Tandem mass spectrometry of polyglycols

International Journal of Mass Spectrometry and Ion Processes, 90 (1989) 119-129 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

TANDEM MASS SPECTROMETRY

R.P. LATTIMER

Center, Brecksville,

OH 44141 (U.S.A.)

and H. BUDZIKIEWICZ

Institute for Organic Chemistry (Received

OF POLYGLYCOLS

*

The B.F. Goodrich Research and Development H. MONSTER

119

University of Cologne, 5000 Cologne 41 (F.R. G.)

13 June 1988)

ABSTRACT Fast atom bombardment-tandem mass spectrometry has been carried out with low molecular weight samples of poly(ethylene glycol) and poly(propylene glycol). Sodium attachment ions generated from either polymer were found to be extremely stable. Decomposition could be induced only by high energy (keV) collisional activation; dissociation was minimal even for cations with 3-keV translational energy. The fragmentation that does occur can be explained by 1,Chydrogen elimination with concurrent C-O bond cleavage. Proton attachment ions, however, were found to be much less stable. These decomposed easily upon low energy collision (57 ev) to yield fragment ions which can be explained by charge-site initiation. From an analytical viewpoint, it appears that tandem mass spectrometry, combined with a desorption ionization method, is very useful for direct analysis of the chemical structure of individual components in an oligomeric mixture.

INTRODUCTION

Applications of tandem mass spectrometry (MS/MS) in polymer science have been fairly slow to evolve. Three specific areas can be considered, however, in which at least some MS/MS work has been reported: (1) identification of organic additives in compounded polymers [l]; (2) identification of volatile pyrolyzates in polymer pyrolysis studies [2-51; (3) characterization of individual oligomers in low molecular weight polymers [6-lo]. In all of these areas, the principal advantage of using MS/MS is that more information is available from mixtures than can be obtained by direct analysis using conventional (single-stage) mass spectral methods. A current

* Author

to whom correspondence

0168-1176/89/$03.50

should be addressed.

0 1989 Elsevier Science Publishers

B.V.

120

thrust in polymer analysis is to reduce the time and cost involved by analyzing the sample directly, rather than first undertaking preliminary separation steps such as solvent extraction and chromatography. Mass spectrometry (MS) is an especially effective means for direct polymer analysis [11,12], particularly when desorption ionization and MS/MS methods are employed. This report focuses on the third MS/MS application mentioned above, i.e., low molecular weight polymers. Oligomers are always mixtures. It would often be useful to obtain a mass spectrum (fragmentation pattern) for a particular oligomeric component in a low molecular weight polymer system. The only conventional way to do this is by separating the components chromatographically and trapping a specific oligomer; this is laborious, time consuming and not always successful. On-line liquid chromatography-mass spectrometry or supercritical fluid chromatography-mass spectrometry would be more direct approaches, but these are also somewhat time consuming since appropriate experimental conditions need to be developed. With MS/MS it is possible to obtain the mass spectrum of a single oligomer directly from the original sample. In this approach we would normally use a desorption ionization method to obtain abundant molecular ions, e.g., M+’ or (M + Na)+. These would then be collisionally dissociated and subjected to a second stage of mass analysis. This would prove useful in (1) analysis of chemical structure, (e.g., the determination of monomers and end groups and whether the polymer is linear, cyclic or branched); (2) studies of sequence distribution (in cases where there is a copolymer); (3) studies of mechanisms (e.g., the investigation of mass spectral fragmentation pathways). The literature is rather sparse in the area of MS/MS of low molecular weight polymers. Craig and Derrick [6-81 have conducted some initial field desorption-tandem mass spectrometry studies with polystyrene. Catlow et al. [9] conducted preliminary electron impact-tandem mass spectrometry measurements on an antistatic agent which contained a poly(ethylene glycol) (PEG). Montaudo et al. [lo] used fast atom bombardment-tandem mass spectrometry (FAB-MS/MS) to study some polyamide and polyester oligomers. have conHydroxyl-terminated polyglycols, H(O-CH,-CHX).OH, stituted the class of low molecular weight polymers most often studied by mass spectrometry. Several desorption ionization methods have been used, with varying degrees of effectiveness; field desorption-mass spectrometry [13-161, electrohydrodynamic ionization-mass spectrometry [17-191, laser specdesorption-mass spectrometry [20-231, 252Cf plasma desorption-mass trometry [24], chemical ionization-mass spectrometry [25] and fast atom bombardment-mass spectrometry [26].

