1 Introduction

The C6–C12 low molecular weight hydrocarbons (LMWHs) are an important part of petroleum, consisting of different compound classes (n-, iso-, cyclo-alkanes, and aromatics). Various parameters based on the chemical and isotopic compositions of these LMWHs have been widely utilized to make oil/source correlations (Bjorøy et al. 1994; Ten Haven 1996; Odden et al. 1998; Whiticar and Snowdon 1999; Obermajer et al. 2000; Wever 2000), assess the thermal maturity of oils and condensates (Thompson 1983; Mango 2000), determine the source allocation of mixed oils (Chung et al. 1998; Rooney et al. 1998), and identify various secondary alterations of crude oils (George et al. 2002; Pasadakis et al. 2004; Zhang et al. 2005). The characterization of these light compounds is also a powerful tool for tracing the source of petroleum-related contaminants and understanding the environmental processes that control the transport and fate of these contaminants (Dempster et al. 1997; Kelley et al. 1997; Gray et al. 2002; Kolhatkar et al. 2002; Mancini et al. 2002, 2008; Smallwood et al. 2002; Zwank et al. 2003).

Liquid petroleum hydrocarbons, particularly LMWHs, lose mass through vaporization, which can occur in a wide variety of settings, including during the weathering of oil spills (i.e., before sampling), and during sampling, transportation, and storage. The different vaporization behavior of each component of an oil sample will alter the sample’s chemical and isotopic composition, thus likely influencing the application of some identification methods based on these properties (Cañipa-Morales et al. 2003). It is therefore essential to clarify the possible effects of vaporization on the composition of oil samples prior to the interpretation of the data.

Numerous studies have examined the effects of evaporation on the composition of LMWHs (Thompson 1987, 1988; Cañipa-Morales et al. 2003). Strict collection and preservation procedures are required to avoid the evaporation of a crude oil sample to facilitate the accurate determination of its LMWHs distribution, because even minor evaporation would affect the quality of the data (Cañipa-Morales et al. 2003). Although compound-specific isotope analysis (CSIA) has become a powerful tool for oil characterization and correlation (Chung et al. 1998; Odden et al. 1998; Harris et al. 1999; Whiticar and Snowdon 1999), few studies have considered the effect of evaporation on the δ13C values of LMWHs (Harrington et al. 1999; Shin and Lee 2010).

In addition, different volatile organic compounds have shown different carbon isotope fractionation trends during evaporation. For example, the enrichment of 13C in the vapor fraction was reported for the evaporation of benzene, toluene, ethylbenzene, and xylene (collectively known as BTEX) (∆13Cvapor–liquid ≈ +0.2‰) (Harrington et al. 1999), trichloroethylene (∆13Cvapor–liquid = +0.1‰ to +0.7‰) (Poulson and Drever 1999), chlorinated aliphatic hydrocarbons (∆13Cvapor–liquid = +0.31‰ for trichloroethene and ∆13Cvapor–liquid = +0.65‰ for dichloromethane) (Huang et al. 1999), and MTBE (tert-butyl methyl ether, ∆13Cvapor–liquid = +0.2–0.5‰ in different physical contexts) (Kuder et al. 2009). The enrichment of 13C in the vapor phase could be explained by higher vapor pressure of 13C-substituted organic compounds relative to 12C-substituted organic compounds (Baertschi et al. 1953; Narten and Kuhn 1961; Jancso and Van Hook 1974). In contrast, some evaporation experiments have shown that progressive evaporation considerably enriches the remaining liquid fraction in 13C, with ∆13Cvapor–liquid = −0.58‰ and −0.41‰ for benzene and toluene, respectively (Shin and Lee 2010). Kinetic fractionation was evidently dominant in controlling the carbon isotopic fractionation during these evaporation experiments.

