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Acrylic compound characterization by oxidative pyrolysis, atmospheric pressure chemical ionization-tandem mass spectrometry
Journal of Analytical and Applied Pyrolysis, 17 (1990) 127-141 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
127
ACRYLIC COMPOUND CHARACTERIZATION BY OXIDATIYE PYROLYSIS, ATMOSPHERIC PRESSURE CHEMICAL IONIZATION-TANDEM MASS SPECTROMETRY
A. PETER
SNYDER
U.S. Army Chemical Research, Development and Engineering Aberdeen Proving Ground, MD 21010-5423 (U.S.A.) (Received
June 27th, 1989; accepted
September
Center, SMCCR-RSL,
lst, 1989)
ABSTRACT Oxidative pyrolysis, atmospheric pressure chemical ionization (Py-APCI) in conjunction with tandem quadrupole mass spectrometry (MS/MS) was investigated to characterize various acrylate and methacrylate compounds. The Py-APCI mass spectra of these compounds were dominated by the protonated molecular ion and depending on the acrylate, either methacrylic or acrylic acid was produced as the primary fragment ion. These observations from the headspace analysis of the monomers were similar to those of the mass spectra of their respective polymer/copolymer compounds under oxidative pyrolysis processing conditions. In particular, Py-APCI-MS/MS produced daughter ion mass spectra of the polymer and selected copolymer forms of the four butyl methacrylate isomers. These spectra could be differentiated by visual inspection. A commercially-available acrylic thermoplastic resin was investigated for its qualitative and quantitative composition. n-Butyl methacrylate, methyl methacrylate and ethyl acrylate, despite identical m/z 101 protonated molecular ions for the latter two compounds were identified in the resin by interpretation of the MS/MS mass spectra. Acrylate isomers; atmospheric methacrylate isomers; pyrolysis,
pressure chemical oxidative; tandem
ionization; butyl mass spectrometry.
methacrylate
isomers;
INTRODUCTION
Alkyl and polyalkyl acrylates play important roles in commodities such as coatings for a wide variety of metals, paints, lubricating oils, composites, thermoplastic/molding resins and dental resins. Because of the extensive use of acrylic compounds in the manufacturing industry, the literature documents their qualitative and quantitative characterization with a number of laboratory benchmark techniques. These instrumental techniques include gas chromatography (GC) [l-7], mass spectrometry (MS) [S], GC/MS [9], nuclear magnetic resonance [7,10] and infrared spectroscopy [6,10]. 0165-2370/90/$03.50
0 1990 Elsevier Science Publishers
B.V.
128
Thermal methods, pyrolysis (Py) in particular, and direct injection of liquid alkyl acrylate monomers and monomer mixtures are the predominant methods of sample introduction for GC, MS and GC/MS. It has been stated that Py-capillary GC is the most efficient method for the qualitative characterization of the monomers in an acrylic copolymer because of the highly reproducible GC monomer profiles observed upon pyrolysis [6]. Investigations that have included MS in the analysis of acrylic monomers and polymers have relied on either electron ionization (EI) [8,9] or chemical ionization (CI) [9] methods. Isobutane CI [9] yields primarily the protonated molecular ion (MH+) of alkyl acrylate and methacrylates from monomer and polymer pyrolyzates. EI yields extensive fragmentation [8,9] of the alkyl acrylate and methacrylates and produces little to negligible amounts of the molecular ion (M+). Methane CI [9] of these compounds appears to yield mass spectra with elements similar to that of EI and isobutane CI mass spectra. The present work focuses on the potential of atmospheric pressure chemical ionization (APCI)-tandem mass spectrometry (APCI-MS/MS) [11,12] in the characterization of liquid and solid acrylic materials. APCI is largely characterized by the formation of a reactive medium containing proton in the positive ion mode. In common hydrates in the form of H(H,O)i practice, n lies in the range of 1-4. Soft ionization of the analyte generally takes place and results in the formation of protonated molecular ions (MH+). With this technique a series of acrylic compounds were analyzed belonging to the acrylate and methacrylate families, and. the four butyl methacrylate isomers were investigated for their differentiation by the MS/MS method. Oxidative pyrolysis was used for introduction of solid polymeric samples into the APCI source [13-151, while a simple headspace sampling proved effective for liquid compounds. The information obtained was applied in the qualitative and quantitative characterization of a commercial acrylic thermoplastic resin.
EXPERIMENTAL
Materials Poly(methy1 methacrylate) (PMMA), poly(ethy1 methacrylate) (PEMA), poly(isobuty1 methacrylate) (PIBMA), poly( n-butyl methacrylate) (PBMA), and the butyl methacrylate/isobutyl methacrylate copolymer (PBIBMA) were purchased from Scientific Polymer Products, Ontario, NY, U.S.A. Poly( tert.-butyl methacrylate) (PTBMA), tert.-butyl methacrylate (TBUMA), and sec.-butyl methacrylate (SBUMA) were obtained from Polysciences, Warrington, PA, U.S.A. Methyl methacrylate (MMA), n-butyl methacrylate (NBUMA), isobutyl methacrylate (IBUMA), acrylic acid (AA), ethyl acrylate
129
(EA), poly(ethy1 acrylate) (PEA), and ethyl methacrylate (EMA) were obtained from Aldrich Chemical Co., Milwaukee, WI, U.S.A. and methacrylic acid (MA) was purchased from Alfa products, Danvers, MA, U.S.A. A commercial, thermoplastic resin was purchased from Rohm and Haas Co., Philadelphia, PA, U.S.A. Complete qualitative and quantitative characterization of the resin is proprietary [16]. However, the Material Safety Data Sheet (MSDS) of the compound lists its composition as containing the methyl methacrylate monomer and an acrylic copolymer. Methods A SCIEX (Toronto, Canada) TAGA 6000 APCI triple quadrupole (tandem) MS/MS system was used as the mass spectral analyzer. Basic references for the methods and parameter settings are noted elsewhere [17] and daughter ion mass spectra (MS/MS mode) were obtained with an argon collision gas pressure of 6.0 X 10e5 torr in the center quadrupole. Sample pyrolysis was conducted with a Pyroprobe Model 122 power supply (Chemical Data Systems, Oxford, PA, U.S.A.) and Pyroprobe @ platinum coil probe. All pyrolyses were conducted in the pulsed mode with the pulse rate control in the off position. The final set temperature was 700 o C, and the total heating time was 20 s. 0.1-0.5 mg of the polymers was sandwiched between quartz wool plugs in the quartz tube sample holder and the latter was inserted into the coil of the heating probe. The Pyroprobe was interfaced to the APCI source of the MS/MS system as shown in Fig. 1. The pyrolysis probe was fitted with perforated positioning rings and was inserted into a glass tube to permit a uniform gas flow over the sample region. The end of the Pyroprobe coil was positioned within the glass tube so that the end of the coil was about 1 cm from the corona discharge needle tip in the
PYROPROBE \
t EXHAUST
Fig. 1. Schematic diagram of the Pyroprobe in the atmospheric pressure ion source of the mass spectrometer.
130
ion source. A ‘zero-air’ flowrate of 150 ml/mm was used to effect oxidative heating and pyrolyzate removal from the heating zone to the ion source. The pressure in the APCI source was lowered from 0.98 atm (ambient) to 0.97 atm with the chamber exhaust motor (Fig. 1) which also facilitated pyrolyzate removal from the heating zone to the ion source [18]. Monomer standards were introduced into the analyzer by drawing the headspace vapor from each respective liquid directly into the ion source. PEA was obtained as a solution in toluene. A few milliliters of the solution were vacuum-dried at 50 o C for 24 h to a thick, clear paste. A pinpoint drop was applied to the quartz wool in the quartz tube for thermal processing. The mass spectra contained in the total ion current (lo-15 individual mass spectra) were signal averaged with a subsequent background subtraction. All analyses were done in single or replicate fashion.
