Direct Analysis of an Oligomeric Hindered Amine Light Stabilizer in

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Anal. Chem. 2004, 76, 697-703

Direct Analysis of an Oligomeric Hindered Amine Light Stabilizer in Polypropylene Materials by MALDI-MS Using a Solid Sampling Technique To Study Its Photostabilizing Action Yoshihiko Taguchi,† Yasuyuki Ishida,‡ Hajime Ohtani,*,† and Hideki Matsubara§

Department of Applied Chemistry, Graduate School of Engineering, and Research Center for Advanced Energy Conversion, Nagoya University, Nagoya 464-8603, Japan, and Aichi Industrial Technology Institute, Kariya 448-0003, Japan

A novel method for the direct analysis of small amounts of an oligomeric hindered amine light stabilizer (HALS) occluded in polypropylene (PP) material was developed to study its photostabilizing action on the basis of matrixassisted laser desorption/ionization mass spectrometry (MALDI-MS) using a solid sampling technique while avoiding troublesome solvent extraction. In this sampling protocol, the powdered mixture of PP composite sample containing trace amounts of an oligomeric HALS, Adekastab LA-68LD (MW ) 1900), and the matrix reagent (dithranol) was spotted on the sample plate, then ion exchanged water was deposited onto the mixture to make a suspension, and finally, the dried mixture adhered on the plate was subjected to MALDI-MS measurement. On the mass spectrum thus obtained by the solid sampling MALDI, the molecular ions of the HALS desorbed from the PP composite were clearly observed as three major series of the HALS components in the range up to about m/z 7000 with little interference by the PP substrate and the other additives. Moreover, in the MALDI-MS spectra for the UV-exposed sample, the satellite peaks around the major HALS components proved to enhance significantly, reflecting the oxidized HALS species at the tetramethylpiperidine units to cause the photostabilizing action. In addition, hydrolyzed HALS species were also observed for the irradiated sample. These results suggest that not only the oxidation reaction but also the hydrolysis or decomposition of the oligomeric HALS components competitively proceed in the PP composites during UV exposure. Hindered amine light stabilizers (HALSs) are commonly added to various polymeric materials for outdoor use as excellent radical scavengers on the order of 0.1% or less for the prevention of photodegradation of the materials.1,2 There have been a number of arguments about the stabilization mechanisms of the polymer * To whom correspondence should be addressed: (phone) +81-52-789-3560; (fax) +81-52-789-4666; (e-mail) [email protected]. † Department of Applied Chemistry. ‡ Research Center for Advanced Energy Conversion. § Aichi Industrial Technology Institute. (1) Zweifel, H. Stabilization of Polymeric Materials; Springer: Berlin, 1997. (2) Hamid, S. H. Handbook of Polymer Degradation, 2nd ed.: Marcel Dekker: New York, 2000; Chapter 1. 10.1021/ac030270a CCC: $27.50 Published on Web 12/30/2003

