The use of the word “feasibility” in the title of this presentation is fully justified because one of the greatest challenges in graphite furnace (GF) based spectral techniques is preventing the evaporation of highly volatile analyte compounds before the measurement step. The challenge especially concerns the organic analyte forms with a boiling point below 100 °C [1]. Such a situation occurs in the case of the determination of fluorine contained as alkyl fluorides in alkylate [2].

Alkalyte is a valuable component of gasoline, called a “liquid gold”, as its properties closely resemble the required properties of commercial gasoline, and its environmental impact is relatively low [2]. Approximately half of alkylation installation units worldwide use concentrated HF as a catalyst. Alkyl (C3-C4) fluorides can be formed during improper alkylation installation work, possessing environmental hazards. Moreover, the fluorides easily undergo decomposition, releasing corrosive HF.

Nowadays, one of the methods of F determination is based on the formation of a monofluoride (e.g. GaF or CaF) in a graphite furnace and measurement using high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS GFMAS) [3]. Efforts have been undertaken to determine organic fluorides dissolved in xylene. Unfortunately, many attempts were unsuccessful, and no signal was registered even for a high concentration of various investigated model F compounds (from 1-fluoropentane up to 1-fluoronaphthalene, with b.p. from 63 up to 215 °C, respectively).

Finally, some promising results were obtained in measurements of natural alkylate samples (expected b.p. of F compounds: ~10-70 °C) with CaF as the target molecule and permanent zirconium modifier. The key point was using “calcium oil” (i.e. an oil commercially available as a Ca standard) as the source of Ca. A large amount of formed carbonaceous residue (usually undesired in the work with GF) was observed to have an important role in keeping volatile F forms. Unexpectedly, it turned out that higher pyrolysis temperature (e.g. in some cases 1200 °C) can be advantageous as due to effective decomposition of the “Ca oil”, residue of strong absorption properties can be formed. The registered signals were proportional to the content of F in reference samples, previously investigated using flame molecular absorption spectrometry or combustion ion chromatography [4]. There is still a lot to do and a lot to explain. For example, no signal of F standards was measured (i); it is difficult to control the processes of F stabilization in alkylate analysis (ii), and the stabilization effect of “Ca oil” changes with the GF lifetime (iii). However, the investigations show unknown faces and still untapped potential of GF-based techniques.

References

[1] Z. Kowalewska, Spectrochim. Acta Part B, 2011, 66, 546-556.

[2] Z. Kowalewska, K. Brzezińska, J. Zieliński, J. Pilarczyk, Talanta, 2021, 227, 122205.

[3] M. Resano, E. Garcia-Ruiz, M. Aramendia, M.A. Belarra, J. Anal. At. Spectrom., 2019, 34, 59-80.

[4] Z. Kowalewska, K. Brzezińska, Fuel, 2022, 309, 122197.