Front. Environ. Sci. Eng. 2013, 7(1): 31–42 DOI 10.1007/s11783-012-0463-2
RESEARCH ARTICLE
A new procedure combining GC-MS with accelerated solvent extraction for the analysis of phthalic acid esters in contaminated soils Tingting MA1,2, Ying TENG1,2, Peter CHRISTIE3, Yongming LUO (✉)1,2, Yongshan CHEN4, Mao YE2,5, Yujuan HUANG1 1 Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China 2 Graduate University of the Chinese Academy of Sciences, Beijing 100049, China 3 Agri-Environment Branch, Agri-Food and Biosciences Institute, Newforge Lane, Belfast BT9 5PX, United Kingdom 4 State Key Laboratory of Environment Simulation and Pollution Control (Joint), Tsinghua University, Beijing 100084, China 5 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2012
Abstract An optimized procedure based on gas chromatography-mass spectrometry (GC-MS) combined with accelerated solvent extraction (ASE) is developed for the analysis of six phthalic acid esters (PAEs), which are priority soil pollutants nominated by United States Environmental Protection Agency (USEPA). Quantification of PAEs in soil employs ultrasonic extraction (UE) (USEPA 3550) and ASE (USEPA 3545), followed by clean up procedures involving three different chromatography columns and two combined elution methods. GC-MS conditions under selected ion monitoring (SIM) mode are described and quality assurance and quality control (QA/ QC) criteria with high accuracy and sensitivity for target analytes were achieved. Method reliability is assured with the use of an isotopically labeled PAE, di-n-butyl phthalate-d4 (DnBP-D4), as a surrogate, and benzyl benzoate (BB) as an internal standard, and with the analysis of certified reference materials (CRM). QA/QC for the developed procedure was tested in four PAE-spiked soils and one PAE-contaminated soil. The four spiked soils were originated from typical Chinese agricultural fields and the contaminated soil was obtained from an electronic waste dismantling area. Instrument detection limits (IDLs) for the six PAEs ranged 0.10–0.31 µg$L–1 and method detection limits (MDLs) of the four spiked soils varied from a range of 20–70 µg$kg–1 to a range of 90– 290 µg$kg–1. Linearity of response between 20 µg$L–1 and 2 mg$L–1 was also established and the correlation coefficients (R) were all > 0.998. Spiked soil matrix showed relative recovery rates between 75 and 120% for Received September 25, 2011; accepted April 1, 2012 E-mail:
[email protected]
the six target compounds and about 93% for the surrogate substance. The developed procedure is anticipated to be highly applicable for field surveys of soil PAE pollution in China. Keywords phthalic acid esters, quality assurance and quality control, soil type, accelerated solvent extraction, certified reference materials
1
Introduction
Phthalic acid esters (PAEs) are the most commonly used plasticizers worldwide and have been marketed for several decades [1,2]. Due to their low Henry coefficients, high boiling points and low vapor pressures, PAEs have strong adsorption to soils and sediments, which resulted in their persistence in the environment [3]. PAEs and their metabolites and degradation products have shown important environmental hormonal effects and potential risks to ecosystems and human health as well as the potential to contaminate air, soils, groundwater and sediments around the world [4–9]. Once enter the soil ecosystem, PAEs may also be transferable to the edible parts of food crops, and thus, enter the human food chain. Quantification of PAEs in soils often employs microwave assisted extraction (MAE), Soxhlet extraction (SE), ultrasonic extraction (UE) or accelerated solvent extraction (ASE) [9–14]. SE and UE are traditional methods with low instrumental costs and easy operation [12–14], but UE has shown higher efficiency than SE in PAE [11]. Both MAE and ASE have been found to be rapid low solvent replacements for existing extraction methods [15]. ASE
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was chosen for this study because of its higher efficiency and recoveries in the extraction of PAE compounds from soil when compared with other available extraction methods [16,17]. Sample clean-up is important in the analysis of organic pollutants to improve precision in the measurements. Commonly used detection approaches of PAEs in extracts include gas chromatograph (GC), high performance liquid chromatography (HPLC), fluorescence and ultraviolet spectrophotometry, gas chromatograph-mass spectrometer (GC-MS), and thin layer chromatography. Novel techniques such as HPLC-MS and tandem mass spectrometry have also been employed for the detection of PAEs. Among these detection techniques, GC-MS has been well accepted for its reliability in detecting both PAE compounds and their metabolites. GC-MS is more accurate than GC and the procedure is simpler than HPLC, excluding the need for a mobile phase in addition to shortened retention times of the target compound analyzed. GC-MS also has higher sensitivity than fluorescence or ultraviolet spectrophotometry. It is not surprising that GCMS, especially under Selected ion Monitor (SIM) mode, has become the most widely used technique for the detection and quantification of organic pollutants in complex environmental matrices [10,13,14,18–21]. The increasing use of plastic products such as agricultural films is a major source of PAE contamination in many agricultural soils. In addition to the four typical agricultural soils examined in the present study, a fifth soil was collected from Taizhou City in Zhejiang Province, China, an area acknowledged for its electronic waste dismantling sites and multiple soil contamination by persistent organic pollutants [22–25], for quality assurance and quality control (QA/QC) evaluation. The aims of the present study were to optimize an analysis procedure for six PAEs (dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DnBP), butyl benzyl phthalate (BBP), bis (2-ethylhexyl) phthalate (DEHP) and di-n-octyl phthalate (DnOP)) with GC-MS determination by comparing PAE extraction efficiency of UE and ASE, and selecting better sample clean up techniques in four spiked agricultural soils; to check QA/QC results of the optimized method for the analytes in four matrix soils; to evaluate the repetition of PAE analysis on samples of a contaminated soil from an ewaste dismantling site and to test the reliability of this method by making comparison between the test soil samples and certified reference materials (CRM).
