2653
Anal. Chem. 1887, 59, 2653-2658
Fast Atom Bombardment Combined with Tandem Mass Spectrometry for Structural Studies of Substituted Cyclic Perfluoroalkanesulfonates Sharon L. Hunt, Fred E. Behr, L a r r y D. Winter, a n d Philip A. Lyon* 3M, 201BW09, St. Paul, Minnesota 55144 Ronald L. Cerny, Kenneth B. Tomer, and
M.L. Gross
Midwest Center for Mass Spectrometry, University of Nebraska-Lincoln,
Fast atom bombardment (FAB) and tandem mass spectrometry were demonstrated to be useful for characterlrlng cyclk perlluoroalkanesulfonates. Abundant parent Ions appeared In both the posltlve and negatlve Ion FAB spectra, yleldlng molecular welght and counterlon Informatlon. Because these stable parent Ions undergo llttle fragmentation, collision actlvated dlssoclatlon (CAD) spectra were acqulred to provide structural Information. The CAD spectra of parent anlons yield lnformatlon to verity the cyclic nature of the compounds and to elucldate the slze, number, and nature of the rlng substltuent. Relathre abundances of CAD fragments suggest greater stablltty for the cyclk carbanlons compared to thelr straight chain analogues.
Ionic fluorochemicals constitute an unusual and commercially important class of anionic surfactants. Fluorochemical surfactants are both extremely stable and highly surface active, two properties essential to many industrial applications. Perfluoroalkanesulfonates are able to perform as surfactants even in harsh environments. These compounds are surface active in such diverse media as strong acid, strong base, highly ionic, and oxidizing solutions. The fluorocarbon sulfonic acids or salts show no thermal decomposition up to 350 "C. Although the synthetic route to the compounds included in this study begins with pure hydrocarbons, a mixture of materials results from the process of electrochemical fluorination (1-3). Extensive purification is normally not possible nor necessary for the compounds to satisfy the requirements of most commercial surfactant applications. Because their properties as surfactants depend on the nature of the mixture, characterization methods are required. Previous work has demonstrated the utility of fast atom bombardment and tandem mass spectrometry for the characterization of straight chain perfluoroalkanesulfonatesalts (4) and other cationic and anionic surfactants (5, 6). In this paper we expanded upon the perfluoroalkanesulfonate study by demonstrating the applicability of this technique for cyclic and substituted cyclic compounds. The structures of the perfluoroalkanesulfonateshave a large impact upon their surface activity. The presence of CF, groups greatly improves the surface tension reducing ability of the fluorocarbons. Zisman (7)has reported the critical surface tension of CF3terminated fluorocarbon groups to be 6 dyn/cm, the lowest ever reported. Although the terminal group (CFJ has considerableimpact upon the surface energy, the length of the carbon chain is also important. Fluorocarbons containing less than six carbons were found to be impractical as surfactants (9). This paper is a report of the mass spectra of positive and negative ions generated by FAB from a series of nine cyclic perfluoroalkanesulfonates. Information obtained from the 0003-2700/87/0359-2653$01.50/0
Lincoln, Nebraska 68588
normal FAI3 mass spectra is compared to that resulting from collisionally activated dissociations. Differentiation of isomers and homologues is addressed and the saturated cyclic systems are compared with aromatic analogues. EXPERIMENTAL SECTION The mass spectra were obtained with the Kratos MS-50 triple analyzer mass spectrometer at the University of Nebraska (8)or the Kratos MS50RF TA recently installed at 3M (9). The Kratos MS50RF TA was designed and built to possess both high maw range and high-resolutioncapabilities by using an EBE configuration. An inhomogeneous fully laminated magnet was fitted to a standard double focusing MS-50 that functions as the fiist analyzer (MS-I)of the tandem. This magnet has a mass range in exceea of 8OOO at fullacceleratingvoltage (8 kV). The first mms spectrometer of both