Mass Spectrometry of Polymers: Polypropylene Glycol

Transcription

1 Mass Spectrometry of Polymers: Polypropylene Glycol Gregory M. Neumann, Peter G. Cullis, and Peter J. Derrick La Trobe LTniversity, Physical Chemistry Department, Bundoora, Victoria 3083, Australia Z. Naturforsch. 35 a, (1980); received July 30, 1980 Dedicated to Professor H.D.Beckey on occasion of his 60th. birthday Mass spectra have been measured, using field desorption technique, for polypropylene glycols with nominal mass-average relative molecular masses of 1000, 2000, 3000 and Ions of up to mjz 7400 have been observed. Major peaks in spectra correspond to (M-f H) + ions. (M + Na) + ions are observed if sodium iodide is added to sample. There are strong peaks corresponding to fragmentation of polymer chains close to ir termini. It is suggested that se fragmentations are induced by strong electric fields at emitter. 1. Introduction mass Mass spectrometry is, almost by definition, a method f o r measuring molecular masses precisely, and molecular mass is perhaps most important property of a polymer. Yet, use of mass spectrometry in polymer chemistry has f o r most part been restricted to analysis of volatile products from degradation of polymers [1]. That mass spec- trometry has not become a standard method f o r determining molecular masses of polymers is due to difficulties created by low volatility and high masses of se compounds. Recently re have been some preliminary results, using specialized ionization techniques, which suggest that mass spectrometry may soon make a much more significant contribution to polymer chemistry. Using field desorption technique [2] and a thin wire coated with silicon microneedles as emitter, Matsuo et al. [ 3 ] have shown that gaseous pseudomolecular ions of up to mjz can be formed f r o m polystyrene. Our interest in mass spectrometry of syntic polymers was stimulated by fact that polyvinyl alcohol ions could be field desorbed f r o m sharp edge emitters [ 4, 5 ]. The purpose of this paper is to present results of a detailed investigation of field desorption of polypropylene glycols of nominal mass-average relative molecular masses 1000, 2000, and ( f r o m here on, samples will be referred to as mass-1000, mass-2000, and mass-4000 mass-3000 respectively). These results show how, in addition to determination of molecular distributions, field desorption mass spec- trometry can elucidate both molecular structures of polymers and chemistry of macromolecular ions. The szecialized techniques used with non-volatile compounds generally rely on f o r m i n g and desorbing ions f r o m a solid probe, reby circumventing need f o r volatilization of neutral prior to ionization [6, 7 ]. In field desorption [ 7 ], strong electric fields are used f o r this purpose, although it is usually also necessary to gently heat sample. The mechanism of field desorption, which is quite simple in case of atoms [ 8 ], is more complex in case of organic compounds [ 2 ]. Neverless, field desorption approach has been shown to possess an efficacy f o r production of molecular ions f r o m rmally labile, non-volatile compounds [9], which is far beyond that of any or ionization method apart f r o m californium-252 plasma desorption [ 6 ]. An obstacle to be surmounted in applying mass spectrometry to polymers is magnitude of magnetic field strength needed to deflect and analyse massive molecular ions. Consider ions of m / z , and a magnetic sector of radius mm. If ir translational energy were ev, a magneticinduction of over 4 T would be required to transmit and focus such ions. Fields of this magnitude have not been realised in mass spectrometry experiments. Extending energy mass leads to range b y reduced lowering sensitivity, particularly true in case of field and ion this is desorption. Here strong field, necessary to achieve significant Reprint requests to Dr. P. J. Derrick, Physical Chemistry Department, La Trobe University, Bundoora, Victoria 3083, Australien ,/ 80 / $ 01.00/0. ionization, also acts to accelerate ions to higher energies, so that beam, which is not collimated, must in fact be retarded if low energy ions are - Please order a reprint rar than making your own copy. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.v. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with Max Planck Society for Advancement of Science under a Creative Commons Attribution 4.0 International License.

