Anal. Chem. 1988, 60, 2613-2620
2613
Quantitative Determination of Trace Levels of Cationic Surfactants in Environmental Matrices Using Fast Atom Bombardment Mass Spectrometry J. R. Simms and T. Keough* The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 398707, Cincinnati, Ohio 45239-8707
S. R. Ward, B. L. Moore, and M. M. Bandurraga The Procter & Gamble Company, Ivorydale Technical Center, 5299 Spring Grove Avenue, Cincinnati, Ohio 45217
Two fast atom bombardment mass spectrometry methods have been evaluated for the quantltatlon of part-per-bllllon levels of catlonlc surfactants In aqueous envlronmental matrices lncludlng sewage Influents, sewage effluents, and rlver water. The flrst method, low-resolution full scannlng, allows the slmultaneous quantltatlon of targeted components. The second, hlgher resolullon slgnal averaglng, Is more accurate and reproduclbk than full scannlng but allows quantltatlon of only a slngle component at a tlme (wlth exlstlng soflware). Average recoveries of theoretlcal spike, over the concentratlon range of 10-500 ppb, varled from 86 to 107% dependlng upon the method used. The coeffklents of varlatkn averaged 5-14%. Fast atom bombardment mass spectrometry has a llmlt of quantltatlon that Is 1 to 2 orders of magnitude lower than exlstlng chromatographic techniques and sample throughput Is, conservatively, 5 tlmes hlgher. A potentlal llmltatlon of thls method Is the requlrement of suitable, preferably stabletsotope labeled, Internal standards.
Surfactants are among the most versatile industrial chemicals and are used in numerous consumer products including motor oils, pharmaceuticals, detergents, fabric softeners, hair care products, and paints (I). Annual worldwide production exceeds lo9 lb with anionics representing about 80% of the total. Cationics comprise less than 10% of total surfactant production, but they are the most rapidly growing segment of the surfactant market (2). Given the commercial significance of surfactants, it is not surprising that considerable effort has been devoted to their characterization. Spectrophotometric and titrimetric methods are the most commonly used techniques to quantitate surfactants ( 2 , 3 ) . Separations techniques have also been used for both purification and quantitative purposes (4). Recently, mass spectrometry (MS) has emerged as a powerful tool for the characterization of nonvolatile organic compounds including surfactants. This has occurred largely because of the development of several desorption ionization (DI) methods. Excellent general reviews of these techniques have recently been published by Burlingame and co-workers (5, 6), Pachuta and Cooks (7),and Busch (8). Two books, presenting both theory and applications, have also appeared (9, 10) as have specific reviews of each of the individual methods (11-15). Sensitivity, molecular specificity, and direct applicability to complex mixtures are among the principal advantages of mass spectrometry based methods for the characterization of surfactants. Weber and co-workers (16) have obtained com-
* Author to whom correspondence should be addressed. 0003-2700/88/0360-2613$01.50/0
plete mass spectral data on as little as 5 pg of various surfactants by using field desorption (FD) and a specially designed tandem mass spectrometer. More typically, a few nanograms of material is required for structural analysis (17). Intact molecular ions are usually the base peaks in the DI mass spectra of surfactants. These ions reportedly provide direct information on the molecular weight distributions of cationic (18, 19), anionic (20-22), and nonionic surfactants (23). Furthermore, mass spectral fragmentation behavior can be used to determine the type of surfactants present in a mixture, the length and number of alkyl chains, the degree of ethoxylation, and the presence of isomers (19,21,23). Lyon and co-workers (10, 19) successfully used tandem mass spectrometry (MS/MS) to unambiguously differentiate isobaric quaternary ammonium salts containing different combinations of fatty alkyl substituents. Furthermore, Jensen has used these same techniques to locate double bonds, branch points, hydroxy and epoxy substituents, and cyclopropane and cyclopropene rings in surfactant molecules (24). However, isomer differentiation must be done with caution. For example, the MS/MS fragmentation patterns of sodium