tandem instruments is capable of delivering a highly resolved ion beam ( R = 100000 static) that can be sampled by an intermediate detector in the third field free region (for normal FAB survey spectra) or passed into a collision cell. The cell of the MS5ORF TA was constructed with entrance and exit lenses to permit fragmentation products to be refocused into the third analyzer (an ESA), which functions as MS-11. A standard Kratos FAB source equipped with an Ion Tech atom gun was used. The samples were dissolved in glycerol or triethanolamine for analysis. A small drop of the sample solution was placed on the copper target of the FAB direct insertion probe and bombarded with &keV xenon atoms. The ions produced were accelerated through 8 kV into the analyzer region of the mass spectrometer. CAD spectra were obtained by selecting the appropriate ion with MS-I at R = 3000 resolution and then introducing sufficient helium into the collision cell located in the third field-free region to give 50% reduction of the intensity of the selected ion beam. The CAD spectra, obtained by scanning MS-11, were signal averaged and processed with software written at the University of Nebraska (IO). Each CAD spectrum reported here was an average of at least 10 20-8 scans. Most chemicals used in this study were obtained from 3M Industrial ChemicalsProducts Division. Others are commercially available from other sources (Fluorochem Ltd., Glossop, Derbyshire). The compounds were used without further purification. The purity of these materials is discussed in the Results and Discussion section of this paper. RESULTS AND DISCUSSION The purpose of this paper is to develop an understanding of the desorption and gas-phase ion chemistry of cyclic perfluoroalkanesulfonates(see Table I) so that suitable strategies for analysis and structure determination can be devised. To that end, mass spectra of both positive and negative ions that are desorbed were taken. These spectra are listed in Table I1 and 111, respectively and will be discussed first. The collisionally activated decompositions (CAD) are a means to study the ion chemistry and they are discussed later. Mass Spectra of Positive and Negative Ions Desorbed by FAB. The positive and negative ion mass spectra generated by FAB of the samples are relatively simple (see Figure 0 1987 American Chemical Society
2654
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
Table I Compound Name
Structure
Compound
(f>
pertluo(ocyclohexanes~lfonafe
SO,K
I
Table 111. Relative Abundance of FAB-Generated Negative Cluster Ions of Perfluoroalkysulfonate Salts
Fragment
@-
111
CF,
1v
v
C2F,
VI
s-C4F,
VI1
KSO,CF,
a a a a
SOIK
p-trifluoromelhylpe~tluorocylohelanesulfonale
C,F,SO,K
pertluorocyclohexylletrafluoroelhanesultonale
[L+38]
-@-
12
[MI
p-penlafluoroethylperflu~~ocyclohexanesultonale
S03K
SO,K
p-nonalluoro-1 -melhylpropylperlluorocyclohexanesullonale
CF,SO,K
perfluorocylohexyl~l.4-di(dlll~0r0melhanesullonale)
12
16
33
15
11
~
37
30
~
1 '
1
39 1
01
38
'
171
-
2
65
Compound VI1 is a disulfonatethus an additional metal should be added to each cluster (i.e., L- for other compounds is analogous to LM- for the difunctional compound).
.i
perll~~~Oben~enesuIfonale
@-
benzenesultonale
SO,K
*
'O
1
'O
1
IO 20
CF,+O,K'
4
-
Table 11. Relative Abundance of FAB-Generated Positive Cluster Ions of Perfluoroalkysulfonate Salts '-,
;
100,
SO,K
IX
1 I
iL-381
0 .,
VI11
, ,
p~rtl~o~ocycloheXyldIfluoromelhanesultonale
CF,SO,K
I1
'\
,I
l 489LK)K'
I
Compound1
Fragment'
1 4~
[LM2-38]'
-6 1
41
3 0 ' 1 2 ' 261 921
I [LM2-18]' [LM,]'
'
I
1
100
I
-I 100
'
5 '
7
[LM2-38+Gly]'
3
1 4 ~7 ,
36
IO0
200
300
4OE
IOC
6ca
':c
tl/?
I
100
100
100
100
7
i -
2
5
3
3
4
7
12
131
-
-
I -
-
[ LM, + Gly ] '
4
5
[LM2+38tGly].
1
-
2
13
8
9
L2M;
7 ' 17'
~
100
100
Figure 1. FAB spectrum of positive ions from compound 111.
I
[LM2+38]'
[LMZ-18+Gly]+
527
-
1
3 ,
~
6 1
5
1
11
1 -
81
-
2
-
7
- 1
21
1
-
1
-
6 1
-
2
-
-
539
19
Compound VI1 is a disulfonate thus an additional metal should be added to each cluster (i.e., LM2+for other compounds is analogous to LM3+ for the difunctional compound).