RESULTS AND DISCUSSION
Characterization
of acrylic compounds
Fig. 2 presents the APCI and Py-APCI mass spectra of acrylate and methacrylate compounds. The monomers were observed by headspace analysis, and the polymer compounds were analyzed by quartz tube pyrolysis. Structures of the monomer compounds are presented in Fig. 3. Methyl methacrylate (Fig. 2A) predominates as the protonated monomer (MH+) species at m/z 101 with minor peaks at m/z 69 and 73. Ethyl acrylate (Fig. 2B) is characterized by the MH+ species at m/z 101 as the base peak, a protonated acrylic acid molecule at m/z 73 (vide infra), and a low intensity feature at m/z 118, which could represent the hydrated monomer ion. Ethyl methacrylate (Fig. 2C) is represented by the MH+ and protonated methacrylic acid species (vide infra) at m/z 115 and 87, respectively. All four butyl methacrylate isomers (Fig. 2D,E) produced identical APCI mass spectra except for sec.-butyl methacrylate (Fig. 2E). The isomers produced the MH+ ion at m/z 143 as well as the abundant methacrylate fragment at m/z 87 (vide infra). sec.-Butyl methacrylate contains the m/z 87 ion as the base peak with an m/z 143/87 intensity ratio of approximately 50% while the other three isomers show an identical 125% ratio, with m/z 143 as the base peak. Upon oxidative pyrolysis of these compounds in polymer form, simple as well as more complex mass spectra were observed (Fig. 2F-.I). PMMA produced the MH+ as the base peak, as well as the m/z 115, 127, 141, 155, 169, 187, 201 and 215 series of lower intensity masses (Fig. 2F), and m/z 69 and 73 are also observed. PEA has its Py-APCI mass spectrum shown as Fig. 2G and its abundance pattern is skewed to higher mass. The m/z 143, 155, 189 and 201 dominate the spectrum, while the low intensity features are
131 II&
B-...,...!‘...1,...,...,...,..,~...,.””, ila-
MA
A
EnA
c
87 B-...,...,..I!,...,...,..,,..,,...,..:”’1 IIs+ 87
NBIJM II IBUM TBUM
b...,!..,.......~..,.. Ilk
I.,
.I...,‘.“‘>
143
SBUM
E
Pm
F
~...,c~..,...,...;..‘.‘.‘...‘.“.”~”’-. ilk i=
L,&,,I,
,.!_, .J+,d_+.
.:’
,:,.
331
.., PEA
G
PBM I PIBRt PBIBM
IIO-
RESIN BI
0-e : ‘. , iI%
143
I
.
.
K .
PmA+PBML
Fig. 2. APCI mass spectra from that of (A) MMA, (B) EA, (C) EMA, (D) representative mass spectrum of the NBUMA, IBUMA and TBUMA monomers, (E) SBUMA. Py-APCI mass spectra of (F) PMMA, (G) PEA, (H) PEMA, (I) representative mass spectrum of PBMA, PIBMA and PBIBMA, (.I) PTBMA, (K) resin and (L) a 37.0: 1.0 mixture (w/w) of PMMA and PBMA.
132 CHZ=F-c02c~2cH3
~H,=~-co~H
H
H AA
NBUMA
EMA
‘: CH,
IBUMA
CH,=y-CO,(CH,),CH, CHs
CHs
MMA
Ch
MA
CH2=F-CO&H&H3
CHs
CH,=~-CO$ZH+H3
CH3 EA
CH,=y-C02CH,
CH2=y-C02H
H CH2=~-COz-+-CH3 CH3
CHzCH3
SBUMA
FH3 CH2=F-C02-F-CH3 CH3
CH3
TBUMA
Fig. 3. Structures of monomer acrylate and methacrylate compounds. The m/z value for the protonated molecular ion of each are as follows: AA (73), EA (lOl), MA (87), MMA (lOl), EMA (115) and the butyl methacrylate isomers (143).
found in the m/z 73, 85, 101, 115 and 127 lower molecular weight species. The Py-APCI mass spectrum of PEMA (Fig. 2H) has the MHf ion at m/z 115 as the base peak and the intensity of the methacrylate ion (vide infra) at m/z 87 is ca. 40% that of m/z 115. Higher molecular weight features are observed at relatively low abundance. Identical Py-APCI mass spectra are observed for PBMA, PIBMA and PBIBMA and a representative mass spectrum is presented in Fig. 21. The mass spectra of the n- and iso-polymer compounds shown in Fig. 21 are very similar with their respective headspace-sampled, monomer mass spectra (Fig. 2D). Poly( tert.-butyl methacrylate), on the other hand, forms a complex array of mass clusters with a prominent display of the MH+ and methacrylate (vide infra) ions, m/z 143 and 87, respectively (Fig. 25). In comparison to other CI techniques, the APCI mass spectra of these acrylic compounds more closely resemble that of their respective methane CI mass spectra (ref. 9, Table V), as opposed to their respective electron ionization (ref. 9, Table IV) and isobutane CI (ref. 9, Table VI) mass spectra. Application
to an acrylic resin -
qualitative analysis
The APCI mass spectra in Fig. 2A-J were subsequently used as a basis for the characterization of a commercial thermoplastic resin that is used as a modifier for poly(viny1 chloride). Fig. 2K presents the Py-APCI mass spectrum of the thermoplastic compound and an analysis of the constituent components follows. Prominent masses include m/z 87, 101, 143 and a
133
15
D 59
29 0-
.,.. 10.e
28.8
.‘.‘, 33.8
: :+A.‘. , . .‘: ! 58.8
40.8
I..
68.0
w
:I, :. . 70.8
I..
Ea.8
.
I..
98.8
.-! ?
103.8
Fig. 4. m/z 101 daughter ion mass spectra from that of (A) a representative of MMA and PMMA, (B) a representative of EA and PEA, (C) resin and (D) a 25.0 : 1.0 mixture (v/v) of MMA and EA.
number of minor features. The presence of m/z 101 and the m/z 115, 127, 141, 155, 169, 187, 201 and 215 series strongly suggested the presence of PMMA. m/z 87 suggested the presence of a methacrylate other than PMMA and m/z 143 pointed to the butyl methacrylate isomer series and/or PEA. An MS/MS analysis of m/z 101 is presented in Fig. 4. The MMA and PMMA compounds produced identical daughter ion mass spectra of m/z 101 and a representative daughter ion spectrum is given in Fig. 4A. Comparison of Fig. 4A and the MS/MS spectrum of the resin in Fig. 4C shows a close, but not identical match. In particular, the alkyl m/z 27/29 ratio is different, as well as the relative intensities of m/z 55 and 73 to that of the m/z 41 base peak. Therefore, it was postulated that another protonated molecular ion of m/z 101 was also present in the acrylic resin. Ethyl acrylate was chosen and the m/z 101 daughter ion mass spectra of EA and PEA were found to be identical. A representative mass spectrum is shown in Fig. 4B, and indeed, m/z 27,29, 55 and 73 are the primary fragments in the m/z 101 daughter ion mass spectrum of the acrylate. Even though PMMA and PEA both produced the m/z 101 protonated monomer peak, daughter ion analysis provided strong evidence of their presence in the resin compound. The ion m/z 73 was deemed useful as further support for the origin of m/z 101 in the resin compound because this mass is also found in MMA, EA, PMMA and PEA (Fig. 2). The MS/MS mass spectra of m/z 73 in these compounds and acrylic acid are found in Fig. 5. The m/z 73 daughter ion mass spectra of both MMA and PMMA were identical and are shown in
n/i!
Fig. 5. m/z 73 daughter ion mass spectra from that of (A) a representative of MMA and PMMA, (B) a representative of EA and PEA, (C) acrylic acid, (D) resin and (E) a 25.0: 1.0 mixture (v/v) of MMA and EA.
