Anal. Chem. 1997, 69, 972-976
Evaluation of a Mirror-Polishing Technique for Fluorocarbon Polymer Surfaces for Reduction of Contamination from Containers Used in Ultratrace Analysis Miyuki Takenaka,* Masaru Hayashi, Isao Suzuki, Yuji Yamada, Koji Takamatsu, and Mokuji Kageyama
Research and Development Center, Toshiba Corporation, 1 Komukai, Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan
A mirror-polishing technique for fluorocarbon polymer surfaces using high-precision diamond cutting tools was developed. The goal of this technique was the reduction of ultratrace elemental analysis contamination levels of containers fabricated from such mirror-polished materials. Remarkably smooth inner surfaces with degrees of flatness of 0.1 µm peak-to-valley (PTV) for containers fabricated from mirror-polished PTFE materials were obtained, in contrast to degrees of surface flatness of more than 30 µm PTV for commercially available PTFE containers. (Here, PTV denotes the difference between the highest peak and deepest valley in a scanned area of 10 × 10 µm.) Extractable impurity levels for mirror-polished PTFE container surfaces were reduced by more than 1 order of magnitude relative to those of unpolished PTFE containers. The surface conditions of the PTFE containers were observed by atomic force and scanning electron microscopy. The microphotographs so obtained suggest that the degree of surface smoothness of the containers is proportional to their ultratrace metallic contamination levels.
Reduction of the blank levels of analytical procedures has long been an important consideration in analytical chemistry.1,2 Continuous improvements in the sensitivity of analytical techniques such as inductively coupled plasma mass spectrometry (ICPMS)3,4 have necessitated the mitigation of contamination levels in many ultratrace procedures.5-9 The contamination levels of containers fabricated from fluorocarbon polymers, such as polytetrafluoroethylene (PTFE), fluorinated ethylenepropylene (FEP), and perfluoroalkoxy (PFA), have been rendered sufficiently low for (1) Patterson, C. C.; Settle, D. M. Proceeding of the 7th IMR Symposium; LaFleur, P. D., Ed. Natl. Bur. Stand. (U.S.) Spec. Publ. 1976, 422, 321-351. (2) Moody, J. R.; Lindstorm, R. M. Anal. Chem. 1977, 49, 2264-2267. (3) Gray, A. L. Analyst (London) 1975, 100, 289-301. (4) Houk, R. S.; Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1980, 52, 2283-2297. (5) Paulsen, P. J.; Beary, E. S.; Bushee, D. S.; Moody, J. R. Anal. Chem. 1988, 60, 971-975. (6) Hutton, R. C.; Eaton, A. N. J. Anal. Atom. Spectrom. 1988, 3, 547-564. (7) Henshaw, J. M.; Heithmar, E. M.; Hinners, T. A. Anal. Chem. 1989, 61, 335-342. (8) Lindstrom, R. M.; Byrne, A. R.; Becker, D. A.; Smodis, B.; Garrity, K. M. Fresenius J. Anal. Chem. 1990, 338, 569-571. (9) Morley, N. H.; Statham, P. J.; Burton, J. D. Deep Sea Res. 1993, 40, 10431062.
972 Analytical Chemistry, Vol. 69, No. 5, March 1, 1997
most analytical procedures.10 However, extant contamination levels are too high for applications related to the production of materials requiring ultralow contamination levels.11-13 For example, picogram-level trace element contaminations are becoming increasingly detrimental to the operation of integrated silicon semiconductor devices.14-17 In general, the blank level of a given sample is affected by such factors as the purity of sample-treating solutions, container purity, and general analytical environment. There are several concepts pertinent to the reduction of blank levels. Among these are improvements in acid-washing processes for surfaces that come into contact with samples, control of the ambient analytical environment, and control of the purity of sample-treating solutions. These considerations have been thoroughly developed, and there is little prospect for further progress in these and related areas as has been demonstrated by Patterson and Settle.1 Assuming that contamination is related to the dissolution of metals in the fluorocarbon polymers, other factors pertinent to the mitigation of blank levels are reduction of the metallic concentration levels in the polymers and minimization of the contact area between sample and polymer surfaces. The aim of this investigation was to fabricate analytical containers from fluorocarbon polymers in a manner such that analytical blank levels were reduced by minimization of the contact area between sample and polymer surfaces. Such minimization was achieved by polishing the PTFE surfaces with high-precision diamond cutting tools. The dissolved metallic impurity concentrations in the polymer material removed by the mirror-polishing process were determined by electrothermal atomic absorption spectrometry (ETAAS) and ICPMS. In addition, the surface condition of the containers was verified by atomic force microscopy (AFM) and scanning electron microscopy (SEM). (10) Howard, A. G.; Statham, P. I. Inorganic Trace Analysis: Philosophy and Practice, 6th ed.; Wiley: Chichester, U.K., 1993; Chapter 2. (11) Gretzinger, K.; Kotz, L.; Tsho¨pel, P.; To¨lg, G. Talanta 1982, 29, 10111018. (12) Imbalzano, J. F.; Moody, J. R.; McKenzie, R. J. Solid State Technol. 