Polar compounds in diesel soot and historic monument surfaces

Diesel soot, an important contributor to atmospheric aerosols, consists of a complex mixture of compounds. However, a very limited knowledge on the organic composition of aerosol and the water soluble fraction exists. A considerable portion of polar (oxygenated) organic compounds remained unanalysed, and additional analyses are needed to identify the polar compounds. Column fractionation of a diesel soot results in the obtaining of a methanol-soluble polar fraction enriched in aromatic polycarboxylic acids. This analysis provides a comprehensive dataset and important clues for solving the chemical nature of the polar (oxygenated) aerosol fraction and for identifying the polar compounds deposited on the surfaces of historic monuments. 2 MATERIAL AND METHODS A fractionation protocol was applied to a sample of diesel soot obtained from the exhaust pipe of a community bus with seven years of service. The sample was obtained by scrubbing the exhaust pipe, and although this sample may not be representative of a real (fresh) diesel exhaust, since the soot material that accumulated over time in the exhaust pipe of the bus experienced modifications, the sample is representative of an aged diesel soot that undergo chemical and physical transformations, thus providing a range of primary and secondary reaction products, which is well suited for the evaluation of our analytical protocol and for the study of the large amount of unknown organic species present in diesel soot. Twenty g of diesel soot was extracted in a Soxhlet apparatus with dichloromethane-methanol (2:1) during 70 h. The extracts were evaporated under vacuum at low temperature (below 40C) and redissolved in dichloromethane-methanol (2:1). The resulting extract was chromatographed using a silica column, eluting with hexane (fraction I), hexane-dichloromethane (1:1) (fraction II), dichloromethane (fraction III), and finally with methanol (fraction IV). Details of the fractionation protocol and analyses were reported elsewhere (Gaviño et al. 2004). Briefly, fraction I contained mainly a homologous series of n-alkanes in the range C14–C37, n-alkenes (C15–C27), alkylcyclohexanes (C9–C22), and alkylbenzenes (C11–C24). Pristane and phytane, diagenetic products of phytol and molecular markers of petroleum including α,β-hopane biomarkers were also identified. Fraction II contained a complex mixture of polycyclic aromatic hydrocarbons (PAH), including some of their alkylated derivatives. This PAH mixture included sulphur-, oxygen-, and nitrogen-containing polycyclic aromatic species. Fraction III was dominated by the series of n-fatty acids (C8–C30). In addition, polycyclic aromatic quinones and some other compounds coeluted with the n-fatty acids. Fraction IV contained the polar compounds. Fractions IIII will be not discussed in detail in this paper. In addition, no attempts to quantify the different fractions were made because the primary purpose of this work was to find a protocol providing a good separation and identification of polar compounds in complex matrices. Fraction IV was methylated with tetramethyl ammonium hydroxide (Sigma-Aldrich) (TMAH) and injected directly into a GC-MS Fisons GC 8000/MD 800. Alternatively, TMAH thermochemolysis of fraction IV was accomplished as described by Saiz-Jimenez (1994). In the analytical procedure used in this work, the carboxylic acids were recovered as the corresponding methyl esters and the hydroxyls as methoxyls. Throughout this paper they are referred to as acid and hydroxyls, their original forms, rather than as derivatised methyl esters and methoxyls. The compounds were identified by comparison of their mass spectra with a self-compiled data bank of compounds from a variety of macromolecules containing polar compounds (Martin et al. 1979, Saiz-Jimenez 1994). In addition, identification was achieved by computer analysis using the National Bureau of Standards and Wiley libraries, with the computer matching being checked against standards whenever possible. 3 RESULTS AND DISCUSSION To ascertain the nature and presence of polycarboxylic acids, fraction IV from the diesel soot was subjected to thermochemolysis at 300°C. Thermochemolysis has been used for the characterization of humic substances and the polar functional groups of their building blocks, as well as for the analysis of complex macromolecular materials (e.g. lignins) containing carboxyl and hydroxyl groups (Saiz-Jimenez 1994). Using this approach, the oxygenated functional groups can be protected from further degradation during GC-MS analysis due to the use of TMAH. Fraction IV upon TMAH thermochemolysis yielded a high number of compounds. No attempt was made to identify all compounds but the most abundant or representatives. The tentative identification of the most characteristic compounds is shown in Figure 1. The number of resolved peaks in the thermochemolysis chromatogram was much higher than those obtained by room temperature TMAH methylation and direct injection of fraction IV in the CG-MS (data not shown), which suggests that a number of