2 G. M. Neumann et al. Mass Spectrometry of Polymers required for mass analysis [2, 10]. Our response to this problem of analysing massive ions has been to employ a magnetic sector of large radius (nominally 780 m m ). This in turn influences overall scale of mass spectrometer. Finally, massive ions of a given energy move at relatively slow velocities, leading to insensitivity of particle multiplier detectors [ 1 1 ]. Special techniques might refore be desirable f o r detection of massive ions [ 1 1 ], although in work reported here a particle multiplier proved to be satisfactory for ions of up to m/z Experimental The emitters employed for field desorption were formed from tungsten wire ( 1 0 p m diameter), using standard activation technique [ 1 2 ]. Activation converts wire substrate to tungsten carbide ( W C ) [ 1 3 ]. The magnitude of emitter heating current was set manually, and feedback stabilised to better than ± 1 0 pa. The physical position of an emitter in ion source was fixed, however cathode slit could be adjusted sideways so as to maximise mass-resolved signal [ 1 4 ]. All mass spectra were recorded with a potential of 8 kv on emitter and cathode at earth potential. The design and construction of double-focussing mass spectrometer employed have been outlined elsewhere [14, 15] and will be discussed in detail in relation to performance in a future paper [ 1 6 ]. The magnetic sector radius is nominally 780 mm and electric sector radius is 1000 mm. The widths of source and collector slits were fixed at 0.25 mm and 0.40 mm respectively, which gave a mass resolution of approximately The magnetic induction was sensed using magnetoresistor circuits. Up to m/z 1000, mass calibration of polypropylene glycol spectra was based on field ionization mass spectra of perfluorokerosene and perfluorotributylamine. Up to m/z 2500, re were sufficient peaks in mass spectra of glycols to allow "counting" of peaks. Above m/z 2500, mass assignment rests on reproducibility of measured values of magnetic induction. The uncertainty in mass assignments above m/z 2000 is placed at , e. g. uncertainty in m/z 5000 is + 5. If, however, it is assumed that type of ions formed above m/z 2500 with mass-2000, and samples are same as those formed with mass-1000 sample, mass assignments at all masses are reliable to better than of an m/z unit. The polypropylene glycol samples were purchased from Aldridi. Gel permeation chromatography was used to test purity. Samples were loaded onto emitters eir as pure compounds or dissolved in a solvent (acetone, water or ethanol/water). In certain experiments, various amounts of mineral acid were added to sample solutions to encourage protonation. 3. Results The polypropylene glycols could be field desorbed at room temperature. Passing a heating current of up to 15 ma through emitter in general enhanced ion emission (see below). The total emitted ion currents could be as high as 10 na, and could persist for some hours. The usual concept of a "best anode temperature" [ 9 ] was not relevant to se experiments, since ion desorption occurred smoothly over a broad range of heating currents. The field desorption mass spectra of polypropylene glycol samples are shown in Figures 1 3. Regions of mass-1000 spectrum are shown in greater detail in Figure 4. The large numbers of peaks in each spectrum can be arranged into a relatively small number of series of peaks, in which successive members are separated by 58 m/z units. 58 a. m. u. is repeat unit of polypropylene glycol. Indeed, only peak in spectra which does not appear to be part of some series is m/z 87 peak, origin of which is unclear. The alteration in peak intensity on moving along a series is generally gradual as is evident from Figures 1 3. This allows concept of an "intensity envelope" to be used to mean distribution curve which would result from drawing a continuous line through tops of all peaks in any particular series. There appears to be 6 strong series, composed of peaks corresponding to ions of general formulae (A/ + l ) +, (Af 1 7 ) ( M ) t, (M-31)+, (M ) + and (M 104) t - M devotes relative molecular masses of polymer, which are given by formula M = n (n is an integer). All six series occur with each of four samples of polypropylene glycol, however ( M ) + and ( M ) t series are very weak in mass-3000 and mass4000 spectra and are not shown in Figure 3.

3 1092 G. M. Neumann et al. Mass Spectrometry of Polymers 1092 R CH, HO+CH 2 CHO L Jn -In _Li LL ill nil mte Li J_L Ii,.Ii.1,.1..1 I l I T Fig. 1. The fielet desorption mass spectrum of mass-1000 polypropylene glycol. Emitter heating current was 13.0 ma. 13 C isotope peaks have been omitted. I I i.i i i i m/z Fig. 2. The field desorption mass spectrum of mass-2000 polypropylene glycol. Emitter heating current was 13.0 raa. 13 C isotope peaks have been omitted m/z Fig. 3. The field desorption mass spectra of mass-3000 and mass-4000 polypropylene glycol. Emitter heating current was 13.0 ma for mass-3000 and 15.0 ma for mass Only peaks in (M-f (M 18)+ and (M 31)+ series are shown. 13 C isotope peaks have been omitted.

