Anal. Chem. 1997, 69, 2931-2934
Direct Current Glow Discharge Mass Spectrometry for Elemental Characterization of Polymers Wim Schelles and Rene´ Van Grieken*
Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerpen, Belgium
A direct current glow discharge mass spectrometer has been used for a novel application, the sputtering and subsequent analysis of polymers. This was made possible by the application of a secondary cathode, a tantalum diaphragm placed in front of the nonconducting sample. Different types of polymers were measured (polytetrafluoroethylene, polycarbonate, and poly(vinyl chloride)). Important to note is that the mass spectra obtained are predominantly characterized as atomic, a major difference from the radio frequency GDMS spectra of polymers reported earlier. This facilitates quantitative elemental analysis for several reasons. Characterization of polymers has been focused predominantly on structural aspects or organic constituents. For these purposes, techniques such as infrared and Raman spectroscopy, nuclear magnetic resonance, gas and gel permeation chromatography, and light and electron microscopy are mostly used.1-4 However, in particular cases, elemental analysis should be considered as well. Polymers are used, e.g., in surgery, in alimentary packing, or as recipients for ultrapure chemicals. In these applications, the additives in the polymers can play a dominant role in the successful and safe use of the material. These (inorganic) additives include color pigments, plasticizers to provide flexibility, and stabilizers to protect the polymer from degradation due to heat or UV radiation (e.g., Ba, Ca, Cd, Mg, Pb, Sn, Zn).5 These components are added in concentrations usually ranging from ppm to percentage levels. Qualitative and quantitative determination of the additives can be most important from the health and environmental points of view. Different techniques are currently being used for the elemental analysis of plastic materials. A general overview can be found in the literature.6 Wet chemical techniques like classical atomic absorption spectrometry (AAS)7,8 and inductively coupled plasma (1) Bark, L. S., Allen, N. S., Eds. Analysis of Polymer Systems; Applied Science Publ.: London, 1982. (2) Williams, E. A. Polymer molecular structure determination. In Materials Science and Technology, Characterization of Materials; Lifshin, E., Ed.; VCH Publ.: New York, 1992; Chapter 10. (3) Campana, J. E.; Sheng, L.; Shew, S. L.; Winger, B. E. Trends Anal. Chem. 1994, 13, 239-247. (4) Ortner, H. M.; Xu, H. H.; Dahmen, J.; Englert, K.; Opfermann, H.; Go¨rtz, W. Fresenius’ J. Anal. Chem. 1995, 355, 657-664. (5) Hemmerlin, M.; Mermet, J. M. Spectrochim. Acta 1996, 51B, 579-589. (6) Marshall, J.; Carroll, J.; Crighton, J. S. J. Anal. At. Spectrom. 1991, 6, 305R314R. (7) Belarra, M. A.; Azofra, M. C.; Anzano, J. M.; Castillo, J. R. J. Anal. At. Spectrom. 1988, 3, 591-593. (8) Hoffmann, P.; Paller, G.; Thybusch, B.; Stingl, U. Fresenius’ J. Anal. Chem. 1991, 339, 230-234. S0003-2700(97)00186-8 CCC: $14.00
© 1997 American Chemical Society
(ICP) optical emission and mass spectrometry (OES and MS)9 are mostly invoked if high accuracy is required. Standards solutions can then be used to create calibration graphs. The necessity of (time-consuming) digestion and the inherent chance of contamination are, however, major disadvantages. Time-saving solid sampling techniques have, therefore, also been used for the analysis of polymers: destructive techniques like graphite furnace AAS10 and laser ablation ICPOES5 and ICPMS11 and nondestructive ones like instrumental neutron activation (INAA)8 and X-ray fluorescence (XRF).8 Of all these techniques, XRF is mostly used as a routine tool for production control because of its ability to provide fast and semiquantitative results. It suffers, however, from a rather low sensitivity, resulting in limits of detection between 1 and 100 ppm, whereas limits of detection in the low ppm and subppm ranges have been reported for the other techniques. The use of sputter-based mass spectrometric techniques for the characterization of polymers has also been described in the literature, e.g., secondary ion (SIMS)12-14 and glow discharge (GDMS) mass spectrometries.15 The resulting mass spectra are predominantly formed by molecular species (revealing structural information), but elemental information can, in principle, also be acquired. For example, recently Teflon (polytetrafluoroethylene, PTFE) has been measured by radio frequency (rf) glow discharge mass spectrometry.15 In that case, the major peaks in the mass spectrum are CF+, CF2+, CF3+, C2F5+, etc. Moreover, because of the sputter-ablation, these technique can also be used for depthprofiling purposes. In the present study, direct current (dc) GDMS has been used for the direct (i.e., without a digestion step) sputtering and characterization of polymers. Because the concept of a glow discharge, in which the sample acts as a cathode in a low-pressure discharge, seems to preclude the analysis of nonconducting materials, a specific approach was needed: the so-called secondary cathode technique makes it possible to analyze insulators with a dc discharge by applying a conducting diaphragm (the secondary cathode) in front of the nonconducting sample.16 Due to the continuous, in situ sputter-redeposition of the secondary cathode (9) Fordham, P. J.; Gramshaw, J. W.; Castle, L.; Crews, H. M.; Thompson, D.; Parry, S. J.; McCurdy, E. J. Anal. At. Spectrom. 