Anal. Chem. 2008, 80, 6626–6632
Quantification of Hydroxyl Content in Ceramic Oxides: A Prompt γ Activation Analysis Study of BaTiO3 Vahit Atakan,*,† Chun-Wei Chen,† Rick Paul,‡ and Richard E. Riman† Department of Materials Science and Engineering, Rutgers University, 98 Brett Road, Piscataway, New Jersey 08854, and Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Thermogravimetric analysis (TGA) and Fourier transforminfrared spectroscopy (FT-IR) techniques for water content determination were compared with a neutron characterization technique, prompt γ activation analysis (PGAA). Residual H content in the samples, heat treated at various temperatures, was measured with PGAA and compared with the results obtained from TGA. Two major difficulties in TGA were overlapping of mass loss regimes due to removal of different species and residual water that could not be removed, even though the samples were heated above 900 °C. After 3 h of heat treatment at 900 °C, 0.007% mass fraction H remained in the sample. FTIR spectra confirmed the presence of H semiquantitatively. It is concluded that residual H remains even after high-temperature treatments. Solution-based powder production techniques such as hydrothermal and sol-gel methods are gaining popularity since they lower the reaction temperature and provide better particle size and morphology control when compared to solid-state reactions. However, due to the low temperature nature of these reactions, and presence of hydroxyls and H+ in the reaction medium, residual hydroxyls are incorporated into the solution-based powders. Residual hydrogen in ceramic oxides is named differently depending on the technique used to characterize it. For example, when TGA is used for measurement, the term residual water is used as the designation since hydrogen leaves the system as water when heated. In the case of FT-IR, the term hydroxyl is preferred due to the interaction of the laser with O-H vibration modes. In prompt γ activation analysis (PGAA), neutrons interact with the nucleus of the hydrogen atom and the term residual hydrogen is preferred. The presence of residual hydrogen affects the properties of materials and attracted the attention of the materials community. Depending on the applications, the presence of H either improves or degrades the properties of materials. Hydrogen defects in oxides were first reported in 1954 and 1956 for ZnO.1,2 It was * Corresponding author. Phone: +1 732 445 5754. Fax: +1 732 445 6264. E-mail:
[email protected]. † Rutgers University. ‡ National Institute of Standards and Technology. (1) Mollwo, E. Z. Phys. 1954, 138, 478–88. (2) Thomas, D. G.; Lander, J. J. J. Chem. Phys. 1956, 25, 1136–42.
6626
Analytical Chemistry, Vol. 80, No. 17, September 1, 2008
found that the presence of hydrogen increases the conductivity of ZnO single crystals. In the mid-1960s, Stotz and Wagner3 reported that hydrogen in the oxide is incorporated as protons (H+) at interstitial sites thus increasing conductivity. In 1980, Takahashi and Iwahara4 reported that in the presence of water vapor or hydrogen atmosphere Yb-doped SrCeO3 as well as some other perovskites are protonic conductors at high temperatures. Besides ceramic oxides, residual hydrogen also affects the properties of metals. It was found that residual hydrogen with a mass fraction as low as 50 mg kg-1 in titanium alloys decreases the cracking strength5 and also causes embrittlement in steel6 at concentrations as low as 10 mg kg-1. While a property like conductivity is enhanced, residual hydroxyls may also simultaneously degrade another property of the same material. In the case of hydrothermally synthesized BaTiO3 thin films, it was observed that the presence of hydroxyls increases the dielectric constant of the material while causing higher dielectric losses.7 Waser8,9 studied the diffusion and solubility of hydrogen defects in BaTiO3 ceramics. Diffusion coefficients of hydrogen defects were determined by thermal desorption studies (TDS) and FT-IR experiments. It was shown that hydrogen ions did not move as hydroxides but as protons. The grain boundaries did not act as dominant sites for hydrogen ions, but the presence of a second phase had increased the solubility of hydrogen defects. Hydroxyls have interesting effects on the crystal structure of the materials as well. Vivekanandan10 claimed that residual hydroxyls in the oxygen sublattice resulted in strains allowing metastable cubic BaTiO3 powders to form at room temperature. Qi11 studied the paraelectric-ferroelectric phase transition of BaTiO3 as a function of structure hydroxyls and concluded that structure hydroxyls were responsible for the metastable paraelectric BaTiO3 phase. Study of residual hydrogen is important in geology and mineralogy because it can provide very useful information about (3) (4) (5) (6) (7)
