Anna Maria
Sulej-Suchomska
*ab,
Żaneta
Polkowska
a,
Tomasz
Chmiel
a,
Tomasz Marcin
Dymerski
a,
Zenon Józef
Kokot
b and
Jacek
Namieśnik
a
aGdańsk University of Technology, Faculty of Chemistry, Department of Analytical Chemistry, 11/12 G. Narutowicza St., 80-233 Gdańsk, Poland
bPoznan University of Medical Sciences, Faculty of Pharmacy, Department of Inorganic and Analytical Chemistry, 6, Grunwaldzka Str., 60-780 Poznań, Poland. E-mail: sulejsuchomska@ump.edu.pl; Fax: +48 61 854 66 09; Tel: +48 61 854 66 16
First published on 3rd May 2016
A fundamental aspect of airport operations is the pollution caused by airport runoff waters. Polycyclic aromatic hydrocarbons (PAHs) are one of the most important groups of xenobiotics which are commonly found in runoff water originating from airports. Only very limited data on the analysis of airport runoff water have been published until now. Therefore, a reliable and accurate analytical method based on headspace solid-phase microextraction (HS-SPME) coupled with comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC × GC-TOF-MS) for simultaneous determination of 16 PAHs in airport runoff water was developed. The optimization of the HS-SPME procedure resulted in the following extraction conditions: extraction time of 45 min, temperature of 70 °C, salt addition of 3.0 g, and desorption time of 10 min at 270 °C. The recovery values obtained using this method (63–108%) mostly fell within the acceptable range for the analytical procedures. This indicates that HS-SPME is a suitable and efficient tool for the extraction of PAHs from airport wastewater, the latter being characterized by a very complex matrix composition. In addition, the developed procedure exhibited satisfactory selectivity, accuracy and a low MDL (0.22–2.20 ng L−1). It should be emphasized that the presented procedure is new with respect to the determination of toxic, mutagenic and carcinogenic analytes in the original environmental samples, elaboration of the detailed metrological characteristics, and the diversity of places from which runoff water samples are collected. The validated analytical protocol was successfully applied to determine the aforementioned organic pollutants in real samples collected from different international airports. Regardless of the airport location, chrysene, phenanthrene and pyrene were the most abundant PAH compounds detected in all analyzed samples (1.8–26.3 μg L−1). The presented methodology can be used for tracking the environmental fate of PAHs and assessing the impact of airports on the environment.
The development of appropriate techniques for analyte isolation and enrichment is necessary in order to obtain reliable information about the content of analytes in airport wastewater. The use of various extraction techniques for the preparation of samples from the original material, such as airport runoff water, may have a significant effect on the final determination of analytes. So far, mostly the procedures based on liquid–liquid extraction (LLE) and solid-phase extraction (SPE) have been used to determine PAHs in different types of runoff water samples.15–18 Both the aforementioned extraction techniques are generally effective, but have some drawbacks, e.g. they require a large volume of the aqueous sample for isolating the analytes (up to few liters). LLE is time-consuming and labor intensive, and it uses large volumes of high-purity solvents. In the case of SPE, a relatively small amount of solvent is required to rinse the sorption media, however, the sorbent bed and/or sorbent pores can be blocked by the particles of the suspension present in the sample.18–20 It is always advisable to develop new techniques that have an advantage over the old ones, and ensure successful application of the new approach. Therefore, green sample preparation techniques such as solid-phase microextraction (SPME) have been introduced to extract a broad spectrum of volatile and semi-volatile organic compounds, including PAHs, from different types of environmental samples.19,21–23 SPME does not have any of the aforementioned limitations because it is a solvent-free technique that combines sampling, sample clean-up and preconcentration into a single step.19,22,24 SPME is currently recognized as the most useful preparation technique for determining the PAH levels in complex matrices such as oily wastewaters, natural stream water or sludge.23,25,26