© 2004 American Chemical Society

systems by HALS.3-7 Generally, it is known that the photodegradation of a given polymeric material proceeds after a certain induction time when the stabilizers are lost from the substrates by their decomposition or migration during practical use. Therefore, it is important to analyze the stabilizers remaining in the materials for the lifetime prediction of the polymeric materials in relation to their structural changes during the stabilizing actions. HALSs in polymer materials have been generally analyzed by means of gas chromatography (GC) or liquid chromatography after their extraction from the substrate materials.8-13 However, the extraction process is sometimes troublesome especially for the oligomeric or polymeric HALSs, which are often employed in practical use because of their lower migration out from the substrate materials. Moreover, the quantitative recovery of the higher molecular weight components in the HALS is not thoroughly attained by ordinary solvent extraction because of their lower solubility and possible decomposition during the extraction process. Recently, the authors have developed a novel method to determine an oligomeric HALS in PP materials using reactive thermal desorption-gas chromatography (RTD-GC) in the presence of an organic alkali, tetramethylammonium hydroxide [(CH3)4NOH].14,15 This technique allowed the rapid and highly sensitive determination of the oligomeric HALS in polypropylene (3) Shilov, J. B.; Battalova, R. M.; Denisov, E. T. Dokl. Akad. Nauk SSSR 1972, 207, 388-389. (4) Klemchuk, P. P.; Gande, M. E. Polym. Degrad. Stab. 1988, 22, 241-274. (5) Klemchuk, P. P.; Gande, M. E.; Cordola, E. Polym. Degrad. Stab. 1990, 27, 65-74. (6) Zahradnickova, A.; Sedlar, J.; Dastych, D. Polym. Degrad. Stab. 1991, 32, 155-176. (7) Step, E. N.; Turro, N. J.; Klemchuk, P. P.; Gande, M. E. Angew. Makromol. Chem. 1995, 232, 65-83. (8) Newton, I. D. In Polymer Characterisation; Hunt, B. J., James, M. I., Eds.; Blackie A & P: Glasgow, 1993; pp 8-36. (9) Crompton, T. R. Manual of Plastics Analysis; Plenum Press: New York, 1998; pp 3-9. (10) Scheirs, J. Compositional and Failure Analysis of Polymers: John Wiley & Sons: Chichester, U.K., 2000; pp 242-248. (11) Freitag, W. Fresenius Z. Anal. Chem. 1983, 316, 495-496. (12) Caceres, A.; Ysambertt, F.; Lopez, J.; Marquez, N. Sep. Sci. Technol. 1996, 31, 2287-2298. (13) Vandenburg, H. J.; Clifford, A. A.; Bartle, K. D.; Carroll, J.; Newton, I.; Garden, L. M.; Dean, J. R.; Costley, C. T. Analyst 1997, 122, 101R-115R. (14) Kimura, K.; Yoshikawa, T.; Taguchi, Y.; Ishida, Y.; Ohtani, H.; Tsuge, S. Analyst 2000, 125, 465-468.

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(PP) on the basis of its methylated fragments in the resulting chromatograms without using any sample pretreatment such as solvent extraction and subsequent derivatization. Moreover, the structural changes of the HALS in PP materials during UV irradiation observed by RTD-GC were discussed in terms of the degrees of oxidation of the tetramethylpiperidine moiety of HALS.16 However, since the RTD-GC method inherently involved the decomposition of the HALS molecules, it was impossible to obtain direct information about the whole molecular structure of HALS by this technique. On the other hand, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been widely utilized as a soft ionization MS technique, which often provides the mass spectra mainly composed of molecular-related ion peaks with little fragmentation even for polymeric compounds. Because MALDIMS can provide the absolute molecular weight of each component in complex mixtures, it has been widely applied for the detailed characterization of chemical structures such as end groups and copolymer compositions of various synthetic polymers.17-20 The ordinary MALDI-MS method usually requires a homogeneous sample/matrix crystal prepared by the “dried droplet method” from appropriate solutions, which has been believed to be essential for highly efficient MALDI of the sample components. On the contrary, it has been recently reported that the molecular ions of polymers are also observed by means of solid sampling protocols for MALDI-MS without dissolving the polymers.21-25 Skelton et al. successfully analyzed an insoluble polyamide sample by MALDI-MS using a pressed pellet from the powdered solid mixture of the polymer sample and a matrix reagent.21 Przybilla obtained the MALDI mass spectra of insoluble polycyclic aromatic hydrocarbons deposited onto a sample holder as a suspended mixture of the ground analyte and matrix in a nonsolvent.22 Marie et al. observed MALDI mass spectra of poly(ethylene glycol) oligomers and a fluorinated polymer using solventless sample preparation such as fusion of the analyte and matrix and matrix vapor deposition onto the analyte.23 Trimpin demonstrated “solventfree” MALDI mass spectra of synthetic polymers such as polystyrene, poly(methyl methacrylate), and polyetherimide along with those of a insoluble pigment and bovine insulin, which were observed by applying the premixed analyte/matrix/cationizing salt powder mixture onto the target followed by laser desorption.24 They also characterized an insoluble ladder-type poly(dithiathioanthrene) by the “solvent-free” MALDI-MS.25 As for the MALDI-MS measurements of additives in the polymer material, the additives are usually separated from the (15) Taguchi, Y.; Ishida, Y.; Ohtani, H.; Tsuge, S.; Kimura, K.; Yoshikawa, T. J. Chromatogr., A 2003, 993, 137-142. (16) Taguchi, Y.; Ishida, Y.; Ohtani, H.; Tsuge, S.; Kimura, K.; Yoshikawa, T. Polym. Degrad. Stab., in press. (17) Raeder, H. J.; Schrepp, W. Acta Polym. 1998, 49, 272-293. (18) Nielen, M. W. F. Mass Spectrom. Rev. 1999, 18, 309-344. (19) Scrivens, J. H.; Jackson, A. T. Int. J. Mass Spectrom. 2000, 200, 261-276. (20) Murgasova, R.; Hercules, D. M. Int. J. Mass Spectrom. 2003, 226, 151162. (21) Skelton, R.; Dubois, F.; Zenobi, R. Anal. Chem. 2000, 72, 1707-1710. (22) Przybilla, L.; Drand, J.; Yoshimura, K.; Rader, H. J.; Mullen, K. Anal. Chem. 2000, 72, 4591-4597. (23) Marie, A.; Fournier, F.; Tabet, J. C. Anal. Chem. 2000, 72, 5106-5114. (24) Trimpin, S.; Rouhanipour, A.; Az, R.; Rader, H. J.; Mullen, K. Rapid Commun. Mass Spectrom. 2001, 15, 1364-1373. (25) Leuninger, J.; Trimpin, S.; Rader, H. J.; Mullen, K. Macromol. Chem. Phys. 2001, 202, 2832-2842.