cationized linear alkyl benzenesulfonates (LAS) suggest that commercial LAS samples are comprised mainly of the 2-isomer with minimal amounts of internal isomers (16). As pointed out by Swisher (4),this conclusion conflicts with well-established results obtained with chromatographic procedures. In addition to the structural studies just noted, DI mass spectrometry has also been used to quantitate surfactants in complex matrices. Levsen and co-workers (25,26)used FDMS to identify surfactants in surface waters and to establish biodegradation mechanisms. Shiraishi et al. (27) also used FD-MS to identify and quantitate nonionic surfactants in river waters. Fast atom bombardment (FAB) mass spectrometry, a technique that is experimentally much more convenient than FD-MS, has also been used for quantitative studies; however, most reported quantitative FAB-MS applications involve the determination of single-targeted compounds from biological matrices. Representative examples, taken from the recent literature, are summarized in Table I (29-59). Quantitative applications of FAB-MS to compounds in environmental matrices are rare. Ligon and Dorn have quantitated NO< and HSO, in rain water (55,56)while Rivera and ceworkers have determined polyglycols (58) and sulfonated dyes (59) in environmental water samples. FAB-MS can be used, in favorable cases, to quantitate trace targeted compounds from complex matrices; however, a number of factors can limit the applicability of this method. For example, analyte response may be largely suppressed due to sample contaminants having higher surface activity than the analyte of interest (29,43,47,48,60-62).The ultimate quantitation limit can be determined, particularly for biological assays, by availability of sample (39). Furthermore, 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1 1988
surfactants on the relative FAB-MS responses of C12TMACand DTDMAC. Synthesis of Stable Isotope Standards. [2H3]Dodecyltrimethylammonium iodide ( [2H3]C12TMAI)was prepared by rematrix compound type ref action of (dimethy1amino)dodecane (22.2 g, Fluka AG, Buchs, steroid sulfates plasma 29, 30 Switzerland) with CD31(23.40 g, KOR Isotopes, Cambridge, MA) UDP-glucuronic acid standard solutions 31 in MeOH. The amine was diluted with 5.55 g of MeOH in a standard solutions 32 quaternary ammonium salts foil-covered round bottom flask. This solution was placed in a aqueous environmental 33, 34 water bath and about half of the CD31was added dropwise until standard solutions 35 the reaction products solidified. The products were diluted with standard solutions 20 alkylbenzenesulfonates 16.65 g of MeOH, heated in a water bath at 40 "C, and the urine 36, 37 acylcarnitines remaining CD31was added. The mixture was stirred 1h at 40 38 standard solutions cimetidine "C and then at 60 "C overnight. The MeOH was then removed caudate nucleus tissue 39 peptides by purging under a stream of dry argon. 40 tooth pulp tissue [2H3]DTDMAIwas prepared by reaction of ditallowmethylamniotic fluid glycerolphosphocholines 41 amine (3.04 g, Armak Chemicals Division of Akzo Chemie America, 42 standard solutions Chicago, IL) with CD31 (1.4 g) in 1-propanol. The amine was HL60 cells 43 diluted with 3.04 g of 1-propanol and completely solubilized by 44 plasma cholesterol sulfate addition of -40 drops of hexane. The CD31was added dropwise bile 45 bile acids and the reaction mixture stirred overnight at 40 "C. The 1standard solutions betaines 46 propanol was then removed by purging with dry argon. 47 standard solutions amino acids plasma 48 Environmental Sample Collection. Grab samples of sewage cured paint photostabilizer 49 influent and treated effluent were collected from the Colerain 50 plasma, urine benzazepine metabolite Heights Sewage Treatment Plant in Cincinnati, OH. River water cyclophosphamide, ifosfamide urine 51 samples were obtained from the Little Miami River near Xenia, tomato fruit a-tomatine 52 OH. All samples were preserved with 1%formalin and 10 pg/mL 53 human lung fatty acids of a nonionic surfactant to reduce adsorption of the cationic 54 chicken fat monensin a sodium salt surfactants to the inside of the polyethylene storage containers. rain water inorganic anions 55, 56 All samples were stored at 4 "C prior to preparation. 