THERMAL SAMPLING TECHNIQUES FOR THE ANALYSIS OF POLYMERS BY GC-MS

Transcription

1 JPACSM 100 THERMAL SAMPLING TECHNIQUES FOR THE ANALYSIS OF POLYMERS BY GC-MS CDS Analytical, Inc. Oxford, PA ABSTRACT Gas chromatography, especially when coupled with mass spectrometry, has become a ubiquitous workhorse in the analytical laboratory, providing rapid separation and identification of organic volatiles in a wide array of sample types. Although samples are generally prepared by dissolving them in a solvent and then injecting the solution into the GC, careful control of sample temperature may be used as a sample preparation technique instead. From a sample preparation standpoint, this has several advantages, including sensitivity and ease of automation. In particular, it permits the analysis of low molecular weight organics without the use of solvents and the analysis of polymers by gas chromatography. As an organic material is heated, it passes through a series of changes which may include the volatilization of intact molecules such as solvents, residual monomers, plasticizers, or other additives. If the temperature is increased further, bonds in the molecules of the material may dissociate, breaking the original material down into smaller, frequently volatile molecules. These processes have significant analytical appeal, since they permit selective analysis of various organic components in a complex material as well as the application of techniques usually reserved for volatile compounds to completely non-volatile substances. Examples are shown of the identification of trace-level organic compounds in manufactured goods, the identification of polymers by pyrolysis, and how this information may be used to distinguish random and block copolymers. INTRODUCTION Any organic compound which is identified by gas chromatography or gas chromatography-mass spectrometry (GC-MS) is done so only because it is sufficiently volatile to pass through the GC column and enter the detector. Analytes are generally injected using a syringe and vaporized in the injection port; however, many sample preparation techniques volatilize the analytes upstream of the injection port and deliver them already in the vapor phase, ready for chromatography. A major advantage of such techniques is the elimination of the injection solvent, for two important reasons. First, the solvent dilutes the analytes and consequently lowers sensitivity; secondly, the analytes may in fact be compounds frequently used as solvents, elute early, and may be lost in the injection solvent front. The use of purge and trap analysis 1 for the detection of trace level organic pollutants in water 2 is a classic example of a pre-vaporization technique. Thermal desorption as a sample preparation tool is essentially a purge and trap analysis of a solid material 3. In general, the sample is warmed to volatilize the small molecules trapped in the sample matrix while a stream of inert gas removes the analytes to a trap for collection. The trap may be a sorbent (such as charcoal, Tenax, and so on) or cryogenic. By venting the purge gas though the trap, a large volume of headspace may be concentrated onto the trap, producing an increase in sensitivity. The trap is then flushed with GC carrier gas and heated to revaporize the analytes, which are delivered to the injection port already volatilized. Careful control of the sample temperature permits some selection of the analytes in the headspace. Most important is the selection of a temperature at which the organic compounds of interest are removed from the sample matrix without causing chemical damage to the analytes or to the sample itself. At elevated temperatures, the sample matrix may degrade, producing volatile compounds in addition to the compounds already present. In general practice, dynamic headspace analyses are performed at temperatures well below those which will cause chemical changes in the sample matrix, most often in the range of C. For analytical pyrolysis on the other hand, chemical degradation of the sample matrix is the whole point, so much higher temperatures are required. At very high temperatures, the bonds in large molecules (polymers in particular) are broken and smaller molecules produced. If these molecules are small enough to pass through a GC column, but still retain structural information about the original macromolecule, they are analytically significant and permit the GC to be used in the analysis of polymers 4. In fact, almost all organic polymers 5, both natural and synthetic 6, produce specific compounds when pyrolyzed and may be differentiated and studied using GC-MS. Many polymers regenerate monomer, frequently along with higher oligomers, and even those which do not create an identifying array of volatiles. Polymers may begin this degradation process at temperatures as low as 350 C, but in general, analytical pyrolysis is performed in the range of C.

2 JPACSM 101 DISCUSSION Dynamic Headspace Sample preparation for dynamic headspace is generally very simple 7. If the sample material is a raw polymer such as a powder, a portion (usually less than a gram) is weighed and placed into a tube which is heated and purged. Manufactured goods such as containers, disk drives, packaging materials, electrical components, and food products may be sampled whole and intact by placing the piece into a heated vessel and purging the surrounding atmosphere to the trap. Commercial systems are available with vessels almost one liter in volume, but it must be remembered that the larger the volume, the longer the vessel must be purged to collect analytes quantitatively. The purging efficiency also depends on the vessel temperature, purge flow rate, and analyte volatility. Therefore, a small tube containing a gram of polymer for solvent residue analysis might be purged at 150 C for 20 minutes, but a one liter vessel containing a computer part purged at 85 C for lubricants or plasticizers might require several hours. Figure 1 shows the chromatogram of the volatiles purged from a small circuit board purged at 100 C for 20 minutes. Under these conditions, the analytes collected were mostly aromatics, including benzene, toluene, xylenes, benzaldehyde, and dichlorobenzene. The GC was equipped with a 5% phenyl methyl silicone column programmed from 40 to 300 at 10 C/minute. The chromatogram in Figure 2 was produced by heating a copper wire coil from a disk drive assembly. The coil was heated to 200 C for 20 minutes and purged at 100 ml/minute to the trap. The insulation used on the wire released many organics under these conditions, including acetic acid, phenolics, and hydrocarbons. When a homopolymer is pyrolyzed, bonds holding the macromolecule together are broken and smaller molecules are produced. For many polymers, including polystyrene 8, polyolefins 9 such as polyethylene, and acrylates such as butyl acrylate, these molecular fragments include the monomer and larger oligomers, such as dimer, trimer, etc. This is represented in the diagram in Figure 3, where M represents a monomer of the polymer, M-M a dimmer, and so on. As the bonds break, and the fragments produced are small enough to be volatile; they leave the polymer matrix and are carried to the GC. Figure 1. Dynamic headspace analysis of a printed circuit board at 100 C.

