Anal. Chem. 1991, 63, 2095-2099
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Identification of [ (Alkyloxy)polyethoxy]carboxylates in Raw and Drinking Water by Mass Spectrometry/Mass Spectrometry and Mass Determination Using Fast Atom Bombardment and Nonionic Surfactants as Internal Standards Francesc Ventura Aigiies de Barcelona, P. Sant Joan 39, 08009 Barcelona, Spain
Daniel Fraisse Seruice Central d’dnalyse CNRS, BP-22 F-69390 Vernaison, France Josep Caixach and Josep Rivera* Laboratori Espectrometria de Masses, Departament Quimica Ambiental, CID-CSIC, J. Girona 18, 08034 Barcelona, Spain
FABMS and FABMSIMS have been applled for the accurate mass measurement of two acklic metabolites of linear alcohol polyethoxylates present In raw and drlnklng water extracts that exhlbH the same exact masses and together wlth thelr correspondlng alcohols give the same serles of peaks. Nonylphenols and thelr acldk metabolhes, present In real samples, have been chosen as reference Ions for accurate mass measurement, thus avoldlng manlpulatlon of the sample.
INTRODUCTION Surfactants are a class of compounds with environmental concern owing to their widespread use and their high levels usually detected in water. The worldwide production of surfactants in 1988 (1)was around 2.8 million metric tons,with nonionics representing more than 29% of the total surfactant production. Their importance and bulk production, however, are growing faster in the surfactant market than that for anionics and cationics. The ether class accounts for about 73% of the total nonionic surfactants. The most commonly used members of this class are alkylpolyethoxylates, mainly nonylphenols (NPnEO’s) and linear alcohols (LAnEO’s). Nonylphenol polyethoxylates are probably the most studied nonionic surfactants in environmental analysis. This interest arises from their biotransformation under aerobic and anaerobic conditions into toxic metabolites (2) by degradation of the polyethoxyl chain, often including carboxylation of the terminal ethoxyl unit (NPnEC). Figure 1shows the structures, acronyms, and nomenclature used. Thus, since Sheldon and Hites (3) first reported the presence of alkylphenol polyethoxylates in surface waters, other authors (4-6) also identified them in raw and drinking waters, wastewaters (7-9), sewage sludges (lo), and municipal landfill leachates ( 1 1 ) . Acidic metabolites were also detected in sewage sludges (12) and raw and drinking waters (13). Moreover, halogenated residues of NPnEO’s and NPnEC’s can be produced during chlorination in wastewaters or in drinking water treatment plants, and therefore, halogenated derivatives were detected in raw water (3), chlorinated wastewaters (14-16), and tap water (13). On the other hand, NPnEO’s have recently been widely replaced in the US. Germany, and Switzerland (18) by linear alcohol polyethoxylates (LAnEO’s). At present, these compounds account for more than 19% of the 1988 total
(In,
0003-2700/9 1/0383-2095$02.50/0
surfactant production (I). Although they have been identified in raw and drinking waters (13,191,they are seldom reported in the literature in spite of their increasing production. The corresponding acidic metabolites of LAnEO commonly described in the literature (17,20) are those produced by carboxylation of the alkyl chain (LAnAC). Acidic metabolites of LAnEO of the same type as NPnEC have so far not been reported to occur in water to our knowledge. Although GC/MS and HPLC have been commonly used for the identification and quantification of NPnEO and NPnEC surfactants (3, 21, 22), the use of fast atom bombardment (FAB) has proved to be very useful for the characterization of nonvolatile organic compounds including surfactants (23, 24). Thus, qualitative identification of individual surfactants and brominated derivatives in raw and drinking waters by FABMS has been accomplished (13,19). In addition, recently Keough et al. (25) quantitatively determined trace levels of cationic surfactant5 in sewage effluents and river water, whereas Wemery and P e d e (26) did the same with the anionic alkylbenzenesulfonate surfactants. This paper reports the qualitative identification of LAnEOs and LAnEC’s in raw and drinking water extracts of Barcelona (N.E. Spain) by accurate mass measurements using FABMS and MS/MS.