121

In a FAB-MS study of PEG and poly(propylene glycol) (PPG) [26], it was found that intense (M + Na)+ ions could be produced via cation attachment. While the oligomer quasi-molecular ion envelope gave a rough indication of the molecular weight distribution of the polymer, the data could not be used to determine accurate molecular weight averages. More specifically, lower mass oligomer intensities tended to be overemphasized in the distribution owing to fragmentation reactions. Two basic types of mechanisms were proposed to account for the FAB mass spectra. The first is a thermal-type mechanism in which C-O bonds are cleaved to yield lower mass PEG/PPG oligomers and other products. The second is a charge-siteinitiated cleavage of (M + H)+ ions; this also yields lower mass PEG/PPG oligomers. In this study we report the FAB-MS/MS analysis of low molecular weight PEG and PPG samples. One objective was to assess the usefulness of MS/MS for analysis of this kind of oligomeric mixture. A second objective was to investigate the mechanisms by which PEG/PPG cation attachment ions fragment upon collisional activation. EXPERIMENTAL

A Finmgan MAT H-SQ 30 mass spectrometer [27] of BEQQ geometry was used for FAB-MS/MS analysis. This instrument has four collision cells. In our study, collisional activation was carried out in the third field-free. region (FFR3), located between the electrostatic sector and the first quadrupole, for high energy activation, using helium as the collision gas, and in the “collision quadrupole” (Ql) for low energy activation, using air as the collision gas. The collision gas pressure was adjusted to an observed reduction of the parent ion intensity by 50%70%. The fast atom gun provided xenon atoms at an energy of 8 keV. The copper FAB target was attached to the tip of the direct introduction probe. Key instrumental parameters were as follows: accelerating voltage 3 kV, magnetic sector resolution 1000, ion source temperature 30’ C. Scans for both the magnetic sector and mass-separating quadrupole were controlled by the data system (Finnigan SS-300). Data acquisition and output were also under computer control. Two low molecular weight polyglycols were examined: PEG 600 (Baker) and PPG 41993 (Waters). The number of average molecular weights (as supplied by the manufacturers) were 600 and 790 respectively. The viscous liquid samples were placed directly on the FAB target for analysis. In some experiments, 1 ~1 of a 2 M solution of NaI in methanol was added to the sample on the probe.

122 RESULTS AND DISCUSSION

Initial PEG 600 experiments were carried out with added NaI (to promote cation attachment). This gave an intense FAB mass spectrum of sodium attachment ions. A daughter ion scan of m/z 569, activated at low collision energy (= 51 eV) in Ql, is shown in Fig. 1. This ion is (M + Na)+ for the dodecamer H(O-C,H,),,OH. As can be seen, there are no appreciable fragment ions; essentially all of the ion current still resides in the precursor ion (m/z 569). This indicates that the sodium attachment ion is extremely stable towards collisional dissociation. Daughter ion experiments were also carried out at high collision energy (3 keV) by the use of FFR3. The (M + Na)+ ions at m/z 569 (dodecamer) and m/z 613 (tridecamer) were examined, and these precursors did show some dissociation at the higher collision energy. Figure 2 is the daughter ion scan of m/z 613. Two very weak but distinguishable fragment ion series are observed, which are labeled B and C in Fig. 2. It appears that these ions may be formed via remote charge-site ion decomposition as shown in Scheme 1. There are close analogies between this type of mechanism and that described by Adams and Gross for the high energy collisional activation of cationized long chain fatty alcohols [28] and acids [29]. The mechanism involves a 1,4-hydrogen elimination with the formation of two terminally unsaturated chains. For PEG, sodium attachment daughter ions can be formed with either a vinyl (B series) or a formyl (C series) end group. It should be noted that in Fig. 2 the daughter ions are mostly higher mass ions. This could be due simply to a statistical effect. That is, upon C-O bond cleavage (Scheme 1) the sodium ion is statistically more likely to reside on the longer chain end. Remote charge-site ion decompositions are a matter of current debate

350

400

450 m/z

Fig. 1. Daughter ion scan (51 eV) for m/z

500

550

569 [ethylene glycol dodecamer (M+Na)+

1.