4 1093 G. M. Neumann et al. Mass Spectrometry of Polymers 1 Mn = HO-CH-CH2-O-CH-CH2-O- CH-CH2-0j^CH-CH 2-0H I A5 ' SOD 831 i i. i 250 L A L i i k >i m/z Fig. 4. Two regions of field desorption mass spectrum of mass-1000 polypropylene glycol. Emitter heating current was 12.0 raa. The (Af ) + is designated as such rar than (Af 4 5 ) b e c a u s e it appears that this fragment is derived f r o m polymer molecule whose mass is 103 a. m. u. greater than that of fragment ion. This conclusion is based on fact that intensity envelope f o r (M ) + series is very similar to that of (Af + 1 ) + series, except that former is shifted to lower masses b y 104 m/z units. The intensity envelopes of (Af 1 8 ) t and (Af 104) t series are also similar to that of [ M + 1 ) + series. That of ( A f ) t series is shifted to lower mass by 19 m/z units, compared to { M + 1 ) + series, and that of ( M ) t b y 105 m/z units. The intensity envelopes of (M 3 1 ) + series, which includes mjz 4 5, m / z 103, m/z 161, m/z 2 1 9, m/z and m / z 3 3 5, and ( A f 1 7 ) + series, which includes m/z 59, m / z 117, m/z 175, m/z 2 3 3, m/z and m / z 3 4 9, are evidently very different from that of (A/ -f 1 ) + series 15m, 13mA ~ O "3 10mA (Figures 1 3 ). There is admittedly a degree of inconsistency in naming of se last two series, in that ( M ) + and ( M ) + are not intended to signify that fragment ions are derived f r o m polymer molecules whose masses are 31 or 17 a. m. u. greater than those of fragments (see b e l o w ). The peak intensities in mass spectra of all f o u r samples were dependent upon heating current. Figure 5 shows mass spectra of mass-1000 sample at a number of different heating currents. The room-temperature ( 0 m A ) mass spectrum was run first. The heating current was n switched on, and a mass spectrum run at 10.0 m A. The 11.5 m A, m A and 15.0 m A spectra were subsequently run in that order. It is evident f r o m Fig. 5 that higher heating currents favour desorption of higher mass ions, and this was case with all samples. If heating current was returned to a lower value after a mass spectrum had been run at i 115mA-, 10mA h^ji.r m/z Fig. 5. Field desorption mass spectra of mass-1000 polypropylene glycol at a number of different emitter heating currents. Only intensity envelopes or ( M + 1 ) + a n d (M 31)+series are shown.

