Anal. Chem. 1990, 62, 2581-2586
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Determination of Linear Alkylbenzenesulfonates and Dialkyltetralinsulfonates in Water and Sediment by Gas ChromatographyIMass Spectrometry Michael L.Trehy,* William E. Gledhill, and Robert G. O r t h
Monsanto Company, St. Louis, Missouri 63167
A method for the determlnatlon of linear alkylbenzenesulfonates (LAS) and diaikyitetrallnsulfonates(DATS) In water and sediment samples by gas chromatography/mass spectrometry (GC/MS) Is descrlbed. Sulfonates are converted to their trlfluoro sunonate derivathres In a two-step derlvatizatlon procedure. I n the flrst step the sulfonates are converted to their sulfonyl chlorides by treatment wlth phosphorus pentachloride and then afler extractlon converted to their corresponding trlfluoroethyl derivatives by the addition of trlfluoroethanol. Ouantitatbn Is by an Internal standard technique. I n this work a C9 dlaikyitetrailnsulfonate Internal standard was used as the Internal standard. The derlvatlzation GC/MS method permits the detectlon and quantltation of LAS and DATS, which are mlnor components present In LAS. The llmk of detectlon for both LAS and DATS In water by electron capture negative chemlcai ionization GC/MS is estimated to be 0.001 mg/L and In sediment 0.5 mg/kg for LAS and 0.2 mg/kg for DATS. Enhanced sensitivity and selectlvity due to GC/MS permit the determlnatlon of LAS and DATS, at the low concentrations encountered In environmental samples.
INTRODUCTION Linear alkylbenzenesulfonates (LAS) are extensively used as surfactants in the detergent industry. Biodegradation of LAS and its components is of interest due to this broad usage in detergents and subsequent direct disposal to wastewater treatment facilities. Commercial LAS is a complex mixture of components including DATS (1-4) and residual linear alkylbenzenes (LAB) (4,5). Dialkyltetralins (DAT) are minor components present in LAB in the range from less than 170 to approximately 5 % . DAT undergo sulfonation along with LAB during conversion of LAB to LAS. DATS have similar retention times to LAS and are consequently difficult to determine by current liquid chromatography (LC) techniques in the presence of the 20-100-fold higher concentration of LAS. This method was developed as an improved procedure to determine the low environmental concentrations of LAS and DATS. Analytical methods for the determination of LAS are numerous. The methylene blue active substances (MBAS) method (6) is generally used for anionic detergents. However, this method is subject to interferences from other anionic detergents and is not suitable for the specific determination of LAS in the environment (4). Detection of LAS (7-9) and LAS intermediates (9-13) by LC methods employing UV and fluorescent detection is suitable for detecting low concentrations of LAS. The fluorescent LC methods are capable of detecting 0.01 mg/L of LAS (14,15)in water and 0.1 mg/kg in solid samples ( 1 4 ) but are not suitable for the detection of DATS. Fast atom bombardment mass spectrometry has also been used to quantitate LAS in detergents and environmental samples (16). The added selectivity of mass spectrometry allowed quantitation without prior chromatography in the 0003-2700/90/0362-2581$02.50/0
complex detergent mixture. Qualitative confirmation of the presence of trace concentrations of LAS type compounds has been carried out by NMR techniques (17).However, puantitation was carried out by a nonspecific titration procedure that would be subject to interferences and quite likely to yield high values. LAS is not sufficiently volatile to permit direct analysis by GC techniques. In order to determine LAS in environmental samples by GC methods, derivatization techniques have been developed. Desulfonation of LAS in boiling phosphoric acid (18) has been successfully employed for the analysis of environmental samples ( 19-21). The desulfonation technique has also been used for the determination of LAS biodegradation intermediates (22-24). The desulfonation GC technique