The selection of a suitable separation technique is the next stage of an appropriate analytical procedure used for the detection, identification and quantitative determination of analytes, often found at trace or even ultratrace levels, in samples characterized by their complex matrix composition. Chromatographic techniques play an increasingly important role in this area. PAHs are commonly analyzed by gas chromatography coupled with mass spectrometry (GC-MS) and flame ionization detection (GC-FID) as well as high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS), spectrofluorometric detection or UV-diode array detection (HPLC-UV-DAD).27,28 The analysis of runoff water or any other samples with a complex matrix composition by means of a single chromatographic technique, e.g. one-dimensional GC, may be insufficient. This is due to the fact that the investigated compounds can be indistinguishable from the baseline noise or hidden in larger peaks of co-eluted sample components. In our study, this crucial problem was resolved by employing comprehensive two-dimensional gas chromatography (GC × GC), which has been shown to be a powerful high-resolution technique well suited for the separation of target compounds present in highly complex sample matrices, such as runoff water.22,29 The benefits of multidimensional systems applied in this kind of research study include highly improved sensitivity, large separation power, and high selectivity.22,30 A detailed description of the working principles and references to the application of GC × GC can be found in a number of articles.31–35 The GC × GC chromatograms are characterized by very narrow peaks which require the use of a fast scanning detector.22,32 The time-of-flight mass spectrometry (TOF-MS) detector has been commonly used to allow successful identification of numerous compounds.22,32,36,37
The purpose of this study was to develop, optimize and validate the procedure for determining polycyclic aromatic hydrocarbons in the samples of airport runoff water based on headspace SPME (HS-SPME) coupled with GC × GC-TOF-MS. To the best of our knowledge, only very limited data have been published on the analysis of airport runoff water. The developed and validated analytical procedure was successfully applied to determine hazardous PAHs in the original samples of runoff water. The samples were collected from three international airports located in different geographical regions and characterized by different levels of activity (number of flights/passengers per year).
The fibre was conditioned at the injection port at 320 °C for 60 min, according to the manufacturer's specifications. Prior to extraction, the sample solution in the vial was maintained at the target temperature for 15 min under continuous agitation (700 rpm). This incubation step was intended to promote the transfer of volatile compounds from the sample solution to the headspace. All the solutions were stirred at a constant rate (700 rpm) throughout the extraction period. Following the extraction, the fibre was withdrawn into the needle, removed from the sample vial and immediately transferred into the GC injector port for thermal desorption. In order to avoid sample carryover, SPME fiber was post-baked at 270 °C for 5 min with the injector in the splitless mode. Moreover, the fiber and chromatographic blanks were run periodically during the analysis to confirm the cleanliness of the GC × GC system. Each extraction was performed in triplicate. All HS-SPME steps were carried out automatically by using a Gerstel MultiPurpose Sampler (MPS) using the Gerstel MAESTRO software.
A schematic representation of the analytical procedure used to determine PAHs is shown in Fig. 1.
Fig. 1 Diagram of the analytical procedure for determining PAHs by using SPME and the GC × GC-TOF-MS system. |
Airport/sample number | Location of sampling site | ||
---|---|---|---|
International Large Airport PL | International Small Airport PL | International Large Airport UK | |
1 | Influent of a river | Vicinity of an airport terminal | De-icing area (1) |
2 | Effluent of a river | De-icing area | A river in the vicinity of the airport |
3 | Municipal water catchment area | Machinery stock, parking places | De-icing area (2) |
4 | CARGO water catchment area | Runway | De-icing area (3) |
5 | Airport ramp | Parking places | De-icing area (4) |
6 | Car park | The periphery of an airport | A road near the airport |
7 | De-icing area | Car park | — |
8 | Airport ramp | — | — |
The samples of runoff water were mostly collected manually during continuous atmospheric precipitation lasting for at least 5 h. The samples were collected in 1000 mL bottles made of dark glass by using a syringe (100 mL) with Teflon tubing. Prior to use, the syringes and tubing were rinsed with ultrapure water and sampled runoff water.