Fig. 5A. EA and PEA produced identical m/z 73 daughter ion mass spectra and are represented in Fig. 5B. Fig. 5C shows that of acrylic acid. Figs. 5B and C are very similar in mass spectral structure and thus the m/z 73 ion in the acrylate Py-mass spectrum originates from acrylic acid. The m/z 73 daughter ion mass spectrum of the resin (Fig. SD) has the same ions with similar intensity ratios as that of a simple summation of Figs. 5A and C. For example, the m/z 15/43 ratio of MMA (Fig. 5A) and the resin (Fig. 5D) are 0.18 and 0.17, respectively, while the m/z 27/55 and 27/45 ratios are an identical 1.2 and 3.0, respectively, for both acrylic acid (Fig. 5C) and the resin. These observations provide more evidence that the m/z 73 feature observed in the Py-APCI mass spectrum of the resin belongs not only to PMMA but also to acrylic acid. The direct evidence for acrylic acid in the resin’s m/z 73 Py-APCI-MS/MS mass spectrum and the indirect observation of ethyl acrylate in the m/z 101 daughter ion mass spectrum of PMMA provide good evidence for the presence of EA in the thermoplastic material under investigation. PEA shows an intense m/z 189 in its Py-mass spectrum in Fig. 2G while it is virtually absent in that of the resin (Fig. 2K). Reasons for this observation can include (a) the EA portion of the resin is composed predominantly of the EA monomer as opposed to the PEA polymeric form and/or (b) that this pyrolysis product may be particularly
135
Fig. 6. m/z 87 daughter methacrylate compounds methacrylic acid.
ion mass spectrum from that of (A) a representative in Fig. 2 with the exception of MMA and PMMA
from all and (B)
sensitive to the pyrolysis processes of the other components and/or (c) its proton ionization efficiency may be lower in comparison to the many other resin pyrolysis products. The abundant m/z 87 and 143 ions in the Py-APCI mass spectrum of the resin (Fig. 2K) indicated the presence of a butyl methacrylate isomer(s) as well as the PEA polymer for the m/z 143 ion. All methacrylate compounds in this study, with the exception of MMA and PMMA, produced an m/z 87 ion (Fig. 2), and their daughter ion mass spectra of m/z 87 were identical (Fig. 6A). The MS/MS mass spectrum in Fig. 6B confirmed the identity of the m/z 87 ion in all of the respective compounds as the protonated methacrylic acid (MH+ = 87). As stated above, the m/z 143 peak in question in the thermoplastic resin was postulated as a butyl methacrylate isomer or isomers as well as PEA. Figs. 7A-H present the MS/MS mass spectra of the m/z 143 ion for each of the monomer, polymer and copolymer butyl methacrylate isomers. The respective monomer and polymer forms of each butyl methacrylate isomer produced similar daughter ion mass spectra of the m/z 143 (MH+) ion (Fig. 7). Nevertheless, the MS/MS mass spectrum of each isomer can be distinguished by a visual inspection; this takes into account the relative abundances of m/z 29, 41, 45, 57 and 87. It is interesting to note that a 50 : 50 mixture of NBUMA and IBUMA produces an m/z 143 daughter ion mass spectrum (data not shown) identical to that of the pyrolyzed copolymer (Fig. 7E). The m/z 143 daughter ion mass spectrum of the copolymer (Fig. 7E) can also be visually differentiated from its respective monomer and homopolymer MS/MS spectra (Figs. 7A-D) chiefly by the relative intensity ratio of m/z 41 and 57. The m/z 143 daughter ion mass spectrum of PEA is shown in Fig. 71 and it displays significantly different ion abundances with respect to the butyl methacrylate compounds. Furthermore, the butyl methacrylate compounds contain the m/z 57 and 59 daughter ions (the former of which is most likely the butyl fragment itself) and they are absent in the case of PEA (Fig. 71). PEA contains an m/z 115 daughter ion which
136 11041
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138
145
IdZ
Fig. 7. m/z PIBMA,
143
daughterion mass spectraof (A) NBUMA, (B) PBM& CC)IBUm, CD) (F) SBUMA, (G) TBUMA, (H) PTBMA, (I) PEA and (J) resin.
(E) PBIBMA,
is not found in the m/z 143 daughter ion mass spectra of the butyl methacrylate and resin compounds. The daughter ion mass spectrum of m/z 143 from the thermoplastic resin (Fig. 7J) most closely matches that of NBUMA and PBUMA (Figs. 7A,B). Minor differences between NBUMA/
137
PBUMA (Figs. 7A,B) and the resin (Fig. 75) are that the m/z 43 and 55 are found in the nz/z 143 daughter ion mass spectrum of the resin and are absent in that of the n-butyl methacrylate. These ions could originate from the m/z 143 Py-APCI-MS/MS of the EA/PEA portion of the resin. The relatively low abundance of these ions in the m/z 143 Py-APCI-MS/MS mass spectrum of the resin indicates a low contribution of PEA to this mass spectral feature. The NBUMA and PBMA compounds, taken together, produced an average m/z 41/57 ratio from their respective m/z 143 daughter ion mass spectra (Fig. 7) of 1.27 (standard deviation of 0.01 and the number of determinations (N) was 2). The IBUMA and PIBMA compounds produced an average m/z 41/57 ratio of 0.71 (0.11, n = 2), and PBIBMA and the 50 : 50 NBUMA/IBUMA liquid mixture produced a similar ion ratio of 0.95 (0.04, N = 2). The resin produced an average m/z 41/57 ratio of 1.42 (0.25, N = 4) which overlaps with that of the n-butyl methacrylate species. The other compounds in Fig. 7 differ from the resin (Fig. 75) in terms of the ratios of m/z 29, 41, 45 and 87 daughter ions to a greater extent than that of the nand iso-butyl methacrylates. These observations show in particular the selectivity power of the MS/MS technique [19] for a series of compounds as similar as that of the butyl methacrylate isomers. The above constitutes a qualitative analysis of the most abundant ions observed in the Py-APCI mass spectrum of the acrylic resin. However, other acrylic compounds could be present in the resin, and therefore all masses of greater than 5% abundance in the resin’s Py-APCI mass spectrum were compared to that of possible MH+ ions in the acrylate and methacrylate
A
n/z Fig. 8. m/z resin.
115 daughter ion mass spectra of (A) PMMA, (B) PEMA, (C) PEA and (D)
138
series [9]. Only m/z 87, 101, 115 and 143 could be considered as candidates. The m/z 87, 101 and 143 were already investigated, and therefore attention was placed on the m/z 115 ion. Initially, it was assumed that in the Py-APCI mass spectrum of the resin (Fig. 2K), m/z 115 originated from the PMMA portion of the polymer (Fig. 2F). However, a closer inspection was performed with the m/z 115 daughter ion mass spectra of PMMA, PEMA, PEA and the resin, Figs. SA-D, respectively. The m/z 115 MS/MS mass spectrum of the resin (Fig. 8D) does not match that of any of the three test polymers (Figs. 8A-C), but an approximation occurs with that of PMMA (Fig. 8A) and either PEMA (Fig. 8B) or PEA (Fig. 8C) taken together. The m/z 115 daughter ion mass spectra of PEMA and PEA are somewhat similar. However the resin (Fig. 8D) displays the m/z 39, 41 and 69 peaks which are found to be more prominent in PEA as opposed to PEMA. The m/z 55 ion in the resin (Fig. SD) appears to originate from PMMA and PEA. Even though there is a closer correspondence between the m/z 115 daughter ion mass spectra of the resin and PEA as opposed to the resin and PEMA, it is possible that based solely on this evidence, PEMA also could be present in the resin. However, the MSDS of the resin states that the compound is composed of PMMA and an acrylic copolymer, and PMMA is clearly observed. Together, the MS and MS/MS analyses show that the acrylic copolymer component appears to be composed of EA/PEA and the n-butyl isomer of butyl methacrylate. Quantitative
analysis of the acrylic resin
Based on the abundance of the major ions in the resin’s Py-APCI mass spectrum (Fig. 2K), mixture experiments were designed so as to arrive at the relative ratios of the MMA, EA and NBUMA constituents in the resin. It is well known that resultant yields of acrylic compounds in polymer systems strongly depend on their form [1,4]. That is, a mixture of two different acrylic homopolymers can yield a ratio of their respective monomers that differs from that of a copolymer with the same percentage of each monomer. An interesting observation in the Py-APCI mass spectrum of the resin (Fig. 2K) is the different ratio of m/z 87/143 in comparison to that of NBUMA and PBMA (Figs. 2D and I respectively). The resin has a value of 1.81 which is similar to the 1.92 value of sec.-butyl methacrylate (Fig. 2E), while the n-butyl compounds exhibit an approximate 0.8 ratio (Table 1; Figs. 2D and I). Despite the similar m/z 87/143 ratios, the MS/MS mass spectra of the m/z 143 ions preclude the presence of SBUMA in the resin compound. However, this difference in the ratio of m/z 87/143 could arise from either (a) a methacrylate in addition to PBMA being present in the resin which contributes to the abundance of the m/z 87 ion or (b) upon oxidative pyrolysis the acrylic compound preferentially yields a greater amount of
139 TABLE
1
Mass spectral
ion abundance
Preparation
NBUMA SBUMA MMA + NBUMA PMMA + PBMA PMMA + PBMA Resin l ** * By volume. ** By weight. *** Two determinations
ratios of methacrylate
Ratio of constituents 19.2 : 1.0 l 19.2 : 1.0 l * 37.0 : 1.0 ** -
m/z
compound
preparations
ratios
87/101
143/101
87/143
0.19 0.99 0.45 0.47 (0.08)
0.23 0.44 0.25 0.31 (0.11)
0.80 1.92 0.80 2.24 1.81 1.59 (0.31)
were made (standard
-
deviation).
