Cristina M. M.
Almeida
a and
Luís Vilas
Boas
bc
aFaculdade de Farmácia da Universidade de Lisboa, Laboratório de Hidrologia e Análises Hidrológicas, Av. das Forças Armadas, 1649-019, Lisboa, Portugal. E-mail: calmeida@ff.ul.pt; Fax: 00351–217946470
bInstituto de Tecnologia Química e Biológica (ITQB), Apartado 127, 2784-505, Oeiras, Portugal
cInstituto Superior Técnico, Av. Rovisco Pais, 1049-001, Lisboa, Portugal. E-mail: lboas@iqb.unl.pt
First published on 17th November 2003
The analysis of BTEX and other substituted benzenes in water samples using solid phase microextraction (SPME) and quantification by gas chromatography with flame ionization detection (GC-FID) was validated. The best analytical conditions were obtained using PDMS/DVB/CAR fibre using headspace extraction (HS-SPME) at 50 °C for 20 min without stirring. The linear range for each compound by HS-SPME with GC/FID was defined. The detection limits for these compounds obtained with PDMS/DVB/CAR fibre and GC/FID were: benzene (15 ng L−1), toluene (160 ng L−1), monochlorobenzene (54 ng L−1), ethylbenzene (32 ng L−1), m-xylene (56 ng L−1), p-xylene (69 ng L−1), styrene (35 ng L−1), o-xylene (42 ng L−1), m-dichlorobenzene (180 ng L−1), p-dichlorobenzene (230 ng L−1), o-dichlorobenzene (250 ng L−1) and trichlorobenzene (260 ng L−1). This headspace SPME-GC-FID method was compared with a previously validated method of analysis using closed-loop-stripping analysis (CLSA). The headspace SPME-GC-FID method is suitable for monitoring the production and distribution of potable water and was used, in field trials, for the analysis of samples from main intakes of water (surface or underground) and from the water supply system of a large area (Lisbon and neighbouring municipalities).
Organic compounds in water derive from three major sources: the breakdown of naturally occurring organic materials, domestic and economic activities, and reactions that occur during water treatment and distribution.
Domestic and economical activities may be sources for a large number of synthetic organic chemicals (SOCs) to wastewater discharges, agricultural runoff, urban runoff, and leachate from contaminated soils. Most of the organic contaminants identified in water supplies as having adverse health concerns are part of this group. Some of the SOCs are also VOCs (volatile organic chemicals). This term “volatile organic chemical” refers to the characteristic evaporative abilities (or the vapour pressure) of these compounds, which can also apply when they are dissolved in water. Three broad groups of VOCs have been found in drinking water. One group includes compounds found in petroleum products, especially aromatics like benzene, toluene and xylenes.1 Major sources of these compounds are leaks in fuel and gasoline tanks and piping due to deficient installation or corrosion especially in old underground storage tanks. Another group is the halogenated VOCs, used as solvents and degreasers. Their former use as septic tank cleaners also accounts for many instances of private well contamination.1
The third group includes some of the chlorinated organic disinfection by-products, particularly the trihalomethanes. They easily volatilize from water used in the home and can be absorbed by inhalation. The air in bathrooms can represent a major source of exposure because of volatilisation from shower and bathwater. In addition, dermal absorption (across the skin) can occur in bathwater because the VOCs are lipophilic. The combination of dermal and inhalation absorption can represent as much or more exposure to VOCs as ingestion. Their lipophilicity also enables VOCs to cross into the brain and can cause a reversible anesthetic effect, including dizziness, nausea, and cardiac depression.1
High exposure to most VOCs over many years causes brain damage and moderate exposure can alter kidney and liver function or cause damage.1,2 The World Health Organization (WHO) defined guidelines values for BTEX and other substituted benzenes in water in order to limit human risks and to retain the aesthetic quality of drinking water: benzene (10 µg L−1), toluene (700 µg L−1), xylenes (500 µg L−1), ethylbenzene (300 µg L−1), styrene (20 µg L−1), monochlorobenzene (300 µg L−1), o-dichlorobenzene (1000 µg L−1), p-dichlorobenzene (300 µg L−1) and trichlorobenzene (20 µg L−1).3 For substances that are considered to be carcinogenic, such as benzene, the guideline value is the concentration in drinking water associated with an excess lifetime cancer risk of 10−5 (one additional cancer per 100000 of the population ingesting drinking water containing the substance at the guideline value for 70 years).3 For related aromatic compounds, concentrations of the substance at or below the health-based guideline value may affect the appearance, taste or odour of the water.3