EXPERIMENTAL SECTION [ (Lauryloxy)polyethoxy]acetic acid and [ (myristy1oxy)poly-
ethoxylacetic acid were a gift of Eytesa (Barcelona, Spain) and were used as received. Sampling and Extraction Procedures. Acidic and base + neutral fractions of raw and drinking water extracts were obtained by a classical analytical scheme. The extracts represent monthly average water samples, and organic pollutants were obtained by passing water (ca 2000 L) through Amberlite XAD-2 or granular activated carbon of the same type as used in the water works plant of Barcelona. Both fractions were analyzed by GC/MS and then by FABMS. The acidic compounds were analyzed as methyl esters after conventional derivatization with BF3/MeOH (27). FAFJMS. Mass spectra were obtained with a MS9 VG updated or a ZAB-SEQ (VG instruments, Mancheeter U.K.) mass spectrometer. FAB was performed with a 1.1-mA beam of a cesium-ion gun. Thioglycerol saturated with NaCl was a suitable matrix for real samples. For accurate mass measurement, a mixture of glycerol and thioglycerol (1:l)with NaCl was used. The mass spectrometer at a resolution of 5000 was scanned at 10 s/decade for limited mass ranges of 50-55 amu for each unknown. An average of five scans was recorded by using VG continuum acquisition software in a VG 11/250J data system. 0 199 I American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 03, NO. 19, OCTOBER 1, 1991 Table I. Accurate Mass Measurement of Compounds Present in Base Neutral Fractions of Raw Water
+
unknown
LAnEO
CmH2m+l -O-(CH2-CH2-O),-H
LAnEC
CmHz,+~-O-(CH~-CH~-O),-CH~-COOH
LAnAC
HOOC-(CHz),.,-O-(CH2-CH,-O),-H
mlz
ion
calcd
obsd
error, ppm
399 385
[M+ Na]+ [M+ Na]+
399.3086 385.2930
399.3097 385.2938
2.1 2.1
f r
n = number of ethoxy units and 12 S m i 15
9:
Flgure 1. Molecular formulae and acronyms used for the studied
compounds.
I
I
6.
‘
1.
n. II. 6.
a. 8.
P.
Flgure 3. FA6 (+) spectrum. A+cidic fraction (as methyl esters)of raw water. (0)NPnEC [M Na] = 447, n =: 3; (+) LAnEO, LAnEC,
+
or LAnAC compounds: (V)fragments of NPnEC.
Figure 2. FA6 (+) spectrum. Base entering the water works plant. [M
+ neutrals fraction of raw water
+ Na]’
peaks. (0)NPnEO; (0) polyethoxylated lauryl alcohol; (*) polyethoxylated tridecyl alcohol. Exact mass measurement was also performed by using manual peak matching. In this case, the instrument resolution was set at 12500 (width at 10% height). For the base + neutral fractions, the references used as internal standards were the NPnEO’s present in the samples, whereas for the acidic fraction the previously identified NPnEC’s were chosen for the same purpose. Mass Spectrometry/Mass Spectrometry. MS/MS spectra were obtained at 10 eV with the ZAB-SEQ instrument, which uses a rf-only quadrupole collision cell and a quadrupole analyzer. Argon was employed as the collision gas and its pressure adjusted to an observed reduction of 50% of the parent ion. A mixture of D’IT/DTE (31)was used in order to obtain daughter ions from [M + H]+ peaks, usually more informative in MS/MS than [M + Na]+.