1 1 0 ~ ~
I
1 for an example) as expected from previous work on perfluoroalkanesulfonates (4). The sputtered ions arise from five sources: solute-solute clusters, solute-solvent clusters, solvent-solvent clusters, chemical impurities, and fragmentation. Using FAB in the positive ion mode, we find that the most abundant high-mass ion desorbed is LM2+,where L represents the perfluoroalkylsulfonate ion and M is the metal counter ion (usually K+). Solute-solute clusters of the type (LM),M+ (series A) are formed by the addition of LM neutrals to LM2+. The mass range of the mass spectrometer used for these experiments limited the observable clusters to n = 3 for smaller compounds and n = 2 for the larger molecular weight species. In extended mass range, available with the 3M mass spectrometer, compound V exhibits clusters up to (LM)8M+ (Figure 2).
866
1608
2468 MI2
3298
moa
Flgure 2. Extended range FAB spectrum of positive ions from compound V.
Another cluster ion series (B) is observed 38 mass units lower than the predominant (LM),M+ clusters. These ions could arise from three of the five sources outlined above. The ions could be solute-solute clusters with a hydrogen substituted for one of the potassium counter ions resulting in (LK),H clusters instead of (LK),K clusters. Another source for these ions is a chemical impurity having one additional site of unsaturation, either a bridge or a double bond in the ring. Third, the ions could be fragments formed by a fluorine loss from the (LM),M+ cluster ions. The addition of sodium chloride to the sample-glycerol mixture resulted in the dis-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
placement of the potassium ions by'the sodium ions. In the ethyl-substituted compound, C2F5C6FI0SO3K, the major ion shifted from m/z 539 to 507, confirming the sodium substitution. Just as the A series was shifted by 32 mass units, there was a corresponding displacement in the B series, indicating that the B series clusters contain only potassium counter ions and are not hydrogen containing. The same mass shifts were observed when sodium exchange was performed on the butyl compound VI. Thus, the B series ions are deficient in two fluorine atoms with respect to the A series. The B series originates either from fragmentation or impurities. Determination of an olefinic impurity in these cyclic perfluoroalkanesulfonates was not successful by NMR because of the complexity of the fluorine NMR spectra. However, the mass spectrum of FAB-generated negative ions from compound V contains ions of m / z 463,38 u lower than the mass of the ethylcyclohexanesulfonate anion ( m / z 501). Because no counterions are present in the ion of m/z 501, this eliminates the possibility that the B series ions contain hydrogen as one of their counter ions. The CAD of the series A ions (discussed later) do not show the Fz losses expected if the series B ions resulted from fragmentation. Series C ions appear 38 mass units higher than the (LM)&Z+cluster ions. A chemical impurity accounts for this cluster series found in the mass spectrum of FAB-generated positive and negative ions from all of saturated cyclic compounds studied. The mass shift suggests that the impurity is most likely a ring-opened form of the cyclic compound. The CAD of these ions contain structural information and will be discussed later in this paper. Cations at [(LM),M - 181' and anions a t [L(LM), - 181also desorb from solutions of several of the compounds. Because the compounds were formed by electrochemical fluorination of hydrocarbon starting materials, these ions are probably low-level chemical impurities resulting from incomplete fluorination. Neither proton nor fluorine NMR could confirm the presence of hydrogen-containing compounds in these samples. However, the concentrations of these impurities may be below the detection limits of NMR. The composition of this chemical impurity was determined by peak matching the ion of m/z 421 present in I. The measured mass fits to within 2.1 ppm of theoretical mass of C6F9HSO3K2. Lower mass ions of m/z 131 and 169 were previously observed in perfluoroalkanesulfonate spectra (4).These masses are commonly associated with fragment ions C3F5+and C3F7+ that are produced by electron ionization of perfluorinated hydrocarbons. However, ions of these masses that are produced by FAB appear indicative of the matrix and counter ion and not the fluorocarbon nature of the analyte. Sodium exchange shifted m / z 131 to 115. Since glycerol was used as the matrix, m / z 131 is likely the addition of one potassium to glycerol. Glycerol clusters also account for the ions of m/z 169 and 223, probably C3H,03K2and C3H6O3K3,respectively. The mass spectra of negative ions generated by FAB are simple and the sulfonate anion (L-) is the most abundant ion. As in the positive ion spectra, solutesolute cluster ions of the form L(LM),- are observed. Clusters of the form [L(LM), - 381-and [L(LM), + 381-are also observed and are analogous to ion series B and C present in the spectrum of positive ions (see Table 111). A unique member of the cyclic perfluoroalkanesulfonates, compound VI1 incorporates two sulfonate moieties. The FAB-generated positive ion spectrum of this difunctional compound contains two predominant high-mass ions of m / z 601 and 639 (Figure 3). These solutesolute cluster ions may correspond to LK2H+and LK3+,where L is now a dianionic moiety. A small amount of a third counterion variation, LKH2+,may also be present. Ions identified as hydrogen-
2655
ILKAK'
+
K- 0,scq
@CFSO, -K+
ILKJH'
185
3 c
501
lLKHIH'563 261
L ?PO
300
400
ti/ 2
Figure 3. FAB
SO0
103
spectrum of positive ions from compound V I I .