In this work, we studied the BTEX and other substituted benzenes in water. A number of aromatic and certain aliphatic hydrocarbons have been detected in ground and surface waters. The origins of these compounds in water supplies are frequently unknown and often controversial, but in the case of surface waters the sources usually are fuel and/or oil spills. In this group of compounds, benzene has been extensively studied.3 The major sources of benzene in water are atmospheric deposition (through rain and snow) and chemical plant effluents (with minor contributions from urban runoff and sewage plants).3 Acute exposure to benzene results in central nervous system depression. Although most studies of benzene toxicity have involved exposure by the inahalation route, limited animal studies suggest that exposure by other routes of administration leads to similar sequelae. Chronic exposure to benzene leads to haemopoietic tissue changes in the form of anaemia and leukopenia.2,3
Benzene and lower alkylbenzenes are volatile and comparatively unreactive in the environment, but because of the suppression of the evaporation process, they can be present in groundwater at higher levels than those usually found in surface-water.3
Due to the concern with toxicological properties of benzene and related compounds, there is an interest in the development of simple and specific analytical procedures to measure them in water2,4,5 and air samples.6–9 In water samples, a pre-treatment is often necessary to isolate the components of interest from sample matrices, to purify and concentrate the analytes. Solid-phase micro extraction is a very convenient technique used for the analysis of volatile and semivolatile compounds in liquid samples. SPME requires no solvents or complicated apparatus. The theory of SPME both in immersion and or in headspace technique (HS) has been described in detail by Zhang and Pawliszyn.10–12
The HS-SPME technique is usually applied in an equilibrated situation with the analytes distributed between the fibre coating and HS gas present in a sealed vial. In theory, the HS-SPME has several advantages: the fibre does not come in contact with the sample, the background adsorption and matrix effects are reduced, which also enhances the life expectancy of SPME fibre. The extraction by SPME is influenced by several factors: sample matrix, stirring, temperature, the amount of sample, the size of the HS vial, the ratio of the HS to aqueous phase and the position of the coated fibre in the HS, which can all affect the time required for the analyte to equilibrate between the HS vial contents and the SPME fibre coating.11,13 The effects of these factors and the advantages of HS-SPME vs immersion-SPME have been studied by several authors using many groups of compounds and many sorts of samples.14–24
CLSA is an alternative method for concentration of BTEX and other substituted benzenes in water samples. This method is a powerful method for trace enrichment of volatile and semi-volatile compounds in water25–27 and was validated for these compounds.28 In this method there is a continuous recycling of the sample headspace in a closed circuit through a trapping cartridge.
A method using headspace SPME-GC-FID was validated and compared with the CLSA method. The headspace SPME-GC-FID was applied to the analysis of samples of water of main intakes (surface and underground) and supply (pipelines, nets and reservoirs) of municipal water of Lisbon. These compounds are potentially toxic and therefore their presence should be monitored both in environmental water and water for human consumption.
A fibre holder for manual use was purchased from Supelco (Bellefonte, USA). Microextraction fibres were also from Supelco and coated with five different films: poly(dimethylsiloxane) (PDMS) 7 and 100 µm, poly(acrylate) (PA) 85 µm, poly(dimethylsiloxane/divynylbenzene) (PDMS/DVB) and poly(dimethylsiloxane/divynylbenzene/carboxen) (PDMS/DVB/CAR). All fibres were conditioned in the hot injector of the gas chromatograph according to instructions provided by the supplier: PDMS 7 µm (320 °C/1 h), PDMS 100 µm (250 °C/0.5 h), PA 85 µm (300 °C/2 h), PDMS/DVB 50/30 µm (250 °C/0.5 h) and PDMS/DVB/CAR 50/30 µm (270 °C/2–4 h).
The stirring and heating of water samples was performed using a hot/stirring plate, DataPlate® Digital, Biomolecular Inc (Cole-Parmer International, Vernon Hills, USA) with stirrer bars 13 mm × 3 mm from Azlon (Bibby Sterrilin, Staffordshire, UK).
The sample temperature during analysis by SPME was monitored using a 5″ thermometer for SPME from Supelco (Bellefonte, USA).
