A d . Chem. 1001, 63,1179-1182
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TECHNICAL NOTES Liquld Chromatographic Determination of Linear Aikylbenzenesutfonates in Aqueous Environmental Samples Antonio Di Corcia,* Marcello Marchetti, and Roberto Samperi Dipartimento di Chimica, Uniuersitd "La Sapienza" di Roma, Piazza Aldo Moro, 5 00185 Roma, Italy Antonio Marcomini Dipartimento di Scienze Ambientali, Uniuersitd di Venezia, Calle Larga S. Marta 2137,Z-30123 Venezia, Italy
Linear allrylbenzenesulfonates &AS) are anionic surfactants of major use in detergent formulations. Commercial LAS materials are mixtures of various alkyl homologues that may vary from Cgto CI4(1)and phenyl positional isomers. Because of their widespread use, LAS have been found in many different environmental compartments (2, 3). The total amount of LAS in water is usually measured by the standard methylene blue method (4). This method is sensitive, simple, and inexpensive but nonspecific, as it suffers from many interferences including naturally occurring compounds, such as humic substances. Starting in 1975 (5, 61,high-performance liquid chromatography (HPLC) has been increasingly employed for the measurement of individual, intact LAS. HPLC coupled with fluorometric detection enabled Nakae et al. (7)to determine LAS in river waters without any preconcentration and prepurification. This method, however, can be applied only to relatively clean aqueous samples having rather high contents of LAS. In order to extend the chemical analysis of LAS in aqueous environmental matrices in the low parts per billion (ppb) region, preconcentration techniques by liquid-solid extraction with various adsorbing materials, such as XAD-8 (81, an anion exchanger (91, and chemically bonded silica (10-121, have been developed. In order to improve the selectivity of the method for determining trace levels of LAS in complex aqueous environmental matrices, two analytical methods involving the use of a nonspecific sorbent for concentrating LAS followed by purification with an anion exchanger have been proposed (13,141. Although selective, these methods are time-consuming, cumbersome, and prone to errors, as they require excessive manipulation of the sample. In addition, the method developed by Castles et al. (14) is rather expensive, as it employs as many as five sorbent cartridges for determining LAS in river water samples. Cartridges containing Carbopack B, which is an example of graphitized carbon black, have been successfully used for extracting various analytes from aqueous environmental samples (15-18). Although this sorbent behaves as a natural reversed phase, the presence on its surface of some particular, positively charged chemical groups (19) enables it to act as both a nonspecific and an anion-exchange adsorbent. This singular feature was exploited for separating by stepwise elution estrogens from their acidic metabolites (20) and base-neutral pesticides from acidic ones co-extracted from water samples (21). The purpose of this work has been that of developing a selective, rapid, simple, and unexpensive method for the HPLC analysis of LAS in actual water samples from different sources. LAS were extracted by a Carbopack cartridge from *Towhom correspondence should be addressed. 0003-2700/91/0363-1179$02.50/0
water samples without any sample pretreatment. Before desorbing U S ,a suitable washing step succeeded in removing from the sorbent surface basic, neutral, and weakly acidic compounds, which may interfere with the subsequent quantification of LAS.
