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Comparative analysis of synthetic polymers using combinations of three analytical methods
Forensic Science International, 25 (1984) Elsevier Scientific Publishers Ireland Ltd.
COMPARATIVE COMBINATIONS
J. ANDRASKO,
57
51-70
ANALYSIS OF SYNTHETIC POLYMERS OF THREE ANALYTICAL METHODS
L. HAEGER,
The National Laboratory
USING
A.C. MAEHLY and L. SVENSSON
of Forensic Science, S-581 01 Linkiiping (Sweden)
(Received May 5,1983) (Revision received January 16, 1984) (Accepted January 19, 1984)
Summary The use of three analytical techniques for forensic investigation of synthetic polymers is described. These techniques are: pyrolysis infrared spectrometry (PyIR), pyrolysis gas chromatography (PyGC) and energy dispersive X-ray micro probe analysis (EPMA). Whereas the first two methods have been described previously, the microanalytical procedure is new. It may be used in combination with one of the other two techniques for the discrimination between polymer samples. Key words: Polymers; Analysis; Pyrolysis; Infrared spectrometry; Electron probe microanalysis.
Gas chromatography;
Introduction
In view of the very widespread use of synthetic polymers nowadays a surprisingly small number of publications on their forensic analysis have appeared so far. The examination of plastics in a forensic science laboratory has usually two aims: -- to identify the chemical - to discriminate between
type of polymer and queried and control samples.
The first type of investigation may sometimes be used, e.g. in order to determine the kind of polymer used in the insulation of a burnt cable or to report which gases or fumes are formed in a fire. However, it is of more general application since it is essential to identify the chemical type of polymer when matching plastics (queried and control samples). The second type of investigation is also often encountered in fire cases when the origin of some plastic material is to be determined. The situation may be more complicated when the sample from the scene is partially burnt. Obviously, the comparison of queried and reference samples can be of importance for linking a suspect to a crime scene. 0379-0738/84/$03.00
o 1984 Elsevier Scientific Publishers Ireland Ltd Printed and Published in Ireland
58
There are a number of techniques available for the characterization and analysis of polymers. The subject has recently been reviewed by Maehly and Stromberg [l] . Who also summarized the methods suitable for the small samples encountered in forensic cases. In this study two of the methods introduced earlier into forensic laboratories for examination of polymers were used - pyrolysis gas chromatography (PyGC) [2,3] and pyrolysis infrared spectrophotometry (PyIR) [43. The (EPMA) of pyrolysis third method reported here - X-ray microanalysis residues - can be used in combination with each of the other methods to discriminate between queried and reference samples. Materials and Methods Polymer samples Technically pure polymers were obtained from the National Swedish Institute for Material Testing. Finished plastic products were kindly given to the laboratory by a number of industrial companies. In addition some plastic products used at our laboratory were analyzed. Pyrolysis infrared spectropho tometry (PyIR) A small rheostat-controlled electric furnace was used for heating 4-lo-mg samples in Pyrex tubes (8 X 75 mm). It was calibrated by measuring the steady state maximum temperature at various voltage settings, using a thermoelement. A Perkin-Elmer 257 grating Infrared Spectrophotometer was used for recording IR-spectra of the pyrolyzate collected on 0.5 inch KBr-discs. The technique for obtaining the spectra is shown in Fig. 1. The IR spectra were coded on punch cards according to the system published by Curry et al. [5].
1x8 CM Fig. 1. The experimental arrangement for obtaining IR-spectra of polymer samples.
59
Pyrolysis gas chromatography (PyGC) Pyrolysis gas chromatography was carried out using two different systems. In System A, a Pye-Unicam 104 gas chromatograph with a Curie-point pyrolyser was employed. A wire with a Curie point of 610°C and a heating period of 10 s were chosen. The column (1.7 m X 2 mm i.d. glass) was filled with Porapak Q (So-100 mesh). The injection port temperature was 2OO”C, that of the flame ionization detector (FID) was 290°C and the oven temperature was programmed from 100°C to 200°C at 7”C/min. Some analyses as well as studies to discriminate between polymer samples were carried out by using a 2.7-m column packed with 5% OV-17 on Carbochrome Q. The oven temperature was programmed from 50°C to 300°C at lO”C/min. In System B, the gas chromatograph was a Sigma 2 Perk&Elmer instrument equipped with an exit splitter, leading to a FID and a nitrogen-phosphorus detector (NPD). The column (3m X 0.8 mm i.d. copper) was filled with Porapak Q, 80--100 mesh. The carrier gas was Nz at 30 ml/min. The pyrolysis was carried out in a CDS-pyroprobe 120, where the sample is placed in a silica tube and heated by a platinum coil (rate of heating 75”C/ms). The injector was kept at 2OO”C, the detectors at 250°C and the oven temperature was programmed as follows: 5 min at 9O”C, G”C/min to 22O”C, 11 min at 22O”C, lO”C/min to 24O”C, 15 min at 240°C. Energy dispersive X-ray micro probe analysis (EPMA) A Jeol JSM-35 scanning electron microscope in conjunction with a PGT1000 microanalyzer was used for X-ray microanalysis. The residues from the pyrolysis tubes for IR-analysis or the residues from the silica tubes in the CDS-pyroprobe 120 were scraped out with the help of a spatula and transferred to carbon stubs for EPMA. In some experiments, when only the discriminating power