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3.6 d. Near-infrared spectroscopic determination of hydrous species in glasses: an interlaboratory comparison (C. Romano, M. Nowak and D.B. Dingwell, in collaboration with H. Behrens/Hannover and F. Holtz/Orleans)

Infrared spectroscopy has been widely applied for the quantification of water in natural and synthetic glasses. In particular, absorption bands in the near-infrared (NIR) have been found to be very useful because they enable different hydrous species to be distinguished. A band at about 4500 cm-1 has been assigned to OH groups associated with tetrahedral cations and a band at about 5200 cm-1 to H2O molecules. For a quantitative determination of the concentration of hydrous species by NIR spectroscopy, the molar extinction coefficients for the bands of interests must be known, which are dependent on the anhydrous glass composition. Generally, it is assumed that errors in absolute absorbancies determined by FTIR techniques are small. However, it is still unclear to which extent the specific measurement conditions influence the spectroscopic results. Furthermore, it has not been tested to which extent published values of molar extinction coefficients can be applied to measurements with other IR spectrometers without additional calibration. In order to characterize possible sources of errors we have performed an interlaboratory comparison using identical samples.

The near-infrared absorption bands at 5200 cm-1 and at 4500 cm-1 were used to specify concentrations of water species in glasses of alkali feldspar composition MAlSi3O8 (M=Li, Na, K). To allow an accurate quantitative evaluation of the water species in hydrous glasses, we have determined the compositional dependence of the density and the linear and integral extinction coefficients of the glasses. For each feldspar composition, 8 to 28 samples with various amounts of water have been synthesized. All samples were quenched isobarically to avoid desorption of water during cooling. Water contents of the glasses were analyzed by Karl Fisher titration. Spectra of the same samples collected by four different FTIR micro spectrometers vary by up to 10 % relative in peak intensities. The differences are attributed to the specific measurement conditions (e.g. opening angle of the objectives, characteristics of the detectors) applied in the laboratories. In order to reduce the uncertainty in determination of water species and total water by FTIR spectroscopy, we have performed a calibration of spectrometers against a reference system. For the evaluation of linear and integral molar extinction coefficients we have chosen the FTIR spectrometer (Bruker IFS88) in Hannover as the reference system. All spectroscopic data were recalculated on the basis of this spectrometer. For compositions of the system MAlSi3O8 the extinction coefficients are strongly dependent on the alkali cation and vary non-linearly along the binary joins of the system. The extinction coefficients of both the 4500 and the 5200 cm-1 bands are significantly lower for a strongly peralkaline glass than for glasses with feldspar compositions. Probably, for the peralkaline composition only a part of the water species contribute to the NIR absorption bands. Variation of species concentration for glasses of the system MAlSi3O8 with same water content are attributed to difference in the fictive temperature of the glass which depends on cooling rate, water content and anhydrous composition of the glass.

This study provides new experimental data for the calibration of the near-infrared spectroscopic technique for determining the water species and total water concentration in silicate glasses. These data can be used in future studies to form part of an enlarged data set describing the compositional dependence of the extinction coefficients and densities. With the help of such a data set it will be possible to measure the water content of natural samples much more precisely by NIR spectroscopy.

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