EXPERIMENTAL SECTION Reagents. A commercial LAS product (Marlon A) was supplied by Chemische Werke Hiils AG, Marl, FRG. Marlon A is a mixture containing only Clo-C1sLAS homologues and four to six so-called phenyl isomers per homologue. The exact percentage weight of each LAS homologue in the commercial product was determined by injecting known amounts of Marlon A from a calibrated methanolic solution, measuring peak areas produced by any LAS homologue, and assigning the same fluorescence quantum efficiency to any LAS homologue. As measured by us, the percentage weights were 12.8%, 41.8%, 36.1%, and 9.3%, respectively, for Clo-LAS,Cll-LAS, C12-LAS,and CIS-LAS. 1Nonylbenzenesulfonate (l-Cg-LAS)was synthesized by direct sulfonation at 70 "C of n-nonylbenzene (Fluka AG, Buchs, Switzerland). The reaction product was recrystallized in water/ethanol. 3-Tetradecylbenzenesulfonate(3-C14-LAS)was supplied by Unilever, Port Sunlight, U.K., and used without further purification. However, preliminary experiments showed that, according to previous works (1,3,12),Cl4-LAS was virtually absent in the aqueous matrices considered, likely due to the fact that it is contained in low amounts in detergent formulations and that this highly hydrophobic compound is readily adsorbed on sediments (22). For this reason, CI4-LASwas not further considered in our work. Two stock standard solutions of LAS were prepared by dissolving 200 mg of Marlon A and 200 mg of Cg-LAS in 100 mL of methanol. A working standard solution was prepared by mixing 27 mL of the former solution with 3 mL of the latter one. For HPLC, distilled water was further purified by passing it through a Norganic cartridge (MiUipore,Bedford, MA). Methanol of HPLC grade was from Carlo Erba, Milan, Italy. Sodium perchlorate and tetramethylammonium hydroxide (TMAOH) were from Fluka. All other chemicals were of analytical reagent grade (Carlo Erba). Apparatus. A 6-cm X 1.4-cm-i.d. polypropylenetube was filled with 500 mg of Carbopack B (12M00-mesh size). Polyethylene frits were used. All the materials cited above were supplied by Supelco, Bellefonte, PA. The extraction cartridge was fit directly into a side-arm fiitering flask where vacuum was done by a water PUP.
Procedure. Collected river waters, sewage treatment plant effluents, and primary influent samples were immediately preserved with 1% formalin (37% formaldehyde) to prevent further microbial degradation of LAS present in the sample. For recovery studies, extraction of spiked samples was conducted about 3 h after the addition of LAS. Before extraction, each sample was vigorously shaken to ensure adequate mixing and suspension of solid particles. When aliquots of the sample were withdrawn without any mixing, lower recoveries of the more hydrophobic LAS, that is, C12 and CIS-LAS,were observed. 0 1991 Amerlcan Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 11, JUNE 1, 1991 Cll
Table I. Distribution of LA9 between the Washing and Eluant Phases on Extracting Increaoing Volumee of a Tap Water Sample. Spike Level: 5 mg/L
Jj
70recove*
c12
Cia
-
0
4
8
1
2
time (min)
Flguro 1. Chromatogram of a commercial Clo-C,S-LASproduct. Column: 25 X 0.46 cm, 5-pm C, DB; eluent, 0.1 mol/L NaCQ in methand/water (8:2, by volume); flow rate, 1.5 mL/mln; amount of total LAS Injected, 0.4 pg.
The Carbopack cartridge was sequentially washed with 5 mL of the eluant system for LAS (see below), 2 mL of methanol, 30 mL of aqueous HCl (0.1 mol/L), and 2 mL of water. Samples of influents (10 mL), effluents (50 mL), and river waters (200mL) were passed through the cartridge at 30 mL/min. After the sample was passed through the cartridge, the cartridge was washed with 5 mL of distilled water, 1mL of methanol, and 10 mL of methylene chloride/methanol (80:20, by volume) acidified with formic acid (15 mmol/L). LAS were eluted from the cartridge by passing drop by drop 5 mL of methylene chloride/methanol (80:20, by volume) basified with TMAOH (5 mmol/L). The extract was then dried at 50 OC under a nitrogen stream and the residue reconstituted with 0.5 mL of methanol/ water (80:20, by volume). Fifty microliters of this solution was then injected into the HPLC apparatus. HPLC Apparatus. A Model 5000 liquid chromatograph (Varian, Walnut Creek, CA) equipped with a Rheodyne Model 7125 injector having a 50-pL loop and with a Model 650-10s fluorometric detector (Perkin-Elmer, Corp., Norwalk, CT) was used. Peak areas and retention times were obtained with a column LCI-100 integrator (Perkin-Elmer). A 25-cm X 4.6." filled with bpm particle size, CgDB (deactivated base) reversed phase (Supelco) was used. In contrast to the C18column, which is commonly used to separate homologous components of LAS by HPLC (1,7,10,12,23,24),the C8 column co-elutes all of the isomers of each L4S homologue into a single peak. This apparent weakness is, in