## 3 Results and Discussion
### Method Validation  
Validation covers the entire analytical procedure, including biological sample preparation (acid digestion) as well as instrumental parameters (time/temperature program, reagents amount, wavelength) that provided the highest signal response from GaF. The selected reagent concentrations and volumes are listed in Table 2, together with their mode of application. Sodium acetate was used as to create a buffer and prevent losses of fluorine in the form of HF during the drying and pyrolysis stages. The improved procedure, which includes the process of acid digestion, allows for the determination of fluorine in samples at low pH values of their solutions (up to \(C\)(H\({}^{+}\)) = 7 mol/L), which is unique and advantageous in the context of the analysis of biological samples. Notably, fluorine determination independent of its solution pH is inaccessible by the widely used ISE method. The equation for quantification of F taken from the calibration curve was y = 0.102x + 0.1302, where x--the concentration of fluorine; y--integrated absorbance. The coefficient of determination (\(R^{2}\)) and the characteristic mass (\(m_{0}\)) of the reported method were 0.9992 and 8.7 pg of \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode** & **Components** & **Concentration** & **Volume [\(\mu\)L]** \\
**of Application** & & & \\ \hline Thermal deposition & Pd + Zr & 0.15\% m/v Pd, & \\  & & 30 mg/L Zr & 5 \\ Thermal deposition & Ga in HNO\({}_{3}\) & 20 g/L & 25 \\ Added with the sample & Ru in HC1 & 0.2 g/L Ru & 5 \\ Added with the sample & Ga in HNO\({}_{3}\) & 2 g/L & 10 \\ Added with the sample & CH\({}_{3}\)COONa & 1 g/L & 5 \\ Added in step 7 & sample or standard & variable & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Concentrations and volumes of the modifier solutions and the Ga reagent used for thermal pretreatment and injection together with the sample. Figure 6: Time- and wavelength-resolved absorbance spectrum during the analysis of F (NaF SRM) in a form of GaF (pixels correspond to the wavelength range between 211.1215 nm and 211.3732 nm) without modifiers (**A**) 1 ng F in water, and with modifiers: (**B**) 0.2 ng F in water; (**C**) 1 ng F in water; (**D**) 1 ng F added to bovine liver matrix. Modifiers: \(m\)(Pd) = 7.5 \(\mu\)g; \(m\)(Zr) = 0.15 \(\mu\)g; \(m\)(Ru) = 1.0 \(\mu\)g; \(m\)(Ga\({}_{\text{petter}}\)) = 500 \(\mu\)g; \(m\)(Ga\({}_{\text{sol}}\)) = 20 \(\mu\)g; \(m\)(CH\({}_{3}\)COONa) = 5.0 \(\mu\)g.  
F, respectively. The calculated LOD of the method was constituted on the basis of three times the standard deviation of 10 repetitive blank measurements (using all the reagents listed in Table 2 and replacing the sample with deionized water) divided by the slope of the calibration curve. Thus, the LOD for 20 \(\mu\)L of sample volume was 0.43 \(\mu\)g/L of F and 8.5 pg of absolute F, respectively. This is comparable with other reports that applied HR-CS GF MAS with GaF formation (4 pg-33 pg of absolute F [2, 14, 27, 28, 30, 33]). It is worth mentioning that such a low LOD cannot be reached using other molecule-forming agents (Ca, Sr or Al) [15, 17, 19]. The linear range of calibration was found for F concentrations to be 2.0 \(\mu\)g/L-2000 \(\mu\)g/L (40 pg-40 ng of F). In order to check the robustness of the proposed procedure to potential matrix effects, first a digest of biological CRM (NIST 1577c or MODAS-5) was added directly into the furnace. The concentration of F was below the limit of detection, so its content in CRMs could be neglected. Then the same set of samples was spiked with a known amount of F standard before acid digestion and they were analyzed according to the proposed procedure. In addition, the samples with high fluorine content were measured in different dilutions and selected results were compared with the fluorine reference values for available water CRMs. An acceptable recovery factor (93-106%) confirmed the negligible influence of matrix components on the determination of fluorine in biological samples (Table 3).