## 3 Results and Discussion
### Method Development
#### 3.1.4 Optimization of the Amount of Gallium  
The last step of the optimization process was the search for the highest yield of GaF molecules formation by adding increasing amounts of gallium(III) nitrite monohydrate as a molecule-forming agent. For this purpose, we used the combined mode of the application of gallium as molecule forming agent solution, meaning that part of gallium was deposited on the surface of graphite along with the Pd/Zr modifier (step 1-5 in Table 1), and part of gallium was added directly to the sample, then introduced together at step 7 (Table 1).  
In order to optimize the amount of gallium introduced in both modes, firstly different masses, up to 600 \(\upmu\)g of Ga, were deposited onto the graphite surface, when 20 \(\upmu\)g of Ga were still added with the sample as in our previous work [34]. The results show that the integrated absorbance grows as the amount of Ga grows (Figure 4), indicating that the formation of GaF is correlated with the mass of Ga deposited on the graphite. A value of 500 \(\upmu\)g Ga(III) (25 \(\upmu\)L of the 20 g/L solution) was selected for further investigation due to the following technical limitations: (1) it is not recommended to inject more than 50 \(\upmu\)L of all solutions into the graphite tube with integrated PIN platform; (2) the solutions of Ga(NO\({}_{3}\))\({}_{3}\) become turbid at the Ga(III) concentration higher than 25 g/L.  
The alternative option is the addition of gallium(III) nitrite monohydrate solution into the furnace together with the sample solution. In this work we use the combined mode of the use of gallium, as to support the GaF formation on two levels, deposited on the graphite surface and being present in the solutions. Two amounts of Ga (60 \(\upmu\)g and 800 \(\upmu\)g) were selected for the evaluation of the performance of gallium as GaF molecular formation agent.  
According to Figure 5, samples with a low amount of Ga used for pretreatment have usually 20-30% lower light absorbance than those from the second set with a high mass of Ga even at high contents of Ga taken together with the sample directly before the measurement. In order to obtain much more robust results and higher signal intensity it is recommended to introduce higher masses of Ga during thermal pretreatment than together with the sample.  
Figure 6 shows an example of the time-resolved molecular absorbance spectrum in the wavelength range of 211.1215 nm-211.3732 nm obtained during the analysis of F in the form of GaF under different conditions. A sharp absorption band from gallium monofluoride with a spectral width of around 8 pm is situated in the middle of the analyzed spectral range. A signal that appears afterwards and at lower wavelength comes from Ga atoms [38] due to their presence in the entire system [33]. Some other signals from Ga atoms may be expected in the target spectral range, but they are significantly smaller, than the one that can be seen on Figure 6, thus can be neglected. The application of the proposed modifiers set suppresses this signal, so it does not interfere with the band of GaF. As it can be seen at Figure 6, no other signals of interferences can be found in the studied spectral range. In particular, the signal from phosphorus monoxide could originating from the biological matrix, but it is not observed (Figure 6D). The signal at 211.248 nm is proportional to the amounts of gallium. It is essentially important that plots on Figure 6C,D for water and bovine liver digest, respectively, are very similar, which means that the possible matrix effects are limited.