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Figure 1. Structure of Adekastab LA-68LD.

substrate through solvent extraction to prepare a sample/matrix solution suitable for the dried droplet method. To the best of our knowledge, there have been no reports concerning the direct analysis of the additives in a polymer by solid sampling MALDIMS. In this work, a novel solid sampling protocol was developed for the direct MALDI-MS investigation of an oligomeric HALS occluded in a PP material to avoid the troublesome solvent extraction. The powdered mixture of the matrix reagent and the PP sample containing the HALS fixed on the sample plate through on-plate suspending was subjected to MALDI-MS measurement. The observed mass spectra were interpreted in terms of chemical structure and molar mass distribution of the trace oligomeric HALS added to the PP material. Finally, by using this technique, the structural changes of the HALS in the PP material induced by UV exposure were investigated to discuss its stabilization mechanisms. EXPERIMENTAL SECTION Materials. Samples used in this work were the same as those in our previous work for the RTD-GC method.16 Adekastab LA68LD (Asahi Denka Kogyo, Tokyo, Japan, average MW ) ∼1900) was used as the oligomeric HALS. Figure 1 shows the chemical structure of Adekastab LA-68LD. A commercially available PP with melt flow index of 1.0 without containing any additives was used as the substrate polymer. A PP model sample containing 1.0 wt % of the oligomeric HALS along with antioxidants, Irganox 1010 (Ciba Specialty Chemicals, Basel, Switzerland) and Irgafos 168 (Ciba Specialty Chemicals), whose contents were 0.1 wt %, respectively, was prepared by kneading the PP powder with the additives at 180-190 °C for 5 min. A typical matrix reagent, 1,8,9-trihydroxyanthracene (dithranol; Aldrich, Milwaukee, WI), was empirically chosen as the optimum matrix for MALDI of the HALS and used without further purification. UV Exposure Test. A thin film with 200-µm thickness of the compounded PP sample was prepared by compression molding at 185 °C for 3 min. A piece of the PP film (about 20 mm × 80 mm) fixed on holders was UV-exposed in a Fade Meter (Suga, Tokyo, Japan) at the wavelength range longer than 300 nm corresponding to daily sunlight at 63 °C for either 100, 200, 350, 400, 500, or 700 h. A carbon arc lamp was used as the UV source with which exposure for 200 h corresponded to that by normal sunlight outdoors for ∼1 year. Solid Sampling for Direct HALS Analysis by MALDI. Figure 2 illustrates the procedures for solid sampling MALDIMS measurement of the PP composite. (1) The PP composite and

