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Post
Workshop Summary
Session
2: Analytical Challenges
Topic Leader: Monica Mazurek
Contributors: Bernd Simoneit, Stephen Wise,
Michelle Schantz and Joellen Lewtas
I. CONNECTION TO WORKSHOP
GENERAL TOPICS
Presentations and posters
comprising Session 2: Analytical Challenges link primarily to
the following workshop topics:
Topic 2. What are the conventional
and emerging methods for collecting and analyzing organic carbonaceous
aerosols, and how do we assess those methods for their ability
to fulfill the needs of public health, climate, and modeling?
Topic 3. How do we assess accuracy
and precision, and what criteria should be met for regulatory
or other purposes?
This
workshop session addresses the current state-of-the-science for
molecular marker analysis in aerosol complex mixtures. Invited
presentations and posters focus on the detection and measurement
of molecular markers in atmospheric fine particles.
One common analysis method for organic mixtures employs Gas Chromatography/
Mass Spectrometry (GC/MS) to identify and measure molecular markers. Although GC/MS is a fairly routine application for organic compounds
associated with ambient particles, little information about the
precision of these measurements has been provided for the parts-per-billion
determinations of single marker compounds in urban particular
matter (PM) (Li et al., 2004). Such information is critical input
to current source apportionment models since the uncertainty of
analytical measurement itself is the primary quantifiable uncertainty
in source receptor models. Regulatory groups must understand underlying
measurement and precision factors relating to organic marker ambient
mass concentrations before requiring and implementing any control
strategies on specific urban sources of PM.
Three
critical elements underlying reliable identification and measurement
of source specific molecular markers are: 1) an analytical method
having well documented measurement precision and accuracy; 2)
authentic standards for verification of the mass abundance of
marker compounds, routine calibration of mass detector instruments,
and for monitoring compound recovery throughout the analytical
protocol; and 3) expanded and dedicated mass spectral libraries
for molecular markers enabling improved interpretation of compound
mass spectra and for verification of known and new molecular
tracers.
Invited
presentations in “Session 2. Analytical Challenges” focus on:
1) components of the collection and analytical protocol necessary
for identification and quantitation of molecular markers in
fine particle samples; 2) standard reference materials, new
standards for aerosol marker compounds, and results of intercomparison
laboratory trials for molecular markers in urban dust standard
reference material; and 3) advanced mass spectrometric interpretation
methods for current and new target compounds in complex organic
mixtures from airborne particles. Summaries for the invited
presentations are presented in Section 2.
II.
BRIEF
OVERVIEW OF CURRENT KNOWLEDGE
Detection
and measurement of organic compounds at the molecular level
is now a routine practice for many air quality ambient monitoring
studies. Molecular tracer analysis provides a powerful approach
for linking major emissions with observed concentrations of
fine particles. Quantitative estimates of major emissions to
observed fine particle ambient mass concentrations are developed
by using mass ratios of marker concentrations to the total organic
aerosol ambient mass. Rogge et al., (1993) first described the
molecular marker approach for fine particle apportionment work
using ambient marker concentrations and emissions profiles measured
for metropolitan Los Angeles. Further development of the mathematical
model was published by Schauer et al., (1996) linking emissions
inventories, emissions sources chemical compositions, and ambient
concentrations of organic molecular tracers. Identification
of molecular markers in ambient fine particles is incorporated
into current research and monitoring activities on the sources
and compositions of fine particles, including the current Supersites
project funded by the U.S. Environmental Protection Agency.
Typically,
a single molecular marker compound comprises only a minute fraction
of the organics fine particle mass fraction with ambient ratios
of marker mass to total organics in units of ng m-3 and
mg m-3, respectively. For example, ambient mass ratios
of hopane fossil fuel biomarkers to organics can
range from [0.08-2.14]x10-3 for metropolitan New
York City (Mazurek et al, 2004) to [0.87-3.50]x10-3
for metropolitan Los Angeles (Schauer et al., 1996). Consequently,
the analytical protocol for detecting and quantifying a molecular
marker within ambient particulate matter must be sensitive and
precise.
Nearly
two decades have passed since the gas chromatography/mass spectrometry
based analytical protocol was developed by Mazurek and coworkers
and applied to ambient fine particle samples (Mazurek et al.,
1987; Mazurek and Simoneit, 1981; Rogge et al., 1993) and emission
source samples (Mazurek et al, 1989, 1993; Hildemann et al.,
1991) over the period 1982 to 1984 in metropolitan Los Angeles.
Many innovations in molecular level analysis have occurred since
this time involving advances in instrumentation, separation
of complex organic mixtures, improvements in measurement accuracy
and precision, and in strategies for improved interpretation
mass spectrometric data for target marker compounds.
III.