Due to safety regulations, assembly and locating of any devices at the airport platform is not possible. These conditions result from the necessity of maintaining the regular schedule of work related to the proper airport functioning and more stringent procedures introduced recently in connection to increased air traffic while maintaining the high service standard and the safety level of air operations.40
After each sample collection, the samples of airport runoff water were transported to the laboratory and stored at 4 °C until further analysis.41
Fig. 2 Effects of the extraction temperature (A), extraction time (B), ionic strength (C), and desorption time (D) on the efficiency of the HS-SPME procedure for the determination of PAHs. |
In the present study, five different temperature levels were examined, i.e. 50, 60, 70, 80 and 90 °C. Preliminary experiments on the effect of extraction time on the SPME efficiency were carried out for 30 and 60 min at different temperatures in order to evaluate the possible relationships between the extraction time and temperature. This approach allowed for performing the optimization of extraction temperature using the most appropriate extraction time (data not included). A longer extraction time has been accepted as sufficient for the extraction of PAHs with relatively low and medium molecular weights (LMW; MMW), and as fully suitable for the effective extraction of high-molecular-weight (HMW) PAHs. HS-SPME is very sensitive to temperature variations and therefore it is crucial to ensure conditions which minimize even minor differences between consecutive analyses. Consequently, in order to maintain constant analytical conditions and reduce possible variations between chromatographic runs, each analysis was preceded by 15 min incubation at 70 °C. PAHs show a wide range of volatility, and the release of these with lower volatility to the headspace during extraction is limited. In general, Henry's constants and diffusion coefficients of PAHs increase with increasing temperature, and consequently the vapor pressure and the concentration of the analytes in the headspace increase.42 On the other hand, since the adsorption of analytes is an exothermic process, the partition coefficients decrease with increasing temperature. Because of that the extraction capacity is a compromise between these two factors. Fig. 2a shows the effect of extraction temperature ranging between 50 and 90 °C for selected PAH compounds, which displayed different extraction behaviors. The low-ring PAHs reached equilibrium at a temperature of 50 °C. As shown in the figure, an increase in temperature had a negative effect on the extraction of the most volatile PAHs, especially naphthalene. However, the extraction efficiency of MMW- and HMW-PAHs characterized by high diffusion coefficients increases at temperatures above 50 °C. Similar observations have also been reported by others.19,42,46Fig. 2a illustrates that the highest extraction yield of 5- and 6-ring PAHs occurred at a temperature of 90 °C. Thus, an extraction temperature of 70 °C was selected as the optimal SPME parameter for efficient extraction of all investigated PAHs.
The extraction time is the next crucial parameter in the SPME process. Since HS-SPME is an equilibrium extraction technique, the time required for reaching equilibrium determines the maximum amount of the target analyte that can be extracted by the fibre, which affects the sensitivity of the technique.19,24 Therefore, it is important to determine the time required for reaching the equilibrium state.42 In this work, the sorption time profiles were studied by measuring the peak area as a function of exposure time. The fibre was exposed to the headspace of model samples during five different extraction periods. Fig. 2b illustrates the equilibrium time profiles of selected PAHs. The extraction times longer than 70 min were not applied in order to avoid the excessive time of routine analysis and reduce the interval between consecutive GC × GC runs to a minimum. Based on the obtained results, it can be concluded that, in general, LMW- and MMW-PAHs reached the highest extraction yield after 45–50 min. On the other hand, PAHs characterized by high molecular mass reached equilibrium after 50 min extraction. The equilibrium times for the analyzed PAH compounds increased with increasing molecular mass, which is in agreement with the previous studies on porewater samples from a sediment core,25 rainwater and stormwater samples.46 This finding mainly results from the fact that HMW PAHs are characterized by low diffusion coefficients, and consequently slower mass transfer. Therefore, a longer time is required to reach equilibrium and provide sufficient SPME yield. Taking into account the obtained results and a suitable extraction period to ensure continuous running time of the GC × GC system, an extraction time of 45 min was selected as the optimal parameter for isolating PAHs via HS-SPME.
