Anal. Chem. 1989, 6 1 , 1195-1201
1195
ARTICLES Thermogravimetric Analyzer/Atmospheric Pressure Chemical Ionization Tandem Triple Quadrupole Mass Spectrometer System for Evolved Gas Analysis R. Bruce Prime* IBM (H20/025),5600 Cottle Road, S u n Jose, California 95193 Bori S h u s h a n
S C I E X , Thornhill, Ontario, L 3 T lP2, Canada
Thermogravlmetry (TO) coupled wlth sequential mass spectrometry (MS/MS) offers structural ldentlflcatlon of compounds evolving durlng thermally stlmulated processes while maintaining temporal resolution between the two analytlcal procedures. MS/MS identlflcatlon of coevolvlng compounds, combined with the weight-loss data obtained from TG, furnlshes a means of dellneatlng complex thermotytlc pathways. The compatlblllty of TG (a predominantly atmospherlc pressure technlque) wlth atmospherlc pressure chemical ionization (APCI) permlts an Interface that Is easlly coupled and decoupled. The control of both Instruments and all data acquisltlon and analysis are performed by a common mlnlcomputer, and thls Integration permlts a “user friendly” Interface between the two techniques.
INTRODUCTION Thermogravimetry (TG) is one of the most widely used of the thermoanalytical techniques ( I ) . Its applications include compositional analysis, characterization of cure processes (e.g. solvent loss, deblocking of protecting groups, and evolution of condensation reaction products), characterization of pyrolytic processes (e.g. polymer-based ceramics and carboncarbon composites), decomposition, and the kinetics of various weight loss processes. Although TG itself is used extensively, the chemical information one obtains from the technique, Le. the exact mechanism of polymer degradation and other associated weight loss processes, is largely speculative. Other more rigorously analytical techniques must be coupled to thermogravimetry in order to furnish the chemical information needed to elucidate complex degradative pathways. One of the most universal, sensitive, and specific of chemical detectors is the mass spectrometer (MS). Pyrolysis and other thermolytic degradation techniques have been succesfully coupled to MS and used to identify and characterize polymeric materials and their formulations (2). It is, therefore, not surprising that this technique has enjoyed a protracted use as a chemical detector for TG (3-5). The coupling of a thermogravimetric analyzer, a predominantly atmospheric pressure technique, with the conventional mass spectrometer, a high vacuum technique, is naturally very challenging, and methods of coupling are about as numerous as users of TG/MS (6-9). Conventional mass spectrometry does pose some limitations
on the TG/MS analysis. For example, the conventional method of ionization, standard 70-eV electron impact (EI), is vulnerable to the presence of oxygen, since ion source filaments tend to oxidize rapidly and burn out. This obviates the use of oxidative atmospheres within the TGA for all but limited time periods making TG/EI-MS not well suited for studies of combustion and other thermooxidative processes or simulating polymer processing under air. Furthermore, E1 ionization causes extensive fragmentation of components entering the ion source from the TG producing an overlapping of fragment and molecular ion signals. This makes identification of individual compounds extremely difficult, especially when several complex compounds coevolve, e.g. during polymer degradation. The latter problem has been partially overcome in the past by trapping the TG effluent and subjecting that mixture to gas chromatographic/mass spectrometric (GC/MS) analysis. However, using such a procedure provides little or no temporal resolution of Components as they emerge from the TG apparatus and the technique is time-consuming and there are limitations imposed by the chromatography of the thermolysate components; that is, many polar or reactive compounds that are evolved during thermolysis are incompatible with gas chromatography and would not emerge from the GC column for MS analysis. One method of ionization that has circumvented some of the limitations of EI-MS has been the use of chemical ionization (CI) (10, 11). This method of ionization supplies to the mass spectrometer pseudomolecular ions indicative of the molecular weight of the neutral component. However, identification of a particular compound solely on the basis of molecular weight is highly speculative and, in addition, the problems of ion source filament lifetime with use of oxidative atmospheres in the TG system remain using conventional CI. One type of C1, referred to as atmospheric pressure chemical ionization (APCI), has, however, been used successfully for the ionization and subsequent MS analysis of chemicals evolved from the TGA even when air is used as the TG purge gas (8, 12). The application of APCI-MS to the analysis of volatile products from the Curiepoint pyrolysis of biopolymers (13) and nitramine explosives (14) have recently been described as well. Under APCI, trace compounds are converted to their pseudomolecular ions a t atmospheric pressure by well-characterized charge transfer reactions (15-18) causing little or no fragmentation of ions formed (see Instrument Design and
0003-2700/89/0361-1196$01.50/0@ 1989 American Chemical Society
1196
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11. JUNE 1, 1989
CAD Gas
UHP N.