containing clusters may also be explained by the presence of an unsaturated species. The mass spectrum of FAB-generated negative ions from KSO3CF2C6FloCF2SO&also shows two predominant ions. These ions, of m / z 523 and 561, could contain hydrogen and potassium counterions, respectively, or the ion of m / z 523 ion could be a member of the B series. The high-resolutionMS-I was required to resolve the ambiguities concerning the m / z 601 positive ion and the m/z 523 negative ion. High-resolution peak matching of the m/z 523 negative ion revealed a doublet. One of the peak masses matched that of C$14S206H to within 0.5 ppm, whereas the more abundant peak matched C8FI2S206Kto within 1.5 ppm. The exact mass determination confirmed the presence of both the compound containing a hydrogen counterion and the unsaturated compound (series B). The presence of the three peak pattern in positive ion mode illustrates the utility of FAB in identifing difunctionality. In addition, the pattern may be used to deduce the counterion identity. The mass shift observed between positive and negative ions modes confirms the counterion species. Little fragmentation, other than F loss, is observed upon FAB desorptionof the positive or negative ions of these cyclics. For negative ions, loss of sulfur trioxide occurs in all but three of the saturated cyclics. In the three compounds that do not lose neutral SO3,the sulfonate is not directly attached to the ring. Lack of this neutral loss may be used to distinguish between isomeric compounds such as I1 and I11 or IV and V by using only conventional mass spectrometry. Collisionally Activated Decompositions. Information on molecular weight, purity, counterions, and difunctionality is available from the mass spectra of FAB-generated positive and negative ions. However, the minimal fragmentation of the desorbed ions results in a limited ability to determine structural characteristics. This situation also applies to fatty acid carboxylates and other surfactants (11). Collision activation and tandem mass spectrometry of the carbanions were used to obtain more structural information on the compounds and their impurities. The CAD spectra of the cyclic perfluoroalkanesulfonates (see Figure 4 for examples) differ from their straight chain analogues. The amount of fragmentation is severely reduced, as expected for a cyclic compound. The decompositions are dominated by the loss of sulfur trioxide to form the perfluoroalkane carbanion. The carbon-sulfur bond was also a major fragmentation site in the straight chain fluorochemicals previously studied (4).However, the charge is retained on sulfur trioxide in the straight chain compounds, whereas the fluorocarbon retains the charge in the cyclics. Loss of sulfur trioxide with charge retention on the fluorocarbon occurs as
2656
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
99
,
,,, , ,,
,
,
A 'CC
50
!/1
m 200
I5C
d 250
300
35c
261,
I 5C
2CO
ISC
'@P
250
300
3SC
40C
45C
M/Z
Figure 4. CAD spectrum of negative ions from the followlng: (A) m l r 361, the sulfonate anion of compound I; (e) m l r 41 1, the sulfonate anion of compound 11; (C) m l r 461, the sulfonate anion of compound IV. 10
,
:j
1
n 1
2
3
4
5
i
1 .