(8) (9) (10) (11)
Stotz, S.; Wagner, C. Ber. Bunsen-Ges. Phys. Chem. 1967, 70, 781–8. Takahashi, T.; Iwahara, H. Rev. Chim. Miner. 1980, 17, 243–53. Meyn, D. A. Metall. Trans. 1974, 5, 2405–14. Chevallier, J.; Aucouturier, M. Annu. Rev. Mater. Sci. 1988, 18 (5), 219. Slmovich, E. B. ; McCormic, M. A. Proceedings of the 5th International Conference on Solvo-Thermal Reactions; East Brunswick, NJ, July 22-26, 2002, pp 165-70. Waser, R. Phys. Chem. 1986, 90, 1223–30. Waser, R. J. Am. Ceram. Soc. 1988, 71 (1), 58–63. Vivekanandan, R.; Kutty, T. Powder Technol. 1989, 57, 181–92. Qi, L.; Lee, B. I.; Badheka, P.; Wang, L. Q.; Gilmour, P.; Samuels, W. D.; Exarhos, G. J. Mater. Lett. 2005, 59 (22), 2794–8. 10.1021/ac800020z CCC: $40.75 2008 American Chemical Society Published on Web 08/06/2008
the Earth’s crust. Rossman,12–15 Smyth,13 Bell,16 Snyder,17 and Kurosawa18 showed that hydrogen is dissolved as OH- in anhydrous minerals. Various researchers studied the effects of H on the properties of natural minerals. Kushiro,19 Kohlstedt,20 and Jung21 studied the effect of H on the melting point; Paterson,22 Karato,23 and Kohlstedt20 studied the effect of H on rheology. Karato24 studied the effect of H on conductivity; Inoue25 studied the effect of H on seismic velocity. In order to understand the effect of reaction conditions on residual hydroxyl content and to control the amount of residual hydroxyls, and hence to better engineer the final properties of materials, quantification of hydroxyls need to be performed reliably. Different types of characterization techniques like thermogravimetric and differential thermal analysis, (TG-DTA3),26–36 FT-IR,26,28–31,35–43 TDS,8,9,44–46 secondary ion mass spectrometry (SIMS),44 Raman spectroscopy,34 and nuclear magnetic resonance (NMR)32 have been used for characterization of hydroxyls. Many researchers26–36 used TG-DTA and FT-IR to specify the types, to (12) (13) (14) (15) (16) (17)
(18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46)
Aines, R. D.; Rossman, G. R. Geology 1984, 12, 720–3. Smyth, J. R.; Bell, D. R.; Rossman, G. R. Nature 1991, 351, 732–5. Bell, D. R.; Rossman, G. R. Science 1992, 255, 1391–1397. Bell, D. R.; Rossman, G. R. Contrib. Miner. Petrol. 1992, 111, 161–78. Bell, D. R.; Ihinger, P. D.; Rossman, G. R. Am. Miner. 1995, 80, 465–74. Snyder, R. A.; Taylor, L. A.; Jerde, E. A.; Clayton, R. N.; Mayeda, T. K.; Deines, P.; Rossman, G. R.; Sobolev, N. V. Archaen Am. Miner. 1995, 80, 799–809. Kurosawa, M.; Yurimoto, H.; Sueno, S. Phys. Chem. Miner. 1997, 24, 385– 95. Kushiro, I. J. Petrol. 1969, 13, 311–34. Hirth, G.; Kohlstedt, D. L. Sci. Lett. 1996, 144, 93–108. Karato, S.; Jung, H. Sci. Lett. 1998, 157, 193–207. Chopra, P. M.; Paterson, M. S. J. Geophys. Res. 1984, 89, 7861–76. Karato, S.-I.; Paterson, M.; FitzGerald, J. D. J. Geophys. Res. 1986, 91, 8151– 76. Karato, S. Nature 1990, 347, 272–3. Inoue, T.; Weidner, D. J.; Northrup, P. A.; Parise, J. B. Sci. Lett. 1998, 160, 107–13. Hennings, D.; Scherinemacher, S. J. Eur. Ceram. Soc. 1992, 9, 41–6. Hennings, D.; Metzmacher, C.; Scherinemacher, S. J. Am. Ceram. Soc. 2001, 84 (1), 179–82. Wada, S.; Suziki, T.; Noma, T. J. Cer. Soc. Jpn. 1995, 103 (12), 1220–7. Noma, T.; Wada, S.; Yano, M.; Suziki, T. J. Appl. Phys. 1996, 80 (9), 5223– 33. Wada, S.; Suziki, T.; Noma, T. Jpn. J. Appl. Phys. 1995, 34, 5368–79. Begg, B.; Vance, E.; Nowotny, J. J. Am. Ceram. Soc. 1994, 77 (12), 3186– 92. Abitch, H.; Voltzke, D.; Schneider, R.; Woltersdorf, J. O. Lictenberger Mater. Chem. Phys. 1998, 55, 188–92. Lencka, M.; Park, B.; Riman, R. Unpublished work. Clark, J.; Takeuchi, T.; Ohtori, N.; Sinclair, D. J. Mater. Chem. 1999, 9, 83–91. Sakabe, Y. Y.; Wasa, N. Characterization of Ultrafine Batio3 powder for multilayer ceramic capacitors (direct communication). Xia, C.; Shi, E.; Zhong, W.; Guo, J. J. Cryst. Growth 1996, 166, 961–966. Vivekanandan, R.; Philip, S.; Kutty, T. Mater. Res. Bull. 1986, 22, 99–108. Fukai, K.; Hidaka, K.; Aoki, M.; Abe, K. Ceram. Int. 1990, 16, 285–90. Kajiyoshi, K.; Ishizawa, N.; Yoshimura, M. J. Am. Ceram. Soc. 1991, 74, 369–74. Hennings, D.; Rosenstein, G.; Scherinemacher, S. J. Eur. Ceram. Soc. 1991, 8, 107–15. Nakamishi, K.; Solomon, P. H.; Furutachi, N. Infrared Absorption Spectrosc. 