57 plasma calcium Sample Preparation. Samples were prepared for FAB-MS aqueous environmental 58 polyglycols analysis with the following procedure: aaueous environmental 59 sulfonated dves 1. A 13.5-pg portion of [2H3]C12TMAI and either 11.6 or 58.0 Fg of [2H3]DTDMAI,depending on analyte level, were added to 200-mL aliquots of well-mixed samples. The [2H3]internal background ions produced from the viscous liquid FAB matrix standards have iodide counterions while the analytes have (36)or from the sample matrix itself may interfere with either chlorides. After correction for the differences in molecular weights, analyte or standard response requiring selective isolation the internal standard concentrations are equivalent to 10 pg of procedures (29),higher mass resolution (36),or MS/MS (39, [2H3]C12TMACand either 10 or 50 bg of [2H3]DTDMACin the 50) to gain the necessary specificity. Of course, all of these 200-mL aliquots. In the text and figures, the stated internal approaches are ineffective if quantitation is limited by natstandard concentrations have been corrected for this difference urally occurring background levels of the internal standard in counterion weight. (56). 2. The samples, in 250-mL glass beakers, were evaporated to Given the potential advantages and limitations of this dryness on a steam bath. method, we felt it was important to evaluate the routine 3. The cationic surfactants were extracted from the dried residue by first rinsing the bottom and sides of the glass beaker application of FAB-MS for the quantitation of cationic surwith 10 mL of 80/20 HPLC Grade MeOH (Burdick and Jackson, factants at parts-per-billion (ppb) levels in sewage influents Muskegon, MI)/Certified Grade CHC1, (Fisher, Cincinnati, OH). and effluents and river waters. If the technique proved to be This solution was transferred to a glass Luerlok syringe with an applicable to surfactants in these matrices, we then wanted: attached Alumina "B" cartridge (Millipore Corp., Waters Chro(1)to compare the precision, accuracy, limits of quantitation, matography Div., Milford, MA). The cartridge eluant was coland speed of the FAB-MS approach with our well-established lected into a 20-mL glass vial. chromatographic methods (63)and (2) to determine the effect 4. The extraction step was repeated with 5 mL of 10/90 of MS instrumental parameters such as scan range, scan speed, MeOH/CHC13. The final extraction was conducted with 5 mL and resolution on precision, accuracy, and limits of quantiof MeOH. tation. This latter information should provide a guide to the 5. The combined eluants were then evaporated to dryness on a steam bath. capabilities and limitations of various FAB-MS techniques 6. Samples were transferred to 1-mL glass Reacti-Vials (Pierce applied to environmental samples. Chemical Co., Rockford, IL) after dissolution in 1 mL of 10/90 EXPERIMENTAL SECTION MeOH/CHCl* The solution was evaporated to dryness on a Pierce Reacti-Therm evaporator. Surfactants. Commercial samples of C12TMAC (a), 7. Step 6 was repeated with 1 mL of MeOH. DTDMAC (b), and LAS (c) were used for method development The prepared samples were stored at room temperature. Just and standard preparation. The purity of C12TMAC(98.6%, prior to analysis, the cationic surfactants were solubilized by sonication after addition of 100 pL of HPLC grade MeOH (EM C,2H&'J(CH&CI ( C ~ H Z ~ + ~ ) & " C H ~ C)IZ~C H~Z ~ C S H ~ S O ~ N ~ Science, Cherry Hill, NJ). Two drops of glycerol (99.5%,Fisher) n = 16, 18 were added directly to the MeOH solutions. The methanol was (a) (b) (C) then removed under a stream of dry argon and 1-2-pL aliquots Eastman Kodak Co., Rochester, NY) was established with tiwere analyzed by FAB-MS. This corresponds to loading about 10 ng of analyte onto the FAB probe, for a sample originally tration. DTDMAC (Akzo Chemical, McCook, IL) was purified by repeated recrystallizationfrom hexane and ethyl acetate until containing 1 ppb of the surfactant. a purity >99% was achieved. Purity was established with Samples were prepared for HPLC analysis in the same manner high-performance liquid chromatography (HPLC) and titration. as for FAB-MS except that the [Q3] internal standards were not The purity of the LAS sample (Aldrich Chemical Co., Milwaukee, added to the samples and the samples were not transferred to WI) was determined to be -81% by titration. This material was the 1-mL Reacti-Vials. After the eluants were collected from the not further purified since LAS was not a specific analyte in this Alumina "B"cartridge and evaporated (step 5 ) , they were filtered study. It was used only to establish the influence of anionic by using the following procedure: Table I. Reuresentative Auulications of Quantitative __ FAB-MS
