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Figure 8 shows a similar reconstructed pyrogram for Nylon 6. Although cyclopentanone can be detected in the pyrogram of Nylon 6 using the limited mass search, the great difference in the relative amounts in the two pyrograms can be used to easily differentiate the two polymers.
LITERATURE CITED Cole, H. M.; Peterson, D. L.; Sijake, V. A,; Smith, D. S. Rubber Chem. Techno/. 1966, 39(2), 259. Hu, J. Chih-An. Anal. Chem. 1977, 49, 537. Vukovic, R.; Snjatovic, V. J. folym. Sci., Part A-7, 1970, 8, 139. Iglauer. N.; Bentley, F. J. Chromatogr. Sci. 1974, 12. 23. Williams, R. L. Anal. Chem. 1973, 45, 1076A. Eustache, H.; Robin, N.; Daniel, J. C.; Carrega, M. Europ. folym. J. 1978, 14, 239; Chem. Abstr. 1978, 89, 180458. Hrabak, F.; Mitera, J.; Kubelka, V.; Bezdek. M. Europ. folym. J. 1978, 14, 219; Chem. Abstr. 1978, 89, 180564. Hughes, J. C.; Wheals, 8. B.; Whitehouse, M. J. Forensic Sci. 1977, 10, 217. Pattacini, S. C. "The Identification of Cured Rubber Compounds Using Infrared Spectroscopy", ferkin-Elmer Bull. Oct 1975, No. 52. Happ, G. P.; Maier, D. P. Anal. Chem. 1964, 36, 1678. Futrell, J. H. "Pyrolysis Einhorn, I. N.; Chatfield, D. A.; Mickelson, R. W.; and Combustion Products of Nomex, Durette and Tedlar Polymers"; Flammability Research Center: University of Utah, 1974: FRC/UU-41, UTEC 75-022.
GUM Pyrolysis of a chewing gum sample produced a very characteristic pyrogram shown in Figure 9. T h e four most intense peaks according to mass spectra were shown to be butene, acetic acid, limonene, and cinnamaldehyde.
CONCLUSION PGCMSDS analysis provides unequivocal mass spectral information for the identification and characterization of a variety of intractable organic solids. An advantage over pyrolysis mass spectrometry is that pure spectra are obtained. Emphasis on mass spectra rather than retention time alone provides for a more positive identification of the parent materials.
RECEIVED for review February 6, 1979. Accepted March 27, 1979.
Determination of Conjugated Polyenes in Solid Poly(viny1 c hIor ide) by Selective Photooxidation P. Kohn, C.
Marechal, and J. Verdu"
Department Materiaux, ENSAM, 75640 Paris Cedex 13, France
The irradiation of a poiy(viny1 chloride) film in the near-uitravioiet range leads to the selective destruction of the preexisting conjugated polyenic sequences by oxidative processes. The evolution of the transmittance spectrum during the exposure can be used to determine the initial concentration of these sequences at the moi/L level.
Despite the high absorptivity of polyconjugated sequences, -(CH=CH),with n 2 4, in the electronic spectrum, low concentrations of these structures in poly(viny1 chloride) (PVC) are difficult t o determine by absorption spectrophotometry for two reasons: (1) It was difficult t o find solvents able to dissolve a large range of samples differing in their molecular weight or their tacticity and which do not give any interaction with the PVC macromolecules (highly polar solvents such as hexamethylphosphoramide) or with the proper conjugated sequences (forming charge-transfer complexes such as tetrahydrofuran (1)).
(2) I n solid state (films), the turbidity of the samples, depending on their morphological state, can mask weak absorption bands. T h e determination of these structures is of practical interest related to the problems of initial coloration and thermal or photochemical stability of commercial resins. A method based on the resonance Raman effect has recently been developed for the determination of large polyenes which absorb in the visible range (2, 3 ) . This method is very sensitive, but at present its extension to t h e shorter polyenes which absorb in near-UV (n = 4-7) presents some difficulties. The aim of this paper is to propose an inexpensive method based on the high reactivity of polyenes toward photo0003-2700/79/0351-1000$01 .OO/O
oxidation. Their disappearance in cast films exposed to near-UV irradiation was followed by UV-visible spectrophotometry.
