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Anal. Chem. 1985, 57, 2984-2989
(5) Karger, B. L.; Snyder, L. R.; Horvath, C. I n “An Introductlon to Separation Science”; Wliey: New ‘fork, 1974. (6) Scott, R. P. W.; Kucera, P. J . Chromatogr. Sci. 1975, 13, 337-346. (7) Scott, R. P. W. J . Chromatogr. Scl. 1980, 18,297-306. (8) Synder, L. R. J . Chromatogr. 1981, 6 , 22-52. (9) Snyder, L. R. J . Chromatogr. 1982, 8 , 319-342. (IO) Snyder, L. R. J . Chromatogr. 1971, 83, 15-44. (11) Popl, M.; Dolansky, V.; Mostecky, J. J . Chromatogr. 1874, 91, 649-658. (12) Hurtublse, R. J.; Allen, T. W.; Sllver, H. F. Anal. Chim. Acta 1981, 126, 225-227. (13) Lindsey, A. J.; Pash, E.; Stanbury, J. R. Anal. Chim. Acta 1956, 15, 291-293. (14) Engelhardt, H.; Wiedeman, H. Anal. Chem. 1973, 45, 1641-1446. (15) Giles, C.H.;Easton, I. A. Adv. Chromatogr. 1988, 3 , 67.
(16) Lee, M. L.; Novotny, M. V.; Bartle, K. D. I n “Analytical Chemistry of Polycycllc Aromatic Compounds”: Academic Press: New ‘fork, 1981. (17) Brockman, H.; Schodder, H. Chem. 8 e r . 1941, 7 4 8 , 73. (18) Brockman, H. DISCUSS.Faraday Soc. 1949, 7 , 58.
RECEIVED for review February 25,1985. Resubmitted August 12,1985. Accepted August 12,1985. This work was supported by the U.S. Department of Energy, Contract No. AC0676RLO-1830, with Pacific Northwest Laboratory, and the Department of Energy, Division of Biomedical and Environmental Research, Contract No. DE-AC02-79EV10237, with Brigham Young University.
Fast Atom Bombardment and Tandem Mass Spectrometry for Characterizing Fluoroalkanesulfonates Philip A. Lyon* 3M, Central Research Laboratories, 201 -BS-05, St. Paul, Minnesota 55144 Kenneth B. Tomer and M. L. Gross Midwest Center for Mass Spectrometry, Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588
A serles of perfluoroalkanesulfonateswere examlned by uslng fast atom bombardment (FAB) Ionization comblned wlth tandem mass spectrometry (MWMS). Both positive and negative ion FAB spectra yleld lnformatlon for determlnlng molecular weight and ldentlfylng counterlons. Abundant parent Ions are desorbed and undergo mlnlmal fragmentation. Structural informatlon Is obtained from the collision actlvated dlssoclatlon (CAD) spectra of selected parent Ions. Comparlsons of colllslonally actlvated decomposltlons are made wlth hydrocarbon analogues. The fluoroalkanesulfonates undergo at least two remote charge slte fragmentations. The more facile Is loss of C,Fzn+, followed by losses of a perfluoroalkene. A second, less abundant serles of fragments Is formed by losses of the elements of CnF2n+z,a process that may be analogous to the parallel ellmlnallons of the elements of C,H,, +2 from carboxylates and alkyl sulfates. Perfluoroalkanesulfonates containing a single hydrogen atom have also been determlned by uslng FAB MS/MS and their fragmentation pathways elucidated. The comblnatlon of FAB and MS/MS should be useful for analysis of mixtures of fluorinated surfactants.