of EPMA was studied the polymer samples were ashed in a crucible at 650°C for lo-15 min. The sample holder for scanning electron microscopy (made from brass) was covered with a plastic cup (coated by carbon) to prevent interfering signals from Cu and Zn. Results and discussion The aim of this project was to evaluate the use of EPMA in combination with PyIR or PyGC for the identification of polymers present in small samples of plastics and for discriminating between them. PyIR It was felt that the PyIR analysis method as introduced to forensic analysis of polymers by Smalldon [4] should be standardized. The effect of the pyrolysis temperature and its duration was first studied. Somewhat to our surprise, these parameters seemed to have very little influence on the resulting IR-spectra. Samples of polystyrene were heated (under otherwise identical conditions) to 34O”C, 45O”C, 550°C and 600°C respectively. Below 3OO”C,
60
the sample simply melted and no fumes could be observed. At 340°C pyrolysis took place very slowly and the fumes could only be collected with difficulty. At the three highest temperatures, the pyrolyzate could be collected with ease and in adequate amounts. No differences could be observed between the PyIR spectra obtained at 45O”C, 550°C and 6OO”C, respectively. Similar results were obtained for several other polymers. The melting and the pyrolysis of different polymers occur at different temperatures (if the polymer melts at all). The kinetics and the yield of pyrolysis of plastics have been reported in several studies (e.g. [6] ) and was not the aim of this work. Our results for many polymer samples indicate that the pyrolysis should be carried out at temperatures of 450°C or higher to obtain sufficient amounts of pyrolyzate. In all, 33 different kinds of polymers were analyzed by PyIR in this study (cf. Table 1). The sensitivity of this method varies with the type of polymer. Some polymers, e.g. polystyrene, polyamides and polyvinylchloride give distinct IR-specta using about 1 mg of sample. Other polymers require larger sample amounts. Melamine-formaldehyd (MF) polymer gave no observable IR spectrum under the conditions described in this work. Some other polymers gave spectra which were not very distinct (cf. Table 1). The reproducibility of the method was tested using five pieces of the same sample of polymer, polystyrene and polyamide-6, respectively. The samples were pyrolyzed at 450°C and the pyrolyzate collected on 5 KBr discs in succession. No significant differences in the 5 Py IR-spectra obtained for each of the polymers were observed. Finally, samples of the same kind of polymer supplied by different manufacturers were analyzed in order to investigate possible differences between their PyIR spectra. Samples of polyamide-6 from three manufacturers showed no measurable differences between their PyIR spectra (Fig. 2). Similar results were obtained for ABS, CP, LDPE, PA-6,6, PA-12, PC, PMMA, POM, PP, PS, PUR, PVC, SAN and SB (the numbers of samples from different manufacturers analyzed by this technique are listed in Table 1). PyIR may be useful for identification of unknown polymer samples, but, the discrimination power for samples of the same kind of polymer is low. EPMA
In order to increase the discrimination power of the polymer analysis, the PyIR technique was combined with an energy-dispersive X-ray microanalysis in a scanning electron microscope. Differences in the elemental composition of different polymer samples might be expected, due to variations in additives in the manufacturing process, such as pigment dyes, fillers etc. The sensitivity of the X-ray microanalysis must be increased by removing the organic matrix of the polymer samples. If the amount of sample is sufficiently large PyIR and X-ray microanalysis may be carried out independently from each other. In that case, part of the sample is ashed in a crucible. Heating to 650-700°C
61 TABLE 1 INVESTIGATED
POLYMERS
AND SOME OF THEIR RECORDED
DATA
Abbreviation
Name of polymer
PYIR-~ spectra
Pyrogram
ABS AMMA ASA CA CAB CP EVA HDPE LDPE MDPE MF PA PA-6 PA-6,6 PA-6,lO PA-l 1 PA-12 PBT PC PE PETP PF PMMA POM PP PPO PPO/SB PS PSU PUR PVC SAN SB
Acrylonitrile-butadiene-styrene Acrylonitrile-methyl methacrylate Acryloester-styrene-acrylonitrile Cellulose acetate Cellulose acetate-butyrate Cellulose propionate Ethylene-vinyl acetate Polyethylene (high density) Polyethylene (low density) Polyethylene (medium density) Melamine-formaldehyde Polyamide Polyamide 6 Polyamide 6, 6 Polyamide 6, 10 Polyamide 11 Polyamide 12 Polybutene-terephthalate Polycarbonate Polyester Polyethylene terephthalate Phenol-formaldehyde Poly-methyl methacrylate Poly-oxymethylene Polypropylene Poly-phenyleneoxide Polyphenylene oxide/styrene-butadiene Polystyrene Polysulfone Polyurethane Polyvinyl chloride Styrene-acrylonitrile Styrene-butadiene
4 1 1 1 1 2 1 lb 3b lb 1= 1 5 2 1 1 2 1 3 1 1 1
1 1
;:: 4 1 1 5 1 2 2 3 3
1 1 3 1 1
1 1 1
1
1 3 4
3
1 3 2
aIR-spectra of products from different manufacturers. bThese spectra were not very distinct. ‘No IR-spectra was obtained.
lo-15 min has been sufficient for complete ashing of most plastics. If the sample amount is small needing the whole of it to be used for the PyIR analysis, the pyrolysis residue in the Pyrex tube can be analyzed. The residue is simply scraped out (with the help of a spatula) and transferred to carbon stubs for X-ray microanalysis. For best results the PyIR should be carried out at high temperatures (at least 550°C) and for a period longer than that necessary for collecting the vapours only.
for
62
Fig. 2. PyIR for three samples of polyamide-6 from different manufacturers. detailed part of the IR spectra is shown.
