Total concentration analysis of polycylic aromatic hydrocarbons in aqueous samples with high suspended particulate matter content

Abstract

The European water framework directive (WFD) requires priority pollutants to be measured in the whole water sample and not only in the dissolved phase. However, it does not give clear definitions on how to achieve this. To overcome this limitation, a new methodology of sample preparation procedure for the analysis of polycyclic aromatic hydrocarbons on the basis of extraction disks is introduced here. The automatable procedure includes a “one-step” extraction of the analytes both dissolved in the liquid phase of the sample and sorbed to suspended particulate matter. The latter is extracted concurrently with the solid-phase extraction (SPE) material within the elution step of the procedure. Separation, identification, and quantification of the analytes is performed by GC–MS. Results from surface water samples spiked with certified sediment up to 1000 mg/l are presented in this work and compared with results derived from liquid–liquid extraction (LLE). Most measured values are within or at least near certified uncertainty limits of the sediment. The SPE disk method shows much higher recoveries and better precision (relative standard deviations between 2% and 11%) than the standard LLE method. For all substances under investigation, the limits of quantification achieved range between 0.001 and 0.005 μg/l.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants, naturally occurring in coal, crude oil and gasoline and their byproducts, e.g. coal tar or creosote. In addition, PAHs are formed in all incomplete combustion processes of organic materials, such as wood or fossil fuels [1], with atmospheric emissions dominated today by anthropogenic activities [2]. PAHs are of particular concern because of their proven or potential carcinogenicity [3].

Consequently, the EU water framework directive (WFD) lists in its annex X the whole group of PAHs as priority hazardous substances (represented by the parameters benzo[a]pyrene, benzo[b]fluroanthene, benzo[k]fluoranthene, benzo[ghi]perylene, ideno[1,2,3-cd]pyrene) and additionally comprises the individual compounds naphthalene, anthracene, and fluoranthene [4], [5]. Annual average environmental quality standards (AA-EQS) values for individual PAHs range from 0.002 to 2.4 μg/l and are defined for the concentration in the whole water sample [6], which includes suspended particulate matter (SPM) present in the sample. For compounds such as PAHs, in particular the larger molecular weight compounds that sorb strongly to environmental solids, the fraction bound to particles may be substantial, depending on content and composition of particulate organic matter in the surface water samples [7], [8], [9], [10], [11], [12]. In such cases, aqueous PAH concentrations reported for filtered samples (freely dissolved fraction) may deviate considerably from the total concentration in the sample and thus would not fulfil the reporting requirements of the WFD.

Furthermore, PAHs, represented by naphthalene, pyrene and benzo[a]pyrene have also been suggested as selected stormwater priority pollutants recently [13]. In stormwater, deviations between freely dissolved and total concentration for pyrene and benzo[a]pyrene will be high since stormwater always carries a rather large SPM load. In conclusion, in order to meet the requirements of the WFD, it is necessary to develop suitable methods for the analysis of PAHs in the whole water sample, including the fraction bound to SPM [14].

If analytical results for the whole water sample are reported they are typically determined separately for water and SPM after filtration [9], [11], [15], [16]. Filtration is generally required when conventional solid-phase extraction (SPE) cartridges are used for extracting turbid water samples because SPE cartridges will easily plug when a substantial amount of particulate matter is present [17]. With liquid–liquid extraction (LLE) techniques, emulsions typically create problems which could adversely impact the proper extraction of the suspended material [18], [19]. LLE has also been reported to be less efficient in the extraction of low-molecular weight PAHs [20] although these results are somewhat counterintuitive. Indeed, we could not corroborate this finding in our study.

A potential means to overcome these problems is the use of solid-phase extraction in another format, i.e., as extraction disks. Although advantages of extraction disks for extraction of PAHs such as higher possible sampling rates and less susceptibility to clogging have been known for years [21], [22], [23], they are still not as widely used as cartridge-type SPE in routine water analysis. Reasons for this may include difficulties in at least partial automation and a smaller number of available phases. In the framework of international standardization of analytical methods with reference to water analysis (ISO/TC 147–Water Quality), currently drafting of an international standard takes place that deals with the analysis of PAHs in water containing heavy loads of particulate matter. Member bodies of the corresponding working group decided to implement a solid-phase extraction procedure based on extraction disks to overcome the problems of LLE mentioned above [24].

The main objective of the presented study was therefore to develop a method based on SPE disk extraction that allows processing of water samples with a varying load of SPM for subsequent analysis of PAHs in the whole water sample according to the requirements of the WFD. The method introduced in the paper includes a new approach of sample preparation by combination of single extraction steps of both the analytes dissolved in the liquid phase of the sample and those adsorbed to the suspended particulate matter. Beside this the complete sample preparation is fully automatable. To this end, the effectiveness of the method was evaluated for the 16 PAHs listed by the US Environmental Protection Agency (EPA) as priority pollutants, including all PAHs listed in the context of the EU WFD, when the particulate matter and the SPE disk were both extracted and eluted with the SPE extracting solvent. Investigated parameters include the maximum total amount of SPM that can be handled by the SPE disk, the selection of a suitable non-halogenated solvent for extracting the SPE disk, and the potential to simplify the method by omission of a solvent concentration step. Furthermore, to substantiate advantages of the SPE disk method over LLE procedures recoveries with both methods were thoroughly compared.