Anal. Chem. 1997, 69, 1204-1210
Determination of Aromatic Compounds in Water by Solid Phase Microextraction and Ultraviolet Absorption Spectroscopy. 2. Application to Fuel Aromatics Brian L. Wittkamp and Steven B. Hawthorne
Energy & Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58202-9018 David C. Tilotta*
Department of Chemistry, University of North Dakota, P.O. Box 9024, Grand Forks, North Dakota 58202-9024
Solid phase microextraction is coupled with ultraviolet absorption spectroscopy to determine fuel aromatic hydrocarbons from unleaded gasoline, jet fuel (JP4), and no. 1 diesel fuel in water. A rectangular “chip” of poly(dimethylsiloxane) (OV-1) was used as the sorbent medium to selectively partition the various fuels from real water matrices without interferences from naturally occurring organics (e.g., humic and fulvic acids). Equilibration times are under 45 min for the majority of aromatic compounds and range from 150 to 300 min for the three fuels. However, it is shown that the fuels can be quantitated at significantly shorter extraction times (45 min), resulting in only a loss of ∼2× in the detection limits. Detection limits for unleaded gasoline, JP4, and no. 1 diesel fuel (obtained at their 100% equilibration times) are 4.9, 17, and 9.0 ppb, respectively. In addition, recovery data for the aromatic components from unleaded gasoline, JP4, and no. 1 diesel fuel from two real water matrices are in the range of 87-106%, with RSDs in the range of 5.0-10%. Part 1 of this series1 focused on the methodology for determining aromatic species in water by solid phase microextraction (SPME) coupled with ultraviolet (UV) absorption detection. Particular emphasis was placed on developing a “sorbent chip” device for use in the SPME procedure. This device consists of a small rectangular solid of poly(dimethylsiloxane) chromatographic stationary phase (OV-1, ∼80 µL volume) that is used to selectively partition low- to nonpolar organic compounds from water. As shown in part 1, the sorbent chip provides for a substantial sensitivity enhancement when aromatics in water are determined using UV spectroscopy. As reported in part 1, the method of SPME coupled with UV detection was evaluated with the use of 11 aromatic compounds commonly found in fuels to determine the optimum operational characteristics (e.g., geometry of the sorbent chip, analyte equilibration times, and sample handling considerations).1 The goal of part 2 is to develop a method, using the procedures developed in part 1, for determining the total aromatic concentra(1) Wittkamp, B. L.; Hawthorne, S. B.; Tilotta, D. C. Anal. Chem. 1997, 69, 1197-1203.
1204 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
tion in water samples that have been contaminated with various fuels (e.g., unleaded gasoline, JP4, and no. 1 diesel fuel). In addition, the advantages of utilizing SPME for the selective determination of three different fuels in two different “real” water matrices will be addressed. EXPERIMENTAL SECTION Instrumentation. A Shimadzu Model UV 260 (Columbia, MD) UV-visible spectrophotometer was used for all experiments. The spectrometer was operated in the absorbance mode with 2-nm spectral resolution. The spectral region examined in this study was 220-300 nm, unless otherwise noted. The scan speed was fixed at 86 nm/min. The quartz cells used in this study (ICL, Garfield, NJ) possessed a 3.5-mL volume with a path length of 1.0 cm. Both the reference and sample cells were masked with black electrical tape to avoid UV radiation from diverting around the extraction medium. The mask size is the same as reported previously in part 1 of this series.1 Reagents and Samples. Commercially available poly(dimethylsiloxane) (OV-1, Alltech, Deerfield, IL) was chosen as the solid phase extraction medium for this study. Prior to use, the air voids in the OV-1 were removed by dissolving it in toluene as outlined in previous work from our laboratory.1,2 Upon removal of the air voids from the OV-1, the medium was cut into rectangular shapes (referred to as sorbent chips) measuring 10 mm × 2 mm × 4 mm ((0.5 mm for all dimensions) with the use of a razor blade. For ease of performing parallel extractions, six sorbent chips were used throughout the study. The resulting average volume of the six sorbent chips was calculated to be 80 ( 2 µL. Excluding the fuels, all compounds were spectrophotometric grade and were used as purchased from the Aldrich Chemical Co. (Milwaukee, WI). The unleaded gasoline sample (certificate 11105008) was obtained from the American Petroleum Institute (Washington, DC) and was stored in 1.5-mL autosampler vials with septa caps. The JP4 and no. 1 diesel fuel were obtained from local sources and were stored in glass vials with Teflon-lined screw-top caps. When not being used, the gasoline, JP4, and no. 1 diesel fuel were stored at 4 °C to maintain sample integrity. (2) Wittkamp, B. L.; Tilotta, D. C. Anal. Chem. 1995, 67, 600-605. S0003-2700(96)00762-7 CCC: $14.00