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
1. The dried residues in the 20-mL vials were reconstituted with 2 mL of HPLC mobile phase. 2. With a disposable pipet, the bottom and sides of the vial were rinsed with the mobile phase. 3. The mobile phase was transferred to a 5-mL glass syringe with an attached Acro LC13 filtering disk (Gelman Sciences,Ann Arbor, MI). 4. The eluant was collected into a new vial. 5. Steps 1-3 were repeated with the initial 20-mL vial and the eluant was collected into the vial used in step 4. 6. The combined eluant was dried on a steam bath with nitrogen. A solution of known volume, between 1and 10 mL, was prepared prior to sample analysis. High-Performance Liquid Chromatography. HPLC analyses were conducted with equipment previously described (63). Separations were obtained with a 25-cm Whatman Partisil 5 PAC (Whatman, Chicago, IL) and a Brownlee AminoSpheri-10 guard column (Brownlee Labs, Inc., Santa Clara, CA). The optimal mobile phase used for quantitation of ClzTMAC was 88/12 CHC13/MeOH; for DTDMAC, it was 96/4 CHC13/MeOH. All quantitative experiments were conducted under isocratic conditions with a flow rate of 1mL/min. Quantitative measurements were not attempted until late-eluting components from the previous injection had been eluted from the column. These components increase overall analysis time, limiting to four the number of samples that can be analyzed in an 8-h day. FAB-MS. All mass spectral data were obtained on a Vacuum Generators ZAB-2F reverse-geometry, double-focusing mass spectrometer operating at an ion acceleratingvoltage of 6 kV. Fast atom bombardment was accomplished with a 1-mA beam of 7.5kV Xe atoms (99.995%, Matheson Gas Products, Inc., Joliet, IL), produced with a modified saddle-field ion source (Ion Tech, Ltd., Teddington, UK). The m m spectrometer was operated in one of two modes: The first utilized magnet scanning at 10 s/decade from lo00 to 50 u. For these experiments,the mass resolution was 1OOO. The second technique involved magnet scanning at 700 s/decade over a limited mass range, 5 to 10 u, centered on the ions of interest. The mass resolution was between 2000 and 6000 for these limited mass scan experiments. With this latter procedure, all samples were analyzed for one surfactant, the magnet was then reset, and the samples were reanalyzed for the other surfactant. All data were acquired and processed with a Vacuum Generators ll-25OJ data system. When the low-resolution full scan (FS) mode was used, five to ten spectra were centroided and then averaged. With the limited mass scan approach, about 20 noncentroided spectra were automatically coadded using VGs “continuum acquisition” software (64). Quantitation. HPLC quantitation of ClzTMAC and DTDMAC in unknown samples was achieved by using external standards. Standard solutions were prepared in the HPLC mobile phase with analyte concentrations ranging from 2 ppm (2 pg/mL) to 30 ppm (30 pg/mL). Calibration curves were generated by injecting 50 pL of each standard solution and determining the response (chromatographicpeak area) versus the amount injected. The best fit to the calibration data was obtained by using a h e a r least-squares analysis. FAB-MS standard solutions were prepared in distilled water (distilled 3 times) by mixing 67 ng/mL of [*H3]ClzTMAIand 58 ng/mL of [*H3]DTDMAIwith varying quantities (0.0005-1.0 pg/mL) of unlabeled ClzTMAC and DTDMAC. This is equivalent to adding 50 ppb of [2H3]ClzTMACand 50 ppb of [*H3]DTDMAC to varying quantities (0.5-1000 ppb) of the unlabeled analytes. All calibration data were corrected for this difference in analyte and standard counterions. FAB-MS calibration curves were obtained by plotting the ratio of analyte response to internal standard response as a function of the corresponding ratio of concentrations. For ClzTMAC,the intensity ratio of m/z 228 to m / z 231 was monitored while, for DTDMAC, the ratio of intensities of m / z 522 to m / z 525 was determined. Unknown samples were then accurately quantitated by reference to these calibration curves (see text).
