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Anal. Chem. 1992, 64, 2862-2865
Analysis of Phosphite Polymer Stabilizers by Laser Desorption/ Electron Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Xinzhen Xiang,?James Dahlgren,g William P. Enlow,$and Alan G. Marshall*~+J Departments of Chemistry and Biochemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210, and General Electric Company, Parkersburg Center, Fifth and Avery Streets, Parkersburg, West Virginia 26102
Detectbn and quantltatlon of phosphlteantloxldantsIn polvmers Is a challenglng analytlcal problem: infrared absorption at reallstlc antloxldant concentratlon (250 ppm to 2 % ) may be obscured by absorptlon from the polymer Itself, and the antloxldant tends to fragment extendvely In massspectrometrk analysls. I n thls work, ULTRANOX 626 dlphosphlte [bls(2,4dl-terl-butylpheny1)pentaerythrltol dlphosphlte] and Its correspondlng diphosphate oxldatlon product, XR-2502, as well as the phosphtte additive, WESTON 618 dlphosphlte (dlstearyi pentaerythrltoldlphosphlte) have been successfully analyzed by NdYAG laser desorption 1.064 p ) electronlonlzatlonFourler transform Ion cyclotron resonance mass spectrometry (LD/ EI/FT/ICR/MS). For each of the Isolated addltlves, the molecular Ion (M’) was observed as the predomlnantspecles wlth vlrtually no fragmentatlon. Moreover, abundant molecular Ions were detectedfor ULTRANOX 626 dlphosphlte In a mixed polymer of poly(ethylene terphthalate), polypropylene, and acrylonitrlie-butadlene-styrene at addltlve concentratlon as low as 0.1 % by dlrect analysis of the polymer fllm when the probe was heated to about 200 OC prlor to laser desorption. The elevated sample temperature appears to Increase the free volume of the polymer, In turn facllltatlng release of laser desorbed/lonlzed addltlves. LD/EI/FT/ICR/MS thus offers a sensltlve and accurate means for detectlng nonvolatlle phosphlte addltlves at typlcal concentratlonIn solld polymers, wlthout the need for any chemlcai pretreatment.
INTRODUCTION As industrial thermoplastics are melt processed, they undergo oxidation reactions leading to changes in molecular weight and color. Phosphite antioxidants’-4 are generally considered to be secondary antioxidants, and their function is to control polymer molecular weight and color. Phosphite stabilizers are used in most common thermoplastics a t levels from 250 ppm to 2 5%. Typical phosphite loadings are often less than 1000 ppm. Phosphite stabilizers react with hydroperoxides, peroxy radicals, alkoxy radicals, and olefinic and carbonyl moieties; in addition, phosphites form coordination complexes with metals, changing their potential a~tivity.~,~
* Author to whom correspondence may be addressed. +
Department of Chemistry, The Ohio State. University.
* Department of Biochemistry, The Ohio State University.
General Electric Company. (1) Schwetlick, K. Pure Appl. Chem. 1983,55,1629-1636. (2)Schwetlick, K.; Konig, T.; Rilger, C.; Pionteck, J.; Habicher, W. D. Polym. Degrad. Stab. 1986,15, 97-108. (3)Schwetlick,K.; Pioteck, J.;Koenig, T.;Habicher, W. D.Eur.Polym. J. 1987,23,383-388. (4) Pobedimskii, D. G.; Kurashov, V. I.; Kirpichnikov, P. A. J . Polymer Sci.: Polym. Chem. Ed. 1983,21,55-66. 8
Since the additive level in the polymer affects its stability, the analysis of polymer additives immediately poses two basic analytical questions: first, how much of the additive got into the polymer during compounding; and second, how much of the additive that was added remains as the original phosphite form? Conventional methods for isolation and detection of phosphite additives include extraction/liquid chromatography, X-ray fluorescence, ash/UV analysis, and/or Fourier transform infrared (FT/IR) spectroscopy.6 However, FT/IR analysis of the intact polymer may be precluded for polymers which themselves have strong infrared absorption. X-ray fluorescence and the ash/UV method suffer from their incapability of differentiating the original phosphite from other forms of phosphorus-containingcomponents which are generated