© 1997 American Chemical Society
One of the two real water samples was obtained from a local river and the other sample was obtained from a local stagnant pond. The water samples were collected in 4-L glass containers with Teflon-lined screw-top caps. Headspace was minimized within these containers to ensure sample integrity. The water samples were stored at room temperature. Water solutions of the pure analytes and the three different fuels were prepared by first spiking the appropriate amount of each compound into 0.5 mL of spectroscopic grade methanol (Fisher Scientific Co., Itasca, IL). Then, the analyte in methanol solution was quantitatively delivered to 500 mL of distilled, deionized water. The resulting analyte spiked water solution contained 0.1% methanol by volume. The “real-world” water samples were each spiked with the unleaded gasoline, JP4, and no. 1 diesel fuels in the same fashion as delineated above. Procedures. The pure analytes and the three different fuels, spiked in water, were extracted in 50-mL round-bottom flasks (with ground glass stoppers) as described in part 1 of this study. In addition, the cleaning procedure for the sorbent chips was the same as used in part 1. Response factors of the 11 analytes examined in this study were obtained from the slopes of their calibration plots. These calibration plots were constructed by performing extractions of a specific concentration and plotting the resulting absorbance signal (measured at 247, 254, and 260 nm) vs concentration. The corresponding slopes (obtained at 247, 254, and 260 nm) were normalized to yield each analyte’s relative response factor at these wavelengths. Quantitation of the three different fuels was achieved by the same method as described previously for the 11 fuel aromatic compounds (part 1). A detection limit for each fuel was obtained by spiking a water sample with each fuel such that the resulting concentration of the solution produced a signal (of the extracted fuel) which measured 4-6× the instrumental baseline noise. The detection limit was defined as that concentration which produced an absorbance signal that measured twice the peak-to-peak noise. GC/MS analyses were performed using a Hewlett-Packard Model 5972 instrument equipped with a 30-m HP-5 column (250µm i.d., 0.25-µm film thickness). The oven temperature at injection was 40 and 60 °C for the jet fuel and diesel fuel, respectively, followed by a temperature ramp at 8 °C/min. RESULTS AND DISCUSSION Selection of Analytes. Principally non- to low-polar species from the fuels will be selectively extracted (over more polar species) because a relatively nonpolar extraction medium was chosen for this study. Furthermore, as determined in part 1 of this series, the absorption of radiation in the UV region of 220300 nm is primarily observed from aromatic compounds. Therefore, the UV absorbance signal observed for the various fuels is mainly due to the non- to low-polarity aromatic compounds and not due to the numerous other fuel components (i.e., aliphatic hydrocarbons). The non- to low-polarity species chosen to represent the aromatic compounds present in the various fuels are shown in Table 1 and were primarily selected based on their relative weight percent (wt %) in a certified unleaded gasoline sample.3 With the exception of naphthalene and 1-methylnaphthalene, only the major contributors (i.e., 1.00 wt % and greater) were included in this (3) American Petroleum Institute (API 91-1), certificate 111790-54, 1990.
Table 1. Relative Concentrations of Selected Aromatic Species Present in Unleaded Gasoline compound
wt %a
compound
wt %a
benzene tolueneb ethylbenzene o-xylene m-xyleneb p-xyleneb
1.22 7.68 3.37 2.64 5.31 2.13
1-methyl-3-ethylbenzene 1-methyl-4-ethylbenzene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene 1-methylnaphthalene naphthalene
2.34 1.06 1.10 3.37 0.69 0.53
a Values were obtained from ref 3. b Values reported in ref 3 include small contributions from coeluting aliphatic hydrocarbons.
