Anal. Chem. 1994,66, 699-705
Quantitative Determination of Linear Primary Alcohol Ethoxylate Surfactants in Environmental Samples by Thermospray LWMS K. A. Evans,.*t S. T. Dubey,t L. Kravetz,t I. Dzldlc,t J. Gumulka,t R. Mueller,t and J. R. Stork* Westhollow Research Center, Shell Development Company, P.O. Box 1380, Houston, Texas 2225 1, and Finnigan MAT Institute, 4450 Carver Woods Drive, Cincinnati, Ohio 45242
A new thermospray liquid chromatography/mass spectrometry (LC/MS) method for quantitativedeterminationof trace levels of alcohol ethoxylates (AE)in dilute aqueous environmental samples has been developed. This method has been validated for use in aquatic safety assessments to determine concentrations of total AE and individual AE species. This validation was accomplished by determination of recoveries of spiked AE samples from waste treatment plant effluents and receiving waters. The method distinguishes highly branched propylenebased AE from isomeric linear ethylene-based AE. Surfactants are used in large volumes in a broad variety of household and commercial detergents and cleaning products. After use, the surfactants are usually disposed into a wastewater treatment system and the effluents ultimately are released into surface waters. Biodegradation and other removal mechanisms greatly reduce the mass and concentration of surfactant reaching the environment. Linear primary alcohol ethoxylates (AE) are an increasingly important class of nonionic surfactants used principally in household cleaning products. They have the general formula RO(CH2CH20),H where R is an alkyl group (typically with 12, 13, 14, or 15 carbon atoms). Commonly these AE compounds consist of a mixture of varied alkyl groups (R) and with a range or distribution of ethylene oxide (EO) groups (see Table I). Linear primary AE, which can be derived from Ziegler or modified-oxo processes, and from oleochemical sources, is widely used in household products. Highly branched AE, typically derived from propene or butene oligomers, is more commonly used in nonhousehold applications. Prior research’ has shown that the linear AE can biodegrade more quickly and extensively than the highly branched AE under normal sewage treatment conditions. It is therefore of interest to differentiatethese materials in environmental analyses in order to understand the removal efficiency and impact of AEs from different sources in the environment. The method reported here allows the resolution of linear AE from highly branched AE isomers. Efforts to characterize surfactants and their degradation intermediates in environmental matrices have increased in + Shell Development Co. Finnigan MAT Institute. (1) Kravetz, L.; Salanitro, J. P.; Dorn, P.B.; Guin, K. F. J. Am. Oil Chem. Soc. 1991, 68, 610. 8
0003-2700/94/0366-0699$04.50/0 0 1994 American Chemical Society
Table 1. Analytlcal Reference Materlalr Used alkyl carbon no. distrib surfactant acronym range avg
linear Clsls AE-9 EO” linear C11 AE-9 EOb branched CISAE-7 EO‘
N25-9 N1-9 BAE13-7
12-15 11 11-15
13.5 11 13
%3 9 9 7
“NEODOL 25-9 (Shell Chemical Co., Houston, TX). The NEODOL 25-9 AE was made from a Clz.16 ethoxylated to an average of nine ethylene oxide (EO) units per mole of alcohol. The Clz.16 alcohol was derived from a C11-14 olefin by hydroformulation using a proprietary catalyst. The resulting alcoholcontains a roximately 80% linear alkyl oups and 20% 2-alkyl groups. fgEODOL 1-9 Houston, TX). Made by laboratory scale (Shell Chemical ethoxylation of Exxal 13 alcohol (Exxon Chemical Co., Baytown, TX).
&.,
recent years. Trace analyses of these compounds are necessary to evaluate and ensure the environmental safety of the surfactants as they are actually used. Several mass spectrometric methods have been recently developed to characterize trace level concentrations of surfactants in municipal waste streams. These methods include mass spectrometric determinations of anionic, cationic, and nonionic surfactants.2-’0 Many of these mass spectrometric methods are for the qualitative characterization of environmental samples and do not report quantitative results. Prior quantitative MS techniques have been reported for cationic,” anionic,12 and alkylphenolnonionic13surfactants. None of these prior reports involved quantification of trace levels of AE nonionic surfactants in environmental samples. Prior quantitative (spectrophotometric) methods for analysis of trace levels of AE in environmental matrices have been reported.’”16 These methods lack the specificityfor individual (2) Shiraishi, H.; Otsuki, A.; Fuwa, K. Bull. Chem. Soc. Jpn. 1982, 55, 1 4 1 s 1415. (3) Stephanou, E. Chemosphere 1984, 13, 43-51. (4) Rivera, J.; Fraisse, D.; Ventura, F.; Caixach, J.; Figueras, A. Fresenius Z . Anal. Chem. 1987, 328, 577-582. (5) Ventura, F.; Figueras, A,; Caixach. J.; Espadaler, I.; Romero, J.; Guardiola, J.; Rivera, J. Wuter Res. 1988, 22, 1211-1217. (6) Ventura, F.; Caixach, J.: Figueras, A,; Espadaler, I.; Fraisse, D.; Rivera, J. Wufer Res. 1989, 23, 1191-1203. (7) Rivera, J.; Caixach, J.; Espadaler, I.; Romero, J.; Ventura, F.; Guardiola, J.; Om,3. Wuter Supply 1989, 7 , 97-103. (8) Schroder, H. F. Vom Wusser 1989, 73, 11 1-136. (9) Schroder, H. F. Vom Wuyasser 1990, 108, 121-144. (10) Schrodcr, H. F. Wuter Sci. Technol. 1991, 23, 339-347. (11) Simms, J. R.; Keough, T.; Ward, S. R.; Moore, B. L.; Bandurraga, M. M. Anal. Chem. 1988.60, 2613-2620. (12) Field, J.A.;Leenheer, J.A.;Thorn,K.A.;Barber,L.B.;Rmtad,C.;Macalady, D. L.; Daniel, S. R . J. Contum. Hydrol. 1992, 9, 55-78. (13) Otsuki, A,; Shiraishi, H. Anal. Chem. 1979, 52, 2329-2332.