The most
Figure 3 shows the X-ray spectra obtained for two polystyrene samples from the same manufacturer but coloured differently. The pigments used gave very different spectra as might be expected. The PyIR spectra of these samples were indistinguishable from each other. It is, of course, usually not necessary to analyse two samples which differ in colour in order to discriminate between them, but in actual fire investigations the colour of burnt material is not always preserved. X-ray analysis may also distinguish between samples of colourless polymers. The spectra of two colourless polypropylene samples from two different manufacturers are shown in Fig. 4. The difference in elemental composition of these two samples is significant. A series of experiments was performed for investigating the discrimination power of the X-ray microanalysis of polymers. About 60 polymer samples were analyzed using this technique. Twenty elements were encountered in these samples. Na. Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Co, Fe, Cu, Zn, Se, Sr, Cd, Ba and Pb.
63
Ti
Fig. 3. EPMA of two polystyrene samples, one coloured red, the other blue. The samples originate from the same manufacturer and were ashed prior to the analysis.
Fifteen of these samples were colourless polymers from two or sometimes three different manufacturers. The polymers were ashed in a crucible at 650°C for 15 min and analyzed by X-ray microanalysis. It was found that if any ash was obtained, discrimination between samples of the same kind of polymer supplied by different manufacturers was achieved (however, in one instance, two of three SAN samples were found practically indistinguishable). The amount of ash varied greatly, however, and for five of the samples no ash was obtained or no elements were detectable. Analysis of transparent samples generally requires relatively large amounts (>50 mg) to obtain clear-cut results. In order to evaluate the discrimination power of EPMA for samples of the same colour 10 samples of red, 7 of blue and 8 of green colour were chosen from our collection of plastics. The samples in each colour group were very similar in colour shade, but all of them were from the same manufacturer, since only one manufacturer (BASF) supplied us with many samples of different shades. Several different polymers thus appeared in each of the groups. Table 2 shows the results and the discrimination achieved in this manner. It may be interesting to note that 9 out of 10 red coloured plastics contained Cd and 7 contained Se. Seven out of 8 green coloured plastics contained Cd. Nevertheless, due to the presence of other elements, the discrimination
64
.
POLYPROPEN
(STELLANA)
POLYPROPEN
(STAT.PROV.ANST.)
I
d, Mg
Iv\. St
6)
Ca Ca
Fig. 4. The EPMA of pyrolysis residues from two colourless polypropylene samples made by different companies. The lower trace stems from a PP sample from the National Institute for Material Testing, the upper trace from a sample distributed by a commercial firm (Stellana Plast AB, Lax%, Sweden). Both residues contain Si and Ca but have otherwise widely different elemental compositions.
between the samples using this single technique is satisfactory. However, one pair of red plastics and one pair of green plastics were found to be indistinguishable in an actual case. The data are, of course, not sufficient for a statistical evaluation. Therefore, another experiment was performed. Seventeen red coloured plastics of very similar shades were collected from various plastic products found in the laboratory. Ten of these samples contained Cd and only two contained Se, which indicates that Cd and Cd f Se pigments are not used very frequently by all manufacturers. All samples except two showed qualitative or large semiquantitative differences in their elemental composition. The results of this experiment illustrate the discrimination power of X-ray microanalysis (Table 3). The number of samples is, however, too small to calculate the discriminating power of this method in a real case. PyGC
PyGC of polymers is a well established analytical method in forensic science. Most of the PyGC measurements performed in this study (Table 1) were carried out with a Curie-point pyrolyzer and a Porapak-Q column
65 TABLE
2
EPMA OF RED, BLUE SAME MANUFACTURER Elemental
Type of
AND GREEN
composition
COLOURED
POLYMER
SAMPLES
FROM
THE
(signal strength)
polymer Strong
Medium
Weak
Cd > S Si > Cd Cd,S Cd Cd Cd > Fe Ti Cd Cd Cd
Ba,Se,Si S Ti > Mg,Zn S S Ti > S Zn,Fe S> Se &Se Ca,S > S&S
Fe Se Al,Si,P Ba,Mg,Se,P,Fe Se,Ba Si,Se,Al Al,Si,S,K,Ca Ti > Fe,Zn Zn Mg
Si Ti Cr,Al,Ca,Si Ti Ti Ti Si > S,Al
Ti Al,Si,S,Ca Co > T&S cu > s Cd > S,Cu
K,Cu Na, K Na,Mg,K Mg,Si,Ca,K,P,Al P,Fe Fe,Cu K,Fe,Cu
Cd S > Si > Ca Cd &Cd Ti > Cd Cd > S Cd > S,Ti Fe
Ti,Fe,Zn Cd,Al S,Ti Si,Ti S Ti
Traces
Red plastic products
PPa POM ABS ABS SAN’ PE PS PS PS PA
Fe Si,Ti Ti
Fe
Blue plastic products
POM PS PA ABS ABS SAN PE
Ti > Na
cu Cl
Al,Si,K,Ca
Green plastic products
PE PA SAN ABS ABSa PP ABSa SAN aSamples
found
indistinguishable
> S
Cu Ti,Zn Zn Al,Zn Zn Mg,Zn,Cu Zn,Si,Fe
Al,Si Mg,Fe Al,Si,Ba Mg,P,Fe Mg,Al,Si,P Al Mg,Al,P Al,Si,S,CI,Cu
by this method.
(System A). System B, developed by one of us (L.S.) also uses a Porapak-Q column, but gives additional information in all cases where nitrogen contaming polymers are analyzed, such as ABS, AMMA, ASA, MF, PA (all types), PUR and SAN (abbreviations cf. Table 1). An example is shown in Fig. 5. The use of this system in an actual case may be found in Ref. 1. The PyGC systems described above are suitable for identification of polymers if reference chromatograms are available. We prepared reference PyGC
66 TABLE
3
THE DISCRIMINATION POLYMERS STUDIED PRODUCTS
POWER OF X-RAY MICROANALYSIS AND ON 1’7 UNKNOWN SAMPLES OF RED COLOURED
Number of indistinguishable samples in one group
Number EPMA
PyGC
1 2 3 4 5
15 1 -
6 1 1 -
6
-
1
16
9
Total
number
of groups
of different
PyGC OF PLASTIC
groups
chromatograms for many polymer samples by System A (cf. Table 1). Of course, if another kind of chromatographic column is used a new series of reference chromatograms must be collected. When PyGC is used for comparing suspect and reference polymer samples, other GC-columns than those packed with Porapak-Q should be employed. The number of pyrolysis peaks analyzed on a conventional packed column
Fig. 5. PyGC of polyamide FID. Lower trace: NPD.
polymer.