protonated methacrylic acid (m/z 87) than in the individual polymer or monomer states. The qualitative analysis indicated that the only methacrylate yielding an m/z 87 in the resin was PBMA, and therefore the latter hypothesis was tested by preparing a solution of MMA and NBUMA. A 19.2: 1 proportion by volume of MMA and NBUMA, respectively, was required in order to produce an m/z 143/101 ratio of 0.23 which was similar to that of the resin (Table 1). This phenomenon is common in APCI gas phase chemistry. The apparent disparity in concentration vs. mass spectral abundance occurs when compounds in a mixture have significantly different proton affinities [20-231. However, the m/z 87/143 ratio of 0.8 was identical to that of the pure NBUMA monomer rather than the 1.59 average ratio found in the resin compound (Table 1). Since a simple mixture of the headspace-sampled MMA and NBUMA did not provide a similar m/z 87/101 ratio with that of the pyrolyzed resin, Py-APCI mass spectra were obtained with mixtures of the PMMA and PBMA polymers. Polymer mixtures with a greater than nineteen-fold by weight of PMMA to that of PBMA produced an m/z 87 of greater relative intensity than m/z 143 (Table 1). A mixture of 37.0 : 1.0 of PMMA : PBMA, respectively, provided similar m/z ratios (Table 1) with that of the resin. The Py-APCI mass spectrum (Fig. 2L) of the 37.0 : 1.0 polymer mixture of PMMA and PBMA, respectively, was very similar to that of the resin (Fig. 2K). As noted with other pyrolysis procedures [1,4], oxidative pyrolysis also produces significantly different amounts of subcomponents with respect to individual acrylic homopolymers and homopolymer mixtures as observed in Figs. 21 and L, respectively. A similar analysis was conducted for EA with respect to MMA except the indicators that were used for comparisons were the m/z 73 (Fig. SD) and m/z 101 (Fig. 4C) APCI-MS/MS mass spectra of the resin to that of a mixture of MMA and EA. A ratio of 25.0 : 1.0 of MMA to EA by volume, respectively, was necessary in order to produce m/z 73 (Fig. 5E)
140
and 101 (Fig. 4D) daughter ion mass spectra that were both similar to that of the respective resin daughter ion mass spectra. The quantitative analysis of the resin has shown that upon formation of the MMA and NBUMA monomers, complex pyrolysis interactions occur to the extent that the ratios of these monomers present in the resin cannot be determined simply by a mixture of monomers. This was expected due to the results of prior acrylate investigations [1,4] and homopolymer mixtures of PMMA and PBMA indicate that their relative ratio in the resin is approximately 37.0 : 1.0. This ratio was found to differ substantially from a simple monomer mixture. The isobaric nature of the MMA and EA monomers, on the other hand, provided the means with which to directly compare the relative ion intensities in their m/z 73 and 101 daughter ion mass spectra with that of the respective resin counterparts. This method allowed a simple liquid mixture of the monomers to yield a direct comparison of the MMA to EA monomer, respectively, in the resin.
CONCLUSIONS
Oxidative Py-APCI-MS/MS was shown to provide several advantages in the characterization of acrylic polymer compounds. Oxidative pyrolysis readily produced the key monomer building blocks of acrylate and methacrylate polymers as well as useful fragment information. More importantly, the daughter ion mode was shown to be particularly useful in the qualitative analysis of the methacrylate isomers, as well as in solid polymer and monomer mixtures or formulations. The experimental analysis, from the loading of a sample to the production of an averaged mass spectrum, can be performed routinely in a 5-7 minute time frame and the experimental protocol is straightforward in design. For the quantitative characterization of acrylic mixtures, one of the strategies was to compare superimposed patterns of daughter ions from components with the same MH+ ion from a mixture/preparation with that of the daughter ion mass spectral patterns of the pure target/suspect compounds. This approach together with the information from MS analyses was decidedly useful in obtaining the relative quantitation of the individual constituents in solid acrylic materials.
ACKNOWLEDGEMENTS
The author wishes to thank Linda G. Jarvis for the preparation and editing of the manuscript and Shirley A. Liebman for a critical evaluation of the manuscript.
141 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
J. Strassburger, G.M. Brauer, M. Tryon and A.F. Forziati, Anal. Chem., 32 (1960) 454. R.L. Gatrell and T.J. Mao, Anal. Chem., 37 (1965) 1294. H. McCormick, J. Chromatogr., 40 (1969) 1. E.C. Ferlauto, M.K. Lindemann, C.A. Lucchesi and D.R. Gaskill, J. Appl. Polym. Sci., 15 (1971) 445. H. Ohtani, T. Asai and S. Tsuge, Macromolecules, 18 (1985) 1148. M.W. Raynor, R.M. Waring and I.L. Davies, J. Chromatogr. Sci., 25 (1987) 104. J.J. Shen and E. Woo, LC.GC, Mag. Liq. Gas Chromatogr., 6 (1988) 1020. N. Gjos, K. Urdal, I.E. Ruyter and I.J. Sjovik, Anal. Chim. Acta, 149 (1983) 87. A. Homa, J. Taborsky and J. Churacek, J. Chromatogr., 360 (1986) 89. L.A. O’Neill and G. Christensen, J. Paint Technol., 47 (1975) 46. P.H. Dawson, J.B. French, J.A. Buckley, D.J. Douglas and D. Simmons, Org. Mass Spectrom., 171 (1982) 205. V.J. Caldecourt, D. Zakett and J.C. Tou, Int. J. Mass Spectrom. Ion Phys., 49 (1983) 233. S.A. Liebman, A.P. Snyder, J.H. Kremer, D.J. Reutter, M.A. Schroeder and R.A. Fifer, J. Anal. Appl. Pyrolysis, 12 (1987) 83. A.P. Snyder, J.H. Kremer, H.L.C. Meuzelaar and W. Windig, J. Anal. Appl. Pyrolysis, 13 (1988) 77. A.P. Snyder, J.H. Kremer, S.A. Liebman, M.A. Schroeder and R.A. Fifer, Org. Mass Spectrom., 24 (1989) 15. B. Anson, personal communication, Rohm and Haas Co., Philadelphia, PA, February, 1989. A.P. Snyder, J.H. Kremer, H.L.C. Meuzelaar, W. Windig and K. Taghizadeh, Anal. Chem., 59 (1987) 1945. A.P. Snyder, G.A. Eiceman and W. Windig, J. Anal. Appl. Pyrolysis, 13 (1988) 243. J. Johnson and R. Yost, Anal., Chem., 57 (1985) 758A. V.J. Vandiver, C.S. Leasure and G.A. Eiceman, Int. J. Mass Spectrom. Ion Proc., 66 (1985) 223. M.T. Bowers, Gas Phase Ion Chemistry, Academic Press, New York, 1979. J. Sunner, G. Nicol and P. Kebarle, Anal. Chem., 60 (1988) 1300. A.H. Lawrence, Anal. Chem., 61 (1989) 343.
