A simple analytical method for the simultaneous determination of multiple organic pollutants in sediment samples

Graphical abstract


Introduction
The extensive usage of synthetic chemicals has resulted in their ubiquitous presence in every corner of the world, and due to their toxicities to humans and organisms, some of them are identified as emerging organic pollutants [4]. Simultaneously, some organic compounds unintentionally derived from precursors are also ubiquitous in the natural environment, such as polycyclic aromatic hydrocarbons (PAHs), which mainly originate from vehicular emissions and biomass/coal combustion [7]. The presence of traditional persistent organic pollutants (POPs) and emerging organic pollutants in the environment poses a great threat to human health and aquatic systems. Consequently, there is great concern about their occurrence and distribution in the environment. As reported in some reviews (Larivière et al., 2017; [5]), extensive studies have been carried out focusing on different classes of contaminants based on the development of specific analytical methods, but these have been limited to organic pollutants originating from different sources. In the present study, several groups of contaminants widely detected in sediment in the Pearl River Delta were chosen as target compounds, including PAHs, organochlorine pesticides (OCPs, namely HCHs and DDTs), synthetic musks used as fragrance materials in domestic products and personal products, UV filters used in personal products and consumer goods, as well as organophosphate esters (OPs) used as flame-retardants and plasticizers.
In order to understand their co-occurrence, possible ecological risk, and ultimate fate in the environment, simple and effective analytical methods are highly desirable. This study is aimed at optimizing and validating a sensitive method for the simultaneous determination of the abovementioned organic pollutants in sediment. Furthermore, to prove the applicability and robustness of the method, four sediment samples have been analyzed.

Chemicals and materials
Five groups of organic chemicals were chosen as target compounds, comprising 16 PAHs, 7 OCPs, 9 synthetic musks, 5 UV filters, and 7 OPs.
Silica gel (70-230 mesh) was obtained from Merck Co. (Darmstadt, Germany), activated at 180 C for 12 h, deactivated with 3% redistilled water, and kept in n-hexane before use. Neutral alumina (100-200 mesh, Shanghai Wusi Chemical Reagent Co., China) was continuously soxhlet-extracted with MeOH and CH 2 Cl 2 for 48 h, then activated at 250 C for 12 h, deactivated with 3% redistilled water, and kept in n-hexane prior to use. Anhydrous Na 2 SO 4 was placed in a furnace at 450 C for 4-6 h before use.

Extraction, separation, and purification
Approximately 10 g of Dry sediment samples, spiked with 50 ng surrogate standards (d8-Nap, d10-Ace, d10-Phe, d12-Chr, d12-Per, d27-TNBP, d15-TPHP, d12-TCEP and d15-MX), and then soxhletextracted with CH 2 Cl 2 (200 mL) for 72 h. Activated copper was used to remove elemental sulfur potentially present in the sediment samples. The extracts were concentrated with a rotary evaporator to a volume of about 1 mL, then the solvent was exchanged into n-hexane, and concentrated to approximately 1 mL finally.
A glass chromatographic column (0.8 cm i.d. Â 30 cm), fitted with a Teflon stopcock, was packed consecutively with neutral alumina and silica gel (1:2) and anhydrous Na 2 SO 4 . The final extracts were loaded onto the chromatographic column for further separation and clean-up. Mixed solvents with different polarity were used to elute and recover the target compounds. Firstly, n-hexane (15 mL) was used to transfer the final extract onto the chromatographic column and discarded due to no target compounds being present in this fraction. Fraction 1 was then collected in n-hexane/CH 2 Cl 2 (3:1, v/v; 50 mL), eluting PAHs, DDTs, HCHs, and two nitro musks (MA and MX); Fraction 2 was collected in nhexane/CH 2 Cl 2 (1:1, v/v; 60 mL) for polycyclic musks, MK, and UV filters; Fraction 3 was collected in EtOAc (100 mL) for OPs.