Table 2. Equilibration Times of Various Fuels equilibration time (min) compound
90%
100%
unleaded gasoline JP4 no. 1 diesel fuel
120 150 190
150 180 300
study. Relative concentrations of aromatic compounds present in this unleaded gasoline sample below 1.0 wt % were ignored. Naphthalene and 1-methylnaphthalene were included because they have large molar absorptivities and are the most concentrated polycyclic aromatics present in the fuels used in this study. The total weight percent of the 11 compounds listed in Table 1 equals ∼31 wt % (including the small contributions from the coeluting aliphatic hydrocarbons), and the total aromatic content for this unleaded gasoline sample is reported to be 39 wt % (as provided in the certificate of analysis). Therefore, the 11 aromatic compounds chosen for this study represent 81% of the total aromatic content in this unleaded gasoline sample. Equilibration Times of the Fuels. Table 2 presents the 90 and 100% equilibration times (within the RSDs of the measurements) for unleaded gasoline, JP4, and no. 1 diesel fuel. As shown in Table 2, the 100% equilibration times are 150, 180, and 300 min for unleaded gasoline, JP4, and no. 1 diesel fuel, respectively. These extraction times are ∼6× longer than those observed for most of the 11 representative aromatic compounds, which arises from the fact that the fuels are composed of numerous aromatic compounds and the time required for each fuel to reach equilibrium is dependent upon each compound. The progressively longer equilibration times for these fuels are expected based on their differing SPME-UV responses (as discussed below). Similarly as reported in part 1 of this series, the 90% equilibration times for the aromatic compounds are typically 1.5× shorter than the 100% equilibration times. Likewise, the 90% equilibration times for the fuels are 1.5× shorter than the 100% equilibration times. Use of the 90% equilibration time for each fuel reduces the time spent for extraction to 120, 150, and 190 min for unleaded gasoline, JP4, and no. 1 diesel fuel, respectively. As observed with the 11 aromatic species, the 90% equilibration time for the three fuels is a factor of 1.5× shorter than the time required for 100% equilibration. Nonequilibrium Calibration. Although a 1.5× reduction in the extraction time is observed for 90% equilibration, an even shorter extraction time would make this method more practical. Therefore, a calibration curve was constructed for each fuel using a 45-min extraction time because the majority of the individual aromatic compounds (as shown in part 1 of this two-part report) Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
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Table 3. Calibration Data for Selected Fuels at Their 100% and 45-Min Equilibration Times 45-min extractiona compound
slopec
y-intd
100% equilibrationa,b slopec
y-intd
unleaded gasoline 1.02 × 7.63 × 1.99 × 5.76 × 10-4 JP4 4.20 × 10-4 1.60 × 10-3 7.01 × 10-4 3.88 × 10-3 no. 1 diesel fuel 4.45 × 10-4 3.53 × 10-3 1.12 × 10-3 4.82 × 10-3 10-3
10-3
10-3
a R2 for all compounds, 9.99 × 10-1. b 100% equilibration times correspond to 150, 180, and 300 min for unleaded gasoline, JP4 and no. 1 diesel fuel, respectively. c Units of AU/ppb (wt/wt). d y-int, y-axis intercept, units of AU.
