaction Catalysts. Analysis of spent catalysts from the US Department of Energy's coal liquefaction plant at Wilsonville showed the preponderance of aza-aromatic species on the catalyst.

• Storage Stability of Diesel Fuels. FIMS analysis of sediments obtained from storage of diesel fuels has provided valuable clues to the chemistry of sediment formation and possible strategies for managing the problem.

• Pyrolysis/FIMS of Polymers and Other Materials. The heated probe inlet provides an opportunity to perform pyrolysis studies as well. FIMS analysis of the pyrolysate can supplement thermogravimetric studies, which indicate weight losses but give no information on the nature of the species being evolved. The mass spectral data of the pyrolysis products provided by FIMS have been used to verify the structure of a polymer and to understand the mechanisms leading to its cleavage.

SRI continues to provide FIMS analysis as a service for many clients. These analyses could be performed either under a research project or can be set up as a stand-alone analytical service for a minimum of five samples. Stand-alone service is available at U.S. $500 per sample for FIMS analysis only, or at US $1,000 per sample, which includes a report providing data interpretation drawn from our experience with the FIMS analysis of wide-ranging materials. For analysis on a continuing basis, we can arrange for open-ended purchase orders for amounts over U.S. $5,000. Although only 40 mg or less of the sample are needed for the analysis, for ease of handling we appreciate receiving 5-10 mg.

A typical analysis report includes a collection of spectra acquired at various temperature intervals, and a SUM spectrum computed by adding the ensemble together. Numerical reports giving the intensity of various peaks are arranged in a matrix of 14 columns so that homologous series differing by a methylene unit appear in the same column. Reports can also be provided in computer-readable forms.

Selected FIMS References

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Technical Contact:
Ripudaman Malhotra
(650) 859-2805
ripudaman.malhotra@sri.com