Direct Current Glow Discharge Mass Spectrometryfor 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 hasbeen used for a novel application, the sputtering andsubsequent analysis of polymers. This was made possibleby the application of a secondary cathode, a tantalumdiaphragm placed in front of the nonconducting sample.Different types of polymers were measured (polytetra-fluoroethylene, polycarbonate, and poly(vinyl chloride)).Important to note is that the mass spectra obtained arepredominantly characterized as atomic, a major differencefrom the radio frequency GDMS spectra of polymersreported earlier. This facilitates quantitative elementalanalysis for several reasons.

Characterization of polymers has been focused predominantlyon structural aspects or organic constituents. For these purposes,techniques such as infrared and Raman spectroscopy, nuclearmagnetic resonance, gas and gel permeation chromatography, andlight and electron microscopy are mostly used.1-4 However, inparticular cases, elemental analysis should be considered as well.Polymers are used, e.g., in surgery, in alimentary packing, or asrecipients for ultrapure chemicals. In these applications, theadditives in the polymers can play a dominant role in thesuccessful 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 toheat or UV radiation (e.g., Ba, Ca, Cd, Mg, Pb, Sn, Zn).5 Thesecomponents are added in concentrations usually ranging from ppmto percentage levels. Qualitative and quantitative determinationof the additives can be most important from the health andenvironmental points of view.

Different techniques are currently being used for the elementalanalysis of plastic materials. A general overview can be found inthe literature.6 Wet chemical techniques like classical atomicabsorption spectrometry (AAS)7,8 and inductively coupled plasma

(ICP) optical emission and mass spectrometry (OES and MS)9

are mostly invoked if high accuracy is required. Standardssolutions can then be used to create calibration graphs. Thenecessity of (time-consuming) digestion and the inherent chanceof contamination are, however, major disadvantages. Time-savingsolid sampling techniques have, therefore, also been used for theanalysis of polymers: destructive techniques like graphite furnaceAAS10 and laser ablation ICPOES5 and ICPMS11 and nondestruc-tive ones like instrumental neutron activation (INAA)8 and X-rayfluorescence (XRF).8 Of all these techniques, XRF is mostly usedas a routine tool for production control because of its ability toprovide fast and semiquantitative results. It suffers, however, froma rather low sensitivity, resulting in limits of detection between 1and 100 ppm, whereas limits of detection in the low ppm and sub-ppm ranges have been reported for the other techniques.

The use of sputter-based mass spectrometric techniques forthe characterization of polymers has also been described in theliterature, e.g., secondary ion (SIMS)12-14 and glow discharge(GDMS) mass spectrometries.15 The resulting mass spectra arepredominantly formed by molecular species (revealing structuralinformation), but elemental information can, in principle, also beacquired. For example, recently Teflon (polytetrafluoroethylene,PTFE) has been measured by radio frequency (rf) glow dischargemass spectrometry.15 In that case, the major peaks in the massspectrum are CF+, CF2

+, CF3+, C2F5

+, etc. Moreover, because ofthe sputter-ablation, these technique can also be used for depth-profiling purposes.

In the present study, direct current (dc) GDMS has been usedfor the direct (i.e., without a digestion step) sputtering andcharacterization of polymers. Because the concept of a glowdischarge, in which the sample acts as a cathode in a low-pressuredischarge, seems to preclude the analysis of nonconductingmaterials, a specific approach was needed: the so-called secondarycathode technique makes it possible to analyze insulators with adc discharge by applying a conducting diaphragm (the secondarycathode) in front of the nonconducting sample.16 Due to thecontinuous, in situ sputter-redeposition of the secondary cathode

(1) Bark, L. S., Allen, N. S., Eds. Analysis of Polymer Systems; Applied SciencePubl.: London, 1982.

(2) Williams, E. A. Polymer molecular structure determination. In MaterialsScience and Technology, Characterization of Materials; Lifshin, E., Ed.; VCHPubl.: 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.; Gortz,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, 305R-

314R.(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.

(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) Vollkopf, U.; Lehmann, R.; Weber, D. J. Anal. At. Spectrom. 1987, 2, 455-458.

(11) Marshall, J.; Franks, F.; Abell, I.; Tye, C. J. Anal. At. Spectrom. 1991, 6,145-150.

(12) Feld, H.; Leute, A.; Zurmuhlen, 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.

