Anal. Chem. 1999, 71, 897-904
Selective Determination of Aromatic Sulfonates in Landfill Leachates and Groundwater Using Microbore Liquid Chromatography Coupled with Mass Spectrometry Marc J.-F. Suter,* Sonja Riediker, and Walter Giger
Swiss Federal Institute for Environmental Science and Technology (EAWAG), Swiss Federal Institute of Technology (ETH), Ueberlandstrasse 133, CH-8600 Du¨ bendorf, Switzerland
Aromatic sulfonates (AS) are large-volume chemicals used in many technical processes of, for instance, the textile industry or construction. A LC/MS method for the selective determination of AS in environmental samples, based on a single-quadrupole MS, was developed and validated. The central point of this technique is the use of the compound-specific fragment ion SO3•- as marker for aromatic sulfonates. This negatively charged SO3 radical, together with the fact that AS undergo loss of SO2, allows screening for AS in complex matrixes, even in the presence of sulfate anions. Calibration curves generated from LC/MS data showed good linearity over 3 orders of magnitude, with an absolute limit of detection of ∼1 ng. The relative standard deviation for mean areas obtained from reconstructed ion chromatograms ranged from 2.9 to 8.6%. Unlike UV detection, this LC/MS method gives similar response for both naphthalene- and benzenesulfonates. The method presented was successfully applied to landfill leachates and groundwater, downstream of a landfill. Furthermore, this technique allowed identification of an unknown AS found in drain samples.
used in foundry molds. Naphthalenesulfonates are intermediates in dye production and monomers of one class of concrete admixtures. They have been found in surface waters4 and landfill leachates.5,6 These chemicals have a potentially high mobility in an aquatic environment, caused by a low pKa of the sulfonic acid group (e.g., pKa ) 0.57 for naphthalenesulfonic acid, NS7) and small octanol-water partition constant (Kow ) 0.115 for NS8). Therefore, their fate and behavior after release into the environment need to be understood. Much work has focused on the environmental impact of anionic surfactants9-14 and fluorescent whitening agents.15,16 However, there is only limited data on aromatic sulfonates (AS) in leachates from landfills, where most of the production byproducts and much of the construction debris are expected to be finally deposited. For example, Kim et al. showed that up to 69% of the total organic carbon found in leachates from a hazardous waste landfill could be attributed to 4-chlorobenzenesulfonate, a DDT manufacturing byproduct.17 They were using anion-exchange chromatography particle beam mass spectrometry for the tentative identification of an additional seven AS. Furthermore, they identified toluene-
Sulfonated organic compounds are used in a wide variety of technical products. Well-known representatives of this compound class are the concrete admixtures naphthalene- and melaminesulfonate-formaldehyde condensates and lignosulfonate. A total of 2920 t of these so-called superplasticizers were used in Switzerland in 1992,1 qualifying them as high production volume chemicals. Furthermore, chlorolignosulfonates are produced as waste materials by pulp mills.2 Another group of sulfonated chemicals that fall into the class of high production volume chemicals is the anionic surfactants, with a global production of linear alkylbenzenesulfonates in the order of 2.8 × 106 tons in 1995.3 Sulfonated dyes and fluorescent whitening agents represent another large fraction of sulfonates, along with p-toluenesulfonate
(4) Kok, S. J.; Kristenson, E. M.; Gooijer, C.; Velthorst, N. H.; Brinkman, U. A. T. J. Chromatogr., A 1997, 771, 331-341. (5) Riediker, S. Ph.D. Thesis, ETH Zu ¨ rich, No. 12974, 1998. (6) Suter, M. J.-F. In Selected Topics and Mass Spectrometry in the Biomolecular Sciences; Caprioli, R. M., Malorni, A., Sindona, G., Eds.; Kluwer Academic Publishers: Dordrecht, 1997; pp 559-573. (7) Weast, R. C., Ed. Handbook of chemistry and physics; CRC Press: Cleveland, OH, 1976. (8) Greim, H.; Ahlers, J.; Bias, R.; Broecker, B.; Hollander, H.; Gelbke, H.-P.; Klimisch, H.-J.; Mangelsdorf, I.; Paetz, A.; Scho¨n, N.; Stropp, G.; Vogel, R.; Weber, C.; Ziegler-Skylakakis, K.; Bayer, E. Chemosphere 1994, 28, 22032236. (9) Reiser, R.; Toljander, H. O.; Giger, W. Anal. Chem. 1997, 69, 4923-4930. (10) Gonzalez-Mazo, E.; Honing, M.; Barcelo, D.; Gomez-Parra, A. Environ. Sci. Technol. 1997, 31, 504-510. (11) Suter, M. J.-F.; Reiser, R.; Giger, W. J. Mass Spectrom. 1996, 31, 357-362. (12) Borgerding, A. J.; Hites, R. A. Anal. Chem. 1992, 64, 1449-1454. (13) Field, J. A.; Miller, D. J.; Field, T. M.; Hawthorne, S. B.; Giger, W. Anal. Chem. 1992, 64, 3161-3167. (14) Trehy, M. L.; Gledhill, W. E.; Orth, R. G. Anal. Chem. 1990, 62, 25812586. (15) Stoll, J.-M. A.; Giger, W. Anal. Chem. 1997, 69, 2594-2599. (16) Poiger, T.; Field, J. A.; Field, T. M.; Giger, W. Environ. Sci. Technol. 1996, 30, 2220-2226. (17) Kim, I. S.; Sasinos, F. I.; Stephens, R. D.; Brown, M. A. Environ. Sci. Technol. 1990, 24, 1832-1836.