RESULTS AND DISCUSSION Calibration and Quantitation. Typical calibration data
I
2815
in
i(n u
b=0.23
0
4
8 12 16 [DOJ/[D31
20
.‘uccu
L“J
m=O 89,
0
4
8 12 16 [DOI/[D31
20
Flgure 1. Typical FABMS calibration data for C,,TMAC and DTDMAC. The internal standard level is 50 ppm.
for Cl,TMAC and DTDMAC are summarized in Figure 1. These data, obtained on two separate occasions about 6 months apart, illustrate the overall reproducibility achieved with FAB-MS. The ClzTMAC calibration curves are linear over the full range of concentrations with correlation coefficients >0.9998. However, the DTDMAC curves significantly deviate from linearity when the analyte concentration is more than 10 times that of the standard. Over the range of concentration ratios between 0.01 and 10.0, the correlation coefficients for the DTDMAC calibration curves are >0.9994. The linear portions of the calibration data were fit with least-squares lines. Unknown concentrations were then calculated from the least-squares relationships once the intensity ratios of the analytes to standards had been experimentally measured. This approach, while generally successful, proved to be inaccurate for samples containing less than 25 ppb of either analyte. This resulted because the highest level standards tended to tilt the calibration curves, lowering the slope and raising the y intercept. For these lowest level samples, much improved accuracy was achieved by using only the lowest level standards for quantitation. The internal standard level was increased to 250 ppb for samples having DTDMAC concentrations greater than 1000 ppb. This ensured that the experimentally measured intensity ratio fell on the linear portion of the calibration curve. Effects of Surface Activity Differences. Sample matrix components may suppress the response of analytes relative to their FAB-MS responses observed in standard solutions. Furthermore, in homologous mixtures of analytes, severe discrimination against the lower weight homologues is often observed (29,36,43,47,48). Ligon and Dorn (61,62) attribute the variations in response to differences in surface activities of the components present in the analyte mixture. Furthermore, i t was shown that the response variations can be minimized if relatively large quantities (20-fold molar excess) of an oppositely charged surfactant is added to the solution prior to analysis. The reported role of the added surfactant is to dominate the glycerol surface layer so that differences in the surface activities of the analytes become unimportant. Some of the environmental matrices under consideration typically contain an excess of anionic surfactants relative to the cationics. This reflects, in part, the high levels of production of anionics relative to cationics (63,65,66). We expect that the relatively high levels of anionic surfactants will minimize response differences between ClzTMAC and DTDMAC (61,62). To study the role of surfactant hydrophobicity on relative FAB-MS response, we analyzed standard solutions containing a low level of ClzTMAC in the presence of a high level of DTDMAC, Figure 2. The spectrum in Figure 2a was obtained from a distilled water solution containing 10 ppb ClzTMAC, 50 ppb [2H3]C1zTMAI,750 ppb DTDMAC, and 250 ppb [2H3]DTDMAI. The FAB matrix was glycerol and we expected that ClzTMAC would be suppressed because i t was a minor component in the bulk solution and because
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
80 4
I
MIZ
MIZ
Flgure 2. FAB mass spectra of standard solutions containing: (a) 10 ppb CI,TMAC and 750 ppb DTDMAC in glycerol; (b) 10 ppb Cl,TMAC, 750 ppb DTDMAC, and 1000 ppb LAS in glycerol; (c) 10 ppb C,,TMAC and 750 DTDMAC in thioglycerol; (d) 10 ppb C1,TMAC, 750 ppb DTDMAC, and 1000 ppb LAS in thioglycerol.