3 JPACSM 102 Figure 2. Dynamic headspace analysis of disk drive coil at 200 C. Figure 3. Bond breaking during pyrolysis of a homopolymer. There are several different types of pyrolyzers for GC, but sample preparation is similar for all of them. Since the whole sample may be vaporized and transferred to the GC column, the sample must be quite small, usually µg. This small piece is either placed directly onto the heater of the pyrolyzer or into a small sample holder, such as a quartz tube, which is inserted into the pyrolyzer. The pyrolysis instrument is then connected to the GC at the injection port and the sample heated very rapidly as the GC program begins. A typical pyrolysis program takes only a few seconds, so no GC programming changes are needed when converting from syringe injection to pyrolysis injection. Figure 4 shows the range of products made when polyethylene is heated to 750 C. Since polyethylene is just a long hydrocarbon chain, the products made are shorter hydrocarbons, including an alkane, an alkene, and a diene of each possible carbon chain length. The largest peak at nine minutes is decene, which is between smaller peaks for decane and decadiene. Similarly, polypropylene generates oligomers, but since the polypropylene chain has a methyl group on alternate carbons, the oligomers are branched and the pattern is distinct from that for polyethylene. Figure 5 shows the pyrogram for polypropylene, in which the tallest peak (at five minutes) is the trimer of propylene, dimethyl heptene. When pyrolyzing copolymers, the results are greatly dependent on whether the sample is a random or a block copolymer. In a random copolymer, the two monomers will exist as part of the same polymer chain, whereas for a block copolymer (or a polymer blend) there are regions of one homopolymer mixed with regions of the other. This is shown diagrammatically in Figure 6 for a styrene/butyl acrylate copolymer, in which S stands for the styrene monomer and B stands for a molecule of butyl acrylate.

4 JPACSM 103 Process Analytical Chemistry Figure 4. Pryogram of polyethylene, showing triplets (diene, alkene, and alkane) for oligomers, each set of which is longer by CH 2 than the one eluting just before it. Figure 5. Pyrogram of polypropylene at 750 C.

5 JPACSM 104 Figure 6. Random (top) and block (bottom) diagram of styrene/butyl acrylate copolymer. When pyrolyzed, both the random and block copolymers of styrene and butyl acrylate produce monomer, and larger oligomers, with the differences shown in Figure 7. The block copolymer for example, will produce only the trimers S-S-S and B-B-B, but the random copolymer can produce these plus the mixed trimers such as S-S-B, S-B-B, B-S-B, and many others. Consequently, a pyrogram of a random copolymer will be more complex, showing more peaks for dimers and trimers than the corresponding block copolymer. Figure 8 is a pyrogram of a random copolymer of styrene and butyl acrylate, showing multiple peaks for the trimers as well as peaks for both monomers and the dimers. Other copolymers show the same behavior. In the case of ethylene/propylene copolymers, the effect is clear. For block copolymers, oligomers of just the polypropylene and polyethylene are seen in the pyrogram, and the chromatogram looks like a pyrogram of polyethylene superimposed onto a pyrogram of polypropylene. For a random copolymer however, there are so many possible isomers for higher oligomers that the pyrogram is a complex mix of hydrocarbons, which does not resemble polyethylene or polypropylene. In Figure 9 a section of the pyrogram from 20 to 30 minutes for all four polyolefins is shown. The block copolymer clearly shows peaks from both the pure polypropylene and the pure polyethylene, but the random copolymer is much more complex, since there are so many possible combinations of ethylene and propylene monomers to make these oligomers. Figure 7. Formation of oligomers during pyrolysis of random (top) and block (bottom) copolymers.

6 JPACSM 105 Figure 8. Pyrogram of a random copolymer of styrene and butyl acrylate. Figure 9. Polyethylene, polypropylene, block and random copolymers. Partial programs from 20 to 30 minutes.

7 JPACSM 106 CONCLUSIONS The power of a GC/MS system to separate and identify organic compounds may easily be applied to the study of macromolecular systems even though the sample materials are comprised of molecules too large for normal gas chromatography. Careful control of the sample temperature permits the isolation and analysis of adsorbed or occluded small molecules in the polymer matrix, facilitating the analysis of residual solvents, monomers, and additives without the use of solvents. Application of more thermal energy causes pyrolysis of the macromolecule, generating fragments which may be studied by GC/MS. These fragments retain structural and compositional information about the polymer and allow not only the identification of the polymer, but analysis of copolymers, molecular structure, tacticity, and the randomness of copolymers. REFERENCES 1. T.A. Bellar, et al., J. Am. Water Works Assoc., 66, 703 (1974). 2. D.H. Ahlstrom, et al., Anal. Chem, 47, 1411 (1975). 3., LC.GC, 16, 9, 812 (1998). 4., J. Chrom. A, 842, 207 (1999). 5. D.L. Zoller et al., Anal. Chem., 71, 866 (1999). 6. F.C-Y. Wang, Anal. Chem., 71, 4776 (1999). 7. C.M. Cuppett, et al., LC.GC, 17, 6, 532 (1999). 8. F.C-Y. Wang and P. B. Smith, Anal. Chem., 68, 3033 (1996). 9. M. Phair, Rubber World, 215, 5, 30 (1997).