1978, 29–34. Slater, J. C. Phys. Rev. 1950, 78, 748–61. Spitzer, W. G.; Miller, R. C.; Kleinman, D. A.; Howarth, L. E. Phys. Rev. 1962, 126, 1710–21. McIntyre, P. C.; Ahn, J-H.; Becker, J.; Wang, R-V.; Gilbert, S. R.; Mirkarimi, L. W.; Schulberg, M. T. J. Appl. Phys. 2001, 89 (11), 6378–88. Chornik, B.; Fuenzalida, V. A.; Grahmann, C. R.; Labbe, R. Vacuum 1997, 48 (2), 161–4. Lisoni, J. G.; Piera, F. J.; Sanchez, M.; Soto, C. F.; Fuenzalida, V. M. Appl. Surf. Sci. 1998, 134, 225–8.
estimate the amount, and to determine the complete removal temperature of hydroxyls in hydrothermally synthesized barium titanate powders. The main disadvantages of TGA, an indirect measurement technique, are the overlapping of mass loss regimes due to removal of different species upon heating and the presence of residual hydroxyls, even at high temperatures. The requirement of extraction of the species of interest makes TGA an indirect method. FT-IR is a powerful technique to identify and differentiate hydroxyl group types. FT-IR can detect residual hydroxyls when TGA fails. In the infrared spectra, the peak around 2800-3600 cm-1 was assigned to surface hydroxyls.26,37–41 At 3509 cm-1, the stretching vibration of OH- with an intramolecular hydrogen bond is observed.41 The bending vibration of a structure hydroxyl37 appears in the IR spectrum at 650 cm-1 whereas the bending vibration of the O6 octahedron42,43 is seen at 550 cm-1. Even though FT-IR can provide very useful information about hydroxyls, quantification of FT-IR data is problematic. Quantification of FT-IR data is done using the BouguerBeer-Lambert Law.47 C)
A εL
(1)
where, C ) concentration, A ) the maximum height or the area of an optical absorbance band, ) the molar absorptivity coefficient for the corresponding band, and L ) the path length of the light through the material. The main problem for quantification of FT-IR data is the difficulty of obtaining the absorption coefficient. This makes FTIR a semiquantitative technique rather than a quantitative one. Karl Fischer titration (KFT) is widely used for measuring water content in liquid samples. The detection limit is as low as parts per million (mg/kg). To apply this technique to solid samples, samples are heated in a furnace to extract the hydroxyls in a humidity free atmosphere and liberated gases are transported to a KFT reaction chamber with a dry carrier gas. The requirement of thermal desorption of hydroxyls in both TGA and KFT decreases the reliability of these techniques in quantification of hydroxyls, in cases where desorption is incomplete. PGAA is a powerful elemental analysis technique especially for detection and quantification of light elements like H, B, N, Si, P, S, and Cl. The high penetration power of neutrons and γ rays enables PGAA to detect elemental H in milligram per kilogram levels. As well as the milligram per kilogram level detection, mass fractions as high as >1% mass fraction H can also be measured by PGAA.48 The main advantage of PGAA over the other techniques is its nondestructive nature. Furthermore, PGAA is not sensitive to the chemical state of the elements and hence can be used for quantification of total H. In PGAA, the sample is irradiated continuously with a neutron beam. Nuclei formed due to the capture of neutrons have excitation energies equal to the binding energy of the added neutron. Hydrogen atoms absorb neutrons and emits γ rays, which have an energy of 2223.4 keV according to the following reaction. 1
H(n,γ) f 2H
(2)
(47) Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectroscopy; Wiley: New York, 1986; p 338. (48) Paul, R. Analyst 1997, 122 (March), 35R–41R.