Experimental
conditions:
System
B. Upper
trace:
67
(f. ex. OV-17) is much higher than that on Porapak Q. This was shown by Wheals and Noble [3] and is here illustrated in Fig. 6 for the sample of SAN. We are now in the process of introducing capillary columns in our PyGC instruments to increase the discrimination power of the analysis. A comparison between PyIR and PyGC methods, respectively, for polymer analysis shows that the latter has higher sensitivity, particularly when systems with conventional packed columns or capillary column are employed. Discrimination between polymer samples from different manufacturers could be achieved by PyGC but not by PyIR. We have found clear semiquantitative differences in peak heights in chromatograms obtained for three colourless SAN polymer samples from different manufacturers. On the other hand, one could not distinguish between various samples of polyethylenes and polypropylenes. Only slight differences were observed by capillary PyGC between differently coloured polypropylenes from the same manufacturers. The residue after pyrolysis may be analyzed by EPMA. In that case, the pyroprobe is superior to the Curi-point pyrolysis unit. With the pyroprobe it is easier to find the residue and transfer the pyrolyzed sample for further analysis. However, in most cases the amount of material is sufficient to be analyzed by PyGC and EPMA independently. The polymer sample for EPMA is then simply ashed in a crucible as described above. In order to estimate the discriminating power of PyGC, a series of 17 red coloured plastics (the same samples as these for X-ray microanalysis above)
b
a
Time ,mtnutes
Fig. 6. PyGC of SAN-polymer: (a) chromatogram obtained on a Porapak-Q column; and (b) chromatogram obtained on a OV-17 column. The experimental conditions are those described in the text for System A.
68
were analyzed. The results are shown in Table 3. System A using a column packed with 5% OV-17 was employed. It is clear that PyGC alone has a limited ability to discriminate between the samples. As an example, six of the materials investigated were identified as polyethylene but all of them were indistinguishable by the PyGC used here. By contrast, EPMA could discriminate between all of the samples studied with the exception of one pair (two of the polyethylenes). The results obtained by EPMA and PyGC for three of the polymer samples are shown in Fig. 7. All three samples were
i
I
Fig. 7. Three samples of red coloured polymers (a, b, c) analyzed by EPMA (part A) and PyGC (part B). The experimental conditions are those described in the text. For PyGC System A was employed. The samples were identified as polystyrene. Samples a and b were found indistinguishable by PyGC but clearly distinguishable by EPMA.
69
identified as polystyrenes (by PyGC). Two of the samples (a,b) were found indistinguishable by PyGC, but clearly distinguishable by EPMA. In this particular study the combination of PyGC and EPMA could not discriminate between more samples than the use of EPMA alone. Since PyGC, unlike EPMA, reveals the type of polymer and also distinguishes between some samples of the same kind of polymer (here the use of capillary columns is favourable), the combination of PyGC and EPMA must be considered very useful for discriminating processes. On the other hand, PyIR is a rapid technique for the identification of polymers. An IR spectrum may be recorded in a few minutes, while a complete PyGC analysis requires approximately 1 h. PyIR may also be used by laboratories that do not own PyGC equipment. For discriminating purposes, this method must be combined with some other technique capable of determining the elemental composition of the pyrolyzed residue. Polymer samples encountered in fire cases are frequently partly burnt. In such cases special care must be taken in interpreting the analytical results. The elemental composition of burnt polymers may be influenced by contamination from other burning materials. Thus, if the sample amount is sufficiently large, the analysis of the surface layer should be avoided. PyGC and PyIR may be used to identify the type of polymer also for quite heavily burnt samples. The possibility of discriminations between samples of the same kind of polymer by PyGC is, however, significantly reduced. Figure 8
0
I I
/
300 -
200
100
-
t PC)
Fig. 8. PyGC analysis (system and (B) partly burnt sample.
I
300
A) of a black coloured
200
100
t PC1
polystyrene
sample.
(A) pure sample;
70
illustrates the changes in PyGC chromatograms of an identical sample - pure (A) and partly burnt (B). The burnt sample may easily be identified as polystyrene but some minor peaks present in the chromatogram for the pure polymer disappeared after the burning. (Similar results were observed for other types of polymers.) References 1 A. Maehly and L. Stromberg, Chemical Criminalistics, Springer-Verlag, BerlinHeidelberg-New York, 1981. 2 B.B. Wheals, and W. Noble, Forensic applications of pyrolysis gas chromatogra\phy. Chromatographia, 5 (1972) 553-557. 3 W. Noble, B.B. Wheals and M.J. Whitehouse, The characterization of adhesives by pyrolysis gas chromatography and infrared spectroscopy. Forensic Sci., 3 (1974) 163-174. 4 K.W. Smalldon, The identification of paint resins and other polymeric materials from the infra red spectra of their pyrolysis products. J. Forensic Sci. Sot., 9 (1969) 135-140. 5 AS. Curry, J.F. Read and C. Brown, Simple infrared spectrum retrieval system. J. Pharm. Pharmacol., 21 (1969) 224-231. 6 M.T. Sousa Pessoa de Amorim, C. Bouster and J. Veron, Pyrolysis of propylene. II. Kinetics of degradati0n.J. Anal. Appl. Pyrol., 4 (1982) 103-115.