6
7
carhn#
8
9
e
i
1011
12
Flgure 5. CAD product ratios ([SO,-] l [M - SO,]- and fragments)vs
number of carbons in compound: 0,perfluoroalkanes; A, cyclic perfluoroalkanes (compounds I, 11, and IV). both a unimolecular and a collision-induced process. Carbanion Stability. The C-S bond in perfluoroalkanesulfonates can undergo either homolytic or heterolytic cleavage. If homolytic cleavage occurs,a sulfur trioxide radical anion is formed. However, under heterolytic cleavage, sulfur trioxide is lost as a closed shell neutral molecule and the charge remains on the perfluoroalkyl portion. Relative abundances of the products of these two processes are plotted in Figure 5. It is obvious from the data in Figure 5 that as the length of the perfluoroalkyl chain increasea, homolytic cleavage begins to dominate. The two mechanisms (homolytic and heterolytic cleavage) become competitive at Cs. Although there is relatively little change in the ratio of gas-phase decomposition products for the longer chain compounds, the relative abundance of the sulfur trioxide radical anion increases dramatically in collisionally activated trifluoromethanesulfonate. Whereas the trifluoromethyl anion is more stable than the methyl analogue, it appears to be the least stable of the perfluorodkyl anions included in this study. Literature values for the electron affinities (EA) of perfluoroalkyl compounds start at 1.9-2.1 eV for CF3 (12, 13) and increase with chain length. This trend is consistent with the fragment ratios plotted in Figure 5. However, the observed large deviation
of the methyl moiety is not predicted by electron affinities (CF3- = C2F5-= 2.1 eV). On the basis, of the EA of sulfur trioxide (1.7 eV) (14),all of the perfluoroalkylsulfonatesshould yield predominantly perfluorocarbanions via heterolytic cleavage of the C-S bond. Several effects have been proposed to explain the stabilities of halocarbanions including induction, polarizability, d-orbital participation, steric factors, the +R effect or p-orbital electron feedback, Murrell's 7-repulsion theory, and hyperconjugation (HCJ). The influences on fluorocarbanion stability are limited to induction, hyperconjugation, feedback (back-bonding),A repulsion, and geometry. Induction alone does not account for the large difference in stabilities between the CF3- and CF,CF3- anions because the inductive effects of fluorine and fluoroalkyl groups are approximately equivalent (15).Pople's molecular orbital calculations (16)suggest that the withdrawal of electrons through the u system and the *-system feedback are most effective at the a site. More recently Klabunde and Burton (17) have invoked the +R effect to explain the unusual trends in the acidity of primary, secondary, and tertiary perfluorocarbanions. They concluded that an a fluorine is capable of a +R effect that more than compensates for its inductive effect and leads to a net destabilization of the carbanion. This back-bonding should be most apparent in the trifluoromethyl carbanion with three CY fluorines and less important in the higher homologues. Hyperconjugation or "no-bond" resonance has been proposed as an additional stabilizing mechanism in fluorocarbanions. Andreades (18), in his study of fluorocarbon acidities, found decreasing stability in the fluorocarbanion series tertiary > secondary > primary. The stability of 3" and 2" anions was explained in terms of p fluorine hyperconjugation (HCJ). The relative importance of HCJ in carbanion stability is unclear because literature contains articles that both support (19-21) and refute (22-24) the mechanism. The factors that influence the stability of acyclic perfluorocarbanions also explain the results plotted in Figure 5 for the cyclic perfluoroalkylsulfonates. Collision-activated dissociation of compounds I, 111, V, and VI yields almost entirely homolytic cleavage and stable perfluorocarbanions. The stability of these anions can be attributed to a combination of inductive effects, hyperconjugation, and lack of CY fluorine destabilization. The stabilizing effects of the cyclohexyl moiety are reduced as difluoromethylene groups are interjected (Figure 4). The ratio of product anions from compound IV approaches that of the long chain acyclics. CAD of Minor Components. A comparison of the CAD spectrum of compound I to the open-chain species at [L(LM) + 381- and the n-perfluorohexanesulfonate shows several differences (see Figures 4A and 6). The CAD spectrum of the acyclic impurity resembles the spectra of the cyclic compounds more closely than it resembles that of straight-chain sulfonates. Whereas the m / z 80 ion (SO3-) is the most abundant fragment of the straight-chain compound, it has a relative abundance of 2% in both compound I and its openchain impurity. The spectral differences between the saturated compounds (see Figure 6) suggest that the stability of the perfluorocarbanion in the open-chain impurity is similar to that of the cyclic compound (i.e., the a carbon in the impurity is tertiary). A tertiary carbon bonded to the sulfur trioxide group requires that the ring opening occur at positions other than the 1,2 or 1,6 bond. Scarcity of fragmentation precludes further characterization of the impurity. Whereas the CAD spectra of the acyclic impurities in compounds I and I11 resemble the CAD of the cyclic compounds, CAD spectra of the acyclic impurity [L + 381- in compounds I1 and IV closely resemble those of straight-chain reference compounds. Figure 7 graphically illustrates the different spectra obtained
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
2857
n-C,F,,SO, -K*
50
100
150
200
250
300
3iO
400
449
C,F,,SO;K+ 50
100
150
200
250
300
350
400
169
MI2
Flgwe 8. CAD spectrum of negative ions from the following: (A) m l r 399, the perfluorohexanesulfonateanion; (E) rnlz 399, an impurity in compound I.