DTDMAC is much more hydrophobic. Surprisingly, C12TMACwas not suppressed. In fact, its relative response was larger than anticipated. The intensity of the ClzTMAC molecular cation was -5% that of DTDMAC even though the bulk solution contained only -3% of Cl,TMAC on a molar basis. The same mixture was reanalyzed after addition of lo00 ppb LAS (Figure 2b). With LAS present, the relative response of ClzTMAC increased even further, to -30% that of DTDMAC. We believe the initial high relative response of C12TMACresults partly because it is more soluble in glycerol than DTDMAC. The glycerol solution presented to the FAB-MS ion source is enriched in C12TMACrelative to the original sample. The critical role that the FAB solvent exerts on relative response was evaluated by reanalysis of a C12TMAC and DTDMAC mixture similar to that used to produce Figure 2a. However, in these experiments, thioglycerol was used as the FAB matrix. The resulting spectrum (Figure 2c) was more in line with our initial expectations, i.e., abundant DTDMAC ions were observed while the ClzTMAC ions were completely suppressed. Even addition of lo00 ppb LAS failed to produce a measurable response for ClzTMAC (Figure 2d). Finally, the mixture containing ClzTMAC and DTDMAC in glycerol was diluted with an equal volume of thioglycerol and reanalyzed (not shown). The resulting mass spectrum revealed complete suppression of ClzTMAC in contrast to the results in Figure 2a. Clearly, the choice of FAB matrix exhibits a profound influence on the observed FAB spectra. The presence of oppositely charged surfactants also influences the spectra, but to a lesser extent. All environmental samples were analyzed by using glycerol as the matrix to maximize the relative response of C12TMAC. Furthermore, we did not find it necessary to add anionic surfactants to enhance the observation of low levels of C12TMAC. The accuracies of the FAB-MS methods are discussed in detail below. However, we want to emphasize here that the methods are accurate even when quantitating low levels of C12TMACin the presence of high levels of DTDMAC. This fact was established with several spiking experiments, Table 11. On two separate occasions ClzTMAC was spiked, in the concentration range of 10-500 ppb, into influent samples also spiked with DTDMAC so that the DTDMAC concentrations
Table 11. Recovery of ClzTMAC Theoretical Spike from Influent Sewage in the Presence of High Levels of DTDMAC
IC,,TMACl,"nbm b analysis date background added found 5/86 5/86 12/86 5/86 12/86
33.5 33.5 43.8 33.5 43.8
10 50 200 250 500
43.5 83.1 218.9 258.9 496.0
recovery: % 100 99 88 90 88
aThe influent sewage samples were spiked with DTDMAC so that the DTDMAC concentrations ranged between 1400 and 1800 ppb. bData were acquired with low-resolution full scanning on 5/86 and with continuum acquisition on 12/86. cRecovery ( W )= 100 X (found - initial background)/added. ranged between 1400 and 1800 ppb. The observed ClzTMAC signals were intense and the recoveries of the spiked quantities were excellent, 88-100%. Accuracy and Precision. Table I11 summarizes the recovery results obtained from several environmental and distilled water samples that were spiked with known quantities (5-500 ppb) of ClzTMAC and DTDMAC. The analysis date, data code, matrix, number of samples analyzed, and spiked levels are indicated in columns 1-5. Columns 6 and 7 summarize quantitative results obtained on C12TMACwith the full-scan (FS) and continuum acquisition (CA) methods. Columns 8 and 9 summarize ClzTMAC recovery results. Similarly, columns 10 and 11 summarize quantitative DTDMAC results while the remaining columns contain the DTDMAC