The closed loop stripping was carried out with an apparatus (Brechbühler AG, Switzerland) including a control unit, water bath thermostat, water-bath glass container, glass bottle inlet/outlet assay with 2 rotulex connections, pump, glass retainer for charcoal, precision charcoal filter and heater. This technique was optimised for BTEX and other substituted benzenes28 using 1.5 mg of charcoal (particle size: 0.05–0.1 mm). After 90 min of purging, the charcoal trap was eluted with 20 µL of solvent (carbon disulfide) and 2 µL of eluate injected in a gas chromatograph.
The reagent water producing apparatus was a Millipore, Milli Q Gradient A10 (Molsheim, France).
The hydrochloric acid and sodium thiosulfate (pro-analysis grade) supplied by Merck (Darmstadt, Germany) were used for sample preservation.
When there were delays between sampling and analysis, 4 drops of 6 M HCl were added to the sample (40 mL) for its preservation and for samples containing residual chlorine it was also necessary to add a reducing agent: 3 mg sodium thiosulfate were added to the sample (40 mL) for its preservation.29 After collection, the samples were refrigerated at 4 °C immediately and they were analysed within 14 days.29 The water samples were allowed to reach room temperature before starting the analysis.
On the beginning of each working day, a column blank was followed by a fibre blank and a reagent water blank to detect any possible laboratory contamination.
The temperature effect in extraction using HS-SPME was studied by analysing three replicates of standard solutions under the same conditions (PDMS/DVB/CAR fibre, 40 mL, 0.5 µg L−1 of each analyte, 10 min, 1500 rpm) but using different temperatures (25, 30, 35, 40, 50 and 60 °C).
The repeatability of the analytical procedure (as relative standard deviation, RSD%) was estimated analysing six replicates of reagent water spiked with an intermediate standard solution of BTEX and other substituted benzenes (5, 10, 15, 20 and 50 µL).
The apparent recovery was determined by analysing six replicates solutions containing two concentration levels of each compound. These solutions were prepared spiking 40 mL of tap water with 10 µL (N = 6) and 20 µL (N = 6) of an intermediate standard solution. The tap water without fortification was analysed in the same conditions.
The amounts of analyte present in the spiked sample and in water without fortification were estimated from the calibration curve. The difference between amount of analyte present in spiked and unspiked sample was divided by the amount of spike to calculate the apparent recovery.
Fig. 1 presents the chromatogram of a solution containing the target compounds with the corresponding retention time (tR) and symbols used to represent them.
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Fig. 1 Chromatogram of BTEX and other substituted benzenes by GC/FID. Identification of the compounds by their retention time (tR) and symbols, including the surrogate (Surr) |
1-Chlorooctane was used as surrogate in the optimization of analysis of BTEX and other substituted benzenes by CLSA28 and the same compound was included in solutions used for the optimisation of conditions by SPME. 1-Chlorooctane could not be used as surrogate in SPME analysis because its behaviour is different from the analytes. However we retained the term surrogate to refer to 1-chlorooctane as it could be used to detect major changes in the fibre behaviour during the time required to carry out the experiments.
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Fig. 2 Comparison of response HS-SPME analysis with different fibres |
The responses achieved with PDMS/DVB and PDMS/DVB/CAR fibres were very similar, with the exception of benzene and toluene, with a response higher for the PDMS/DVB/CAR fibre; therefore, all subsequent experiments were performed with this fibre coating.
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Fig. 3 Influence of the temperature in extraction by HS-SPME with PDMS/DVB/CAR coated fibre |
The most efficient extractions were obtained at 50 and 60 °C. Higher temperatures increase the concentration of the analytes in the headspace but decrease the partition coefficients between the fibre coating and the headspace,10 and a temperature of 50 °C appears most suitable for HS-SPME extraction of BTEX and other substituted benzenes.
There are no significant differences between the results obtained for these three extraction times (P-values >0.05 at the 95% confidence level). An exposure time of 20 min was considered suitable and convenient for a chromatographic run of 25 min.
Fig. 4 presents the results obtained by sampling a solution containing BTEX and other substituted benzenes from headspace phase using a PDMS/DVB/CAR fibre with and without stirring and keeping all the other conditions. There were no differences in results for static and stirred samples, except for the surrogate.