* Corresponding author: (phone) +41 1 823 5479; (fax) +41 1 823 5028; (email)
[email protected]. (1) von Arx, U. Bauprodukte und Zusatzstoffe in der Schweiz; Buwal: Bern, 1995. (2) Bulterman, A. J.; Van Loon, W. M. G. M.; Ghijsen, R. T.; Brinkman, U. A. T.; Huitema, I. M.; De Groot, B. Environ. Sci. Technol. 1997, 31, 19461952. (3) Ainsworth, S. J. Chem. Eng. News 1996, 74 (Jan 22), 32-54. 10.1021/ac980911f CCC: $18.00 Published on Web 01/20/1999
© 1999 American Chemical Society
Analytical Chemistry, Vol. 71, No. 4, February 15, 1999 897
Figure 1. Schematic representation of the major fragmentation reactions of N-1-S and N-1,6-dS. The fragmentation of N-1-S follows two major pathways: Formation of deprotonated 1-naphthol and neutral SO2, through rearrangement of the sulfonate moiety (A) and formation of a neutral naphthalene radical and the SO3•- ion through fission of the C-S bond (B). N-1,6-dS undergoes the same type of fragmentations: rearrangement (m/z 287 f 207 f 143 and m/z 206 f 142) and bond fission and radical formation (m/z 287, 309, 143 f 206 and m/z 207 f 80).
sulfonate in a groundwater monitoring well.18 Betowski et al. identified AS in groundwater samples from superfund sites, using thermospray (TSP).19 Both groups stated that a major fraction of compounds is not detected by routine environmental analysis, which mostly covers volatile and semivolatile pollutants, and that LC/MS is a necessary addition in this regard. The gas-phase ion chemistry of organic sulfonates, and especially of sulfonated azo dyes, has been studied by various groups, using negative chemical ionization,11,20 fast atom bombardment (FAB),21-24 electrospray ionization (ESI),25-28 or matrixassisted laser desorption/ionization postsource decay mass spectrometry.28 Straub et al. compared TSP, particle beam (PB), and ESI performance for the determination of azo dyes and found that (18) Kim, I. S.; Sasinos, F. I.; Richi, D. K.; Stephens, R. D.; Brown, M. A. J. Chromatogr. 1991, 589, 177-183. (19) Betowski, L. D.; Kendall, D. S.; Pace, C. M.; Donnelly, J. R. Environ. Sci. Technol. 1996, 30, 3558-3564. (20) Binkley, R. W.; Flechtner, T. W.; Tevesz, M. J. S.; Winnik, W.; Zhong, B. Org. Mass Spectrom. 1993, 28, 769-772. (21) Monaghan, J. J.; Barber, M.; Bordoli, R. S.; Sedgwick, D.; Tyler, A. N. Org. Mass Spectrom. 1982, 17, 529-533. (22) Smith, J. D.; O’Hair, R. A. J.; Williams, T. D. Phosphorus, Sulfur Silicon Relat. Elem. 1996, 119, 49-59. (23) Richardson, S. D.; McGuire, J. M.; Thruston, A. D. J.; Baughman, G. L. Org. Mass Spectrom. 1992, 27, 289-299. (24) Richardson, S. D.; Thruston, A. D. J.; McGuire, J. M.; Weber, E. J. Org. Mass Spectrom. 1993, 28, 619-625. (25) Bruins, A. P.; Lars, O. G. W.; Henion, J. D.; Budde, W. L. Anal. Chem. 1987, 59, 2647-2652. (26) Straub, R.; Voyksner, R. D.; Keever, J. T. J. Chromatogr. 1992, 627, 173186. (27) Schro¨der, H. F. J. Chromatogr., A 1997, 777, 127-139. (28) Sullivan, A. G.; Garner, R.; Gaskell, J. Rapid Commun. Mass Spectrom. 1998, 12, 1207-1215.