Analytical Chemistry, Vol. 80, No. 17, September 1, 2008
6627
This excitation energy is released by emission of γ rays. Emitted γ rays are detected by a high-resolution Ge detector. Every applicable element has specific excitation energies. Intensity of the observed γ ray is proportional to the amount of the corresponding element. The amount of H is calculated by using the data obtained from an appropriate standard material. The detailed principles of PGAA are described elsewhere.49–56 In this paper, quantification of residual hydroxyls in hydrothermally synthesized BaTiO3 powder by PGAA and KFT were compared with TGA to determine if the combination of PGAA and KFT can be used for quantitative characterization of hydroxyls in ceramic oxides. EXPERIMENTAL SECTION Hydroxyls were classified as surface and structure hydroxyls. Surface hydroxyls were divided into two subgroups, chemically and physically adsorbed surface hydroxyls. Chemically adsorbed surface hydroxyls were defined as the first layer of hydroxyls on the surface that are chemically bonded to BaTiO3. Physi-adsorbed surface hydroxyls were defined as the consecutive water layers that are physically adsorbed on surface hydroxyls. Structure hydroxyls were defined as the hydroxyls located within the sample. Total H ) H(surface water and hydroxyls) + H(structure hydroxyls) (3) Total H content and surface water were measured by PGAA and KFT, respectively. Structure hydroxyls were estimated using eq 3. These results were compared with the results obtained from TGA. Hydrothermally synthesized BaTiO3 powder (high-purity barium titanate, batch no. 00912-0, Ferro Corporation South Plainfield, NJ) was used for hydroxyl content characterization. TGA (PerkinElmer TGA7 HT, Shelton, CT) of the as received powder was done under flowing dry air atmosphere (dry air, Airgas Corporation, Piscataway, NJ) with a constant flow rate of 20 mL/min. Approximately 20 mg of sample was heated up to 1300 °C with a constant heating rate of 10 °C/min. The balance sensitivity of the TGA is 10 µg, the balance accuracy is better than 0.1%, and the weighing precision is up to ±10 mg/kg. The biggest uncertainties in TGA measurements were based on baseline instability. An empty pan was run exactly under the same conditions as the sample. For error estimates in TGA, the change in the mass of the empty pan was recorded at the temperature of interest, e.g., 25, 400, 650, 900, and 1200 °C. It was then subtracted from the (49) Lindstrom, R. M. J. Res. Natl. Inst. Stand. Technol. 1993, 98, 127. (50) Failey, M. P.; Anderson, D. L.; Zoller, W. H.; Gordon, G. E.; Lindstrom, R. M. Anal. Chem. 1979, 51, 2209. (51) Anderson, D. L.; Failey, M. P.; Zoller, W. H.; Walters, W. B.; Gordon, G. E.; Lindstrom, R. M. J. Radioanal. Chem. 1981, 63, 97. (52) Lindstrom, R. M.; Zeisler, R.; Vincent, D. H.; Greenberg, R. R.; Stone, C. A.; Mackey, E. A.; Anderson, D. L.; Clark, D. D. J. Radioanal. Nucl. Chem. 1993, 167, 121. (53) Paul, R. L.; Lindstrom, R. M.; Vincent, D. H. J. Radioanal. Chem. 1994, 180–263. (54) Mackey, E. A.; Anderson, D. L.; Chen, H.; Downing, R. G.; Greenberg, R. R.; Lamaze, G. P.; Lindstrom, R. M.; Mildner, D. F. R.; Paul, R. L. J. Radioanal. Nucl. Chem. 1996, 203 (2), 411. (55) Lindstrom, R. M.; Paul, R. L. J. Radioanal. Nucl. Chem. 2000, 243, 181–9. (56) Lindstrom, R. M. Biol. Trace Elem. Res. 1994, 597, 43–5.