2
Materials and methods
2.1
Chemicals and standards
The six PAE standards, DMP (98.0%), DEP (99.9%), BBP (99.0%), DnBP (99.1%), DEHP (99.6%) and DnOP
(99.6%), as well as di-n-butyl phthalate-d4 (DnBP-D4, isotope surrogate standard, 100 µg$mL–1) and benzyl benzoate (BB, internal standard, 5 mg$mL–1) were obtained from AccuStandard Inc. (New Haven, CT, USA). CRM 119-100 (BNAs - Sandy Loam 6) and 136100 (BNAs - Clay 1) were purchased from RT Corporation (Laramie, WY, one of the original Proficiency Test providers recognized by United States Environmental Protection Agency (USEPA)). The solvents used (acetone and hexane) were analytical reagents obtained from chemical reagent companies in Shanghai and Nanjing and were re-distilled in an all-glass system to remove trace impurities before use. Hexane of HPLC grade was purchased from Tedia Company Inc. (Fairfield, OH, USA). Anhydrous sodium sulfate (Na2SO4, reagent grade), which was used to eliminate water from samples, neutral alumina (Al2O3, reagent grade), neutral silica gel and diatomite were obtained from Sinopharm Group Co. Ltd., Shanghai and were all dried in a muffle furnace at 400°C for 6 h and stored in desiccators before use. The individual standard stock solutions were prepared in hexane at a concentration of 1 mg$mL–1 and stored at – 20°C. Solutions for calibration curves were prepared by dilution of stock solution with hexane to the required concentrations before use. The chromatogram of six PAE standards with the internal standard (1 mg$L–1 each) is shown in Fig. 1. 2.2
Sample preparation
Instruments used in sample preparation include ASE 200 (Dionex Corporation, Sunnyvale, CA), rotary evaporator R-215, vacuum controller V8-50, vacuum pump V-700 (Büchi Labortechnik AG, Flawil, Switzerland), Agilent 7890 GC-5975 MSD with CTC auto-sampler and EI source (Agilent Technologies, Santa Clara, CA) carrying Chemstation workstation, low speed automatic balance centrifuge LDZ5-2 (Beijing Era Beili Centrifuge Co., Ltd., China), numerically controlled ultrasonic cleaner KQ600DB (Kunshan Ultrasonic Instruments Co., Ltd., Shanghai, China), and micro vortex mixer WH-3 (Shanghai Huxi Analytical Instrument Factory, China). Equipment made of plastic was not used throughout the sample preparation procedure to minimize the risk of high background readings. All glassware were washed using the following procedure to minimize adsorption on glass surfaces. First, every item was immersed and washed with soap and water in a laboratory ultrasonic washer. After air drying, glassware with tick marks was immersed in sulphuric acid (H2SO4, 98%, reagent grade) and washed with distilled water, and then rinsed with ultra-pure water before drying in an oven at 50°C. Glassware without tick marks was placed in a muffle furnace for 6 h at 400°C. All glassware was then thoroughly rinsed with acetone: hexane (volume ratio, 1:1) before use.