EXPERIMENTAL Many samples, differing by their polymerization mode, are used in this study. @-Caroteneand thermal degradation experiments are performed with a "bulk" PVC having an average molecular weight: MN = 39 000. The films of thickness 50 to 200 pm are cast from 5 g/L tetrahydrofuran (THF) solution. The residual solvent and its antioxidant are extracted in soxhlet by the diethyl ether. The solvents are Baker BAR grade. The @-carotene (Serlabo used without purification) is introduced in the T H F solutions. The partially thermodegraded sample is obtained by extrusion of the polymer in the presence of dibutyltin dilaurate (2.5% in weight) at 150 "C in a Brabender extrusiograph. The polymer is recovered from the extrudate by THF-methanol reprecipitation. The photooxidation is performed in a reactor equipped with a fluorescent lamp-Osram L 40 W 70 emitting from 300 to 450 nm, a continuous, gauss-shaped spectrum, which peaked at 365 nm. The short-wavelength part is filtered off by a glass plate (cutoff at 310 nm, 50% transmittance at 340 nm). The evolution of the film transmittance during the UV exposure is followed with a Varian 635 D spectrophotometer. All measurements are made at 23 f 1 "C. The difference between the final (asymptotic) and the initial absorbance at a given wavelength is characteristic of the concentration of the polyene which absorbs at this wavelength. The absorptivities found in the literature ( 4 ) are used for the estimation of the concentrations. RESULTS (1) Study of @-CaroteneDoped Films. We studied PVC films containing, respectively, 0.5, 1, and 2% by weight of @-carotene. During the UV exposure, we followed the evo0 1979
American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
1001
I
0.10
10
1000
100
rnl"
Figure 1. Evolution of the 450-nm absorbance during the photooxidation for three @-carotenedoped samples. The film number is the same as in Table I
Table I. Application of the Method in P-Carotene Doped Films sample no. carotene concn, % WIW thickness, fim initial transmittance at 4 5 0 nm, 7% total variation' for absorbanceb
1 0.5
2 0.5
3 1.0
4 5 1.0 2 . 0
6 2.0
129 1 2 6 128 129 130 24.3 25.4 7 . 9 7.1 0 . 6
129 0.7
3 . 3 5 3.27 6 . 3 4 6 . 5 3 1 2 . 9 6 12.47
a Taken after 2 8 2 0 min of exposure. the 2 . 3 - g m peak absorbance.
Normalized by
lution of the 450-nm band absorbance (Figure 1). Each experiment was performed twice to verify the reproducibility. T h e results are summarized in Table I. T h e absorbance decreases effectively during the exposure, showing the polyene destruction, T h e asymptotic value, attained after approximately 1 day of exposure, corresponds closely to the absorbance of a n undegraded film (0.053), showing that no secondary HC zip elimination interferes with the carotene destruction. T h e quantitative results (Table I) show a satisfactory linearity and reproducibility. (2) Example of Tetraene Determination in Two Suspension Polymers. T h e two samples differ only by the fact t h a t one, SO, is polymerized in the presence of oxygen (440 ppm), whereas for S, the polymerization medium has been carefully deoxygenated. T h e tetraenes, -(CH=CH)*- (A, 312 nm, tmax 73 000 L mol-' cm-' ( 4 ) ) ,are determined from the extinction coefficient of the films (AAIAth), using many samples of thickness (th) between 100 and 200 l m , and from the selective photooxidation experiment. T h e evolution of the 312-nm absorbance during t h e UV exposure is shown in Figure 2. As in the previous experiment, the horizontal asymptote confirms the selectivity of the photochemical process: the existing polyenes are destroyed but the irradiation does not create new conjugated sequences by the HC zip elimination process. T h e results are summarized in Table 11. (3) Example of the Characterization of the Polyene Length Distribution in a PVC Extrudate. Figure 3 shows
'
20 LO 60 80 min Figure 2. Evolution of the 312-nm absorbance during the photooxidation 0
for two suspension polymers Tr.
i501 I
0
50
3
7 3 0 1
I
I
25-/
01
1
200
300
1
LOO
500 n m
X
Figure 3. Evolution of the transmittance spectrum of a thermodegraded PVC film during the photooxidation. The numbers in the figure indicate the duration of the UV exposure in hours
Table 11. Tetraene Determination in Two Suspension Polymers by Means of (a) Film Extinction Coefficient and (b)Selective Photooxidationa a b samE, E, ple cm-' C, mol/L cm-' C , mol/L s
SO
68 X 92 X
5 6.7
1.5 3.5
21 X 47 X
' E is the extinction coefficient AA/Ath, where A = absorbance, th = thickness in cm, and C is the concentration. Table 111. Polyene Length Distribution in a PVC Extrudate Emax,
L
Amax
n
mol-! cm-'
312 329 345 371 395 421 445 467 485
4 5 6 7 8 9 10 11 12
73x 121 x 1 3 8 1 7 4 204 x 2 3 3 261 x 2 9 2 3 2 0
1 0 3
103 ~ 103 ~103
io3
~ 103 103 ~ io3 ~103
C , Gmol/L 120 80
60 50 45 35 28 20 13
the bleaching of a 45-lm film during the UV exposure. T h e destruction of the preexisting polyenes is achieved after
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
approximately 1 day of exposure. The location of the peaks in the initial spectrum corresponds closely to the literature data ( 4 ) : eight peaks between 300 and 500 nm for the eight conjugated polyenes -(CH=CH),- from n = 4 to 12. The results are summarized in Table 111.