equilibrated within 45 min. The equation of the line for each fuel’s calibration plot (at their wavelength maxima of 266, 272, and 272 nm for unleaded gasoline, JP4, and no. 1 diesel fuel, respectively) at both 100% equilibration and an extraction time of 45 min can be seen in Table 3. As is expected, the major difference between quantitating these three fuels at 45 min vs at 100% equilibration is reduced method sensitivity. The sensitivity for each fuel (slope of calibration plot) decreases by a factor of 2.0×, 1.6×, and 2.0× when extracting gasoline, JP4, and no. 1 diesel fuel for 45 min vs their 100% equilibration times, respectively. As the extraction time is decreased, less time is allowed for the aromatic species present in each fuel to completely partition into the sorbent chip. Thus, the reduced amount of aromatics partitioning into the sorbent chip results in a lower absorbance signal in comparison to the signal obtained with a longer extraction time (i.e., 100% equilibration), resulting in a poorer detection limit for each fuel. The detection limit for unleaded gasoline, JP4, and no. 1 diesel fuel is 9.7, 27, and 18 ppb by weight (as total fuel concentration in water) for a 45-min extraction, respectively. However, the detection limits for these fuels at an extraction time corresponding to their 100% equilibration time are 4.8, 17, and 9.0 ppb for unleaded gasoline, JP4, and no. 1 diesel fuel, respectively. Thus, extracting unleaded gasoline, JP4, and no. 1 diesel fuel at 45 min results in a factor of 3.3×, 4.0×, and 6.7× reduction, respectively, in the time spent for the extraction with only a loss of a factor of ∼2.0× in the detection limit. It is interesting to note that the SPME-UV responses for the different fuels are fairly similar, despite the different fuel compositions. For example, the gasoline contains ∼39 wt % aromatics, which are accounted for mostly (>75% of the total aromatics) by toluene, C2-, C3-alkylbenzenes.2 As shown in Figure 1, JP4 jet fuel contains a similar proportion and distribution of aromatics. However, no. 1 diesel fuel contains relatively low amounts of aromatics, mostly naphthalene and alkylnaphthalenes (Figure 1). In contrast to gasoline and jet fuel, the majority of diesel components are alkanes which show no (or significantly minor) UV response. Based solely on aromatic content, it would be expected that the combined SPME-UV response of diesel fuel would be quite low compared to gasoline and jet fuel. However, naphthalene and alkylnaphthalenes both partition more into the sorbent chip than single-ring aromatics (i.e., have higher KD’s), and have higher molar absorptivities. Thus, the combination of these two effects results in a similar sensitivity for all three of the fuels tested. This similarity in response indicates that reasonable quantitative results can be obtained for water contaminated with unknown fuels, at least if the fuel was gasoline, jet fuel, or diesel fuel. 1206 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
Response Factors. In efforts to calibrate this system for the determination of the aromatic components from unleaded gasoline, JP4, and no. 1 diesel fuel in water, the response for each of the 11 aromatic compounds at various wavelengths was obtained to determine which wavelength provides the most uniform signal (i.e., similar SPME-UV response) for all the aromatic compounds. The response factors for the compounds were obtained by constructing calibration plots for each compound from their corresponding absorbance signals at 247, 254, and 260 nm. The resulting slopes of the calibration plots (obtained at each wavelength) were normalized relative to naphthalene (most sensitive compound). We selected 247, 254, and 260 nm for the analyte response study for two reasons. The first is because benzene has five major UV absorption maxima (243, 247, 254, 260, and 273 nm), and three of these absorption maxima are observed with this method. Thus, if benzene is to be included as one of the aromatic compounds, then one or more of these wavelengths must be utilized. Second, 254- and 260-nm observation wavelengths were chosen because they are common detection wavelengths in commercially available high-performance liquid chromatographic (HPLC) UV detector systems. Therefore, to examine the possibility of utilizing a simpler chromatographic detector, we included these wavelengths in our study. It should also be recalled from the discussion in part 1 of this series that HPLC detector systems have noise levels which are ∼1 order of magnitude lower than that typically observed with the spectrophotometer used in this study. As shown in Table 4, the absolute and normalized (N in Table 4) response factors for the 11 aromatic compounds are not equivalent (at any observation wavelength), which arises from the difference in the magnitude of each analyte’s KD and value (as mentioned previously in part 1). The higher the KD value and value of an analyte, the greater the observed absorbance signal will be and thus a greater SPME-UV response. However, Table 4 shows that 247 nm is that wavelength at which the