123

272 1 A. I”“““‘I”“““‘I”“~““I”“““‘l”“~““l’”’l” 50 100

II

I

I

I

150

A, 200

2?6

1

11

250

319 A

I

300

613

4;3 3& ’

377

III~llIIII

c 38’

1.1 .

I

IIII

350

4y9

+

4;9 4!6

1111,,,,,,‘,,,,,,,,,,,,,,,,,,1,,,,,,,,,1,1 400

450

1,

*

m/z

Fig. 2. Daughter ion scan (3 keV) for m/z

500

5;3

6Bsl 5i7

1

I

550

ALa.

( 600

613 [ethylene glycol tridecamer (M+Na)+

1.

in the literature and mechanisms other than that shown in Scheme 1 may be possible. These are, however, typical high energy processes. Proton attachment ions can be obtained for ethylene glycol oligomers by analyzing the PEG directly (without addition of NaI). The (M + H)+ ions obtained via FAB-MS are considerably less intense than the (M + Na)+ ions obtained with NaI present, but these are still intense enough to perform MS/MS experiments. The Ql (low collision energy) daughter ion scan for the ethylene glycol decamer (m/z 459) is shown in Fig. 3. (A daughter ion scan for the undecamer, m/z 503, was also run, with very similar results.) The spectrum in Fig. 3 is somewhat noisy (owing to the fairly weak intensity

/?!9 H\CH-CH2+O-CzH&OH]+

Na [H-~O-C~HQ~~O-HC

c

‘G&O

A. m/z 44(m+n+21+41 i No [H -(-O-C2H4+mO-CH=CH2]++ B. m/z44(m+I)

OCH-CH2-kO-C2H&,OH

+23 or

H -(-O-C+H4$-,O-CH=CH2

+

Na [OCH-CH2+0-C2H&, C. m/z 44(n+ll+39

Scheme 1.

OH]+

124

80

100

120

140

160

280

300

320

340

360

Fig. 3. Daughter ion scan (51 eV) for m/z

180

m/z

380

200

400

220

420

240

440

260

460

459 [ethylene glycol decamer (M+ H)+ 1.

of the precursor m/z 459) but the principal daughter ions are nevertheless quite clear. There is only one prominent series (labeled D) and only the lower mass members of the series are prominent. These fragment ions are formed logically by cleavage due to charge-site-initiation (an inductive effect) as shown in Scheme 2. The carbonium ion shown in the scheme may undergo rearrangement to a more stable species. Since the molecule is hydroxyl terminated on both ends, only one ion series is produced. In comparing the FAB-MS/MS results for the (M + Na)+ ions with those for the (M + H)+ ions, it may be noted that the sodium attachment ions are much more stable than the proton attachment ions. That is, while the proton attachment ions could easily be fragmented at low collision energy (51 ev), the sodium attachment ions could only be dissociated to a

HO+C2H4-0+mCH2-CH~-~++C~H4-O+H A. m/z 44(m+n+l)

+ I9

1 HOfC2H,0+mCH,-CH; D. m/z 44(mtl

Scheme 2.

+

I+ I

HO+C2H,-O-);;H

I”“““‘I”“““‘I”“““‘I”“““‘I”“““‘I”””’

50

100

150

450

460

350

Fig. 4. Daughter

200

250

300

6?1

m/z

ion scan (3 keV) for m/z

621 [propylene

glycol decamer

(M + Na)+ 1.

very small extent even at high collision energy (3 keV). It may also be noted that the mechanisms of collisional dissociation are different. While the proton attachment ions follow a charge-site-initiated mechanism (Scheme 2), the sodium attachment ions appear to fragment via a 1,4-hydrogen elimination pathway (Scheme 1). Experiments with PPG 41993 were carried out both with and without added NaI. A daughter ion scan for m/z 621 (3 keV collision energy, FFR3) is shown in Fig. 4. This ion is (M + Na)+ for the PPG decamer. The daughter ions are extremely weak (barely above background level), but