Figure 2. Procedures for the solid sampling MALDI-MS measurement of PP composite. (1) PP composite and matrix reagent (dithranol) are mixed and ground by freezer mill; (2) the powdered mixture is spotted on a well in the sample plate; (3) the mixture is suspended in ion-exchanged water using a plastic needle for stirring and then dried; (4) the MALDI-MS measurement is performed by irradiating N2 laser to the sample.

the matrix reagent (dithranol) were mixed in solid state with a 1:2 weight ratio and ground in liquid nitrogen into fine particles of ∼30 µm or less in diameter using a freezer mill, Spex model 6750 (Spex, Metuchen, NJ). Total grinding time was 15 min. with a 1-min interval after 1 min of grinding to prevent heating of the sample. (2) A few hundred micrograms of the powdered mixture was spotted on a well in the sample plate, and (3) 2 µL of ion-

exchanged water was added onto the sample mixture, and then the mixture was suspended using a plastic needle for stirring. The suspension was dried to adhere the mixture on the sample plate, and the powder excess was removed by blowing to avoid pollution of the ion source. (4) Finally, the sample plate was introduced into the ionization chamber of the mass spectrometer to cause selective MALDI of the HALS components by laser irradiation. Conventional Solution-Based Sampling for MALDI of Solvent Extracts from the PP Material and Intact HALS. The HALS component in the PP composite sample before UV exposure was conventionally extracted with chloroform for 24 h using a Soxhlet apparatus. The chloroform solution of the extracts from the cryomilled PP sample powder was concentrated to ∼1 mg of recovered HALS in 1 mL of solution by rotary evaporator. In addition, a standard solution of the intact HALS was prepared by dissolving ∼1 mg of Adekastab LA-68LD in 1 mL of chloroform. The sample solution (intact HALS or extract from the PP material) was mixed with an equal volume of the chloroform solution of dithranol (∼15 mg/mL). Then, 1 µL of the mixed solution was applied to the sample plate, air-dried, and subjected to conventional solution-based MALDI-MS measurements to obtain the reference data for those obtained by solid sampling MALDI-MS. MALDI-TOF-MS Instrument. MALDI-TOF mass spectra were recorded using a Voyager-DE RP mass spectrometer (Applied Biosystems Japan) equipped with a pulsed nitrogen laser

Figure 3. MALDI mass spectrum of Adekastab LA-68 LD obtained by conventional solution-based MALDI-MS. The weight ratio of sample/ matrix was 1:10.

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Figure 4. MALDI mass spectrum of the HALS components in PP composite sample containing 1.0 wt % of HALS before UV irradiation: (a) solvent extracts from the sample obtained by solution-based preparation method; (b) HALS components directly desorbed from the PP composite obtained by solid sampling technique.

(λ ) 337 nm, 3-ns pulse width, and 3-Hz frequency) and a delayed extraction ion source. Laser beam intensity was experimentally attenuated to the level just above the threshold for the ionization of the oligomeric HALS. Ions generated by the laser desorption were introduced into the flight tube with an accelerating voltage of 15 kV operated in the positive linear mode. The delay time was set at 150 ns. All mass spectra were acquired by averaging 100 individual laser shots. RESULTS AND DISCUSSION MALDI-MS Measurements of Adekastab LA-68LD in PP composite. Figure 3 shows a typical MALDI mass spectrum of intact Adekastab LA-68LD measured by the conventional dried droplet method in linear mode along with the assigned structures of the major components. Table 1 summarizes the calculated molar mass for the assigned structures and the observed peak top m/z values of the major components. Three series of the HALS components are mainly observed as the molecular ions (M+) on the mass spectrum in the range up to about m/z 8000. Here, the precise m/z values of b1 used as an internal standard for mass calibration were determined by an additional MALDI-MS measurement in reflector mode, confirming the observed ions are mostly M+. Among these, the constituents designated with bn, which are the HALS molecules completely end capped with tetramethylpiperidyl groups, show the most intense peaks. In addition, the compounds containing a methoxy substituent (an) 700