in the stabilization process. Extraction-based analysis may not necessarily give an accurate measure of the additive concentration in the original polymer due to incomplete extraction or reactions due to extraction technique. Mass spectroscopy has obvious advantages of high sensitivity and selectivity, providing structural information and the prospect of direct detection of the molecular or pseudomolecular [e.g., (M + H)+] ion by various “soft” ionization methods. Laser desorption (LD),7,8 secondary ion mass spectrometry (SIMS),+12fast atom bombardment (FAB),13 plasma desorption (PD),14J5and electrospray (ESI)16J’ ionization have been used to detect and identify various nonvolatile compounds in For example, Lat(5)Tannahill, M. M.; Enlow, W. P. GE Specialty ChemicalsTechnical Bulletin. Limitations of Liquid Chromatographic and FTIR Determinations for Ultranox’ 626 in Polyolefins; GE Plastics, 1988. (6)Reeder, M.; Enlow, W.; Borkowski, E. In Polyolefins VI International Conference; Society of Plastic Engineers: Houston, TX, 1989; pp F181. (7)Hillenkamp, F. Int. J . Mass Spectrom. Ion Phys. 1982,45,305313. (8)Hillenkamp, F. Int. J . Mass Spectrom. Ion Processes 1987,78, 53-68. (9)Benninghoven, A.; Japers, D.; Sichtermann, W. Adu. Mass Spectrom. 1978,7 4 1433-1436. (10)Lodding, A. R.; Fisher, P. M.; Odelius, H.; N o r h , J. G.; Sennerby, L.; Johansson, C. B.; Chabala, J. M.; Levi-Setti, R. Anal. Chim. Acta 1990,241, 299-314. (11)Pauw, D.E.Mass Spectrom. Rev. 1986,5,191-212. (12)Pachuta, S.J.; Cooks, R. G. Chem. Reu. 1987,87,647-669. (13)Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. M. J. Chem. Soc., Chem. Commun. 1981,325-327. (14)Torgerson, D. F.; Skowronski, R. P.; Macfarlane, R. D. Biochem. Biophys. Res. Commun. 1974,60,616-621. (15)Sundqvist, B. U. R.; Macfarlane, R. D. Mass. Spectrom. Reu. 1985, 42,421-460. (16)Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.; Fenn, J. B. Anal. Chem. 1985,57,675-679. (17)Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F. Mass Spectrom. Reu. 1990,9, 37-70. (18) Lattimer, R. P.; Harris, R. E. Mass Spectrom. Rev. 1985,4,369390. (19)Lattimer, R. P.; Harris, R. E.; Rhee, C. K.; Schulten, H.-R. Anal. Chem. 1986,58,3188-3195. (20)Hsu, A. T.; Marshall, A. G. Anal. Chem. 1988,60,932-937. (21)Asamoto, B.;Young, J. R.; Citerin, R. J. Anal. Chem. 1990,62, 61-70.
0003-2700/92/0364-2862$03.00/0 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, 1992
timer et al.18J9 reported the use of mass spectrometry for the analysis of rubber additives by direct rubber analysis and by prior extraction. Hsu et al.20applied laser desorption Fourier transform ion cyclotron resonance mass spectrometry (LD/ FT/ICR/MS) to identify dyes in poly(methy1methacrylate). More recently, Asamoto et a l . 2 1 and Johlman et al.22 have analyzed polymer additives by LD/FT/ICR/MS. Although the latter results show that LD/FT/ICR/MS can offer high mass range and highly abundant pseudomolecular ions [usually (M + K)+l, mass spectrometry has in general not been notably successful for detecting molecular ions from phosphite additives. In this work, we focus on the detection of phosphite additives by laser desorption followed by electron ionization (LD/EI/FT/ICR/MS). The mass spectra of the three compounds, ULTRANOX 626 diphosphite, its corresponding diphosphate, XR-2502, and WESTON 618 diphosphite are presented. (ULTRANOX and WESTON are registered trademarks of GE Specialty Chemicals, Inc.) The feasibility of direct analysis of a phosphite additive in a polymer thin f i i is demonstrated for phosphite additives in polypropylene (PP),poly(ethy1ene terphthalate) (PET), and acrylonitrilebutadiene-styrene (ABS) polymers. ULTRANOX@626 diphosphite ( M w 604) X
XR - 2502 (Mw636)
W E S O N @618 &phosphite ( M w 732)