As mentioned before, enhanced ionic strength due to the addition of salt has been reported to increase the extraction efficiency; this phenomenon is known as the “salting-out” effect. In the present work, potassium chloride was used for adjusting the ionic strength. Different amounts of KCl (1.0 g, 2.0 g, and 3.0 g) were added to the model solution in order to determine its optimal ionic strength. Higher doses of salt were not tested because of the possible saturation of the sample solution. The results of the extraction of selected PAHs from samples containing different amounts of added salt are shown in Fig. 2c. The obtained data indicate that the addition of KCl had a strong positive effect on the extraction efficiency, resulting in increased sensitivity of the analytical method. All of the investigated PAHs reached the highest extraction yield when 3.0 g of KCl was added to the model sample. Naphthalene was an exception because a slightly higher extraction efficiency was observed for the addition of 2.0 g of salt. Other authors also reported that the values of partition coefficients of PAHs increased with increasing ionic strength of sample solution, resulting in higher SPME yields.45,46 Therefore, the addition of 3.0 g of salt has been chosen as the optimal extraction parameter for determining PAHs in runoff water samples by the proposed method.
Suitable desorption time and temperature are important parameters that ensure a complete desorption of analytes from the fibre and prevent the occurrence of the memory effect or carryover. In the present study, desorption times were optimized by inserting the fibre into the injection port for a period of 5–15 min (Fig. 2d). The desorption of analytes was carried out at a temperature of 270 °C. Temperatures above 270 °C are not recommended due to the thermal instability of the septum, which may cause a noticeable blank signal as well as gas leak. Optimal results were obtained for a desorption time of 10 min (Fig. 2d). Moreover, in order to eliminate any sample carryover, the post-baking of SPME fibre for additional 5 min was applied. Also, the blank sample runs for the fibre were performed between two consecutive sample analyses to check the background level of the GC × GC system. Under these conditions, no sample carryover was observed.
As described above, the following HS-SPME conditions were applied: an extraction time of 45 min, an extraction temperature of 70 °C, a desorption temperature of 270 °C, a desorption time of 10 min, addition of 3.0 g KCl, and sample agitation at 700 rpm. A 2D chromatogram of the model sample extracted via HS-SPME under the optimized conditions is shown in Fig. S1 (see ESI†).
GC oven temperature program is an important parameter that affects the extent of the analyte resolution. Prior to the application of the GC × GC system, different temperature programs for 1D GC separation were evaluated with respect to their effect on the behavior of PAHs. The best 1D separation was achieved with an initial temperature of 50 °C held for 0.2 min; then ramped at 10 °C min−1 to 200 °C; ramped again at 5 °C min−1 to a final temperature of 300 °C; and held for 3 min. After several attempts, the temperature program of the secondary column was optimized as follows: an initial temperature of 80 °C held for 0.2 min; then ramped at 10 °C min−1 to 230 °C; ramped again at 5 °C min−1 to 330 °C; and held for 5 min. A further effort to reduce the run time by increasing the ramping rate disturbed the separation of analytes, and thus has been abandoned. We also attempted to optimize different flow programs but this approach did not offer any meaningful improvement in the resolution of the above co-eluting compounds.