898 Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
TSP gave poor response for the disulfonated azo dye Acid Orange 10, when compared to ESI, while the dye was thermally degraded when PB with electron ionization was used.26 This clearly illustrates the advantages of ESI. Many researchers found that AS frequently undergo loss of SO2 in the negative ion mode and that very often a negatively charged SO3 radical is formed. Figure 1 shows major fragmentation pathways observed for naphthalenemono- and -disulfonates (N-1-S and N-1,6-dS, respectively). Loss of neutral SO2 from AS occurs through a rearrangement of the sulfonate group (A and m/z 206 f 142).20,22 A similar rearrangement produces neutral SO3 from disulfonates (m/z 287 f 207). On the other hand, the negatively charged SO3 radical is formed by fission of the C-S bond (B and m/z 143 f 206). Using tandem mass spectrometry and various ionization techniques, several groups have shown that a similar fragmentation pattern could be observed for all sulfonates investigated.20,22-25 Schro¨der used the loss of SO2 to screen for AS by ESI-MS/MS,27 while Bruins et al. scanned for parents of SO3•-, again using ESI-MS/MS.25 Straub et al. showed that similar fragmentations could be induced without having to rely on sophisticated MS/MS techniques.26 They made use of the fact that in an electrospray interface fragmentations can be induced by adjusting the interface potentials in such a way that ions collide with neutral gas molecules. This technique is often referred to as in-source collisional activation. We have developed and validated an analytical method based on these findings that allows the selective determination of organic sulfonates in landfill leachates and groundwater, using microbore
HPLC coupled to an electrospray interface on a benchtop singlequadrupole mass spectrometer. EXPERIMENTAL SECTION Chemicals and Materials. Naphthalene-1-sulfonate (N-1-S), benzenesulfonate (BS), and p-toluenesulfonate (pTS) were purchased from Fluka AG (Buchs, Switzerland), naphthalene-2sulfonate (N-2-S) and naphthalene-1,6-disulfonate (N-1,6-dS) were from Chem Service (West Chester, PA), 4-aminonaphthalene-1sulfonate (4-NH2-N-1-S) was from Merck (Darmstadt, Germany), and 4-chlorobenzenesulfonate (4-Cl-BS) was from Aldrich (Steinheim, Germany). All standards were p.a. grade, except N-1-S (technical grade, 75%). Toluenedisulfonate (TdS) was synthesized by reacting pTS with concentrated H2SO4 for 3.5 h at 160 °C.29 Bidistilled water (40 mL) was then added to the reaction solution and the pH adjusted to 7-9, using saturated NaOH. Na2SO4 precipitated out after adding 120 mL of 2-propanol. The remaining solution was filtered and the filtrate evaporated to dryness. The white residual was then recrystallized from methanol/diethyl ether and analyzed by HPLC/UV, HPLC/MS, and 1H NMR.5 The HPLC solvents were HPLC grade and purchased from Scharlau (Barcelona, Spain). NH4Ac, dry Na2SO4, Na2SO3, and 32% HCl were p.a. grade (Merck). Reagent-grade tetrabutylammonium hydrogen sulfate, formaldehyde solution (37%), high-purity methylene chloride, and ascorbic acid (p.a. grade) were obtained from Fluka AG. Landfill Sites. Two Swiss landfill sites were investigated. Landfill A is situated in the canton of Aargau. It is a disused hazardous waste landfill, containing 36% industrial waste, 29% construction refuse, 10% municipal solid waste incinerator bottom ash, and 25% oil-contaminated soil. The landfill is covered but has no bottom seal. The leachate is collected and pretreated in an on-site wastewater treatment plant. Samples were taken from the drainage and a groundwater observation well ∼100 m downstream of the disused landfill. Landfill B is situated in the canton of Zurich and contains 40% industrial waste and 50% construction waste. It is sealed with a bituminous bottom, and its leachate is collected and introduced into the municipal sewage treatment plant. Samples were taken from the drainage. Sample Preparation. Environmental samples were prepared according to the procedure developed by Altenbach and Giger30 and modified by Riediker.5 Field samples were stabilized with 1% formaldehyde solution, filtered through 0.45-µm pore size cellulose nitrate (Satorius GmbH, Go¨ttingen, Germany), then enriched on 1 g of ENVI-Carb (Supelco, Buchs, Switzerland), and preconditioned with 5 mL of eluent (50 mM NH4Ac in methanol/methylene chloride 1:4), 3 mL of methanol, 75 mL of ascorbic acid solution (57 mM in 0.1 M aqueous HCl); 250 mL of groundwater or 100 mL of leachate was used. The solid phase was then washed with methanol, and the analytes were washed off with 10 mL of eluent. The extracts were dried at 60-75 °C under a gentle stream of nitrogen. They were then redissolved in 1 mL of aqueous HPLC phase and injected onto the HPLC column. Direct Infusion Experiments. Direct infusion experiments were performed, using a Harvard syringe pump model 22 (Harvard Instruments, Gams, Switzerland), together with Hamilton gastight syringes (Hamilton, Reno, NV), at a flow rate of 20 µL/min. (29) Taylor, P. W.; Nickless, G. J. Chromatogr. 1979, 178, 259-269. (30) Altenbach, B.; Giger, W. Anal. Chem. 1995, 67, 2325-2333.
