engleski

High performance size-exclusion chromatography in characterization of low molecular mass oil compounds

High performance liquid chromatography ( HPLC ) is an important tool for the analysis of samples belonging to oil, hydrocarbon processing and polymer industries. The low molecular mass ( MM ) compound, oils and petrochemicals are mainly separated and characterized by the normal phase ( NP ) and reverse phase ( RP ) HPLC, but the high performance size-exclusion chromatography ( HPSEC ) is one of the technique used in characterization and MM distribution ( MMD ) determination of polymers. HPSEC is just one of the liquid column chromatographic technique in which molecules are separated according to their size ( not MM ) in corresponding solutions. As in all chromatographic techniques, columns are &#8220; heart&#8221; of the SEC system and they dictate the measured efficiency and accuracy. The choice of column packing depends on purpose of the separation ( analytical or preparative ). Pore size is usually the most significant parameter to be considered, since it dictates the range of MM separation. For separating the sample components that extend over more than two decades of MM, a set of bimodal pore size distribution is optimum. Column of the smallest single pore size ( e. g. 50 &Aring ; ) are desirable for separation measuring mixtures of small molecules ( <5000 Da ). Once the pore size is selected, resolution can be improved by coupling together two or more columns of the same pore size, and maximum calibration linearity and fit are obtained using the columns of equal pore volume ( equal separation capacity ). At the end of an HPSEC measurement, molecules of different sizes are separated and their concentration detected as a function of the retention volume ( Vr ). The first step in extracting MM information from HPSEC is to establish a calibration curve relating the retention volumes to the MM of samples. There are several types of approach in constructing calibration curves : narrow MMD standards ( usually polystyrene standards ), wide MMD standards, universal calibration or measurement using absolute detectors. Oil and its products ( petrochemicals ) are very complex mixtures and they contain a huge number of aliphatic, cyclic and aromatic hydrocarbon compounds. Their size ranges differ from very small hydrocarbons such as propane to very large ( super molecular ) asphaltenes ( defined as toluene soluble but n-heptane insoluble ). The quality of crude oil is, among other properties, determined by the amount of high MM fraction ( Bp > 350° C ). Process technologies such as catalytic hydrogenation for the conversion of heavy crude oil into lighter fractions, have to be accepted to each supply feedstock, according to the amount of these high MM compounds. The determination of MMD provides information about conversion into the desired products with lower MM and boiling point distribution, because there is a correlation between these two quantities.. Generally, there are three techniques to measure MM averages or MMD : cryoscopy, vapour pressure and HPSEC. The polystyrene standards ( narrow and wide MMD ), due to structural differences are not applicable to oil and oil fraction characterization by HPSEC. Even though there is a standard method partially covering this area ( D 6579-00 Standard practice for MM averages and MMD of hydrocarbons and terpane resin by SEC ), correct calibration procedure is a difficult task. As it is known that hydrocarbon molecules in oil and oil product samples consist mainly of alkyl chain, aromatic rings and combination of both, calibration with corresponding species gives more realistic results. Taking into account a group-type determination obtained by analytical NP HPLC, it is much easier to choose the corresponding calibration curve for MMD determination of particular samples. Two different column set but with corresponding porosities were checked for MMD of different oil and oil product samples. THF was used as a mobile phase and refractive index detector for monitoring of response. According to the results of NP HPLC a different calibration sets ( polystyrene, n-.alkane, alkyl benzene and aromates ) were used for calculation of MMD data. Results obtained for number average of molecular mass (Mn) for some samples by SEC were compared to the values from cryoscopy technique. Calibration with n-alkane gave reasonable results but a results calculated by PS calibration are also very close.

HPSEC; MM of oil compounds

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