General
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LC/MS/MS
Analysis of Ambient Air Samples: Determination of PAH-Quinones
and Algae-Derived Toxins in Ambient Air
Jake
D. McDonald, Dean Kracko, and Yung-Sung Cheng
Lovelace Respiratory Research Institute, Albuquerque,
NM
High
performance liquid chromatography coupled to a triple quadrupole
mass spectrometer (LC/MS/MS) was employed for the determination
of two unique classes of organic compounds; polycyclic aromatic
hydrocarbon (PAH)-quinones and brevotoxin (toxins in marine algae
that are found in ambient air in coastal areas) compounds. Both
of these organic compound classes have been implicated for an
association between exposure (by inhalation) and acute respiratory
health, and both of these classes pose unique analytical challenges
that are alleviated from the utilization of LC/MS/MS. Simultaneous
description of these methods allows an illustration of the range
of the instrument application, and the challenges/considerations
taken during method development of different compounds. PAH-Quinones,
especially the higher molecular weight benzo(a)pyrene-quinone
isomers, are typically present in ambient air at extremely low
concentrations (e.g. pg/m3). Their analysis by gas chromatography
is possible, but their low concentrations (relative to background)
and poor chromatography (without targeted derivitization) decrease
the selectivity and sensitivity of that technique. Analogous to
selected ion monitoring in GC/MS analysis, LC/MS/MS analysis of
select parent:daughter ion pairs offers superior selectivity by
screening most of the ions in the between the first and third
quadrupole. The result is a “clean” chromatogram of
the target analytes without prior “clean-up” of the
sample matrix. Enhanced sensitivity is provided by implementation
of a recently developed atmospheric pressure photoionization source.
In contrast, a heated nebulizer ionization source was required
for the analysis of brevotoxins. These compounds are too large
to analyze by GC (m.w. > 800 amu), and their concentrations
are too low to analyze by LC with non selective detectors. Immuno-sorbent
assays (ELISA) have been developed for these compounds, but their
lack of specificity has left the results difficult to interpret.
The LC/MS/MS was configured to produce a sensitive (e.g. pg/µl)
assay that removed matrix interferences (by parent:daughter ion
discrimination. The approaches for LC/MS/MS method development,
and the range/limitations of the technique for ambient air analysis,
will be placed in context in these method descriptions.
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Measurement
of monosaccharide anhydrides in ambient aerosols for estimation
of the wood smoke component
Roy
W. Dixon and Jennifer R. Cruz
Chemistry Department, California State University,
Sacramento
In cities in the central valley of California, high aerosol mass
concentrations occur during periods of stagnant air in winter.
Smoke from residential wood combustion may contribute significantly
to aerosol mass during such time periods. Past studies have shown
that wood smoke contains high concentrations of levoglucosan (1,6-anhydro-ßD-glucopyranose)
as well as the presence other monosaccharide anhydrides. We are
investigating using monosaccharide anhydride concentrations to
estimate the fraction and concentration of aerosol mass originating
from wood smoke. Twenty-three air filter samples of 24 hour duration
were collected using California Air Resource Board PM2.5 (particulate
matter under 2.5 mm in diameter) samplers located in Fresno and
Chico, CA in January, 2001. Whole filters were extracted with
methanol and water and analyzed using a novel method employing
high performance liquid chromatography (HPLC) with aerosol charge
detection. The HPLC system used a heated ligand-exchange column
with a 100% aqueous eluent. In aerosol charge detection, the effluent
from the HPLC column is converted to an aerosol and detected using
an electrical aerosol size analyzer. Levoglucosan, mannosan, and
an unidentified peak were well resolved while other sugars and
galactosan were affected by some peak overlap or contaminant peaks.
Levoglucosan was easily detected in all samples, while several
other compounds were observed in samples more heavily affected
by wood smoke. Concentrations of levoglucosan ranged up to 9.5
mg m-3, among the highest reported ambient concentrations, and
some of the aerosol mass concentrations were above national standards.
Major ions concentrations also were measured on some concurrently
collected air samples. We are investigating simple mass apportionment
models using one or two composite sources of wood smoke and levoglucosan,
mannosan and potassium concentrations. The biggest challenges
appear to be the variability in source levoglucosan fractions
between hardwood and softwood smoke sources, collection of appropriate
source data, and low signal to noise ratios in mannosan measurements.
Levoglucosan was observed to correlate with both mannosan and
potassium at each site. Higher mannosan to levoglucosan ratios
observed in the Fresno samples are attributed to a greater fraction
of smoke from softwood combustion. Wood smoke appeared to be a
significant component of the aerosol mass in Fresno and a dominant
component in Chico.
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Identification
and quantification of biogenic marker compounds for secondary
organic aerosol formation
Thorsten
Hoffmann
Johannes Gutenberg University of Mainz
Institute of Inorganic and Analytical Chemistry
55128 Mainz Germany
Organic
contributions to the atmospheric particle phase are known to derive
from primary and secondary sources. Together these sources can
account for 20 to 60% of the total PM2.5, depending on the site
and time of the measurements. A significant fraction is believed
to derive from natural sources. However, despite the high mass
contribution of organic compounds, quantitative information about
their origin and formation pathways are still rare (e.g. anthropogenic
vs. natural, primary vs. secondary). The analytical challenge
is always connected with the fact that the organic aerosol fraction
is comprised of hundreds of individual compounds. This is also
true for secondary organic aerosol components from natural sources.
In
the past few years, several analytical techniques have been developed
to identify and quantify specific marker compounds for biogenic
SOA formation. Typically, these compounds represent low molecular
weight, highly polar organics, such as multifunctional carboxylic
acids. For the investigation of organic aerosol formation in chamber
studies, mass spectrometric on-line techniques have successfully
been utilized. The high time resolution of these
techniques often enable a direct insight into the chemical mechanisms
leading to condensable products, especially when MS/MS capabilities
are applied. Nevertheless, due to the complex composition of ambient
tropospheric aerosols, the low concentration of individual marker
compounds and the undesired fragmentation using classical ionization
techniques of existing real-time aerosol measurement instruments,
typically offline methods have to be employed for field measurements
of organic tracer compounds.
Focused
on biogenic SOA marker, this presentation tries to summarize the
current state-of-the-art of on-line and off-line methods for the
analysis of polar secondary organic aerosol components. As a part
of that, recent results of our group using an on-line aerosol
measurement techniques based laser desorption in combination with
a soft ionization techniques will be presented at the workshop.
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