CAD--/
/
.,J ("lranrparenf'quad)
CwogenicJ Vacuum Pump
Flgure 1. Schematic of me TAGA 6000 tandem triple quadrupole MS/MS system.
Thermogravimetric
Figure 2. Schematic representation of the Interface between the hrmcgravimetric analyzer (TG) and the SCIEX T A W Tandem Mass Spectrometer
!
The ion optics, "curtain gas" interface, and unique cryogenic pumping system of this instrument have heen designed to he fuUy compatible with an APCI ion source (see Figure 1) (19-22). The physical TGbmaas spectrometer interface is shown schematically in Figure 2. The APCI source of the SCIEX system permits the thermogravimetric analyzer to operate under a variety of atmospheric environments (e.g. air or nitrogen). The TG furnace tube is physically connected to an all-glass transfer line by a ball-and-socket joint permitting easy coupling and decoupling. After passing through a 2 cm section of 0.5 mm i.d. capillary, a portion (0.6100%)of the lMt200 mL/min TG effluent is m i e d into the APCI ion source by a 2-3 L/min stream of preheated nitrogen or zero air carrierlreagent gas. The amount of TG effluent allowed into the ion source is controlled to *50 sL/min by a micrometer valve connected in series with a device that measures the pressure drop across the capillary restriction (this pressure drop is directly proportional to the flow across the capillary). The relatively high gas velocities and the fact that the whole interface assembly is heated to -220 OC help to avoid condensation of less volatile materials on the interface walls. Chemicals entering the ion source from the transfer line are converted to their pseudomolecular or molecular ions via wellcharacterized APCI reactions in a current-regulated Corona discharge (16-22). In the positive ion mode,ionization is usually by proton transfer from hydrated protons, viz. M + H+4H20).
Flgure 3. TGlMSlMS system components
Operation below). The pseudomolecular ions thus formed are further analyzed and identified by using tandem mam spectrometry. This paper will demonstrate the many advantages of such an instrumental combination in the analysis and characterization of polymers and products of their cure and degradation. EXPERIMENTAL SECTION The thermogravimetric analyzer/mass spectrometer system used in these studies consists of an APCI ion souce coupled to a tandem triple quadrupole mass spectrometer (I+%?) (MS/MS) in order to identify compounds emanating from two different commercially available TG systems (23,24). The purpose of this paper is to deaerihe the above system in detail and to demonstrate its operational characteristicswith the TG/MS and TG/MS/MS analysis of three polymeric material% a trihlock polymer consisting of styrene-isoprenestyrene (SIS) units, cellulose acetate, and a phenolic resole resin. Instrument Design and Operation. The triple quadrupole mass spectrometer used in this study is the SCIEX TAGA 6000.
-
MH+.(H,O),
+ (n- rn)HZO
(1)
where M is the thermolysate component in the carrierlreagent gas. This ionization mechanism functions when either air or nitrogen is used as the APCI reagent gas. Those compounds whose proton affinities are greater than that of water are detected as MH' at concentrations as low as several parts per trillion (v/v) in the carrier gas stream. For compounds with high gas-phase acidities, negative ion mode APCI works best. Such chemicals are most often ionized by proton abstraction using Oz- reagent ions, viz. M+O;-[M-HI-
(2)
Thk mode of ionization functiom hy wing air as the APCI reagent gaa. Thus, compounds with gazphaee basicities greater than that of oxygen are detected as [M - HI-. Direct electron capture hy very electron affinate compounds can also occur. Another mode of ionization used in APCI is that of charge transfer where a charge transfer reagent is added to the reagent gas stream at partsper-million levels. Benzene is such a reagent and it ionizes any constituent with an ionization potential lower than that of itself, i.e. 9.25 eV. Thus only aromatic or highly conjugated compounds can he ionized to form molecular ions, viz.
M + CeHe'.