is capable of detecting 0.01 mg/L of LAS in environmental waters (25). LAS can also be determined after conversion to either sulfonyl chlorides (26-29) or methyl sulfonates (30, 31). Methyl sulfonate derivatives of alkylbenzenesulfonates have been prepared by reaction with diazomethane (32) and by conversion of the sulfonyl chloride to methyl sulfonates in a two-step procedure (31). In this paper, we describe improvements to the methyl sulfonate method described by Hon-nami et al. (31)for LAS and demonstrate the general applicability of the method for LAS type compounds. Trifluoroethyl sulfonates are prepared instead of methyl esters in order to enhance sensitivity and selectivity for electron capture negative chemical ionization GC/MS. This permits the determination of the environmental concentrations of LAS and DATS at low microgram per liter concentrations. The LC techniques with either fluorescent detection or UV detection cannot analyze environmental samples for both LAS and DATs due to coelution. The desulfonation techniques for GC/MS are less sensitive and less selective and therefore more susceptible to interference. An additional advantage of chemical ionization is that there is less fragmentation than in electron impact (EI) ionization. The decrease in fragmentation results in an increase in sensitivity in selected ion monitoring mode (SIM) and simplifies interpretation of the spectra. EXPERIMENTAL SECTION Reagents and Materials. Water samples were extracted with 6-mL C8 solid-phase extraction (SPE) columns purchased from Analytichem International containing 500 mg of the bonded-phase material. LAS standards were prepared from an industry blend of LAB and sulfonated by Stepan Co. (Monsanto Log No. 3513278-A). A mixture of LAS isomers was prepared so that the full range of chain lengths from Clo to CI4LAS could be used for the determination of the response factors. The 1-butyl-4-methyltetralinsulfonate (C9DATS) internal standard and the Clo to C14DATS standard used for estimation of the concentration of LAS and DATS in the environment were provided by Vista Chemical Co. Samples. Samples of river water, wastewater, and sediment were collected by Procter & Gamble and preserved by the addition of formaldehyde. Preserved samples of water and sediment 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO.
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onto SPE column Rinse SPE column with deionized water
1
Extract aliquot of water or diluted sediment extract
2
Elute Wnh 80% acetonitrile and evaporate to dryness
3
Prepare sullonylchloride derivatives
i
4
5
+ 4
Add phosphorus pentachloride cap and heal at 103 OC for l o miniites Extract with 2 ml of pentane pentane to clean vial and evaporate io
;E;;;;:
Prepare trifluoroefhyl derivatives
Concentrate and analyze oy GC
i MS
Add 0 2 ml lrilluoroelhanol and heat at 70 O C lor ?C minutes Add 2 mi pentane and 2 ml a i deionized water and enract
Flguro 1. Flow diagram of the LASIDATS isolation and derivatization
procedure. contained 1% and 3% formaldehyde, respectively. Apparatus. Analyses were performed on a Hewlett-Packard 5985 gas chromatograph/mass spectrometer with negative chemical ionization capabilities and a Teknivent data system. Water LAS/DATS Isolation and Concentration Procedures. A 60-mL reservoir was attached to the Analytichem International C8SPE column and then activated by passing first 10 mL of acetonitrile and then 10 mL of deionized water through the column while attached to a vacuum manifold. Approximately 0.8 pg of the Cg DATS internal standard dissolved in acetone was added to an aliquot of the water sample. For wastewater treatment plant effluents and river waters, a 50-mL sample was generally used while a 5-mL sample was used for raw wastewater. The sample was mixed and transferred to the reservoir and drawn through the SPE column. The column was rinsed with 10 mL of deionized water, the reservoir was removed, and a 5-mL receiving Reacti-vial was placed below the SPE column. LAS and DATs were eluted with 4 mL of acetonitrile. Acetonitrile