SUBJECT
MATTER OF THE PRESENTATIONS BY TOPIC
A.) A Critical Assessment of the Molecular-Level
Analytical Protocol for Ambient Fine Particles
Monica
A. Mazurek,
Department of Civil and Environmental Engineering, Rutgers
University, 623 Bowser Road, Piscataway, NJ, 08854-8014.
Organic
molecular tracers in fine particulate matter constitute only a
minute fraction of aerosol mass. Given the sub parts-per-billion
concentrations of organic marker compounds present in most urban
atmospheres, the analytical protocol for detection and measurement
is detailed and requires high precision and accuracy. An essential
feature of the molecular analysis protocol involves a thorough
quality assurance/quality control (QA/QC) plan. The QA/QC plan
examines sampling, and filter handling and preparation steps evaluated
also at the molecular level with identical instrumentation for
compound detection and quantification. Typically, quadrupole electron-impact
mass detection is used with pre-separation by high resolution
gas chromatography (GC).
Although
the GC/MS molecular marker technology was developed in the early
1980’s, no general criteria have been developed for how accurately
and precisely a marker compound must be measured, what are critical
detection limits, or what surrogate analytes must be incorporated
into a sample to monitor method, instrument, and analyst performance.
Each of these factors is critical for producing molecular marker
measurements of known quality (Budde, 2001). Finally, additional
method validation steps, including laboratory duplicate sample
aliquots, performance check standards, and field duplicate samples,
generally are not conducted for molecular marker characterization
work, but are essential to improving ambient mass concentration
measurements. This presentation addresses these analytical protocol
elements as key challenge areas for molecular marker measurement
and identification in fine particle samples.
B.) Reference Material and Quality Assurance Needs
to Support Organic Speciation Measurements in Air Particulate
Matter
Stephen
A. Wise and Michele M. Schantz, National Institute of Standards and Technology
(NIST), Analytical Chemistry Division, 100 Bureau Drive Stop
8392, Gaithersburg, MD 20899-8392; Joellen Lewtas,
USEPA, NERL, Manchester Lab, 7411 Beach Dr. E., Port Orchard,
WA 98366.
One
of the first environmental matrix Standard Reference Materials
(SRMs) developed by the National Institute of Standards and Technology
(NIST) for determination of organic species was SRM 1649 Urban
Dust, an ambient total suspended particulate matter sample collected
in Washington DC in the late 1970’s. Since SRM 1649 was issued
in 1981, it has found widespread use in the particulate matter
(PM) measurement community, and NIST has assigned values for over
100 organic species in this material. However, there is a growing
need for additional reference materials to support organic speciation
of PM, particularly for the fine particulate matter fraction and
representative of contemporary combustion sources. NIST is collaborating
with the U.S. Environmental Protection Agency (EPA), with input
from a group of investigators involved in EPA’s PM research program
and related studies, to develop additional SRMs and to provide
interlaboratory comparison exercises to improve the accuracy and
comparability of organic speciation measurements. SRM activities
include development of both PM matrix and calibration solution
SRMs for organic species of interest in PM characterization. For
development of a future PM-matrix SRM, efforts are underway to
obtain a suitable quantity of a fine PM either through collection
of fine PM or size fractionation of existing total suspended particulate
material to provide fine particulate fraction. We are assessing
also the suitability of a fine PM on filter media SRM, which was
developed for carbon measurements, for organic speciation measurements.
Calibration solution SRMs containing a wide range of organic species
are under development including: polycyclic aromatic hydrocarbons
(PAHs) (two redesigned solutions with an expanded list of 53 PAHs
and alkyl-substituted PAHs), aliphatic hydrocarbons, nitro-substituted
PAHs (redesigned and expanded list of compounds), hopanes/steranes,
and 13C-labeled levoglucosan for use as an internal standard.
In addition to the SRMs developed in conjunction with EPA, several
additional PM-matrix SRMs for organic speciation are currently
in progress including: SRM 2585 Organic Contaminants in House
Dust and SRM 1650b Diesel Particulate Matter. In addition to the
SRM development activities, two NIST/EPA interlaboratory comparison
studies have been conducted to assess and improve the comparability
measurements of organic species in PM. This presentation will
discuss these SRM and quality assurance activities and their potential
impact on improving the accuracy of organic speciation measurements
for PM characterization.
This
work has been funded in part by the U S Environmental Protection
Agency. It has been subjected to Agency review and approved
for publication.
C.) Characterization
of novel organic tracers in aerosols by mass spectrometry
Bernd
R. T. Simoneit, Oceanic and Atmospheric Sciences, Oregon State University,
104 Oceanography Administration Building, Corvallis, OR 97331-5503.
Organic
compounds in aerosols are useful as tracers for assessment of
sources, alteration and fate in indoor, urban and global air sheds.