Anal. Chem. 1997, 69, 2931-2934

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atoms, a thin conducting layer is formed on the sample; penetra-tion of the bombarding ions (and, indirectly, also of the fast atoms)through this conducting layer allows atomization of the underlyingnonconducting sample. The major advantages of this secondarycathode technique are its simplicity and low cost, combined withsatisfying analytical figures of merit. On the other hand, a possibleblank contribution and restricted discharge conditions for stableatomization of the nonconductor are disadvantageous.17-20 Thistechnique has previously been proven to be useful for the traceanalysis (sub-ppm limits of detection) of, e.g., Macor, nuclearmaterials, and ZrO2.16-21 In those cases, it could be considered avaluable alternative for rf GDMS, the technique that has nowbecome the main and probably most appealing approach forelemental (ultra)trace analysis of solid nonconducting materials.22-24

Although promising, the secondary cathode technique is not yetusable for solving analytical problems on a routine base. Themeasurement of a new type of matrix, in this case polymers, whichis completely different from those measured before (i.e., inorganicsamples) can, therefore, be seen as a challenge and an appealingnew application of dc GDMS.

EXPERIMENTAL SECTIONGlow Discharge Mass Spectrometer (GDMS). The GDMS

work reported in this study was performed with a VG9000 double-focusing glow discharge mass spectrometer (VG Elemental,Thermo Instruments, Winsford, England). This instrumentationhas already been described in detail elsewhere.25 A typicalworking resolution of 3500-4000 (5% peak height) has been used.The detection system consists of a combination of a Faraday cupand a Daly detector, providing a dynamic range of about 10 ordersof magnitude. The “new flat cell” 26 was used for all themeasurements. The cell was cryogenically cooled to reduce thebackground due to residual gases. The glow discharge wassupported by high-purity argon (Air Liquide, 99.9997%).

Secondary Ion Mass Spectrometry (SIMS). The compara-tive SIMS measurement was performed with a double-focusingCAMECA IMS3F mass spectrometer (Cameca, Paris, France),already described in detail elsewhere.27 Cs+ ions with a bombard-ing energy of 14.5 keV were used as primary ions (87 Na); theprimary ion beam was scanned over a raster of 20 × 20 µm2, and

the area analyzed had a diameter of 13.3 µm. A mass resolutionof about 300 has been used for the measurements.

Materials. For this methodological study, polymers of techni-cal quality have been used (i.e., no certified nor high-puritymaterials): polytetrafluoroethylene (PTFE, 0.5 mm thick; FluorSeals, Gummelo Del Monte, Italy), polycarbonate (PC, 0.5 mmthick; from a manufacture that wishes to remain anonymous), andpoly(vinyl chloride) (PVC, 1 mm thick; Goodfellow, Cambridge,UK). The secondary cathode used was made of 0.25 mm thicktantalum (Goodfellow).

RESULTS AND DISCUSSIONA 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 onthe characteristics of the atomization process with relevance tothe actual elemental analysis. As a consequence, this study doesnot aim at a fully analytical evaluation of dc GDMS for polymeranalysis, which would require a profound comparison with variousother analytical techniques (since no reliable polymer standardsfor elemental analysis are yet available). Instead, the goal of thisreport is to show to the analytical chemist a new potential use ofdc GDMS.

Three different types of materials were used to evaluate thesuitability of dc GDMS for the analysis of polymers: PTFE, PC,and PVC. Structural, electrical, and thermal characteristics arelisted in Table 1.28 For all these materials, operating conditionspreviously used for the atomization of glass with dc GDMS wereapplied.17 This implies a tantalum secondary cathode with a 4mm orifice, a 7.5 mm anode opening diameter (as determined bythe hole size in the sample holder front plate), a 0.5 mm thickTeflon spacer (between the anode and the secondary cathode)and a 3 mA/0.6 kV discharge. Successful atomization of the PTFEand the PC could be obtained. In the case of PVC, high matrixsignals 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 reasonfor these unwanted effects is the low upper working temperatureof PVC (see Table 1). The PVC melts as a consequence of theheating by the energetic particle bombardment. This could,indeed, also be noticed visually after sputtering. Liquid nitrogencooling is applied to the anode body (i.e., the discharge cell), andalthough it indirectly cools the sample as well, it could not preventthese effects. Therefore, other operating conditions were appliedto PVC, namely those used for the analysis of Macor with atantalum secondary cathode.19 The electrode configuration is thesame as that for the analysis of PTFE and PC, but the dischargeconditions differ significantly: a 0.6 mA/1.2 kV discharge is used.

(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.

(27) Benninghoven, A.; Rudenauer, F. G.; Werner, H. W. Secondary Ion MassSpectrometry, Basic Concepts, Instrumental Aspects, Applications and Trends.Chemical Analysis, Vol. 86; Wiley: New York, 1987; Chapter 4, pp 603-605.

(28) Goodfellow Cambridge Ltd., UK, Catalogue 1995/1996, pp 444-450.