1995, 10, 303-309. (10) Vo ¨llkopf, U.; Lehmann, R.; Weber, D. J. Anal. At. Spectrom. 1987, 2, 455458. (11) Marshall, J.; Franks, F.; Abell, I.; Tye, C. J. Anal. At. Spectrom. 1991, 6, 145-150. (12) Feld, H.; Leute, A.; Zurmu ¨ hlen, R.; Benninghoven, A. Anal. Chem. 1991, 63, 903-910. (13) Hercules, D. M. Mikrochim. Acta (Wien) Suppl. 1985, 11, 1-27. (14) Briggs, D. Static SIMSsSurface Analysis of Organic Materials. In Practical Surface Analysis, Vol. 2, Ion and Neutral Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: New York, 1992; Chapter 7. (15) Shick, C. R., Jr.; DePalma, P. A., Jr.; Marcus, R. K. Anal. Chem. 1996, 68, 2113-2121. (16) Milton, D. M. P.; Hutton, R. C. Spectrochim. Acta 1993, 48B, 39-52.
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Table 1. Bulk Polymers Measured and Their Electrical and Thermal Characteristics
polytetrafluoroethylene (PTFE) poly(vinyl chloride) (PVC) polycarbonate (PC)
repeating unit
electrical resistivity (Ω‚cm)
upper working temperature (°C)
-CF2-CH2CHCl-OC6H4C(CH3)2C6H4OCO-
1018-1019 1016 1014-1016
180-260 50-75 115-130
atoms, a thin conducting layer is formed on the sample; penetration of the bombarding ions (and, indirectly, also of the fast atoms) through this conducting layer allows atomization of the underlying nonconducting sample. The major advantages of this secondary cathode technique are its simplicity and low cost, combined with satisfying analytical figures of merit. On the other hand, a possible blank contribution and restricted discharge conditions for stable atomization of the nonconductor are disadvantageous.17-20 This technique has previously been proven to be useful for the trace analysis (sub-ppm limits of detection) of, e.g., Macor, nuclear materials, and ZrO2.16-21 In those cases, it could be considered a valuable alternative for rf GDMS, the technique that has now become the main and probably most appealing approach for elemental (ultra)trace analysis of solid nonconducting materials.22-24 Although promising, the secondary cathode technique is not yet usable for solving analytical problems on a routine base. The measurement of a new type of matrix, in this case polymers, which is completely different from those measured before (i.e., inorganic samples) can, therefore, be seen as a challenge and an appealing new application of dc GDMS. EXPERIMENTAL SECTION Glow Discharge Mass Spectrometer (GDMS). The GDMS work reported in this study was performed with a VG9000 doublefocusing glow discharge mass spectrometer (VG Elemental, Thermo Instruments, Winsford, England). This instrumentation has already been described in detail elsewhere.25 A typical working resolution of 3500-4000 (5% peak height) has been used. The detection system consists of a combination of a Faraday cup and a Daly detector, providing a dynamic range of about 10 orders of magnitude. The “new flat cell” 26 was used for all the measurements. The cell was cryogenically cooled to reduce the background due to residual gases. The glow discharge was supported by high-purity argon (Air Liquide, 99.9997%). Secondary Ion Mass Spectrometry (SIMS). The comparative SIMS measurement was performed with a double-focusing CAMECA IMS3F mass spectrometer (Cameca, Paris, France), already described in detail elsewhere.27 Cs+ ions with a bombarding energy of 14.5 keV were used as primary ions (87 Na); the primary ion beam was scanned over a raster of 20 × 20 µm2, and (17) Schelles, W.; De Gendt, S.; Muller, V.; Van Grieken, R. Appl. Spectrosc. 1995, 49, 939-944. (18) Schelles, W.; De Gendt, S.; Maes, K.; Van Grieken, R. Fresenius’ J. Anal. Chem. 1996, 355, 858-860. (19) Schelles, W.; Van Grieken, R. Anal. Chem. 1996, 68, 3570-3574. (20) Schelles, W.; Van Grieken, R. J. Anal. At. Spectrom. 1997, 12, 49-52. (21) Betti, M.; Rasmussen, G.; Koch, L. Fresenius’ J. Anal. Chem. 1996, 355, 808-812. (22) Duckworth, D. C.; Marcus, R. K. Anal. Chem. 1989, 61, 1879-1886. (23) Duckworth, D. C.; Donohue, D. L.; Smith, D. H.; Lewis, T. A.; Marcus, R. K. Anal. Chem. 1993, 65, 2478-2484. (24) Marcus, R. K.; Harville, T. R.; Mei, Y.; Shick, C. R., Jr. Anal. Chem. 1994, 66, 902A-911A. (25) Robinson, K.; Nayler, R. Eur. Spectrosc. News 1986, 68, 18-22. (26) van Straaten, M.; Gijbels, R.; Vertes, A. Anal. Chem. 1992, 64, 1855-1863.
