Studio della caratterizzazione chimica di composti organici polari nel particolato atmosferico

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. 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