Anal. Chem. 1980, 52, 1245-1248 (12) Mills, P. A. J. Assoc. Off. Anal. Chem. 1961, 4 4 , 171. (13) Chlorinated Dioxin Task Force, Michigan Division Dow Chemical U.S.A., "The Trace Chemistries of Fire-A Source for the Entry of Chlorinated Dioxins into the Environment". 1978. (14) Buser, H. R.; Bosshardt, H. P.; Rappe, C. Chemosphere 1978, 165. (15) Olie, K.; Vermeulen, P. L.; Hutzinger, 0. Chemosphere 1977, 455. (16) Rappe, C.; Buser, H. R.; Bosshardt, H. P. The 23rd Collaborative In-
1245
ternational Pesticide Analytical Council (CIPAC) Symposium, Baltimore, Md., June 1979.
RECEIVED for review September 4, 1979. Accepted February 28, 1980.
Pyrolysis Gas Chromatographic-Mass Spectrometric Identification of Polydimethylsiloxanes John C. Kleinert' and Charles J. Weschler" Bell Laboratories, Holmdel, New Jersey 07733
Pyrolysis of polydimethylslloxanes (980 O C , 1 s) yields a series of cyclic dimethylslloxanes that are separated and detected using gas chromatography-mass spectrometry. Trace amounts of polymer can be quantitatively analyzed by selectively monitoring the Isotopic cluster of ions at m / e 207, 208, and 209 present in the mass spectrum of the major pyrolysis product, hexamethylcyclotrisiloxane. Application of this technique to polydlmethylsiloxane determination in the range from to lo-'' g Is demonstrated. The amount of detected materials increases as the viscosity of the fluid increases, and this limits accuracy to about an order of magnitude. However, if the viscosity of the polydlmethylsiloxane is known, the technique permits analyses accurate to within 10%.
Approximately 235 million pounds of silicones were produced in 1978 ( I ) , almost one tenth the amount of nylon produced during the same year (2). Silicones are found in a vast array of products, including synthetic lubricants, paint resins, polishes, coolants, fuser oils, plastics, brake fluids, sealants, surfactants, and dielectrics. Polydimethylsiloxanes are easily the largest class of silicones produced. These materials have low surface tensions, about 17-22 dynes/cm, and spread or creep over all types of surfaces ( 3 ) . As a consequence, they are particularly troublesome contaminants that can destroy adhesion, make repainting or refinishing difficult, interfere with the wetting of solders, and even stress crack polyethylene. Creepage is of special concern to the electronics industry, where silicone contamination on operating electrical contacts can lead to equipment failures (4).An analytic capability to detect such contamination is obviously valuable. A further application of trace silicone analyses is suggested in a recent report by Pellenberg (5). Silicones are totally synthetic, ubiquitous, and possess excellent thermal and chemical stability. Given the above, silicones may serve as sensitive tracers for anthropogenic additions to the environment. This report describes the application of pyrolysis gas chromatography-mass spectrometry to the detection of polydimethylsiloxanes. Fluids ranging in viscosity from 20 to 30 000 centistokes have been examined. Pyrolysis products have been identified, and correlations have been made between Summer research student. Present address: Department of Chemical Engineering, Princeton University, Princeton, N.J. 08544. 0003-2700/80/0352-1245$01 .OO/O
the amounts of major products and the amount of starting material. As little as 0.1 ng of a given polydimethylsiloxane has been detected using the described techniques.
EXPERIMENTAL Equipment and Procedures. The pyrolysis studies were performed using a Chemical Data Systems "Pyroprobe 100" interfaced to a Hewlett-Packard 5992A gas chromatograph-mass spectrometer equipped with a single stage jet separator. The pyrolysis products were separated on a 1.22 m X 2.0 mm i.d. glass column packed with 1%SP-2250 on 10G120 mesh Chromosorb W-HP. The GC oven temperature was held at 50 "C for 1 min and then programmed to 220 "C at 8 "C per min. The injection port was held at 150 "C, the pyrolysis interface was held at 100 "C, and the helium carrier gas flow was maintained at 20 cm3/min. A platinum ribbon probe was used for the majority of the pyrolysis studies, but a few experiments were performed using a coil probe and a quartz sample tube. Pyrolyses were carried out with rise times of approximately 75 "C/ms for the ribbon element and 1 "C/ms for the coil element, an upper temperature limit of 980 "C (unless otherwise stated), and total pyrolysis times of 1 s for the ribbon probe and 5 s for the coil probe. Reported temperatures are corrected final temperatures. In a typical run, 5.0 pL of silicone-containing solution were spread evenly over the platinum ribbon. The ribbon was fired twice at 70 "C (10 s) to remove solvent and then sealed in the pyrolysis interface for a minimum of 15 min (for air to be purged) before initiating the run. Materials. Linear polydimethylsiloxanes ranging in viscosity from 20 to 30000 centistokes were obtained from a number of suppliers including Dow Corning, General Electric. Union Carbide, Harwick, and Petrarch Systems. Silicone solutions of known concentration were prepared using freshly distilled 1,2,2-trifluoro-1,2,2-trichloroethane(Freon 113). When using the coil probe, the