2932 Analytical Chemistry, Vol. 69, No. 15, August 1, 1997
the area analyzed had a diameter of 13.3 µm. A mass resolution of about 300 has been used for the measurements. Materials. For this methodological study, polymers of technical quality have been used (i.e., no certified nor high-purity materials): polytetrafluoroethylene (PTFE, 0.5 mm thick; Fluor Seals, Gummelo Del Monte, Italy), polycarbonate (PC, 0.5 mm thick; from a manufacture that wishes to remain anonymous), and poly(vinyl chloride) (PVC, 1 mm thick; Goodfellow, Cambridge, UK). The secondary cathode used was made of 0.25 mm thick tantalum (Goodfellow). RESULTS AND DISCUSSION A first condition to perform an elemental analysis of polymers with dc GDMS is to sputter-atomize the material in a stable way. Because this has not been reported before, we will focus here on the characteristics of the atomization process with relevance to the actual elemental analysis. As a consequence, this study does not aim at a fully analytical evaluation of dc GDMS for polymer analysis, which would require a profound comparison with various other analytical techniques (since no reliable polymer standards for elemental analysis are yet available). Instead, the goal of this report is to show to the analytical chemist a new potential use of dc GDMS. Three different types of materials were used to evaluate the suitability of dc GDMS for the analysis of polymers: PTFE, PC, and PVC. Structural, electrical, and thermal characteristics are listed in Table 1.28 For all these materials, operating conditions previously used for the atomization of glass with dc GDMS were applied.17 This implies a tantalum secondary cathode with a 4 mm orifice, a 7.5 mm anode opening diameter (as determined by the hole size in the sample holder front plate), a 0.5 mm thick Teflon spacer (between the anode and the secondary cathode) and a 3 mA/0.6 kV discharge. Successful atomization of the PTFE and the PC could be obtained. In the case of PVC, high matrix signals could be measured, but they decreased quite rapidly. Moreover, discharge instabilities were often noticed shortly (i.e., 10-20 min) after ignition of the discharge. The obvious reason for these unwanted effects is the low upper working temperature of PVC (see Table 1). The PVC melts as a consequence of the heating by the energetic particle bombardment. This could, indeed, also be noticed visually after sputtering. Liquid nitrogen cooling is applied to the anode body (i.e., the discharge cell), and although it indirectly cools the sample as well, it could not prevent these effects. Therefore, other operating conditions were applied to PVC, namely those used for the analysis of Macor with a tantalum secondary cathode.19 The electrode configuration is the same as that for the analysis of PTFE and PC, but the discharge conditions differ significantly: a 0.6 mA/1.2 kV discharge is used. (27) Benninghoven, A.; Ru ¨ denauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry, Basic Concepts, Instrumental Aspects, Applications and Trends. Chemical Analysis, Vol. 86; Wiley: New York, 1987; Chapter 4, pp 603605. (28) Goodfellow Cambridge Ltd., UK, Catalogue 1995/1996, pp 444-450.
Figure 1. Raw signal intensities for some signals measured over >1 h for the analysis of PTFE. 9, C+; 2, F+; ×, CF+; 0, Fe+. See text for discussion.