CH3

/5

No[HSO-CH&HhO-HC

H,

c

‘7-O

CH3

CH3

CH-hH+O-CH2-hHt,OH]+

H3C’ A. m/z 44(m+n+2)

c

y43

CH3

Na[HfO-CH2-CH+,O-CH=CH-CH3]++ B. m/z 58(m+l)

+4l

CH3

0CH-hH~O-CH2-;H~OH

+23 OT

C”3

H+O-CH,-hH%O-CH=CH-CH3

CH3

C. m/z 58(n+l)

Scheme 3.

CH3

+ Na[OCH-kH$O-CH&H+,OH]+ +39

126

2Q3

031

450

500

550

600

650 m/z

Fig. 5. Daughter ion scan (51 eV) for m/z

700

750

800

831 [propylene glycol tetradecamer

(M+ H)+ 1.

nevertheless two fragment ion series can be distinguished (labeled B and C). These series can be explained by a 1,4-hydrogen elimination pathway (Scheme 3) which is completely analogous to that for PEG (Scheme 1). The fragment ions from methine C-O cleavage contain either a propenyl (B series) or a formyl (C series) end group. (It should be noted that the end groups would be isopropenyl and acetyl in the case of cleavage of a methylene C-O bond.) The daughter ion scan for m/z 831 (51 eV collision energy, Ql) is shown in Fig. 5. This ion is (M + H)+ for the propylene glycol tetradecamer. (Daughter ion scans were also obtained for other (M + H)+ ions, with similar results.) There is only one prominent ion series (labeled D). These ions can be explained via a charge-site-initiated mechanism (Scheme 4) which is analogous to that for PEG (Scheme 2). The parent ion scan (51-eV collision energy, Ql) for m/z 175, a D series fragment ion, is shown in Fig. 6. The Figure shows that there are two types of precursors. One is the protonated molecular ion series (labeled A), and the other is the fragment ion series D. This shows that the D series ions are capable of depolymerizing (unzipping) during collisional activation, with the neutral loss of ethylene oxide oligomers. This depolymerization phenomenon

127

y3

y3

HO-+CH2

-CH-O+mCH,

-CH

A. m/z

58(m+n+l) 1

HOfCH2-CH-O+mCH2--C+H 0. m/z 58(m+l)

+ +

-0%

H

+ 19

y3

y3

y3

7

-_S+CH2-CH

CH I 3 HO+CH2-CH-O-f;yH

I

Scheme 4.

III 200

250

300

350

400

450

977 ,,;,,,,, 4,;

llllll

600

lllll+ 550

500

650

700

750

Fig. 6. Parent ion scan (51 eV) for m/z

800

m/z

850

900

950

,,,.,,

~,,,

1000

175 (PPG 41993).

has been reported earlier for collisionally induced dissociation chains, including polystyrene and polyols [6].

of polymer

CONCLUSION

The FAB-MS/MS results in this report show that once a polyglycol sodium attachment ion is ejected into the vapor phase and leaves the ion source region, it is a very stable species. Decomposition can be induced only by high energy (keV) collisions, and even then the dissociation may be minimal. This effect is probably due to relatively high barriers to fragmenta-

128

tion, and it is typical for remote charge control processes. It may well be related to the mechanism of energy uptake. In any case, (M + Na)+ ions in the vapor phase are not likely contributors to the FAB fragmentation patterns of polyglycols. Proton attachment ions, in contrast, are much less stable. (M + H)+ ions decompose easily upon low energy collision to yield low mass fragment ions which can be explained by charge-site initiation (Schemes 2 and 4). These types of reactions may be responsible for many of the prominent low mass fragment ions that are observed in the FAB mass spectra of polyglycols [26]. A final note of caution is that the experiments described in this report were conducted at low mass resolution, both in the magnetic sector and the quadrupole portions of the instrument. Thus, it is possible that some of the collisionally induced ions observed could represent unresolved multiplets. In this case, more than one decomposition pathway would be operative. From an analytical viewpoint, it appears that MS/MS combined with a desorption ionization method is very useful for direct analysis of the chemical structure of individual components in an oligomeric mixture. Questions involving monomer types, end groups, branching and sequence distributions may be addressed. MS/MS provides a rapid means for this type of analysis, since time-consuming physical or chemical separations of components are not required. ACKNOWLEDGMENT