compounds appear to have relatively similar normalized response factors (a 53-fold span from benzene to naphthalene). In contrast, the normalized response factors for the other wavelengths have a greater span (i.e., 90- and 200-fold for 254 and 260 nm, respectively). With the exception of benzene (lowest response) and 1-methylnaphthalene and naphthalene (highest responses), the remaining aromatic species have normalized response factors in the range of 0.083-0.137 AU/ppb at 247 nm with an average response of 0.106 AU/ppb. Thus, observation of the analyte signal at 247 nm will yield the most uniform analyte response for this system. Selectivity. Both partitioning and spectroscopic considerations govern analyte selectivity for SPME coupled with UV detection. The partitioning component for analyte selectivity stems from matching the polarity of the analyte to that of the sorbent chip. With the use of poly(dimethylsiloxane) (a nonpolar phase) as the sorbent chip, principally non- to low-polar analytes will be selectively extracted from a water matrix. The spectroscopic factor concerning analyte selectivity in this application arises from the fact that the absorption of UV radiation is primarily observed for compounds with conjugated double bond systems (i.e., aromatic species). Therefore, when these two considerations are taken together, the absorbance signal observed from an extraction of a fuel from water is due to the nonpolar aromatic species and not from the numerous other aliphatic compounds
Figure 1. GC/MS analysis of JP4 jet fuel (top) and no. 1 diesel fuel (bottom). The numbers on the diesel fuel chromatogram refer to the chain length of the major n-alkanes. All significant aromatic hydrocarbons are labeled in both chromatograms. The unidentified peaks are aliphatic hydrocarbons.
present in the fuel. However, as noted in part 1, the spectra of the aromatics are similar and cannot readily be distinguished from one another. Since benzene is an important environmental contaminant, an experiment was conducted to determine the criteria required to selectively determine benzene from a fuel matrix, in water, by SPME-UV. JP4 was selected as the fuel matrix because it has a very low benzene concentration but the highest concentration of the other alkylbenzene components (of the three fuels studied in this work). Therefore, observance of UV absorption bands from benzene would be a direct result of the benzene in the spike and not the benzene originally in the JP4. Mixtures of 10, 25, and 50% (by volume) benzene in JP4 were spiked into distilled, deionized water such that the concentration of JP4 in each water sample remained constant. These solutions were extracted in triplicate, and a plot of the benzene UV absorbance signal vs concentration was constructed. The benzene UV absorbance was obtained by linearly correcting the benzene
signal’s baseline for the JP4 background. From this plot, it was found that the least amount of benzene that can be selectively determined in JP4 is 16% (by volume). Thus, to selectively determine the amount of benzene in unleaded gasoline, the effective concentration of benzene must be increased from 1.03% (which is benzene’s volume percent in unleaded gasoline) to 16%. Unfortunately, an effective concentration of benzene corresponding to 16% in unleaded gasoline would result in total absorbances too strong (i.e., matrix aromatics plus benzene) to remain within the linear dynamic range (LDR) of this system. The major limitation in these determinations were the relatively poor signal-to-noise (S/N) ratios of the measured spectra. Since the entire spectrum is required to perform the baseline correction (as discussed above), a solution to avoid exceeding the LDR while still providing a spectrum with a high S/N ratio would be to use a UV spectrophotometer equipped with a multichannel detector [such as a charge-coupled device (CCD) or a photodiode array detector (PDA)]. It is well-known that multichannel detectors Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
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Table 4. Analyte Response Factors responsea
a
Nb
compound
247 nm
254 nm
260 nm
247 nm
254 nm
260 nm
benzene toluene ethylbenzene o-xylene m-xylene p-xylene 1-methyl-3-ethylbenzene 1-methyl-4-ethylbenzene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene 1-methylnaphthalene naphthalene
6.41 × 2.77 × 10-4 3.26 × 10-4 4.10 × 10-4 4.40 × 10-4 3.28 × 10-4 4.59 × 10-4 3.45 × 10-4 2.79 × 10-4 3.26 × 10-4 2.16 × 10-3 3.35 × 10-3
7.01 × 4.53 × 10-4 4.82 × 10-4 5.78 × 10-4 5.98 × 10-4 3.87 × 10-4 3.42 × 10-4 1.13 × 10-3 3.86 × 10-4 5.13 × 10-4 6.06 × 10-3 6.43 × 10-3
5.19 × 5.60 × 10-4 5.71 × 10-4 7.34 × 10-4 7.00 × 10-4 1.25 × 10-3 4.55 × 10-4 1.52 × 10-3 5.31 × 10-4 6.68 × 10-4 9.31 × 10-3 1.05 × 10-2
0.019 0.083 0.097 0.122 0.131 0.098 0.137 0.103 0.083 0.097 0.645 1.00
0.011 0.077 0.075 0.089 0.093 0.056 0.053 0.166 0.060 0.080 0.940 1.00
0.0049 0.053 0.054 0.070 0.067 0.120 0.043 0.145 0.051 0.064 0.890 1.00
10-5
10-5
10-5
Absolute response factor [units of AU/ppb (w/w)]. b Normalized response factor, relative to naphthalene.