Tingting MA et al. ASE/GC-MS method for PAEs analysis in soils
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Fig. 1 Chromatogram of the six PAEs and the internal standard at 1 mg$L–1
Quantification of PAEs in soil consisted of the following steps: extraction, concentration, clean-up, second enrichment and determination. UE and ASE for the detection of six PAEs in soils were evaluated. 1）UE Sample extraction was performed using modified USEPA Method 3550 [26]. A 10.00 g of soil was used as blank or spiked with surrogate standard and 6 PAE standards, and then transferred to clean glass centrifuge tubes. A 30 mL of acetone:hexane (volume ratio 1:1) was added and mixed on a vortex mixer for 1 min before equilibrating overnight. UE was then carried out in a 25°C water bath for 30 min at full power, followed by centrifugation of the tubes at 1500 r$min–1 for 5 min. The supernatant obtained was filtered through filter paper into round bottom flasks. An additional 20 mL of acetone: hexane (volume ratio 1:1) was added for another extraction for 15 min under the same conditions. The extraction was repeated for another time and the supernatant was retained. 2）ASE A Dionex ASE200 with 11 mL stainless steel cells and Polyurethane foam adsorption column (PUF) plugs was used for sample extraction. The cells and vessels were rinsed with 10 mL of the acetone:hexane (volume ratio, 1:1) mixture and dried gently with nitrogen before use. Sample extraction was performed using a modified USEPA Method 3545 [27]. Briefly, 2.00 g of blank or spiked soil was mixed with about 3 g of diatomite and packed in a cell in which quartz sand was used to fill the gaps at both ends.
After closing with stainless steel screw caps, a mixture of acetone:hexane (volume ratio 1:1) was used as an extraction solvent and the extraction was done as follows: the cells were preheated for 5 min, followed by purging with nitrogen stream for 120 s and 5 min static extraction under extraction pressure of 10343 kPa psi at 100°C. The extraction process was repeated to complete two cycles. The combined elution volume was 36 mL for the two cycles. After extraction, the filtered liquid from both cycles were combined and the volume was reduced on a rotary evaporator to 1–2 mL (35 kPa, 40°C water bath at 80 r$min–1). Then approximately 3–4 mL hexane was added to the round bottom flask and evaporated to less than 1 mL (but not to dryness) under the same conditions. The recovery rates of both approaches were determined after the extracts were concentrated and made up to 1 mL. Column chromatography clean-up was conducted in a glass column (1 cm 26 cm). In the optimization of clean up procedures, a number of chromatography column packing materials including Florisil, activated carbon, silica gel and alumina, and different combined pre-washing and elution solvents have also been evaluated. Six clean-up methods were selected (Table 1). All the washing solutions were collected and reduced to less than 1 mL (35 kPa, 40°C water bath at 80 r$min–1) by rotary evaporator. 10 μL of internal standard was added before hexane (HPLC grade) was used to bring the final volume to 1 mL. Samples were transferred into GC vials and stored at – 20°C before detection.
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Table 1
Clean-up methods using columns and elution solutions specified
method
packing materials (from bottom to top)
elution solutions
one
2 g Na2SO4 + 5 g florisil
15 mL hexane for pre-washing + sample loading + 40 mL hexane
two
2 g Na2SO4 + 5 g florisil + 0.5 g activated carbon + 1 g Na2SO4
15 mL hexane + 15 mL acetone:hexane (1∶4 v/v) for pre-washing + sample loading + 40 mL acetone:hexane (1∶4 v/v)
three
2 g Na2SO4 + 6 g neutral Al2O3 + 12 g neutral silica gel
15 mL hexane for pre-washing + sample loading + 40 mL hexane
four
2 g Na2SO4 + 5 g florisil
15 mL hexane + 15 mL acetone:hexane (1∶4 v/v) for pre-washing + sample loading + 40 mL acetone:hexane (1∶4 v/v)
five
2 g Na2SO4 + 5 g florisil + 0.5 g activated carbon + 1 g Na2SO4
15 mL hexane for pre-washing + sample loading + 40 mL hexane
2 g Na2SO4 + 6 g neutral Al2O3 + 12 g neutral silica gel
15 mL hexane + 15 mL acetone:hexane (1∶4 v/v) for pre-washing + sample loading + 40 mL acetone:hexane (1∶4 v/v)
six
2.3
GC-MS application
2.4
Analysis of individual PAEs in soil samples was performed using GC-MS by USEPA Method 8270C [28] with modification and an Agilent 7890GC-5975 MSD. The MS operating parameters were as follows: ionization under electron impact (EI) mode at 230°C with detector voltage at 1.012 kV; scan mode under selected ion monitoring mode and transfer line 280°C. The m/z values of characteristic ions of analytes are listed in Table 2. A DB-5 (30 m 0.25 mm 0.25 µm) fused silica capillary column with helium (purity > 99.999%) as a carrier gas at 1.2 mL$min–1 was used to separate the compounds. The injector temperature was set at 250°C. The GC oven temperature was programmed as follows: initial temperature of 50°C was held for 1 min, increased at a rate of 15°C min–1 to 200°C, then held for 1 min, then increased at a rate of 8°C min–1 to 280°C, then held for 3 min. Post run was at 285°C for 2 min. Under selected ion monitoring mode, non-pulse injection with a volume of 1 μL each in split-less mode was carried out. Table 2
