FT-Raman spectroscopy of quantitative analysis soluble salts

In comparison with other spectroscopic techniques, Raman spectroscopy has not been widely applied to problems of quantitative analysis. In fact, few literature is dedicated at quantitative studies. The aim of this paper is to describe new approaches for using Raman spectroscopy to quantify soluble salts in efflorescences. Quantitative Raman spectroscopy for this study was conducted using a 180° scattering geometry MultiRAM FT-Raman spectrometer (Bruker Optik GmbH) equipped with a diode-pumped Nd:YAG laser operating at 1064 nm, capable of delivering power to the sample in the range of 0 – 500 mW. A Michaelson interferometer and an high-sensitivity, liquid-nitrogen cooled Ge diode detector were used. Preliminary experiments were carried out to examine the relationship between the absolute intensity of the Raman active SO4= ν1 mode (1008 cm-1) and the spectrometer laser power [Nicolae Buzgar et al., 2009]. Over the range of 0.0 – 400 mW, the scattering intensity response was measured to be linear with a correlation of R2 = 1. Subsequent quantitative experiments were run with the laser power at a constant set value, i.e 250 mW. Preliminary experiments were carried out also to examine the effects of excitation time of the 1064 nm laser [Vickers and Mann, 1991]. Spectral data were analyzed over a range from 98 to 5.000 scans to examine the effects of heating and instrument drift. Preliminary results indicated that experiments showed minimal added thermoluminescence background response with additional number of scans (i.e., beyond 2960 or 30 min. scan). Scans in the range of 98-2960 (or 1-30 min. scan) produced comparable signal-to-noise. It is important to point out that a consistent background correction method be employed when quantifying the SO4= 1008 cm-1 peak. The obtained spectra were well defined (i.e., 1008 cm-1 peak width at half height = 9 cm-1) to allow the selection of 100 cm-1 as the low wavenumber cutoff point for baseline correction. After this preliminary study a calibration curve was built for sulfates. A number of mixtures with different percentages of gypsum mixed with KBr were analyzed. Gypsum content was plotted versus spectral band intensity of SO4= ν1 peak and a line was fitted to the data. For our particular setup (i.e., solid sample, 494 scans, and invariant laser power at 250 mW), the equation y = 1.0913x – 1.0793 described the linear behavior, where y is the gypsum weight (%w) and x is the ν1, background-corrected band intensity at 1008 cm-1 (Fig. 1). The reasonably good correlation (R2 = 0.99) showed the possibility to evaluate the sulfates content in efflorescences by Raman analysis. The results obtained with Raman Spectroscopy were compared with the results obtained by Ionic Chromatography, showing a good correlation.