Spectroscopic fingerprinting techniques as rapid alternatives to conventional analytical methods for biomonitoring of air pollution with lichens

The process of environmental air analysis is a difficult task in the step of sample handling and monitoring, because of expensive air sampling and analyzing equipments are needed [1–6]. An advantageous technique is the use of pollution bioindicators and/or bioaccumulators which permit an easier sampling and facilitate the sample treatment and analysis steps in laboratory.

Lichens are extremely sensitive to the presence of substances that alter the composition of the atmosphere and for this reasons are excellent biomonitors of air pollution.

Lichens as bioindicators and/or bioaccumulators of air pollution have been used in several studies, during the last 30 years, since Hawksworth & Rose in 1970 related SO2 concentration with the presence/absence of lichens in an area [7]. More recently lichens were used to biomonitor trace element contamination in Pisa Province (Tuscany, Italy) [8] and in Veneto (NE Italy) [9], to determine depositions of heavy metals in the Shetland Islands (Antarctica)[10] and Zinc concentrations in western Ireland  and in Finland [11]. Other examples of lichens used as bioindicators and/or bioaccumulators were to identify polycyclic aromatic hydrocarbons (PAHs), phenols and trace metals in the Holy Cross Mountains (SouthCentral Poland) [12].

The evaluation of the impact of polycyclic aromatic hydrocarbons is receiving increasing attention due to their mutagenic and carcinogenic properties that can seriously threaten human health (IARC, 1983)[13]. The US Environmental Protection Agency (EPA) addressed 16 PAHs as priority pollutants to be monitored (USEPA, 1993)[14].

Recent research on PAH pollution monitoring by lichens suggests the possibility of relating PAH values in lichens with their atmospheric concentrations which would give the chance to use this information for regulatory purposes.

Juri Nascimbene et al. in a recent study [15] verified the biological impact of polycyclic aromatic hydrocarbon (PAH) emissions due to traffic along the roads leading to seven passes of the Dolomites (SE Alps), which were recently declared a UNESCO World Heritage Site. Thalli of the epiphytic lichen Pseudevernia furfuracea, collected at increasing distances from the roads, were used as biomonitors and sixteen EPA priority PAHs (USEPA, 1993) were identified and quantified in lichen samples by applying Automated Soxhlet extraction followed by SPE clean up and gas chromatography coupled with mass spectrometry (GC/MS) analysis. The results of this study showed the effectiveness of lichen biomonitoring to reveal both large scale and local patterns of traffic PAH pollution in mountain areas where chemical–physical monitoring is often hindered by logistic constraints.

In another study, Augusto et al. [16] compared PAH concentrations and profiles in lichens with those of soil, air and pine needles in a petro-industrial region. The lichen samples were analyzed after Soxhlet extraction, by high-performance liquid chromatography (HPLC), using two columns, coupled to an ultraviolet fluorescence detector (FLD) and to an ultraviolet/visible detector (DAD/V-UV).

The limiting factor in these approaches is the use of traditional analytical methods (GC/MS, HPLC) very expensive and time-consuming.

Near-infrared spectroscopy offers a number of important advantages over traditional chemical methods, because it is a rapid and non-destructive method and it requires minimal or no sample preparation. Moreover, NIRS is less expensive because no reagents are required and thus no waste is produced.

In our previous work [‘NIR spectroscopy as a tool for discriminating among lichens exposed to air pollution’, M. Casale et al., Chemosphere, accepted, doi. 10.1016/j.chemosphere.2015.03.095], we have shown that NIR spectroscopy is able to generate a ‘fingerprint’ of lichens capable of discriminating between lichen samples according to their exposure to pollutants.

Our goal is to propose NIRS as a rapid and simply tool for the determination of  polycyclic aromatic hydrocarbon (PAHs) in lichen thalli, biomonitors of air pollution.