Indoor and urban pollution research has been reported mainly in
the U.S. and European literature and both organic and inorganic
tracers have been applied.
Progress
in defining new organic tracers in aerosols was mainly due to
instrument development (GC-MS sensitivity) and the applications
of the biomarker compounds elucidated in the geologic record
by organic geochemists, the natural compounds characterized
by natural product chemists, and the synthetic compounds from
the chemical industry (Simoneit, 1999). Mass spectrometry (MS)
is the analytical method of choice and compound identifications
must be coupled initially with comparisons to authentic standards
or structure proofs by MS, NMR and syntheses. It is now routine
to analyze total extracts (both organic or aqueous) directly
by GC-MS after suitable derivatization of the polar compounds.
Preparation of separated polarity fractions (by LC or TLC) remains
an option for selected samples to gain additional functional
group information. Derivatization is typically carried out by
methylation and /or trimethylsilylation. This can involve MS
interpretation because the derivatives (especially TMS) are
not necessarily in the library or the free compound or acetate
derivative MS may be archived. High temperature GC-MS can also
be applied for high molecular weight compound identification
( e.g., wax esters to C40, alkanes to C100).
The
processes of MS interpretation and data evaluation (identification
of the compounds in a mixture analyzed by GC-MS) will be illustrated
here with some examples. Smoke from burning of contemporary
(biomass, refuse, etc.) and fossil fuels are a global problem
and the mass spectrometric identification of tracers for this
process is discussed (Simoneit el al., 1999). Soil resuspension
and erosion is another unquantified emission source and its
contribution to the ACE-Asia aerosols is presented in another
example (Simoneit et al., 2003). Also, a brief discussion of
what not to do with organic tracer analyses is included (Simoneit,
2003). The total extract GC-MS analysis method with selected
derivatization is a powerful tool for determining aliphatic
homologous lipids, natural products, fossil fuel components,
secondary oxidation products, PAH, UCM, phenols, saccharides,
etc. and thus attaining an overview of the major and key organic
tracers in aerosol PM.
IV. MAJOR FINDINGS AND RECOMMENDATIONS OF SESSION
2: ANALYTICAL CHALLENGES
Workshop
presentations identified the following areas for further research
where the greatest capacity for advancement exists for enhancing
current science and technology for air pollution research and
complementary disciplines:
1.)
A chemical species mass balance provides a quantitative
framework for assigning PM organic substances; from bulk carbon
fractions (total carbon, elemental carbon, organic carbon)
and chemical compound groups (alkanes, PAHs, alcohols, aldehydes,
alkanoic acids, dicarboxylic acids) to individual marker compounds.
The chemical species inventory is essential for relating
PM source profiles to receptor site concentrations for OC,
EC and molecular tracers emitted from primary emission sources.
Although only a minor fraction of the total PM OC is
identified at a molecular level, the chemical mass balance
of PM carbonaceous species is a quantitative description that
will accommodate new analytical technologies and new molecular
tracers for comparison to existing bulk carbon and molecular
level ambient PM data.
2.)
Organic carbon (OC) and elemental carbon (EC) are
critical bulk chemical measurements of ambient PM.
Mass ratios are constructed routinely for molecular
marker concentrations to sample OC and EC concentrations for
source apportionment applications, ambient PM chemical compositions,
and emission source chemical compositions.
Currently 15 methods are used operationally to measure
the OC and EC fractions of PM. Method intercomparisons using ambient PM samples
and certified standards would link OC and EC results generated
from the suite of measurement approaches.
3.)
Incorporating more rigorous statistical design in PM
collection and analysis protocols should improve current knowledge
of method precision and bias for molecular marker concentrations
in ambient PM samples and in emission source profiles.
Co-location of duplicate PM samplers would increase confidence
of PM organic chemical compositions.
Simultaneous deployment of organic chemical species collectors
and alternative bulk, chemical group, and molecular level analysis
methods would improve current knowledge of overall bias and
precision.
4.)
New suites of certified reference materials for urban
PM are becoming available to the measurement and analysis
community through the National Institute of Standards and
Technology. These reference materials are available from NIST for individual
laboratory use. Additionally,
NIST and US EPA are distributing the certified urban PM materials
to PM research groups as part of laboratory trials. Results from the first two trials will be published
soon, identifying factors which allow for greater precision
and accuracy of molecular marker identification and quantitation
in urban PM. NIST is soliciting recommendations from PM
research groups for additional chemical standards that are
either to expensive for individual groups to purchase and
prepare, or are not available from commercial suppliers.
The new NIST chemical standards will assist laboratories
identify and measure key marker compounds in PM ambient and
emission source studies. The NIST certified reference materials and the new chemical standards
will improve current measurement and analysis methods and
also assist with the development and validation of emerging
technologies.