provide spectra with higher S/N ratios than photomultiplier tubes (PMTs) because of the multiplex advantage from the greater number of spectral resolution elements (e.g., 1024 elements vs 1 element).4 Typically, increases in the spectral S/N ratios obtained from a multichannel detector, in comparison to the S/N ratios obtained with a PMT, are on the order of N1/2, where N is the number of spectral resolution elements (e.g., a detector with 1024 spectral resolution elements would provide a S/N enhancement of 32× vs a PMT). Calibration Standards. The SPME-UV method can be calibrated for the determination of the aromatic components from fuels in water by use of a specific fuel as the calibration standard if the fuel is known or use of a general calibration standard (e.g., one of the 11 fuel aromatic compounds) if the fuel is unknown. Obviously, the first approach will yield the most accurate analytical results. However, as discussed above, the similarity in responses for gasoline, jet fuel, and diesel fuel indicates that reasonable quantitation can be performed even if the fuel is unknown. The use of any one of the three fuels to quantitate any of the others will result in an error of not more than 3×. A useful general calibration standard should have a similar SPME-UV response as the fuel(s). Therefore, the optimum general calibration standard would be an analyte that possesses an average SPME-UV response with respect to the differing relative concentrations of these aromatic species in various fuels. However, the concentration of the aromatic species present in the three fuel matrices examined in this study is only known for the unleaded gasoline sample (from the certificate of analysis, ∼39 wt %) and is not known for the JP4 and no. 1 diesel fuel samples. Thus, the unleaded gasoline sample was used to determine a general calibration standard for the determination of these fuels in water. The general calibration analyte was selected based on the closest match of the standard analyte’s slope (response) to that of the three fuels at an observation wavelength of 247 nm (which was determined to be the optimum observation wavelength). The concentration of each calibration standard that was prepared from unleaded gasoline was reduced by 39%, which is the weight percent of the aromatic species present. Then, the slope of the calibration plot for unleaded gasoline (2.31 × 10-3) was compared to the slopes of the calibration plots for the aromatic compounds (shown in Table 4). (4) Ingle, J. D.; Couch, S. R. Spectrochemical Analysis; Prentice-Hall: Englewood Cliffs, NJ, 1988; p 160.
