engleski

THE ROLE OF CALIBRATION PROCEDURE IN SEC OF SMALL NON-POLYMERIC COMPOUNDS

Crude oil and its products are very complex mixtures and they contain a huge number of aliphatic, olefinic, naphthenic, aromatic and alkyl aromatic hydrocarbon compounds. Typically, oil and fractions have been difficult to analyse by simple techniques due to their complex nature. Molecular mass ( MM )is one of the bulk properties that are indicative of molecular size and structure. There are three techniques used to measure MM averages and MM distribution ( MMD ) ; cryoscopy, vapour pressure osmometry, and size exclusion chromatography ( SEC ). The distribution of MM is suitable for characterising oil fractions since the molecular distribution is closely related to the boiling point distribution. SEC has been used for a long time as a technique for MMD determination of polymer samples, but the technique has also been applied for characterisation of crude oil and its fractions, especially higher MM component-asphalthenes. Beside the MMD determination, SEC of small oil and oil fraction molecules is used to make qualitative comparison between samples, to isolate fraction for subsequent characterisation by other techniques and to perform quantitative analyses. Using SEC, the MM range of crude oils can also be estimated and different crude oils qualitatively differentiated. HPSEC is just one of the liquid column chromatography techniques in which molecules are separated according to their size in corresponding solutions. At the end of an HPSEC experiment, sample molecules are separated and their concentration detected as a function of the elution volume ( Ve ). The first step in extracting MM information from HPSEC is to established a calibration relating the Ve to the MM of the measured sample. In case of polymer samples there are several types of approach in constructing the calibration curves: Calibration with narrow MMD polymer standards Calibration with broad MMD polymer standards Universal calibration SEC measurements with absolute detection ( LS ). The polymer standards alone ( narrow or broad MMD ) are not applicable to oil and its fractions characterisation by HPSEC due to structural differences. The main problem when using selected SEC technique is to discriminate between the different hydrocarbon families that are eluting at different rates owing to their varying geometrical molecular size. To overcome the deviation due to structural differences, calibration curves were sometimes constructed based on polymer standards in the upper region ( lower Ve ) and n-alkane hydrocarbon in the lower region ( higher Ve ) of the curve [ 1 ] ( Figure 1 ). When for pure hydrocarbon the MM of three homologous HC groups ( n-alkanes, naphthenes and alkyl benzenes ) is drawn versus the carbon number, the difference between MM of HC from different groups is small. It is known that the aromatic HC displays an anomalous deviation in SEC on styrene-divinylbenzene gel. The elution range of aromatics lies beyond the size separation range of the gel and the effect is generally denoted as the gel adsorption effect. Introduction of an alkyl group into the polyaromatic HC molecules brings about a significant change in the elution characteristic, due to alkyl part of the molecules obeys the separation principle more strictly [ 2 ] ( Figure 2 ). Taking in account a group-type determination of aromatics obtained by analytical NP HPLC, it is much easier to choose the corresponding calibration curve for MMD determination of particular samples. [ 1 ] A.Gratzfeld-Huesgan, HP HPLC Application Note 88-3 [2 ] N. Šegudović, High perfoemance size-exclusion chromatography in characterisation of low molecular mass non-polymeric compounds, 13th ISSS, Lipica, Slovenia, September 27-29. 2007. invited lecture Book of abstract p. 38-39.

SEC of small MM compound; calibration in SEC; HC calibration standards

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