5.)
Interpretation and validation of mass spectra for
marker compounds in PM complex organic mixtures will benefit
from the increased availability of authentic standards such
as those produced from NIST. Novel marker compounds from major emission
sources (anthropogenic, biogenic, synthetic, geogenic) and
from secondary photochemical processes are important for improving
the detail of PM organic chemical composition studies.
Opportunities exist for merging dedicated mass spectral
libraries, converting these to electronic formats, and increasing
access by the PM research community.
Because such an effort is not fundamental research,
but more a synthesis and digital conversion process, funding
mechanisms should be coordinated by federal and private sources
to coordinate and support this necessary research tool for
PM organic chemical molecular level research efforts and monitoring
programs.
References
Budde,
William L. Analytical Mass Spectrometry: Strategies for Environmental
and Related Applications. American Chemical Society, Washington,
DC. 386 pp., 2001.
Hildemann,
L. M., M. A. Mazurek, G. R. Cass, and B. R. T. Simoneit, Quantitative
characterization of urban sources of organic aerosol by high-resolution
gas chromatography, Environ.Sci.Technol., 25, 1311-1325, 1991.
Li,
M., S. McDow, D. Tolerud, M. A. Mazurek, Quantitation, detection,
and measurement precision of organic molecular markers in urban
particulate matter, Aerosol Science & Technology, submitted
2004.
Mazurek,
M. A., G. R. Cass, and B. R. T. Simoneit, Interpretation of
high-resolution gas chromatography and high-resolution gas chromatography/mass
spectrometry data acquired from atmospheric organic aerosol
samples, Aerosol Science & Technology, 10, 408-420, 1989.
Mazurek,
M. A., L. M. Hildemann, G. R. Cass, B. R. T. Simoneit, and W.
F. Rogge, Methods of analysis for complex organic aerosol mixtures
from urban emission sources of particulate carbon, in Measurement
of Airborne Compounds: Sampling, Analysis, and Data Interpretation,
edited by E. D. Winegar, pp. 177-190, American Chemical Society
Symposium Series, CRC Press, Inc., Boca Raton, FL, 1993.
Mazurek,
M. A., M. Li, S. McDow, J. Graham, D. Felton, C. Pietarinen,
A. Leston, S. Bailey, Speciation of Organics for Apportionment
of PM2.5 (SOAP) in the New York City Metropolitan Area, presented
at the Mid-Atlantic Regional Air Management Association 2004
Science Meeting, January 27-29, 2004, Baltimore Maryland.
Mazurek,
M. A. and B. R. T. Simoneit, Characterization of biogenic and
petroleum-derived organic matter in aerosols over remote, rural
and urban areas, in Identification and Analysis of Organic Pollutants
in Air, edited by L. H. Keith, pp. 353-370, Ann Arbor Science/Butterworth,
Boston, MA, 1984.
Mazurek,
M. A., B. R. T. Simoneit, G. R. Cass, and H. A. Gray, Quantitative
high-resolution gas chromatography and high-resolution gas chromatography/mass
spectrometry analyses of carbonaceous fine aerosol particles,
Int.J.Environ.Anal.Chem., 29, 119-139, 1987.
Rogge,
W. F., M. A. Mazurek, L. M. Hildemann, G. R. Cass, and B. R.
T. Simoneit, Quantification of urban organic aerosols at a molecular
level: Identification, abundance and seasonal variation, Atmos.Environ.,
27A, 1309-1330, 1993.
Schauer,
J. J., W. F. Rogge, L. M. Hildemann, M. A. Mazurek, and G. R.
Cass, Source apportionment of airborne particulate matter using
organic compounds as tracers, Atmospheric Environment, 30, 3837-3855,
1996.
Simoneit,
B.R.T. Organic matter in eolian dusts over the Atlantic Ocean.
Marine Chemistry 5, 443-464. 1977.
Simoneit,
B.R.T. A review of biomarker compounds as source indicators
and tracers for air pollution. Environ. Sci. and Pollut. Res.
Int. 6, 159-169, 1999.
Simoneit,
B.R.T. Polemic response to Mayol-Bracero et al. (2001) Atmos.
Environ. 36, 5259-5263, 2002.
Simoneit,
B.R.T., J.J. Schauer, C.G. Nolte, D.R. Oros, V.O. Elias, M.P.
Fraser, W.F. Rogge and G.R. Cass. Levoglucosan, a tracer for
cellulose in biomass burning and atmospheric particles. Atmos.
Environ. 33, 173-192, 1999.
Simoneit,
B.R.T., Kobayashi, M., Kawamura, K. et al., Saccharides, lipids
and oxidation products in Asian dust and marine aerosols of
the East Asia/Pacific region. Geochim. Cosmochim. Acta 67, A437,
#2003.
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