-
M*' + CeHe
(3)
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
i]V,!-=y-q Intensity
80
am 2
Time
1 E
T
(5)
lBBB
553
1658
1-
38a
58a
488
,9/
70 60
RELATIVE ABUNDANCE
("C)
'T
100'
Percent
A ,I
40 30 20
340
10
Percent
158
1
@
K
V
I
0
15
Time is) 0
588
1-
1658
588
1000
100
50
10,1
Time is) i6sa
Figure 4. Real-time data displays for TG/MS run of SIS triblock copolymer: (top left) log CI reagent ion intensity in counts/second (for m lz = 78' of benzene) versus time; (top right) percent weight versus temperature; (bottom left) percent weight versus time; (bottom right) temperature versus time.
200
150
250
300
'pi
901 80:
-1
37OOC 204
50
688
1197
70'
,
350
400
425O C
RELATIVE ABUNDANCE
401 30; 201 1 o!
207
I
I
Flgure 6. APCI benzene charge transfer mass spectra of SIS triblock copolymer: (top) at 370 OC,data normalized to m/z = 136' intensity; (bottom) at 425 OC, data normalized to m / z = 104' intensity.
Abundance Relative
:::I 4ooo
I
,
,~il, ll,!
_.,,
,
,
2000
55
(I,,
;[!,,
!I,
68
,
I,,
. (I_
,,j
107
0 0
10
20
30
40
50
60
70
80
11000 Relative Abundance
I
Relative Abundance Relative Abundance
93
MONOTERPENE
(FROM ISOPRENE)
'
.
STYRENE
90 100 110 120 130
4000 2000
27
39
77
'7'
136
M/Z
Figure 7. Identification of monoterpene from SIS degradation by MSlMS: (top) MS/MS CAD spectrum of 136' from SIS triblock thermodegradation; (bottom) 70-eV mass spectrum of a-pinene from EPAINIH spectral data base.
No matter what the method of APCI, ions at atmospheric pressure are quickly surrounded by water molecules, which are attached to the ion by dipole interactions and weak van der Waals forces. Such clustering could lead to very complex mass spectra if the clustered ions were left intact. One property of the present APCI ion source design is its ability to dehydrate these clustered ions by two processes: passing the clustered ions through a nitrogen gas "curtain", followed by subjecting the clustered ions to accelerating electric fields during the free-jet expansion of gases into the high-vacuum analyzer. The latter process requires a special "cluster-breaker" lens, which is a standard feature of this APCI ion source design. Besides desolvating the cluster ions, the nitrogen gas curtain prohibits particulate matter and un-ionized material from going near the sampling orifice or into the vacuum chamber/analyzer. The combination of these two declustering processes renders ions formed by APCI into simple pseudomolecular ion species, which are then focused into the high vacuum analyzer portion for MS or MS/MS analysis. By use of tandem mass spectrometry (MS/MS) and collisionally activated decomposition (CAD), these pseudomolecular ions can be further structurally analyzed (25-27). Thus, ions of a particular massto-charge ratio are selected by the first quadrupole (Ql),collided
with a neutral target gas such as argon in Q2 (a process referred to as CAD), producing "daughter" ions that are indicative of the precursor ion's structure. Scanning Q3 generates daughter ion spectra that serve as chemical fingerprints used to identify these ions and the corresponding neutrals entering the ion source. This process is especially useful at distinguishing between ions of the same mass but different structure, which can coevolve during complex pyrolysis reactions. In order to bring the TG operation and data acquisition fully under the control of the mass spectrometer's computer, a slave microprocessor and electrical interface were employed ("TG Interface" and "Slave Microprocessor" in the block diagram of Figure 3). The TG interface is comprised of a power supply, signal handling, and 16-bit digital to analog converter (DAC) while the slave microprocessor is comprised of a 280 CPU with 64K RAM, two 14-bit analog to digital converters (ADC), and one 16-bit DAC channels. Together these two components are called a single module serial controller or SMSC (Omnitherm Corp., Chicago IL). In order to drive the SMSC by the mass spectrometer's DEC PDP 11 minicomputer, a CPM compatible program must first be downloaded to the slave microprocessor via an RS-232 serial port by using a nonstandard protocol at 9600 baud. The user then
I198
ANALYTICAL CHEMISTRY, VOL. 61. NO. 11, JUNE 1. 1989
Relalive
Abundance
Relative
4 1
Isi
RELATIVE AMOUNT
4 MIZ
m/z
Figure 8. Average of APCIMS spectra obtained between 360 and 410 "C during the Umrmodegradation of cellulose acetate (maximum weight loss): (top)display betmeen 15 and 300 a m ; (bottom) display between 150 and 300 amu. Percents refer to degree of acetylation of indicated bns, see text. (All data normalized to m l z = 6 t + intensity.)