samples were placed in quartz tubes 2 mm X 25 mm. These tubes were rinsed with distilled Freon 113 and air fired at 900 "C to ensure cleanliness before use. A similar procedure was used to clean the platinum ribbon (35 mm X 1.5 mm X 0.0127 mm) prior to each analysis with the ribbon probe. RESULTS AND DISCUSSION A series of exploratory pyrolyses were conducted with 30 000 centistoke polydimethylsiloxane to determine optimum operating conditions. With a fixed total pyrolysis time of 1 s, 5.0-pL samples of 0.04% polydimethylsiloxane solutions were pyrolyzed a t upper temperature limits of 700, 800,900, and 980 "C. The higher the final temperature, the more abundant the pyrolysis products detected, although the relative distribution of pyrolysis products remained essentially unchanged. With a fixed upper temperature of 980 "C, the total pyrolysis time was varied from 0.1 to 5 s. The abundance of detected products increased with increasing time intervals for t 2 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8. JULY 1980
1246 A
A
'I
R T . 3 2 MIN
-L
-I
50
100
150
200
250
300
350
400
450
500
200
250
300
350
400
450
500
'8"
R T = 2 9 MIN
Y VI
z
x
50
K Y
100
I
a 0
" I-
Y
+ P Y
150
"C
R T = 5 3 MIN
355
E
"
0 'I R T
I
I
I
I
I
0
5
10
15
20
8 2 MIN
I
, 429
"E "
TIME ( M I N I
R T a10 7 M I N
Figure 1. Pyrogram of 30 000 centistoke polydimethylsiloxane (980 o c , 1 s)
times u p to 1 s; longer total pyrolysis times produced no significant enhancement. As a rough indicator of the extent of pyrolysis, repeated runs were made on the same sample without removing the probe from the interface. At temperatures lower than 980 "C or a t time intervals less than 1 s, pyrolysis products could still be detected on repeat runs. However, a t 980 OC and 1 s, pyrolysis products could only be detected on the first run and repeat runs produced pyrograms with no major peaks. Consequently, the conditions chosen for subsequent analyses with the ribbon probe were an upper temperature limit of 980 OC and a total pyrolysis time of 1 S.
Figure 1 shows a typical pyrogram resulting from the pyrolysis of polydimethylsiloxane under the chosen conditions. There are five distinct peaks, and the mass spectrum associated with each is shown in Figure 2. Certain features observed in these mass spectra are worth noting. T h e ions at masses 207, 281, 355, 429, and 503 are likely formed by the loss of a methyl group from the parent ion. Characteristic fragments formed due to rupture of the Si-0 bonds have m / e 73, (CHJ3Si+, and m / e 147, (CH3)2Si+OSi(CH3)3 (6). The ions with m / e 267, 341, and 415 are likely bicyclic species, e.g., CH3
CH3
/
CH3
0
\
C" 3
formed in the electron impact induced decomposition of cyclic dimethylsiloxanes (7). The spectra for the species that elute at 1.2, 2.9, and 5.3 min match literature spectra (8, 9) for hexamethylcyclotrisiloxane,D,, octamethylcyclotetrailoxane, D4, and decamethylcyclopentasiloxane, D, (cyclodimethylsiloxanes are sometimes represented as D, where D =
503
Table I. Identities and Relative Distribution of Major Pyrolysis Products in Figure 1 peak
mol
!PI
C
compound name
100
222
136
B
36
296
171
C
13
370
205
D
8
444
236
E
6
518
265
hexamethylcyclotrisiloxane octamethylcyclotetrasiloxane decamethylcy clopentasiloxane dodecamethylcyclohexasiloxane tetradecamethylcy cloheptasiloxane
letter
amounta
A
wt
a Contribution of hexamethylcyclotrisiloxane arbitrarily assigned a value of 100.
(CH&SiO). Literature spectra could not be found for the species with retention times of 8.2 and 10.7 min, but the obvious pattern in the mass spectra (see Figure 2) and the systematic progression in retention times suggest that these species are D6and D7. The above observations are consistent with earlier reports (7, 9, 10) t h a t identify cyclic dimethylsiloxanes as thermal degradation products of polydimethylsiloxanes. Table I lists the relative amounts, molecular weights, and deduced identities for the cyclic species responsible for the lettered peaks in Figure 1. Initially, we hoped to relate the relative distribution of the major pyrolysis products to the viscosities of the polydimethylsiloxanes. However, for the range of viscosities studied in this report, 20 to 30000 centistokes, the product distribution remained fairly constant. Although this precludes a simple measure of viscosity, it does eliminate a variable in the analysis of dimethylsilicones. It is interesting to note that the product distribution shown in Table I is similar t o that observed by Thomas and Kendrick under strikingly different conditions (420 "C in vacuo for 5 h) (IO). Observing any of the cyclic dimethylsiloxanes, following pyrolysis of an unknown, should normally be sufficient to indicate the initial presence of a dimethylsilicone. For the
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
1247
Table 11. Reproducibility of Polydimethylsiloxane Determinations by Pyrolysis GC/MS
fluid viscosity 20 ctsksa 30 000 ctsks
peak area in ion pyrogram, mle 207.1 run no. 3 4
1
2
131 000
119 000
145 000
59 000
52 000
57 000
5
mean
125 000
132 000
58 000
47 000
130 400 (t9700) 55 000 (i 5000)
a
1.95 Qg of polydimethylsiloxane in each run.
38.9 ng of polydimethylsiloxane in each run.
’O’
E
iE l