was evaporated by purging with nitrogen and heating at 70 "C. Sediment LAS/DATS Isolation and Concentration Procedures. Samples were air-dried, ground in a mortar and pestle, and sieved through a 50-mesh screen. Approximately 1g of sieved sample was then transferred to a polyethylene centrifuge tube, approximately 1 pg of Cg DATS internal standard dissolved in acetone was added, and the sample was mixed. Subsequently, 10 mL of methanol was added, and the solution was sonicated for 10 min. Samples were then centrifuged for 10 min at 6000 rpm, and the extract was transferred to 50-mL graduated cylinders and diluted to volume with deionized water. A 60-mL reservoir was attached to the Analytichem International Cs SPE column and activated by passing first 10 mL of acetonitrile and then 10 mL of deionized water through the column while attached to a vacuum manifold. The sample was mixed and transferred to the reservoir and then drawn through the SPE column. The column was rinsed with 10 mL of deionized water, the reservoir was removed, and a 5-mL receiving Reacti-vial was placed below the SPE column. LAS and DATS were eluted with 4 mL of acetonitrile. Acetonitrile was evaporated by purging with nitrogen and heating at 70 O C . LAS/DATS Derivatization Procedure. Approximately 100-200 mg of phosphorus pentachloride was added to the dry sample residue in 5-mL Reacti-vials, and the vials were capped with Teflon-lined septa and transferred to a heating block at 100 "C for 10 min. The Reacti-vials were removed from the heating block and allowed to cool. After the vials had cooled, 2 mL of ptentane was added and extracted for 30 s in a vortex mixer. The pentane was transferred to another Reacti-vial and blown down to dryness with a stream of nitrogen at ambient temperature. Residue from the pentane extracts was then dosed with 0.2 mL of trifluoroethanolthat had been previously dried over anhydrous potassium carbonate, and the vial was capped with a Teflon-lined septum. Vials were heated at 70 "C for 20 min and then allowed to cool. Derivatives were extracted by adding 2 mL of pentane and 1.5 mL of deionized water and vortex mixing for 30 s. The pentane layer was transferred to a clean Reacti-vial, evaporated to dryness with a stream of nitrogen, and redissolved in 0.2 mL of isooctane, and the solution was analyzed by GC/MS. A flow diagram for the extraction and derivatization procedure is shown in Figure 1. Mass Spectrometric Analysis. LAS and DATS can be determined either by electron impact (EI) ionization or by chemical ionization techniques. The trifluoroethanol derivative was chosen
to enhance electron capture negative chemical ionization (NCI). Due to the high electron affinity of the trifluoroethyl group, NCI is more selective and sensitive than E1 and was the ionization procedure of choice. The source was typically maintained at 110 "C for NCI and 150 O C for EI. In NIC the source was operated at approximately0.5 Torr as indicated by a Pirani gauge. Selected ion monitoring (SIM) was used to increase sensitivity. In NIC the following ions were monitored for LAS: m / z 380,394, 408, 422, and 436. For DATS, m / z 378,392,406,420, and 434 were monitored, and for the CgDATS, m/z 364 was monitored. These ims correspond to the Clo,Cll, Clz, CI3,and C14homologues of LAS and DATS, respectively. In the E1 mode, LAS can be determined by the ions at m / z 253 and 267, while DATS can be determined from the ions at m / z 279 and 307. Gas Chromatographic Conditions for LAS a d DATS. Either a J&W DB-5 15-m capillary column with a 0.25 mm i.d. and 0.25-pm film thickness or a J&W DB-1 15-m capillary column with a 0.25 mm i.d. and 0.1-pm film thickness was used for the analyses with helium carrier gas. The temperature program for the DB-5 column had an initial temperature of 125 "C for 1min, and then the temperature was ramped at a rate of 5 "C/min to 230 "C and held at 230 "C for 5 min. When the DB-1 column was used, the temperature program for the DB-1 column had an initial temperature of 100 "C for 1min, and then the