Previous experimental studies have investigated the evaporation of LMWHs mainly by simulating the evaporation of one or two pure components in each experiment and determining their composition and isotope fractionation at different stages during the evaporation (Huang et al. 1999; Poulson and Drever 1999; Shin and Lee 2010). The effect of the matrix in which the vaporization of a hydrocarbon is studied has seldom been discussed in these experimental studies. However, crude oils and oil products generally comprise a complicated mixture of hydrocarbons, and the matrix effect of the other components may to some extent influence the evaporation behavior of each individual compound.

Additionally, evaporation in natural environments commonly involves other media such as water and soils. A few studies have explored the effects of mixture with water and adsorption to soil on carbon isotope fractionation (Harrington et al. 1999; Slater et al. 1999; Höhener et al. 2003; Schüth et al. 2003; Bouchard et al. 2008a, b). No significant carbon isotope fractionation was observed during the equilibrium vaporization of aqueous solution of toluene and trichloroethylene (Slater et al. 1999), the soil adsorption of BTEX (Harrington et al. 1999) and the sorption of halogenated hydrocarbon compounds (trichloroethene, cis-dichloroethene, vinylchloride) and BTEX compounds onto activated carbon, lignite coke, and lignite (Schüth et al. 2003). However, significant fractionation has been observed after passing some volatile organic compounds across alluvial sand (e.g., ∆13Cvapor–liquid is −2.14 ± 0.22‰, −1.73 ± 0.52‰, and −1.55 ± 0.45‰ for n-pentane, n-hexane, and benzene, respectively) (Bouchard et al. 2008b), and an unsaturated soil zone (Bouchard et al. 2008a). Therefore, the matrix effects (of both mixing and soil diffusion) on the evaporation fractionation should be better understood prior to utilizing the δ13C value of volatile organic compounds.

The main purpose of this research is to gain better insights into carbon isotope fractionation during the vaporization of C6–C12 LMWHs and to determine the influences of multi-component mixtures and soil diffusion on the vaporization of these hydrocarbons. Three series of laboratory vaporization experiments were conducted at room temperature. The first investigated the vaporization of a mixture of C6–C12 LMWHs by assessing their vaporization characteristics in a mixture compounds. The second series compared the vaporization of individual pure single-phase compounds. The third series examined vaporization in soil.

2 Experimental

2.1 Reagents and chemicals

n-Hexane (n-C6, 99%), n-heptane (n-C7, HPLC grade, 99+%), n-octane (n-C8, 98+%), n-nonane (n-C9, 99%), n-decane (n-C10, 99%), n-undecane (n-C11, 99%), n-dodecane (n-C12, 99+%), benzene (99%), ethylbenzene (99%), o-xylene (99%), methylcyclohexane (MCH, 99%), and deuterated n-octane (n-C8D18, 99%) were purchased from Alfar Aesar China (Tianjin) Co., Ltd. n-Pentane (n-C5, 99%) and toluene (99%) were purchased from Qianhui Chemicals and Glassware Co. Ltd. (Guangzhou, China).

The soil used here was first freeze-dried, pulverized with an agate mortar, and then sieved. The fraction with particle sizes less than 100 mesh was heated at 250 °C in an oven for 4 h to eliminate any naturally present LMWHs and microbes. The main minerals in the soil were quartz, chlorite, feldspar, illite, calcite, and dolomite. The total organic carbon of the soil was 0.12%.

2.2 Vaporization experiments

Three series of vaporization experiments were designed in which three pure compounds (n-hexane, n-nonane, and n-dodecane) and a mixture of compounds were progressively volatilized in the laboratory. The mixture was of twelve LMWHs, including C6–C12 n-alkanes, MCH, and BTEX. The mixture was prepared by adding the 12 pure compounds (n-C6, 700 μL; benzene, 700 μL; n-C7, 400 μL; MCH, 400 μL; toluene, 400 μL; n-C8, 300 μL; ethylbenzene, 300 μL; o-xylene, 300 μL; n-C9, 200 μL; n-C10, 200 μL; n-C11, 200 μL; and n-C12, 200 μL) to a 4-mL glass vial capped with an aluminum–rubber seal.