HPLC/UV/MS. The HPLC used for LC/MS was a HewlettPackard Series 1100 (Hewlett-Packard Schweiz AG, Urdorf, Switzerland) with a variable-wavelength UV detector, set to 220 nm. The HPLC column (250 × 2 mm) and precolumn (5 × 2 mm) used was a Hypersil ODS, with 5-µm particles (Knauer GmbH, Berlin, Germany). Eluent A was 10 mM aqueous NH4Ac, pH 6.5, and eluent B was methanol. The eluents were degassed using an on-line degasser DG4 from Henggeler Analytic Instruments (Riehen, Switzerland). The column was conditioned with 5 mL of eluent of initial composition. The gradient was run from 5 to 50% B in 15 min, then to 100% B in 3 min, and finally back to initial conditions in 3 min, giving a total run time of 21 min. The flow rate through the column was 250 µL/min, resulting in a back pressure of 110-130 bar at initial conditions. The column was kept at 40 °C. Sample volumes of 20 µL were injected. Mass Spectrometry. All mass spectra were acquired on a Platform LC single-quadrupole mass spectrometer, using electrospray ionization (Micromass UK Ltd., Manchester, UK). Full-scan spectra were acquired in negative ion mode, scanning from m/z 50 to 600 at 1 s/scan. The mass range was calibrated and the sensitivity of the instrument tested using 1 mM CsI or 2.5 mM NaNO3 infused at a flow rate of 20 µL/min. The electrospray interface temperature was set to 150 °C and the nitrogen gas flow to 500 L/h. The needle and cone voltages were optimized to give maximum signal intensity for the target analyte signal. Throughout all experiments, the needle voltage ranged from -3 to -5 kV and the cone voltage (CV) from 0 to 120 V. The so-called “pepper pot” was used as counter electrode for all experiments. Low- and high-mass resolution of the quadrupole was set to 14 and the ion energy to 1, giving roughly unit mass resolution over the whole mass range. The multiplier was operated at 650 V. Quantitation. The relative signal intensities at a given setting were calculated as the fraction of the sum of nine significant ions for N-1,6-dS (m/z 309, 287, 245, 222, 207, 206, 143, 142, 80) and three significant ions for N-2-S (m/z 207, 143, 80). The spectra were averaged over 10 scans and then an average of 10 background spectra subtracted. Calibration curves and limits of detection were determined by diluting a stock solution, containing the seven AS at concentrations of ∼5 mM. Absolute amounts down to 0.01 nmol (injections of 20 µL) were then determined for all sulfonates. The limit of detection was defined as the amount that gives a signal-to-noise ratio of 3. Nuclear Magnetic Resonance. NMR data were acquired on a Bruker ASX 400 NMR spectrometer (Bruker AG, Fa¨llanden, Switzerland). The sample was dissolved in methanol-d4. The signals measured were 2.70 (3H, CH3), 7.32 (1H, aromatic, J ) 7.9 Hz), 7.77 (1H, aromatic, J ) 2 Hz, 7.9 Hz), and 8.47 ppm (1H, aromatic, J ) 8.47 Hz). RESULTS AND DISCUSSION Optimization of the Electrospray Parameters. Conventional HPLC methods for the separation of AS on a C18 column use ion-pairing chromatography with tetrabutylammonium hydrogen sulfate (TBA‚HSO4) as ion-pairing reagent.30 This leads to the formation of strong [AS + TBA] ion pairs and, consequently, loss of analyte signal in the MS detector, if the clusters are neutral, as Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
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Figure 2. Direct infusion electrospray mass spectra of N-1,6-dS in 5 mM aqueous TBA, showing an abundant cluster ion at m/z 528 (a) and in 10 mM NH4Ac (b). X corresponds to an unidentified matrix ion.
