Anal. Chem. 1907, 5 9 , 2024-2027
2024
Identification of the Ions Produced by Fast Atom Bombardment Mass Spectrometry in Some Polyesters and Polyamides Albert0 Ballistreri,' Domenico Garozzo,' Mario Giuffrida,*and Giorgio Montaudo'* Dipartimento di Scienze Chimiche, Universitci di Catania, Viale A. Doria 6, 95125 Catania, Italy, and Istituto per la Chimica e la Tecnologia dei Materiali Polimerici, Consiglio Nationale delle Ricerche, Viale A . Doria 6, 95125 Catania, Italy
Fast atom bombardment (FAB) mass spectrometry was applied to the analysis of two polyesters, poly(ethyiene adlpate) and pdy(ecaprolactone), and two polyamides, Nylon 6.6 and Nylon 6. The peaks present In the spectra of the crude polymers were Identified as corresponding to protonated molecular ions of preformed cyclic ollgomers and of low molecular welght compounds contained in the polymer samples; these specles were desorbed intact from the glycerol matrlx under FAB condltons. When the polymers lnvestlgated were accurately purltied from low molecular welght compounds, no significant peaks were observed in their FAB mass spectra. Instead, the FAB mass spectra of the mixtures extracted from the polymers were found to be very similar to those obtalned for crude polymers.
The technique of fast atom bombardment mass spectrometry (FABMS), although introduced in 1981 ( I ) , has already become widely used for the analysis of polar organic molecules, including biomolecules of relatively high molecular weight (2, 3). The mass spectrometric characterization of polymers by pyrolysis followed by gas-phase ionization methods (EI, CI) is well-known (4-6). Recently, Doerr e t al. ( 7 , 8 ) have reported the FAB mass spectra of some aliphatic polyesters obtained by using glycerol as matrix. Peaks observed in these spectra were tentatively interpreted as corresponding t o the products originating by thermal degradation or by ionic fragmentation reactions induced by atom bombardment into the polyesters investigated. However, aliphatic polyesters are known to contain sizable amounts of cyclic oligomers formed in the polymerization reaction (9-12), and since those authors ( 7 , 8 ) did not check the identity of the ions appearing in their FAB spectra by comparison with authentic samples or by tandem mass spectrometry, the possibility that they were actually looking a t preformed oligomers cannot be a priori ruled out. We have therefore investigated the FAB spectra of two polyesters poly(ethy1ene adipate) (PEA) and poly( t-caprolactone) (PCL),and of two polyamides, Nylon 6 and Nylon 6.6. Our results show that the peaks appearing in the FAB mass spectra of these samples are not due to a fragmentation of the polymer chain but arise instead from preformed low molecular weight species, which are desorbed intact from the glycerol matrix under FAB conditions. When the polymers investigated were accurately purified from low molecular weight compounds, no significant peaks were observed in FABMS conditions.
EXPERIMENTAL SECTION Materials. PEA and PCL were obtained from Aldrich Chemical Co., Inc. Poly(hexamethy1eneadipamide) (Nylon 6.6) and poly(t-caprolactam) (Nylon 6) were obtained from EGA Universitl di Catania. Consiglio Nazionale delle Ricerche. 0003-2700/87/0359-2024$01.50/0
Chemie. t-Caprolactam was supplied from Fluka AG. Mass Spectrometry. A double focusing Kratos MS 50s equipped with the standard FAE3 source and a DS 55 data system was used to obtain mass spectra. The fast atom bombardment gun (ION TECH) was operated with a 7-8 kV argon beam. The instrument was scanned from m / z 1000 to 60, with a scan rate of 10 s/decade. Accelerating voltage was 8 kV. Cesium and rubidium iodide (50/50 w/w) were used for computer calibration. The resolution was 2000. Samples to be analyzed were dissolved in a suitable solvent (HCl for nylons and tetrahydrofuran (THF) for polyesters). A drop of the polymer solution was placed on the copper target end of the direct insertion probe and mixed with glycerol. No polymer particles appeared to precipitate out when the glycerol was added. Peak intensity values shown in mass spectra, computed after subtraction of the contribution from the glycerol matrix, represent the average of five separate mass spectra. Collision activated decomposition (CAD) B/ E scans were performed by using a linked scan unit at a scan rate of 20 s/decade and registered on a UV oscillograph. The collision cell is in the first field free region. Metastable decompositions were activated by using helium as collision gas. The pressure in the collision cell was such as to reduce the ion beam to 10% of its usual value. Polymers LC Analysis. Oligomers Extraction and MS Identification. GPC and/or HPLC analyses showed that all polymer samples investigated here contained sizable amounts of low molecular weight components, in agreement with current literature (12-14). It should be realized that even a very small amount (on a weight basis) of oligomer content may produce intense peaks in the FAB mass spectra. Assuming for the polymer a degree of polymerization of about 100-200,1% of oligomer content in the polymer would yield a 1:l mixture on a molar basis. Finely powdered, crude polyamide samples (Nylon 6 and Nylon 6.6) were extracted with methanol for 40 h. Methanol was evaporated at 60 "C under vacuum and the residue dried under vacuum, overnight. Polyesters (PEA and PCL) were extracted with ethyl ether. Polymers were recovered as powders and dried under vacuum at room temperature. Polyesters extractions were repeated several times until no peaks were detectable in the FAB spectra. In fact, the first extraction left trace amounts of oligomers, which still appeared in the spectra. The FAB mass spectra of PEA and PCL extract correspond to the first extraction, and this accounts for the differences with the FAB mass spectra of crude PEA and PCL samples reported. Ten milligrams of each soluble material extracted from crude polymers was dissolved in a suitable solvent and subjected t o semipreparative LC analysis. The eluted fractions were collected separately, dried, and analyzed by MS. Compounds extracted from Nylon 6 were identified as cyclic monomer, dimer, and trimer. Compounds extracted from Nylon 6.6 were identified as cyclic monomer, dimer, and trimer. Compounds extracted from PEA were identified as listed in Table I. Compounds extracted from PCL were identified as listed in Table 11. Gel permeation chromatography (GPC): four columns of pStyrage1 (in the order lOOO-,500-, 10000-, 100-p\pore size) were used in the case of polyesters; eluent was THF. High-performance liquid chromatography (HPLC): a Lichrosorb RP 85 (Supelco) column was used in the case of polyC 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59,NO. 17, SEPTEMBER 1, 1987
2025
Table I. Cyclic and Open-Chain Oligomers Present in Poly(ethy1ene adipate)" MH+ - 18, m/z
MH+, mlt
compound n = l n=2 n = 3 n=O n = l n=2 n = l n=2 n=3 n = l n=2 n=3
H-fOfCH2~O-COiCh~tyCO~OfCH~~0H
HO-COfCH2~SCO+OICH2fi-O-CO-tCH2~C0~OH
O-fCH,+O-COtCH2+C0
H+O~CH,~O-CO+CH~~+C+
235 407
L
147 319 491 173 345 517 191 363 535
f,
217 389 561 129 301 473 155 327 499
2 f,
" Arrows and asterisks indicate transitions confirmed by metastable peaks. Table 11. Cyclic and Open-Chain Oligomers Present in Poly( ecaprolactone)
t
I%
'I
MH+, m / z
compound
173
100129
191
60O t C H , ~ C O"
H-€O+CH2gCO+OH
~~
n = l n=2 n=3 n=4 n=5 n=6 n=2 n = 3 n=4 n = 5 n=6
115 229 343 457 571 685 247 361 475 589 703
345
2'7
147
319
20-
.
18,
'1 ,[: 20
~
amides; eluent was 40/60 CF3CH20H/H20(density, 1.186).
RESULTS The FAB mass spectrum of crude PEA is reported in Figure
389 407
,
i,
235 ~ I I d . . I . .,
473 491
.
b
517 535 .,,L
400
,
,,.561
,
, 600
500
~
m/z
Figure 1. Fast atom bombardment mass spectrum of PEA.
1. This spectrum is practically coincident with the FAB mass spectrum reported previously by other authors (8). The spectrum can be interpreted by assuming that oligomers contained in the polymer sample are desorbed under FAB conditions and that the polymer does not contribute significantly to the spectrum. In fact, the spectrum shows peaks corresponding to protonated cyclic oligomers (m/z 173 n172) and to protonated low molecular weight compounds such as open chain oligomers, diglycols, and dicarboxylic acids (Table I). Other peaks present in the FAB mass spectrum in Figure 1are due to the loss of 18 amu (presumably water) from ions corresponding to protonated cyclic or open chain oligomers. In this case metastable transitions are found in the spctra at mlz 200.4 (transition 235 217), m / z 371.8 (transition 407 389), m / z 113.2 (transition 147 129), m/z 284.0 (tran301), and m / z 138.9 (transition 173 155). sition 319 Fragment ions and relative transitions are shown in Table I. The presence of open chain compounds (Table I) in the FAB mass spectrum is not surprising. In fact, cyclic esters (formed in the polymerization process) are easily subject to hydrolysis during polymer work up and/or storage (12). In order to confirm the hypothesis that the peaks in Figure 1 originate from PEA oligomers, the CAD B / E linked scans of the peak at m / z 173 in the FAB spectrum of the polymer and of the quasi-molecular ion (M + H)+of an authentic sample of cyclic monomer were performed. The resulting two MS/MS spectra (omitted for brevity) were identical, allowing the assignment of the structure of the cyclic monomer to the peak a t m / t 173 in the mass spectrum of the PEA. The FAB mass spectrum of an accurately purified sample of PEA (where the oligomers had been extracted as indicated
+
- -
- -
-
I Y, loo/
,389
20
,363
,
407 400
473
-535
560
_S6I1579
b 600
m/z
Figure 2. Fast atom bombardment mass spectrum of mixture extracted from PEA.