6628
Analytical Chemistry, Vol. 80, No. 17, September 1, 2008
actual reading and converted into % mass fraction. The difference between the actual % mass fraction and the subtracted actual % mass fraction was estimated as the error in TGA measurements. Two sets of samples were prepared for PGAA measurements. Samples are named with a number corresponding to the heat treatment temperature and a letter indicating the atmosphere in which heat treatments were done, where c indicates a humidityfree controlled atmosphere and an ambient atmosphere is indicated with letter a. The samples in the first set of experiments were heat treated under an ultrahigh-purity (UHP) nitrogen atmosphere (ultrahighpurity nitrogen gas 99.999%, Matheson Tri Gas, Montgomeryville, PA) at 400, 650, 900, and 1200 °C. A 50 mL alumina combustion boat (AD-998 CoorsTek Corporation, Golden, CO) was used during heat treatment experiments. The boat was loaded with the sample and inserted into the tube furnace which was then evacuated with a vacuum pump and flushed with UHP nitrogen gas. The flushing process was repeated three times. A gas flow rate of 20 mL/min, heating rate of 10 °C/min, and a dwell time of 3 h was used at the specified temperature under UHP nitrogen atmosphere for all of these samples. The samples were cooled down to room temperature at the natural cooling rate of the furnace. UHP nitrogen gas flow remained during cooling. Samples were taken out of the furnace in a humidity-free glovebag, which was prefilled with UHP nitrogen gas. Samples were sealed into Teflon bags in the glovebag. A Teflon bag was chosen as a sample holder since it is H free. Teflon bags with the dimensions of 1 cm × 4 cm were prepared from a Teflon sheet obtained from the National Institute of Standards and Technology Center for Neutron Research facility, NCNR. An electrically powered sealing machine was used to seal all bags. Sealed bags were kept in a dry desiccator prior to the analysis. The second set of experiments consisted of five samples. They were, namely, “as received” and heat-treated samples, which were treated at 400, 650, 900, and 1200 °C. Unlike the first set, the heat treatments in the second set were done in a box furnace under ambient atmosphere. Samples were cooled down to room temperature at the natural cooling rate of the furnace. The amount of water adsorbed on the samples upon cooling or exposure to ambient atmosphere was calculated by subtracting the set 1 result from set 2. Identical heating rates and dwell times were used. PGAA measurements were done at the NCNR cold neutron PGAA facility. A sealed Teflon bag containing a sample was suspended at the center of a U-shaped aluminum sample holder by using Teflon strings in such a way that the sample holder is aligned with the collimator aperture with the sample lying at the center of the neutron beam. The sample holder was placed at a 45° angle with respect to both the detector and the neutron beam. The sample holder was covered with a magnesium sample box and evacuated to eliminate hydrogen gas and water vapor present in the ambient atmosphere. A pinhole was punched in the Teflon bag just prior to the measurement to outgas the bag and hence to prevent possible explosion of the bag upon evacuation of the PGAA chamber. The sample box was then positioned in the neutron beam by means of an electrical motor. The neutron beam had a flux of 8.8 × 108 cm-2 s-1. Prompt γ rays were detected by a high-purity germanium detector which is surrounded by a bismuth germanate Compton shield. The resolution of the detector
is 1.9 keV. Canberra Nuclear Data Software on a VAX station 3100 was used for collecting γ ray spectra up to 10 MeV. Acquisition time ranged from 1.5 h to as long as 24 h depending on the H content and the amount of sample. The raw data were processed with the SUM56 program written at NIST. The background was hand fitted by defining a background region on the left and right-hand side of the valley of the peaks of interest. The area under the H and Ti peaks were then integrated by using the SUM program to obtain counts. The peaks at 2223.4 and 1381 keV were used for H and Ti, respectively. The relative sensitivity of Ti to H in units of counts s-1 mg-1 was measured as 0.452 by using a NIST-prepared mixture containing known amounts of Ti, graphite, and urea. Counts were divided by the live time (LT) to obtain counts per second (CPS). CPS (counts s-1) values of H and Ti were corrected for detector dead time pile up factor (PF), defined as corrected counts per second (CCPS), which is described as follows: CCPS ) (CPS)(PF)
(4)
(5)
where, LT ) live time, DT ) dead time, P ) pile up constant (dimensionless), determined experimentally. An empty bag was run to obtain a correction for trace levels of H in the background. The CCPS for the empty bag was 0.07 counts per second with a 1s uncertainty of 2% based on counting statistics, which corresponds to (5 ± 0.1) µg. The CCPS of the empty bag was subtracted from the CCPS obtained from the sample. The ratio of mg of H to mg of Ti is calculated by using eq 6.