In the first vaporization experiment, aliquots (approximately 200 μL) of the mixture of compounds were delivered into a series of 4-mL glass vials and weighed. Each vial was then placed in a fume cupboard to allow open vaporization without any agitation, and an air conditioner was used to control the room temperature at 24 ± 1 °C. At intervals up to 72 h, vaporization was measured. For the GC measurement, the vials were then weighed and filled with n-pentane. They were tightly capped with aluminum–rubber seals, shaken in an ultrasonicator for 10 min to increase the dissolution of the residue hydrocarbons into the n-pentane solvent, and then kept in a freezer prior to analysis.

The second series of vaporization experiments was conducted similar to the first, although instead of the artificial oil mixture, pure compounds were individually studied (n-hexane, n-nonane, or n-dodecane). Aliquots (about 100 μL) of each individual pure compound were added to a series of 4-mL glass vials, and the following procedures for the vaporization experiment were identical to those of the first series.

The third series of vaporization experiments was designed to reveal the effect of diffusion through soil on the vaporization of LMWHs. A certain amount of soil (1 or 2 g) was added to 4-mL glass vials containing a pure hydrocarbon (n-hexane or n-nonane). Subsequent procedures were as in the preceding series. Finally, the residual compounds were ultrasonically extracted into pentane solvent from the soils.

After vaporization, the concentrations and δ13C values of the target compounds in the unaltered original sample and the evaporated residual aliquots were measured by directly injecting n-pentane solutions containing the target compounds into gas chromatography (GC) and gas chromatography–isotope ratio mass spectrometry (GC–IRMS) apparatus. The remaining fraction (F) of each component was calculated by measuring the weight (pure compound) or concentration (artificial oil) of the corresponding compound before and after vaporization.

2.3 Gas chromatography (GC)

GC analyses were performed on an Agilent 7890 GC instrument equipped with a split/splitless injector, an HP-PONA fused silica capillary column (50 m × 0.20 mm i.d. × 0.50 µm), and a flame ionization detector. The temperatures for both injection and detection were set at 300 °C. Nitrogen (99.999%) was used as the carrier gas at a maintained constant flow rate of 1.0 mL/min. The injection was operated in split mode (10:1). The GC oven temperature was programmed to rise for 5 min from 35 to 50 °C at a rate of 4 °C/min, and then to 180 °C at 8 °C/min. C6–C12 LMWHs were quantified by integration of the peak areas. The response factors of these hydrocarbons relative to the internal standard (n-C8D18) were calculated based on the peak area ratios of each C6–C12 hydrocarbon compared with the internal standard.

2.4 Gas chromatography–isotope ratio mass spectrometry (GC–IRMS)

Carbon isotopic compositions of LMWHs were measured with a gas chromatograph (Agilent 6890) equipped with a DB-5MS column (50 m × 0.25 mm i.d. × 0.25 μm) coupled to an isotope ratio monitoring mass spectrometer (GV IsoPrime). Helium was used as the carrier gas with a maintained constant head pressure of 8.5 psi. The GC oven temperature was programmed to be initially held at 35 °C for 5 min, then raised to 50 °C at 1.5 °C/min, held for 3 min, increased to 53 °C at 0.5 °C/min, and finally increased to 200 °C at 25 °C/min and held for 2 min. The combustion by-product (H2O) was removed by passing the analyte stream through a selectively permeable membrane (Nafion™) with a dry He counter flow. Carbon isotope ratios were computed by five pulses of CO2 reference gas with known δ13C values (−22.5‰, VPDB), which were injected via the interface to the IRMS instrument at the beginning and end of each analysis. A standard mixture of n-alkanes (C12–C32) from Indiana University with known isotopic composition was used daily to monitor the performance of the instrument. The reported isotopic data represent the arithmetic means of at least two replicate analyses, and the repeatability is better than ±0.3‰ (VPDB).