Modulation is a key step in GC × GC separation, which consists of three tasks: trapping, focusing and releasing small fractions of the effluent from the first column to the second one. The duration of a single complete cycle of these modulation events is called the modulation period (PM). The optimization of PM is important as it ensures the appropriate peak shape, high sensitivity and separation through cryofocusing. Therefore, the modulation periods of 2, 3, 4, 5 and 6 s with the hot pulse duration set at 20% of PM were investigated. In order to select the optimal PM that provides comprehensive separation of individual analytes as well as their separation from other volatile components of the sample matrix, the airport runoff water sample spiked with a standard mixture of PAHs was used as a model in all tests. In general, it was observed that with increasing PM the peak height increased, while the peak area was not significantly affected. This finding indicated the enhancement of the S/N ratio, and thus a higher sensitivity of the method. Fig. 3 shows the experimental verification of the effect of PM on the GC × GC separation of selected PAHs. For example, phenanthrene (Ph) and anthracene (An) that had the same retention time on the 2D column were well separated on the 1D column. The separation of these two analytes is well preserved using a PM ≤ 5 s (three modulations per 1D peak; PM = 5 s), as the two compounds yielded two separate spots (Fig. 3d). However, in some cases, a longer modulation period led to deterioration or a complete loss of the separation achieved on the 1D column, e.g. similar to Ph and An, chrysene (Chry) co-eluted with benzo(a)anthracene (BaA) at PM = 6 s. On the other hand, shortening the modulation period to less than 5 s caused the wraparound effect. For short modulation periods of 2, 3 or even 4 s, this effect was observed when the 2D retention of an analyte became longer than PM, which caused some or all of these analytes to elute during the successive modulation cycle(s). For example, fluoranthene (Flt) and pyrene (Py) appeared as partially wraparound peaks in the total ion current (TIC) chromatograms obtained for the PM value of 2 and 4 s (Fig. 3a and c). The same problematic effect was also reported for fluorene (Flu) for PM = 3 s. The wraparound effect disturbs the structure of 2D chromatograms and may lead to co-elution of analytes eluting in the following modulation cycle(s). For these reasons, the PM value of 5 s was chosen as the optimized modulation period in this study.
Fig. 3 GC × GC contour plots (TIC mode) of airport runoff water samples spiked with PAH standards obtained at different modulation periods: (a) 2 s; (b) 3 s; (c) 4 s; and (d) 5 s. |
Analytes | Concentration range used for calibration [μg L−1] | Coefficients of the calibration curve (y = ax + b) | Coefficient of determination, R2 | MDL | MQL | Intra-day precision [%] | Inter-day precision [%] | Recovery [%] | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
I | II | ||||||||||||
PAHs | I | II | a | b | a | b | I | II | [ng L−1] | ||||
Naphthalene | 0.01–10 | 10–100 | 5774412 | 130797 | 5920862 | −9549526 | 0.9979 | 0.9987 | 0.23 | 0.70 | 5.5–7.1 | 3.2–29 | 77–97 |
Acenaphthylene | 12380813 | 1412444 | 11939174 | 4184651 | 0.9954 | 0.9970 | 0.34 | 1.03 | 5.7–8.9 | 2.3–30 | 74–96 | ||
Acenaphthene | 7491619 | 1406948 | 7541935 | −7308033 | 0.9873 | 0.9968 | 0.58 | 1.73 | 5.6–9.4 | 23–28 | 75–95 | ||
Fluorene | 10957828 | 1379730 | 10689221 | −4738740 | 0.9915 | 0.9974 | 0.47 | 1.42 | 5.9–7.6 | 5.9–30 | 70–103 | ||
Phenanthrene | 3994454 | 9015966 | 3752905 | 39079632 | 0.9869 | 0.9899 | 2.2 | 6.50 | 6.0–9.9 | 12–30 | 79–108 | ||
Anthracene | 27972896 | −795103 | 27560297 | 1685462 | 0.9885 | 0.9929 | 0.55 | 1.65 | 6.1–10.4 | 11–31 | 71–90 | ||
Fluoranthene | 13885150 | 5136377 | 14324786 | −17284631 | 0.9987 | 0.9994 | 1.63 | 4.89 | 7.8–11.3 | 12–25 | 69–87 | ||
