**2. Materials and methods**

The study objects were the oil and natural bitumens of various-age deposits of Tatarstan fields and heavy residues of oil refining: vacuum residue (VR) from TAIF-NK OJSC oil refinery; asphalt (A-1) after tar propane deasphalting at the ANK Bashneft OJSC refinery; asphalt (A-2) after tar propane deasphalting at the NK Rosneft refinery; atmospheric residues of heavy oils from the Ashalchinskoe (AR-1) and Zyuzeyevskoe (AR-2) fields.

All oils and natural bitumens were separated from emulsion water and mechanical impurities by centrifugal process. Organic solvents of hch and chda classes were additionally treated and desiccated by employing widely known methods.

Asphaltenes were extracted by using a common methodology by means of diluting with 40× hexane excess with further flushing to remove sedimented resins and oils in a Soxhlet apparatus. Oils and resins were separated by means of column chromatography, using an activated granulated large-pore silica gel as an immobile phase, with the grain size of 0.2–0.5 mm, as well as the hexane/benzene mixture at 85:15 as an eluent.

Vanadium and nickel concentrations in oils and asphaltenes were measured by means of direct flame atomic absorption spectrometry using AAS-1N spectrophotometer, with approved standard samples of metal concentration in oil products used as blank solutions.

To extract asphaltenes by a boiling solvent, a 1 g sample of asphaltenes was placed into a roundbottom flask with back flow condenser, 200 mL of extractant was added, and the mixture was boiled for 1 h. After cool-down, the mixture was filtered. The resulted extract was dried in vacuum. To extract asphaltenes by sedimentation extraction, a 1 g sample of asphaltenes was diluted in 10 mL of benzene. A total volume of 100 mL of extractant was added to the resulting solution, which was then boiled with a back flow condenser for 10 min. After the solution cooldown, asphaltenes were filtered. The resulting extract was dried in vacuum.

The concentration of vanadyl porphyrins in extracts from asphaltenes was calculated for the absorption band of 575 nm according to the following formula:

$$\mathbf{C}\_{\rm vp} = 0.187 \mathbf{h} \cdot \mathbf{V} / \mathbf{m} \cdot \mathbf{l} \tag{1}$$

where 0.187 is the conversion factor describing the medium absorption; h is the height of absorption α-band maximum for 575 ± 5 nm; m is the extract sample, g; V is the porphyrin extract volume to be reached, mL; l is the flask thickness, cm.

Matrix-assisted laser desorption/ionization (MALDI) mass spectra of extracts from asphaltenes were obtained by UltraFlex III TOF/TOF mass-spectrometer in linear mode. The data were processed by using FlexAnalysis 3.0 software. The sample was ionized by nitrogen laser radiation (wave length of 337 nm) with the energy of 19 eV. Positively charged ions were recorded. A metallic target was used. Sinapinic acid was used as a matrix. Molecular ions of VPs of various homotypes are presented as peaks with the weight of 373 + 14n amu and 375 + 14n amu (where n is the number of methylene groups in pendent groups). The share of each homotype was calculated by means of internal normalization by using the peak intensity of molecular ions.

Primary porphyrinic extracts were obtained from heavy petroleum residues solutions in benzene with the concentration of 10 wt% by treating with 10× excess of concentrated sulfuric acid. The extract was separated by filtering in a Shott funnel and after being neutralized with sodium hydrate water solution, it was retreated with three portions of tetrachloromethane. The lower layer was separated and dried above anhydrous sodium sulfate. After dried solution filtering and solvent stripping, the primary porphyrinic extract was obtained.

Adsorption-chromatographic separation of primary porphyrinic extracts was carried out in a glass column 1:100 cm by using an activated granulated large-port silica gel as an immobile phase and the 0.5% isopropil alcohol solution in benzene as an eluent. The volume of the eluent sampled was 10 mL. The obtained fractions were combined according to eluent colors. Fraction spectra were obtained in Specord UV-VIS spectrophotometer with the range from 400 to 650 nm.