QA/QC
2.4.1
Soil matrix selection
Spiked soils. The four typical agricultural soils with relatively low levels of PAEs used for QA/QC confirmation of the procedure were a red soil collected from Yingtan Red Soil Ecological Experimental Station of the Chinese Academy of Sciences (CAS), Jiangxi Province, China a yellow brown soil collected in Nanjing, Jiangsu Province, a brown soil from Shouguang, the largest vegetable production area in Shandong Province, and a black soil (chernozem) collected from Hailun AgroEcological Experimental Station of the CAS, Heilongjiang Province, China. Field contaminated soil. The contaminated soil (paddy soil) was obtained from Taizhou in Zhejiang Province, a site known to be polluted by electronic waste dismantling. Selected physico-chemical properties of all five soils are shown in Table 3. According to the United States Department of
Average relative response factors (RRF) and linearity for the six target compounds based on internal standard linear equation
m/z of characteristic ions
correlation coefficient (R)
RRF (RSD, n = 7)
DMP
y = 127.9x + 87.52
163.0, 77.0, 194.0
0.9987
0.02713
DEP
y = 402.9x + 139.3
149.0, 77.0, 222.0
0.9994
0.01820
DnBP
y = – 527.5x + 128.6
149.0, 223.0, 278.0
0.9981
0.07516
BBP
y = – 2319x + 88.91
149.0, 91.0, 206.0
0.9984
0.07543
DEHP
y = – 2372x + 59.75
149.0, 167.0, 249.1
0.9997
0.08395
DnOP
y = – 309.5x + 58.04
149.0, 167.0, 279.1
0.9993
0.1154
compound
Table 3
Some physico-chemical properties of the five experimental soils
soil type
organic matter /(g$kg–1)
available potassium /(mg$kg–1)
available nitrogen /(mg$kg–1)
Available phosphorus /(mg$kg–1)
clay content /(% v$v–1)
pH
red soil
9.1
114.8
64.3
17.9
51.4
4.7
brown yellow soil
14.6
102.8
96.8
14.4
16.7
7.4
brown soil
17.6
125.7
132.3
18.4
10.4
8.6
black soil
48.7
123.1
239.7
17.6
34.9
7.2
paddy soil (from Taizhou)
36.5
118.6
177.5
15.1
38.8
5.8
Tingting MA et al. ASE/GC-MS method for PAEs analysis in soils
Agriculture (USDA) classification system [29], these soils belong to Ultisols, Alfisols, Alfisols, Histosols and Hortic Anthrosols, respectively. All soils were collected from the top 15 cm of the soil profile. After collection, soil samples were stored in paper bags instead of plastic bags at < – 20° C after air drying and passing through a 60-mesh sieve. 2.4.2
Instrument QA/QC analysis
Instrument detection limits (IDLs) or Limit of Detection (LOD). The instrument detection limits were calculated as: IDLs ¼ 3Q N =I,
(1)
where Q refers to sample concentration and N/I is the noise to signal ratio of the instrument (Table 4). Calibration curve. Standard solutions of seven different Noise -to- signal ratios and IDLs
Table 4
S/Na)
IDL /(μg$L–1)
DMP
195.27
0.31
DEP
260.73
0.23
DnBP
315.07
0.19
BBP
283.09
0.21
DEHP
226.49
0.26
DnOP
630.74
0.10
compound
Note: a) S/N is the average of seven replicate analyses
concentrations (including 20, 50, 100, 200, 500, 1000 and 2000 μg$L–1) were used to build calibration curve with the help of the response factor of the internal standard. The six PAEs were quantified using the response factor of the internal standard which was calculated daily. The relative standard deviation (RSD) of the relative response factors (RRF) of each substance at different concentrations must be less than 25%. The RRF for the six target compounds based on the internal standard is outlined in Table 2. Standard solution validation. Newly built calibration curves were used to analyze newly prepared standard solutions with known concentration and the difference between the real value and the detected value should be less than 20%. 2.4.3
Method QA/QC analysis
Method detection limits (MDLs) or Limit of quantification (LOQ). With continual analysis of seven spiked matrices with a concentration of 5-fold IDL in four soils, the MDLs of the six PAEs in each soil were calculated as: MDLs ¼ S tðn – 1,0:99Þ ,
(2)
where S is the standard deviation (SD) of the seven spiked matrices at the concentration of 5-fold IDLs, n – 1 denotes
35
the degrees of freedom and 0.99 refers to the confidence interval of 99% [30]. Blanks and spiked samples in QA/QC analysis. QA/QC analysis during which surrogate was always added and performed by means of three types of sample: whole procedure blanks (method blanks, performing blank sample without soil using the same reagents and procedure as soil samples) to account for background readings and errors at every step, including sample preparation and instrument analysis, in addition to reagent background, glassware background, human contamination and other factors over the whole procedure; soil matrix blanks (performing the whole procedure with four different soil types) to elucidate the background of each soil; spiked matrix (performing the whole procedure with four different soil types by adding 0.1 mg$kg–1 of analytes which is almost 10-fold MDLs) for quality control by checking recovery rate, standard error of the mean (SEM) and RSD. CRM soil samples. A 10.00 g of CRM soil samples was put through the same procedure as regular soil samples without spiking as described above. The confidence interval used for the results was 95%. For every 14 sample analyses of each soil type, 2 whole procedure blanks, 2 soil matrix blanks and 2 CRM, one CRM 119-100 (BNAs-Sandy Loam 6) and one 136-100 (BNAs-Clay 1) were analyzed. QA/QC of sample recovery and SEM of parallel samples were reported after every 14 samples tested. One standard solution consisting of the six target compounds and the internal standard at 1 mg$L–1 was analyzed every day to ensure QA/QC of sample analysis.
