In recent years there has been increasing interest in the public and in the authorities
on properties and origin of atmospheric particulate matter (PM) because of it has
been established its negative influence on environment and human health¹. In
particular, it is becoming more important to understand the mechanisms that lead to
the formation and distribution of atmospheric aerosol. It is a critical problem for
analytical chemistry, because the particles consists of more than thousands of
substances, both organic and inorganic, so it has not yet been realized a total
speciation². Moreover, the processes of atmospheric transport of air masses and the
transformation of compounds, which are subjected to oxidizing agents and chemical
reactions, make it essential to identify the sources even when they are very distant
from the point of collection of PM. For this reason, rather than characterizing all the
substances, the modern approaches tend to identify specific substances (markers) that
are characteristic of specific sources, in order to distinguish between biogenic and
anthropogenic sources and estimate the oxidation level of the substances produced
from primary sources (SOA: Secondary Organic Aerosol)³-⁵.
The aim of this work was to develop an innovative multi-residue method for the
analysis of a complex system like the atmospheric aerosol in order to obtain
information about the sources, the anthropogenic contribution and the oxidation level
(SOA), identifying and quantifying a wide range of compounds.
This work had focused on a group of substances present in the PM that could provide
relevant information: the soluble organic fraction (WSOC) in PM, that is the most
bioavailable, may be useful in order to estimate the risk on human health. The main
components of this fraction are the dicarboxylic acids and sugars³, ⁶.
These compounds are present at very low concentrations (average few ng/m3 of air
sampled), for this reason there is critical need for efficient and sensitive analytical
techniques; gas chromatography coupled with mass spectrometry (GC-MS) is the
mostly used because of its great sensitivity and excellent separation and
identification capabilities on highly complex matrices such as PM. Moreover, it is
necessary a preliminary process of derivatization in order to raise the volatility of
polar compounds for GC analysis.
Therefore, this work started considering and comparing the most used derivatization
techniques, the silylation and esterification, in order to choose the one that would
reach the level of sensitivity, specificity and applicability necessary for
environmental analysis⁷-⁹. From the results, the best technique is silylation with N,
O-bis (trimethylsilyl)-trifluoroacetammide (BSTFA): the developed method permits
to analyze 15 carboxylic acids with detection limits lower than 3ng/m3.
The study was extended in order to analyze other compuonds present in WSOC: in
particular sugars, that are specific molecular markers that provide important
information on sources of organic compounds in PM.
The derivatization reaction was optimized, in particular the parameters that affect the
yield and the sensitivity of the analytical method, such as time and reaction
temperature and the amount or composition of the reagents. We used a mathematical/statistical approach to assess simultaneously the effect of these factors
with a small number of experimental tests. We used an experimental design model: a
central composite design¹⁰. This allowed the development of a method for the
simultaneous determination of 15 acids and 7 sugars present in atmospheric particles
in concentration level less than 10ng/m3 in most cases.
The procedure was successfully applied to a series of samples of atmospheric
particles, both fine (PM2.5) and ultrafine (PM1), within an integrated environmental
monitoring of a local area around Bologna (Project Moniter , developed by ARPAER):
consisted of two sampling campaigns, one on summer and one on winter, over
8 sites with urban, rural or intermediate characteristics. The analytical data obtained
have provided significant information on the contribution from anthropogenic urban
transport and domestic heating, the contribution due to agriculture and the state of
formation of organic substances originating from processes in the atmosphere (SOA).
In the development of innovative methods applicable to environmental monitoring, it
has also studied a new method for analysis of PM samples, based on direct thermal
desorption, with in situ derivatization and gas chromatography coupled with mass
spectrometry time of flight (IDTD-GC-TOFMS)¹¹-¹³. This study was conducted in
collaboration with Prof. Ralf Zimmermann and his research group at the Helmholtz
Zentrum in Munich (Germany). This technique enables the analysis of environmental
samples avoiding the most common extraction treatments because the analytes are
thermally directly desorbed from filter and can enter the chromatographic column.
The addition of the silylating agent on the filter permits a gas-phase reaction of polar
compounds. The application of IDTD-GC-TOFMS in PM2.5 samples simultaneously
led to the identification of both polar (such as sugars) and apolar species (e.g. alkanes
and polycyclic aromatic hydrocarbons), with great advantage in terms of time
consuming and loss of analytes during sample pretreatment.
In conclusion, it had been developed analytical methods suitable for environmental
monitoring in order to obtain information on chemical composition that can be
integrated with other information, i.e. physical, meteorological, environmental, for an
enhanced approach to the study of particulate air pollution and its impact on the
environment and human health.
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