Strontium monofluoride – a new molecule to measure fluorine using high-resolution continuum source flame molecular absorption spectrometry and its application for gasoline analysis

Abstract

In this work, for the first time, strontium monofluoride was generated in an air-acetylene flame of a high-resolution continuum source molecular absorption spectrometer with the aim of fluorine determination via molecular absorption. The particular task was the determination of organic fluorine in gasoline/its components dissolved in xylene. Sequential measurements in the area of the most intensive absorption of the molecule were executed to select the spectral range for analysis. The acquired spectra differed from the ones from the literature, especially the emission spectra. The best and comparable sensitivity has been registered near 651 nm and 663 nm in measurements of rotations from the range of the X2Σ+ → A2Π electronic transition of the SrF molecule. Unfortunately, SrOH molecules were also spontaneously formed and absorbed radiation in the selected area. No other spectral interferences were found, except absorption by the SrBr molecule, which could have been formed in the flame despite a relatively low energy bond of 330 kJ/mol. The dependence of generation of the SrF and the SrOH molecules on the flame conditions was studied for pure solvent (xylene) and solutions of investigated samples of gasoline type. The effect of the SrOH molecule was overcome by blank subtraction or the least square background correction. The last method had to be used to overcome the SrBr effect. Due to the various combustion properties of various hydrocarbons and their different behaviour in flame, applying the simplified standard addition method became indispensable. As a source of strontium, an oil Sr standards or Sr acetylacetonate (SrAA) were investigated. The crucial benefit of SrF molecule application was the relatively good solubility (and relatively low price) of SrAA. Under the selected conditions, the characteristic concentration of fluorine was 1.9 and 2.0 mg/L, while the instrumental detection limit was 0.5 and 0.8 mg/L for the 651 and 663 nm ranges, respectively. The analytical technique enables the analysis of samples diluted to a small degree, e.g. 1:3 v:v and the best method detection limit is 1.3 mg/L. The proposed method is fast and cheap.