3
Results and discussion
3.1
Comparison of different extraction methods
UE and ASE were used to compare extraction efficiencies of partly-open and closed extraction systems. The selection of extraction methods was based mainly on the spiked matrix recovery of the six target compounds under the optimized clean-up and determination conditions. The results showed that the recovery rate of the six target PAEs ranged from 80% to 110% by UE and 90% and 110% by ASE (Fig. 2). ASE gave smaller SEM values among quadruplicate sub-samples. The obtained data indicate that ASE is a more reproducible and reliable extraction method than classic UE in PAE extraction, with consistent recovery rates and lower RSD values. ASE also require a smaller soil sample and about half of the extraction volume used in UE [31], which resulted in better recovery rates, in part, due to lower background readings from the solvents [11]. Moreover, sample extraction was carried out in a closed system, limiting the chance of exposure to the laboratory atmosphere which may be a source of background contamination in the
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Front. Environ. Sci. Eng. 2013, 7(1): 31–42
Fig. 2 Comparison between ASE and UE for extraction of the six PAEs. Each point is the mean value of quadruplicates; error bars denote SEM values
extraction of the PAE compounds. The extracts resulting from ASE were filtered, which obviated the exposure of PAE solutions in the laboratory air during the intermediate centrifugation and filtration steps of UE [32]. Although UE is applicable to most environmental samples and requires a relatively short period of time, it may lead to PAEs contamination and cause background readings by the laboratory air during the three filtration steps. At high temperature and pressure, with a suitable solvent and multiple extraction cycles, ASE guarantees a quick, high recovery and simply operated extraction, which is gaining increasing acceptance for the extraction of trace environmental pollutants. In a previous study GC-MS coupled with ASE and gel permeation chromatographic purification to analyze six PAEs in soils achieved recoveries ranging from 65.5% to 104.4% [31]. The recoveries are further improved using the developed procedure in this study. For the determination of PAEs (BBP, DnBP, DEHP and DnOP) in plastic tablecloth, a method combining ASE and GC-MS gave a detection limit of 1.9 mg$kg–1 with recoveries between 89.0% and 95.5% and good reproducibility and precision [33]. Quantification of DEHP and DnOP in PVC film by ASE-RP-HPLC showed recoveries between 87% and 108% [34]. ASE has also been applied in the extraction of PAEs in sediment and fish samples, with recoveries between 70.1% and 109.1% for the six prior pollutants nominated by USEPA [35]. These results confirm the applicability of ASE for PAE extraction, and are consistent with results obtained in this study. 3.2
Comparison among different clean-up methods
Comparison among six combined column chromatography clean-up methods suggested that all of these methods could be used in the cleanup of the six PAEs tested (Fig. 3). However, method six gave higher (near 100%) average
recoveries of the six target compounds and smaller SEM (Fig. 4). Methods one to six are the clean-up combinations in Table 1. Each point is the mean value of quadruplicates. Error bars are SEM values. The comparison of three combinations of different chromatography packing materials was carried out by means of accelerated solvent extraction. The first and third combinations are mostly employed in soil sample cleanup, while the second is more often used for pesticide samples [36–38]. All the results were compared so that both soil and plant sample clean-up could be confirmed. Due to the differences in polarity, elution solution with polarity variance revealed higher efficiency for the six target compounds and method six was validated for further studies. Of the procedures tested, the combination of ASE and method six for sample clean-up was the most optimized method for the detection of the six target PAE compounds in soil. 3.3
QA/QC parameters
The instrumental quality parameters, IDLs, linearity and repeatability, and the method quality parameters, MDLs, trueness (recovery) and precision (RSD), for the detection of the six PAEs with the selected optimized procedure are shown below. 3.3.1
Instrumental quality parameters
The IDLs under the analytical conditions tested in this study were calculated by injecting 20 μg$L–1 of each target compound 7 times and the signal-to-noise values obtained by the ChemStation method varied from 0.10 to 0.31 μg$L–1 (Table 4). The analytical method meets the requirements of trace PAE analysis of environmental samples.