Fgure 9. (Top) Derlvattve of weight loss during thermogravlmetric analysis of celiulose acetate at 20 'Clmin in air In Du Pont 951 mermogaVmebic ana@er, wiih recarsbucted ion mermOqam for me 187+ Ion. (Bottom) MSlMS daughter ion spectrum for the 187+ ion. The str&es shorn above the onresponding peaks in the spectrum and the neutral losses demonstrate the elucidative powers of CADMSIMS spectra. Data normalized to m l r = 43+ intensity.
sends the TG heating pmgram, consisting of up to ten individual isothermal/ramp methods in the form of a Method Table to the slave microprocessor. Upon reaching the "START SAMPLING TEMPERATURE", MS and TG data are recorded onto the PDP-11's 14-MByte storage disks. The frequency of TG weight and temperature data point acquisition is specified by the user in 0.5s intervals. Typically TG data are stored every 3-6 s (ca one data point per de), and, since compounds tend to evolve from the thermogravimetric analyzer over relatively long time periods, MS scans were performed only every -@/, s, taking ahout 1s to scan the l(t400 amu scan range.
real-time display of up to four graphs comprised of any two user-selected TG or MS parameters as abscissa and ordinate. Examples of real-time data displays for the TG/MS analysis of SIS are provided in Figure 4. After aquisition, TG or maas spectral data can be displayed on the mass spectrometer's data system. In addition to the real-time display of TG data discussed above and shown in Figure 4, comparable postacquisition data displays as well as the derivative of weight loss versus time or temperature can be plotted (see Figure 5). The thermodegradation of SIS proceeds through a two-step mechanism that is not readily apparent from the plot of weight percent vs temperature ( F i e 4, bottom left). From the fmt derivative of the weight loss (Figure 5, top) the bimodal degradation can be observed by a maximum occurring a t -370 "C and a poorly resolved maximum a t -425 "C. If the background subtracted AF'CI-MS spectra are plotted for these two degradative steps, drastically different chemical phenomena are observed a t the two temperatures a8 illustrated in Figure 6. The 370 "C spectrum shows products of the degradation of polyisoprene. These products were identified by MS/MS as described below and include monomeric isoprene (m/z = 68) as well as higher order isoprene oligomers (e.g. monoterpenes, 136+;sesquiterpenes, 204+; and diterpenes, 272+). The 425 "C spectrum is considerably simpler, showing the presence of only two major components identified hy MS/MS as styrene (l04+) and a styrene dimer a t 207+. The MS data can he displayed as in Figure 6, i.e. as spectra, or the ion intensity for any individual ion(s) can be extracted from the data file and plotted against time or temperature. Such reconstructed ion "thermograms" mshown in Figure 5, bottom for all ions (total ion thermogram), and the 136+ and 104+ ions. This type of display serves to clearly differentiate the two degradative processes that occur between 350 and 450 "C. The first is a degradation of the isoprene block into mono- and oligomeric units (as depicted by the maximum in the 136' trace) and the second is the unzipping of the remaining polystyrene into almost exclusively monomeric units. In order to confirm the structure of these molecular ions, MS/MS spedra were obtained on the 10-12 major ions during
RESULTS AND DISCUSSION Styrene-IsopreneStyrene (SIS) Triblock Copolymer. The first example is Shell Kraton SIS triblock polymer. A 3.7-mg sample was weighed into the Du Pont 951 TGA and heated under a nitrogen purge of 100 mL/min from 200 to 600 "C a t 15 'C/min heating rate. Approximately 1%of the TG effluent was carried into the ion source by a 3 L/min stream of zero air containing a few parts per million of benzene charge transfer reagent. Within the ion source, ionization of molecular species evolving from the TG takes place in a corona discharge by 9.25-eV benzene charge transfer-a very soft ionization proceas producing primarily molecular ions. These ions are then focused through the gas curtain and sampling orifice and into the high vacuum analyzer portion for MS or MS/MS analysis. In order to ensure the optimum performance of the TG system and mass spectrometer during a TG/MS or TG/ MS/MS run, certain key parameters must be monitored and displayed in real time. For example, the reagent ion (benzene, 78' in this case) intensity versus time is usually displayed to ensure proper ion 8ource and m w spectrometer performance. In such a display there is often a depletion of ionizing reagent evident, which is coincident with the evolution of material from the themogravimetricanalyzer due to ionization of the thermolysate. Other real time displays that are possible are concerned typically with TG parameters, e.g., weight percent, time, and temperature. The graphics package provided by the MS/MS data system is flexible enough to accomplish the