in the Experimental Section) did not yield significant peaks beyond those due to the liquid matrix. The FAB mass spectrum of the mixture of oligomers extracted from PEA, as reported in the Experimental Section, is shown in Figure 2. It can be noted that this spectrum is similar to that obtained for crude polymer (Figure 1); in fact the peaks present in the spectrum can be assigned to protonated molecular ions of cyclic oligomers and open chain compounds (Table I). The FAB mass spectrum of crude PCL is shown in Figure 3. The spectrum shows intense peaks, which can be interpreted as corresponding to protonated cyclic oligomers ( m / z
2026
ANALYTICAL CHEMISTRY, VOL. 59,
NO. 17, SEPTEMBER 1, 1987 I% 4I
I% f
227
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361
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I
I
Ill
I
,.J
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229
A
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.L
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i'
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ifz
760
660
Flgure 5. Fast atom bombardment mass spectrum of Nylon 6.6.
4
lrti
133
500
m/2
Flgure 3. Fast atom bombardment mass spectrum of PCL.
3
Ll
1 I 300
I1
I . . l 400
'
I
b
m/2
r
457
1
343 I
L
475 ..L.
.. Flgure 6. Fast atom bombardment mass spectrum of Nylon 6.6, after treatment with methanol.
of the polymer and of the molecular ion (MH)+of an authentic sample of cyclic monomer were performed; the two resulting MS/MS spectra (omitted here for brevity) were essentially identical, allowing the assignment of the structure of the cyclic monomer to the peak at m / z 227 in the polymer. To further this interpretation, a sample of Nylon 6.6 was treated with methanol to extract the cyclic monomer and dimer contained (13, 14). The FAB mass spectrum of this treated sample is reported in Figure 6. It can be noted that the peak at m / z 227 (cyclic monomer) is missing, whereas the peak at m / z 679 (cyclic trimer) is more intense than the peak a t m / z 453 (cyclic dimer). This provides conclusive evidence that the peaks in the FAB mass spectrum in Figure 5 correspond to cyclic oligomers contained in the polymeric sample and desorbed under these conditions. The FAB mass spectrum of crude Nylon 6 is reported in Figure 7. The spectrum shows an intense peak a t m / z 114 and less intense peaks a t m / z 227 and m / z 340. The CAD B/E scans of the peak a t m/z 114 present in the polymer and in an authentic sample of +caprolactam were performed; the two MS/MS spectra (omitted for brevity) are essentially identical, allowing the assignment of the structure of the cyclic monomer to the ion at m / z 114 in the FAB spectrum of the polymer. Therefore, also in the case of Nylon 6, peaks present in the FAB mass spectrum (Figure 7) correspond to the protonated cyclic monomer (mlz 114), dimer ( m / z 227), and trimer ( m / z 340), respectively, contained in the polymer sample.
Anal. Chem. 1987, 59, 2027-2033
I
Registry No. PEA (SRU), 24937-05-1; PEA (copolymer), 24938-37-2; PCL (SRU), 25248-42-4; PCL (homopolymer), 24980-41-4; Nylon 6, 25038-54-4; Nylon 6.6, 32131-17-2.
,114
60i I
LITERATURE CITED
1% F
X
5
l 20 j
340 250
2027
3b0
350
-
m I1
Flgure 7. Fast atom bombardment mass spectrum of Nylon 6.
CONCLUSIONS Our results show that FAB mass spectra of the four polymers investigated show peaks due to the low molecular weight compounds already present in the polymer system. Peaks due to selective fragmentation of the polymeric backbone are not detectable in the mass spectra obtained in FAB mode. Recent studies (15-17) on the mechanism of formation of ions in FABMS indicate that the organic species emitted by the liquid matrix are those which do not collide directly with the atom beam,which is in contradiction with the hypothesis of a selective fragmentation of the polymers in the condensed phase with a subsequent desorption and cationization of the fragments originated. The latter process probably occurs in SIMS where a selective fragmentation of nylons has been reported by Bletsos et al. (18).
