Ph.D. Thesis
Title Development of two-step laser mass spectrometry
as a competitive analytical-chemical method
Advisers Prof. Renato Zenobi
Thesis Committee Prof. Renato Zenobi (ETH, Zurich), prof. Walter
Giger (EAWAG, Dubendorf), PD Dr. Urs Baltensperger (PSI, Villigen)
Essay
Two-step laser mass spectrometry (L2MS) is an innovative
technique for the direct analysis of selected compounds in complex
mixtures such as environmental samples. In the first step, an infrared
laser pulse desorbs intact neutral molecules from the sample surface,
or ablates them from a solid matrix. In the second step, a pulse from
a tunable ultraviolet laser is used for resonance-enhanced multiphoton
ionization (1+1 REMPI) of the desorbed species; this soft ionization
scheme prevents fragmentation of the analytes. Mass analysis is then
performed in a reflectron time-of-flight mass spectrometer. The mass
spectra are dominated by intact parent ions of those mixture components
that strongly absorb the selected ultraviolet laser wavelength. Major
advantages are the detection limit in the low attomole range and minimal
need for sample preparation, therefore giving the possibility to measure
large numbers of samples within short periods of time.
In an initial characterization stage, the potential
capabilities offered by L2MS for both quantitative and qualitative
measurements were assessed in this thesis. A new, fast, and convenient
sample preparation procedure was developed that allows accurate and
reproducible control over the amount of analytes desorbed by each
laser pulse. Quantitative L2MS measurements were demonstrated for
polycyclic aromatic hydrocarbons (PAHs) over three orders of magnitude
using this sample preparation method. The wavelength (238 - 310 nm)
dependence and the laser pulse energy dependence of the 1+1 REMPI
ion yield was then studied for 17 PAHs. This data allows one to determine
which wavelengths are optimal to measure with high sensitivity and
selectivity the analytes of interest using L2MS or related methods.
Chemical analyses of atmospheric aerosol particles were
performed during extended field measurement campaigns. First, the
aerosols emitted by all major emission sources including Diesel vehicles,
gasoline vehicles, residential heating, wood fires, and cigarettes
were characterized. For each emission source, tracer mass spectral
patterns were identified, notably based on specific PAHs and other
polycyclic aromatic compounds (PACs). Only small sample quantities
were necessary and the measurements were performed within minutes.
L2MS was found to be a valuable alternative to more laborious chemical
analysis techniques that often require extensive sample preparation.
These results were then applied to the study of the
dynamic behavior of aerosol-bound PACs in urban air over the course
of several whole days, both next to a street with heavy traffic and
in a city park. L2MS allowed measurements with 15 minute time resolution.
Large variations in particle concentration and chemical composition
were observed, reflecting the contributions from Diesel trucks and
gasoline-powered cars to urban aerosols. The photochemically induced
decay of oxygenated polycyclic aromatic hydrocarbons (OPAHs) was observed
in real time. In a similar way, the dynamic behavior of aerosol-bound
PACs in air was studied over the course of a whole year at four sites
representative of the different types of aerosol exposure in Switzerland.
This project was completed in the framework of a collaboration between
several research institutes in order to achieve a chemical characterization
of the aerosol samples that would be as thorough as possible. L2MS
allowed the measurement of about 1000 samples, which would not have
been possible within a reasonable amount of time using chromatographic
techniques. A strong seasonal dependence of the amount of aerosol-bound
PACs was observed.
Finally, to exemplify the versatility of L2MS, studies
were undertaken to demonstrate the applicability of L2MS to the chemical
analysis of contaminants in environmental water. This thesis establishes
L2MS as a competitive analytical-chemical method that can provide
innovative contributions to unanswered analytical problems due to
its-specific advantages.