recovery data. Table IV summarizes the precision studies conducted from the various environmental matrices. Again, the analysis date, code, matrix, and number of samples analyzed are given in columns 1-4. The levels of ClzTMAC spiked into the samples are given in column 5. Columns 6-11 summarize results obtained on ClzTMAC. Columns 6-8 contain the FS results while columns 9-11 contain the corresponding CA data. The levels of DTDMAC spiked into the samples are listed in column 12. Columns 13-15 summarize the precision results obtained on DTDMAC with the FS method while columns
ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
2617
Table 111. Recoveries of Theoretical Spike for ClzTMACand DTDMAC
analysis date code 1/86 5/86
matrixa no. of samples
a b
RW
C
RW
d e f g h
12/86 5/86
12/86 12/86 12/86 1/86
RW Id
0
DW
1 2
P q r
E
X
E
Z
aa bb cc dd ee
5/86
Id RW Id
ff
12/86
gg hh ii jj
12/86
-
10 -
20
20 -
20
-
2 5 3 3 5 3 3 5 5 5 3 3 5 3
Id
Y
5 -
5 5
RW
[CIZTMACI found, ppb FSb CA 4.0 7.8
[DTDMAC]
found, ppb FS CA
recovery: % FS CA
50
-
50
-
50
-
50
-
200
-
250
-
250
DW
2
-
I
2 5 5
500
-
500
recovery, % FS CA
76 2.0
0.0
8.0
160
2.8
125
82
2.0 6.2 23.5 27.9 0.0
0.0
115
86
10.9
99
90
113
77
11.9 31.6
10
3
E
W
5/86
-
10
RW
V
12/86
5
2
2 2 5
U
5/86
-
-
DW
t
5/86
5
5 5 5
k 1 m n
8
5/86
-
2 5 3 5 2 2
i j
12/86
analyte added, ppb
1.7 7.9 33.9 43.5
0.7 4.8
0.0
0.0
11.5 1.7 21.4
8.6 0.7 18.6
0.0
0.0
22.5
15.4
16 42 24.8 31.9
88
141
11.0
109
110
15.9 30.7
98
74
100
2.0 52.8
102 2.0 48.7
0.0
42.2 33.5 83.1
84
93
99
0.0
0.0
42.5 35.9 179.8
43.8 218.9
85 72
57.6 791.7 977.2 2.0 269.2 1397.5 2063.3
88
0.0
251.6 33.5 258.9
100 90
1192.3 1340.3
115 93
74
107 266
0.0
472.2 35.9 396.0
43.8 496.4
86/66'
94 72
91
99' 20.88
8d
791.7 1494.4
1192.3 1726.7
5.38
141
107
107f 638
98' 288
"RW, river water; I, sewage influent; E, sewage effluent; DW, distilled water. bFS,full scan low-resolution method; CA, higher resolution continuum acquisition method. 'Recovery = 100 X (found - initial level)/added. dThese samples were spiked so the DTDMAC concentration ranged from 1400 to 1800 ppb. They were also spiked with low levels of ClzTMACto see how accurately ClzTMAC can be measured in the presence of high DTDMAC levels (see Table I1 and text). 'Total number of samples that were prepared and analyzed for C12TMAC (86) and DTDMAC (66). 'Average recovery. Standard deviations. f
Table IV. Precision of Replicate ClzTMAC and DTDMAC determinations
analysis date 1/86 5/86 12/86 1/86 5/86 12/86 5/86 1/86 5/86 12/86 5/86 5/86 12/86
code matrixa a b c d e f g
RW RW RW
h i
E E
j k m
I I RW I
0
1
1
1 I I
no. of samples 5 5 5 5 5 5 5 5 5 3 5 3 3 5
CTgTMAC .FS CA addedb found' sd CVe found s 5 5
7.8 8.0
CV
1.3 16.2 0.3 4.1
DTDMAC FS CA added found s CV found s 5 0
5 0 0 0
50 50 50 200 250 250 500
7.9 1.6 19.4 18.8 2.5 13.3 33.5 1.5 4.5 35.9 5.7 15.9 42.5 1.8 4.2 52.8 0.8 1.5 83.1 6.7 8.1 179.8 22.3 12.4 251.6 7.8 3.1 258.9 51.4 19.9 396.0 88.8 22.4
4.8
0.1 12.2
43.8
1.6 3.7
5 0
50
17.6 23.5 27.9
0.6 3.4 3.2 13.1 2.6 9.4
CV
24.8 31.9
0.4 1.5 2.1 6.6
791.7 101.4 12.8 1192.3 57.3 2.0 3.5
33.7 2.8
218.9 15.0 6.9
200
1347.9 102.9 8.6 977.2 222.7 22.8 1304.0 89.4 6.9
496.4 43.7
250 500
2063.3 352.4 17.1 1494.4 566.6 37.9 1726.7 124.1 7.2
0
8.8
14.38 5.08 11.28 7.99 59/43f ORW, river water; I, sewage influent; E, sewage effluent; DW, distilled water. bThe concentration of analyte added to the native sample. Samples with 0 ppb added are native samples that were not spiked. 'Surfactant level found in the native or spiked sample. dStandard deviation. 'Coefficient of variation. /Total number of samples that were prepared and analyzed for ClzTMAC (59) and DTDMAC (43). 8 Average.