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On the basis of the above results, 1-methylnaphthalene was selected as a useful, general calibration standard for this method to determine the concentration of unleaded gasoline in water, because its slope at 247 nm (2.16 × 10-3) was the closest match to unleaded gasoline’s slope of 2.31 × 10-3 at 247 nm. In addition, 1-methylnaphthalene was easy to use because it is a liquid. To check the efficacy of this standard, the UV absorbance signal from an extraction of a 23 ppb solution of unleaded gasoline in water (corrected concentration which reflects the fact that unleaded gasoline contains 39 wt % of aromatic species) was fitted to the calibration plot for 1-methylnaphthalene. The resulting concentration of unleaded gasoline (based on the calibration curve for 1-methylnaphthalene) corresponds to 27 ppb, which is to within 13% of the original aromatic concentration in water (23 ppb). Analysis of Surface Waters. The “real” water samples used in this study were obtained from an area river and a local stagnant pond. Total residual solids, after air evaporation, were determined to be 280 and 15 000 mg of solids/L for the river water matrix and the pond water matrix, respectively. Both the pond water and the river water were yellowish in color, and both samples were visibly cloudy (due to the suspended solids). Both water samples were extracted with the sorbent chip in triplicate before spiking them with the fuels to ensure that they were clean of extractable aromatics. Following the extractions, the UV spectra of the sorbent chips were void of any UV absorption bands, thus suggesting that there are no interferences from species originally present in the water samples. Unleaded gasoline, JP4, and no. 1 diesel fuel were individually spiked into the two different “real” water matrices at concentrations of ∼1 ppm and extracted. The absorbance values of the different fuels obtained from the real water SPME extractions were compared to the fuels’ absorbance values obtained from SPME extractions in distilled, deionized water. For convenience, the absorbance values for the fuels were measured at their maximum absorbance wavelengths of 268, 272, and 272 nm for unleaded gasoline, JP4, and no. 1 diesel fuel, respectively. It should be noted that the recovery does not depend on the observation wavelength. The recovery data (concentration determined by SPME-UV compared to the spiked concentration) for the various fuels in the different water matrices can be seen in Table 5. As shown in this table, the recoveries determined for the three fuels from the different water matrices range from 87 to 106% of the known values, with RSDs ranging from 5.0 to 10%. The recoveries for
Table 5. Recovery Data for Unleaded Gasoline, JP4, and no. 1 Diesel Fuel from Various Water Matricesa fuel
water matrix
% recoveryb
% RSD
unleaded gasoline
river pond river pond river pond
105 106 95 89 87 93
9.9 9.4 10 10 5 6
JP4 no. 1 diesel
a Although SPME is not an exhaustive extraction technique, we use the term “recovery” to be consistent with current practice. b Values resulted from a single extraction of three separate analyte spiked water solutions at water concentrations of ∼1 ppm.
unleaded gasoline for both water matrices are above 100% (e.g., 105 and 106%), while the JP4 and no. 1 diesel fuel are below 100%. However, all the recoveries for each fuel in both water matrices are within the RSDs of the measurements. It was interesting to note that before any extractions were performed in the real water matrices, the sorbent chips were visibly clear. Following the extractions in the real water matrices (∼30 extractions), the sorbent chips turned from clear to a darkcolored material, which resulted from the accumulation of solid particulates from the water matrix throughout the course of the extractions. The presence of adsorbed solid particulates on the outer surface of each sorbent chip did not appear to substantially affect the sorbent chip’s extraction efficiency. However, the sorbent chips were discarded when they became dark. Detection Limits in Real Samples. An important advantage of using SPME in conjunction with optical spectroscopy is the selective analyte preconcentration (based on the analyte’s KD value), which provides detection limits within the low-ppb range. Table 6 is a list of the detection limits obtained with and without SPME for three aromatic compounds and three fuels each in two real water matrices. Benzene was selected for this study because it has the poorest KD and values and has the highest degree of spectroscopic fine structure. Conversely, naphthalene was chosen because of its low degree of fine structure and its relatively high KD and values. Lastly, toluene was selected as a representative aromatic compound which has intermediate fine structure, KD value, and value. As shown in Table 6, the detection limits with SPME preconcentration for all the species examined from real water samples are approximately 1-5 orders of magnitude lower than the detection limits without SPME. In fact, the aromatic components from both the JP4 and diesel fuels cannot be determined in the
Figure 2. UV spectra illustrating SPME preconcentration from a real pond water sample spiked with gasoline (0.86 ppm water concentration). The upper spectrum was obtained with SPME preconcentration (through the sorbent chip), the middle spectrum was obtained without SPME preconcentration (i.e., directly in the water sample), and the lower spectrum is of the pond water blank. It should be noted that all spectra have been offset for clarity.