Appreciation is expressed to The B.F. Goodrich Co. and to the Deutsche Forschungsgemeinschaft for support of this work. REFERENCES 1 R.P. Lattimer, Rubber Chem. Technol., 61 (1988) 658. 2 B. Shushan, B. Davidson and R.B. Prime, Anal. Calorim., 5 (1984) 105. 3 A. Ball&ret-i, D. Garozzo, M. Giuffrida and G. Montaudo, Polym. Degrad. Stab., 16 (1986) 337. 4 G. Montaudo, E. Scamporrino, C. Puglisi and D. Vitalini, J. Anal. Appl. Pyrol., 10 (1987) 283. 5 A. Ballistreri, D. Garozzo, M. Giuffrida and G. Montaudo, J. Anal. Appl. Pyrol., 12 (1987) 3. 6 A.G. Craig and P.J. Derrick, J. Chem. Sot., Chem. Commun., (1985) 891. 7 A.G. Craig and P.J. Derrick, J. Am. Chem. Sot., 107 (1985) 6707. 8 A.G. Craig and P.J. Derrick, Aust. J. Chem., 39 (1986) 1421. 9 D.A. Catlow, M. Johnson, J.J. Monaghan, C. Porter and J.H. Scrivens, J. Chromatogr., 328 (1985) 167. 10 A. Ballistreri, D. Garozzo, M. Giuffrida, G. Montaudo, A. Filippi, C. Guaita, P. Manaresi and F. Pilati, Macromolecules, 20 (1987) 1029.

129 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

H.-R. Schulten and R.P. Lattimer, Mass Spectrom. Rev., 3 (1984) 231. R.P. Lattimer, R.E. Harris and H.-R. Schulten, Rubber Chem. Technol., 58 (1985) 577. T. Matsuo, H. Matsuda and I. Katakuse, Anal Chem., 51 (1979) 1329. G.M. Neumann, P.G. Culhs and P.J. Derrick, Z. Naturforsch., Teil A, 35 (1980) 1090. J. Saito, S. Toda and S. Tanaka, Shitsuryo Bunseki, 28 (1980) 175. R.P. Lattimer and G.E. Hansen, Macromolecules, 13 (1980) 776. S.-T.F. Lai, K.W.S. Chan and K.D. Cook, Macromolecules, 13 (1980) 953. K.W.S. Chan and K.D. Cook, Org. Mass Spectrom., 18 (1983) 423. K.W.S. Chan and K.D. Cook, Macromolecules, 16 (1983) 1736. D.E. Mattem and D.M. Hercules, Anal. Chem., 57 (1985) 2041. C.L. Wilkins, D.A. Wei& C.L.C. Yang and CF. Ijames, Anal. Chem., 57 (1985) 520. R.S. Brown, D.A. Weil and C.L. Wilkins, Macromolecules, 19 (1986) 1255. R.J. Cotter, J.P. Honovich, J.K. Olthoff and R.P. Lattimer, Macromolecules, 19 (1986) 2996. B.T. Chait, J. Shpungin and F.H. Field, Int. J. Mass Spectrom. Ion Processes, 58 (1984) 121. R.D. Smith, H.T. Kahnoski and H.R. Udseth, Mass Spectrom. Rev., 6 (1987) 445. R.P. Lattimer, Int. J. Mass Spectrom. Ion Processes, 55 (1983/1984) 221. Firmigan MAT GmbH, Postfach 144062, D-2800 Bremen 14, F.R.G. J. Adams and M.L. Gross, J. Am. Chem. Sot., 108 (1986) 6915. J. Adams and M.L. Gross, Anal Chem., 59 (1987) 1576.