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
Table I. Some Products of the Thermal Degradation of Cellulose Acetate Analyzed by TG/MS/MS
ION
STRUCTURE
I
lo0l 7
NAME
61'
CH3C0, H
ACETIC ACID
103'
CH 3COOCOCH 3
8
ACETIC ANHYDRIDE
109'
1199
BENZOQUINONE
401
I
\
U
127'
169'
DIHYDROXYCYCLOHEXADIENONE
Qbai OH
187'
100
200
300
ACETOXYHYDROXYCYCLOHEXADIENONE
ACETOXYDIHYDROXYCYCLOHEXENONE
229'
analogous to above
DIACETOXYHYDROXYCYCLOHEXENONE
247'
analogous to above
)IACETOXYDI HY DROXYCYCLOHEXANONE
400
600°C
500
Thermogravimetric analysis of phenolic resole resin, showing weight losses due to cure (30-300 "C) and decomposition (400-650 "C), 10 'C/min in air.
Flgure 10.
117
Relative Abundance
i
45
I
61
Negative Ions
thermogravimetry (TG/MS/MS). The MS/MS spectra were obtained during the TG run by sequentially and repetitively allowing ions of preselected mass-to-charge ( m / z ) ratios through the first quadrupole mass filter. In the second quadrupole these preselected ions were collided with a neutral target gas (argon) at a gas density of -300 x 10l2atoms/cm2. The fragmentation spectrum obtained by scanning the last quadrupole can be compared to similarly obtained spectra of standards contained in the system's MS/MS library or compared directly to standard spectra obtained by EI-MS appearing in conventional mass spectral libraries. Comparison of CAD-MS/MS spectra to EI-MS library spectra often results in a good match especially for CAD-MS/MS of odd-electron molecular ions. For example, a CAD-MS/MS to EI-MS comparison was performed to confirm the structure of the 136' ion as that of a monoterpene. Figure 7 shows the CADMS/MS spectrum of 136' from the TG/MS/MS experiment, while the mass spectrum below it is the 70-eV E1 spectrum of a-pinene as obtained from the EPA/NIH Mass Spectral Data Base. Although the spectra are not identical, the fragmentation patterns are similar enough to infer that the 136+ ion from the thermodegradation of isoprene has a predominantly monoterpene structure. In addition the above CADMS/MS spectrum was observed to be almost identical with that of limonene in the system's MS/MS library (not shown). Cellulose Acetate. To illustrate the comparable temporal response characteristics between the thermogravimetric analyzer and mass spectrometer, the structural information contained in an MS/MS spectra, and the ability of the TG/APCI-MS technique to furnish microstructural information, TG/MS and TG/MS/MS runs on cellulose acetate were performed. For TG/MS, a 12.4-mg sample of cellulose acetate was heated in a Du Pont 951 TGA from 100 to 600 O C at 20 OC/min heating rate. The TG apparatus was purged with air at 200 mL/min and 1.3% of the TG effluent was
OH
41, allyl
0 /
135
91 IM-HZOl HC
/ 147 IM-HZOI H'
107, (FV1-HZO)H'
/
CH20H
187 (M-HzOI Hi
Flgure 11. Positive and negative ion mass spectra of gases evolved
during cure of phenolic resole resin. Structures shown were determined from MS/MS daughter ion spectra.
transferred to the mass spectrometer ion source. The APCI reagent in this experiment was the hydrated protons formed in the corona discharge of the zero air carrier gas. Pyrolysate evolving from the thermogravimetric analyzer was, therefore, ionized by proton transfer to form the protonated molecular ions MH'. Cellulose acetate exhibited maximum weight loss due to thermal degradation between 360 and 410 "C (28) and the average of mass spectra recorded between these temperatures is shown in Figure 8. The complexity of this spectrum underscores one of the limitations of conventional TG/MS. Since each peak corresponds to the protonated molecular ion of a neutral evolved during thermolysis, only molecular weight
1200
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
ec;;$;A ;
l l q
//---ks dwldT
Relative 95 MH'
300
330
360
390
420 450
480
510
540
570
600
630
Temperature ("Cl
110 100 90
1
80
CHJOH MH'
Relative Abundance
50
Relative Abundance
1
100
200 Temperature
300
400
-
50 40 -
I
I
70 60
30 20
("C)
7
Total Positive
-
Figure 12. Total and reconstructed ion thermograms of phenolic resin during cure demonstratlng presence of unreacted staring material (e.g. phenol), volatile resin components (e.g. monomethylol allyl phenyl ether), and early decomposltion products (e.g. methanol).