Perfluoroalkanesulfonates have considerable commercial interest as anionic surfactants ( 1 ) . As is characteristic of most industrial surfactants, they are provided as complex mixtures. Extensive purification is normally not possible nor necessary for the compounds to satisfy the requirements of most commercial surfactant applications. Nevertheless, their properties as surfactants depend on the nature of the mixture, and characterization methods are required. But as for other surfactants, their ionic nature increases considerably the problems of separation and analysis. Perfluoroalkanesulfonates may have other uses as mass calibrants in mass spectrometry. Perfluoroalkane mixtures are well-known calibrants for positive electron ionization mass spectra. However, the requirements of mass standards have become more demanding in order to cope with the enhanced
abilities of new mass spectrometers equipped with desorption ionization and mass analyzers which can deal with molecules up to and greater than 10000 amu. The perfluorotriazines satisfy the requirement up to approximately mass 2000, and they can be used for both electron ionization and desorption ionization. To press beyond mass 2000, inorganics such as CsI have been extensively used as calibrants for fast atom bombardment (FAB) mass spectra. Recently, Heller et al. (2) and Roberts and White (3) demonstrated the ability of perfluoroalkanesulfonates to be desorbed as high mass clusters under conditions of FAB and field desorption (FD), respectively. Hydrocarbon sulfonates were reported earlier (4)to desorb as L,M,+,+ clusters, where L is the alkylsulfonate and M is a counter metal ion; so the phenomenon of clustering is not surprising. What is significant is the higher abundance high mass ions produced from the fluoroalkanesulfonates as compared to CsI. Apart from the ion clustering exhibited by the fluoroalkanesulfonates, little attention has been paid to other mass spectral features of these compounds. Previously, we have demonstrated the power of FAB combined with tandem mass spectrometry (MS/MS) for characterizing both anionic (4) and cationic (5) surfactant mixtures. In this paper we report extending the combined technique to the fluoroalkanesulfonates. The fragmentations of collisionally activated fluoroalkanesulfonates are deciphered and compared with the decomposition reactions of the hydrocarbon analogues.
EXPERIMENTAL SECTION Chemicals used in this study were obtained from 3M Commercial Chemicals Division but most are commercially available from other sources (e.g., PCR Research Chemicals, Gainesville, FL, or Fluka Chemical Corp., Hauppauge, NY). The compounds were used without further purification. The purity of these materials is discussed in the Results section of this paper. The mass spectra were obtained with a Kratos MS-50 triple analyzer mass spectrometer (6). The instrument is comprised of a high-resolution MS-I (a standard Kratos MS-50) follwed by an electrostatic sector, 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
0003-2700/85/0357-2984$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
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Table I. FAB Generated Positive and Negative Cluster Ions of Perfluoroalkanesulfonate Salts
compound (CF3S03)2Ba C2F5SO3K CIF,SO3K C5FllS03K C6F13S03K
C8F11S03K C10F21S03K
C12F25S03K CF2H-CF2-S03Na CF3CFHCF2S03Na C6F1,CFHCF2SO3Na
mass of ligand 149 199 299 349 399 499 599 699 181 231 431
relative abundance, % [L,M,J negative ion [(LM),M]+ positive ion n = l n=2 n=3 n=4 n=5 n=6 n i l n=2 n=3 n=4 n=5 n=6 n=7 a
100 100 100
18 16
8 6
3 4
100
18 12
b b b b b
27
2
18 4
1 b
100 100 100
23
6 8 b b b 12
18
9
4 2
7
5
b
0.7
1 b b b b b b 2 1 b
b b b b b b
0.9 b b
100 100
59
100
39 39 27 17
55
100
100 100 100
b
100 100
63 70 39
100
4 4
3 4 b b b b b 5 5 b
2 2 b b b
9 8 2
1 b b b b b b 2 2 b
0.4 b b b b b b
b b b b b b b
0.8
0.1
b b
b b
Not available (divalent metal ion). *Exceeded calibrated mass range for this experiment. of the sample solution was placed on the copper target of the FAB direct insertion probe and bombarded with 8-keV xenon atoms. The ions produced were accelerated through 8 keV into the analyzer region of the mass spectrometer. Normal spectra were acquired at low mass resolution ( R = 3000) by scanning MS-I and leaving MS-I1 fixed to pass all ions. 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 between MS-I and MS-I1 to give 50% reduction of the intensity of the selected ion beam. CAD spectra, obtained by scanning MS-11, were signal averaged and processed with a standard DS-55 data system using software written at the University of Nebraska laboratory (7). Each CAD spectrum reported here was an average of at least ten 20 s/scan spectra.