EXPERIMENTAL SECTION Materials. All samples reported in this work were provided by GE Specialty Chemicals. The preparation of samples of the pure additives was as follows: typically -20 mg of pure additive was dissolved in 1 mL of methylene chloride and added dropwise to the mass spectrometer probe tip. Subsequent evaporation of the solvent left a uniform thin sample film on the probe tip. Polymer samples were prepared by hot pressing to form a thin film which was then affixed to the probe tip. Mass Spectrometry. FT/ICRmassspectrawere obtained on a standard 3.0 T Extrel2000 FT/ICR mass spectrometer (Extrel FTMS, Madison, WI) modified with Helix CryoTorr 8 Cryopumps (CTI Corp., MA) in place of the usual diffusion pumps on both the source and analyzer sides of the dual trap. Laser desorption was produced by a Continuum Model YG660A Nd:YAG laser operated at 1.064 pm, at a power level of -50 mJ in 10ns. Details of the mechanical and electronic interface between the mass spectrometer and a Continuum ModelYG-660NdYAG aredescribedelsewhere.23 Sincelaser irradiation typically produces 1000-fold excess of neutrals over ions, best results were obtained by ionizing the laserdesorbed neutrals by means of an electron beam (70 eV, 5-ms
-
(22) Johlman,C.L.;Wilkins,C.L.;Hogan,J.D.;Donovan,T.L.;Laude, D.A,, Jr.; Y o w e f i , M.-J. Anal. Chem. 1990,62, 1167-1172. (23) Liang, Z.; Ricca, T. L.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1991,5, 132-136.
Scheme I
2863
a+
Ultranox’ 626 diphosphlte (M.W. 604)
u u
oxidation
oxidation
duration, emission current of 5 pA measured just behind the filament) passing through the ICR ion trap. Creasy24 has used the same LD/EIFT/ICR/MS technique for the analysis of neutral oligomers. The trapping voltage was typically 2 V. A delay of 3-5 s after ion formation allowed neutrals to be pumped away before excitation/detection. Broadband frequency sweep excitation for 3.8 ms at a sweep rate of 200400 Hz ps-l was followed by collection of a 16K time-domain transient, digitized in direct-mode at a bandwidth of 2000 kHz, padded with an additional 16K of zeroes, and discrete Fourier transformed (without apodization) followed by magnitude calculation to produce an 8K magnitude-mode FT/ ICR mass spectrum.
RESULTS AND DISCUSSION Mass Spectra of P u r e Additives. The LD/EI/FT/ICR mass spectrum of the ULTRANOX 626 diphosphite additive is shown in Figure la. The molecular ion (M+) at mass-tocharge ( m l t ) 604 is the principal ionic species, in contrast to the pseudomolecular (M + K+) ion observed in highest abundance for other kinds of additives21722 by LD/FT/ICR/ MS (i.e., no electron ionization following laser desorption). By optimizing the laser power (to -50 mJ in -10 ns in this case), it is possible to generate abundant molecular ions with virtually no fragmentation; higher laser power induces significant fragmentation. Scheme I shows the two-stage oxidation of ULTRANOX 626 diphosphite to form the diphosphate compound, XR 2502. The LD/EI/FT/ICR mass spectrum in Figure l b shows the predominant molecular ion signal (mlz 636) for the diphosphate, XR 2502, along with residual signals from incompletely oxidized phosphite precursor at m/z 620 (half-oxidized monophosphate) and m/z 604 (unoxidized phosphite). Figure ICshows the LD/EI/FT/ICR mass spectrum of a second phosphite additive, WESTON 618 diphosphite. Although the molecular ion (m/z 732) is readily observed, its abundance is lower than for ULTRANOX 626 diphosphite or XF 2502, presumably because WESTON 618 diphosphite has saturated C18 hydrocarbon chains in place of aromatic rings and therefore fragments more easily. Similar behavior has been reported for other phosphite additives.21J2 Phosphite/Phosphate Ratio. Since only the phosphite form of the additive is effective as an antioxidant, it is clearly useful to determine the ratio of phosphite to phosphate, in order to find out how much of the original additive is still in ita active (phosphite) form. To that end, Figure 2 shows an LD/EI/FT/ICR mass spectrum of a 1:l mixture of the phosphite additive, ULTRANOX 626 diphosphite, and its corresponding diphosphate, XR 2502. Since both the diphos(24) Creasy, W. R. In Proceedings of the 38th ASMS Conference on Mass Spectrometry and Allied Topics; American Society Mass Spectrometry: Tucson, AZ, 1990; pp 850-851.