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Fig. 4 Comparison of extraction of BTEX and other substituted benzenes between static and stirred samples by HS-SPME with PDMS/DVB/CAR coating fibre |
The response obtained by HS-SPME was about nine times higher than the response obtained by direct immersion of the fibre. The highest ratio was observed for o-dichlorobenzene (∼28) and the lowest for benzene (∼3). These results confirmed HS-SPME as most suitable for the analysis of volatile organic compounds.4,30
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Fig. 5 Comparison of extraction of BTEX and other substituted benzenes between immersion and headspace SPME with PDMS/DVB/CAR coating fibre |
The linear range of FID coupled with the HS-SPME procedure for a PDMS/DVB/CAR was studied by analysis of fourteen solutions containing all standards at different concentration levels (between 15 ng L−1 and 15 µg L−1). Under these chromatographic conditions, m-xylene and p-xylene co-elute (tR = 13.26 min) as do styrene and o-xylene (tR = 14.31 min). Therefore, we prepared two groups of fourteen solutions for the linearity study: one group containing all standards including m-xylene and styrene and the second group containing all standards including p-xylene and o-xylene. The study of linearity included the statistical linearity test determining the test value PG31 required for the F-test. If PG ≤ F, the non-linear calibration function does not lead to a significantly better adjustment: the calibration function is linear. If PG > F, the working range should be reduced as far as possible to receive a linear calibration function; otherwise the information values of analysed samples must be evaluated using the non-linear calibration function.
For most compounds in this study, the three higher concentration levels are not well represented by a linear calibration function and these observations may be explained, as due to deviation of equilibrium or limited fibre capacity and this is confirmed by PG values (these PG values were larger than tabulated F values of Fisher/Snedecor).
The peak areas obtained for these high concentration levels were smaller than what would be expected for a linear plot. To avoid the reduction of the concentration range, a polynomial equation (second or third degree) was fitted to the calibration data. If the highest concentration is not considered, the trichlorobenzene and the p-dichlorobenzene show linear calibration functions, with PG values of 0.024 and 1, respectively [F(1,10)95% = 5].
The analysis of non-linear calibration curves of target compounds showed a good fit by an equation of second degree with squared correlation coefficients (R2) between 0.9981 and 0.9993 with the exceptions of benzene and toluene. Removing the last point, the toluene also presents a good fit with a polynomial function of second degree.
The study of linearity is required in procedures for chromatographic method validation. The fitting of non-linear calibration curve may extend the working range although this is unusual in routine analysis where dilution of samples is preferred in order to keep concentrations within the working range established according to the linearity criteria. The linear ranges, the PG values and F values of Fisher/Snedecor for each compound are given in Table 1.
Compound | Linearity range/µg L−1 | R 2 | N | SE | m | SE(m) | b | SE(b) | PG | F |
---|---|---|---|---|---|---|---|---|---|---|
a R 2 = squared correlation coefficients, m = slope, SE(b) = standard error of the intercept, N = number of data points, SE(m) = standard error of the slope, PG = test value, SE = standard error, b = intercept, F = value of Fisher/Snedecor (tabled value) | ||||||||||
B | 0.049–0.37 | 0.9937 | 4 | 1317 | 96307 | 5430 | 2841 | 1252 | 0.83 | F(1,1)95% = 161 |
T | 0.37–3.7 | 0.9953 | 6 | 8383 | 92769 | 2864 | 19607 | 5887 | 0.03 | F(1,3)95% = 10 |
CB | 0.58–2.8 | 0.9969 | 11 | 5142 | 89335 | 1674 | 2000 | 2149 | 0.1 | F(1,8)95% = 5.3 |
EB | 0.027–2.7 | 0.9967 | 11 | 9431 | 173297 | 3299 | 3861 | 3940 | 0.2 | F(1,8)95% = 5.3 |
m-X | 0.075–3.8 | 0.9947 | 9 | 16433 | 181980 | 4206 | 13522 | 6399 | 5.7 | F(1,6)95% = 6.0 |
p-X | 0.073–1.9 | 0.9952 | 8 | 10906 | 225995 | 6422 | 10416 | 6013 | 10−4 | F(1,5)95% = 6.6 |
St | 0.13–6.4 | 0.9953 | 9 | 18357 | 117310 | 2957 | 15893 | 8695 | 1.1 | F(1,6)95% = 6.6 |
o-X | 0.059–2.3 | 0.9945 | 9 | 13569 | 219347 | 6209 | 8186 | 6612 | 5.3 | F(1,6)95% = 6.0 |
m-dCB | 0.28–5.5 | 0.9971 | 9 | 8141 | 79559 | 1629 | 8114 | 4338 | 0.04 | F(1,6)95% = 6.0 |
p-dCB | 0.35–6.9 | 0.9971 | 9 | 10570 | 82883 | 1686 | −3034 | 5633 | 0.4 | F(1,6)95% = 6.0 |
o-dCB | 0.34–6.7 | 0.9963 | 9 | 10592 | 75934 | 1740 | 6070 | 5645 | 3.6 | F(1,6)95% = 6.0 |
tCB | 0.33–6.6 | 0.9959 | 10 | 9832 | 67666 | 1536 | 379 | 4657 | 0.085 | F(1,7)95% = 5.6 |
The squared correlation coefficients (R2) of BTEX and other substituted benzenes were between 0.9937 and 0.9971. The linear range of BTEX and other substituted benzenes was one order of magnitude except for ethylbenzene and (m-, p-, o-)xylene (two orders of magnitude).