Tingting MA et al. ASE/GC-MS method for PAEs analysis in soils
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Fig. 3 Comparisons among six combined column chromatography clean-up methods
Fig. 4 Average recovery of the six target compounds in method six. Each point is the mean value of quadruplicates, error bars are SEM values
Linearity was examined over the range of 0 to 2 mg$L–1 by testing standards at concentrations 20, 50, 100, 200, 500, 1000 and 2000 μg$L–1, respectively. The linearity range was acceptable according to the correlation coefficient (R) of the six PAEs. Three of the compounds, namely DEP, DEHP and DnOP, showed linear responses (R≥0.9986) over the concentration range tested (Table 2). A calibration curve was developed for each prepared standard solution of 20 μg$L–1. Of standards tested, the detected value ranged from 19.060.08 μg$L –1 to 21.160.06 μg$L–1. The SEM values were less than 10%. RSD values for repeatability at 20 μg$L–1 of each compound were less than 3% for DMP and DEP, and over 9% for DnOP, with intermediate values for the remaining tested compounds.
3.3.2
Method quality parameters
Validation extraction and clean up methods were employed for MDL calculation in the four soils (Table 5). The results meet the requirement of trace PAE analysis of environmental samples, which gives a lower limit of this analysis of around μg$kg–1 level. 3.3.3
Matrix blanks
The whole procedure blank results of the six PAEs were between 1.40.2 ng and 37.14.1 ng for the optimized extraction and clean up method. However, these concentrations were converted to 0.50.1 µg$kg –1 and 18.542.0 µg$kg–1 in soil samples, resulting in less
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Table 5
MDLs of the six PAEs in the four soils/(mg$kg–1)
soil type
DMP
DEP
DnBP
BBP
DEHP
DnOP
red soil
0.04
0.06
0.07
0.03
0.03
0.02
yellow brown soil
0.06
0.05
0.08
0.07
0.10
0.11
brown soil
0.12
0.05
0.08
0.09
0.12
0.10
black soil
0.09
0.11
0.12
0.13
0.29
0.19
Note: MDLs are the average of seven analytes
Fig. 5
Quantity of the six PAEs in the whole procedure blanks. Each point is the mean value of septuplicates, error bars are SEM values
Background factors are often considered the bottleneck for high accuracy and precision analysis of PAEs in the laboratory, especially in the case of DnBP and DEHP [39]. Experimental materials, such as polyethylene tubing, can contain up to 2 107 μg$kg–1 of DEHP and airborne pollution in the laboratory might bring about a total PAE concentration to 300–700 ng$m–3. As a result, DnBP and DEHP in 1 mL of cyclohexane would increase from 1.7 and 0.6 μg$L–1 to 3.7 and 2.4 μg$L–1, respectively, following 0.5 h of exposure to air [39,40]. Aiming to minimize these interferences, ASE was employed instead of UE to reduce the volume of solvent and air contact time. All solvents were re-distilled before use and all glassware was rigorously washed and rinsed following the steps described above. However,
background interference (Fig. 5). DnBP and DEHP had higher values and SEM which indicates that solvents and subsequent sample analysis procedure might introduce more background readings that interfere with detection of these two target compounds. A special phenomenon appeared in the recovery of BBP; its recovery exceeded 100% (Fig. 4) with relatively low background readings (Fig. 5). This could be due to its low concentration in the chemicals used in the analysis and air in the testing environment as its concentration was below the detection limit (Table 6), or its weak binding to the environmental matrix, or a combination of both. In the analysis of PAEs, solvents, glassware and the time of air exposure could affect the limit of detection and quantification by altering background contamination. Table 6 compound