when the isomeric acyclic impurities are collisionally activated. This is consistent with the correlations mentioned above relating the tertiary nature of the a carbon to its effect on fragmentation. The three fragment ion series, C,F2,S03-, C,F2,-1SOf, and CnFzn+l-,observed for the straight-chain references (4)are also found in the CAD spectra of these acyclic impurities. Fragmentations forming distonic radical anions predominate upon activation, and the carbanion fragment series is least abundant. Structure Elucidation. Tandem mass spectrometry can also be used to differentiate between isomers such as I1 and I11 or IV and V. The monosubstituted compounds (I1 and IV) not only have enhanced sulfonate anions but also show a significant perfluorocyclohexyl carbanion of m/z 281 (Figure 4, parts B and C). For compound IV, this carbanion is the most abundant fragment. Both the sulfonate anion and the perfluorocyclohexyl anion appear a t less than 1% in the disubstituted isomers (111 and V). Ions of m / z 130 (CF2S03) and m / z 180 (C2F4S03)can also be used diagnostically to separate these isomer pairs. In this study, fragment ions attributable to ring substituents are significant only for the species with p-butyl substitution. In the CAD spectrum of the secondary butyl compound (VI), fragments C3Ff (m/z 169) and C4F< (m/z 219) are enhanced in abundance. As expected, these two ions are also the most abundant carbanions in the CAD spectrum of pentafluorobutanesulfonate. The size of the ring substituent may be inferred from the abundance of the carbanion fragments whereas the cyclic nature of the compounds is apparent from the absence of other fragment ions. Collision activation of K03SCF2C6FlOCFzSO3from the disulfonate species VI1 yields a unique spectrum. The most abundant fragment ion of m/z 442 results from the loss of neutral S03K. This compound produces significantly more fragmentation than its monofunctional homologues. Although an ion of m / z 130 is present (characteristic of the CFzS03 group), its abundance is not significantly increased by the presence of two such functionalities in the molecule. Perfluorobenzenesulfonateand benzenesulfonate were included in this study to evaluate the effect of fluorination on the CAD spectra. The major fragmentation for each of the aromatics, compounds VI11 and IX, occurs at the C-S bond. However, for benzenesulfonate, the charge is retained by sulfur trioxide, whereas for the pentafluoro compound, the charge remains on the ring, thus showing the ability of fluorocarbons to stabilize the negative charge. Benzenesulfonate, and to a
Figure 7. CAD spectrum of negative ions from the following: (A) rnlr 449, an impurity In compound 111; (E) rnlz 449, and impurity in compound 11.
lesser extent the fluorinated analogue, rearrange to lose SO2 as an alternative fragmentation pathway. This rearrangement does not appear to be unique for ions containing the aromatic system but is also observed in several unsaturated compounds. The rearrangement was reported the for the perfluoroheptenesulfonate anion (4)and is also observed in the CAD of the unsaturated cyclic impurities (series B ions).
