Worldwide Solutions - February 2008 - (Page 13) GERSTEL Solutions Worldwide Application Chemistry of PAHs The simplest PAHs, as defined by the International Union on Pure with two rings are more soluble in water and more volatile. Because of and Applied Chemistry (IUPAC) [IUPAC nomenclature for fused-ring these properties, PAHs in the environment are found primarily in soil systems], are phenanthrene and anthracene. Smaller molecules, such and sediment, as opposed to in water or air. PAHs, however, are also as benzene and naphthalene, are not formally PAHs, although they often found in particles suspended in water and air. Natural crude oil are chemically related they are called one-ring (mono) and two-ring and coal deposits contain significant amounts of PAHs, as do com(di) aromatics. bustion products and smoke from naturally occurring PAHs may contain four-, five-, six- or sevenforest fires. member rings, but those with five or six are most PAH toxicity is very structurally dependent, with common. PAHs composed only of six-membered isomers (PAHs with the same formula and number of rings are called alternant PAHs. Certain alternant rings) varying from being non-toxic to being extremely toxic. Thus, highly carcinogenic PAHs may be small PAHs are called „benzenoid“ PAHs. The name or large. One PAH compound, benzo[a]pyrene, is nocomes from benzene, an aromatic hydrocarbon table for being the first chemical carcinogen to be diswith a single, six-membered ring. These can be covered (and is one of many carcinogens found in benzene rings interconnected with each other by cigarette smoke). The EPA has classified seven PAH single carbon-carbon bonds and with no rings recompounds as probable human carcinogens: benz[a] maining that do not contain a complete benzene anthracene, benzo[a]pyrene, benzo[b]fluoranthene, ring. benzo[k]fluoranthene, chrysene, dibenz[a,h]anthraThe set of alternant PAHs is closely related to Benzo[ghi]perylene (C22H12) cene, and indeno[1,2,3-cd]pyrene. a set of mathematical entities called polyhexes, which are planar figures composed by conjoining regular hexagons of identical size. PAHs containing up to six fused aromatic rings are often known as “small” PAHs and those containing more than six aromatic rings are called „large“ PAHs. Due to the availability of samples of the various small PAHs, the bulk of research on PAHs has been of those of up to six rings. The biological activity and occurrence of the large PAHs does appear to be a continuation of the small PAHs. They are found as combustion products, but at lower levels than the small PAHs due to the kinetic limitation of their production through addition of successive rings. Additionally, with many more isomers possible for larger PAHs, the occurrence of specific structures is much smaller. PAHs possess very characteristic UV absorbance spectra. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomers, each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). The extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain large PAHs also exhibiting semi-conducting and other behaviors. PAHs of three rings or more have low solubilities in water and a low vapor pressure. As molecular weight increases, aqueous solubility and vapor pressure decrease. The aqueous solubility decreases approximately one order of magnitude for each additional ring. PAHs Naphthalene (C 10 H 8 alls) ), constituent of mothballs), mothballs), sting mconsisting of two coplanar six-memnbered rings sharing an edge, is another aromatic hydrocarbon. By al e formal convention, it is not a true PAH, though is referred to as yclic a bicyclic aromatic hydrocarbon. Hs PAHs are lipophilic. Their nce presence has been reported ble in edible oils from different parts of the world. ource: Source: Wikipedia compound is the most volatile of the ones determined. If the solvent mixture were optimized further, the performance for naphthalene could probably be improved. Repeat injections of a standard using the 1 mL syringe yielded very respectable RSD values though slightly higher than when a 100 µL syringe was used. This means it was possible to use one syringe (1 mL) for the entire process, including sample introduction, allowing everything to be automated in one batch withGERSTEL Solutions Worldwide – February 2008 out manual intervention. If the 100 µL syringe is required for sample introduction to the GC, the sample preparation step is performed for the complete batch using the 1 mL syringe and the syringe then quickly exchanged to perform the sample introduction using the same sampler. If a dual rail MPS (PrepStation) is used, the syringe exchange step can be avoided since the two robot arms can be equipped with different syringes for different parts of the process. The upper robot performs the SPE step using a 1 mL or 2.5 mL syringe while the lower robot performs the LVI sample introduction using a 100 µL syringe. The Prep Ahead function of the MPS under MAESTRO software control enables sample preparation and analysis to be performed in parallel ensuring best possible productivity of the system. Oliver Lerch: “The determination of PAHs can be automated in a very simple and efficient manner using this system. It is all a question of having the right tool and the right method.” 13
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