fact, an advantage, as it simplifies interpretation and quantification of the chromatogramsand enhances detection limits by increasing the peak height of each LAS homologue. Figure 1shows a chromatogram obtained by injecting a methanolic solution of Marlon A. The mobile phase was a premixed water/methanol(2080, by volume) solution containing sodium perchlorate (0.1 mol/L). One liter of mobile phase was recycled four times before substituting it with one freshly prepared. The column effluent was monitored with an excitation wavelength of 225 nm and an emission wavelength of 290 nm, with a spectral band-pass of 10 nm. The flow rate was 1.5 mL/min. Quantitation. Individual CQ-Cla-LASpeaks in both unspiked and spiked samples were quantified through the use of external standards. Calibration plots were constructed from measurements of peak areaa versus known amounts of the alkyl homologuea. For this purpose, appropriate and known volumes of the working standard solution containing CQ-C,,-LAS were taken, and after
vol, mL
washing phase
eluant phase
50 100
4
200
21
97 94 80
Mean values were calculated from three determinations. methanol removal, the residue was reconstituted with 0.5 mL of the same solution mentioned above. The response of the fluorometric detector is linearly related to injected amounts of each LAS homologue from 0 to 2 pg. Safety Considerations. Some safety precautions were followed in this study. The preparation of the eluant mixtures for washing the Carbopack cartridge and for eluting LAS from it waa prepared in a hood, as it contained a chlorinated solvent. Also, safety goggles were used when handling TMAOH.
RESULTS AND DISCUSSION Recently, it was shown that Carbopack B offers an advantage over conventional anion exchangers in that inorganic anions cannot compete with organic ones for adsorption on the exchange sites (21). On the other hand, if compared to resin-based anion exchangers, the exchange sites on the Carbopack surface are few in number (20). Therefore, on extracting highly contaminated aqueous samples containing large amounts of LAS, it may occur that the ion-exchange sites of the Carbopack material are saturated and LAS are in part adsorbed on the predominant, unspecific adsorption sites of the sorbent. As a consequence, this fraction of LAS is lost, as it is co-eluted with nonacidic or weakly acidic compounds on washing the Carbopack cartridge. The extent of this effect was assessed by extracting increasing volumes of a tap water sample spiked with 5 mg/L of LAS. Data reported in Table I show that a significant saturation effect of the anion-exchange sites of the sorbent took place when extracting 200 mL of a water sample with a high content of LAS. In order to avoid such an unwelcome effect, the appropriate water volume submitted to the extraction procedure by the Carbopack cartridge was varied, depending upon the nature of the aqueous sample. That is, volumes of 10,50, and 200 mL were considered for water samples taken from raw sewage, treated sewage, and river water, respectively. It may occur that a source of water containing a low level of LAS is occasionally contaminated by an anomalously high m o u n t of other organic acids coming from an industrial spill. As a consequence, these co-extracted organic acids could saturate the anion-exchange sites of the Carbopack material artificially, reducing the concentration determined for LAS. In order to evaluate this effect, a river water sample was divided in aliquots; one was analyzed as such and the other two aliquota were analyzed after artificially contaminating them respectively with 10 and 20 mg/L of a commonly used acidic herbicide, such as 2,4-dichlorophenoxyacetic acid (2,4-D). Measurements were made in triplicate. The resulta obtained showed that the determination of any individual LAS was not affected by the presence in water of 10 mg/L of 2,4-D, whereas a 28% and 16% loss respectively of C,-LAS and C,,-LAS was evident when analyzing the river water sample containing 20 mg/L of 2,4-D. In terms of total LAS, however, this loss was negligible, as the presence of 20 mg/L of 2,4-D in the water sample did not interfere with measurement of the most abundant LAS, that is Cll-LAS and C,,-LAS. The accuracy of this method is reported in Table 11. LAS were added a t the indicated concentrations to the individual samples of each matrix considered and assayed. When an
ANALYTICAL CHEMISTRY, VOL. 63, NO. 11, JUNE 1, l 9 Q l
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Table 11. Accuracy of the Method for the Determination of LAS in Aqueous Environmental Matrices
sample
no. of samples
influent sewage effluent sewage river water
4 4 7
concn range, mg/L
LAS added, mg/L
5.0-10 2.2-1.0 0.0414.67 0.10-2.0 0.0114.28 0.020-1.0
av I recovery
SW
91 f 6.1 96 f 4.2 93 f 3.8
Arithmetic mean i 1 standard deviation.
