Anal. Chem. 1992, 64, 1449-1454
1449
Quantitative Analysis of Alkylbenzenesulfonate Surfactants Using Continuous-Flow Fast Atom Bombardment Spectrometry Anthony J. Borgerding and Ronald A. Hites’
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A quantltatlve method for the analysis of linear alkylbenzenesulfonates (LAS) has been developed using contlnuousflow fast atom bombardment and tandem mass spectrometry. Simultaneous analysls of all LAS homologues Is posslble by scanning the parent Ions of m/r 183, which Is a product Ion common to all LAS sublected to colllslonally Induced dlrrsoclatlon. Slmllarly, branched alkylbenzenesunonates (ABS) can be analyzed by scanning the parent Ions of m/z 197. Samples were concentrated using solid-phase extractlon on C18dlsks. Because Separation Is not required, samples can be analyzed In 4 mln or less udng flow Injection analysls. Quantltatlon lo llnear from 1 to at least 100 ng; the detectlon llmlt k 1 ng Injected, corresponding to 0.5 pg/L In 1-L water samples. This method has been used to determlne LAS concentratlonsIn samples taken from a wastewater treatment plant and In river water. I n this actlvated sludge treatment plant, removal of LASfromwastewater occurred by adsorption tosludge as well as by bkdegradatlon. Relatlve concentrations of longer chain homologues In sludge were hlgher than In the plant Influent, lndlcatlng that these homologues accumulate on sludge. LAS adsorbedto sludge were eventually degraded In the plant’s aerobic sludge dlgester.
INTRO DUCT1ON Alkylbenzenesulfonateshave been the most commonly used anionic surfactants since their introduction in the 1940s. Initially, these compounds were manufactured with branched alkyl chains (abbreviated as ABS). However, due to their resistance to biodegradation and consequent episodes of foamingin rivers and wastewater treatment plants, they were replaced by linear alkylbenzenesulfonates (LAS), which biodegrade rapidly. Approximately 270 000 metric tons of LAS are produced in the United States each year,’ most of which is used in household detergents and introduced into the environment with municipal waste. ABS has continued to be used in third world countries and is a potential source of pollution in these countries. In the United States, the presence of ABS in groundwater has been used as an indicator of the groundwater’s age.2 Traditionally, LAS and ABS have been measured using the methylene blue m e t h ~ da, nonspecific, ~ complexometric method incapable of differentiating individual homologues. It is also prone to interferences from naturally present anionic material. Chromatographic techniques have also been used in the last 15 years to study LAS, including gas chromatog(1)Rapaport, R.;Eckhoff, W. Enoiron. Sci. Technol. 1990,9,12451257. Jr.;Thom,K.Anal.Chem. (2)Thurman,E.; Willoughby,T.;Barber,L., 1987,59,1798-1802. (3) Toei, K.; Fujii, H. Anal. Chim. Acta 1977,90,319-324. 0003-2700/92/0364-1449$03.00/0
raphy of derivatized LAW5 and liquid chromatography using fluorometric6-8 or UV detection.9 Partial separation of positional isomers of LAS (isomers due to ring attachment a t different carbons in the alkyl chain) is possible using Cle HPLC packing.6 Rapid analysis and quantitation of LAS by homologue chain length has typically been done using C8 HPLC packings.7~9 Analysis of mixtures of ABS is more difficult, yieldinga complex,unresolved group of peaks which prevents the quantitation of individual homologues.9 Compared to the techniques above, mass spectrometry is more versatile and generally more rapid, since it can be used to analyze LAS and ABS without prior chromatographic separation. Alkylbenzenesulfonates have been qualitatively analyzed using fast atom bombardment (FAB) tandem mass spectrometry10and field desorption mass spectrometry.11 This paper discusses the use of continuous-flow FAB (CFFAB) coupled with tandem mass spectrometry to quantitatively analyze alkylbenzenesulfonate surfactants. CFFAB has been shown to be more sensitive than static FAB12and to allow easy and rapid quantitation by flow injection analysis. In addition, CFFAB reduces surface activity effects which occur in static FAB.l3 Our technique, which utilizes parention scans, takes advantage of the different fragmentation patterns of LAS and ABS to selectively analyze each of these classes. All LAS or ABS homologues are analyzed simultaneously by flow injection; thus, samples can be analyzed more rapidly than is possible by using chromatographic techniques. Becauseof the selectivity of the technique, sample preparation is simple, usually requiring only concentration by solid-phase extraction of the analyte from liquid samples or methanol extraction using sonication from solid samples. EXPERIMENTAL SECTION Materials and Standards. LAS were obtained as a mixture of free sulfonic acids from Alfa Products. Individualhomologues were isolated from this mixture using a preparative LC system that consisted of a Waters M-6000Areciprocating pump, a Regis semiprepcolumn packed with lo-” Cls material, and a Waters Model 440 absorbance detector. Fractionscorrespondingto each homologue were collected, and the solvent was removed under a stream of nitrogen. Spectrophotometry-grademethanol and HPLC water were purchased from EM Science. Glycerol (