Table 1. Bulk Polymers Measured and Their Electrical and Thermal Characteristics

repeating unit electrical resistivity (Ω‚cm) upper working temperature (°C)

polytetrafluoroethylene (PTFE) -CF2- 1018-1019 180-260poly(vinyl chloride) (PVC) -CH2CHCl- 1016 50-75polycarbonate (PC) -OC6H4C(CH3)2C6H4OCO- 1014-1016 115-130

2932 Analytical Chemistry, Vol. 69, No. 15, August 1, 1997

The input power (0.72 W) is 2.5 times lower than that for thePTFE and the PC analysis. In a first approximation, one can statethat the input heat is, therefore, also 2.5 times lower and thusthat 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 thata successful and particularly long-lasting measurement of the PVCcannot be guaranteed under the above-mentioned conditions,unlike for PTFE and PC. It seems that an even lower power hasto be used for the analysis of low-melting polymers (e.g., alsopolyamide, polyethylene, and polystyrene, all with an upperworking temperature of less than 100 °C).28 However, the (useful)data obtained for PVC are reported because they confirm the trendnoticed for PTFE and PC.

Stable and reproducible signals could be obtained for PTFEand PC over a long time range. As an example, Figure 1 showsthe raw signal intensities for some signals measured over a periodof more than 1 h for the analysis of PTFE. The stability of thetrace elements is represented by the course of the 56Fe+ signal(Fe has an estimated concentration of 100-500 ppm). Theprecision of the absolute signal intensities was for all measure-ments between 3 and 20% RSD. The obtained matrix signalintensity, represented by the 12C+ signal intensity (30-50% in thesample), was for the three different polymer samples between 1and 6 × 10-12 A. Extrapolation of this value toward the lowerconcentration levels reveals possible limits of detection in the lowppm and sub-ppm ranges. This is based on (1) a low backgroundfrom which peaks of (5 × 10-18 A can clearly be distinguished,for uninterfered isotopes, if integration times of (200 ms/channelare used and (2) the rather uniform elemental sensitivity, typicalfor GDMS, which allows more or less generalization of the data.

However, besides the sensitivity, also the background shouldbe considered when discussing limits of detection. This back-ground can be caused either by the blank contribution due tothe sputtering of the secondary cathode, plasma, or residual gasspecies or by the sample-based molecular clusters. The secondarycathode contribution is important only for specific elements forwhich the impurity level in the tantalum is higher than, e.g., 100ppb. Concentrations of the impurities in the tantalum used havealready been reported.20 Also, the weight factor of this contribu-tion should be taken into account.17 This factor, represented bythe ratio between the signal intensity of the secondary cathodeand the total signal intensity of the sample, was found to be

between 3 and 15. More specific details concerning the influenceof the secondary cathode contribution have been discussedpreviously.19 The presence of plasma gas species, like Ar2+ orAr2

+, or mass peaks due to residual gases, like O2+ or H3O+, is

not typical for polymer analysis and is, therefore, not discussedin this study. The presence of molecular clusters due to thesputtering of the sample is, however, a problem that seemsinherent to polymer analysis, and it is, therefore investigated morethoroughly.

The relative abundances of the most important clustersmeasured during the sputtering of the three different polymersare represented in Table 2. These values are calculated as a ratiobetween the cluster signal intensity (X+) and the sum of theelemental matrix signal intensities (i.e., X+/(C+ + F+) for PTFE;X+/(C+ + Cl+) for PVC; X+/(C+ + O+) for PC). It has previouslybeen demonstrated for nonmetallic samples that the majority ofsample-based clusters seen in the spectra are a result of the sputterprocess 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 majorcause for the presence of clusters in dc GDMS analysis ofpolymers. This is illustrated by the fact that, for example, theabundance of C2F4

+ (a double-repeating polymer unit) is signifi-cantly higher than that of C2F2

+, a cluster with fewer atoms.It should also be noticed that the mass spectrum is predomi-

nantly characterized as atomic; thus, the contribution of thesample-based clusters in the spectrum is rather low. This seemsto be a major point of difference in comparison with the spectrarecently published for polymer analysis with rf GDMS. In thatcase, the molecular cluster peaks formed a fingerprint that couldbe used to characterize and distinguish different polymers. The

Table 2. Average Abundances of the Main ClustersPresent in the Mass Spectrum of Varying Polymers,Relative to the Elemental Matrix Signal Intensitiesa

PTFE PVC PC24C2

+ 1.8 × 10-3 13CH+ 4.9 × 10-2 13CH+ 1.3 × 10-2

31CF+ 3.4 × 10-2 24C2+ 3.3 × 10-3 24C2

+ 1.2 × 10-3

43C2F+ 2.4 × 10-5 47CCl+ 6.3 × 10-4 28CO+ 3.2 × 10-3

50CF2+ 2.4 × 10-3 59C2Cl+ 1.0 × 10-5 44CO2

+ 1.6 × 10-4

62C2F2+ 7.0 × 10-5

100C2F4+ 1.3 × 10-3

a For Teflon, relative to C+ + F+; for poly(vinyl chloride), relativeto C+ + Cl+; for polycarbonate, relative to C+ + O+.