ACKNOWLEDGMENT The authors are indebted to Mark Pellerite of 3M Industrial and Consumer Sector Research Lab and Charles Kolpin of 3M Industrial Chemicals Products Division for their helpful discussions and advice throughout the various stages of this study. The assistance of Tom Kesner in providing NMR data and interpretation is gratefully acknowledged. Registry No. I, 3107-18-4;11, 110205-37-3;111, 374-62-9;IV, 110205-38-4;V, 335-24-0;VI, 110205-39-5;VII, 110205-40-8;VIII, 882-96-2;IX, 934-55-4.
LITERATURE CITED (1) Rozhkov, I. N. In Organic Electrochemistry; Baizer, M. M., Lund, H., Eds.; Marcel Dekker: New York. 1983; p 805. (2) Nagase, F. In Fluorlne Chemistry Reviews; Tarrant, P., Ed.; Marcel Dekker: New York, 1967; Vol. 1, p 77. (3) Abe, T.; Nagase, S. In Preparation, Properties, and Industrial Applications of Organofluorine Compounds; Banks, R. E., Ed.; Horwood: Chicester, UK, 1982; p 19. (4) Lyon, P. A.; Tomer, K. 8.; Gross, M. L. Anal. Chem. 1985, 5 7 , 2964-2989. (5) Lyon, P. A.; Crow, F. W.; Tomer, K. 6.; Gross, M. L. Anal. Chem. 1984, 56, 2278-2284. (6) Lyon, P. A.; Stebbings, W. L.; Crow, F. W.; Tomer, K. B.; Lippstreu, D. L.; Gross, M. L. Anal. Chem. 1984, 56, 8-13. (7) Zisman, W. A. "Relation of Chemical Constitutlon to the Wetting and Spreading of Liquids on Solids"; NRL Report 4932, May 1954. (8) Gross, M. L.; Chess, E. K.; Lyon, P. A,; Crow, F. W.; Evans S.; Tudge, H. Int. J . Mass Spectrom. Ion fhys. 1982, 42, 243-245. (9) Lyon. P. A; Hunt, S. L.; Evans S.; Tudge, H. Presented at the 34th Annual Conference on Mass Spectrometry and Allied Topics, Cinclnnatl, OH, 1986. (10) Crow, F. W. and Lapp, R. L. Presented at the 29th Annual Conference on Mass Spectrometry and Allied Topics, Minneapolis, MN, 1981.
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Anal. Chem. 1987, 59, 2658-2662
(11) Jensen, N. J; Tomer. K. 9.: Gross, M. L.; Lyon, P. A. I n Desorption Mass Spectrometry; Lyon, P. A.. Ed.: ACS Symposium Series: American Chemical Society: Washington, DC, 1985; p 194. (12) Rosenstock. H. M.;Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data Suppl. 1977, 6(1), 1-752. (13) Page, F. M.;Goode. G. C. Negative Ions and the Magnetron; Wiiey: London, 1969. (14) Janousek. B. K.: Braurnan. J. I. In Gas Phase Ion Chemistry;Bowers, M.,Ed.; Academic; New York 1979; Vol. 2, p 87. (15) Sheppard, W. A.; Sharts. C. M. Organic Fluorine Chemistry: Benjamin: New York. 1969: p 36. (16) Pople. J. A.; Gordon, M. J. Am. Chem. SOC. 1967, 89, 4253. (17) Klabunde, K. J.; Burton, D. J. J. Am. Chem. SOC. 1972, 94, 5985. (18) Andreades, S. J. Am. Chem. SOC. 1964. 8 6 , 2003. (19) Sleigh. J. H.: Stephens, R.; Tatiow, J. C J. Floorine Chem. 1980, 15, 411.
(20) Schieyer, P.; Kos, A. Tetrahedron 1983, 39(7), 1141. (21) Dixon, D. A.; Fukunaga, T.; Smart, 9. J. Am. Chem. SOC.1986, 108, 4027. (22) Streitwieser, A., Jr.; Berke, C. M.; Schriver, G. W.; Grier, D.; Collins, J. 9. Tetrahedron, Suppl. 11981, 3 7 , 345. (23) Streitwieser, A., Jr.: Holtz, D. J. Am. Chem. SOC. 1967, 89, 692. (24) Klabunde, K. J. Burton, D. J. J. Am. Chem. SOC. 1972, 9 4 , 820.