information is available. Structural assignment solely on the basis of molecular weight, however, can be a tenuous proposition. In order to unambiguously identify a component without forgoing direct T G m a s s spectral integrity, MS/MS techniques must be employed. Figure 9 (top) shows the derivative TG trace ("DTG") and the reconstructed-ion thermogram (RIT) for the 187' ion, demonstrating the correlation of temporal response between the thermogravimetric analyzer (weight loss represented as DTG) and the mass spectrometer (ionization). Below the reconstructed-ion thermogram is the MS/MS daughter ion spectra of the 187' ion obtained during the evolution of this compound from the TG apparatus. As noted above many such daughter ion spectra can be obtained sequentially and repetitively during a single TG run providing complete structural information of the evolved compounds. A summary of compound identification from the TG/MS/MS analysis of cellulose acetate is provided in Table I. (It is instructive to note that an earlier TG/MS study of cellulose acetate using conventional CI had incorrectly identified the 103' ion as protonated valeric acid on the basis of molecular weight data (IO). The present MS/MS analysis has demonstrated this ion to be due to acetic anhydride instead.) Based on these results a scheme for the thermolytic breakdown of cellulose acetate was proposed (28) which proceeds through the reactive intermediate of structure I.
Relative Abundance
40 30
a0 10 0
10
40 60 80 100 120 140160180 200 2 2 0 2 4 0 2 6 0 2 8 0 3 0 0 m/z
Figure 13. (Top) Derivative of the weight loss during thermooxidative decomposition of phenolic resin. (Middle) Total ion thermogram. (Bottom) Mass spectra of positive ions formed between 450 and 550 OC.
ions obtained from the APCI mass spectrum in Figure 8 and applying eq 1, where I,, values are ion intensities of m / z = n,it should be possible to calculate the degree of acetylation. The calculated degree of acetylation was 55.7 f 1.2%. This value compared favorably with the value of 55.0 f 0.2% measured by the supplier (Celanese Canada Ltd., Cookesville, Ontario). % acetylation = (120s
x 33.3) + (1247 x 66.7) 4-(I289 x 100) (1) I163 + I205 + I247 + I289
Phenolic Resole Resin. This example illustrates the use of the Perkin-Elmer TGS-2 thermogravimetric analyzer interfaced to the APCI tandem mass spectrometer, the examination of weight losses during cure in addition to degradation, and operation of the mass spectrometer in both the positive and negative ion modes. The phenolic resin examined in this study is Methylon 75108, manufactured by the General Electric Co. It is a mixture of mono-, di-, and trimethylol allyl phenyl ethers and their oligomers and byproducts (29). The objectives of this study were characterization of the resin itself, characterization of its cure combining weight loss and evolved gas analysis data, and insight into the cross-linked structure from the nature of the decomposition products. TG/MS and TG/MS/MS analyses were carried out from 30 to 650 OC at 10 OC/min heating rate. The TG instrument was purged with zero air or dry,high-purity nitrogen at 100 mL/min, and 1-5% of the TG effluent was transferred to the mass spectrometer ion source. The APCI reagent was hydrated protons in the positive ion mode and 02-in the negative ion mode, both formed in the corona discharge of the zero air carrier gas. As can be seen in Figure 10 weight losses due to cure occur
,RohoH I (R = -H, -COCH,)
If structure I is indeed the intermediate, then it should be possible to ascertain the degree of acetylation of the cellulose substrate by measuring the relative intensities of the various acetylated analogues of structure I. For example, 100% acetylated cellulose would result in all the ion intensity of I to be at m / z = 289+ (R = -COCH3}, i.e. each sugar monomer unit in the cellulose polymer would have three acetyl groups attached to it. Similarly, the ions a t m / z = 247' (R = 2(COCH3), 1HI, 205' (R = l(-COCH3), 2H},and 163+ (R = H} correspond microscopically to 66.7%, 33.3%, and 0% acetylation, respectively, if we ignore the minor contribution from terminal monomer units. Using the intensities of these four
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
between room temperature and 300 "C, while decomposition occurs between 400 and 650 "C. Figure 11 shows the positive and negative ion mass spectra from gases evolved during cure and the structure of several resin components as determined from their MS/MS daughter ion spectra. It should be noted here that the benzyl alcohols all undergo dehydration after protonation in the positive mode, to form stable ions with a tropylium structure, viz.