The input power (0.72 W) is 2.5 times lower than that for the PTFE and the PC analysis. In a first approximation, one can state that the input heat is, therefore, also 2.5 times lower and thus that the increase in sample temperature is 2.5 times lower as well. Under these “soft” conditions, PVC could successfully be analyzed, but the atomization remained critical. In practice, this means that a successful and particularly long-lasting measurement of the PVC cannot be guaranteed under the above-mentioned conditions, unlike for PTFE and PC. It seems that an even lower power has to be used for the analysis of low-melting polymers (e.g., also polyamide, polyethylene, and polystyrene, all with an upper working temperature of less than 100 °C).28 However, the (useful) data obtained for PVC are reported because they confirm the trend noticed for PTFE and PC. Stable and reproducible signals could be obtained for PTFE and PC over a long time range. As an example, Figure 1 shows the raw signal intensities for some signals measured over a period of more than 1 h for the analysis of PTFE. The stability of the trace elements is represented by the course of the 56Fe+ signal (Fe has an estimated concentration of 100-500 ppm). The precision of the absolute signal intensities was for all measurements between 3 and 20% RSD. The obtained matrix signal intensity, represented by the 12C+ signal intensity (30-50% in the sample), was for the three different polymer samples between 1 and 6 × 10-12 A. Extrapolation of this value toward the lower concentration levels reveals possible limits of detection in the low ppm and sub-ppm ranges. This is based on (1) a low background from which peaks of (5 × 10-18 A can clearly be distinguished, for uninterfered isotopes, if integration times of (200 ms/channel are used and (2) the rather uniform elemental sensitivity, typical for GDMS, which allows more or less generalization of the data. However, besides the sensitivity, also the background should be considered when discussing limits of detection. This background can be caused either by the blank contribution due to the sputtering of the secondary cathode, plasma, or residual gas species or by the sample-based molecular clusters. The secondary cathode contribution is important only for specific elements for which the impurity level in the tantalum is higher than, e.g., 100 ppb. Concentrations of the impurities in the tantalum used have already been reported.20 Also, the weight factor of this contribution should be taken into account.17 This factor, represented by the ratio between the signal intensity of the secondary cathode and the total signal intensity of the sample, was found to be
Table 2. Average Abundances of the Main Clusters Present in the Mass Spectrum of Varying Polymers, Relative to the Elemental Matrix Signal Intensitiesa PTFE 24C + 2 31CF+ 43C
+ 2F 50CF + 2 62C F + 2 2 100C F + 2 4
1.8 × 10-3 3.4 × 10-2 2.4 × 10-5 2.4 × 10-3 7.0 × 10-5 1.3 × 10-3
PVC 13CH+ 24C + 2 47CCl+ 59C
2Cl
+
4.9 × 10-2 3.3 × 10-3 6.3 × 10-4 1.0 × 10-5
PC 13CH+ 24C + 2 28CO+ 44CO + 2
1.3 × 10-2 1.2 × 10-3 3.2 × 10-3 1.6 × 10-4
a For Teflon, relative to C+ + F+; for poly(vinyl chloride), relative to C+ + Cl+; for polycarbonate, relative to C+ + O+.
between 3 and 15. More specific details concerning the influence of the secondary cathode contribution have been discussed previously.19 The presence of plasma gas species, like Ar2+ or Ar2+, or mass peaks due to residual gases, like O2+ or H3O+, is not typical for polymer analysis and is, therefore, not discussed in this study. The presence of molecular clusters due to the sputtering of the sample is, however, a problem that seems inherent to polymer analysis, and it is, therefore investigated more thoroughly. The relative abundances of the most important clusters measured during the sputtering of the three different polymers are represented in Table 2. These values are calculated as a ratio between the cluster signal intensity (X+) and the sum of the elemental matrix signal intensities (i.e., X+/(C+ + F+) for PTFE; X+/(C+ + Cl+) for PVC; X+/(C+ + O+) for PC). It has previously been demonstrated for nonmetallic samples that the majority of sample-based clusters seen in the spectra are a result of the sputter process rather than of recombination processes in the plasma.19,20 It can be seen from Table 2 that this is also likely to be the major cause for the presence of clusters in dc GDMS analysis of polymers. This is illustrated by the fact that, for example, the abundance of C2F4+ (a double-repeating polymer unit) is significantly higher than that of C2F2+, a cluster with fewer atoms. It should also be noticed that the mass spectrum is predominantly characterized as atomic; thus, the contribution of the sample-based clusters in the spectrum is rather low. This seems to be a major point of difference in comparison with the spectra recently published for polymer analysis with rf GDMS. In that case, the molecular cluster peaks formed a fingerprint that could be used to characterize and distinguish different polymers. The Analytical Chemistry, Vol. 69, No. 15, August 1, 1997