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instead of a tetramethylpiperidyloxy unit of the corresponding main components (bn) and those with a spirochain-type terminals (cn) are also observed in fairly strong intensities. These two components are presumed to be the byproducts due to incomplete condensation or partial decomposition during synthesis of the HALS. The mass spectrum indicates that the oligomeric HALS consists of a number of components with at least three kinds of different chemical structures and wide variations of molecular weights. Figure 4 shows MALDI mass spectra of the HALS occluded in the PP composite (a) obtained by the dried droplet method after conventional solvent extraction from the PP material and (b) directly desorbed from PP in the ionization chamber by using the solid sampling technique. On the mass spectrum of the extracts (a), the peak of the main HALS oligomers markedly declined in comparison with those in Figure 3 so that the components in n ) 6 and higher regions were scarcely observed. Moreover, the relative peak intensity of the byproduct a0 significantly increased and some satellite peaks around the major components such as b1 were fairly boosted or additionally observed after the solvent extraction probably due to the decomposition of the larger components. These results suggest that not only the insufficient extraction of the higher molecular weight HALS components but also their undesirable decomposition proceeded considerably during the extraction process.

Scheme 1. Hydrolysis of a Piperidine Moiety of Component b1 To Form Component d1 during UV Irradiation

Table 1. Calculated Molar Mass for the Assigned Structures and Observed Peak Top m/z Values of Major Components of HALS component a0 b0 c0 a1 b1 c1 a2 b2 c2 a3 b3 c3 a4 b4 c4 a5 b5 c5 a6 b6 c6 a7 b7 c7 a8 b8 c8

calculated molar massa 665.9 791.1 938.2 1446.9 1572.1 1719.2 2227.9 2353.1 2500.2 3008.9 3134.1 3281.2 3789.9 3915.1 4062.2 4570.9 4696.1 4843.2 5351.9 5477.1 5624.2 6132.9 6258.1 6405.2 6913.9 7039.1 7186.2

observed m/z value 666.3 791.5 938.6 1447.1 1572.1b 1719.5 2228.2 2353.3 2500.6 3009.5 3134.5 3281.6 3789.9 3915.6 4063.0 4571.9 4696.7 4843.7 5352.9 5478.0 5623.8 6133.0 6258.3 6407.1 6915.1 7039.1b 7185.3

a Isotopic abundance was taken into account. b Used as internal standards for mass calibration: the molecular ions of b1, m/z 1572.1 and b8, m/z 7039.1.

On the other hand, the mass spectrum obtained by direct MALDI-MS measurement of the PP sample (b) was almost identical to that of intact Adekastab LA-68LD shown in Figure 3. This fact suggests that the whole molecular weight range of the HALS components was appropriately ionized during the solid sampling MALDI process through adequate contact between the matrix and the HALS molecules on the surface of the PP substrate. Here, the ions of the substrate polymer components and the antioxidants were scarcely observed on the mass spectrum under the given MALDI-MS conditions. These results demonstrate that MALDI-MS using the solid sampling method enables us to analyze the oligomeric HALS molecules occluded in the PP material directly without causing discriminative loss or decomposition of the HALS components during desorption. By using this technique, therefore, the subtle change in the molecular structure of the HALS components in PP during UV irradiation could be observed in the MALDI mass spectrum to interpret it in terms of the photostabilizing action.