COMPARATIVE COMBINATIONS
J. ANDRASKO,
57
51-70
ANALYSIS OF SYNTHETIC POLYMERS OF THREE ANALYTICAL METHODS
L. HAEGER,
The National Laboratory
USING
A.C. MAEHLY and L. SVENSSON
of Forensic Science, S-581 01 Linkiiping (Sweden)
(Received May 5,1983) (Revision received January 16, 1984) (Accepted January 19, 1984)
Summary The use of three analytical techniques for forensic investigation of synthetic polymers is described. These techniques are: pyrolysis infrared spectrometry (PyIR), pyrolysis gas chromatography (PyGC) and energy dispersive X-ray micro probe analysis (EPMA). Whereas the first two methods have been described previously, the microanalytical procedure is new. It may be used in combination with one of the other two techniques for the discrimination between polymer samples. Key words: Polymers; Analysis; Pyrolysis; Infrared spectrometry; Electron probe microanalysis.
Gas chromatography;
Introduction
In view of the very widespread use of synthetic polymers nowadays a surprisingly small number of publications on their forensic analysis have appeared so far. The examination of plastics in a forensic science laboratory has usually two aims: -- to identify the chemical - to discriminate between
type of polymer and queried and control samples.
The first type of investigation may sometimes be used, e.g. in order to determine the kind of polymer used in the insulation of a burnt cable or to report which gases or fumes are formed in a fire. However, it is of more general application since it is essential to identify the chemical type of polymer when matching plastics (queried and control samples). The second type of investigation is also often encountered in fire cases when the origin of some plastic material is to be determined. The situation may be more complicated when the sample from the scene is partially burnt. Obviously, the comparison of queried and reference samples can be of importance for linking a suspect to a crime scene. 0379-0738/84/$03.00
o 1984 Elsevier Scientific Publishers Ireland Ltd Printed and Published in Ireland
58
There are a number of techniques available for the characterization and analysis of polymers. The subject has recently been reviewed by Maehly and Stromberg [l] . Who also summarized the methods suitable for the small samples encountered in forensic cases. In this study two of the methods introduced earlier into forensic laboratories for examination of polymers were used - pyrolysis gas chromatography (PyGC) [2,3] and pyrolysis infrared spectrophotometry (PyIR) [43. The (EPMA) of pyrolysis third method reported here - X-ray microanalysis residues - can be used in combination with each of the other methods to discriminate between queried and reference samples. Materials and Methods Polymer samples Technically pure polymers were obtained from the National Swedish Institute for Material Testing. Finished plastic products were kindly given to the laboratory by a number of industrial companies. In addition some plastic products used at our laboratory were analyzed. Pyrolysis infrared spectropho tometry (PyIR) A small rheostat-controlled electric furnace was used for heating 4-lo-mg samples in Pyrex tubes (8 X 75 mm). It was calibrated by measuring the steady state maximum temperature at various voltage settings, using a thermoelement. A Perkin-Elmer 257 grating Infrared Spectrophotometer was used for recording IR-spectra of the pyrolyzate collected on 0.5 inch KBr-discs. The technique for obtaining the spectra is shown in Fig. 1. The IR spectra were coded on punch cards according to the system published by Curry et al. [5].
1x8 CM Fig. 1. The experimental arrangement for obtaining IR-spectra of polymer samples.
59
Pyrolysis gas chromatography (PyGC) Pyrolysis gas chromatography was carried out using two different systems. In System A, a Pye-Unicam 104 gas chromatograph with a Curie-point pyrolyser was employed. A wire with a Curie point of 610°C and a heating period of 10 s were chosen. The column (1.7 m X 2 mm i.d. glass) was filled with Porapak Q (So-100 mesh). The injection port temperature was 2OO”C, that of the flame ionization detector (FID) was 290°C and the oven temperature was programmed from 100°C to 200°C at 7”C/min. Some analyses as well as studies to discriminate between polymer samples were carried out by using a 2.7-m column packed with 5% OV-17 on Carbochrome Q. The oven temperature was programmed from 50°C to 300°C at lO”C/min. In System B, the gas chromatograph was a Sigma 2 Perk&Elmer instrument equipped with an exit splitter, leading to a FID and a nitrogen-phosphorus detector (NPD). The column (3m X 0.8 mm i.d. copper) was filled with Porapak Q, 80--100 mesh. The carrier gas was Nz at 30 ml/min. The pyrolysis was carried out in a CDS-pyroprobe 120, where the sample is placed in a silica tube and heated by a platinum coil (rate of heating 75”C/ms). The injector was kept at 2OO”C, the detectors at 250°C and the oven temperature was programmed as follows: 5 min at 9O”C, G”C/min to 22O”C, 11 min at 22O”C, lO”C/min to 24O”C, 15 min at 240°C. Energy dispersive X-ray micro probe analysis (EPMA) A Jeol JSM-35 scanning electron microscope in conjunction with a PGT1000 microanalyzer was used for X-ray microanalysis. The residues from the pyrolysis tubes for IR-analysis or the residues from the silica tubes in the CDS-pyroprobe 120 were scraped out with the help of a spatula and transferred to carbon stubs for EPMA. In some experiments, when only the discriminating power of EPMA was studied the polymer samples were ashed in a crucible at 650°C for lo-15 min. The sample holder for scanning electron microscopy (made from brass) was covered with a plastic cup (coated by carbon) to prevent interfering signals from Cu and Zn. Results and discussion The aim of this project was to evaluate the use of EPMA in combination with PyIR or PyGC for the identification of polymers present in small samples of plastics and for discriminating between them. PyIR It was felt that the PyIR analysis method as introduced to forensic analysis of polymers by Smalldon [4] should be standardized. The effect of the pyrolysis temperature and its duration was first studied. Somewhat to our surprise, these parameters seemed to have very little influence on the resulting IR-spectra. Samples of polystyrene were heated (under otherwise identical conditions) to 34O”C, 45O”C, 550°C and 600°C respectively. Below 3OO”C,