RESULTS AND DISCUSSION Figure 2 shows, as an example, the positive FAB spectra of a base neutral fraction of raw water. Polyethoxylated nonionic surfactants in FAB spectra can be easily recognized by their [M + Na]+ ions separated by 44 units corresponding to different degrees of polyethoxylation. The formation of [M Na]+ ions instead of [M + H]+ is enhanced by salt added to the matrix. Ventura et al. (19) provided a table in the range of m / z 200-800 for rapid identification of surfactants in water samples. Thus, the main identified compounds in Figure 2 are phthalates, NPnEO’s and LAnEO’s. The univocal identification of NPnEOs was confirmed by comparison of the CID-MIKE spectra of the standard and the same peak in the real sample. The presence of nonylphenols and their brominated derivatives (BrNPnEO’s) in tap water with a low degree of polyethoxylation (n = 0-3) was observed in its FAB spectrum and confirmed by GC/MS (E1 mode). One major disadvantage of FAB arises from those compounds that can suppress ionization of others present in the samples and matrix background. Moreover, when two or more compounds give the same series of peaks,reliable identification car, be achieved by comparison of both negative and positive
+
+
FAB, tandem MS, or accurate mass measurement. In this latter technique, external or internal standards can be used, but more accurate mass measurements are obtained with internal standards. In this case, the main problem is finding out which compounds are the most suitable since this may be difficult and time consuming. This standard would require no suppression of the unknown(s),not to be suppressed by the unknown and to give steady signals over a wide mass range. Thus,with this aim, mixtures of different poly(ethy1ene glycols) (28, 29), an amphoteric surfactant (30),and amino acids (31)have been used. Recently, Siege1 et al. (32) used isolated nonionic surfactant fractions to calculate the accurate mass of a glycopeptide. In our case, we overcame the difficulty of introducing appropriate internal standards in our samples by using the polyethoxylated nonylphenols and their acidic metabolites as reference ions in the base + neutral and acidic fractions, respectively. Thus, as they are always present in our samples, spaced only by 44 mass units and with an intensity slightly superior than that for unidentified compounds, no manipulation of the sample was required. In order to evaluate its suitability, we calculated the exact mass of peaks m / z 399 and 385 of Figure 2 that were previously assigned to [M + Na]+ ions of tetraethoxylated tridecyl and lauryl alcohols, respectively. Table I shows the obtained results, the feasibility of the method, and the correct assignation of compounds. We used m / z 375.2506 and 419.2768 as reference ions, which correspond to exact masses of [M + Na]+ ions of NP3EO and NP4E0, respectively. Figure 3 displays the positive FAB spectrum of an acidic fraction of raw water. Acids were prior derivatized to methyl esters and analyzed by GC/MS. The main identified compounds were NPnEC’s ranging from n = 1 to 6 with their fragments at m / z 251,265, and 279, which were the same as those observed in their E1 spectra (6,33). The base peak of the FAB spectrum (not shown) was m / z 117, corresponding to the [CHzCHzOCHzCOOCH3]+ion. Compounds at m/z 383, 427, 471, ..., were primarily assigned to [M + Na]+ ions of polyethoxylated pentadecyl alcohol. The FAB spectrum of the acidic fraction of tap water (see Figure 4) also showed the presence of NPnEC’s and polyethoxylated compounds that were primarily assigned to LAnEO’s. Nevertheless, as no NPnEO’s were present in this
ANALYTICAL CHEMISTRY, VOL. 63,NO. 19, OCTOBER 1, 1991 2097
* *
188-
515
01
?a. 88.
*
a.
I
68.
553 "1
FAB (+) spectrum. Acidic fraction (as methyl esters) of tap water. [M 4- Na]+ peaks. (0)NPnEC; ( * ) BrNPnEC; (*,., A) LAnEO, LAnEC, or LAnAC compounds. Flgure 4.
Table 11. Accurate Mass Measurement of Compounds Present in the Acidic Fractions of Raw and Tap Waters and Assignment of Structures unknown sample m / z ion raw tap
obsd
calcd for LAnEC, error, calcd for LAnAC ppm LAnEO
383 [M + Na]+ 383.2758O 383.2773 383.277gb 413 [M + Na]+ 413.2834 413.2879 427 [M + Na]+ 427.3047O 427.3035
3.9 1.6 9.7 2.8
Figure 5. FAB (+) spectrum of a commercial mixture of [(lauryloxy)polyethoxy]acetic acid and [(myristyloxy)polyethoxy]acetlcacid [M+(analyzed as acid). (). [M+H]+ and (A)[M+Na]+ for CI2; (0) H I + and ( 0 )[M+Na]+ for C,4; (0)for lauryl laurate. 1
i
Continuum acquisition. Manual peak matching.