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
16-18 contain the corresponding CA results. It should be noted that many of these spiked samples were also used in the recovery studies summarized in Table 111. CIzTMACSummary. The average recoveries of theoretical spike were 99% or 86% (Table 111) depending on the method used. The standard deviations (s) about these values are 20.8 and 5.3, respectively. The coefficients of variation (CV's), for replicate determinations, averaged 11.2 or 7.9 (Table IV). The recoveries of the spiked quantities show considerable variation. With the low-resolution FS method, recoveries range from a high of 160% to a low of 72%. At the lowest analyte levels, at least part of this variation results because of an interference that occurs at the same nominal mass as the C12TMACmolecular cation. The interference is clearly evident in the high-resolution (M/hM= 6000) CA spectrum given in Figure 3a. In river water, the response of this interference is comparable to 1-3 ppb of C12TMAC. The signal simply adds to that of authentic ClzTMAC when the data are acquired by using the low-resolution FS method. It is, therefore, not surprising that the FS method yields variable, high values when applied to samples containing 1 1 0 ppb of Ci2TMAC. This interference is resolved with the higher resolution CA approach. Accordingly, at the lowest analyte levels, C12TMACvalues determined with the CA method are lower than those determined with the FS method. Both techniques are generally accurate to f 1 5 % above the 10 ppb level. The FS method is less reproducible than the CA method. This conclusion is based on the average CV's observed for the two techniques, Table IV. An even more direct comparison of the precision of these methods was obtained by analysis of four sets of samples (Table IV; d, g, k, 0) on the same occasion. The average CV was 17.7% with the FS method and only 7.9% using the CA approach. The CA method is more precise than the FS method because 2-5 times as many scans were averaged and because the data were obtained at a much slower magnet scan speed. Therefore, more time was spent actually integrating the ion currents of interest. DTDMAC Summary. The average recoveries of theoretical spike were 107% and 98% (Table 111) depending upon the method used. The standard deviations (s) about these values are 63 and 28, respectively. Clearly, the reliability of the DTDMAC results is worse than that of the C12TMAC results. The extremely large s, obtained with the FS method, results mainly because of errors in measurement at or below the 10 ppb level and above the 1000 ppb level. For example, some river water samples spiked with 5 and 10 ppb of DTDMAC showed recoveries of only 16 and 4270,respectively (Table 111; c, d and e, f). A t these levels, ion statistics may limit the accuracy of the method. On the other hand, the measured DTDMAC concentration in the 12/86 background influent sample was quite different with the FS (791 ppb) and CA (1192 ppb) methods. The value independently determined by HPLC, 1375 ppb, compared with the CA result. Similarly, results obtained on the 200 ppb spike of the 12/86 influent sample (Table 111;bb) were 977 ppb (FS)and 1340 ppb (CA). Finally, the recovery obtained for the 250 ppb spiked influent sample (Table 111; ee, ff) was 2.7 times greater than it should have been. We believe the FS method is inaccurate above the 1000 ppb level because of DTDMAC's limited solubility in glycerol. Poor analyte solubility results in unstable ion currents that have been shown to limit the accuracy of quantitative FAB-MS measurements (35). This stability problem is minimized with the CA method because more time is spent integrating the ion currents of interest. The FS method can be reliably applied to the analysis of samples having DTDMAC concentrations between 10 and at least 250 ppb. For this range of concentrations, the average
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Figure 3. Partlal FAB mass spectra for C,,TMAC
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observed recovery is 10370 and s = 10. On the other hand, the CA method shows reasonably consistent recoveries over the entire concentration range examined. The DTDMAC levels determined with the FS method were less reproducible than the results obtained with the CA technique (Table IV). The average CV's were 14.3 and 5.0, respectively. The discrepancy was even greater for the five sets of samples (Table IV; c, d, g, k, 0) that were analyzed by both techniques on the same occasion. For these samples, the average CV of the FS method was 19.3% compared to 5.0% for continuum acquisition. Again, most of the scatter associated with the full scan data resulted from the analyses of samples containing >lo00 ppb DTDMAC. Comparison with Other Results. The accuracy and precision of previously reported quantitative FAB-MS measurements depend upon the biological matrix, analyte level, and MS technique employed. Recovery of theoretical spike varied from 80% to 130% while the CV's varied from 1 % to 10% (29,35,36,46,50).The results obtained in this study, from the environmental matrices, compare well with the previous quantitative FAB-MS results obtained from biological matrices. FAB-MS is much more sensitive than HPLC (see below) and it yields quantitative results that are comparable to those obtained by HPLC. In this study, above 10 ppb, the average HPLC recoveries of theoretical spike were in the range of 90-95% and the coefficients of variation averaged -7%. The correlation observed between the quantitative results obtained with FAB-MS and the HPLC results is illustrated, for C,,TMAC, in Figure 4. These data were obtained from all the various environmental matrices. The slope of the line equals the expected value of 1.0 end the correlation coefficient is 0.977. Limit of Quantitation. The limit of quantitation achieved in the present study depends upon the andyte and the method used. With the low-resolution FS method, the limit for Cl2TMAC is a few parts per billion due to the chemical interference at the same nominal mass as that of the molecular cation (see Figure 3). However, with a mam resolution of 2000 to 3000, the limit of quantitation for the CA method should be