Compound | HS-SPME | CLSA | ||
---|---|---|---|---|
MDL/µg L−1 | MQL/µg L−1 | MDL/µg L−1 | MQL/µg L−1 | |
Benzene | 0.015 | 0.050 | 0.031 | 0.105 |
Toluene | 0.16 | 0.54 | 0.026 | 0.087 |
Chlorobenzene | 0.054 | 0.18 | 0.025 | 0.083 |
Ethylbenzene | 0.032 | 0.11 | 0.012 | 0.041 |
m-Xylene | 0.056 | 0.19 | 0.018 | 0.061 |
p-Xylene | 0.069 | 0.23 | 0.016 | 0.055 |
Styrene | 0.035 | 0.12 | 0.017 | 0.057 |
o-Xylene | 0.042 | 0.14 | 0.019 | 0.062 |
m-Dichlorobenzene | 0.18 | 0.59 | 0.027 | 0.089 |
p-Dichlorobenzene | 0.23 | 0.77 | 0.020 | 0.067 |
o-Dichlorobenzene | 0.25 | 0.83 | 0.017 | 0.057 |
Trichlorobenzene | 0.26 | 0.87 | 0.024 | 0.081 |
The MDL for BTEX and other substituted benzenes were in the range 0.015–0.26 µg L−1 and the MQL were in the range 0.050–0.87 µg L−1 by HS-SPME followed by GC/FID, using a PDMS/DVB/CAR fibre.
There are significant differences between the quantification limits obtained for these two methods (P value = 0.0021 at the 95% confidence level): they are lower for CLSA than for HS-SPME with the exception of benzene. The differences between quantification limits of the two methods were higher for the compounds with higher retention times. The compound with lowest quantification limit by HS-SPME is the compound with lower retention time, benzene. The quantification limits for the different compounds by CLSA are very similar (<0.1 µg L−1).