for instance in the case of [N-1-S + TBA]0.31 Naphthalenedisulfonates such as N-1,6-dS, however, give a very strong signal for m/z 528, which corresponds to the negatively charged [M + TBA]- cluster (see Figure 2a). This implies that the ESI interface parameters optimize differently for aromatic mono- and disulfonates. Monosulfonates are detected when the cluster breaks apart, which is achieved at high cone voltages, while aromatic disulfonates show an intense [AS + TBA]- cluster ion at low cone voltages, but only weak signals for [M + H]- or [M + Na]-. Due to the involatility of the ion pair reagent, heavy contamination of the electrospray interface leads, however, to loss of signal intensity over time and, for this reason, poor reproducibility. We have developed a HPLC method that uses 10 mM NH4Ac (pH 6.5) as buffer in the aqueous phase. Direct infusion experiments with the disodium salt of N-1,6-dS dissolved in 80% aqueous 10 mM NH4Ac/ 20% methanol (v/v) showed no significant amounts of NH4+ adducts but strong signals for the partially dissociated [M + Na]at m/z 309, the fully desodiated M2- at m/z 143, and the corresponding protonated ion [M + H]- at m/z 287 (see Figure 2b). All aromatic monosulfonates tested were easily separated using this aqueous phase. However, the doubly charged naphthalenedisulfonates elute very close to dead time (e.g., 3.06 min for N-1,6-dS). This means that isomers of naphthalenedisulfonates are not well separated. The use of stationary phases that are more hydrophilic, such as aminopropyl/C18 mixed phases, could help to solve this problem in the future. (31) Suter, M. J.-F.; Riediker, S.; Giger, W. Proceedings of the 44th ASMS Conference on Mass Spectrometry and Allied Topics, 1996, p 160.
900 Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
Figure 3. Relative signal intensity in dependence of the sampling cone voltage for major ions of N-2-S (a) and N-1,6-dS (b), acquired under LC/MS conditions (n ) 3). The signal intensities were calculated as percentage of the sum of all major ions: (N-1,6-dS) 100% ) ∑ m/z 80, 142, 143, 206, 207, 222, 245, 287, 309; (N-2-S) 100% ) ∑ m/z 80, 143, 207. The corresponding structures are shown in Figure 1. The fragment of mass 245 is formed through SO2 loss from m/z 309 and 222 possibly from m/z 287 by rearrangement and loss of a HSO2 radical.
Because the whole method is aimed at obtaining a marker for AS, the formation of the SO3 radical (m/z 80) was investigated under various conditions. The effect of the needle voltage on the
Figure 4. Reconstructed ion chromatograms of a standard mixture, containing seven AS at 0.1-0.2 mM concentration. The bottom trace shows the UV absorption at 220 nm, acquired before the LC effluent enters the mass spectrometer. The RIC for m/z 80 corresponds to the SO3 radical that readily forms at a CV of 80 V. The mass spectra on the right-hand side all show loss of SO2.