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elemental contribution in the rf GDMS spectra reported was, however, at first sight of minor importance. For example, the analysis of a copolymer composed of PTFE and perfluoromethyl vinyl ether resulted in a mass spectrum in which the signal intensity of fragments like CF2+, C3F3+, C2F5+, and C3F5+ was a factor of 7-15 higher than that of C+. In the rf GDMS study,15 the discharge power is responsible for the molecular character of the spectrum. It is suggested that less energy is available to liberate large fragments in the sputtering process if a lower discharge power is applied. In the case of the rf GDMS study, 20 W rf power was applied (which is not necessarily the power that reaches the sample surface); in the present dc GDMS study, less than 2 W is applied. This can explain why the dc GDMS spectra have a much more elemental character. The rather elemental nature of the spectra seems to make dc GDMS less suitable for a general characterization of the polymer material but facilitates, on the other hand, conceptually, quantitative elemental analysis, for several reasons. First, it is clear that cluster peaks can interfere with signals from elements selected for analysis; for example, the 31CF+ peak interferes with the monoisotopic 31P+. These two peaks can be separated with a mass resolution of 1260, but the tailing certainly increases the background. Moreover, when using a dual detection system (as is the case for the VG9000), the high signal intensity of CF+ will automatically force the instrumentation to use the Faraday cup (for high ion currents, >10-13 A) to protect the Daly detector (for low ion currents) from an overload. Second, the procedure of quantification is more straightforward if the matrix signals are atomic. It has been proven for dc GDMS analysis of ceramic materials by applying a secondary cathode that the use of the matrix peak as an internal standard is a valuable tool to obtain acceptable semiquantitative results.19,20 This is especially important if no reference material nor trace elements with known concentrations are available. For PTFE (in which carbon is present for 24% w/w), raw concentrations of a trace element X can, e.g., be calculated according to one of the following formulas:
concn X(%) )
signal intensity X × 24% signal intensity C
concn X(%) ) signal intensity X × 100% signal intensity C + signal intensity F It is obvious that this simple way of estimating the concentration (29) Vieth, W.; Huneke, J. C. Spectrochim. Acta 1991, 46B, 137-153. (30) Schelles, W.; De Gendt, S.; Van Grieken, R. J. Anal. At. Spectrom. 1996, 11, 937-941.
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of the matrix elements is useful only if other (molecular) matrix peaks are negligible. In this approach, a uniform sensitivity is assumed. A more accurate approach would imply the use of relative sensitivity factors29,30 or even calibration graphs.20 In both cases, reliable polymer standards, in which trace elements are certified, are needed. Up to now, no generally accepted polymer standards are known for this purpose. This makes, however, GDMS (dc and rf) even more attractive as a possible tool for polymer analysis, because of its uniform elemental response and, as a consequence, its satisfying standard-free results, as proven previously for other materials.19,20
CONCLUSION Direct current GDMS has successfully been applied to sputtering of varying polymers by means of the secondary cathode technique. There are certain drawbacks in comparison to rf GDMS: the blank contribution due to the sputtering of the tantalum diaphragm and the restricted discharge conditions are the most obvious ones. There are, on the other hand, also some advantages of dc GDMS in comparison to the rf approach. The obtained sample signal intensities (both absolute and relative) do not depend on the thickness of the polymer sample, as is the case for rf GDMS. Moreover, the much more elemental nature of the dc GDMS mass spectrum facilitates quantitative elemental analysis. This seems to be a consequence of the low power applied to the polymer sample: less than 2 W dc power, compared to 20 W rf power. These dc GDMS results may, therefore, also be useful to optimize rf GDMS for elemental analysis of polymers. Generally, one can state that the sputtering of polymers, as demonstrated in this work, can mean a potential increase of the range of applications of dc GDMS and that this technique can be complementary to the existing alternatives.
ACKNOWLEDGMENT W.S. acknowledges financial support from the Flemish “Instituut ter Bevordering van het Wetenschappelijk-Technologisch Onderzoek in de Industrie” (IWT). The authors thank C. Ferauge for performing the SIMS measurement and the handling and interpretation of the SIMS data and K. De Cauwsemaecker and R. Saelens for technical support.
Received for review February 13, 1997. Accepted May 19, 1997.X AC970186T X
Abstract published in Advance ACS Abstracts, July 1, 1997.