5 G. M. Neumann et al. Mass Spectrometry of Polymers I '"'1 M N» HO-^CH2CHO-J-H _Li L J J I L 500 L J I L m/z Fig. 6. Field desorption mass spectrum of mass-1000 polypropylene glycol to which aqueous sodium iodide has been added. Emitter heating current was 10.0 ma. Only peaks in (M + Na)+, ( M + l ) +, (M 31)+ and (M 17)+ series are shown. 13C isotope peaks have been omitted. a higher value, mass spectrum characteristic of results is that se pseudomolecular ions are higher temperature was still observed but at formed to exclusion of molecular ions (M) t. That reduced intensity. Overall, degree of fragmenta- molecular ions are not formed is attributed to tion was found to be insensitive to emitter heating magnitude of field strengths [ 2 ] experienced by current with all four samples. There was a certain polymers. The polymer, existing in a condensed dependence of relative intensities of peaks in phase on emitter surface, is probably adsorbed in (M 1 7 ) + and (M 3 1 ) + series upon heating regions where field strengths are not sufficiently high current, which is shown in Figure 5. Orwise, to induce field ionization. only pronounced effect was that intensities of ( M 18) t peaks fell on raising heating current. Matsuo et al. [ 3 ] have reported that major peaks It is suggested that pseudomolecular ions ( M + H ) + are formed by proton transfer f r o m one polymer molecule to anor, and that this is induced in F D mass spectrum of polypropylene glycol by electric field (of, perhaps, V m - 1 ) (mass-1000) are due to [M + N a ) + ions. ( M + Na) Röllgen, Beckey and colleagues [ 1 7 ] and Block and + [2]. ions were not observed in our experiments, unless colleagues samples were deliberately contaminated reactions can proceed at field strengths below with [18] have found that proton transfer sodium. Figure 6 shows a mass spectrum of threshold mass-1000 sample to which aqueous sodium iodide occur due to force of field on protonated (to a concentration of approximately 1 M ) had been molecule overcoming intermolecular forces hold- for field ionization. Desorption would added. The major peaks at high masses correspond ing to cationized molecules desorption upon heating is attributed to disruption ( M + Na) + ; ( M - f l ) + molecule at surface. The enhanced peaks are still present. No sodium-cationized equiv- of restraining intermolecular forces. The ten- alents of or series of peaks were observed. dency One effect of doping samples with sodium iodide heating was that rate of desorption became more sensitive number of intermolecular bonds, as compared to a to emitter heating current, and, in contrast to ex- larger molecule. That transferred proton origi- periments on polymer in absence of salts, nates f r o m anor polymer molecule can be infer- re was an optimum heating current, or red f r o m fact that ( M + H ) + are formed in "best anode temperature". The polymer has also been cationized using potassium and caesium. 4. Discussion The (M + 1 ) + peaks are assigned to protonated molecules ( M + H ) +, so that an obvious feature of f o r smaller molecules to currents would result desorb from at lower smaller absence of solvent. A distinctive feature of field desorption of polypropylene glycol is that desorption occurs smoothly and in a controllable fashion over a wide range of emitter heating currents This relative insensitivity to (0 to 15 m A ). heating current is

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7 1096 G. M. Neumann et al. Mass Spectrometry of Polymers CHa I pected to be a favourable process. A rmal de- H-(0CHCH 2 ) Oj-CHCHJOH composition does not seem to be involved, since (M 1 8 ) t (DI) CH, EMITTER H (0CHCH2) OH +C 3 H 7 Ö1 n-1 m/z 59 favoured by fact that a stable molecule (poly- propylene glycol) is being formed. If polypropylene glycol formed in proposed reactions (Scheme II and III) were subsequently ionizh)+ ed, ( M + ions would presumbly be produced and contribute to se peaks in spectra. It appears, however, various that neutral field-induced fragments reactions are from not ionized, orwise neutrals f r o m reaction (I) should be visible in mass spectrum. These neutrals have masses (M 102), and if ionized should occur as in mass spectrum. There are no (M 101)+ ions at se masses. The neutral fragments presumably escape f r o m ionization region. The [M 103) + -series of fragment ions might be explained on basis of mechanism (IV). That intensities of se ions are always weak can be explained