Analytical Chemistry, Vot. 66, No. 5, M r c h 1, 1994 099
AE compounds obtained with the method reported here. A combined liquid chromatograph y/gas chromatography (LC/ GC) characterization has also been used at levels above 50 ppm.17 The new LC/MS method reported here can quantify concentrations of linear primary AE in receiving waters and sewage treatment plant influents and effluent at levels of 25100 ppb for total AE and less than 3 ppb for individual AE compounds. Such low detection limits are required for the development of aquatic safety assessments, in which the predicted or measured concentration of a substance in the environment is compared to the lowest concentration shown to produce an observable ecological effect. The determination of ecological effect concentration is largely accomplished through the use of laboratory toxicity testing to identify acute or chronic effects on relevant species and establish the lowest concentration at which the effect is observed. The ratio of this effect concentration to the predicted or measured environmental concentration is the environmental safety factor. However, because of the uncertainty in translating test data into complex natural environments, uncertainty factors of up to IOOX are often applied to the laboratory toxicity data. Trace analyses are therefore needed in the safety assessment to measure concentrations of potentially impacting substances in the more complex natural environment, to calibrate and verify predictive models, and to verify exposure concentrations during aquatic toxicity testing. The first step in the AE analysis procedure involves separation of the nonionic surfactants from the environmental matrix using a Cg solid-phase extraction (SPE) cartridge. After isolation,a reversed-phase thermospray LC/MS method is used to obtain quantitative concentrations of AE. The detection limit for the LC/MS method is in the low nanogram range for individual compounds. When combined with extraction and concentration, the spike concentrations detected were in the range from 0.06 to 2.17 ppb for individual AE compounds. Validation of the method for obtaining the total and individual linear primary AE concentrations in dilute aqueous solutions is described in this report.
EXPERIMENTAL SECTION Six aliquots of grab samples from a sewage treatment plant effluent in Sycamore Township, Cincinnati, OH, were collected. This sewage treatment plant receives wastewater from primarily household sources and utilizes activated sludge for bioremediation. Three were spiked with 102.2 pg/L NEODOL 25-9 (N25-9), a commercial linear primary AE (Table 1). Three were used as unspiked controls. This AE product contains a distribution of alcohol ethoxylates where R ranges from Cl2 to CISand with an average of nine EO groups. The same procedure was followed on six aliquots of river water samples (grab samples collected 1 mi downstream from the same sewage treatment plant effluent). Three river water samples were spiked at a level of 25.7 pg of N25-9/L and three were used as unspiked controls. (14) Saito, T.; Hagiwara, K. Fresenius' Z . Anal. Chem. 1983, 315, 201-204. (15) van Hoof, F. M.; Craenenbroeck, W. J.; Dewaele, J. K. Int. J. Enuiron. Anal. Chem. 1985, 19, 155-164. (16) Inaba, K. I n f . J . Enuiron. Anal. Chem. 1987, 31,63-73. (17) Kravetz, L.; Salanitro, J . P.; Dorn, P. B.; Guin, K. F. J . Am. Oil Chem. SOC. 1991, 68, 610.