(1) Barber, M.; Bordoii, R. S.; Sedgwick, R. D.;Tyler, A. N. J . Chem. Soc., Chem. Commun. 1991, 325. (2) Fenseiau, C. J . Mt. Prod. 1984, 4 7 , 215. (3) Mess Spectrometry In the Health and Life Sciences; Proceedings of an International Symposium, San Francisco, California, Sept. 1984; Buriingame, A. L.. Castagnoii, N., Jr., Eds.; Elsevier: Amsterdam, 1985. (4) Foti, S.; Montaudo, G. I n Analysls of Polymer Systems; Bark, L. S . , Allen, N. S.. Eds.; Applied Science: London, 1982, p 103. (5) Schulten, H. R.; Lattimer, R. P. Mass Spectrom. Rev. 1084, 3 , 231. (6) Montaudo, G Puglisi, C. I n Developments in fo/ymer Degradation; Grassie, N., Ed.; Applied Science: London, 1987; Voi. 7, p 35. (7) Doerr, M.; Luderwaid, I.; Schulten, H A . Fresenlus’ 2.Anal. Chem. 1084, 378,339. (8) Doerr, M.; Luderwaid, I.; Schuiten, H A . J . Anal. Appl. pvrolysls 1085, 8 , 109. (9) Spanagel, E. W.; Carothers, W. H. J . Am. Chem. SOC. 1935, 5 7 , 929. (10) Garozzo, D.;GiuffrMa, M.; Montaudo, G. Macromolecules 1088, 19, 1643. (11) Garozzo, D.;GiuffrMa, M.; Montaudo, 0. folym. Bull. (Berlin) 1988, 75. . ., 353. - ..
(12) Manoiova, N. E.; Gitsov, I.; Veiichkova, R. S.; Rashkov, I. B. Polym. Bull. (Berlin) 1095, 73,285. (13) Gualta, C. Makromol. Chem. 1984, 185, 459. (14) Mori, S.; Takeuchi, T. J . Chromatogr. 1970, 49, 230. (15) Cooks, R. 0.; Busch, K. L. Int. J . Mass Spectrom. Ion fhys. 1983. 53.111. (16) Wong, S. S.; Roiigen, F. W.; Manz, I.; Przybyiski, M. Blomed. Mess Spectrom. 1985, 72, 43. (17) Desorption Mess Spectrometry; Lyon, P. A., Ed.; ACS Symposium Series; American Chemical Society Washington, DC, 1985. (18) Bietsos, I. V.; Hercules, D. M.; Graifendorf, D.; Benninghoven, A. Anal. Chem. 1985, 57, 2384.
RECEIVED for review October 28,1986. Accepted April 6,1987. Partial Financial Support from the Italian Ministry of Public Education and from Consiglio Nazionale delle Ricerche (Roma) is gratefully acknowledged.
Low-Voltage, High-Resolution Mass Spectrometric Methods for Fuel Analysis: Application to Coal Distillates C. E. Schmidt,* R. F. Sprecher, and B. D. Batts’
US.Department of Energy, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236 A low-voltage, high-resolution m a s spectrometric (LVHRMS) method for analyses of complex materiais, such as coaiderived liquids, Is presented. Descriptionsof the computations utliired to convert LVHRMS spectra to weight percent data on individual compound types and the associated computer software are reported. Elemental analyses, carbon number distributions, number average molecular weights, and a measwe of aromatktty have been determined for 10 distillate fractions derived from an HCoai llquld by this technique, and the results were compared with those obtained by classical methods. The changes in dlstrlbution of the principal compound classes over the range of distillates can readily be followed. The LVHRMS method descrlbed can be applied to other complex materials containing heteroatoms.
The application of mass spectrometry to the analysis of complex mixtures was developed by the petroleum industry Permanent address: School of Chemistry, Macquarie University, N o r t h Ryde, New South Wales 2113, Australia.
in the 1950s (1-5). All of these techniques involved the use of mass spectra obtained at 50-70 eV ionizing voltages. “Type” analyses for compound classes such as olefins, saturates, naphthenes, and aromatics were determined by matrix calculations. Robinson and Cook (6) extended the group-type analyses to include 21 compound types in petroleum aromatic fractions, accounting for the entire composition of the sample. Robinson (7) later developed a new low-resolution mass spectrometric procedure for determining up to 25 saturated and aromatic types in petroleum fractions having wide ranges of boiling points (200-1100 OF) and composition. The use of low ionizing voltages (10-15 eV) to limit the spectra to molecular ions was introduced by Field and Hastings (8) and further developed by Lumpkin (9). Upon the commercial availability of high-resolution mass spectrometers, Reid e t al. (10) and Lumpkin (11) extended the mass spectrometric technique for hydrocarbon analysis to isobaric compound types, such as alkylbenzenes and benzothiophenes, and also naphthenobenzenes and pyrenes, employing a mass-resolving power of 1 part in 10 000. Gallegos et al. (12) published the first multicomponent group-type
This article not subject to U.S. Copyright. Published 1987 by the American Chemical Society