5 µL/40 mL (N = 6) | 10 µL/40 mL (N = 6) | 15 µL/40 mL (N = 6) | 20 µL/40 mL (N = 6) | 50 µL/40 mL (N = 6) | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
µg L−1 | Area | RSD (%) | µg L−1 | Area | RSD (%) | µg L−1 | Area | RSD (%) | µg L−1 | Area | RSD (%) | µg L−1 | Area | RSD (%) | |
B | 0.12 | 11120 | 11.0 | 0.25 | 23199 | 5.4 | 0.37 | 25545 | 4.2 | 0.49 | 31734 | 6.1 | 1.23 | 39704 | 7.1 |
T | 0.37 | 34535 | 4.8 | 0.74 | 95312 | 2.2 | 1.10 | 96866 | 3.9 | 1.47 | 125686 | 3.7 | 3.68 | 297345 | 4.5 |
CB | 0.27 | 39160 | 2.9 | 0.54 | 105259 | 3.1 | 0.80 | 107319 | 5.4 | 1.07 | 143278 | 3.0 | 2.68 | 441013 | 3.2 |
EB | 0.25 | 37443 | 2.9 | 0.50 | 103251 | 4.3 | 0.75 | 103654 | 4.8 | 1 | 138809 | 3.4 | 2.50 | 448569 | 3.1 |
m-X | 0.32 | 30229 | 4.0 | 0.64 | 97357 | 3.4 | 0.96 | 85390 | 2.7 | 1.28 | 118359 | 6.7 | 3.20 | 455411 | 3.7 |
St | 0.28 | 16300 | 7.6 | 0.55 | 49567 | 5.6 | 0.83 | 41165 | 4.0 | 1.1 | 58093 | 6.9 | 2.75 | 241539 | 3.7 |
m-dCB | 0.35 | 16024 | 9.0 | 0.69 | 57407 | 2.7 | 1.04 | 45304 | 3.6 | 1.38 | 64432 | 4.1 | 3.45 | 293275 | 4.6 |
p-dCB | 0.34 | 15298 | 5.0 | 0.67 | 51915 | 4.6 | 1.01 | 42257 | 1.9 | 1.34 | 57304 | 3.6 | 3.35 | 251981 | 3.6 |
o-dCB | 0.33 | 62773 | 2.7 | 0.66 | 111137 | 5.0 | 0.98 | 166376 | 7.4 | 1.31 | 225235 | 6.4 | 3.28 | 618481 | 6.4 |
tCB | 0.33 | 11481 | 7.5 | 0.66 | 40398 | 4.4 | 0.99 | 31270 | 6.2 | 1.32 | 95385 | 7.7 | 3.3 | 219683 | 5.7 |
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Fig. 6 Successive extractions on same sample by HS-SPME |
Before two successive analyses on the same sample there was a period of 15 min of stabilization before exposure to the fibre (20 min).
In the second extraction, the response (area count) for the majority of compounds was 25–30% lower; therefore, in these conditions it is not possible to repeat the analysis on the same sample.
These results may be explained as due to depletion of the analyte occurring after each extraction. This explanation is confirmed by the observed concentration ratios discussed in next section.
The area of a peak obtained using SPME for the spiked solution was divided by the area of the corresponding peak of the chromatogram of the standard solution (0.5 µL, direct injection) used for spiking: this quotient was then divided by the dilution factor (spiking volume/40 mL).
The values of estimated concentration ratios should be considered just as operational values because this chemical system is complex: there are three phases (sample, head space and fibre) and the fibre material contains three different polymers (PDMS, DVB and CAR). These values of calculated concentration ratios are presented in Fig. 7.
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Fig. 7 Concentration ratios for different concentrations of spiked samples by HS-SPME with PDMS/DVB/CAR fibre |
The concentration ratios decrease with the increase of the analytes concentration but there is no direct relationship between concentrations ratios and retention times: benzene and trichlorobenzene have low concentration ratios.
A value of ∼30000 for the concentration ratio may be used to illustrate the depletion of analyte in a sample (40 mL) containing 0.50 µg L−1 of analyte. Assuming a volume of 0.5 µL of polymeric material of fibre, the concentration of the analyte in the aqueous solution will be reduced to a calculated value of 0.36 µg L−1 after the first analysis. Each subsequent extraction would reduce the concentration by 28% if the concentration ratio was kept constant.
The apparent recoveries of BTEX and other substituted benzenes for higher concentrations were between 68 and 106% (Table 4) with the exception of styrene.
10 µL/40 mL | 20 µL/40 mL | |||||
---|---|---|---|---|---|---|
µg L−1 | Rec (%) | RSD (%) | µg L−1 | Rec (%) | RSD (%) | |
B | 0.25 | 102 | 14.8 | 0.49 | 101 | 1.4 |
T | 0.74 | 83 | 8.2 | 1.47 | 94 | 3.4 |
CB | 0.58 | 89 | 12.3 | 1.15 | 97 | 1.6 |
EB | 0.54 | 70 | 7.1 | 1.07 | 86 | 1.4 |
m-X | 0.50 | 56 | 2.8 | 1 | 68 | 4.5 |
p-X | 0.59 | 59 | 36 | 1.17 | 71 | 18 |
St | 0.64 | 5 | 6.6 | 1.28 | 11 | 17.6 |
o-X | 0.49 | 67 | 31 | 0.97 | 78 | 25 |
m-dCB | 0.55 | 93 | 17.2 | 1.1 | 98 | 2.0 |
p-dCB | 0.69 | 90 | 14.4 | 1.38 | 99 | 1.4 |
o-dCB | 0.67 | 95 | 14.1 | 1.34 | 106 | 11.5 |
tCB | 0.66 | 94 | 25.1 | 1.32 | 102 | 2.1 |
Most compounds had a recovery higher than 85% for both fortification levels; however, these were better for the higher concentration (fortification of 20 µL). The precision increases with the concentration levels (when the concentration is 5–10 times higher than the minimum quantification level).