?k3;;/,,,J 377
........................
10
169 C3F7
20
RESULTS AND DISCUSSION Perfluoroalkanesulfonates. In a manner similar to alkanesulfonates (4),perfluoroalkanesulfonates yielded simple positive and negative FAB mass spectra. In the positive ion mode, the neutral salt LM was sputtered from the glycerol matrix with an additional cation attached (LM2+,see Table I). These species cluster with additional LM ion pairs to yield species of the form (LM),M+. The simplest cluster LM2+is normally the most abundant of the high mass ions. For example, potassium perfluorobutanesulfonate gives an abundant (C4F@03K)K+(see Figure 1). Higher mass cluster ions are also visible at m / z 715,1053, and 1392 corresponding to the L2K3+,L3K4+,and L4K5+members of the series, respectively. This type of ligand clustering was observed up to very high masses (ca. m / z 15000) when Cs+ was used as the metal ion and perfluorohexanesulfonate as the anion (2). Another type of cluster ion, m / z 469, is seen in the desorption of perfluorobutanesulfonate (Figure l). A molecule of glycerol solvent has desorbed with the ion. The abundance of this ion is relatively low, and higher homologues are not observed. The LK2 + glycerol' species was observed in the positive ion spectra of all the perfluoroalkanesulfonates included in this study. An ion composed of a sulfonate ligand with one potassium and a proton (LKH+) is often observed in the FAB spectrum of perfluoroalkanesulfonates. When observed, the relative abundance of this species is less than 10%. Limited fragmentation to give low mass ions occurs as a result of desorbing these sulfonates. Ions at m / z 131 (C3F5+) and 169 (C3F7+)are characteristic fragments commonly associated with the perfluorokerosene mass calibration compounds. The presence of these ions may function diagnostically for identifying the sample as a fluorochemical. It should be noted that the higher perfluoroalkanesulfonate homologues also fragment to give m / z 131 and 169.
LKd
0
223
0 IO0
200
469 507
271 100
k00
SO0
-, a00
,,,
, ,(15 ::L,
TOO
tl/ z Flgure 1. FAB spectrum of positive ions from potassium perfluoro-
butanesulfonate. The presence of homologues in the positive FAB spectra becomes more apparent with increasing chain length. The spectrum of perfluorodecanesulfonate contains ions of the form (LK)K+ for the C5 through C9 homologues (relative abundance for each species is less than 3%). The presence of homologues is not, however, unexpected. The fluorochemicals used in this study were derived from synthetic hydrocarbon precursors. Even though the feedstocks may have been highly purified, the process of electrofluorination will result in the production of lower molecular weight homologues and possibly chain branched isomers (8, 9). The resulting fluorochemicals had been converted to the sulfonate salts without subsequent purification. The mass spectrum of FAB-generated negative ions from potassium perfluorobutanesulfonate shows the sulfonate anion m/z 299. No fragmentation to give products with masses lower than the sulfonate anion were apparent. In the higher perfluoroalkane homologues, low abundance fragments are produced by losses of F and 2F or F2 from the parent ion. Cluster ions of the form L,K,-l- are also desorbed. Anion clustering for the fluoroalkanesulfonates is similar to that observed for the alkanesulfonates and alkyl sulfates. The presence of homologue impurities detected in the positive ion mass spectrum was also confirmed by examining the spectrum of FAB-desorbed negative ions. For example, C5-C9 compounds found in the sample of perfluorodecanesulfonate desorb as sulfonate anions: mlz 349, 399,449,499, and 549. CAD Spectra. While the positive and negative FAB spectra yield information on the molecular weight, purity, and identity of the metal cation, the CAD spectra of FAB-gen-