127
ACRYLIC COMPOUND CHARACTERIZATION BY OXIDATIYE PYROLYSIS, ATMOSPHERIC PRESSURE CHEMICAL IONIZATION-TANDEM MASS SPECTROMETRY
A. PETER
SNYDER
U.S. Army Chemical Research, Development and Engineering Aberdeen Proving Ground, MD 21010-5423 (U.S.A.) (Received
June 27th, 1989; accepted
September
Center, SMCCR-RSL,
lst, 1989)
ABSTRACT Oxidative pyrolysis, atmospheric pressure chemical ionization (Py-APCI) in conjunction with tandem quadrupole mass spectrometry (MS/MS) was investigated to characterize various acrylate and methacrylate compounds. The Py-APCI mass spectra of these compounds were dominated by the protonated molecular ion and depending on the acrylate, either methacrylic or acrylic acid was produced as the primary fragment ion. These observations from the headspace analysis of the monomers were similar to those of the mass spectra of their respective polymer/copolymer compounds under oxidative pyrolysis processing conditions. In particular, Py-APCI-MS/MS produced daughter ion mass spectra of the polymer and selected copolymer forms of the four butyl methacrylate isomers. These spectra could be differentiated by visual inspection. A commercially-available acrylic thermoplastic resin was investigated for its qualitative and quantitative composition. n-Butyl methacrylate, methyl methacrylate and ethyl acrylate, despite identical m/z 101 protonated molecular ions for the latter two compounds were identified in the resin by interpretation of the MS/MS mass spectra. Acrylate isomers; atmospheric methacrylate isomers; pyrolysis,
pressure chemical oxidative; tandem
ionization; butyl mass spectrometry.
methacrylate
isomers;
INTRODUCTION
Alkyl and polyalkyl acrylates play important roles in commodities such as coatings for a wide variety of metals, paints, lubricating oils, composites, thermoplastic/molding resins and dental resins. Because of the extensive use of acrylic compounds in the manufacturing industry, the literature documents their qualitative and quantitative characterization with a number of laboratory benchmark techniques. These instrumental techniques include gas chromatography (GC) [l-7], mass spectrometry (MS) [S], GC/MS [9], nuclear magnetic resonance [7,10] and infrared spectroscopy [6,10]. 0165-2370/90/$03.50
0 1990 Elsevier Science Publishers
B.V.
128
Thermal methods, pyrolysis (Py) in particular, and direct injection of liquid alkyl acrylate monomers and monomer mixtures are the predominant methods of sample introduction for GC, MS and GC/MS. It has been stated that Py-capillary GC is the most efficient method for the qualitative characterization of the monomers in an acrylic copolymer because of the highly reproducible GC monomer profiles observed upon pyrolysis [6]. Investigations that have included MS in the analysis of acrylic monomers and polymers have relied on either electron ionization (EI) [8,9] or chemical ionization (CI) [9] methods. Isobutane CI [9] yields primarily the protonated molecular ion (MH+) of alkyl acrylate and methacrylates from monomer and polymer pyrolyzates. EI yields extensive fragmentation [8,9] of the alkyl acrylate and methacrylates and produces little to negligible amounts of the molecular ion (M+). Methane CI [9] of these compounds appears to yield mass spectra with elements similar to that of EI and isobutane CI mass spectra. The present work focuses on the potential of atmospheric pressure chemical ionization (APCI)-tandem mass spectrometry (APCI-MS/MS) [11,12] in the characterization of liquid and solid acrylic materials. APCI is largely characterized by the formation of a reactive medium containing proton in the positive ion mode. In common hydrates in the form of H(H,O)i practice, n lies in the range of 1-4. Soft ionization of the analyte generally takes place and results in the formation of protonated molecular ions (MH+). With this technique a series of acrylic compounds were analyzed belonging to the acrylate and methacrylate families, and. the four butyl methacrylate isomers were investigated for their differentiation by the MS/MS method. Oxidative pyrolysis was used for introduction of solid polymeric samples into the APCI source [13-151, while a simple headspace sampling proved effective for liquid compounds. The information obtained was applied in the qualitative and quantitative characterization of a commercial acrylic thermoplastic resin.
EXPERIMENTAL
Materials Poly(methy1 methacrylate) (PMMA), poly(ethy1 methacrylate) (PEMA), poly(isobuty1 methacrylate) (PIBMA), poly( n-butyl methacrylate) (PBMA), and the butyl methacrylate/isobutyl methacrylate copolymer (PBIBMA) were purchased from Scientific Polymer Products, Ontario, NY, U.S.A. Poly( tert.-butyl methacrylate) (PTBMA), tert.-butyl methacrylate (TBUMA), and sec.-butyl methacrylate (SBUMA) were obtained from Polysciences, Warrington, PA, U.S.A. Methyl methacrylate (MMA), n-butyl methacrylate (NBUMA), isobutyl methacrylate (IBUMA), acrylic acid (AA), ethyl acrylate
129
(EA), poly(ethy1 acrylate) (PEA), and ethyl methacrylate (EMA) were obtained from Aldrich Chemical Co., Milwaukee, WI, U.S.A. and methacrylic acid (MA) was purchased from Alfa products, Danvers, MA, U.S.A. A commercial, thermoplastic resin was purchased from Rohm and Haas Co., Philadelphia, PA, U.S.A. Complete qualitative and quantitative characterization of the resin is proprietary [16]. However, the Material Safety Data Sheet (MSDS) of the compound lists its composition as containing the methyl methacrylate monomer and an acrylic copolymer. Methods A SCIEX (Toronto, Canada) TAGA 6000 APCI triple quadrupole (tandem) MS/MS system was used as the mass spectral analyzer. Basic references for the methods and parameter settings are noted elsewhere [17] and daughter ion mass spectra (MS/MS mode) were obtained with an argon collision gas pressure of 6.0 X 10e5 torr in the center quadrupole. Sample pyrolysis was conducted with a Pyroprobe Model 122 power supply (Chemical Data Systems, Oxford, PA, U.S.A.) and Pyroprobe @ platinum coil probe. All pyrolyses were conducted in the pulsed mode with the pulse rate control in the off position. The final set temperature was 700 o C, and the total heating time was 20 s. 0.1-0.5 mg of the polymers was sandwiched between quartz wool plugs in the quartz tube sample holder and the latter was inserted into the coil of the heating probe. The Pyroprobe was interfaced to the APCI source of the MS/MS system as shown in Fig. 1. The pyrolysis probe was fitted with perforated positioning rings and was inserted into a glass tube to permit a uniform gas flow over the sample region. The end of the Pyroprobe coil was positioned within the glass tube so that the end of the coil was about 1 cm from the corona discharge needle tip in the
PYROPROBE \
t EXHAUST
Fig. 1. Schematic diagram of the Pyroprobe in the atmospheric pressure ion source of the mass spectrometer.
130
ion source. A ‘zero-air’ flowrate of 150 ml/mm was used to effect oxidative heating and pyrolyzate removal from the heating zone to the ion source. The pressure in the APCI source was lowered from 0.98 atm (ambient) to 0.97 atm with the chamber exhaust motor (Fig. 1) which also facilitated pyrolyzate removal from the heating zone to the ion source [18]. Monomer standards were introduced into the analyzer by drawing the headspace vapor from each respective liquid directly into the ion source. PEA was obtained as a solution in toluene. A few milliliters of the solution were vacuum-dried at 50 o C for 24 h to a thick, clear paste. A pinpoint drop was applied to the quartz wool in the quartz tube for thermal processing. The mass spectra contained in the total ion current (lo-15 individual mass spectra) were signal averaged with a subsequent background subtraction. All analyses were done in single or replicate fashion.