aqueous environmental sample was spiked with relatively high amounts of LAS, agitated for some minutes and thereafter extracted, lower and irreproducible recovery of the analytes was observed. This loss was more evident for the most hydrophobic LAS, that is, C12-LASand CI3-LAS. The reason for this was not very clear to us. One could speculate that, following the addition of LAS to the aqueous sample, the most hydrophobic ones initially have the tendency to aggregate in micellar forms able to pass partially unretained through the sorbent cartridge and that dissolution of these micelles is a slow process in which the suspended particulate material might play an important role. As reported in the Experimental Section, 3 h of equilibration was sufficient to obtain high and reproducible recovery of added LAS. The precision of the method was measured by repeated analyes (n = 6) of each type of aqueous environmental sample considered. The relative standard deviation obtained were 4.7%, 3.6%, and 3.0%, respectively, for samples of an influent sewage, effluent sewage, and river water. The detection level (signal-to noise ratio = 3) was approximately 0.004 r g for each LAS homologue. Therefore, the detection limit of this method was 0.8 ppb of the total LAS for river water specimens, under the experimental conditions chosen. In terms of accuracy, simplicity, time, and cost of the analysis, this sample preparation method was compared with the other two recently reported extraction procedures. One involves the use of a single (&-bonded silica cartridge (11); the other makes use of C2 cartridges for extracting LAS and a strong anion-exchanger (SAX) cartridge for subsequent isolation of the analytes of interest (14). For this comparative study, water samples from different sources were considered. They were extracted in duplicate by each method under comparison and quantified by HPLC. Quantitative results are reported in Table 111, and Figure 2 shows typical chromatograms obtained by the three extracion procedures on analyzing 200 mL of a river water samples. With respect to the other two analytical procedures, the less selective extraction procedure by the C18 cartridge produced a positive
---
0
4
8
1
2
0
4
8
1
2
0
4
8
1
2
t i m e (min)
Flgure 2. Chromatograms obtained from 200 mL of Po river water (May 1990) contaminated by LAS and exlracted by 50O-mg C,, cartridga (A); four 50O-mg C, cartrklges + SAX cartridge (B); 500-mg Carbopack cartridge (C). The concentrations (pg1L) of LAS homologues determined by our procedwe were C,IAS, 1.6 pg/L C,,IAS, 4.6 PglL; Cti-LAS, 5.5 /.Q/L; Ci&AS, 5.1 NglL; C&AS,
2.1
NglL.
