## 4 Conclusions  
The use of the HPLC-ICP-MS and UHPLC-ESI-Orbitrap-MS/MS methods allowed for the speciation of selenium metabolites in extracts obtained from the cellular biomass of _C. utilis_ ATCC 9950 yeast. The presence of four selenium compounds, for which standards were commercially available, was confirmed using the HPLC-ICP-MS method, by matching the retention times of the recorded selenium signals to the standards. Due to the possibility of signal overlap and the inaccuracy of retention times during HPLC-ICP-MS, each time a formal identification using a more accurate method in ionization conditions through electrospray should be considered. The Orbitrap MS system is a stand-alone mass spectrometry, designed for high-accuracy, high-resolution full scans. The high resolution of the Orbitrap MS combined with good mass accuracy provides good sensitivity and selectivity in analyses (yeast biomass). Using the UHPLC provides a superior chromatographic resolution, leading to improved sensitivity. The identification of selenium metabolites and their fragmentation behavior provides valuable and new information for a better understanding of the action of selenium metabolism in yeast cells. Moreover, structures for compounds with C3H8 NSSe+, C6H10 NO3 Se+, and C6H13 NO3 Se+ formulas have been proposed and found in _C. utilis_, which have not yet been reported. In conclusion, selenium present in yeast in organic form is characterized by better bioavailability than inorganic forms. The chemical form of selenium consumed in a diet is more important for dietetics than the content of this element in the diet.  
Available online at [[http://www.mdpi.com/2076-3417/8/11/2050/s1](http://www.mdpi.com/2076-3417/8/11/2050/s1)]([http://www.mdpi.com/2076-3417/8/11/2050/s1](http://www.mdpi.com/2076-3417/8/11/2050/s1)). Figure S1: Chromatograms registered for a mixture of selenium standards: (a) XIC for Se-methyl-selenocysteine, [M + H]+ _m/z_ 166.9604; (b) XIC for Selenomethionine, [M + H]+ _m/z_ 198.0026; (c) XIC for Selenocysteine, [M + H]+ _m/z_ 336.9194; (d) XIC for \(\gamma\)-glutamyl-Se-methyl-selenocysteine, [M + H]+ _m/z_ 313.0293, Figure S2: MS spectrum recorded for Se-methyl-selenocysteine (SeMetCys) ion, _m/z_ = 183.9871 (t\({}_{\mathrm{R}}\) = 5.19) along with the isotopic envelope characteristic of selenium compounds, Figure S3: MS spectrum recorded for Selenocysteine (SeCys) ion with _m/z_ = 336.9194 (t\({}_{\mathrm{R}}\) = 5.63) along with the isotopic envelope characteristic of selenium compounds, Figure S4: MS spectrum recorded for Selenomethionine (SeMet) ion with _m/z_ = 198,0026(\(\mathrm{t_{R}=6.58}\)) along with the isotopic envelope characteristic of selenium compounds, Figure S5: MS spectrum recorded for \(\gamma\)-glutamyl-Se-methyl-selenocysteine ion \(m/\mathrm{z=313.0293}\) (\(\mathrm{t_{R}=10.58}\)) along with the isotopic envelope characteristic of selenium compounds, Figure S6: Fragmentation spectra of reference substances (Se-methyl-selenocysteine), Figure S7: Fragmentation spectra of reference substances (Selenocysteine), Figure S8: Fragmentation spectra of reference substances (Selenomethionine), Figure S9: Fragmentation spectra of reference substances (\(\gamma\)-glutamyl-Se-methyl-selenocysteine), Table S1: LOD (detection limit) and LOQ (quantification limit) for Se-methionine (SeMet) and selenomethyl-selenocysteine (SeMetCys) obtained through UHPLC-QqQ-MS/MS [mg\(\cdot\)kg\({}^{-1}\)].  
[PERSON] collected and reviewed the literature and was responsible for the drafting and writing of the manuscript. [PERSON] and [PERSON] conceived and designed the experiments. [PERSON] and [PERSON] critically analyzed and reviewed the manuscript.  
This work was financially supported by the Faculty of Food Science at Warsaw University of Life Sciences-SGGW, Warsaw, Poland (accounting records: 505--10-092800-N00287-99).  
The authors would like to thank [PERSON] of Zdrochem Sp. z o. o. for performing parts of the commissioned analyses.  
The authors declare no conflict of interest.