60
the sample simply melted and no fumes could be observed. At 340°C pyrolysis took place very slowly and the fumes could only be collected with difficulty. At the three highest temperatures, the pyrolyzate could be collected with ease and in adequate amounts. No differences could be observed between the PyIR spectra obtained at 45O”C, 550°C and 6OO”C, respectively. Similar results were obtained for several other polymers. The melting and the pyrolysis of different polymers occur at different temperatures (if the polymer melts at all). The kinetics and the yield of pyrolysis of plastics have been reported in several studies (e.g. [6] ) and was not the aim of this work. Our results for many polymer samples indicate that the pyrolysis should be carried out at temperatures of 450°C or higher to obtain sufficient amounts of pyrolyzate. In all, 33 different kinds of polymers were analyzed by PyIR in this study (cf. Table 1). The sensitivity of this method varies with the type of polymer. Some polymers, e.g. polystyrene, polyamides and polyvinylchloride give distinct IR-specta using about 1 mg of sample. Other polymers require larger sample amounts. Melamine-formaldehyd (MF) polymer gave no observable IR spectrum under the conditions described in this work. Some other polymers gave spectra which were not very distinct (cf. Table 1). The reproducibility of the method was tested using five pieces of the same sample of polymer, polystyrene and polyamide-6, respectively. The samples were pyrolyzed at 450°C and the pyrolyzate collected on 5 KBr discs in succession. No significant differences in the 5 Py IR-spectra obtained for each of the polymers were observed. Finally, samples of the same kind of polymer supplied by different manufacturers were analyzed in order to investigate possible differences between their PyIR spectra. Samples of polyamide-6 from three manufacturers showed no measurable differences between their PyIR spectra (Fig. 2). Similar results were obtained for ABS, CP, LDPE, PA-6,6, PA-12, PC, PMMA, POM, PP, PS, PUR, PVC, SAN and SB (the numbers of samples from different manufacturers analyzed by this technique are listed in Table 1). PyIR may be useful for identification of unknown polymer samples, but, the discrimination power for samples of the same kind of polymer is low. EPMA
In order to increase the discrimination power of the polymer analysis, the PyIR technique was combined with an energy-dispersive X-ray microanalysis in a scanning electron microscope. Differences in the elemental composition of different polymer samples might be expected, due to variations in additives in the manufacturing process, such as pigment dyes, fillers etc. The sensitivity of the X-ray microanalysis must be increased by removing the organic matrix of the polymer samples. If the amount of sample is sufficiently large PyIR and X-ray microanalysis may be carried out independently from each other. In that case, part of the sample is ashed in a crucible. Heating to 650-700°C
61 TABLE 1 INVESTIGATED
POLYMERS
AND SOME OF THEIR RECORDED
DATA
Abbreviation
Name of polymer
PYIR-~ spectra
Pyrogram
ABS AMMA ASA CA CAB CP EVA HDPE LDPE MDPE MF PA PA-6 PA-6,6 PA-6,lO PA-l 1 PA-12 PBT PC PE PETP PF PMMA POM PP PPO PPO/SB PS PSU PUR PVC SAN SB
Acrylonitrile-butadiene-styrene Acrylonitrile-methyl methacrylate Acryloester-styrene-acrylonitrile Cellulose acetate Cellulose acetate-butyrate Cellulose propionate Ethylene-vinyl acetate Polyethylene (high density) Polyethylene (low density) Polyethylene (medium density) Melamine-formaldehyde Polyamide Polyamide 6 Polyamide 6, 6 Polyamide 6, 10 Polyamide 11 Polyamide 12 Polybutene-terephthalate Polycarbonate Polyester Polyethylene terephthalate Phenol-formaldehyde Poly-methyl methacrylate Poly-oxymethylene Polypropylene Poly-phenyleneoxide Polyphenylene oxide/styrene-butadiene Polystyrene Polysulfone Polyurethane Polyvinyl chloride Styrene-acrylonitrile Styrene-butadiene
4 1 1 1 1 2 1 lb 3b lb 1= 1 5 2 1 1 2 1 3 1 1 1
1 1
;:: 4 1 1 5 1 2 2 3 3
1 1 3 1 1
1 1 1
1
1 3 4
3
1 3 2
aIR-spectra of products from different manufacturers. bThese spectra were not very distinct. ‘No IR-spectra was obtained.
lo-15 min has been sufficient for complete ashing of most plastics. If the sample amount is small needing the whole of it to be used for the PyIR analysis, the pyrolysis residue in the Pyrex tube can be analyzed. The residue is simply scraped out (with the help of a spatula) and transferred to carbon stubs for X-ray microanalysis. For best results the PyIR should be carried out at high temperatures (at least 550°C) and for a period longer than that necessary for collecting the vapours only.
for
62
Fig. 2. PyIR for three samples of polyamide-6 from different manufacturers. detailed part of the IR spectra is shown.
The most
Figure 3 shows the X-ray spectra obtained for two polystyrene samples from the same manufacturer but coloured differently. The pigments used gave very different spectra as might be expected. The PyIR spectra of these samples were indistinguishable from each other. It is, of course, usually not necessary to analyse two samples which differ in colour in order to discriminate between them, but in actual fire investigations the colour of burnt material is not always preserved. X-ray analysis may also distinguish between samples of colourless polymers. The spectra of two colourless polypropylene samples from two different manufacturers are shown in Fig. 4. The difference in elemental composition of these two samples is significant. A series of experiments was performed for investigating the discrimination power of the X-ray microanalysis of polymers. About 60 polymer samples were analyzed using this technique. Twenty elements were encountered in these samples. Na. Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Co, Fe, Cu, Zn, Se, Sr, Cd, Ba and Pb.