RESULTS AND DISCUSSION
Characterization
of acrylic compounds
Fig. 2 presents the APCI and Py-APCI mass spectra of acrylate and methacrylate compounds. The monomers were observed by headspace analysis, and the polymer compounds were analyzed by quartz tube pyrolysis. Structures of the monomer compounds are presented in Fig. 3. Methyl methacrylate (Fig. 2A) predominates as the protonated monomer (MH+) species at m/z 101 with minor peaks at m/z 69 and 73. Ethyl acrylate (Fig. 2B) is characterized by the MH+ species at m/z 101 as the base peak, a protonated acrylic acid molecule at m/z 73 (vide infra), and a low intensity feature at m/z 118, which could represent the hydrated monomer ion. Ethyl methacrylate (Fig. 2C) is represented by the MH+ and protonated methacrylic acid species (vide infra) at m/z 115 and 87, respectively. All four butyl methacrylate isomers (Fig. 2D,E) produced identical APCI mass spectra except for sec.-butyl methacrylate (Fig. 2E). The isomers produced the MH+ ion at m/z 143 as well as the abundant methacrylate fragment at m/z 87 (vide infra). sec.-Butyl methacrylate contains the m/z 87 ion as the base peak with an m/z 143/87 intensity ratio of approximately 50% while the other three isomers show an identical 125% ratio, with m/z 143 as the base peak. Upon oxidative pyrolysis of these compounds in polymer form, simple as well as more complex mass spectra were observed (Fig. 2F-.I). PMMA produced the MH+ as the base peak, as well as the m/z 115, 127, 141, 155, 169, 187, 201 and 215 series of lower intensity masses (Fig. 2F), and m/z 69 and 73 are also observed. PEA has its Py-APCI mass spectrum shown as Fig. 2G and its abundance pattern is skewed to higher mass. The m/z 143, 155, 189 and 201 dominate the spectrum, while the low intensity features are
131 II&
B-...,...!‘...1,...,...,...,..,~...,.””, ila-
MA
A
EnA
c
87 B-...,...,..I!,...,...,..,,..,,...,..:”’1 IIs+ 87
NBIJM II IBUM TBUM
b...,!..,.......~..,.. Ilk
I.,
.I...,‘.“‘>
143
SBUM
E
Pm
F
~...,c~..,...,...;..‘.‘.‘...‘.“.”~”’-. ilk i=
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,.!_, .J+,d_+.
.:’
,:,.
331
.., PEA
G
PBM I PIBRt PBIBM
IIO-
RESIN BI
0-e : ‘. , iI%
143
I
.
.
K .
PmA+PBML
Fig. 2. APCI mass spectra from that of (A) MMA, (B) EA, (C) EMA, (D) representative mass spectrum of the NBUMA, IBUMA and TBUMA monomers, (E) SBUMA. Py-APCI mass spectra of (F) PMMA, (G) PEA, (H) PEMA, (I) representative mass spectrum of PBMA, PIBMA and PBIBMA, (.I) PTBMA, (K) resin and (L) a 37.0: 1.0 mixture (w/w) of PMMA and PBMA.
132 CHZ=F-c02c~2cH3
~H,=~-co~H
H
H AA
NBUMA
EMA
‘: CH,
IBUMA
CH,=y-CO,(CH,),CH, CHs
CHs
MMA
Ch
MA
CH2=F-CO&H&H3
CHs
CH,=~-CO$ZH+H3
CH3 EA
CH,=y-C02CH,
CH2=y-C02H
H CH2=~-COz-+-CH3 CH3
CHzCH3
SBUMA
FH3 CH2=F-C02-F-CH3 CH3
CH3
TBUMA
Fig. 3. Structures of monomer acrylate and methacrylate compounds. The m/z value for the protonated molecular ion of each are as follows: AA (73), EA (lOl), MA (87), MMA (lOl), EMA (115) and the butyl methacrylate isomers (143).
found in the m/z 73, 85, 101, 115 and 127 lower molecular weight species. The Py-APCI mass spectrum of PEMA (Fig. 2H) has the MHf ion at m/z 115 as the base peak and the intensity of the methacrylate ion (vide infra) at m/z 87 is ca. 40% that of m/z 115. Higher molecular weight features are observed at relatively low abundance. Identical Py-APCI mass spectra are observed for PBMA, PIBMA and PBIBMA and a representative mass spectrum is presented in Fig. 21. The mass spectra of the n- and iso-polymer compounds shown in Fig. 21 are very similar with their respective headspace-sampled, monomer mass spectra (Fig. 2D). Poly( tert.-butyl methacrylate), on the other hand, forms a complex array of mass clusters with a prominent display of the MH+ and methacrylate (vide infra) ions, m/z 143 and 87, respectively (Fig. 25). In comparison to other CI techniques, the APCI mass spectra of these acrylic compounds more closely resemble that of their respective methane CI mass spectra (ref. 9, Table V), as opposed to their respective electron ionization (ref. 9, Table IV) and isobutane CI (ref. 9, Table VI) mass spectra. Application
to an acrylic resin -
qualitative analysis
The APCI mass spectra in Fig. 2A-J were subsequently used as a basis for the characterization of a commercial thermoplastic resin that is used as a modifier for poly(viny1 chloride). Fig. 2K presents the Py-APCI mass spectrum of the thermoplastic compound and an analysis of the constituent components follows. Prominent masses include m/z 87, 101, 143 and a
133
15
D 59
29 0-
.,.. 10.e
28.8
.‘.‘, 33.8
: :+A.‘. , . .‘: ! 58.8
40.8
I..
68.0
w
:I, :. . 70.8
I..
Ea.8
.
I..
98.8
.-! ?
103.8
Fig. 4. m/z 101 daughter ion mass spectra from that of (A) a representative of MMA and PMMA, (B) a representative of EA and PEA, (C) resin and (D) a 25.0 : 1.0 mixture (v/v) of MMA and EA.
number of minor features. The presence of m/z 101 and the m/z 115, 127, 141, 155, 169, 187, 201 and 215 series strongly suggested the presence of PMMA. m/z 87 suggested the presence of a methacrylate other than PMMA and m/z 143 pointed to the butyl methacrylate isomer series and/or PEA. An MS/MS analysis of m/z 101 is presented in Fig. 4. The MMA and PMMA compounds produced identical daughter ion mass spectra of m/z 101 and a representative daughter ion spectrum is given in Fig. 4A. Comparison of Fig. 4A and the MS/MS spectrum of the resin in Fig. 4C shows a close, but not identical match. In particular, the alkyl m/z 27/29 ratio is different, as well as the relative intensities of m/z 55 and 73 to that of the m/z 41 base peak. Therefore, it was postulated that another protonated molecular ion of m/z 101 was also present in the acrylic resin. Ethyl acrylate was chosen and the m/z 101 daughter ion mass spectra of EA and PEA were found to be identical. A representative mass spectrum is shown in Fig. 4B, and indeed, m/z 27,29, 55 and 73 are the primary fragments in the m/z 101 daughter ion mass spectrum of the acrylate. Even though PMMA and PEA both produced the m/z 101 protonated monomer peak, daughter ion analysis provided strong evidence of their presence in the resin compound. The ion m/z 73 was deemed useful as further support for the origin of m/z 101 in the resin compound because this mass is also found in MMA, EA, PMMA and PEA (Fig. 2). The MS/MS mass spectra of m/z 73 in these compounds and acrylic acid are found in Fig. 5. The m/z 73 daughter ion mass spectra of both MMA and PMMA were identical and are shown in
n/i!
Fig. 5. m/z 73 daughter ion mass spectra from that of (A) a representative of MMA and PMMA, (B) a representative of EA and PEA, (C) acrylic acid, (D) resin and (E) a 25.0: 1.0 mixture (v/v) of MMA and EA.