,I
,i"
A
133
383.3137 413.3243 427.3399
B
.*g IS II
' + [ , A
I I 51
fraction, we used NPnEC's as internal standards to calculate the exact mass of the peak at mlz 383 from the raw water acidic fraction (Figure 3) as well as those at mlz 413 and 427 from the acidic fraction of tap water (Figure 4). Table I1 shows the obtained results. The reference ions for raw water were mlz 359.2198 and 403.2460, which correspond to [M + Na]+ ions of the methyl esters of NPlEC's and NF'2EC's. We used mlz 403.2460 and 447.2423, corresponding to NP2EC's and NP~EC'S,respectively, for tap water. These observed accurate masses were in disagreement with the presumed alcohols. Thus, we thought that these compounds could be the sodium methyl esters of acidic metabolites of LAnEO's. We calculated their expected [M Na]+ ions for these compounds with an alkyl chain ranging from C12to CIS. The series obtained for the different degrees of polyethoxylation and alcohols led us to observe the same series of peaks for LA,nEO, LA,&-l)EC and LA,(n-l)AC, m being the number of carbon atoms of the alkyl chain. For example, triethoxylated myristyl alcohol will give the same series of peaks as the methyl esters of [(lauryloxy)diethoxylacetic acid and 12-diethoxymyristic acid. Indeed, as Figure 5 shows, the positive FAB spectrum of a commercial mixture of lauryl and myristyl ether polyethoxyacetic acids gave the expected [M + Na]+ ions. With this assumption, we calculated the exact masses of the alcohols that give the same series of peaks as the real samples (see Table 11). As expected, manual peak matching gave better accuracy than computerized continuum acquisition within an acceptable range of measured errors. The major difference observed in accuracy between our results and those reported by other authors for different products could be explained on the basis that our measurements were made in complex mixtures whereas the examples reported in the literature usually referred to a single compound. Thus, peaks at mlz 383,427, and 413 correspond to acidic metabolites of the LAnEC or LAnAC type and the discarded
+
B 329
221 .I
I58
n
1
E4
JSl
u
N
C
m
267
5.
8. 5.
n. 5.
n. a. 0. 5.
m. 4.
a. 5.
n. El.
a. 15. I1
A
240
0
I I
A
0
284
3 ~ $ 1
a. Y. 45.
a. 5.
1. 5.
a.
Figure 6. (a, top) MS/MS
spectrum of LA5EO (Cl,). Parent ion [M
+ H I + = 435. (b, middle) MSlMS spectrum of LA4EC (C12). Parent ion [M + HI+ = 435. (0)Traces of LASE0 (C,4). (c, bottom) MS/MS spectrum of an unknown compound present in tap water (see Figure 4, marked with an 1). Parent ion [M + HI+ = 435.
compounds were tri- and tetraethoxylated pentadecyl alcohols and tetraethoxylated myristic alcohol, respectively. As both types of compounds had the same empirical formula, accurate
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
Scheme I A
H
0
C 1 4 H 2 9 \ 0 e $ / +
+
oH*o)H
mr ml
: :
1-2
m2
CONCLUSIONS The use of FABMS and MS/MS has allowed us to qualitative identify the presence of different surfactants. Among them, polyethoxylated linear alcohols and their acidic metabolites have been found by means of accurate mass measurement using the previously identified NPnEO and NPnEC compounds as reference ions. The presence of these compounds in real samples, with homologues separated only by 44 mass units and a slight intensity referred to unidentified components, has overcome the difficulty of finding out suitable internal standards and has allowed accurate mass measurements.