on grounds that charge resides on fragment closest to emitter [2]. As this before, EMITTER //A reason CH, CH? H (0CH2CH) 0 ) I n'2 H f o r rupture of ch 3 polypropylene glycols of nominal mass-average relative molecular masses 1000, , and These are polymers of relatively low molecular masses, but re is possibility that technique will be equally successful with compounds of higher masses. It is pointed out that re is no noticeable fall in ionization efficiency on moving f r o m m / z up to m / z The determination o f molecular mass distribu- tions of polymers by mass spectrometry appears to be feasible, at least f o r low molecular masses. What is required are mass standards, which can be used to obtain correction factors f o r pseudo- molecular ion intensities. The long-chain field-induced pseudomolecular ions undergo reactions to an extent which would be quite unusual f o r smaller molecular ions. The fieldinduced fragmentation provides structural informa- Finally, it is emphasised that ion emission can (E) be intense (order of 10 n A ) and stable over lengthy I periods of time ( h o u r s ), opening up possibilities f o r or experiments such as collisional decomposi- 103) + particular C C b o n d (Scheme I V ), rar than any of or C C bonds in chain, is seen as positive charge tending to be localized in this region due to influence of field. The (M 18) t Using technique of field desorption gaseous pseudomolecular ions ( M + H ) + can be formed f r o m chains. > H (0CH2CH) 0=CH2 + CH2OCH2CHOH n-2 (M 5. Conclusion tion about groups at ends of polymer ^ CH 0CH2CH0H + intensities fall on raising heating current. + tion [ 1 9 ] or laser spectroscopy [ 2 0 ] of beam of massive ions. A cknowledgements W e are pleased to acknowledge financial sup- and ( M ) t series of frag- port of Australian Research Grants Committee ment peaks differ f r o m or fragments dis- under Grant N o. C 7 6 / W e are delighted to cussed in that y are odd-electron species. The (M 104)?" is a minor peak in all spectra, be able to express our gratitude to Mr. l o h n Chippindall for his invaluable contribution to this however (M 18) t is a major fragment ion with research project. W e are indebted to Dr. Ken Ghig- mass-3000 and mass-4000 spectra. The forma- gino of Melbourne University f o r gel permeation tion of odd-electron ion [M 18) t chromatography results and f o r valuable discussion even-electron f r o m (A/ + H ) + would not have been ex- [1] (a) D. O. Hummel, in "Polymer Spectroscopy", ed. D. O. Hummel, Verlag Chemie, Berlin (b) C. David in "Degradation of Polymers", Vol. 14 of "Comprehensive Chemical Kinetics", eds. C. H. Bamford and C. F. H. Tipper, Elsevier, Amsterdam of mass spectrometry results. [2] H. D. Beckey, Principles of Field Ionization and Field Desorption Mass Spectrometry, Pergamon Press, Oxford [3] T. Matsuo, H. Matsuda, and I. Katakuse, Anal. Chem. 51, 1329 (1979).

8 G. M. Neumann et al. Mass Spectrometry of Polymers 1097 [4] M. G. Darcy, M. Sc. Thesis, La Trobe University [5] See also R. D. Lattimer, D. J. Harman, and K. R. Welch, Anal. Chem. 51, 1293 (1979). [6] R. D. Macfarlane, C. J. McNeal, and J. E. Hunt, Adv. Mass Spectrom., 8 (1980). [7] H. D. Beckey, Int, J. Mass Spectrom. Ion Phvs. 2, 500 (1969). [8] E. W. Müller, Phys. Rev. 102, 618 (1956). [9] H. D. Beckey and H.-R. Schulten, Agnew, Chem. Int. Ed. 14, 403 (1975). [10] A. J. B. Robertson and B. W. Vinev, J. Chem. Soc. A 88, 1843 (1966). [11] R. J. Beuhler and L. Friedman, Int. J. Mass Spectrom. Ton Phys., 23, 81 (1977). [12] H. D. Beckey, E. Hilt, and H.-R. Schulten, J. Phys. E. Sei. Instrum. 6, 1043 (1973). [13] G. M. Neumann, D. E. Rogers, P. J. Derrick, and P. J. K. Paterson, J. Phys., D: Appl. Phys. 13, 485 (1980). [14] P. G. Cullis, G. M. Neumann, D. E. Rogers, and P. J, Derrick, Adv. Mass Spectrom., 8 (1980). [15] M. G. Darcy, D. E. Rogers, and P. J. Derrick, Int. J. Mass Spectrom. Ion Phys., 27, 335 (1978). [16] A. G. Craig, P. G. Cullis, K. F. Donchi, G. M. Neumann, D. E. Rogers, and P. J. Derrick, to be published. [17] (a) F. W. Röllgen, H. J. Heinen, and U. Giessmann, Naturwiss. 64, 222 (1977). (b) F. W. Röllgen and H. D. Beckey, Z. Naturforsch. 29 A, 230 (1975). (c) F. W. Röllgen and H.-R. Schulten, Z. Naturforsch. 30A, 1685 (1975). [18] N. Ernst, G. Bozdech, and J. H. Block, Int. J. Mass Spectrom. Ion Phys., 28, 33 (1978). [19] F. W. McLafferty, Accounts Chem. Res. 13, 33 (1980). [20] A. Carrington, Proc. Roy. Soc. London 367 A, 443 (1979).