Soil matrix blanks of six PAEs red soil
brown yellow soil
brown soil
black soil
Ca) /(μg$kg–1)
SEM
C /(μg$kg–1)
SEM
C /(μg$kg–1)
SEM
C /(μg$kg–1)
SEM
DMP
39.4
3.2
32.1
0.6
50.0
2.7
27.5
3.3
DEP
3.9
0.2
17.3
0.3
4.9
0.1
11.8
0.3
DnBP
104.4
1.6
142.6
2.1
126.8
2.0
49.2
1.7
–
–
–
–
–
–
–
DEHP
152.3
2.2
194.5
0.6
182.5
7.2
427.0
7.5
DnOP
10.0
1.6
10.8
1.0
9.6
2.4
7.6
1.9
BBP
–
b)
Notes: a) C, concentration; b) – , below detection limit
Tingting MA et al. ASE/GC-MS method for PAEs analysis in soils
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brownish yellow soil, 80.3%–109.3% in the brown soil, and 79.8%–109.5% in the black soil (Table 8). Precision as evaluated by RSD% ranged 0.9%–19.9% (Table 8). The RSD values for the six target compounds were higher in black soil. One possible explanation is the matrix effect of the black soil system in which higher concentrations of nutrients and organic matter may have hampered extraction of the target compounds. However, the recoveries in matrix spiked samples of the four soil types were comparable to the average recovery rates reported by USEPA using EPA method 8270 (Table 8). Differences between soil matrices can significantly affect extraction efficiency. However, it has been reported that high dilution factors will reduce matrix effects that could otherwise interfere with perchlorate quantitation [42]. Soils in which higher recoveries were observed may be a sign that bound compounds were freed from the soil, while low recoveries imply incomplete release from PAEssoil interactions or signal suppression. In the extraction using ASE, high temperature and pressure combined with multiple extraction cycles in enclosed cells reduced differences in recovery between different soil matrixes so that the recoveries of all four test soils were closer to 100% than when using UE.
background DnBP and DEHP concentrations remained challenging to be eliminated. Therefore, whole procedure blanks should always be analyzed when a new batch of samples is analyzed. In the present study, two whole procedure blanks were analyzed for every 14 samples. Recovery of the surrogate standard was 90.5%4.1% which meet the requirements of trace PAE analysis in environmental materials. The backgrounds in the four soils were calculated based on testing of matrix spiked samples for each soil (Table 6). Data showed that DnBP and DEHP had higher background values than DMP, ranging 49.2–142.6 μg$kg–1, 152.3– 427.0 μg$kg–1 and 27.5–50.0 μg$kg–1, respectively (Table 6). Compared with the allowable concentration of the six PAEs in US soils listed in Table 7, DMP and DnBP in the tested four soils, with a few exceptions, exceeded the maximum allowable concentrations set by USEPA. The test results are generally at least as good as other published results from China. DnBP and DEHP in some agricultural vegetable production facilities can reach 3.6 mg$kg–1 and 3.4 mg$kg–1, respectively. In some vegetable and soils in southern China, the concentrations of total PAEs can reach 35.6 mg$kg–1 [41,42], and those results suggest that many soils in east China may exhibit PAE contamination.
