Theory and Application of Dissociative Electron Capture in Molecular Identification

Abstract

The coupling of an electron monochromator (EM) to a mass spectrometer (MS) has created a new analytical technique, EM-MS, for the investigation of electrophilic compounds. This method provides a powerful tool for molecular identification of compounds contained in complex matrices, such as environmental samples. In particular, EM-MS has been applied to the detection of nitrated aromatic compounds, many of which are potent mutagens and/or carcinogens and are considered environmental hazards. EM-MS expands the application and selectivity of traditional MS through the inclusion of a new dimension in the space of molecular characteristicsthe electron resonance energy spectrum. EM-MS also enhances detection sensitivity as well because the entire electron flux of the proper energy can be delivered into the negative ion resonance that is analytically most useful to solving the problem at hand. However, before this tool can realize its full potential, it will be necessary to create a library of resonance energy scans from standards of the molecules for which EM-MS offers a practical means of detection. Unfortunately, the number of such standards is very large and not all of the compounds are commercially available, making this library difficult to construct. Here, an approach supplementing direct measurement with chemical inference and quantum scattering theory is presented to demonstrate the feasibility of directly calculating resonance energy spectra. This approach makes use of the symmetry of the transition-matrix element of the captured electron to discriminate between the spectra of isomers. As a way of validating this approach, the resonance values for twenty-five nitrated aromatic compounds were measured along with their relative abundance. Subsequently, the spectra for the isomers of nitrotoluene were shown to be consistent with the symmetry-based model. The initial success of this treatment suggests that it might be possible to predict negative ion resonances and thus create a library of EM-MS standards.2

Temporary anion states and dissociative electron attachment to nitrobenzene derivatives

“…The results of our low-resolution measurements for 1,2-and 1,3-nitrotoluene are in excellent agreement with the data of Ref. [16].…”

“…This approach, however, requires knowledge of the electron attachment cross section. A recent NIMS study [16] of 25 nitrobenzens explored the suitability of this technique as a tool for molecular identification of electrophilic compounds.…”

Investigation of Nitro-Organic Compounds in Diesel Engine Exhaust: Final Report, February 2007 - April 2008

“…4,5 However, despite the fact that this mixture was useful for assessing the recoveries of two-ring, four-ring, and dinitro NPAHs, it did not account for all ring types. As a result, the internal standard solution was adjusted to include a single-ring (nitrobenzene-d 5 ) and three-ring (9-nitroanthracene-d 9 ) NPAH.…”

“…The extraction scenario was based upon previous work conducted at CSM. 5 To streamline the NH 2 SPE method development, we utilized steps described in Analytical SolidPhase Extraction by James S. Fritz, which outlines techniques for developing SPE methods. 6 The technique developed here is based upon prefabricated 25 mL Alltech ultra-clean cartridges containing 1.5 g of amino phase.…”

“…However, within the error of the measurements there is no difference in total PAHs between B100 and B20 samples, and naphthalene appears to actually increase from B20 to B100. A comparison of the NPAH levels (Table 4-3) with the PAH levels (Table [4][5] shows that all of the NPAH levels (except 9-nitroanthracene) were observed to be approximately two orders of magnitude below the observed levels for the corresponding PAH. In the case of 9-nitroanthracene, this compound was consistently at or below one order of magnitude under the relative concentration of anthracene.…”