temperature was ramped at a rate of 5 "C/min to 210 "C and held at 210 "C for 4 min. Injection port and transfer lines were maintained at 230 OC. LAS and DATS isomers elute over the time period from 13 to 25 min with the DB-1 column. Peaks for each homologue were summed, and an average response factor was calculated. The internal standard eluted at approximately 14-15 min with the DB-1 column. LAS and DATS Quantitation. A known quantity of CB DATS was added to an aliquot of each sample prior to extraction and derivatization. A response factor for each of the 26 major components of the Cl0-Cl4 LAS was not calculated even though the majority of the LAS isomers are separated. Areas for each homologue group were summed, and a response factor for each homologue series was calculated. Response factors were calculated for each of the homologues. Response factors for the DATS were similarly calculated. RESULTS AND DISCUSSION S t r u c t u r e a n d Sources f o r LAS a n d DATS. Linear alkylbenzenes (LAB) are sulfonated to produce LAS for detergent applications. In most developed countries LAS has replaced the slow to biodegrade branched-chain polypropylene-based derivatives. For example, this transition was made by the US.detergent industry in 1965 (4). LAS is a mixture of homologues, and within each homologue series there are isomers due to the different possible locations of the phenyl group on the alkyl chain. Typically, detergents contain LAS with an alkyl chain length that varies from 10 to 14 carbons in length. Since the primary carbon is not substituted by the phenyl ring and sulfonation generally occurs at the para position, there are 26 major isomers in typical commercial LAS: 4 Cl0 LAS isomers, 5 C13 LAS isomers, 5 C12 LAS isomers, 6 CI3LAS isomers, and 6 Ci4 LAS isomers. Structures for the 4 Clo LAS isomers are shown in Figure 2. Dialkyltetralins (DAT) are inadvertently produced during synthesis of LAB. The mixture of alkyl chain lengths used to prepare the homologues for LAB also results in varying alkyl chain lengths for the DAT. However, the structure for the DAT is more complex due to the added possible formation of cis/trans isomers. When DAT is sulfonated to form DATS, there are nonequivalent positions on the aromatic ring, also increases the number of possible isomers. Thus,assuming that the 1- and 4-positions on the aromatic ring are sterically hindered, thereby limiting sulfonation to either the 2- or 3-positions, there are 70 major isomers present in a DATS mixture generated by the use of a Cl0-Cl4 alkylation mixture. The 10 isomers that could be produced from sulfonating the reaction product of decadiene with benzene are shown in Figure 3.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 199.0
Table I. Results for Spiked Water Samples Analyzed by EI/GC/MS and NCI/GC/MS
w s0,o-
LJ s0,oz-(sulfophenyl)decane
conc, mg/L
CH,CH,CH,CH,CH,CH,
CH,CH,CH,CH,CH
fi
CH,CHzCH&H,CH,
% RSD
109 79 97
16 4 5
Linear Alkylbenzenesulfonates (LAS) Analyzed by Negative Chemical Ionization (NCI)
fi
w s0,o-
w s0,o-
% recovery
Linear Alkylbenzenesulfonates (LAS) Analyzed by Electron Impact Ionization (EI)
3-(sulfophenyl)decane
0.0212 0.846 8.46 CH,CH,CH,CH
2583
0.423
98
7
Dialkyltetralinsulfonates (DATS) Analyzed by Negative 4-(sulfophenyl)decane
Chemical Ionization (NCI)
5-(sulfophenyl)decane
Flgure 2. Structures of the 4 major Cl0 LAS isomers of isomers formed by the reaction of decene with benzene and subsequent sui-
fonation. There are 26 major isomers present in an produced by the use of a Cio-Ci4 alkylation mixture.
-
LAS
mixture
CH~QCHZCH&H,CH,C
H,
-
CH,~CH2C~CH2CH,CH,
0.0067 0.0335 0.122 0.201 1.005
89 86 100 83 96
a 16 10 10 12
Table 11. Results for Spiked Sediment Samples Analyzed by NCI/GC/MS
s0,o-
Qsozo-
CISfrRANS
CIS/TRANS
Q) s0,o-
s0,o-
CISfrRANS
CISPRANS
CH,CH,CH,
8
CH,CH,CH,
s0,o-
CISfrRANS
Flgure 3. Structures of the 10 DATS isomers formed from the reaction of decadiene with benzene and subsequent sulfonation. There are 70 major isomers present in a DATS mixture produced by the use of a
alkylation mixture.