bias for the analysis of the Cl0-LAS due to a co-extracted and co-eluted compound, which was identified as a carboxylate metabolite (25) of the nonylphenolethoxylates (NPEOs),the latter being nonionic surfactants. In addition, a broad peak for NPEOs themselves disturbed correct quantification of CI2-LAS. Data obtained by this procedure and those making use of a trap tandem system were in good agreement. However, considering that the latter analytical procedure involves the use of four C2cartridges for extracting 200-mL aliquots of river water samples and complete removal by evaporation of 15 mL of a 1:l water/methanol mixture, the procedure developed by us appears far simpler, more rapid, and less expensive. In addition, this extraction procedure is more adaptable to field sampling, as it does not require any sample pretreatment. Figure 3 shows typical chromatograms on analyzing a sewage influent and a final sewage treatment plant (Rome) effluent. In situ sampling has the advantage over the conventional sample collection in that most contamination and handling
Table 111. Concentrations (mg/L) of Individual C&r-LAS and Total LA9 Found in Water Samples from Different Sources by the Proposed Method and the Other Two Methods Involving the Use of Different Sorbent Cartridges
raw influent (Rome) raw influent (Venice) treated effluent (Rome) treated effluent (Venice) Po river Tevere river Arno river
1.2
4.5
0.28 1.1
3.5
2.0
0.42 11.6
0.88 0.71 0.12
0.15 0.72 0.13 0.052 0.009
1.1
3.6
3.4
2.2
0.65
3.09 0.22 0.74 0.85 0.59 0.19
11.0
1.0
3.2
3.2
2.59 0.27 0.73 0.88
1.9
0.52 9.82
0.66 0.21 2.75
1.06 0.15 0.22 0.13 0.053 0.012 0.565 0.18 0.27 0.16 0.060 0.013 0.683
0.011 0.012 0.009 0.014 0.003 0.049 0.011 0.004 0.009 0.010 0.004 0.038 0.005 0.004 0.009 0.010 0.006 0.034 0.002 0.002 0.002 0.003 0.001 0.010 0.002 0.001 0.002 0.004 0.003 0.012 0.002 0.001 0.002 0.003 0.002 0.010 0.015 0.12 0.063 0.046 0.014 0.258 0.027 0.085 0.096 0.075 0.022 0.305 0.026 0.082 0.10 0.082 0.020 0.310 0.11 0.13 0.094 0.020 0.384 0.020 0.13 0.087 0.056 0.013 0.306 0.025 0.130 0.12 0.089 0.022 0.386 0.030 -
-
-
-
Anal. Chem. 1991, 63, 1182-1184
1182
1
and C were each one extracted and analyzed in triplicate by this procedure. The total amounts of LAS we measured were 125, 118, and 8 pg/L for samples A, B, and C, respectively. Hence, no significant adverse effect occurred during storage of the Carbopack cartridges. Registry No. Water, 1132-18-5.
A
c11
LITERATURE CITED (1) (2) (3) (4)
(5) (6) (7) (8) (9) (IO) (11)
(12) (13)
-u
0
4
8
1
2
0
4
8 1 2 time (min)
Flguro 3. Chromatograms obtained from (A) 10 mL of a sewage influent and (B) 50 mL of a fhal effluent. The total LAS concentratkns were 4.83 and 0.204 mg/L, respectively.
problems are eliminated. The small-volume cartridge could be sealed and shipped to the laboratory for elution and chromatographic analysis. In order to assess the feasibility of on-site sampling by a Carbopack cartridge, a river water sample containing LAS was divided in three aliquots. One was extraded in triplicate with Carbopack cartridges that were air-dried for 5 min before storage (sampleA). A second aliquot was added to 1?% formalin to prevent microbial degradation of LAS (sample B), and a third aliquot was left unaltered (sample C). Both cartridges and water samples were stored for 30 days a t room temperature. Thereafter, the three Carbopack cartridges were submitted to the remaining part of the procedure for HPLC quantification, while samples B
(14) . . (15)
(16) (17) (16)
(19) (20) (21) (22) (23) (24) (25)
