Oak Ridge National Laboratory

Physics Division

Physics Division Seminars

Physics Division Seminars bring us speakers on a variety of physics related subjects. Usually these are held in the Building 6008 large Conference Room, at 3:00 pm on the chosen day, but times and locations may vary. For more information, contact our seminar chairman,

Alfredo Galindo-Uribarri
Tel (Office): (865) 574-6124  (FAX): (865) 574-1268

Thu., September 23, 2010, at 3:00 p.m. (refreshments at 2:40 p.m.)

Bayesian Decomposition of Quadrupole Mass Spectra: What Can be Extracted from the Data?

Thomas Schwarz-Selinger, Max-Planck-Institut für Plasmaphysik, Garching, Germany
Building 6008 Conference Room

Quadrupole mass spectrometry is a widespread method that allows characterizing gas mixtures for a wide range of applications and promises a large dynamic range and high sensitivity. However, unambiguous identification and quantification of components of an unknown gas mixture from measured spectra is a challenging task due to fragmentation of the molecules by electron impact in the ionizer. The pedestrian subtraction method and/or least-square evaluation are often used for the decomposition of multi-component mass spectra. These, however, suffer from the excessive error buildup and provide only poor and sometimes unphysical results like negative concentrations. An advanced analysis technique of mass spectra is presented that applies Bayesian probability theory. The method handles the data and all other information consistently and is able to identify components in unknown mixtures via model comparison. It provides estimates not only for the species concentrations but also for the cracking patterns. In addition, the confidence intervals for both are derived. This method not only allows reliable identification of radicals in an unknown mixture, but also determination of their cracking pattern. The basics of the method are presented together with illustrating examples from low temperature methane plasmas.

Host: Fred Meyer (574-4705)