Chenchen Fang

Application of headspace single-drop microextraction coupled with gas chromatography for the determination of short-chain fatty acids in RuO4 oxidation products of asphaltenes.

In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography-flame ionization detection (GC-FID), was employed to determine short-chain fatty acids (SCFAs) in ruthenium tetroxide (RuO(4)) oxidation products of asphaltenes. Several significant parameters, such as drop solvent type, drop volume, sample solution ionic strength, agitation speed, extraction time, and ratio of headspace volume to sample volume were optimized. Under optimum extraction conditions (i.e., a 3-microL drop of 1-butanol, 20 min exposure to the headspace of a 6 mL aqueous sample placed in a 10 mL vial, stirring at 1000 rpm at room temperature, and 30% (w/v) NaCl content), the reproducibility and accuracy of the method have been tested and found to be satisfactory. The analysis of a real asphaltene sample using this method proved that HS-SDME can be a promising tool for the determination of volatile SCFAs in complex matrices.

An effective pre-treatment method for the determination of short-chain fatty acids in a complex matrix by derivatization coupled with headspace single-drop microextraction

We have developed a sample preparation method involving derivatization combined with headspace single-drop microextraction (HS-SDME) for the determination of short-chain fatty acids (SCFAs) in complex matrices. The derivatization of SCFAs was conducted using the BF3/ethanol method prior to HS-SDME. The HS-SDME extraction conditions for the derivatization products (ethyl esters) of SCFAs were optimized using 1.0μL of dibutylphthalate (DBP), 1000rpm stirring speed, 30% (w/v) NaCl, 20min extraction time, and 7mL of sample solution in a 12mL vial. Quantitative determination of ethyl esters was performed using gas chromatography (GC). Linear calibration curves and excellent reproducibility were obtained using these optimized extraction conditions. Compared with our previous work, the significantly lower detection limits (0.11, 0.017, 0.0060, and 0.0024μg/mL for C2 to C5 SCFAs, respectively) indicate that this new method is suitable for quantitative analysis of SCFAs in complex matrices, such as the RuO4 oxidation products of kerogen or asphaltene.

The application of diamondoid indices in the Tarim oils

This study presents new data for the identification of the source and assessment of the thermal maturity of oils based on the diamondoid indices of oils from the Tazhong and Luntai uplifts in the Tarim Basin in northwest China. The oil samples were divided into three groups according to biomarker characteristics and the abundance and distribution of diamondoids. Group I oils are located along the Tazhong No. 1 fault zone, which contained abundant diamondoids and are of high thermal maturity, suggesting they are late-charged hydrocarbons that were derived from a Middle–Upper Ordovician source. Group II oils are mainly located in blocks close to the Tazhong No. 1 fault zone and are dominated by early formed hydrocarbons from Cambrian–Lower Ordovician source rocks. Group III lacustrine oils comprise relatively low concentrations of diamondoids compared with group I and group II oils. Group III oils were sourced from Jurassic or possibly Triassic units and are hosted by the low-relief Yingmaili section of the Luntai uplift. The thermal maturity of the oils in each group was evaluated using diamondoid parameters; a few group I oils exhibit intense thermal cracking. To estimate the extent of oil cracking using the concentrations of diamondoids in oils, this study proposes a practical approach that facilitates the determination of baseline 4- and 3-methyldiamantane concentrations. Application of this method indicates that the Tarim Basin marine oils contain a baseline concentration of approximately 69 ppm.

Carbon isotopic fractionation during vaporization of low molecular weight hydrocarbons (C6–C12)

Three series of laboratory vaporization experiments were conducted to investigate the carbon isotope fractionation of low molecular weight hydrocarbons (LMWHs) during their progressive vaporization. In addition to the analysis of a synthetic oil mixture, individual compounds were also studied either as pure single phases or mixed with soil. This allowed influences of mixing effects and diffusion though soil on the fractionation to be elucidated. The LMWHs volatilized in two broad behavior patterns that depended on their molecular weight and boiling point. Vaporization significantly enriched the 13C present in the remaining components of the C6–C9 fraction, indicating that the vaporization is mainly kinetically controlled; the observed variations could be described with a Rayleigh fractionation model. In contrast, the heavier compounds (n-C10–n-C12) showed less mass loss and almost no significant isotopic fractionation during vaporization, indicating that the isotope characteristics remained sufficiently constant for these hydrocarbons to be used to identify the source of an oil sample, e.g., the specific oil field or the origin of a spill. Furthermore, comparative studies suggested that matrix effects should be considered when the carbon isotope ratios of hydrocarbons are applied in the field.

The effect of volatile components in oil on evolutionary characteristics of diamondoids during oil thermal pyrolysis

On the basis of the results of simulation experiments, now we better understand the contribution of high carbon number hydrocarbons to diamondoid generation during thermal pyrolysis of crude oil and its sub-fractions (saturated, aromatic, resin, and asphalene fractions). However, little is known about the effect of volatile components in oil on diamondoid generation and diamondoid indices due to the lack of attention to these components in experiments. In this study, the effect of volatile components in oil on diamondoid generation and maturity indices was investigated by the pyrolysis simulation experiments on a normal crude oil from the HD23 well of the Tarim Basin and its residual oil after artificial volatilization, combined with quantitative analysis of diamondoids. The results indicate that the volatile components (≤nC12) in oil have an obvious contribution to the generation of adamantanes, which occurs mainly in the early stage of oil cracking (EasyRo<1.0%), and influences the variations in maturity indices of adamantanes; but they have no obvious effect on the generation and maturity indices of diamantanes. Therefore, some secondary alterations e.g., migration, gas washing, and biodegradation, which may result in the loss of light hydrocarbons in oil under actual geological conditions, could affect the identification of adamantanes generated during the late-stage cracking of crude oil, and further influence the practical application of adamantane indices.