PNNL-13501 - Pacific Northwest National Laboratory

Matrix Assisted Laser Desorption/Ionization Using Resonant Laser Ablation
Study Control Number: PN98046/1292
Gregory C. Eiden
Matrix assisted laser desorption/ionization (MALDI) provides an unparalleled capability to characterize large molecules,
both naturally occurring and synthetic, and has found widespread application in the life sciences and materials analysis.
Our work has pointed the way to a new approach in MALDI that will enable the characterization of materials that could
not previously be detected by MALDI, as well as a new way to characterize the mechanistic components of MALDI ion
formation processes.
Project Description
We have developed a new approach to generating matrix
assisted laser desorption/ionization (MALDI) mass
spectra of large molecules, especially those that have
traditionally been difficult to observe by MALDI. While
the focus has been MALDI detection of synthetic
polymers, this work will impact MALDI more broadly by
enabling polypeptide and nucleic acid MALDI. Our
short-term interest has been in improving MALDI by
making the ionization of analyte molecules more selective
and efficient using resonant ionization of selected
elements in the prepared sample. This is in contrast to the
conventional non-resonant MALDI process using fixed
wavelength lasers, typically the 337 nm nitrogen laser.
The cations formed are chosen for the specificity of their
gas-phase reactivity with the analytes of interest. The
desired reactivity can be simple adduct formation when a
spectrum of parent molecules is needed (weight
distribution measurement of a polymer) or fragmentation
when structural information is needed. Using resonant
ionization, the ion yield is controllable independently of
the ablation yield. By choosing different metal cations,
different gas-phase reactions are selected. We thus gain
independent control over the plume density (via the
ablation rate), the reagent (metal) ion density, and the gasphase
chemistry. By controlling these factors, most of the
processes leading to ion formation can be controlled or at
least manipulated. Our longer-term interests focus on
understanding the ionization mechanism in MALDI
experiments other than synthetic polymers.
Introduction
Measurements of metal cation reactivity are conveniently
made using a plasma source-ion trap mass spectrometer.
Reaction conditions in the trap are drastically different
from those in the laser plume—ion and neutral
temperatures differ, as well as number density, collision
rates, and other factors—however, the reactivity trends
established from ion trap data are useful in guiding the
choice of metal species in the resonant laser ablation
MALDI experiments.
This project is being conducted in collaboration with a
team external to PNNL. Those members of the team are
Dr. Kevin G. Owens of Drexel University (Philadelphia,
Pennsylvania), Dr. Scott D. Hanton of Air Products and
Chemicals, Inc. (Allentown, Pennsylvania), and Dr.
Robert J. Noll of Lawrence University (Appleton,
Wisconsin).
Results and Accomplishments
At Drexel University, we successfully demonstrated
several combinations of polymer and metal salts that yield
MALDI or laser desorption mass spectra using a single
tunable dye laser pulse for both matrix desorption and
metal ionization. Examples include copper with
polystyrene-2450 and silver with polystyrene-1250.
Although we have observed resonance effects (stronger
MALDI adduct signals with the laser tuned to the metal
atomic resonance), the highly variable nature of MALDI
ion signals makes it difficult to quantify the degree of
resonance enhancement.
At PNNL, ion trap reaction studies were made for
numerous metal cations reacting with selected small
organic molecules. We compared the reactivity of
n-octane versus 2,2,4-trimethylpentane with the following
metal cations: Sc + , Ti + , V + , Cr + , Mn + , Ni + , Co + , Cu + , Zn + ,
Y + , Zr + , Mo + , Ag + , and U + . Some of these choices were
dictated by prior knowledge of metal cation reactivity in
beam studies, e.g., propane + Ni + reactivity, or from prior
work where certain metals were reported to enhance
adduct formation in polymer MALDI (polystyrene +
Analytical and Physical Chemistry 15