The apparent recovery of styrene was about 10%, therefore, the method is not suitable to quantify this compound. Depending on the fortification level, the apparent recoveries of xylenes vary between 60 and 70%. Since the retention times for styrene and o-xylene are the same (these compounds were not resolved on the chromatographic column), when a sample has a peak with this retention time, it will probably be m-xylene, although confirmation should be obtained by a complementary analysis: GC with mass spectrometry detector.
The m-xylene and p-xylene have similar apparent recoveries and the same retention time. If a peak occurs in the chromatogram at this retention time, a complementary analysis, with a different column separating these isomers, would be necessary to check which of these compounds is(are) present in the sample.
Compound | SM01A, sample/µg L−1 | SL09A, sample/µg L−1 | ||
---|---|---|---|---|
00/06/08 | 00/07/06 | 00/07/13 | 00/07/13 | |
Toluene | <0.16 | <0.16 | <0.16 | 0.51 |
Ethylbenzene | <0.03 | 0.63 | 0.23 | <0.03 |
m-, p-Xylene | <0.06 | 2.49 | 1.23 | 0.20 |
Styrene/o-Xylene | <0.04 | 1.40 | 0.71 | 0.20 |
o-Dichlorobenzene | <0.25 | <0.20 | 0.76 | 0.59 |
Trichlorobenzene | 1.01 | <0.26 | <0.26 | <0.26 |
These 225 samples are representative of raw and distributed waters and were collected as part of the sampling program for the quality control of EPAL.
Only four samples had measurable concentration of the analytes and three of these were collected at the same sampling site (Table 5). The values presented in Table 5 are much lower than guideline values from the World Health Organization.
The different parameters affecting the HS-SPME of BTEX and other substituted benzenes have been studied: SPME fibres with different coatings were tested and PDMS/DVB/CAR was found as the most suitable fibre for this analysis. The best conditions of extraction by SPME for BTEX and other substituted benzenes with this type of fibre were obtained with 40 mL samples, extracted (headspace) at 50 °C during 20 min without stirring.
The optimized method gave good reproducibilities, ranging from 2.9 to 11%, 2.2 to 5.6%, 1.9 to 7.4%, 3.0 to 7.7% and 3.2 to 7.1% as determined at 0.3, 0.6, 0.9, 1.2 and 3 µg L−1, respectively.
Linearity was studied over a wide range of concentrations, and the minimum detection levels (between 15 and 260 ng L−1) were found suitable for analysis, considering the regulatory levels (Council Directive 98/83/CE and guidelines values of the World Health Organization3). These limits were compared with those obtained by the CLSA technique. The SPME fibre has a smaller capacity and therefore the amount of analyte extracted is smaller than with CLSA, resulting in higher detection levels and quantification limits. However, analysis by CLSA are laborious and slower (90 min per sample), a larger volume of sample (1000 mL) is necessary and carbon disulfide has to be used as solvent. Carbon dissulfide is toxic and has high affinity for laboratory environmental contaminants (it is difficult to obtain good blanks).
For screening of the BTEX and other substituted benzenes in distribution water, GC-FID is suitable for routine analyses but the use of GC-MS would be advisable to confirm identification of compounds and get further quantitative information if such information is needed: mass spectrometry is very sensitive and provides both qualitative and quantitative information.
The optimised HS-SPME procedure was applied in field trials monitoring samples from main intakes of water (surface or underground) and from the water supply system of a large area (city of Lisbon and surrounding area).
In routine analysis for control of public water supplies (raw ant treated water), just a few samples are likely to contain benzene and/or related compounds and the proposed method was easily used for screen testing.
If more detailed analysis are necessary for the few samples expected found with contaminants, the use of GC-MS would be necessary to confirm the identifications and search for possible causes of contamination.
This method can be adapted for an automated chromatographic system to make the process easier.
This journal is © The Royal Society of Chemistry 2004 |