Mass Spectrometric Methods

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Mass Spectrometric Methods

Photoionization Mass Spectrometry

The reactor tube and skimmer of one multiplexed photoionization mass spectrometer at the CRF, which is optimized for the study of low-pressure individual reactions and reaction sequences. Source: D. Osborn, Sandia National Laboratories.

Technical Details

It is difficult to probe a complex chemical system holistically. Although a “perfect” experimental approach doesn’t exist for all systems, several criteria would clearly be advantageous. An optimal technique would have the following characteristics:

Universal: probes all species regardless of their chemical nature
Selective: can identify and quantify different isomers in chemical reactions
Sensitive: can detect minute quantities of a chemical
Time-resolved: follows creation and consumption of species in time
Multiplexed: probes all these aspects simultaneously, in a single experiment

The Combustion Research Facility has been a pioneer in developing multiplexed photoionization mass spectrometry, a technique that meets most or all these goals. We have applied this technique broadly in studies of flame chemistry; unexpected chemical intermediates, such as enols; chemistry on chemistry on Saturn’s moon, Titan; Criegee intermediates important in Earth’s atmosphere; combustion chain branching, chemical mechanisms at high pressures, and heterogeneous catalysis. These studies provide both broad and deep views of chemical reaction mechanisms by their ability to expose and quantify pathways from reactants, through intermediates, and to multiple simultaneous products. Resulting high-fidelity, multiplexed data sets provide important constraints to theoretical models of chemical reactions, which combine with experiments to advance foundational knowledge of chemistry.

Key Contributions

Isomeric-resolved identification of reactive intermediates
Elucidation of detailed reaction pathways in combination with theoretial chemistry

Partners

Marsha I. Lester, University of Pennsylvania
Mitchio Okumura, California Institute of Technology
Carl Percival, Jet Propulsion Laboratory
Andrew Orr-Ewing, University of Bristol
Dudley Shallcross, University of Bristol

PIs: Nils Hansen, Craig A. Taatjes, David L. Osborn, and Leonid Sheps

A custom-built multiplexed photoionization mass spectrometer for the study of chemical reactions at pressures up to 100 atm. Source: Sandia National Laboratories.

PEPICO

Photoelectron Photoion Coincidence Spectroscopy (PEPICO): A New Tool for Chemical Physics of Complex-Forming Reactions

Figure 1: Photoelectron image at a particular kinetic time giving the fingerprint for one chemical species in a reaction. Source: Sandia National Laboratories.

Figure 2: A depiction of the radical thermometer for methyl peroxy radicals (CH3OO). Source: Sandia National Laboratories.

Technical Details

Nature is replete with complex-forming chemical reactions, such as OH + CH₃-CH=CH₂ (propene). The potential energy surface for these reactions is characterize by multiple deep wells (representing different isomers of the OH-propene adduct) with many available bimolecular product channels. To probe such reactions, we developed a new use for a classic technique in chemical physics: photoelectron photoion coincidence spectroscopy (PEPICO). After initiating a chemical reaction with a laser pulse, we use single vacuum ultraviolet photons to ionize all species (reactants, intermediates, and products). We detect both the cations and the electrons formed from this ionization using ion and electron imaging (a technique created at the CRF by David W. Chandler and Paul Houston). The results provide photoelectron spectra that arise from each unique mass-to-charge ratio from the chemically reacting system under study. These photoelectron images (Fig. 1) provide unique fingerprints to identify which chemical isomers are produced at each mass, and allow us to follow reactions as a function of kinetic time.

Our approach provides many new tools to the chemical physicist, such as a Radical Thermometer that measures the internal energy of reactive free radicals, as depicted in Figure 2.

Key Contributions

Multiplexed probing of time-resolved chemical reactions with isomeric selectivity
Breaking through the false coincidence barrier in coincidence spectroscopy

Partners

Balint Sztaray, University of the Pacific
Andras Bodi, Paul Scherrer Institute, Switzerland
Patrick Hemberger, Paul Scherer Institute, Switzerland

PI: David L. Osborn

Tandem Mass Spectrometry

Technical Details

Although a powerful technique, analyzing molecules based on mass does not always enable us to identify specific molecular structures. Photoionization spectroscopy can yield isomer-specific information from mass spectrometry, but as the molecules grow larger in the chemistry leading to soot formation, for example, the photoionization spectra are no longer diagnostic. CRF researchers have used another mass spectrometric method, tandem mass spectrometry for such challenging applications. To implement tandem mass spectrometry, CRF researchers sample molecules from a reactive environment (e.g., a flame) directly into an atmospheric pressure photoionization (APPI) source where they are ionized using 10.0 eV photons from a continuous vacuum-ultraviolet (VUV) Kr discharge lamp. This photon energy is sufficient to ionize the targeted aromatic and highly oxygenated species important in particle formation. The resulting ions, when mass selected and accelerated into the argon-filled collision-induced dissociation (CID) cell, can fragment into a neutral and an ion fragment. Those fragment mass spectra indicate which components exist in the original molecule. For example, as illustrated below, C₃₂H₂₀ molecules observed in samples from a flame are shown to be a mix of C₁₆H₂₀ dimers and mixed (C₁₄H₁₀ + C₁₈H₁₀) clusters.

Diagram of a tandem mass spectrometric analyzer used to investigate the composition of a flame. Molecules are swept from the burner (far left) into the ionization region and ionized by a VUV lamp. The resulting ions can be mass-analyzed in the quadrupole mass spectrometer Q0 and selected masses can be passed through the quadrupole mass filter (QMF) and accelerated into a cell filled with argon gas. Collisions with the argon can break the initial ion into fragment ions, and a second mass spectrometer measures the mass of the fragments using time of flight (TOF) methods (far right). The efficiency of this fragmentation depends on the energy to which the initial ions were accelerated, and the fragmentation pattern reveals the constituents of the original ion. (Source: https://doi.org/10.1021/acs.jpca.8b08947)

Left graph: Tandem mass spectrometry showing fragmentation of initial ions of mass/charge ratio (m/z) = 404.157 (C32H20) sampled from a fuel-rich flame. Fragmentation of the ions into m/z = 202.078 (C16H10) at low collision energies is visible. The fragmentation reveals that C32H20 also contains (C14H10 + C18H10) dimers. Right graph: High-resolution integrated mass spectra at low (from 0-10 eV) collision energies. (Source: https://doi.org/10.1515/zpch-2020-1638)

Key Contributions

Partners: Musahid Ahmed, Lawrence Berkeley National Laboratory

PI: Nils Hansen

At the CRF, researchers apply the power of mass analysis to investigate the details of complex chemical reaction networks such as those that drive combustion, autoignition, and tropospheric autoxidation.” Development and application of synchrotron photoionization methods have proved particularly powerful, and the CRF has benefited from a close collaborative relationship with researchers at Lawrence Berkeley National Laboratory’s Advanced Light Source (https://als.lbl.gov/). Our scientists have developed new mass spectrometers and reactors that have pushed the frontiers of experimental reaction kinetics and combustion chemistry.