Fig. 5A. EA and PEA produced identical m/z 73 daughter ion mass spectra and are represented in Fig. 5B. Fig. 5C shows that of acrylic acid. Figs. 5B and C are very similar in mass spectral structure and thus the m/z 73 ion in the acrylate Py-mass spectrum originates from acrylic acid. The m/z 73 daughter ion mass spectrum of the resin (Fig. SD) has the same ions with similar intensity ratios as that of a simple summation of Figs. 5A and C. For example, the m/z 15/43 ratio of MMA (Fig. 5A) and the resin (Fig. 5D) are 0.18 and 0.17, respectively, while the m/z 27/55 and 27/45 ratios are an identical 1.2 and 3.0, respectively, for both acrylic acid (Fig. 5C) and the resin. These observations provide more evidence that the m/z 73 feature observed in the Py-APCI mass spectrum of the resin belongs not only to PMMA but also to acrylic acid. The direct evidence for acrylic acid in the resin’s m/z 73 Py-APCI-MS/MS mass spectrum and the indirect observation of ethyl acrylate in the m/z 101 daughter ion mass spectrum of PMMA provide good evidence for the presence of EA in the thermoplastic material under investigation. PEA shows an intense m/z 189 in its Py-mass spectrum in Fig. 2G while it is virtually absent in that of the resin (Fig. 2K). Reasons for this observation can include (a) the EA portion of the resin is composed predominantly of the EA monomer as opposed to the PEA polymeric form and/or (b) that this pyrolysis product may be particularly
135
Fig. 6. m/z 87 daughter methacrylate compounds methacrylic acid.
ion mass spectrum from that of (A) a representative in Fig. 2 with the exception of MMA and PMMA
from all and (B)
sensitive to the pyrolysis processes of the other components and/or (c) its proton ionization efficiency may be lower in comparison to the many other resin pyrolysis products. The abundant m/z 87 and 143 ions in the Py-APCI mass spectrum of the resin (Fig. 2K) indicated the presence of a butyl methacrylate isomer(s) as well as the PEA polymer for the m/z 143 ion. All methacrylate compounds in this study, with the exception of MMA and PMMA, produced an m/z 87 ion (Fig. 2), and their daughter ion mass spectra of m/z 87 were identical (Fig. 6A). The MS/MS mass spectrum in Fig. 6B confirmed the identity of the m/z 87 ion in all of the respective compounds as the protonated methacrylic acid (MH+ = 87). As stated above, the m/z 143 peak in question in the thermoplastic resin was postulated as a butyl methacrylate isomer or isomers as well as PEA. Figs. 7A-H present the MS/MS mass spectra of the m/z 143 ion for each of the monomer, polymer and copolymer butyl methacrylate isomers. The respective monomer and polymer forms of each butyl methacrylate isomer produced similar daughter ion mass spectra of the m/z 143 (MH+) ion (Fig. 7). Nevertheless, the MS/MS mass spectrum of each isomer can be distinguished by a visual inspection; this takes into account the relative abundances of m/z 29, 41, 45, 57 and 87. It is interesting to note that a 50 : 50 mixture of NBUMA and IBUMA produces an m/z 143 daughter ion mass spectrum (data not shown) identical to that of the pyrolyzed copolymer (Fig. 7E). The m/z 143 daughter ion mass spectrum of the copolymer (Fig. 7E) can also be visually differentiated from its respective monomer and homopolymer MS/MS spectra (Figs. 7A-D) chiefly by the relative intensity ratio of m/z 41 and 57. The m/z 143 daughter ion mass spectrum of PEA is shown in Fig. 71 and it displays significantly different ion abundances with respect to the butyl methacrylate compounds. Furthermore, the butyl methacrylate compounds contain the m/z 57 and 59 daughter ions (the former of which is most likely the butyl fragment itself) and they are absent in the case of PEA (Fig. 71). PEA contains an m/z 115 daughter ion which
136 11041
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,
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.
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.
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.
,
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.
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,
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,
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,
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138
145
IdZ
Fig. 7. m/z PIBMA,
143
daughterion mass spectraof (A) NBUMA, (B) PBM& CC)IBUm, CD) (F) SBUMA, (G) TBUMA, (H) PTBMA, (I) PEA and (J) resin.
(E) PBIBMA,
is not found in the m/z 143 daughter ion mass spectra of the butyl methacrylate and resin compounds. The daughter ion mass spectrum of m/z 143 from the thermoplastic resin (Fig. 7J) most closely matches that of NBUMA and PBUMA (Figs. 7A,B). Minor differences between NBUMA/
137
PBUMA (Figs. 7A,B) and the resin (Fig. 75) are that the m/z 43 and 55 are found in the nz/z 143 daughter ion mass spectrum of the resin and are absent in that of the n-butyl methacrylate. These ions could originate from the m/z 143 Py-APCI-MS/MS of the EA/PEA portion of the resin. The relatively low abundance of these ions in the m/z 143 Py-APCI-MS/MS mass spectrum of the resin indicates a low contribution of PEA to this mass spectral feature. The NBUMA and PBMA compounds, taken together, produced an average m/z 41/57 ratio from their respective m/z 143 daughter ion mass spectra (Fig. 7) of 1.27 (standard deviation of 0.01 and the number of determinations (N) was 2). The IBUMA and PIBMA compounds produced an average m/z 41/57 ratio of 0.71 (0.11, n = 2), and PBIBMA and the 50 : 50 NBUMA/IBUMA liquid mixture produced a similar ion ratio of 0.95 (0.04, N = 2). The resin produced an average m/z 41/57 ratio of 1.42 (0.25, N = 4) which overlaps with that of the n-butyl methacrylate species. The other compounds in Fig. 7 differ from the resin (Fig. 75) in terms of the ratios of m/z 29, 41, 45 and 87 daughter ions to a greater extent than that of the nand iso-butyl methacrylates. These observations show in particular the selectivity power of the MS/MS technique [19] for a series of compounds as similar as that of the butyl methacrylate isomers. The above constitutes a qualitative analysis of the most abundant ions observed in the Py-APCI mass spectrum of the acrylic resin. However, other acrylic compounds could be present in the resin, and therefore all masses of greater than 5% abundance in the resin’s Py-APCI mass spectrum were compared to that of possible MH+ ions in the acrylate and methacrylate
A
n/z Fig. 8. m/z resin.
115 daughter ion mass spectra of (A) PMMA, (B) PEMA, (C) PEA and (D)
138
series [9]. Only m/z 87, 101, 115 and 143 could be considered as candidates. The m/z 87, 101 and 143 were already investigated, and therefore attention was placed on the m/z 115 ion. Initially, it was assumed that in the Py-APCI mass spectrum of the resin (Fig. 2K), m/z 115 originated from the PMMA portion of the polymer (Fig. 2F). However, a closer inspection was performed with the m/z 115 daughter ion mass spectra of PMMA, PEMA, PEA and the resin, Figs. SA-D, respectively. The m/z 115 MS/MS mass spectrum of the resin (Fig. 8D) does not match that of any of the three test polymers (Figs. 8A-C), but an approximation occurs with that of PMMA (Fig. 8A) and either PEMA (Fig. 8B) or PEA (Fig. 8C) taken together. The m/z 115 daughter ion mass spectra of PEMA and PEA are somewhat similar. However the resin (Fig. 8D) displays the m/z 39, 41 and 69 peaks which are found to be more prominent in PEA as opposed to PEMA. The m/z 55 ion in the resin (Fig. SD) appears to originate from PMMA and PEA. Even though there is a closer correspondence between the m/z 115 daughter ion mass spectra of the resin and PEA as opposed to the resin and PEMA, it is possible that based solely on this evidence, PEMA also could be present in the resin. However, the MSDS of the resin states that the compound is composed of PMMA and an acrylic copolymer, and PMMA is clearly observed. Together, the MS and MS/MS analyses show that the acrylic copolymer component appears to be composed of EA/PEA and the n-butyl isomer of butyl methacrylate. Quantitative
analysis of the acrylic resin
Based on the abundance of the major ions in the resin’s Py-APCI mass spectrum (Fig. 2K), mixture experiments were designed so as to arrive at the relative ratios of the MMA, EA and NBUMA constituents in the resin. It is well known that resultant yields of acrylic compounds in polymer systems strongly depend on their form [1,4]. That is, a mixture of two different acrylic homopolymers can yield a ratio of their respective monomers that differs from that of a copolymer with the same percentage of each monomer. An interesting observation in the Py-APCI mass spectrum of the resin (Fig. 2K) is the different ratio of m/z 87/143 in comparison to that of NBUMA and PBMA (Figs. 2D and I respectively). The resin has a value of 1.81 which is similar to the 1.92 value of sec.-butyl methacrylate (Fig. 2E), while the n-butyl compounds exhibit an approximate 0.8 ratio (Table 1; Figs. 2D and I). Despite the similar m/z 87/143 ratios, the MS/MS mass spectra of the m/z 143 ions preclude the presence of SBUMA in the resin compound. However, this difference in the ratio of m/z 87/143 could arise from either (a) a methacrylate in addition to PBMA being present in the resin which contributes to the abundance of the m/z 87 ion or (b) upon oxidative pyrolysis the acrylic compound preferentially yields a greater amount of
139 TABLE
1
Mass spectral
ion abundance
Preparation
NBUMA SBUMA MMA + NBUMA PMMA + PBMA PMMA + PBMA Resin l ** * By volume. ** By weight. *** Two determinations
ratios of methacrylate
Ratio of constituents 19.2 : 1.0 l 19.2 : 1.0 l * 37.0 : 1.0 ** -
m/z
compound
preparations
ratios
87/101
143/101
87/143
0.19 0.99 0.45 0.47 (0.08)
0.23 0.44 0.25 0.31 (0.11)
0.80 1.92 0.80 2.24 1.81 1.59 (0.31)
were made (standard
-
deviation).