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LDIEIIFTIICRIMS
604
UItmOX@diphOsphite626 (MW I 604)
I
604
Ultranoxs 626 diphosphite in Polypropylene(O.l%)
I
200 214 j
XR-2502 (MW = 636)
400
600
Ultranox’626 phosphite (0.25%) in ABS (Acrylonitrile-Butadiene-Styrene
636
I
200
600
400
Ultranor’626 diphosphite (10%) in Polyethylene Terphfhalate
205
169
Weston@618 diphosphite (MW 732) 627 ,
1I
732
200
400 mlz
600
Flgure3. LD/EI/FT/ICR mass spectra of ULTRANOX 626 diphosphtte in various solid polymers: (a) 0.1% in polypropylene; (b) 0.25% in acryion~iie-butadlne-styrene;(c) 10% In polyethylene terphthaiate.
No pretreatment or extraction was necessary to producethesespectra.
400
200
600
800
mlr Flguro 1. LDIEIIFTIICR mass spectra of pure additives: (a) ULTRANOX 626 diphosphtte;(b)XR 2502; (c)WESTON6 18 diphosphite. Note the prominent molecular Ion in each case. 636
604
214
II
low laser power and high molecular weight of the polymer), molecular ions from the additive in both diphosphite (mlz 604) and diphosphate (mlz 636) form are clearly detectable at -0.05 % each. The signal magnitude increases significantly when the probe is heated to about 200 “C prior to laser desorption. Heating evidently increases the free volume of the polymer to facilitate laser desorption/ionization of the additives. Abundant ions at m/z 191 and 205 correspond to the following fragments:
m h 191
200
400
600
800
1000
m/z Flgure 2. LD/EI/FT/ICR mass spectrum of a 1:l mixture of the diphosphiteaddltlve, ULTRANOX 626 diphosphite,and Its corresponding
diphosphate, XR2502. Note the comparable abundances phosphite and phosphate molecular ions,
of the
phite and diphosphate ions are present in comparable abundance, it appears that the compounds have comparable ionization efficiencies and may thus be quantitated by use of suitable standard mixtures. Spectra of ULTRANOX 626 DiphosphiteinPolymers. Finally, for maximum practical utility, it is important to be able to detect the phosphite additives present in an already cast polymer. For example, Figure 3a shows the LD/EI/FT/ ICR mass spectrum of ULTFtANOX 626 diphosphite ( -0.1 % w/w) in polypropylene. Although no fragment ions from the polymer itself are observed under these conditions (due to
m/z 205
The fragment at m/z 205 is an obvious phosphite bond cleavage product; the other fragment was confirmed to be C13H190+(191.143u)by accurate-massmeasurement (191.145 u) by internal calibration against ions of seven m/z ratios from perfluorotri-n-butylamine. LD/EI/FT/ICR mass spectra of the same additive present at higher concentrations in poly(ethy1ene terphthalate) and acrylonitrile-butadiene-styrene (ABS) polymers are shown in Figures 3b,c. This time, there does not appear to be significant oxidation of the additive, since no signals from the phosphate or diphosphate oxidation products are observed. Spectra of WESTON 618 Diphosphite in Polymers. Although the LD/EVFT/ICR mass spectrum of WESTON 618 (5 % ) in acrylonitrile-butadiene-styrene (ABS) polymer (not shown) did not show a molecular ion, its corresponding oxidation product (m/z764) was observed,along with several fragment ions. In summary, the above results clearly show that LD/FT/ ICR/MS provides for simple and direct identification of the molecular ions of phosphite additives and their oxidized phosphate forms. Moreover, these additives may be detected
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, 1992
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sensitively and accurately in solid polymers without any prior sample pretreatment. Sensitivity (-0.1 % additive in solid polymer) is superior to FTIR detection, aiid is moreover applicable to polymers whose own IR absorption may preclude FT/IR detection of additives.
RECEIVED for review March 30, 1992. Accepted July 10,
ACKNOWLEDGMENT
Registry No. PP (homopolymer), 9003-07-0; PET (SRU), 25038-59-9; ABS (copolymer), 9003-56-9; ultranox 626 diphosphate, 26741-53-7;ultranox 626 diphosphate, 97994-11-1;weston 618 diphosphite, 3806-34-6.
This work was supported by the NSF (CHE-8721498,CHE90-21498) and The Ohio State University.
1992.