relative signal intensities was negligible. For instance, the relative signal intensity of m/z 80, generated from N-1,6-dS under LC/ MS conditions, averaged over experiments at needle voltages of -3, -4, and -5 kV, was 17.8 ( 1.9% (n ) 9; three experiments per needle voltage). A needle voltage of -4 kV was chosen for all screening experiments. The crucial parameter for the induction of fragmentation in the electrospray source is the cone voltage. A high cone voltage causes increased fragmentation of charged molecules, because of the higher energy involved in collisions with neutral gas molecules. Figure 3 shows the change of the relative signal intensities for the SO3 radical and major ions of N-2-S and N-1,6dS as a function of the cone voltage. No SO3 radical formation was observed at cone voltages below 20 V. With higher values, the intensity of the SO3 radical increased up to a setting of 80100 V, where a flat maximum is reached. For this reason, a cone voltage of 80 V was used in all screening experiments because they depend on the SO3 radical formation and the stimulation of SO2 loss. Increasing the cone voltage also induced the formation of the deprotonated 2-hydroxynaphthalene [M - SO2]- at m/z 143 from
N-2-S, through SO2 loss by rearrangement (Figure 3a; Figure 1, pathway A). Correspondingly, the deprotonated molecule M- (m/z 207) decreased as well. The slight increase of M- at CV 120 could be due to fragmentation competing with increasing formation of M- from dissociating [M + Na]0 clusters. The fragmentation behavior for N-1,6-dS with varying collision energy is more complex, since various fragmentation pathways are possible (see Figure 1). The ions [M + H]- at m/z 287, and [M + Na]- at m/z 309, show a behavior similar to M- of N-2-S. Their intensity also decreases with increasing collision energy. As expected, the fragment ions m/z 207 and 80 show a corresponding steady increase that levels off with CV > 80 V. The ion observed at m/z 143 for N-1,6-dS is composed of the fully desodiated molecule M2and the fragment [MH - S2O5]-, corresponding to a loss of SO3, followed by loss of SO2. Consequently, the diminishing signal intensity at low CV corresponds to a decreasing M2- population (through increasing fragmentation), while the following increase at CV > 40 V represents the appearance of the fragment [MH S2O5]-. Validation of the LC/MS Method. A standard mixture of N-1,6-dS, N-1-S, N-2-S, 4-NH2-N-1-S, BS, pTS, and 4-Cl-BS was used Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
901
Table 1. Precision of the LC/MS Determination of AS compounda ion
MSb retention time (min)
SDb,c (min)
N-1,6-dS SO3•M2-, [MH - S2O5][MH - SO3]MH4-NH2-N-1-S SO3•MBS SO3•MPTS SO3•M4-Cl-BS SO3•MN-1-S SO3•[M - SO2]MN-2-S SO3•[M - SO2]M-
3.06
0.01
4.55
0.01
5.25
0.02
10.15
0.02
12.10
0.02
14.72
0.02
15.80
0.02
signald
mean areab,e
UV m/z 80 m/z 143 m/z 207 m/z 287 UV m/z 80 m/z 222 UV m/z 80 m/z 157 UV m/z 80 m/z 171 UV m/z 80 m/z 191 UV m/z 80 m/z 143 m/z 207 UV m/z 80 m/z 143 m/z 207
naf 2771.8 11440.4 5960.4 8740.8 naf 6599.8 19873.2 22415.6 7983.4 20136.4 44537.2 9655.4 27169.8 45782.0 8020.2 18995.0 naf 10625.8 16806.4 28466.8 naf 10877.8 21813.4 35528.2
SDb,c
SDb,c (%)
206.7 981.0 345.4 467.9
7.5 8.6 5.8 5.4
435.6 1047.2 133.4 483.2 1576.5 239.8 478.1 1412.6 195.6 482.4 553.0
6.6 5.3 0.6 6.1 7.8 0.5 5.0 5.2 0.4 6.0 2.9
603.4 606.1 1192.9
5.7 3.6 4.2
649.9 690.6 1129.8
6.0 3.2 3.2
a M stands for the fully deprotonated or desodiated molecule, e.g., M2- for N-1,6-dS and M- for N-1-S. b n ) 5. c SD, standard deviation in min, mAU‚s, arbitrary units, or % of mean area. d UV detection at 220 nm (UV) and RIC traces for selected ions. e mAU‚s for UV data; arbitrary units for MS data. f na, not available; UV signal was saturated, due to better UV response for naphthalenesulfonates when compared to benzenesulfonates.