1986, 35, 135-137. (13) Ortner, H. M.; Xu. H. H., Dahmen, J.; Englert, K. Fresenius J. Anal. Chem. 1996, 355, 657-664. (14) Matsunaga, H.; Hirate, N. Bunsekikagaku, 1988, 37, T 215 - 218. (15) Fabry, L.; Pahlke, S.; Kotz, L.; To ¨lg, G. Fresenius J. Anal. Chem. 1994, 353, 260-271. (16) Takenaka, M.; Hayashi, M.; Kubota, A., Matsushita, Y. Proceeding of the 2nd UCPSS Symposium, Bruges, Belgium, 1994. (17) Krivan, V.; Koch B. Anal. Chem. 1995, 67, 3148-3153. S0003-2700(96)00906-7 CCC: $14.00
© 1997 American Chemical Society
EXPERIMENTAL SECTION Mirror-Polishing Procedure for PTFE Container Surfaces. PTFE rod samples of circular cross section 80 mm in diameter and length 100 mm were cut to form rods 60 mm in diameter and 70 mm long. The rods thus formed were then hollowed out with a cutting machine to form beakerlike containers of inner diameter 55 mm and wall thickness ∼2.5 mm. The inner surfaces of the containers were polished with a diamond bit until a degree of flatness of 0.1 µm peak-to-valley (PTV) was achieved. PTFE containers fabricated following this procedure are henceforth referred to as “mirror-polished PTFE containers”. Washing of Containers. Commercially obtained PFA and PTFE containers and mirror-polished PTFE containers, all of 100 mL capacity, were first soaked in acetone to remove organic contaminants and then bathed in hot concentrated nitric acid (∼100 °C) for 72 h. The containers were then soaked in 0.1 M nitric acid for 72 h at room temperature following the method of Patterson and Settle.1 Measurement of Metallic Impurity Concentrations. After being bathed and soaked in concentrated and dilute nitric acids, the containers were washed with high-purity water. A 20 mL of aliquot of 0.1 M nitric acid was then added to each container, and the containers were maintained at room temperature for 3 h in a PTFE chamber located in a class 10 clean hood to dissolve the metals present on the surfaces of the containers. The 20 mL nitric acid volumes were then evaporated on a hot plate to ∼0.2 mL to concentrate the dissolved metallic contaminants. Eight metallic impurity concentrations (Na, K, Mg, Al, Cr, Fe, Ni, Cu) were measured in solution by ETAAS and ETV-ICPMS. The above-described bathing, soaking, and evaporation treatments and metallic impurity measurements are hereafter denoted collectively as “one washing cycle”. This cycle was repeated several times so that the total amounts of metals dissolved from the fluorocarbon containers could be measured. Samples and Reagents. PTFE and PFA containers were obtained from four different manufacturers. Mirror-polished PTFE containers were fabricated from raw material in the manner described above. Ultrahigh-purity water was derived using an ultrahigh-purity water system manufactured by Nomura Microscience, Tokyo, Japan. Standard solutions of metallic elements used throughout the procedure were obtained from Kanto-Merck, Tokyo, Japan. Concentrated nitric acid (Tamapure AA-10) was obtained from Tama Kagaku, Ltd., Kawasaki, Japan. Care was exercised to avoid contamination of samples by dust, chemicals, and handling. Sample manipulation and solution preparation were conducted in a class 1000-equivalent clean room. Apparatus. A Seiko SPQ-6500 inductively coupled plasma mass spectrometer equipped with an EV-300 transversely heated graphite atomizer (ETV-ICPMS) was used to measure Mg, Al, Cr, Fe, Ni, and Cu concentrations, while a Perkin-Elmer 5100 ZL atomic absorption spectrometer (ETAAS) was used for the measurement of Na and K. A Hatachi S-900 SEM and a NanoScope 200 AFM were used to examine the overall surface condition of the fluorocarbon polymers. A Kosaka ET-30H Profilometer was used for the microroughness measurements of the sample surfaces. Sample Preparation for Surface Observation by AFM and SEM. PTFE and PFA containers were cut into approximately 1 × 1 cm squares. Several squares were washed with acids following the washing cycle procedure described above. Other
Table 1. Amounts of Metals Dissolved from Several Fluorocarbon Polymersa metal amts after a washing cycleb (ng‚20 mL-1 per analysis) MPmetal PTFEc Na K Mg Al Cr Fe Ni Cu
PTFE-1
PTFE-2