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table 11. Perfluorodecanesulfonate Fragment Ion Series
mlz at carbon chain length A B
CraF2nS03CnF2,-1S0C CnF,n+,-
C
580 561 519
530 511 469
480 461 419
430 411 369
bo
I
260
460
360
vz
560
330 311 269
280 261 219
230
180
130
169
161 119
69
formed by only two processes: loss of CzF5 from the sulfonate anion (m/z 599) and loss of C2F4 from (M - F)-. All of the lower mass ions in the homologous series can be produced by at least three reactions; e.g., mlz 430 can be formed from mlz 530, 580, and 599. Thus, the perturbed abundance of mlz 480 is consistent with a mechanism involving sequential losses of perfluoro radicals and perfluoroalkenes. Another possibility for the formation of ion series A is sequential losses of difluorocarbene, CF2 This is unlikely since when given the choice of losing two molecules of CF, or C2F4, the loss of C2F4 is favored by about 70 kcal. Similarly, the formation of mlz 530 could proceed via a two-step process involving loss of F from m / z 599 followed by CF2. However, even though we see loss of F, the formation of m / z 530 is probably a one-step process given that the stability of the CF3 radical is considerably greater than those of F and CF2 (IO). An alternate mechanism would involve ring formation with expulsion of a perfluoroalkyl radical (eq 2). However, the
~ ' " ' " " " ' ' " ' ' " " ' " " " " " ' ' " " " ' " ' A, ' "280 ' '
I
380 361 319
600
Flgure 2. CAD spectrum of negative ions from m l r 599, the perfluorodecanesulfonateanion (see Table I I for masses and structures).
erated negative ions can be utilized to gain structural information. The CAD spectrum of perfluorodecanesulfonate anion (mlz 599, Figure 2) is typical of the long chain perfluoroalkane compounds investigated here. The members of the predominant series of peaks spaced every 50 daltons are separated by the mass of a CF, group (see series A, Table 11). This series of peaks can be counted from mlz 80 (the SO3ion) up to the molecular ion, as can be done for the hydrocarbon analogues, to determine the length of the alkyl chain ( 4 ) . The ion series begins with the loss of F, CF3, and CzF5 from C,F2,+,SO~ by direct homolytic cleavage to generate mlz 580, 530, and 480, respectively. In addition to the direct cleavage reaction, a sequel process must take place whereby the initially formed radical anion undergoes loss of C,F2, (see eq 1). The evidence for the CF~(CF~)&F~CFZ(CFZ)~SO~-+ 'CF~CF~(CF~)~SOS''(Ch)mS0~-
(1)
second process (the loss of C,F2,) is indirect. The homolytic cleavage would be expected to take place with approximately equal facility at each -CF2-CF2- bond. However, the fragmentation, as seen from the CAD spectrum, does not yield a set of equally abundant product ions mlz 130,180,230,280, 330 .... Rather certain ions are favored as a result of a trade-off of their rates of formation and rates of decomposition. For example, mlz 280 is most abundant probably because it can exist as a stable six-membered ring species, 1, instead of as a distonic radical anion as shown in eq 1. As a result, losses of C,F,, from this ion are less likely. o\
$40
CFCS\O
I
C F z ,CF2 CF2
1-
I
rnlz 280 1 N
The abundance of mlz 480 is reduced with respect to its neighboring homologues mlz 430 and 530. This ion can be
R = F, CFJ,C2FS..