to test the performance of the LC/MS. Figure 4 shows, from bottom to top, the UV absorption at 220 nm, the reconstructed ion chromatogram (RIC) for m/z 80, followed by the RICs of the deprotonated molecules of the seven AS, in increasing time of elution. The corresponding mass spectra are drawn on the righthand side of Figure 4. As expected from the optimization of the cone voltage, the spectra show that all AS form the negatively charged SO3 radical (m/z 80) at a CV setting of 80 V. Furthermore, all AS undergo loss of neutral SO2 under these conditions, as depicted by the arrows in Figure 4. The precision of the LC/MS determination was investigated using the same standard mixture. Ion chromatograms were extracted from full-scan spectra for the SO3 radical (m/z 80) and other significant ions (see Table 1). Retention times and mean peak areas are an average of five independent measurements (n ) 5). The standard deviation is expressed in minutes (MS trace) and percent of the mean area (UV and RIC trace), respectively. The deviation in time was below 0.02 min for all analytes. The deviation in mean area determined from UV traces of the benzenesulfonates ranged from 0.4 to 0.6%. UV data for naphthalenesulfonates could not be interpreted, because their strong absorption led to saturated signals. Generally, naphthalenesulfonates give a better UV response at 220 nm, when compared to benzenesulfonates. This can easily be seen in the UV trace at the bottom of Figure 4. The standard deviation for mean areas obtained from RICs was higher and ranged from 2.9 to 8.6%. This is a problem commonly encountered with electrospray, where the stability of the spray directly affects signal noise. Calibration curves were generated for all standards, acquired with a CV of 80 V. For example, both pTS and N-2-S show good linearity over 3 orders of magnitude, for fragment ions and UV 902 Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
data (r > 0.994), while the deprotonated molecules (m/z 171 and 207) scatter slightly more (r > 0.984).32 This agrees with the standard deviations of the relative signal intensity for N-2-S, shown as error bars in Figure 3a (1.00% for m/z 80, 1.19% for m/z 143, and 1.51% for m/z 207). The limits of detection (absolute) for 20µL injections of N-1-S, N-2-S, and pTS onto the HPLC column were determined to be 1.6, 0.7, and 1.8 ng, respectively. One potential source of interference in determining organic sulfonates with the described method are sulfate salts. They are usually present in landfill leachates at concentrations of up to 2.5 g/L.33 Mass spectra of sulfate, as well as sulfite salts, contain a distinct signal at m/z 80. Figure 5 shows the RIC for m/z 80, after injection of 20 µL of 5.7 mM Na2SO4 at pH 6.5. The salt elutes at a dead time of 2.22 min. The inset in Figure 5 represents the corresponding spectrum with typical clusters of the form [nH2SO4 + HSO4]-, generated at a CV of 80 V (X denominates an unidentified matrix ion). Even though the cluster formation is expected to be concentration dependent, the absence of SO2 loss (m/z 64) from any significant ions, the other major indication for organic sulfonates, allows us to distinguish between sulfate salts and AS. Furthermore, because Na2SO4 elutes with the dead time, no interference is expected with most aromatic sulfonates, except with aromatic disulfonates that also elute very near to the front. Sulfite salts coelute with sulfates and can also be identified by their retention time, the presence of typical cluster ions, such as [nH2SO3 + HSO3]-, and the absence of SO2 loss from any (32) Correlation coefficients for pTS: m/z 80 r ) 0.995, m/z 171 r ) 0.983; UV r ) 0.998. N-2-S: m/z 80 r ) 0.996, m/z 143 r ) 0.994, m/z 207 r ) 0.984, UV r ) 0.997 (n ) 1). (33) Ehrig, H.-J.; Endell, R.; Hilger, K.; Klockner, D.; Ru ¨ ffer, H.; Schulz, D.; Weisbrodt, W.; Weitzel, H. Mu ¨ ll Abfall 1988, 2, 67-71.
Figure 5. LC/MS reconstructed chromatogram for m/z 80, obtained from a 20-µL injection of 5.7 mM Na2SO4, with a CV of 80 V. The signal at dead time (2.22 min) corresponds to Na2SO4, determined as SO4- clusters, as can be seen in the inset (X is an unknown matrix ion).
significant ions (data not shown). Determination of Aromatic Sulfonates in Landfill Leachates and Groundwater. We have been making use of the SO3 radical formation in analyzing leachates from a hazardous landfill in Switzerland (landfill A) and groundwater samples downstream of the landfill. Figure 6 shows the total ion chromatogram (TIC) for a drain sample (a), the corresponding RIC for m/z 80 (b), and the RIC for m/z 80 for a groundwater taken from an observation well that is situated ∼100 m downstream of the landfill (c). Most of the signals in the leachate could be identified, based on their retention time and mass spectra. The compounds identified were sulfate (at 2.24 min), BS (5.50 min), pTS (10.48 min), 4-Cl-BS (12.79 min), N-1-S (13.78 min), and N-2-S (16.75 min), while no disulfonates were identified. Clearly the SO3•- trace helps enormously to identify AS in a mixture, whose complexity can be estimated, when looking at the total ion chromatogram (a). Two unknown compounds could be tentatively identified, based on their mass spectra. The signal at 8.67 min shows an M- of m/z 206 with an isotope pattern indicating the presence of one chlorine atom and formation of the SO3 radical. A possible candidate fulfilling these prerequisites is benzenesulfonate with a chlorine atom and an amino group as substituents. The second unknown component elutes at 14.74 min, shows an M- of m/z 185, SO2 loss, and formation of SO3•-. Two additional signals at 15.05 and 15.42 min give identical spectra. Hence, we believe this to be the four isomers of xylenesulfonate, with two isomers possibly coeluting. The unknown components with a retention time of 3.29 and 11.58 min could not be identified so far. Both show an M- of m/z 187 and the formation of SO3•-. The lack of other fragment ions prevents a tentative structural assignment. However, naphthalenedisulfonates can be excluded, because then strong signals for [MH - SO3]- and [MH - S2O5]- would have to be visible in the mass spectrum. When looking at the groundwater sample, taken downstream of the landfill (Figure 6c), nine components, clearly visible in the leachate, are also detected at significant concentrations. Some
Figure 6. Total ion chromatogram and RICs for the negatively charged SO3 radical, obtained from samples taken from a hazardous waste landfill in Switzerland. The top trace shows the TIC of leachate taken from a drain (a). The corresponding RIC for the SO3 radical is shown in the middle (b), while the SO3 radical trace at the bottom was obtained from a sample taken from a groundwater observation well that is situated ∼100 m downstream of the landfill (c).