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 fordiscussion.

Analytical Chemistry, Vol. 69, No. 15, August 1, 1997 2933

elemental contribution in the rf GDMS spectra reported was,however, at first sight of minor importance. For example, theanalysis of a copolymer composed of PTFE and perfluoromethylvinyl ether resulted in a mass spectrum in which the signalintensity 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 characterof the spectrum. It is suggested that less energy is available toliberate large fragments in the sputtering process if a lowerdischarge power is applied. In the case of the rf GDMS study,20 W rf power was applied (which is not necessarily the powerthat reaches the sample surface); in the present dc GDMS study,less than 2 W is applied. This can explain why the dc GDMSspectra have a much more elemental character.

The rather elemental nature of the spectra seems to make dcGDMS less suitable for a general characterization of the polymermaterial but facilitates, on the other hand, conceptually, quantita-tive elemental analysis, for several reasons. First, it is clear thatcluster peaks can interfere with signals from elements selectedfor analysis; for example, the 31CF+ peak interferes with themonoisotopic 31P+. These two peaks can be separated with a massresolution of 1260, but the tailing certainly increases the back-ground. Moreover, when using a dual detection system (as isthe case for the VG9000), the high signal intensity of CF+ willautomatically force the instrumentation to use the Faraday cup(for high ion currents, >10-13 A) to protect the Daly detector (forlow ion currents) from an overload. Second, the procedure ofquantification is more straightforward if the matrix signals areatomic. It has been proven for dc GDMS analysis of ceramicmaterials by applying a secondary cathode that the use of thematrix peak as an internal standard is a valuable tool to obtainacceptable semiquantitative results.19,20 This is especially impor-tant if no reference material nor trace elements with knownconcentrations are available. For PTFE (in which carbon ispresent for 24% w/w), raw concentrations of a trace element Xcan, e.g., be calculated according to one of the following formulas:

It is obvious that this simple way of estimating the concentration

of the matrix elements is useful only if other (molecular) matrixpeaks are negligible. In this approach, a uniform sensitivity isassumed. A more accurate approach would imply the use ofrelative sensitivity factors29,30 or even calibration graphs.20 In bothcases, reliable polymer standards, in which trace elements arecertified, are needed. Up to now, no generally accepted polymerstandards are known for this purpose. This makes, however,GDMS (dc and rf) even more attractive as a possible tool forpolymer analysis, because of its uniform elemental response and,as a consequence, its satisfying standard-free results, as provenpreviously for other materials.19,20

CONCLUSIONDirect current GDMS has successfully been applied to sput-

tering of varying polymers by means of the secondary cathodetechnique. There are certain drawbacks in comparison to rfGDMS: the blank contribution due to the sputtering of thetantalum diaphragm and the restricted discharge conditions arethe most obvious ones. There are, on the other hand, also someadvantages of dc GDMS in comparison to the rf approach. Theobtained sample signal intensities (both absolute and relative) donot depend on the thickness of the polymer sample, as is the casefor rf GDMS. Moreover, the much more elemental nature of thedc GDMS mass spectrum facilitates quantitative elemental analy-sis. This seems to be a consequence of the low power applied tothe polymer sample: less than 2 W dc power, compared to 20 Wrf power. These dc GDMS results may, therefore, also be usefulto optimize rf GDMS for elemental analysis of polymers. Gener-ally, one can state that the sputtering of polymers, as demonstratedin this work, can mean a potential increase of the range ofapplications of dc GDMS and that this technique can be comple-mentary to the existing alternatives.

ACKNOWLEDGMENTW.S. acknowledges financial support from the Flemish “Insti-

tuut ter Bevordering van het Wetenschappelijk-TechnologischOnderzoek in de Industrie” (IWT). The authors thank C. Feraugefor performing the SIMS measurement and the handling andinterpretation of the SIMS data and K. De Cauwsemaecker andR. Saelens for technical support.

Received for review February 13, 1997. Accepted May 19,1997.X

AC970186T(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. X Abstract published in Advance ACS Abstracts, July 1, 1997.

concn X(%) ) signal intensity Xsignal intensity C

× 24%

concn X(%) )signal intensity X

signal intensity C + signal intensity F× 100%

2934 Analytical Chemistry, Vol. 69, No. 15, August 1, 1997