real water samples without the SPME preconcentration. It is interesting to note that a comparison of the detection limits obtained without using SPME for benzene, toluene, and naphthalene in the real water matrices vs distilled, deionized water (i.e., Table 5, part 1) shows that the detection limits in distilled, deionized water are lower by a factor of ∼5×. The reason for the lower detection limits is most likely because of the UV absorption of the naturally occurring organics (e.g., humic and fulvic acids) and increased light scattering. The light scattering is most prominent at shorter wavelengths and results in a sloping baseline which is more difficult to correct. Figure 2 illustrates this feature. This figure consists of a UV spectrum of a gasoline spike in pond water (0.86 ppm concentration) following a 40-min SPME extraction (i.e., in the sorbent chip), a spectrum of the same gasoline spike obtained directly in the water (without SPME), and a spectrum of the pond water blank. The spectra acquired directly through the pond water used distilled, deionized water as the reference material.
Table 6. Detection Limits with and without SPME for Various Aromatic Compounds and Fuels detection limit, ppb (w/w) pond water
river water
compound
+ SPME (% RSD)a
- SPME (% RSD)a
+ SPME (% RSD)a
- SPME (% RSD)a
benzene toluene naphthalene unleaded gasoline JP4 no. 1 diesel fuel
81.0 (5.6) 15.1 (5.0) 0.386 (5.0) 3.65 (12) 14.5 (8.8) 8.35 (6.8)
7350 (7.4) 4080 (4.9) 56.0 (7.2) 7060 (5.9) >100000b >100000b
95.8 (6.2) 15.9 (2.3) 0.359 (4.2) 4.51 (5.1) 16.4 (5.6) 8.34 (4.8)
6780 (5.3) 2950 (6.7) 50.0 (8.5) 3920 (6.7) >100000b >100000b
a Averages and relative standard deviations from triplicate measurements. b Due to lack of spectral contrast, values were defined at solubility limit in water.
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The lower two spectra clearly show the sloping baseline and illustrates the loss of “spectral contrast” caused by the light scattering and absorption. This loss is observed for all the fuels in water, but is most prominent for JP4 and no. 1 diesel fuel. As can be seen by comparing the lower two spectra, the baseline increases significantly throughout the entire spectral range and is void of UV absorptions arising from the gasoline (the spectrum of the unspiked water is identical to the water spiked with gasoline as shown in Figure 2). The same effect of poor spectral contrast in the real water matrices was observed for the case of no. 1 diesel fuel (up to a concentration of 100 ppm in the water samples) and jet fuel. In contrast, the UV spectrum of the extracted gasoline in the SPME chip (the upper spectrum in Figure 2) has excellent spectral contrast. Note the strong absorbance in the 240-280nm region from the fuel aromatics. In contrast to the poorer detection limits achieved without SPME in the “real” vs pure water samples, the high matrix organic and solid content of the pond and river waters did not decrease the method sensitivity for either the pure aromatics or the fuels. This feature can easily be seen by comparing the detection limits found in Table 6 (+SPME) to those found in Table 5 of part 1 of this series of reports (+SPME). Therefore, the extraction of aromatic species from a real water matrix does not appear to be affected by the suspended solids or the dissolved organic matter, and the relatively high KD values of the fuel aromatic compounds permits low ppb detection limits of these compounds in a water matrix.
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Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
CONCLUSIONS Solid phase microextraction coupled with UV absorption detection has been shown to be a useful technique for determining total fuel aromatic species in water. The preconcentration afforded by the sorbent chip allows detection limits of the total fuel aromatic compounds present in unleaded gasoline, JP4, and no. 1 diesel in the low-ppb range. These detection limits are comparable with the detection limits obtained in real water matrices when SPME is utilized. The selectivity of the SPME chip for nonpolar aromatics allows direct quantitation of ppb and higher concentrations of fuels in surface waters by eliminating the interferences caused by more polar UV-absorbing organics (e.g., humics). ACKNOWLEDGMENT Financial support from the U.S. Environmental Protection Agency (Office of Exploratory Research) and the U.S. Department of Energy (METC) is gratefully acknowledged.
Received for review July 30, 1996. Accepted December 20, 1996.X AC9607626
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Abstract published in Advance ACS Abstracts, February 1, 1997.