protonated methacrylic acid (m/z 87) than in the individual polymer or monomer states. The qualitative analysis indicated that the only methacrylate yielding an m/z 87 in the resin was PBMA, and therefore the latter hypothesis was tested by preparing a solution of MMA and NBUMA. A 19.2: 1 proportion by volume of MMA and NBUMA, respectively, was required in order to produce an m/z 143/101 ratio of 0.23 which was similar to that of the resin (Table 1). This phenomenon is common in APCI gas phase chemistry. The apparent disparity in concentration vs. mass spectral abundance occurs when compounds in a mixture have significantly different proton affinities [20-231. However, the m/z 87/143 ratio of 0.8 was identical to that of the pure NBUMA monomer rather than the 1.59 average ratio found in the resin compound (Table 1). Since a simple mixture of the headspace-sampled MMA and NBUMA did not provide a similar m/z 87/101 ratio with that of the pyrolyzed resin, Py-APCI mass spectra were obtained with mixtures of the PMMA and PBMA polymers. Polymer mixtures with a greater than nineteen-fold by weight of PMMA to that of PBMA produced an m/z 87 of greater relative intensity than m/z 143 (Table 1). A mixture of 37.0 : 1.0 of PMMA : PBMA, respectively, provided similar m/z ratios (Table 1) with that of the resin. The Py-APCI mass spectrum (Fig. 2L) of the 37.0 : 1.0 polymer mixture of PMMA and PBMA, respectively, was very similar to that of the resin (Fig. 2K). As noted with other pyrolysis procedures [1,4], oxidative pyrolysis also produces significantly different amounts of subcomponents with respect to individual acrylic homopolymers and homopolymer mixtures as observed in Figs. 21 and L, respectively. A similar analysis was conducted for EA with respect to MMA except the indicators that were used for comparisons were the m/z 73 (Fig. SD) and m/z 101 (Fig. 4C) APCI-MS/MS mass spectra of the resin to that of a mixture of MMA and EA. A ratio of 25.0 : 1.0 of MMA to EA by volume, respectively, was necessary in order to produce m/z 73 (Fig. 5E)
140
and 101 (Fig. 4D) daughter ion mass spectra that were both similar to that of the respective resin daughter ion mass spectra. The quantitative analysis of the resin has shown that upon formation of the MMA and NBUMA monomers, complex pyrolysis interactions occur to the extent that the ratios of these monomers present in the resin cannot be determined simply by a mixture of monomers. This was expected due to the results of prior acrylate investigations [1,4] and homopolymer mixtures of PMMA and PBMA indicate that their relative ratio in the resin is approximately 37.0 : 1.0. This ratio was found to differ substantially from a simple monomer mixture. The isobaric nature of the MMA and EA monomers, on the other hand, provided the means with which to directly compare the relative ion intensities in their m/z 73 and 101 daughter ion mass spectra with that of the respective resin counterparts. This method allowed a simple liquid mixture of the monomers to yield a direct comparison of the MMA to EA monomer, respectively, in the resin.
CONCLUSIONS
Oxidative Py-APCI-MS/MS was shown to provide several advantages in the characterization of acrylic polymer compounds. Oxidative pyrolysis readily produced the key monomer building blocks of acrylate and methacrylate polymers as well as useful fragment information. More importantly, the daughter ion mode was shown to be particularly useful in the qualitative analysis of the methacrylate isomers, as well as in solid polymer and monomer mixtures or formulations. The experimental analysis, from the loading of a sample to the production of an averaged mass spectrum, can be performed routinely in a 5-7 minute time frame and the experimental protocol is straightforward in design. For the quantitative characterization of acrylic mixtures, one of the strategies was to compare superimposed patterns of daughter ions from components with the same MH+ ion from a mixture/preparation with that of the daughter ion mass spectral patterns of the pure target/suspect compounds. This approach together with the information from MS analyses was decidedly useful in obtaining the relative quantitation of the individual constituents in solid acrylic materials.
ACKNOWLEDGEMENTS
The author wishes to thank Linda G. Jarvis for the preparation and editing of the manuscript and Shirley A. Liebman for a critical evaluation of the manuscript.
141 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
J. Strassburger, G.M. Brauer, M. Tryon and A.F. Forziati, Anal. Chem., 32 (1960) 454. R.L. Gatrell and T.J. Mao, Anal. Chem., 37 (1965) 1294. H. McCormick, J. Chromatogr., 40 (1969) 1. E.C. Ferlauto, M.K. Lindemann, C.A. Lucchesi and D.R. Gaskill, J. Appl. Polym. Sci., 15 (1971) 445. H. Ohtani, T. Asai and S. Tsuge, Macromolecules, 18 (1985) 1148. M.W. Raynor, R.M. Waring and I.L. Davies, J. Chromatogr. Sci., 25 (1987) 104. J.J. Shen and E. Woo, LC.GC, Mag. Liq. Gas Chromatogr., 6 (1988) 1020. N. Gjos, K. Urdal, I.E. Ruyter and I.J. Sjovik, Anal. Chim. Acta, 149 (1983) 87. A. Homa, J. Taborsky and J. Churacek, J. Chromatogr., 360 (1986) 89. L.A. O’Neill and G. Christensen, J. Paint Technol., 47 (1975) 46. P.H. Dawson, J.B. French, J.A. Buckley, D.J. Douglas and D. Simmons, Org. Mass Spectrom., 171 (1982) 205. V.J. Caldecourt, D. Zakett and J.C. Tou, Int. J. Mass Spectrom. Ion Phys., 49 (1983) 233. S.A. Liebman, A.P. Snyder, J.H. Kremer, D.J. Reutter, M.A. Schroeder and R.A. Fifer, J. Anal. Appl. Pyrolysis, 12 (1987) 83. A.P. Snyder, J.H. Kremer, H.L.C. Meuzelaar and W. Windig, J. Anal. Appl. Pyrolysis, 13 (1988) 77. A.P. Snyder, J.H. Kremer, S.A. Liebman, M.A. Schroeder and R.A. Fifer, Org. Mass Spectrom., 24 (1989) 15. B. Anson, personal communication, Rohm and Haas Co., Philadelphia, PA, February, 1989. A.P. Snyder, J.H. Kremer, H.L.C. Meuzelaar, W. Windig and K. Taghizadeh, Anal. Chem., 59 (1987) 1945. A.P. Snyder, G.A. Eiceman and W. Windig, J. Anal. Appl. Pyrolysis, 13 (1988) 243. J. Johnson and R. Yost, Anal., Chem., 57 (1985) 758A. V.J. Vandiver, C.S. Leasure and G.A. Eiceman, Int. J. Mass Spectrom. Ion Proc., 66 (1985) 223. M.T. Bowers, Gas Phase Ion Chemistry, Academic Press, New York, 1979. J. Sunner, G. Nicol and P. Kebarle, Anal. Chem., 60 (1988) 1300. A.H. Lawrence, Anal. Chem., 61 (1989) 343.