0
0
C
-
H
LAnEO C41H23
+
C11H23\/\\
H2°+*0iH
ACKNOWLEDGMENT We are grateful to M. Guerra and X. Huguet for assistance in obtaining FAB spectra. Registry No. LAnEC (n = 12), 27306-90-7;LAnEC (n = 14), 38720-61-5; water, 7732-18-5; nonylphenol, 25154-52-3.
n = 5
CgH19\/\\
+ H20+*Oix
n = 4
0
OCH3
mass measurement was not enough to establish which type of compound was present in our extracts, and therefore, tandem MS was employed to distinguish them. Mass Spectrometry/Mass Spectrometry. Figure 6 shows the daughter spectra at 10 eV of the m/z 435 [M + H]+ ion from pentaethoxylated myristil alcohol, [ (1auryloxy)tetraethoxylacetic acid, and a peak present in the FAB spectrum of the acidic fraction of drinking water. This peak corresponds to the next homologue of m / z 413 [M + Na]+ whose exact mass was calculated (see Table II). For this study, MS/MS of [M H]+instead of [M + Na]+ was used in order to obtain more structural information. The daughter ions of both standards can be rationalized according the Scheme I. LAnEO (89,133,177,221)and LAnEC (117,161,205,249) are A-type ions formed by charge site initiated decomposition. LAnEO (285,329) and LAnEC (257,301) are B-type ions formed in the same way as described for A. LAnEO (239) and LAnEC (267) are C-type ions formed by ether cleavage and hydrogen transfer. The fragments described here are in agreement with those obtained for polyglycols by Lattimer et al. (34)and recently for some nonionic surfactants by Kalinoski and Hargiss (35). These authors suggested that a possible mechanism to explain the C-type ions may be analogous to the path described by Vettori et al. (36) in the E1 spectra of linear alcohol polyethoxylates. Comparing both spectra and the compound present in drinking water, the polyethoxylated alcohol could be dis-
+
carded, as we already found by accurate mass measurement. The base peak at m/z 267 and that at m / z 117 of the acidic metabolite of the LAnEC type (Figure 6b) can be considered as diagnostic ions. Thus, m / z 117 assigned to [+CHzCHzOCHzCOOCH3] and m / z 267 t o [HzO+(CHzCHz0)4CHzCOOCH3] cannot be present in acidic metabolites of the LAnAC type. The unknown compound of drinking water (Figure 6c) is therefore [ (lauryloxy)tetraethoxy]acetic acid, although traces of 12-tetraethoxymyristic acid can be rationalized by ions at m / z 239,403, and 417 not formed in metabolites of the LAnEC type. Thus, the compounds present in the acidic fractions were (m) LAnEC (for Clz), (+) LAnEC (for C13),and (A) LAnEC (for C14). With the obtained results, we conclude that NPnEO’s (n = &lo), NPnEC’s (n = 0-7), LAnEO’s (n = 0-8),and LAnEC’s (n = 0-6) were present in raw water, whereas the same compounds with a lower degree of polyethoxylation, BrNPnEO’s (n 5 10) and BrNPnEC’s (n Ilo), were present in tap water.
LITERATURE CITED (1) Greek, B. F. Chem. Eng. News 1990, 68 (5), 37. (2) Swedish EnviromentalProtection Agency. Report 3907,Saltsjhden, Sweden, 1991. (3) Sheldon, L. S.;Hites, R. A. Environ. Scl. Techno/. 1978, 72, 1188. (4) Otsuki, A.; Shirashi, H. Anal. Chem. 1979, 57, 2329. (5) Crathorne, 6.; Fielding, M.; Steel, C. P.; Watts, C. D. Environ. Scl. Techno/. 1984, 78, 797. (6) Rivera, J.; Fraisse, D.; Ventura, F.; Caixach, J.; Figueras, A. Fresenius’ Z.Anal. Chem. 1987, 328, 577. (7) Giger, W.; Stephanou, E.; Schaffner, C. Chemosphere 1981, 10,