Pyrene | 8741436 | −283600 | 9340469 | −48801048 | 0.9979 | 0.9980 | 2.08 | 6.25 | 6.9–12.0 | 2.0–26 | 70–81 | ||
Benz[a]anthracene | 6331871 | −161647 | 6677654 | −8315466 | 0.9980 | 0.9985 | 0.23 | 0.68 | 5.8–12.9 | 4.0–21 | 65–87 | ||
Chrysene | 2361801 | 339560 | 2496327 | −1883088 | 0.9900 | 0.9948 | 0.51 | 1.53 | 5.6–12.5 | 1.3–28 | 63–79 | ||
Benzo[b]fluoranthene | 49585 | 0 | 53546 | −155738 | 0.9721 | 0.9875 | 1.85 | 5.55 | 6.0–11.6 | 5.6–12 | 64–77 | ||
Benzo[k]fluoranthene | 1222702 | 117776 | 1244465 | −377049 | 0.9981 | 0.9975 | 0.22 | 0.67 | 5.7–13.1 | 1.4–23 | 64–79 | ||
Benzo[a]pyrene | 1186529 | −153344 | 1135200 | 300546 | 0.9817 | 0.9841 | 1.67 | 5.01 | 6.9–12.6 | 13–30 | 67–78 | ||
Indeno[1,2,3-c,d]pyrene | 83005 | 13762 | 85907 | 0 | 0.9812 | 0.9847 | 0.70 | 2.11 | 11.7–13 | 16–28 | 64–70 | ||
Dibenz[a,h]anthracene | 11486 | 1450 | 13841 | −76560 | 0.9812 | 0.9857 | 0.71 | 2.12 | 12.0–12.9 | 15–27 | 63–67 | ||
Benzo[g,h,i]perylene | 11760 | 1402 | 13588 | −52378 | 0.9793 | 0.9776 | 0.74 | 2.22 | 12.8–13.5 | 19–26 | 64–71 |
The obtained results indicate that the MDL values of the HS-SPME/GC × GC-TOF-MS procedure elaborated in the present study are generally lower than those reported for the SPE/GC-MS (1–57 ng L−1),18 LLE/GC-MS (1600–7800 ng L−1),49 and SPME/GC-MS (1–29 ng L−1)50 methods previously applied to determine 16 PAHs in water/wastewater samples. The above-mentioned studies also show that the efficiency of the developed analytical procedure is quite similar to that of the SPE/GC-MS method (recovery of 71–86%), and slightly lower than that of LLE/GC-MS (81–106%).50 The precision experiments demonstrated that the repeatability of the proposed methodology is slightly lower than that of the SPE/GC-MS procedure (CV = 1.5–5.2%), and similar to that of the LLE/GC-MS procedure (CV = 6–12%) applied in our previous study.18 The obtained results indicate the suitability of the HS-SPME technique for efficient extraction of PAHs from airport runoff water. Moreover, the MQL values of the proposed analytical procedure are significantly lower than the corresponding values obtained by applying SPE/GC-MS and LLE/GC-MS procedures to determine PAHs in airport runoff waters.18,49,50 This finding confirms the higher sensitivity of the methodology applied in the present study, especially the two-dimensional gas chromatography in comparison to its one-dimensional counterpart. Furthermore, the developed method, being very selective, solves the problem of coelution and interferences that is associated with the analysis of PAHs in airport runoff water samples. As a result of the study, the quantitation of acenaphthylene, acenaphthene, fluoranthene, pyrene, phenanthrene, anthracene, benzo[b]fluoranthene and benzo[k]fluoranthene has been improved in comparison to the outcome of SPE/GC-MS and LLE/GC-MS procedures.18 Also, it should be emphasized that the elaborated HS-SPME procedure is a solvent-free extraction technique as opposed to LLE and SPE; the latter two also require the sample concentration step prior to GC analysis. The advantages and limitations of the procedure for PAH determination in the samples of airport stormwater are presented in Table 3.
Advantages | Disadvantages |
---|---|
Suitable for highly contaminated samples | Qualified staff is necessary for handling the apparatus |
A Small amount of sample is required | Time-consuming |
Relatively high recovery | Low repeatability in some case |
Low MDL | |
Good accuracy | |
Good selectivity | |
Good resolution |
Fig. 5 The content of PAHs in runoff water collected from different airports (International Large PL, Small Airport PL, and International Large Airport UK). |
Fig. 6 GC × GC extracted chromatograms of the samples of runoff water collected on 14 Jan 2013 from (a) the aircraft de-icing area, and (b) the river outflow at the International Large Airport PL. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ay00401f |
This journal is © The Royal Society of Chemistry 2016 |