(CCPSH⁄CCPSTi)(STi⁄SH) ) (mH ⁄ mTi)
(6)
where, CCPSi ) CCPS of species i, Si ) sensitivity of species i, m ) mass of species i. The results were then converted into number of H atoms per Ti atom. During conversion from m(H)/m(Ti) to % mass fraction H, it was assumed that the sample was phase-pure BaTiO3. In 1 mol of BaTiO3, which equals 233.24 g, there is 1 mol of Ti, which equals 47.9 g. For 100 mg of BaTiO3 there is 20.54 mg of Ti. The mass ratio was converted into % mass fraction H in BaTiO3 by multiplying the result obtained from eq 4 by 20.54. The corresponding residual % mass fraction H for BaTiO3 was calculated. These results were then converted into residual % mass fraction H2O for BaTiO3 to predict the expected mass loss for TGA. Uncertainties for all PGAA measurements are 1 s, based on counting statistics and background subtraction. A brief summary of uncertainty calculations for PGAA measurements were given below. The % uncertainty for each peak was obtained from the SUM56 program. The uncertainty in CCPSi was calculated by eq 7. es )
CCPSi × % error 100
eT ) eS + et
(8)
The upper limit of CCPSH equals CCPSHUL ) CCPSSH - CCPSBH + eT
(9)
in which CCPSSH refers to CCPS of H in the sample, and CCPSBH refers to CCPS of H in the background. The lower limit of the CCPSH equals CCPSHLL ) CCPSSH - CCPSBH - eT
(10)
The upper limit of CCPSTi equals CCPSTiUL ) CCPSSTi + eTi
(11)
The lower limit of CCPSTi equals
PF is defined by Knoll57 as PF ) exp[(P)(DT) ⁄ (LT)]
For H, the total uncertainty equals the sum of uncertainty in the H peak and the uncertainty in the background measurement
(7)
(57) Knoll, G. F. Radiation Detection and Measurement; Wiley: New York, 1989; pp 65-99.
CCPSTiLL ) CCPSSTi - eTi
(12)
in which CCPSSTi refers to CCPS of Ti in the sample. The upper and lower uncertainties in H content were calculated by eqs 13 and 14.
upper error )
CCPSHUL CCPSH CCPSTiLL CCPSTi
(13)
lower error )
CCPSHLL CCPSH CCPSTiLL CCPSTi
(14)
The KFT measurement of the “as received” sample was done with a coulometric KF titrator, (controller model 270 and titration module, model 275 KF, Denver Instruments). Approximately 150 mL of anolyte reagent (Hydranal Coulomat AG, 34836, St. Louis, MO) was added into the outer cell compartment of the KFT cell, and 10 mL of catholyte reagent (Hydranal Coulomat CG, 34840, St. Louis, MO) was added into the inner chamber of the generator electrode. A standard material (Hydranal Water Standard 1.00 34828, St. Louis, MO) was used to control the accuracy of the measurement. An amount of 0.4 g of as received powder was ultrasonicated in 13.834 g of anhydrous methanol (dry methanol, Hydranal 3741, St. Louis, MO) for 15 min. The mixture was then centrifuged at 6500 rpm for 15 min. The liquid was then transferred into a syringe. A filter was attached at the tip of the syringe to prevent injection of any residual powder into the KFT reaction chamber. The same procedure was applied to anhydrous methanol alone to measure its background water content to correct the sample measurement. FT-IR spectra of the as received and heat treated samples were collected in diffuse reflectance mode from 4000 to 400 cm-1 (Perkin-Elmer Shelton CT). The comparison of detection limit, repeatability, and accuracy of TGA, KFT, and PGAA are tabulated in Table 1. The identification of certain commercial equipment, instruments, or materials does not imply recommendation or endorseAnalytical Chemistry, Vol. 80, No. 17, September 1, 2008
6629
Table 1. Detection Limit, Repeatability, and Accuracy of TGA, KFT, and PGAA Obtained from Manufacturers technique
detection limit
repeatability
TGA KFT PGAA
N/A 10 µg to 20 mg 2 mg/kg
10 mg/kg 0.1 µg 2%
accuracy