1253. (8) Stephanou, E.; Giger, W. Environ. Scl. Technol. 1982, 76, 800. (9) Reinhard, M.; Goodman, N. L.; McCarty, P. L.; Argo, D. G. J. Am. Water Works Assoc. 1988, 78, 163. (10) Giger, W.; Brunner, P. H.; Schaffner, C. Sclence, 1984, 225, 623. (11) Reinhard, M.; Goodman, N. L.; Barker, J. F. Environ. Sci. Techno/. 1984, 78, 953. (12) Ahei, M.; Conrad, T.; Giger, W. Environ. Scl. Techno/. 1987, 2 7 , 697. (13) VentuFa, F.; Figueras, A.; Caixach, J.; Espadaler, I.; Romero, J.; Guardlola, J.; Rivera, J. WaterRes. 1988. 70, 1211. (14) Bail, H. A.; Reinhard, M. I n Water ChMnat&n; Joiiey, R. L., Ed.; Lewls Publishers: Chelsea MI, 1985;Vol. 5, p 1505. (15) Reinhard, M.; Goodman, N. L.; Morteimans, K. L. Environ. Scl. TechM I . 1982, 76, 351. (16) Ball, H. A,; Reinhard, M.; McCarty, P. L. Environ. Sci. Techno/. 1989, 2 3 , 951. (17) Swisher, R. D. Surfactant Bicdegradation; Marcel Dekker Inc.: New York 1987. (18) Marcomini, A.; Fiiipuzzi, F.; Giger, W. Chemosphere 1988, 5 , 853. (19) Ventura, F.: Caixach. J.; Figueras. A.: Eswdaier. I.; Fraisse. D.: Riv’ era, J. WaterRes. 1989, 2 3 , 1191. (20) Vashon, R. D.; Schwab, B. S. Environ. Scl. Technol. 1982, 16, 433. (21) Gioer. W.: Ahei. M. Anal. Chem. 1885. 57. 2584. (22) G e r , w.; Marcomini, A. Anal. C h m . lS87, 59, 1709. (23) Lyon, P. A.; Stebbings, W. L.; Crow, F. W.; Tomer, K. 6.; Lippstreu, D. L.; Oross, M. L. Anal. Chem. 1984, 5 6 , 8. (24) Lyon, P. A.; Crow, F. W.; Tomer, K. 6.; Gsoss. M. L. Anal. Chem. 1984, 5 6 , 2278. (25) Keough, T.; Simms, J. R.; Ward, S. R.; Moore, B. L.; Bandurraga, M. L. Anal. Chem. 1988, 6 0 , 2613.
Anal. Chem. 1981, 63,2099-2105 (26) Wernery. J. D.; Peake, D. A. RapUCommun. Mess Spectrom. 1989. 3,396. (27) Bleu, K.; King, 0. S. Hendbodc of Derfvetives tor Chromatography; Heyden: Chichester, U.K., 1978. (28) Gilllam, J. M.; Landis, P. W.; Occolowitz, J. L. Anal. Chem. 1984, 56, 2285. (29) Goad, L. J.; Prescott, C. M.; Rose, M. E. Org. Mass Spectrom. 1984, 19, 101. (30) Gilllam, J. M.; Landis, P. W.; Occolowitz, J. L. Anal. Chem. 1983, 55, 1531. (31) Kaise, T.; Watanabe, S.; Ito. K.; Hanaoka, K.; Tagawa, S.; Hirayama, T.; Fukui, S. Chemosphere 1987, 16, 91. (32) Slegei. M. M.; Tseo, R.; Oppenhelmer, S.; Chang, T. Anal. Chem. 1990, 62, 322. (33) Stephanou, E.; Relnhard, M. M.; Ball, H. A. Bbnmd. Envlron. Mess Spectrom. 1088, 15, 275.
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(34) Lattimer. R. P.; Munster. H.; Budzikiewicr, H. Int. J . Mess Spectrom. Ion Processes 1990, 90. 119. (35) Kallnoskl. H. T.; Harglss. L. 0.38th ASMS Conference on Mass Spectrometry and Allied Topics, Tucson, AZ, 1990 p 361. (36) Vettori, U.; Issa, S.; Maffei Facino, R.; Carinl, M. Biomed. Envlron. Mass Spectrom. 1988, 17. 193.
RECEIVED for review January 14,1991. Revised manuscript received June 11, 1991. Accepted June 17, 1991. Financial support was provided by CICYT (Comisi6n Interministerial de Ciencia y Tecnologia) and CSIC (Consejo Superior de Investigaciones Cientificas) Grant PB87-0392.
