Calcium monofluoride as a target molecule for determination of organic fluorine in gasoline and its components using high-resolution continuum source flame molecular absorption spectrometry

Abstract

High-resolution continuum source molecular absorption spectrometry (HR-CS MAS) is widely applied to determine fluorine in various materials, and its graphite furnace option with calcium monofluoride as a target molecule is popular. However, up to now, the idea of using the molecule in flame molecular absorption spectrometry seemed impossible or impractical due to the expected low sensitivity and detectability. The following paper completely changes this view. The CaF molecule has been proven to be an excellent tool for determining organic fluorine in gasoline using flame molecular absorption spectrometry, giving benefits over previously used gallium monofluoride. The following favourable circumstances decide about it: 1) relatively high population of CaF molecules in preferable air-acetylene flame, 2) possibility of using oil calcium “standard” as a source of calcium, which is well dissoluble in both xylene (solvent) and gasoline, 3) extremely good stability of baseline in the used spectrum range, 4) relatively high freedom from spectral and chemical interferences and 5) possibility of summing of many pixels response. The analysis relies on simple sample dilution in xylene, with the addition of oil calcium “standard” to obtain 1500 mg L−1 Ca in measured solutions. Simplified standard addition calibration is used to overcome the effect of variability of samples (various physical properties and combustion characteristics). The signals of four CaF rotational “lines” are summed. The only undesirable spectral effect is a molecular absorption at the second “line” (606.0368 nm), which is attributed to the absorption by the CaOH molecule formed in the flame. The effect can be easily overcome by using the least square background correction, LSBC. The characteristic concentration and the detection limit for solutions are 6 and 0.6 mg L−1, respectively. The detection limit recalculated for the original sample is 2 mg kg−1 in the case of alkalyte, reformate and gasoline, while it is 6 mg kg−1 in the case of volatile isomerizate, mainly due to necessary higher sample dilution. The calibration curve is linear over at least two orders of magnitude. Heptafluorobutanol has been found a suitable standard. The agreement of the results of the proposed method and standard combustion ion chromatography is satisfactory.