Calibration Line for Reflectance Measurements of FTIR

Additional file 1

Calibration line for reflectance measurements of FTIR

Analyses of volcanic glasses using FTIR have become a routine method for rapid and reliable quantification of dissolved H2O content. However, a disadvantage of this technique is that the sample has to be thin sectioned and polished on both sideswhen transmitted light is used. Small-run products are difficult to handle. Furthermore, in the case of arun with both crystals and glass,the absorbance coefficient for pure glass cannot be applied. These problems associated with the transmitted IR method can be resolved at least in part by using the reflectanceIR method.

In order to determine the calibration line of H2O with the reflectanceIR method, additional high-pressure experiments were conducted on basalt from Miyakejima volcano (OFS). The experimental method was thesame as that used for the hygrometer except that oxygen fugacity was not controlledin the additional runs.We considered that experiments were performed at ΔNNO = +2–3, which is similar to experiments conducted by Tatsumi and Suzuki (2009) using SMC-5000.Experimental conditions and results are listed in Additional file 5: Table S4Table A4.Recovered run products were cut intohalves. One half was sliced and doubly polished for analysis with the conventional transmitted IRmethod.The other half was mounted in epoxy resin for thatbythereflectance IR method.All sample surfaces were mirror-polished using 1 μm diamond paste.The analytical method was also the same as that used in experiments for the hygrometer.In the reflectance measurements, we followed the method of Yasuda (2011, 2014)using an aperture size of100 μm with 2048 scans.We analyzed ~10 points per sample to confirm the homogeneityof H2O content. Yasuda (2011) reported that the noise level (1σ)was 0.10 wt.% H2Owith 1024 scans and 100 μm of aperture size. A line connecting 3150 cm−1 with 4400 cm−1 was used as a baseline, and the height at a peak of near 3650 cm−1(Δreflectance3650cm-1)used for estimation of H2O content (Fig.A1).

Fig.A2shows the relation between H2O in the melt measured by transmitted FTIR (abscissa) and that measured by reflectance FTIR (ordinate). Assuming that the calibration line crosses the origin, the line in Fig.A2 is in good agreement with the data points of C1909, C1910, and C1911.In C1909, the error of H2O content determined by transmitted measurements was large, whereas that by reflectance measurement was small. The large uncertainty in the transmitted IR method is due to the largeuncertainty in thickness measurement by micrometer(i.e., the sample has to be very thin to avoid saturation in the absorbance peak). Furthermore,theabsorption coefficient used (Yamashita et al. 1997) was calibrated only between 1.06 and 3.70 wt.% H2O. Therefore,uncertainty of C1909 was significantly large. Yasuda (2011) calibrated the relation of Δreflectance3650cm−1 versus H2O to be between 1.17 and 4.05 wt.% H2O with linear fitting. Our new calibration is nearly identical to that ofYasuda (2011). Using the calibration line in Fig.A2, the H2O content in quenched melt in run products was analyzed by the reflectance method.

Fig.A1

Reflective spectrum of three hydrous glasses.

Fig.A2

Relation between Δreflectance3650cm−1using the reflectance infrared (IR) methodand H2O content for hydrous glasses using the transmitted IR method.

Table A4

Experimental conditions for reflectance measurement of Fourier transform infrared spectroscopy (FTIR).