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The sample preparation procedure involved conditioning the Cg SPE with methanol followed by water, then passing the sample through the cartridge, rinsing the cartridge with dilute HCl(O.001 N) to remove excess formaldehyde added for surfactant preservation, and eluting AE from the cartridge with methanol followed by isopropyl alcohol. After the sample was eluted from the SPE cartridge, the solventswere evaporated using nitrogen at 60 OC. An internal standard (Nl-9), a primary linear C11 AE with an average EO value of 9, was added to the residue. The N1-9 product is similar in chemical properties and structure to the N25-9 (see Table 1 ) . The use of N1-9 compared to N25-9 is low. Therefore the expected concentration of N1-9 in household effluents is negligible. The mixture was then diluted in methanol to a final volume of 1 mL. The use of the N1-9 as internal standard compensated for variations in instrumental response due to ionization efficiency. Thermospray LC/MS analysis is sensitive to minor changes in operating parameters and sample matrices. These changes cause variations in response factors which are best compensated by selecting internal standards which are very similar in chemical composition to the analyte. By using N 1-9, each specificethoxylate ( C I ~ - C I Swas ) referenced to thecorresponding CI1 ethoxylate. The highly branched AE (BAE) used for comparison of chromatographic resolution was an Exxal 13-7 (Table 1). The calculation of concentration factors included adjustments for initial sample volumes, spike amounts, final (concentrated) volumes, and amounts injected. Application of the appropriate adjustments for each sample resulted in concentration factors in the range of 50-200. In addition, the calculated values in nanograms were converted to micrograms and the results reported relative to 1-L starting volumes (pg/ L) as parts per billion (ppb). The standard solutions and concentrated samples were analyzed by injection of 100 pL into the LC/MS system. The HPLC system was a Waters 600 MS with column heater (40 "C), a Supelco LC18 4.6 mm X 25 cm X 5 pm column, and a Waters 590MS for postcolumn addition of ammonium acetate solution. An isocratic mobile phase of 55% water (18 mequiv) and 45% tetrahydrofuran at a flow rate of 0.7 mL/ min was used. An additional 0.6 mL/min of 43 mM aqueous ammonium acetate was added postcolumn. The mass spectrometer was a Finnigan MAT TSQ 700 with Thermospray 2 interface. The thermospray vaporizer was operated at 93 OC and without filament or discharge. The collector voltage was varied from 25 to 45 as a function of the mass range. The mass spectrometer source temperature was 250 OC. The mass range was scanned from 250 to 1050 amu in 3 s using a profile averaging program to average every four scans. The profile average data was converted to centroid data, and integration of peaks corresponding to individual AE was accomplished using Finnigan MAT quantitation programs. Standard solutions were prepared by diluting known amounts of the AE standards (N25-9 and N1-9) to 1 mL in methanol. They were used as standards for our study in the absence of model compounds for the individual AE compounds. These AE standards have been well characterized by LC/GC separation and analysis. The previously published17 LC/GC characterization was done on highly concentrated standards (ppm
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range). Absolute concentrations of individual AEs in each standard were calculated based on data from the LC/GC analyses. The concentration of the N1-9 internal standard, in all of the samples for this study, was kept constant at 26 pg/mL (total of all ethoxylates C11E02-Cl1E018). The concentration of the N25-9 in the standards was varied from 20 to 200 pg/mL (total of all ethoxylates, C12EO2-C15E018). Response factors were calculated for individual AEs by analyzing these standard solutions at concentration levels of 20,50,100, and 200 pg/mL total N25-9 to cover a calibration range of 2-20 pg (injection volume was 0.1 mL). Individual response curves for each AE (AE,) were constructed using the calculated areas (individualAE and correspondinginternal standard area), known reference amounts (from LC/GC wt 9% data), and the calculated response factors. Areas for specific AE compounds were obtained from the respective LC/MS ion chromatograms. Concentrations of individual AE compounds in the environmental samples were then calculated by linear regression using the ratios of area of a specific AE to that of the corresponding internal standard and using the appropriate response curve. Individual quantification files were obtained in this manner for each sample. These were then converted to ASCII format and copied to a PC, and values for precision and accuracy were determined using spreadsheet processing (Lotus 123). The individual concentrations for each AE detected in the triplicate analyses of the spiked samples were averaged. Calculations were performed in Lotus to determine the standard deviation
(SD,1), percent relative standard deviation (7% RSD), and percent recovery (9% REC) for each AE. The data were converted to 3-D graph format displaying the average distribution and the percent recovery values for all 68 AE compounds using Harvard Graphics software.
RESULTS Thermospray LC/MS detection of the AE compounds results in primarily molecular ions of the type (M NH4)+ with some of the lower molecular weight compounds having a significant (M + H)+ ion. Typical thermospray spectra for C I S H ~ ~ O ( C H ~ C H and ~ O )CllH230(CHzCHz0)18H ~H are shown in Panels a and b of Figure 1, respectively. Since each individual AE has a different molecular mass (M), plotting the two ions, MH+ and MNH4+, and determining the area for the detected peaks allows quantification for individual AEs. Selected ion chromatograms obtained from LC/MS analysis of the spiked effluent sample and from a N25-9 standard are shown in Figures 2 4 . These ion chromatograms highlight the separation capabilities of the method. Individual AE compounds are separated by alkyl chain and by ethoxylate chain length. Figure 2 shows the selected ion chromatograms (MH+ and MNH4+ ions) for a linear C12 AE. This figure illustrates the separation obtained for oligomers with the same alkyl chain. Figures 3 and 4 show selected ion chromatograms (again MH+ and MNH4+ ions) for specific ethoxylate chain lengths of 2 and 18, respectively. These figures illustrate the
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AnalyflcalChemlstry, Vol. 66, No. 5, March 1, 1994
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