decrease in entropy accompanying formation of very large rings (greater than eight members) is too prohibitive for this mechanism to produce a series of ions of comparable abundances (11, 12). Thus, the mechanism proposed as eq 1 is favored and is another example of a remote-charge-sitefragmentation; i.e., the reaction is not initiated or promoted by the presence of the charged sulfonate moiety. It is noteworthy that the corresponding alkyl losses were not found for (M - H)- of fatty acids or for alkylsulfates probably because the distonic radical anions are not stable. The presence of fluorine atoms raises the electron affinity and permits observance of relatively stable anion radicals. A second series of ions is formed in the collisionally activated decompositions of the perfluorodecanesulfonate anion. The fragments have the general form C,F2,-,S03-, and their masses are shown as series B in Table 11. The fragments are of similar abundance and spaced at 50-amu intervals. The ions are probably formed by parallel losses of the elements of CF4, C2F,, C3F8,etc. from m/z 599 anion. The eliminations must be occurring at the perfluoroalkyl end of the chain, the end remote from the site of charge localization. Fragmentations involving parallel C2H2n+2 losses have been shown to occur for carboxylate anions upon collisional activation (13). The chemistry does not appear to involve the negative charge, which is highly localized at the carboxylate site, but rather takes place at the other end of the molecule. These remote charge site fragmentations are probably 1,4eliminations of H2 resulting in formation of an alkene and an unsaturated carboxylate. It is likely that the CnF2n+2losses from the perfluoroalkanesulfonates occur in an analogous fashion. This series is less important in the fluorocarbon compared to the hydrocarbon chains. The bond energies of the F-F bond (36.6 kcal) and H-H bond (104 kcal) formed in the fragmentation are consistent with the decrease in im-
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table 111. Comparison of Low Mass Fragment Ions
COf
CHzCOL
mjz 44
58
mjz 80
130
sod-
CH2=CHC0f
CH2=CHCHZCOf
71
85 (not observed)
161
211 (weakly observed)
CH20SOf
1
II
CH2=CH(CH2)2C0
40 e
< 2
K 10 Y
20
IO
0 20a
10
-!
600
500
800
M,'Z
I1
-
I66
-so,
CnFmii- -ClFb, CIFS,etc.* Fragment Carbanions (3) carbon carbanion is not stable and not observed. The high electron affinity of fluorine stabilizes the structure. The fragmentation continues by probably both parallel and sequential losses of C,F2, to generate lower mass carbanions. For collisionally activated perfluorodecanesulfonate,the most abundant carbanions are m / z 519, the intact perfluoroalkyl carbanion, and mlz 169 (C3F7-). The abundance of m / z 169 is high because it may be the termination of the series and the result of fragmentations of many higher mass ions. The fragment ion series illustrated for perfluorodecanesulfonate also occur for the other homologous compounds included in this study, most notably for the perfluorooctaneand perfluorododecanesulfonates. The three fragmentation series become more defined and increasingly prominent as the chain length increases. The only ions that are technically not a part of any of the three fragmentation series described above are m / z 19, fluoride anion, and mlz 99, the fluorosulfonate anion. The relative abundance of these ions may be influenced more by the fragmentation pathways leading to their formation than by the stability of the resulting anion. 2-Hydroperfluoroalkanesulfonates. Comparing mass spectra of analogues often facilitates elucidation of chemical structures and decomposition pathways not intuitively obvious CnhniiSOo'
124
i
/I I 200
a00
800
W Z
j,
, , ,
,
,
, ,
,000
,, ,, , , ,
,,
, , ,
I100
,i" ,409
Figure 3. FAB spectrum from 2-hydroperfluoroheptanesulfonate sodium salt: (A) positive ions and (B) negative ions. from examining a single series. Three straight-chain analogues were available for a study of analogues with a single fluorine replaced by a hydrogen. For each compound, the substitution was made on the carbon fi to the sulfonate moiety giving a series of the form RCFHCF2SO3Nawhere R = H, CF3, and C5Fll*
The spectra of FAB-generated positive and negative ions from these modified perfluoroalkanesulfonates appear very similar to those of the hydrocarbon or totally fluorinated analogues. From the positive ion spectra, it is seen that abundant (LNa)Na+ and a series of higher mass cluster ions (LNa),Na+ are desorbed. Higher mass cluster ions are also
2988
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table IV. Collision Activated Dissociations from m / z 41 1, Perfluoroheptenesulfonate Anion" CnFZn-1
C7Fn C6F11
C5F9 CIF, C3F5
CnFzn-3
m/z
RA, %
331 281 231 181 131
24
C7F11
57 1
C8F9 C5F7 C4F5 C3F3
9 9
CnFZncl
m/z
RA, %
293 243 193
5
m/z
RA, %
219 169 119 69
30 8
4
14
143
2 4
93
C4F9 C6F7
CZF, CF3
8 1
Om/z 80 designated as the base peak (relative abundance (R.A.) = 100%).
