Wavelength-resolved REMPI mass spectrometry in a hostile industrial environment, limitations and promises of the method

Abstract

The complementary REMPI (resonance-enhanced multi-photon ionisation) techniques using either a tuneable laser or a fixed-frequency laser are compared. Our Jet-REMPI apparatus, consisting of a tuneable laser unit (Nd:YAG pump laser and OPO) and a reflectron mass spectrometer, is briefly introduced. This machine has been upgraded to cope with the difficult conditions (such as vibrations, temperature fluctuations and dust) prevailing in an incinerator. On-line measurements of the raw gas have been carried out at the municipal waste incinerator (MWI) in Stuttgart. Although this test series could not be completed the results are encouraging. They show that wavelength resolution can provide valuable information beyond that which can be obtained by fixed-frequency REMPI. Examples discussed include identification of phenanthrene as major constituent of the isomeric pair phenanthrene/anthracene and the structural identification of xylene isomers in the incinerator raw gas. Both examples are also of more general interest, the former with regard to investigation of soot precursors and the latter for quality assessment of refinery products. Spectra of some deuterated aromatics are reported for use of these compounds as standards. In addition, a mass spectrum containing DDT is shown and a chlorobenzene profile measured earlier in a pilot scale incinerator as demonstration of the sensitivity of the instrument. As is to be discussed, significant further sensitivity gains through increase of the beam density are limited because of ion collisions within the sample beam.