63
Ti
Fig. 3. EPMA of two polystyrene samples, one coloured red, the other blue. The samples originate from the same manufacturer and were ashed prior to the analysis.
Fifteen of these samples were colourless polymers from two or sometimes three different manufacturers. The polymers were ashed in a crucible at 650°C for 15 min and analyzed by X-ray microanalysis. It was found that if any ash was obtained, discrimination between samples of the same kind of polymer supplied by different manufacturers was achieved (however, in one instance, two of three SAN samples were found practically indistinguishable). The amount of ash varied greatly, however, and for five of the samples no ash was obtained or no elements were detectable. Analysis of transparent samples generally requires relatively large amounts (>50 mg) to obtain clear-cut results. In order to evaluate the discrimination power of EPMA for samples of the same colour 10 samples of red, 7 of blue and 8 of green colour were chosen from our collection of plastics. The samples in each colour group were very similar in colour shade, but all of them were from the same manufacturer, since only one manufacturer (BASF) supplied us with many samples of different shades. Several different polymers thus appeared in each of the groups. Table 2 shows the results and the discrimination achieved in this manner. It may be interesting to note that 9 out of 10 red coloured plastics contained Cd and 7 contained Se. Seven out of 8 green coloured plastics contained Cd. Nevertheless, due to the presence of other elements, the discrimination
64
.
POLYPROPEN
(STELLANA)
POLYPROPEN
(STAT.PROV.ANST.)
I
d, Mg
Iv\. St
6)
Ca Ca
Fig. 4. The EPMA of pyrolysis residues from two colourless polypropylene samples made by different companies. The lower trace stems from a PP sample from the National Institute for Material Testing, the upper trace from a sample distributed by a commercial firm (Stellana Plast AB, Lax%, Sweden). Both residues contain Si and Ca but have otherwise widely different elemental compositions.
between the samples using this single technique is satisfactory. However, one pair of red plastics and one pair of green plastics were found to be indistinguishable in an actual case. The data are, of course, not sufficient for a statistical evaluation. Therefore, another experiment was performed. Seventeen red coloured plastics of very similar shades were collected from various plastic products found in the laboratory. Ten of these samples contained Cd and only two contained Se, which indicates that Cd and Cd f Se pigments are not used very frequently by all manufacturers. All samples except two showed qualitative or large semiquantitative differences in their elemental composition. The results of this experiment illustrate the discrimination power of X-ray microanalysis (Table 3). The number of samples is, however, too small to calculate the discriminating power of this method in a real case. PyGC
PyGC of polymers is a well established analytical method in forensic science. Most of the PyGC measurements performed in this study (Table 1) were carried out with a Curie-point pyrolyzer and a Porapak-Q column
65 TABLE
2
EPMA OF RED, BLUE SAME MANUFACTURER Elemental
Type of
AND GREEN
composition
COLOURED
POLYMER
SAMPLES
FROM
THE
(signal strength)
polymer Strong
Medium
Weak
Cd > S Si > Cd Cd,S Cd Cd Cd > Fe Ti Cd Cd Cd
Ba,Se,Si S Ti > Mg,Zn S S Ti > S Zn,Fe S> Se &Se Ca,S > S&S
Fe Se Al,Si,P Ba,Mg,Se,P,Fe Se,Ba Si,Se,Al Al,Si,S,K,Ca Ti > Fe,Zn Zn Mg
Si Ti Cr,Al,Ca,Si Ti Ti Ti Si > S,Al
Ti Al,Si,S,Ca Co > T&S cu > s Cd > S,Cu
K,Cu Na, K Na,Mg,K Mg,Si,Ca,K,P,Al P,Fe Fe,Cu K,Fe,Cu
Cd S > Si > Ca Cd &Cd Ti > Cd Cd > S Cd > S,Ti Fe
Ti,Fe,Zn Cd,Al S,Ti Si,Ti S Ti
Traces
Red plastic products
PPa POM ABS ABS SAN’ PE PS PS PS PA
Fe Si,Ti Ti
Fe
Blue plastic products
POM PS PA ABS ABS SAN PE
Ti > Na
cu Cl
Al,Si,K,Ca
Green plastic products
PE PA SAN ABS ABSa PP ABSa SAN aSamples
found
indistinguishable
> S
Cu Ti,Zn Zn Al,Zn Zn Mg,Zn,Cu Zn,Si,Fe
Al,Si Mg,Fe Al,Si,Ba Mg,P,Fe Mg,Al,Si,P Al Mg,Al,P Al,Si,S,CI,Cu
by this method.
(System A). System B, developed by one of us (L.S.) also uses a Porapak-Q column, but gives additional information in all cases where nitrogen contaming polymers are analyzed, such as ABS, AMMA, ASA, MF, PA (all types), PUR and SAN (abbreviations cf. Table 1). An example is shown in Fig. 5. The use of this system in an actual case may be found in Ref. 1. The PyGC systems described above are suitable for identification of polymers if reference chromatograms are available. We prepared reference PyGC
66 TABLE
3
THE DISCRIMINATION POLYMERS STUDIED PRODUCTS
POWER OF X-RAY MICROANALYSIS AND ON 1’7 UNKNOWN SAMPLES OF RED COLOURED
Number of indistinguishable samples in one group
Number EPMA
PyGC
1 2 3 4 5
15 1 -
6 1 1 -
6
-
1
16
9
Total
number
of groups
of different
PyGC OF PLASTIC
groups
chromatograms for many polymer samples by System A (cf. Table 1). Of course, if another kind of chromatographic column is used a new series of reference chromatograms must be collected. When PyGC is used for comparing suspect and reference polymer samples, other GC-columns than those packed with Porapak-Q should be employed. The number of pyrolysis peaks analyzed on a conventional packed column
Fig. 5. PyGC of polyamide FID. Lower trace: NPD.
polymer.
