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Mobile System Features Three Mass Spectrometry Techniques

Photonics Spectra
Dec 2001
Daniel C. McCarthy

There are few chemical species that cannot be identified through one or another of mass spectrometry's myriad forms. However, chemists must select which spectrometric method best applies, based on the targeted species and the method of sample introduction.

Lacking a single spectrometric silver bullet that can perform more than one ionization technique, collaborators based at the Institut für Ökologische Chemie (Institute for Ecological Chemistry) in Oberschleissheim, Germany, have integrated three laser-based ionization techniques into a single mobile system: resonance-enhanced multiphoton ionization, electron-impact ionization and single-photon ionization.

The instrument circumvents the sticky option of choosing between applying a single limited technique and multiple disparate methods.

For example, laser-based resonance-enhanced multiphoton-ionization spectroscopy is a highly selective method for detecting aromatic compounds such as phenols, naphthalene or benzene.

This technique's selectivity prevents it from rendering an overview of a sample's inorganic bulk constituents, such as water, oxygen or nitrogen. For these, the more suitable detection method may be laser-induced electron-impact ionization. However, neither electron-impact nor resonance-enhanced multiphoton ionization is effective in the detection of aliphatic and/or in-organic compounds. These compounds are made evident via single-photon-ionization spectroscopy.

Because each of these ionization methods delivers different selectivities, the new instrument can provide a comprehensive spectrometric characterization of complex samples.

"The integration of three techniques was easy in principle. We just had to use two different [ionization] wavelengths," said Ralf Zimmerman, one of the researchers. "The trick was to use the same laser."

The group designed a system in which the beam from a Minilight-II Nd:YAG laser from Continuum of Santa Clara, Calif., passes through a nonlinear crystal to produce collinear 1064- and 532-nm radiation. A computer-controlled galvo-based scanner directs the collinear beam along one of two optical paths.

Tilted into the beam path, the mirror deflects the beams through an additional nonlinear crystal, shifting and filtering them into 266-nm pulses of 5 mJ. This wavelength is applied to both resonance-enhanced multiphoton ionization and electron-impact spectrometry.

Tilted out of the optical path, the mirror allows the beam to pass through another nonlinear crystal to produce 355-nm light. Focused into a xenon-filled cell, the 355-nm beam is tripled to 118-nm pulses of 10-6 mJ, the wavelength applied to single-photon-ionization spectroscopy.

The system needs an ordinary 220-V/16-A power supply, with no external gas or water cooling required. Mass spectra can be stored in real time on an integrated hard drive.

Potential applications include online process monitoring, such as coffee roasting, where the system could monitor patterns of phenolic compounds as a surrogate for the roast status. It might also monitor exhaust from a generation plant or detect different catalyst effects during chemical processing, where long-chain aliphatic compounds such as octane are visible only through the 118-nm technique.


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