## 1 Introduction  
The interest in the determination of fluorine increased over decades, which is due to its significant role in environment as well as in biological system. Although not a first one, though interesting work was published in 1941 by the U.S. National Bureau of Standards [1] on the quantitative determination of fluorine in organic compounds. Since that time a large number of papers has been published on the analytical procedure used for the determination of fluorine. The application of commonly used optical spectroscopy (inductively coupled plasma optical emission spectrometry--ICP-OES; atomic absorption spectroscopy--AAS) is limited due to the resonance lines of the fluorine being below 100 nm [2], also a very high ionisation potential (17.42 eV) limits the use of both optical techniques but also inductively coupled plasma mass spectrometry (ICP-MS). In practice fluorine has been determined, e.g., by ion-selective electrodes (ISE) [3], ion chromatography (IC) with electrochemical conductivity detector [4], UV-VIS [5] or cavity ring-down spectroscopy (CRDS) coupled with helium microwave-induced plasma [6]. Nonetheless, these techniques do not fulfil the specific requirements to be applicable for fluorine determination in biological objects. Another nonconventional technique that can be applied in F determination is molecular absorption spectrometry (MAS) with the use of high-resolution continuum source (HR-CS MAS), which performance can be increased in the presence of matrix modifiers, and also those with the ability to serve as pretreatment modifiers as it was proposed for GF AAS measurements [7; 8; 9; 10; 11]. Interestingly, this technique is still not sufficiently explored, yet remains important in the field of molecular spectrometry, providing a resolution (k/Dk) of 175,000 in a 0.3 nm\({}^{-1}\) nm range, corresponding to a bandwidth of 1.6 pm per pixel at 200 nm to 6.4 pm at 800 nm [12]. Fluorine is the most studied element via HR-CS MAS with graphite furnace (GF) atomization, among all halogens. This is mostly due to the high sensitivity that is provided by a formation of monofluoride molecules with selected metals. The limit of detection (LOD) for most halogens is at the level of nanograms, while for F a picogram level was achieved. The selectivity of F determination is usually better than for other halogens, as it is known to create stronger bonds with metals [13]. Fluorine can form the following molecules that are stable in gaseous form: GaF [2; 14], CaF [15; 16], SrF [17; 18] or AIF [19; 20], which offer the possibility of generating specific monofluoride molecules for a given matrix. Although AIF, CaF and SrF are known to be more thermally stable [15; 17; 19] than GaF, the last one [14] has been shown to be preferred in fluorine determination by HR-CS GF MAS due to the better sensitivity with the support of tailored modifier compositions and robustness concerning interferences thanks to the high spectral resolution. The only requirement is that the F has to be accompanied by a complex mixture of modifiers for the best performance. Among the main applications of HR-CS GF MAS for F determination, the analyses of water [17], soil [21], coal [22], milk [16], baby food [23], flour [24], tea [15], fish and seafood [25], eye shadow [26] and etching solutions from the photovoltaic industry [20] can be mentioned. In most of these applications Ca was used as a molecule-forming agent and only in some cases were Al or Sr also applied. The samples were placed in the furnace without any treatment or only after a dilution with water. The molecular absorbance of GaF can also be applied for analysis of F in samples with relatively high fluorine content (e.g., toothpaste [27], novel fluorinated biological and environmental compounds [28; 29], drug candidates [30]), as well as in samples with only trace amounts of F, such as drinking, mineral and river water [2; 31; 32] or cancer cells containing a fluorinated drug [33]).