the polar compounds were not amenable to the study by this method. It may be hypothesized that fraction IV actually consists of a very complex and unresolved mixture of relatively small molecules and were released only when subjected to thermochemolysis, denoting that this method was more effective than room temperature TMAH methylation and direct injection on the gas chromatograph. Figure 1. TIC chromatogram of fraction IV from diesel soot as analysed by thermochemolysis. Peak identification: 3 Hydroxybenzene, 4 Phenol, 5 Benzoic acid, 9 Pyridinecarboxylic acid, 11 Hydroxybenzoic acid, 16 Isobenzofurandione, 21 Furandicarboxylic acid, 25 Methylisoindoledione, 28 Thiophenedicarboxylic acid, 29 Benzenedicarboxylic acid, 30 Benzenedicarboxylic acid, 32 Pyridinedicarboxylic acid, 34 Methylbenzenedicarboxylic acid, 36 Unknown, 43 Unknown, 48 Cyanobenzenedicarboxylic acid, 50 Cyanobenzenedicarboxylic acid, 52 Benzenetricarboxylic acid, 53 Benzenetricarboxylic acid, 55 Hexadecanoic acid, 57 Hexadecanoic acid, 59 Naphthalenedicarboxylic acid, 66 Octadecanoic acid, 67 Unknown, 69 Acenaphthenedicarboxylic acid, 74 Dihydroxybenzenedicarboxylic acid, 80 Unknown, 81 Unknown, 85 Benzenedicarboxylic acid dioctyl ester, 92 Fluorenonedicarboxylic acid, 94 Anthracene/phenanthrenedicarboxylic acid, 104 Squalene. All acids were identified as methyl esters and hydroxyls as methoxyls, except for peak 57, which was identified as free acid of hexadecanoic acid. In the complex chromatogram of fraction IV from diesel soot, polycarboxylic acids represented the majority of compounds. In fact, benzenepolycarboxylic acids (including methyl and hydroxyl derivatives) represented a 36 % of the total identified products. Also carboxylic acid derivatives from polycyclic aromatic hydrocarbons (PAH) were present (naphthalene-, fluorene, acenaphthene-, anthracene/phenanthrene-, etc.) representing a 23 % of identified products. If fatty acids and oxygen-, nitrogenand sulphur-heterocyclic carboxylic acids were also considered, all carboxylic acid derivatives amounted for 88 % of the total identified products. The identification of an important amount of carboxylic acid derivatives, including benzenepolycarboxylic acids, in the polar fraction of diesel soot is of interest since as far as we know no previous reports shed light on a complete or cuasi-complete individual characterization of their components. The low number of benzenecarboxylic acids identified by other authors is no satisfactory and does not correspond with the amount of benzenecarboxylic acids expected in aerosols and/or diesel soot (e.g. Gelencser et al. 2000, Subbalakshmi et al. 2000, Kubátová et al. 2009). In this work over 100 compounds have been identified, from which polycarboxylic acids constitute a considerable portion of the polar fraction of diesel particulate matter directly extracted from a bus exhaust. The information obtained on diesel soot composition, and particularly on the polar fraction, is more complete than that reported earlier. This also applies to the composition of the black crusts of Saint Denis basilica in France (Gaviño et al. 2004), where an important number of benzenepolycarboxylic acids were identified as major components of a polar, yellow fraction IV. We show that diesel soot components, and by extension aerosol organic compounds and black crusts components, can be individualized after a fractionation in column, according to the polarity of their components. This fractionation results in obtaining fractions enriched in selected classes of compounds (hydrocarbons, PAH and aliphatic acids) and in a polar fraction enriched in aromatic polycarboxylic acids, which components can be analysed and chromatographed after protection of their functional groups with TMAH. This analysis could provide important clues for solving the chemical nature of the polar (waterand/or methanol-soluble) aerosol fraction and black crusts from monuments. In the aerosols the compounds bearing carboxyls and hydroxyls can react to form esters for which it is needed an inorganic acid as catalyst. This reaction is possible in diesel soot, due to its H2SO4 content. The identification of a high amount of aromatic polycarboxylic acids upon thermochemolysis that were not evidenced by direct injection of the methanolic fraction in a GC-MS indicated that saponification of ester linkages might be a possible mechanism releasing a part of these aromatic polycarboxylic acids. If this hypothesis is confirmed, it will provide a new understanding of the chemical nature and origin of polymeric compounds in aerosols and black crusts. In fact, a number of important scientific issues are critical in air quality and climate change, such as to ascertain the role of carboxylic acids in cloud formation, the interaction of aerosols and volatile organic compounds with other molecules, the polymeric nature of humiclike substances (HULIS), etc. (Graber & Rudich 2006). A better knowledge of the composition of polar fractions in aerosols and particulate matter, as it was shown here for the polar fraction of diesel soot, can help to answer some of these issues.