Experimental
conditions:
System
B. Upper
trace:
67
(f. ex. OV-17) is much higher than that on Porapak Q. This was shown by Wheals and Noble [3] and is here illustrated in Fig. 6 for the sample of SAN. We are now in the process of introducing capillary columns in our PyGC instruments to increase the discrimination power of the analysis. A comparison between PyIR and PyGC methods, respectively, for polymer analysis shows that the latter has higher sensitivity, particularly when systems with conventional packed columns or capillary column are employed. Discrimination between polymer samples from different manufacturers could be achieved by PyGC but not by PyIR. We have found clear semiquantitative differences in peak heights in chromatograms obtained for three colourless SAN polymer samples from different manufacturers. On the other hand, one could not distinguish between various samples of polyethylenes and polypropylenes. Only slight differences were observed by capillary PyGC between differently coloured polypropylenes from the same manufacturers. The residue after pyrolysis may be analyzed by EPMA. In that case, the pyroprobe is superior to the Curi-point pyrolysis unit. With the pyroprobe it is easier to find the residue and transfer the pyrolyzed sample for further analysis. However, in most cases the amount of material is sufficient to be analyzed by PyGC and EPMA independently. The polymer sample for EPMA is then simply ashed in a crucible as described above. In order to estimate the discriminating power of PyGC, a series of 17 red coloured plastics (the same samples as these for X-ray microanalysis above)
b
a
Time ,mtnutes
Fig. 6. PyGC of SAN-polymer: (a) chromatogram obtained on a Porapak-Q column; and (b) chromatogram obtained on a OV-17 column. The experimental conditions are those described in the text for System A.
68
were analyzed. The results are shown in Table 3. System A using a column packed with 5% OV-17 was employed. It is clear that PyGC alone has a limited ability to discriminate between the samples. As an example, six of the materials investigated were identified as polyethylene but all of them were indistinguishable by the PyGC used here. By contrast, EPMA could discriminate between all of the samples studied with the exception of one pair (two of the polyethylenes). The results obtained by EPMA and PyGC for three of the polymer samples are shown in Fig. 7. All three samples were
i
I
Fig. 7. Three samples of red coloured polymers (a, b, c) analyzed by EPMA (part A) and PyGC (part B). The experimental conditions are those described in the text. For PyGC System A was employed. The samples were identified as polystyrene. Samples a and b were found indistinguishable by PyGC but clearly distinguishable by EPMA.
69
identified as polystyrenes (by PyGC). Two of the samples (a,b) were found indistinguishable by PyGC, but clearly distinguishable by EPMA. In this particular study the combination of PyGC and EPMA could not discriminate between more samples than the use of EPMA alone. Since PyGC, unlike EPMA, reveals the type of polymer and also distinguishes between some samples of the same kind of polymer (here the use of capillary columns is favourable), the combination of PyGC and EPMA must be considered very useful for discriminating processes. On the other hand, PyIR is a rapid technique for the identification of polymers. An IR spectrum may be recorded in a few minutes, while a complete PyGC analysis requires approximately 1 h. PyIR may also be used by laboratories that do not own PyGC equipment. For discriminating purposes, this method must be combined with some other technique capable of determining the elemental composition of the pyrolyzed residue. Polymer samples encountered in fire cases are frequently partly burnt. In such cases special care must be taken in interpreting the analytical results. The elemental composition of burnt polymers may be influenced by contamination from other burning materials. Thus, if the sample amount is sufficiently large, the analysis of the surface layer should be avoided. PyGC and PyIR may be used to identify the type of polymer also for quite heavily burnt samples. The possibility of discriminations between samples of the same kind of polymer by PyGC is, however, significantly reduced. Figure 8
0
I I
/
300 -
200
100
-
t PC)
Fig. 8. PyGC analysis (system and (B) partly burnt sample.
I
300
A) of a black coloured
200
100
t PC1
polystyrene
sample.
(A) pure sample;
70
illustrates the changes in PyGC chromatograms of an identical sample - pure (A) and partly burnt (B). The burnt sample may easily be identified as polystyrene but some minor peaks present in the chromatogram for the pure polymer disappeared after the burning. (Similar results were observed for other types of polymers.) References 1 A. Maehly and L. Stromberg, Chemical Criminalistics, Springer-Verlag, BerlinHeidelberg-New York, 1981. 2 B.B. Wheals, and W. Noble, Forensic applications of pyrolysis gas chromatogra\phy. Chromatographia, 5 (1972) 553-557. 3 W. Noble, B.B. Wheals and M.J. Whitehouse, The characterization of adhesives by pyrolysis gas chromatography and infrared spectroscopy. Forensic Sci., 3 (1974) 163-174. 4 K.W. Smalldon, The identification of paint resins and other polymeric materials from the infra red spectra of their pyrolysis products. J. Forensic Sci. Sot., 9 (1969) 135-140. 5 AS. Curry, J.F. Read and C. Brown, Simple infrared spectrum retrieval system. J. Pharm. Pharmacol., 21 (1969) 224-231. 6 M.T. Sousa Pessoa de Amorim, C. Bouster and J. Veron, Pyrolysis of propylene. II. Kinetics of degradati0n.J. Anal. Appl. Pyrol., 4 (1982) 103-115.