I1
TG measures a broad 30-40% weight loss during cure occurring between 80 and 300 "C. The total- and reconstructed-ion thermograms in Figure 12 demonstrate three distinct regions of evolution during cure: volatile starting materials (room temperature to 150 "C), evolved gases associated with the major portion of cure (10C-250 "C), and those associated with the latter portion of cure and/or the first stages of decomposition (200-300 "C).By comparison of the weight loss and ion evolution data it can be concluded that the volatile starting materials, e.g. phenol (95+),and products of late cure/early decomposition, e.g. methanol (33+),comprise only a small portion of the weight loss during cure. In addition, loss of the allyl group was observed in the 100-250 "C temperature region (29). It is speculated that allyl functions as a thermally labile protecting group and that its loss plays a key role in cure. This interpretation is supported by the patent literature (30). Figure 13 compares the derivative of the weight loss with the total ion thermogram during decomposition and shows the average mass spectra of gases evolved between 450 and 550 O C . Decomposition can be seen to occur in three steps, where step 2 produces the most material ionizable in the positive mode. From the DTG trace it is readily observed that the weight loss of step 2, -40% of the cured polymer, is very sharp. The total ion thermogram closely matches the DTG trace, demonstrating that the explosive nature of this step is real and not simply due to a solid piece of material being ejected from the TG sample pan (in fact, the total-ion thermogram demonstrates better temporal resolution than does the DTG curve since some electronic damping is used in acquiring weight loss data). The major ions associated with step 2 have been identified by APCI-MS/MS as phenol (95+) and substituted phenols (log+,123+, 137+, ...). As a further confirmation, these phenols were also detected in the negative mode as [M - HI-. Step 1 accounts for -10% of the cured polymer weight. The broad weight loss beginning close to 350 "C is mostly C2H40(detected as C2H,0+),most likely formed from the decomposition of the methylene-ether bridge. No ions were detected for step 3 in the positive mode, and only carbon dioxide was detected in the negative mode (as the C02.0- adduct). This weight loss step, which accounts for -50% of the cured polymer, is interpreted as pyrolysis of the char formed in decomposition step 2. The compounds detected by APCI-MS are consistent with a predominantly cross-linked structure which is the condensation product of 2,4,6-trimethylol phenol containing both methylene and methylene-ether cross-links. No major differences were observed between air and nitrogen pyrolyses except that in the latter, no C02 is produced.
CONCLUSIONS This paper has illustrated the use of a dual-processor minicomputer based TG/MS/MS system employing APCI
1201
ion source technology. This instrumental combination has exhibited the following features: excellent temporal correspondence between TG and mass spectral data; ability to operate in a variety of oxidizing as well as inert atmospheres; ability to identify and resolve complex products, including isomers, emanating from the TG apparatus; compatibility with a variety of TG instruments; MS/MS library search and identification of unknowns evolved from the TG system. Coupling of APCI-MS/MS to TG vastly increases the amount of chemical information one obtains from either method alone. The rapid identification of coevolving compounds by MS/MS, combined with weight loss data from TG, provides a means of delineating complex thermally activated processes such as decomposition and cure of polymers. The combination of atmospheric pyrolysis with APCI-MS/MS provides an ideal method for assessing the potential hazards of the oxidative degradation of polymer systems used, for example, in building materials or interior furnishings of homes, offices, aircraft, and such. This type of dynamic analysis stands to make important contributions to the areas of polymer science, materials characterization, and combustion toxicology. Registry No. (S) (I) (block copolymer),105729-79-1;Methylon 75108, 9074-30-0; cellulose acetate, 9004-35-7.
LITERATURE CITED
(27) (28) (29) (30)
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RECEIVED for review May 31, 1988. Resubmitted January 24, 1989. Accepted March 8, 1989.