3.3.4
Sample duplication
Table 7 Soil allowable concentrations and cleanup objectives of six
Analysis of the field contaminated soil from Taizhou was carried out in triplicates. It was reported that total concentrations of the six PAEs in this soil ranged 9.1– 50.6 mg$kg–1 at 100 m from the e-waste dismantling and 2.6–17.6 mg$kg–1 at 1000 m away from the disassembly area [43]. Other studies showed that the total PAE concentrations ranged from 12.6 to 46.7 mg$kg–1 at three other sites, namely Fengjiang, Nanshan and Meishu in Taizhou City. DEHP, DnBP and DEP were the major phthalates accounting for more than 94% of total phthalates studied [44]. Our data indicate a total concentration of the six PAE compounds near 4.2 mg$kg–1 (ranging from less than MDLs to 2.70.07 mg$kg–1 for individual PAEs). The sampling site is a field that was used to evaluate phytoremediation of PCBs in the soil with alfalfa [45]. It has been reported that sludge and chemical fertilizers
PAE compounds in the United States allowable concentration /(mg$kg–1)
cleanup objective value /(mg$kg–1)
DMP
0.02
2.00
DEP
0.07
7.10
DnBP
0.08
8.10
BBP
1.22
50.00
DEHP
4.35
50.00
DnOP
1.20
50.00
compound
Sub-samples of the four soils were spiked with about 10fold MDLs of the target compounds and the surrogate standard. Trueness was in the range of 75%–120% for the six PAEs in the four soils tested. Specifically, these values were 80.3%–119.2% in the red soil, 75.0%–112.7% in the Table 8
QA/QC report of matrix samples spiked with PAEs/(0.1 mg$kg–1)
soil type
DMP
DEP
DnBP
BBP
DEHP
DnOP
R /%
RSD/%
R/ %
RSD/%
R/%
RSD/%
R/%
RSD/%
R/%
RSD/%
R/%
RSD/%
red
80.3
3.9
81.5
3.3
90.4
5.3
101.9
2.8
111.5
2.7
119.2
2.8
brown yellow
75.0
1.8
76.5
2.2
80.9
1.9
94.9
1.1
104.3
0.9
112.7
0.9
brown
80.3
3.7
84.9
2.6
99.1
1.8
98.6
1.2
109.3
2.6
108.2
1.0
black
81.1
14.1
79.8
14.8
89.0
16.6
98.0
14.2
105.5
19.9
109.5
13.2
a)
EPA-8270a) Note: a) recovery
93.2
95.8
90.8
91.2
94.2
89.9
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Front. Environ. Sci. Eng. 2013, 7(1): 31–42
Comparison between CRM assigned values and determined values
Table 9 compound
given confidence interval/(mg$kg–1)
determined value/(mg$kg–1)a)
119–100
136–100
119–100
136–100
DMP
8.29–11.17
2.85–3.42
8.640.08
2.930.06
DEP
4.21–5.26
1.29–1.64
4.550.09
1.490.09
DnBP
NEb)
0.64–0.80
NDc)
0.730.08
BBP
12.06–16.34
6.61–8.32
13.750.10
6.840.07
DEHP
7.65–9.94
0.82–0.96
9.380.17
0.920.11
DnOP
4.93–6.54
4.78–5.71
6.310.14
5.470.12
Notes: a) values are meanSEM; b) NE, not exist in the CRM sample; c) ND, not detected
containing PAEs may increase the concentration of PAEs in Ipomoea aquatica up to a range of 2.1 to 7.1 mg$kg–1 [46]. PAEs can be transferred from contaminated soils to plants, such as vegetables, at a concentration range of 1.2– 2.0 mg$kg–1 [47]. The risk of PAE transfer to the edible parts of vegetables and the potentials to enter human food chain deserve special attention for future studies. The results of the surrogate standard show that average recoveries were 92.97%0.02% and SEM of parallel samples were less than 10%.
Acknowledgements This research was supported by the China National Environmental Protection Special Funds for Scientific Research on Public Causes “Studies on the regulation of agricultural soil environmental quality, environmental risk and key control techniques” (No. 201109018) and the Program of Innovative Engineering of the Chinese Academy of Sciences (No. KZCX2-YW-Q02-06-02). We thank Dr. Shiping Deng in the Department of Plant and Soil Sciences at Oklahoma State University in the USA for language revision of this manuscript. We are grateful to the State Key Laboratory of Soil and Sustainable Agriculture at the Institute of Soil Science, Chinese Academy of Sciences, for the supply of Dionex ASE200.
References 3.3.5
Comparison between CRMs
The accuracy of the procedure was tested by analyzing a CRM in which 5 or 6 PAE compounds were present. Comparison between the assigned values and determined values are shown in Table 9. The results indicate that the determined values were generally within the range of the assigned values at given confidence intervals, suggesting that the developed procedure produced quantitative measurements of the tested PAEs in soil and may be used to evaluate PAE pollution in the environment.
4
Conclusions
A comprehensive GC-MS based analytical procedure for the determination of six priority PAEs in soils was developed. ASE for extraction, followed by cleaning up with a compound chromatography column and mixed elution solutions, coupled with analysis using SIM mode of GC-MS and an isotope surrogate, the optimized analysis method of PAEs in different soil matrix has been built up. Results showed that multiple soil matrices can be prepared and determined with high sensitivity. The use of procedure blanks, matrix blanks and spiked matrix blanks limited interferences from solvents, glassware, human operation and decreased noise to signal ratios in the detection. The developed procedure was validated by testing CRM. In application, QA/QC is essential for ensuring high accuracy and reliability of data obtained.
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