Accuracy a n d Precision. The method for LAS and DATS determination was validated in both water and sediment. The concentration procedure increases the concentration of LAS and DATS sufficiently that either EI/GC/MS or NCI/GC/MS may be used for the analysis of environmental samples. Results for water and sediment samples analyzed in sets of five at each concentration are listed in Table I for water samples and Table I1 for sediment samples. Recoveries and relative standard deviations were good in spite of the individual components being present at concentrations as low as 0.0001 mg/L in the DATS 0.0067 mg/L validation sample. Recoveries for water samples were from 79% to 109% with a percent relative standard deviation ranging from 4% to 16% and recoveries for sediment samples were from 64% to 103% with percent relative standard deviations ranging from 3% to 21%. Results for the GC/MS compare favorably with the LC methods. While the LC method is approaching its limit of detection at 0.02 mg/L LAS (15),the NCI/GC/MS method has a limit of detection (LOD) for 50-mL samples of approximately 0.001 mg/L. The LOD was estimated by calculating a signal to noise ratio of 3 for a component and then multiplying the number of components in the mixture. The detection limit for either the NCI- or EI/GC/MS method is conservatively 20 times better than that attainable by the LC method. Added sensitivity is of considerable value since even
% recovery
% RSD
Linear Alkylbenzenesulfonates (LAS) 171 21.7 4.2
v
C,&,
conc, mg/ kg
92 103 76
21 3 11
Dialkyltetralinsulfonates (DATS) 61 73 16 9.18 88 3 1.17 64 7
in areas where treated wastewater is entering the environment the concentrations of LAS are quite low. A survey of German rivers found that the average LAS concentration was only 0.04 mg/L (14). This concentration is easily determined by either NCI- or EI/GC/MS but approaches the method limit of detection by the LC method. The NCI method simplifies estimation of the homologue distribution since a different ion is monitored in NCI for chain length. Fragmentation i n NCI/MS. Formation of the trifluoroethyl sulfonates increases sensitivity by making the derivative electronegative for electron capture methods such as NCI/MS or -GC with an electron capture detector and acts as a marker to assist in the detection of LAS and DATS compounds. Derivatized LAS and DATS have a characteristic MS fragmentation pattern that can be used to identify the LAS and DATS in complex environmental matrices. The negative chemical ionization (NCI) mass spectra are sensitive to temperature effects. The three major ions formed in NCI are the parent ion, the parent ion minus 100 amu corresponding to the loss of trifluoroethanol, and the m / z 163 ion corresponding to SO3CH2CH3. The higher the source temperature the greater the intensity of the m / z 163 ion, which is characteristic of all the derivatized sulfonates. In order to maximize the formation of the more informative higher mass fragments the source temperature was maintained at approximately 110 "C. Quantitation was made on either the parent ion or the parent ion minus 100 amu. LAS and DATS may also be determined by more generally available E1 ionization techniques as well. Major high-mass fragments in E1 for LAS are m / z 267 and 253. The major fragment ion for the 2-(sulfopheny1)alkanes is the m / t 267 ion, while for the internal isomers the major fragment ion is m / z 253. The major fragment ions for the DATS are m / z 307 and 279. When one of the alkyl side chains is a methyl group then the m / z 307 ion is a significant fragment. This is consistent with the formation of the m / z 307 ion from the C9
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
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Figure 4. Reconstructed ion chromatograms for LAS in the influent and effluent of a trickling filter wastewater treatment facility Springvifle, UT, normalized to the highest peak in the time range. Biodegradation is demonstrated by the rapid loss of the external isomers.
Table 111. LAS Concentrations in Water (mg/L) and Sediment (mg/kg) at Three Sites Determined by LC and GC/MS Tec hniqueeD water
sediment
LC
GC/MS
GC/MS
LC