Structural Changes of HALS Molecule in PP Composite during UV Irradiation. Figure 5 shows a mass spectrum of HALS in the PP composite sample after UV irradiation for 700 h obtained by solid sampling MALDI, along with the partially expanded spectra in the n ) 1 region observed for the related PP samples; (a) before irradiation and after UV exposure (b) for 200 h and (c) for 700 h. The whole mass spectrum of the PP sample after UV irradiation was basically similar to that before exposure, which mainly consisted of three series of the peaks corresponding to the major HALS components an, bn, and cn. As shown in the expanded spectra for the n ) 1 region, however, the satellite peaks around the major ones were significantly enhanced after irradiation, although these satellites were also observed as minor peaks even before irradiation. Among these, the m/z values of the satellite peaks designated with a1′, b1′, and c1′ were larger than those of the corresponding major components a1, b1, and c1 by 16 atomic mass units (amu), respectively, whereas that with d1 appeared at lower m/z by 14 amu than that of a1. Basically the same phenomena were also observed in the other regions (n ) 0, 2, and higher). The fact that the difference, 16 amu, corresponds to the mass of an oxygen atom suggests that these satellite components are assigned to be the oxidized products of the major HALS components at a piperidine unit as shown in Figure 5. Moreover, a fairly intense peak of doubly oxidized products (e.g., b1′′) and trace amounts of multiply oxidized ones were also observed in the mass spectra of the highly irradiated samples. This observation demonstrates that the HALS components in the PP composite are primarily oxidized at the piperidine units during UV irradiation. This result is consistent with the generally proposed photostabilizing action of HALS via the oxidation at tetramethylpiperidyl groups.3-7 Here, our previous work by reactive TD-GC inferred that the nitroxyl radicals were primarily formed as the oxidized species of HALS during UV exposure.16 Therefore, the hydroxylamine type of the oxidized species observed in the mass spectra might be formed from the nitroxyl radicals to some extent through protonation during ionization. On the other hand, the m/z value of the other satellite peak d1 was consistent with that of the product having a hydroxyl group instead of a tetramethylpiperidyloxy group of the main component b1. Scheme 1 shows a possible formation pathway of the compound d1, in which the original HALS component b1 is hydrolytically decomposed into d1 with the elimination of a 4-hydroxyltetramethylpiperidine. The notable formation of this type of decomposed product (dn) after UV exposure indicates that a certain amount of the HALS components might be subjected to hydrolysis induced by the moisture in the atmosphere or the PP sample as well as the oxidation during irradiation. Analytical Chemistry, Vol. 76, No. 3, February 1, 2004

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Figure 5. MALDI mass spectrum of HALS in UV-irradiated PP composites for 700 h obtained by the solid sampling method and partial spectra in the n ) 1 region observed for related PP composite samples: (a) before UV irradiation, (b) after UV irradiation for 200 h, and (c) after UV irradiation for 700 h.

Figure 6. Relationship between the relative peak intensities of four HALS components without oxidation in the n ) 1 region and UV irradiation time: 2, a1 with a methoxy end group; b, b1 totally endcapped with tetramethylpiperidine; 9, c1 with a spiro chain-type terminal; ], d1 with a carboxylic end group.

Finally, Figure 6 shows the relative peak intensities in the n ) 1 region among three major HALS components, a1, b1, and c1 plus a hydrolyzed product d1 as a function of UV irradiation time. Here, the total intensities of these four peaks were defined as 100%. 702 Analytical Chemistry, Vol. 76, No. 3, February 1, 2004

As was expected, the relative peak intensity of the hydrolyzed product d1 (]) increased with the increase in irradiation time whereas that of the main component b1 (b) gradually decreased through oxidation and hydrolysis. It is interesting to note, however, that the intensity of one of the original major HALS components a1 (2) with a methoxy end group also slightly increased with UV exposure although the other component c1 (9) did not. This result suggests that the component a1 might be produced to some extent by UV irradiation through the decomposition of larger HALS components. Scheme 2 shows a possible formation pathway of the compound a1, in which the C-C bond in the original main component bn was cleaved through disproportionation. In conclusion, a novel method to analyze directly the small amounts of an oligomeric HALS added in the PP material was developed on the basis of MALDI-MS using the solid sampling technique while avoiding troublesome solvent extraction. The method enabled us to study the structural changes of the oligomeric HALS during UV irradiation, causing a photostabilizing action. At the present stage, however, the solid sampling MALDIMS is inadequate for the quantitative determination of the HALS in the PP material based on the absolute peak intensities. Further

Scheme 2. Possible Decomposition of Main HALS Component bn by UV Irradiation To Form Component a1

study to develop a modified MALDI-MS method using an internal standard, which enables the quantitative analysis of the oligomeric HALS in polymer systems, is currently in progress.

The authors are grateful to Professor Toshio Yoshikawa, Aichi Institute of Technology, for the preparation of the PP composite sample and helpful discussions.

ACKNOWLEDGMENT This work was supported in part by the 21st Century COE Program “Nature-Guided Materials Processing” of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Received for review July 21, 2003. Accepted October 28, 2003. AC030270A

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