DISCUSSION Our method is essentially based on two assumptions: (1)The absorbance decrease during photooxidation is due to the disappearance of preexisting chromophores. This assumption is verified in the case of concentrations as low as 1 ppm (polyene unit per monomer unit)-the change in chemical structure is then too small to induce a significant change in sample morphology (and therefore in light diffusion) or in refractive index (and therefore in reflection losses). Only oxidizable pollutants, such as polyaromatics, can interfere in the dosage, but they can easily be detected by their strong absorption band in the medium UV or by fluorescence. Stable impurities such as colorant or pigment traces do not interfere, giving a relatively major advantage to the Raman resonance method. (2) The photooxidation is selective, i.e., the destruction of the conjugated sequences is favored relative to the HC1 zip photoelimination. As previously shown, this latter process requires a radiation of wavelength shorter than 330-340 nm ( 5 , 6). In our case, due to the active part of the lamp spectrum being filtered off by a glass plate, new conjugated sequences cannot appear a t a significant rate, so that the asymptotic value of the absorbance corresponds effectively to the total disappearance of the polyenes in the sample, and can therefore be used for their quantitative determination. The relative advantage of this method to the direct determination in films using their extinction coefficient lies essentially in the fact t h a t physical factors such as light diffusion, which depend strongly on the film morphology, and which are difficult to appreciate quantitatively, are not taken into account in the
measurement by selective photooxidation. For instance, in the tetraene dosage in two suspension polymers, the difference between the concentration values obtained from the film extinction coefficient and from the photooxidation experiment, i.e., 46 f 1 pmol/L for the two samples, can be attributed to the light diffusion by the films. The overestimation of the concentration caused by the use of the extinction coefficient is therefore very important (100 to 200% in our case); thus, the film turbidity, although low, cannot be neglected a t least in the determination of small polyene concentrations (lower than mol/L). With a routine spectrophotometer, it is possible to easily determine tetraene concentrations of mol/L, corresponding to approximately 4 units per IO7 monomer units, i.e., less than 1 unit per 2000 chains for the majority of the commercial polymers. The method is limited to tetraenes or larger polyenes. For the dienes and trienes, the accumulation of stable oxidation products such as a,P-unsaturated carbonyls or dienones, resulting from the oxidation of larger polyenes, can compensate a t least partially for the absorbance decrease due to the short sequence destruction. This is clearly shown in Figure 3 in which the absorption increases between 200 and 250 nm (dienes), whereas it decreases in the other parts of the spectrum.
LITERATURE CITED (1) R . Schlimper, flaste Kautsch., 13, 196 (1966). (2) G. Pritscher and W. Holtrup, Angew. Makroml. Chem., 47, 11 1 (1975). (3) C. Bassez, Thesis, Lille, France, 1978. (4) F. Sondheimer, D. A. Ben Efrain, and R. Wolovsky, J . Am. Chem. Soc., 83, 1675 (1961). (5) M. A. Golub and J. A. Parker, Makromol. Chem., 85, 6 (1965). (6) R. F. Reinisch. H. R. Gloria, and G. M. Androes, in "Photochemistry of Macromolecules", R. F. Reinisch, Ed., Plenum Press, New York, 1970, p 185.
RECEIVED for review October 30, 1978. Accepted February 12, 1979.
Ozone Interference in the Determination of Nitrogen Dioxide by a Modified Manual Saltzman Method Eduard H. Adema' Central Laboratory, DSM, Geleen, The Netherlands
To determine whether ozone interference occurs in a modified manual Saltzman nitrogen dioxide procedure, several experiments were performed using dynamic mixtures of nitrogen dioxide and ozone. Two types of sample bottle were used in the examination. I n one of them no ozone interference was observed. Apparently the ozone was destroyed before it reached the Saitrman reagent. With the sample bottle of the second type, a negative interference was observed caused by the reaction of NO2and O3in ail probabillty in the aqueous solution of the reagent. However, a positive ozone interference could be observed if the Saltzman reagent contained impurities. No reaction of ozone with the Saitzman reagent or the azo dye agent occurred. The negative interference dependent on the ratio of ozone to nitrogen oxide as found by Baumgardner could not be confirmed. Present address, A g r i c u l t u r a l U n i v e r s i t y , D e p a r t m e n t of E n v i r o n m e n t a l Sciences, S e c t i o n Air P o l l u t i o n , Wageningen, T h e Netherlands. 0003-2700/79/0351-1002$01.00/0
A method of manually determining nitrogen dioxide in ambient air was described in 1954 by Saltzman (1). The method consists of absorption of NOz by a solution of sulfanilic acid, N-(1-naphthy1)ethylenediamine dihydrochloride, and acetic acid. The solution forms a stable color which shows an absorbance maximum a t 550 nm. According to Saltzman, ozone caused an orange to yellow-brown tint which can be regarded as a positive interference. With 30 pL of ozone, the coupling reagent was completely destroyed. It was also found that specially prepared manganese dioxide decomposes the ozone but a t the same time accelerates the oxidation of the available nitrogen dioxide by the ozone. Another investigation on the ozone interference was carried out by Baumgardner et al. ( 2 ) ,who reported a negative interference, the magnitude of which was dependent on the ratio of ozone to nitrogen dioxide. In their study they made use of a Bendix monitor, working on the principle of chemiluminescence, and of a continuously operating Technicon instrument using a modified Saltzman procedure. The decrease of the response in the 1979 American Chemical Society