additional compounds elute, e.g., at 3.08, 9.08, and 19.41 min, none of which could be unambiguously identified. Because the concentrations in the groundwater sample depend on rain incidents and are affected by the complex hydrogeology of the site, no conclusions can be drawn on fate and behavior of the AS in this case. Nevertheless, the results show that the method performs very well for environmental samples. Identification of an Unknown AS. The investigation of leachate from another landfill, containing degradable industrial and construction waste (landfill B), revealed high concentrations of a compound not encountered previously, which elutes at 3.06 min. Figure 7 shows the corresponding mass spectrum, acquired with a cone voltage of 80 V. The base peak at m/z 251 represents MH-. The signal at m/z 125 with isotope peaks separated by 0.5 m/z units is a clear indication for the doubly charged M2-. Loss of SO3 from MH- (m/z 251 f 171) was so far only observed from disulfonates. For instance, N-1,6-dS loses SO3 from MH- (m/z 287) to form m/z 207 (see Figure 4). The remaining ions in the spectrum of the unknown AS show the same intensity distribution as pTS seen in Figure 4 (m/z 171, third spectrum from top). On the basis of these findings, we conclude that the unknown substance found at high concentrations is a toluenedisulfonate. Because TdS is not commercially available, it was synthesized in Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
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Figure 7. Mass spectrum of an unknown compound, eluting at 3.06 min, found in leachates from landfill B, which contains degradable industrial and construction waste.
order to verify this hypothesis. The retention times and mass spectra resulting from injections of this product and the unknown compound were identical. The position of the two sulfonate substituents, however, had to be determined by means other than mass spectrometry, because MS does not usually distinguish between different positional isomers. Using 1H NMR, we could show that the sulfonate groups were in positions 2 and 4. The mass-specific information, available from the LC/MS method presented in this article, proved very helpful in obtaining information on unknown components found, for instance, in leachates from landfills. CONCLUSION The use of the SO3 radical, formed under negative ESI conditions, as a marker ion for aromatic sulfonates was demon-
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strated, using a single-quadrupole MS. Together with the loss of SO2, it is an excellent indication for organic sulfonates. Calibration curves, generated from LC/MS data, showed good linearity over 3 orders of magnitude. The relative standard deviation for mean areas obtained from RICs ranged from 2.9 to 8.6% and was higher than results obtained from UV traces. This is a problem often encountered with ESI. One drawback of the method is the somewhat poor separation of aromatic disulfonates, when C18 columns are used. The use of a different packing material is suggested, to help overcome this problem in the future. Interference from sulfate and sulfite salts could be excluded because these salts form easily recognizable cluster ions, do not undergo loss of SO2, and elute with the dead time of the LC method. Detection limits were in the order of 1 ng and thus comparable to UV sensitivity. Contrary to UV detection, there was a similar response for naphthalene- and benzenesulfonates. Furthermore, the method presented allowed the identification of unknown aromatic sulfonates, found in landfill leachates and groundwater. This method has proven to be very well suited for the quantitative and qualitative analysis of aromatic sulfonates in heavily contaminated landfill leachates and groundwaters. ACKNOWLEDGMENT The authors thank R. Hany (EMPA, Du¨bendorf, Switzerland) for his help in obtaining the NMR information on TdS. Mr. Balmer is acknowledged for his help in collecting samples from landfill B, and Mr. Klumpp (Institut Bachema AG, Schlieren, Switzerland) for generously supplying samples from the hazardous waste landfill A. A. Alder and C. McArdell are greatly acknowledged for their critical review of the manuscript. Received for review August 13, 1998. Accepted December 3, 1998. AC980911F
