Abstract:
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants which are of concern due to their potential human toxicity. They are formed during numerous combustion processes, including biomass burning and diesel vehicular emissions, which are of relevance in developing countries. A novel analytical screening method for atmospheric polycyclic aromatic hydrocarbons (PAHs) was developed in this study based on laser induced fluorescence (LIF) of samples on quartz multi-channel polydimethylsiloxane (PDMS) traps. A tunable dye laser with a frequency doubling crystal provided the excitation radiation, and a double monochromator with a photomultiplier tube detected emitted fluorescence. The method allowed for the rapid (<5 min), cost effective analysis of samples. Those yielding interesting results could be further analysed by direct thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS, with limits of detection of ~0.3 ng.m-3), as photodegradation was minimal (<10% over 5 min irradiation). Without any signal optimization, a LIF detection limit of ~1 ìg.m-3 was established for naphthalene using a diffusion tube (diffusion rate of 2 ng.s-1) and 292 nm excitation. Gas standards which facilitated the uniform distribution of analyte across each of the 22 PDMS tubes were provided by easily constructed diffusion tubes for naphthalene and by a gas chromatographic fraction collection method for the less volatile target PAHs. The methods developed were successfully tested in a number of applications which are of relevance to southern Africa, as emissions from sugar cane burning, household fires, diesel vehicles and industries were monitored. The LIF method allowed for the differentiation between impacted and non-impacted industrial sites, and the importance of naphthalene as an indicator for atmospheric PAHs was verified in that this PAH was the most abundant in the various applications which were investigated. The multi-channel silicone rubber traps were also evaluated theoretically and practically in the denuder configuration, in order to monitor PAHs in both the gas and particle phases, which is important in terms of human health effects. The novel LIF method developed in this study has the potential to serve as a screening tool to avoid the comprehensive and costly analysis of samples which do not contain appreciable levels of PAHs. The experimental procedure is simple and rapid, with acceptably low limits of detection, even with the initial, unoptimized optical arrangement and without extensive time-averaging. LIF also provides selectivity without the need for sample clean-up and separation processes. The LIF method could be further optimized by improving the laser energy stability, as well as by the investigation of possible time resolution techniques. As equipment cost considerations were important, it is possible that the LIF screening method could find application in a centralized environmental laboratory for the southern African region. This would facilitate the widespread monitoring of atmospheric PAHs in a cost effective manner.