I
5
4
I
eo
C,F11CFHCF2SOi
,
281
80
,
I
219
I/
331
0
v-
Flgure 4. CAD spectrum of negative ions from m l z 431, the 2hydroperfluoroheptanesulfonate anion.
present (Figure 3A). The only obvious fragmentations are losses of HF and Fz. On the basis of negative ion spectra, it is concluded that only a sulfonate anion is desorbed, and the desorption is accompanied by losses of HF and F2(Figure 3B). The collisionally activated decomposition spectra of 2hydroperfluoroalkanesulfonates were significantly different than those of the perfluoroalkanesulfonates. At first, it appears surprising that the presence of a single hydrogen would greatly alter the fragmentation of the compounds discussed earlier. However, based on the approximate 100-kcal energy difference between the H F and FF bond energies, the dominating loss of H F can readily be justified. The CAD spectrum of 2-hydroperfluoroheptanesulfonate(Figure 4)is dominated by the loss of HF ( m l z 411) from the molecular anion (mlz 431). This facile neutral loss predominates in both unimolecular (see Figure 3B) and collision-induced processes. The lower mass fragments are produced by alkane and alkene losses to yield the ions m / z 331,293, 243,193,and 143 (M - CzF4, CzF6, C3F8, C4F10,and C5F12, respectively). Direct cleavages yield the radical anions m / z 312,262,212,162 (loss of CnFPn+J,and 130 (loss of C6F12H). The SO3- ion a t m / z 80 in the CAD spectrum is characteristic of both hydrocarbon or fluorocarbonsulfonate salts and a useful indicator of the compound class. The decompositions observed for C5Fl1CHFCF2SO3-,m / z 431,are most likely primary fragmentations originating from the molecular anion. This is supported by the collisionally activated decomposition of m / z 411, the fragment from C5Fl1CHFCF2SO8-produced by loss of HF. There is very little overlap between the ions present in the two CAD spectra of m / z 431 and 411 (compare Figures 4 and 5). The 2-hydroperfluoroheptanesulfonate anion decomposes to yield exclu-
1
347
400
Flgure 5. CAD spectrum of negative ions from mlz 41 1, the perfluoroheptenesulfonateanion.
sively fragments containing the SO3 group with a highly localized negative charge. But once the molecule loses hydrogen fluoride to form a perfluoroalkenesulfonate, the fragmentation shifts almost entirely to give perfluorocarbanion species with only a few decomposition products similar to the perfluoroalkanesulfonates. The most abundant ion is the sulfonate anion m / z 80. A rearrangement results in the formation of the fluorosulfonate anion at m / z 99. Three fragment ion series, their observed masses and relative abundances, are listed in Table IV. The most abundant product ions belong to the series C,F2,-1. These are formed by homolytic cleavage of the carbon chain and produce odd electron or carbanion fragments. The C7F13and C6Flc species are very abundant yet the C5F9-ion is barely detectable. The relative abundance of these fragments would be consistent with the location of the double bond between C2 and C3 carbons (2). Locating the double bond in that position is also supported by the ions observed in the series CnF2,+