Instrumental analyses
The collected fractions were concentrated to a volume of about 1 mL, solvent-exchanged into nhexane, and further concentrated to 200 mL under a gentle N 2 stream. An aliquot of 100 ng internal standard (HMB) was added before instrumental analysis.

Quality assurance/quality control
Recovery studies were performed with mixtures of target compounds in a solvent. For this purpose, n-hexane was fortified with 50 ng mixtures of standards, which were separated on the combined chromatographic column. The recovery values were then calculated by dividing the obtained concentration by the theoretical concentration calculated from the added amounts. Six replicate samples were analyzed, and the recoveries for the target compounds are listed in Table 1.
In addition, blanks (n = 6) were analyzed as real samples. The results indicated that TCEP, TCIPP, and HHCB were detected in all of the blanks below the limits of quantification (LODs), whereas the other target compounds were not found in any of the blanks.
The LODs were calculated as 3Â (S.D./S), where S.D. is the standard deviation of the response acquired for seven replicate injections of standards at concentrations as low as possible, which was established using dilution series of a stock solution of each compound. The limits of quantification (LOQs) were calculated as twice the LODs [2]. Based on the developed analytical method, acceptable results have been achieved in terms of LODs in 10 g of sediment for PAHs (0.09-0.10 ng/g), OCPs (0.08-0.20 ng/g), synthetic musks (0.03-0.04 ng/g), UV filters (0.16-0.33 ng/g), and OPs (0.06-0.18 ng/g).

Validation of the analytical method
To evaluate the effectiveness of the established analytical method, it was applied to four sediment samples. Meanwhile, QA/QC samples, including blanks (solvent, n = 3), spiked blanks (spiked standards in solvent, n = 3), and spiked matrix (standards spiked into pre-extracted sediment, n = 3) were analyzed as real sediment samples.
Due to its high volatility, recovery of d 8 -Nap was not calculated, and Nap was not quantified in the present study. Acceptable recoveries of the other surrogates were achieved with good reproducibility. Specifically, the recoveries of these surrogates were as follows: The results indicated that most of the target compounds were found in the sediment samples. All 16 PAHs were detected in the sediment, with P 15PAHs varying in the range 2145.2-2533.2 ng/g; HCHs and DDTs were found at levels of 0.06-0.95 ng/g and 0.19-2.78 ng/g, respectively. Meanwhile, HHCB and AHTN were measured at concentrations of 1.03-1.69 ng/g and 1.25-1.70 ng/g, respectively, but MX was found to be below the LOD, and the other synthetic musks were not found in any of the four sediments. The results agreed to the current market information which HHCB and AHTN were predominate over the fragrance materials. As for the five UV filters, BP (6.26-10.43 ng/g), OC (0.66-0.97 ng/g), and 4MBC (0.50-0.85 ng/g) were found in all four sediments, and EHMC (0.14-0.69 ng/g) Table 1 The recoveries and RSDs (n = 6) of the target compounds.

Compounds
Recoveries/% RSD/% was found in three of the sediments. However, ODPABA was not detected in any of the samples. The OPs were found in all of the samples at different concentrations. For example, TBEOP was found below the LOD, TBP at 3.39-3.75 ng/g, TPhP at 1.55-3.17 ng/g, and TMPP at 1.45-2.34 ng/g. Simultaneously, three chlorinated OPs, TECP (1.68-23.41 ng/g), TCIPP (1.46-7.78 ng/g), and TDCIPP (3.77-12.26 ng/g), were found in the samples.

Conclusion
Based on soxhlet extraction coupled with separation/clean-up on a chromatographic column packed with neutral alumina and silica gel, a simple and effective analytical method has been developed for the simultaneous determination of five groups of organic pollutants, namely PAHs, DDTs and HCHs, synthetic musks, UV filters, and OPs. The developed analytical method exhibited good recoveries with excellent reproducibilities. By integrating the analytical process and operation, we can obtain more information about the co-occurrence of common contaminants in sediment, using limited samples with less consumption of energy, reagents, time, and manpower.