RECEIVED for review January 15,1987. Resubmitted July 22, 1987. Accepted July 22,1987. This work was supported by 3M and the Midwest Center for Mass Spectrometry, a National Science Foundation Regional Instrumentation Facility at the University of Nebraska-Lincoln (Grant CHE8211164).
Inductively Coupled Plasma Mass Spectrometric Determination of Lead Isotopes Thomas A. Hinners* a n d Edward M. Heithmar
US.Environmental Protection Agency, Environmental Monitoring Systems Laboratory, Las Vegas, Nevada 89193-3478 Thomas M. Spittler
U.S. Environmental Protection Agency, Lexington, Massachusetts 02173 J o h n M. Henshaw
Lockheed Engineering and Management Services Company, Las Vegas, Nevada 89109
Inductively coupled plasma mass spectrometry (ICP-MS) offers the opportunity to measure stable Isotopes of the elements. When the Isotope proportions dtffer s d k h t i y among source materlals, isotope ratio analysis provides a means to dlstlngulsh the orlgln of pollutants such as lead. Relatlve standard devlatlons of 1.1 %, 0.76%, and 0.80% were obtained for an 8 h perlod using a 5-mln measurement Interval for the m / z 208:206, 207:200, and 204:206 lead ratlos, respectively. By use of 95% tolerance Ilmlts, slgnlflcant dlfferences In lead Isotope ratlos were observed for lead ores from Idaho, Mlssourl, and Yugoslavla, as well as for commerkal lead standards. Lead Isotope ratlos for Mlssourl ore differed as much as 19% from values for Natlonal Bureau of Standards “Common Lead Isotopic Standard” (SRM 981). Lead Isotope data obtalned by ICP-MSare consistent wlth palnt as the source of lead In blood for one case study but not for another. An lsotopk detectlon llml of 2 pptr (parts per Amerlcan trllUon) was both calculated and measured for lead.
Inductively coupled plasma mass spectrometry (ICP-MS) entered the commercial instrument market in 1983 after several years of development (1-7). While ICP-MS offers attractive detection limits near 10 pptr for the determination of elemental concentrations, the isotopic proportions of an element may provide information that can distinguish the source of the element. However, the proportions of an element’s isotopes will not have diagnostic value unless source materials have different isotope proportions. Isotopic composition variations on the order of 50% have been reported for boron in nature (8). Differences of 17-36% in isotope
ratios for lead from various geographic areas have been documented (9, 10). Natural materials differ in lead isotope composition because radioactive decay of thorium and uranium yields different lead isotopes and because thorium and uranium were not distributed uniformly in nature. While ICP-MS offers convenience and speed in the determination of isotope ratios, it is not expected to provide the high precision (0.01’70 relative standard deviation) of thermal ionization mass spectrometry (7). The usefulness of ICP-MS for distinguishing the origins of lead (and other polyisotopic elements) by the isotope ratios will depend upon the precision obtainable and upon the variation of the ratios in commercial products and in nature. This study was conducted to assess the performance of an ICP-MS system to distinguish between lead samples by their isotope ratios and to evaluate its potential for determining the source of lead in pollution situations. EXPERIMENTAL SECTION Instrumentation. Isotope determinations were made on an Elan Model 250 ICP-MS instrument purchased from Sciex, Thornhill, Ontario, Canada. Measurements were made with both the original and new ion optics supplied by the manufacturer. The new ion optics differ from the original version by the replacement of the mesh ring lens with an opened-centered front lens, by replacement of the collimating AC rod set with a set of Einzel lenses, and by revised electronics for the plate lens unit and the barrel lens. The new ion optics were designed by Sciex to reduce signal drift and to improve the utility of internal standards. A peristaltic pump (Minipuls 2, Gilson, Middleton, WI) was used for sample uptake. A multichannel mass flow controller (ModelFM4575, Linde Division, Union Carbide Corp., Somerset, NJ) was used for the plasma (outer gas) and nebulizer argon flows, unless noted otherwise. Reagents. Deionized water was obtained from a Milli-Q system (Millipore Corp., Bedford, MA) at, or above, 13 Ma cm. Nitric
0003-2700/87/0359-2658$01.50/00 1987 American Chemical Society