Marcomini, A.; Glger, W. Anal. Chem. 1987, 59, 1709-1715. Waters, J.; Garrigan. J. T. Water Res. 19811, 17, 1549-1582. Kikuchl. M.; Tokal, A.; HoshkJa, T. Water Res. 1086. 20, 643-850. (;reenberg, A. E.; Comners, J. J.; Jenklns, D. StanUa~IMSttwrA for the Examhation of Water end Wastewater, 18th d.;Amerloan PUMlc Health Association: Washington, D.C., 1985; Section 512 A. Kunihto. K.; Nakae, A.; Muto, 0. BuwkIKagnku 1975, 24, 188-192. Takano, S.; Yagi, N.; Kunlhko, K. Vakagaku 1975, 24, 389-394. Nakae, A.; Tsuij, K.; Yamaunaka. M. Anal. Chem. 1980, 52, 2275. Tayloc, P. W.; Niclcless, 0. J . Chromatog*. 1979, 178. 259-269. Saito. T.; Higashi, K.; Hagiwara, K. Z . Anew. Chem. 1982, 313, 21-23. Kkuchi, M.; Tdtai, A.; Yoshide, T. Water Res. 1088, 20, 843-850. Marcomini, A.; Capri, S.; Giger, W. J . chromatog*. 1987, 403, 243-252. MathljS, E.; De k W U . H. Tenslde SUhC&nb, Deterg. 1987, 24 (4), IQ3-IQQ. .- - .- -. Thwman, E. M.; Wllloughby, T.; Barber, L. 6.; Thorn, K. A. Anal. Chem. 1987, 59, 1798-1802. castles. M. A.; Moore. B. L.; Ward. S. R. Anal. Chefn. 1989. 61. 2534-2540. Bacabni, A.; *elti, 0.;Lagena. A.; Petronio, B. M.; Rotatori, M. Anal. Chem. 1980, 52, 2033-2038. Borra, C.; Di Corcia, A.; Marchem, M.; Samperi, R. Anal. Chem. 1988, 58. 2048-2052. Battlsta, M.; Di Corcia, A.; Marchetti, M. Anal. Chem. 1989. 67, 935-939. DI Corcia, A.; Marchetti, M.; Samperi, R. Anal. Chem. 1989, 61, 1383-1367. Campanella. L.; Di Corcia, A.; Samperi, R.; Gambacwta, A. Meter. Chem. 1082, 7. 429-43s. Andreoiini, F.; Borra, C.; Caccamo, f.; DI Corcla, A.; Samperl. R. Anal. Chem. 1987. 59, 1720-1725. Di Corcia, A.; Marchetti, M. Anal. Chem. 1991, 63, 580-585. Hand, V. C.; Williams, G. K. fnvlron. Scl. Technol. 1987, 21. 370-373. Nakae, A.; Tsuij. K.; Yamaunaka, M. Anal. Chem. 1981, 53, 1818. Linder, D. E.; Allen, M. C. J . Am. OII Chem. Soc. 1982, 59, 152. Marcomini, A.; Busettl, S.; Sfriso, A.; Capri. S.; La Noce,T.; Uberatorl. A. Proceedings of the Sixth Internatknai Symposium on Organic MIcropoliutants in the Aquatic Environment, Lisbon 1990; p 125.
RECEIVED for review November 13, 1990. Accepted March 4,1991.
Robust, High-Efticiency, Hlgh-Capacity Cryogenic Trap Carl A. M. Brenninkmeijer DSIR Physical Sciences, Nuclear Sciences Group, P.O. Box 31 312,Lower Hutt, New Zealand Cryogenic traps are frequently used for removing condensable gases from a gas mixture (1). In essence, such traps consist of a piece of U-shaped tubing immersed in a cold bath. The function is based on the condensation of the vapor to be removed on the cold wall when the gas flow passes through the tubing. A t sufficiently low flow rates, the residual concentration of a condensable component is determined only by its vapor pressure in relation to the total pressure. However, in most applications, the effectiveness is less, and multiple loops are employed. A quantitative description of the actual trapping is rather complex (2) and may not be complete. The trap I describe here was developed for removing COz and N20at liquid nitrogen temperature from a flow of air for the isotopic analysis of atmospheric CO. The metal trap replaces about eight loops of glass tubing and has several advantages. The limited effectiveness of a single-loop trap is due to at 0003-2700/9 110383-1182$02.50/0
least two factors. First, the condensed material may be dislodged from the wall by the gas stream and become resuspended in the outflow. Also, as can be easily observed in the case of condensingwater vapor, fog formation can occur with a similar result. Second, trapping is limited by the rather slow process of diffusion to the cold wall. Adding more loops increases the overall efficiency, but at the cost of a higher consumption of coolant. Since most traps are constructed in glass (3), a rather fragile construction results. Stevens et al. ( 4 ) introduced a glass capillary type of trap, the increased efficiency of which is based on the decreased distance over which the condensableshave to diffuse. Also, a larger surface mea is exposed. Horibe et al. (5) introduced a trap with a glass frit and comment on its high efficiency for trapping water in a single pass. The present design is based on similar considerations, and the detailed construction is shown in Figure 1. 0 1991 American Chemical Society