Sample Introduction by On-Line Two-Stage Solvent Extraction and Back-Extraction To Eliminate Matrix Interference and To Enhance Sensitivity in the Determination of Rare-Earth Elements with Inductively Coupled Plasma Mass Spectrometry Mohammad B. Shabani* and Akimasa Masuda Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo 113, Tokyo, Japan An on-line continuous two-stage solvent extraction and back-extraction system for the isolation and concentration of rare-earth elements (REEs) from matrix elements has been developed. Two s t e p of extraction and back-extraction are linked together by muillchannel pumping with the final backextracts in aqueous soiutlon being introduced to the Inductively coupled plasma mass spectrometer (ICPMS). A mixture of 65 % bls(2-ethyihexyi) hydrogen phosphate (HDEHP) and 35 % P-ethyihexyi dihydrogen phosphate (H,MEHP) In heptane to ussd as the extracting agent, and octyl alcohol and nitric acid are used for the back-extraction. Parameters affecting extraction and back-extraction of REEs in the system were examined. These include extraction coil length, pH of sample solution, concentration of compiexing agent, backextraction coil length, amount of octyl alcohol, and concentratlon of nitric acid for back extraction. Potential problems associated with matrix elements and deposition on the Sampiing cone have been overcome. Analytical characteristics represent significant improvement In ICPMS sensitivity and an improvement in the accuracy and precision in the measurement of natural samples with a significant matrix. The entire process of extraction and back-extraction prior to introduction of the sample to the ICPMS can be carried out within 4 min, and a preconcentrationfactor of up to 1 order of magnitude for synthetic seawater has been achieved. By application of the proposed method, geological samples with final diiutlon factors of 5 could be introduced to the nebuilzer of the ICPMS with negilgibie matrix problems. Thls represents a 100-fold improvement in sample senstlivity as compared to direct sample introduction with a dilution factor of 500. Finally, the method has been successtuiiy applied in determinatlon of REEs in synthetic seawater, alkaii-fused JG,, standard rock, and acld-decomposed samples of JB, standard rock and Jiiin chondrite.
INTRODUCTION Inductively coupled plasma mass spectrometry (ICPMS) has been widely recognized as a suitable technique for the 0003-2700/91/0363-2099$02.50/0
determination of trace elements, with particular advantages of its high sensitivity, large dynamic range and low background ( I , 2). However, in a number of papers it has been reported that many problems related to mutual effeds involving matrix interferences exist in this technique (3-5). In particular, it has been noticed that determination of trace elements in natural materials by ICPMS is strongly affected by two problems: suppression or enhancement of ion intensity by the matrix and gradual drift in sensitivity by deposition of matrix elements on the sampling cone (3, 4, 6). In order to reduce the matrix-associated problems mentioned above, a number of approaches for elemental analyses have been suggested. These include matrix matching (7), standard additions (8),and internal standardization (9-1 1). However, these compensation methods have not always been successful since the matrix problems due to concomitant elements are very complicated (4). In addition, because of the low concentrations of many elements of interest in natural materials, the dissolution procedure is also an especially critical step. It has been reported that in some rock types the REEs are contained in mineral phases which are resistant to acid attack (12-15). In such rocks, samples should be decomposed by the alkali fusion method. This method of decomposition, however, results in a solution containing a high level of dissolved solids. Therefore, direct introduction of such a solution to the sampling system of ICPMS is restricted (4, 12). Pretreatment prior to instrumental analysis solves important problems related to matrix and sensitivity in a large number of cases of determination and extends the range of application of the instrument in a rather substantial manner. Solvent extraction has been proved to be an effective mean of increasing sensitivity as well as a means of removing matrix elements (16, 17). Although ICPMS instrumental determination is rapid and can handle a large number of samples, the manual solvent extraction preconcentration is tedious and time-consuming and incompatible with the final rapid determination step. Recently, large advances have been made in developing flow injection systems for on-line solvent extraction with the aim of speeding up and simplifying the preconcentration step. The concept and applicability of the method has undergone rapid 0 1991 American Chemical Society
