## 3 Results and Discussion
### Analytical Performance of the Developed Voltammetric Method  
From previous paragraph, it seems that VK1 and \(\alpha\)-TOH gave nearly the same peak current response (\(I_{\text{p}}\)) for equal concentration level (\(c\)). This fact applies only if they are determined separately using SWAdSV. The separate measurements on SWAdSV of VK1 and VE showed different behavior from the simultaneous ones, based on peak current responses (\(I_{\text{p}}\)), as it is shown in Figure 4. The \(\alpha\)-TOH gave significantly higher peak current response than VK1. An explanation could be found in preferable adsorption of \(\alpha\)-TOH onto the surface of the GCE from 50% MeCN. It was found that this phenomenon negatively affected the final linear ranges. The peak height of the VK1 decreased above a concentration of 8 \(\mu\)mol L\({}^{-1}\) VK1, while the oxidation peak of \(\alpha\)-TOH still increased linearly with increasing content up to 10-\(\mu\)mol L\({}^{-1}\)\(\alpha\)-TOH (Figure 4A, dashed and dotted lines). The linear ranges for VK1 and \(\alpha\)-TOH determination were relatively short, 77-1000 nmol L\({}^{-1}\) for VK1 and 29-1000 nmol L\({}^{-1}\) for \(\alpha\)-TOH, with detection limits (LOD) of 25 and 10 nmol L\({}^{-1}\), respectively. Figure 3: Repetitive SWV of VK1 (red, **A**) and \(\alpha\)-TOH (blue records, **B**) adsorbed onto GCE surface from their 50-\(\mu\)mol L\({}^{-1}\) solution (accumulated from 50% MeCN at stirring speed of 400 rpm for 5 min) in 0.01-mol L\({}^{-1}\) HNO\({}_{3}\) containing 0.1-mol L\({}^{-1}\) KCl (pH 2.08) at \(E_{\text{dep}}=-0.1\) V, \(t_{\text{dep}}=60\) s, \(t_{\text{eq}}=5\) s, \(E_{\text{step}}=5\) mV, \(E_{\text{ampl}}=30\) mV and \(f=80\) Hz. Values of LODs were calculated as three times the standard deviation (\(s\)) of ten replicate measurements (200-nmol L\({}^{-1}\) VK1 and 50-nmol L\({}^{-1}\)\(\alpha\)-TOH) divided by the slopes of corresponding regressions (\(k\)). Limits of quantification (LOQ), presented as the first values of corresponding linear ranges, were calculated as 10 \(s\)/\(k\). The above-mentioned linear ranges are described by equations of calibration curves as follows: \(I_{\rm P}\) (\(\mu\)A) = 0.611 \(c\) (\(\mu\)mol L\({}^{-1}\)) \(-\) 0.080 with coefficient of determination (\(R^{2}\)) 0.9920 for VK1 and \(I_{\rm P}\) (\(\mu\)A) = 6.333 \(c\) (\(\mu\)mol L\({}^{-1}\)) \(-\) 0.012 with \(R^{2}\) = 0.9993 for \(\alpha\)-TOH. Statistically insignificant (negligible) values of intercepts (\(q\)) allow the use of the standard addition method for the simultaneous voltammetric determination of VK1 and \(\alpha\)-TOH in selected food supplements. Moreover, additional linear ranges for higher concentrations were found: 1.0-7.0-\(\mu\)mol L\({}^{-1}\) VK1 and 1.0-10-\(\mu\)mol L\({}^{-1}\)\(\alpha\)-TOH described by regression equations \(I_{\rm P}\) (\(\mu\)A) = 1.964 \(c\) (\(\mu\)mol L\({}^{-1}\)) \(-\) 1.046 with \(R^{2}\) = 0.9997 and \(I_{\rm P}\) (\(\mu\)A) = 3.416 \(c\) (\(\mu\)mol L\({}^{-1}\)) + 1.4105 with \(R^{2}\) = 0.9973, respectively. Comparable analytical parameters were obtained with previously reported adsorptive stripping voltammetric methods (Table 1). The precision of the developed SWadSV, referring to the mutual agreement between repeated measurements, was calculated for eight replicate measurements of equal vitamin content (50-\(\mu\)mol L\({}^{-1}\)) and relative standard deviations (RSD) of 4.7% and 6.6% for VK1 and \(\alpha\)-TOH were achieved, respectively.