2.5 Quantification of isotope fractionation during vaporization and diffusion across soil

The experimental isotope factors can be determined using the following Rayleigh equations:

$$R = R_{\text{o}} \cdot F^{\alpha - 1}$$
(1)
$${ \ln }\,A = \, \left( {\alpha {-}1} \right) \cdot { \ln }\,F$$
(2)
$$A = \, \left( {\updelta^{13} {\text{C}}_{\text{R}} + \, 1000} \right)/\left( {\updelta^{13} {\text{C}}_{\text{I}} + \, 1000} \right)$$
(3)

where F is the mass fraction of the original compound remaining, R and R o are the 13C/12C value of the individual compound at a specific F (F < 1) and at F = 1, respectively, and α (equal to R vapor/R liquid) is the vapor–liquid fractionation factor. δ13CR and δ13CI are the δ13C values of the residual and initial compound, respectively. For each compound of the artificial oil, the α values were calculated by linear regression of lnF versus lnA.

The corresponding isotope enrichment factors can be calculated according to:

$$\varepsilon (\permil) = \left( {\alpha {-}1} \right) \cdot 1000$$
(4)

The uncertainty was characterized using the standard uncertainty of the slope obtained by linear regression using the least-squares method, which was performed by the data analysis tool installed in Microsoft Excel.

The theoretical fractionation factor between the diffusion coefficients is given by the following equation (Craig 1953; Cerling et al. 1991; Bouchard et al. 2008c; Jeannottat and Hunkeler 2012):

$$\alpha_{t} = \frac{{D_{\text{h}} }}{{D_{\text{l}} }} = \sqrt {\frac{{\left( {{\text{MW}}_{\text{h}} + {\text{MW}}_{\text{a}} } \right) \cdot {\text{MW}}_{\text{l}} }}{{\left( {{\text{MW}}_{\text{l}} + {\text{MW}}_{\text{a}} } \right) \cdot {\text{MW}}_{\text{h}} }}}$$
(5)

where D represents the diffusion coefficient, MW the molecular weight, and the subscripts l, h, and a represent light isotopes only, molecules with one heavy isotope, and air, respectively (MWa = 28.8 g/mol in this case).

3 Results and discussion

An evaporation system with constant boundary conditions will usually have a constant evaporation rate for a single liquid (one component) with respect to time (Stiver and Mackay 1984). In contrast to the linear evaporation of a pure compound, the evaporative loss of a mixture by total weight or volume is either logarithmic (approximately seven or more components) or a square root function (between about five and seven components) with time (Fingas 1997). This implies that the evaporation behavior of a given component is probably different between its pure state (single-component liquid) and when it is in a mixture due to the occurrence of intermolecular interactions. Therefore, the carbon isotope fractionations of LMWHs during evaporation from a mixture and from soil were investigated to explore their evaporation behavior under conditions resembling practical situations.

3.1 Carbon isotope fractionations of LMWHs during the progressive vaporization of artificial oil

To eliminate the possible influence of co-elution and other factors on the measurement of the LMWHs, a mixture consisting of twelve pure standards (n-C6, benzene, n-C7, MCH, toluene, n-C8, ethylbenzene, o-xylene, n-C9, n-C10, n-C11, and n-C12) was selected here to replace a real oil.

The progression of the remaining mass fractions (F) and carbon isotope compositions of the individual C6–C12 LMWHs in the volatilized residues of the mixture are summarized in Table 1. The mass losses are being observed against time (h). n-C6, benzene, n-C7, MCH, toluene, n-C8, ethylbenzene, and o-xylene in the mixture showed expected mass losses, with the lighter compounds being the most volatile, and these compounds in the residual liquid were enriched in 13C. However, the heaviest compounds (n-decane, n-undecane, and n-dodecane) after 72 h showed mass losses of 66%, 16%, and 1% and δ13CR−I values of 0.6, 0.4, and 0.2‰ (within the CSIA error), respectively.

Table 1 Remaining fraction and the corresponding carbon isotopic composition of individual LMWHs in the residual mixture compounds during progressive vaporization (24 ± 1 °C)
Full size table

Based on their vaporization rates and variations in δ13C values, the considered LMWHs fall into two categories: a lighter C6–C9 fraction and a heavier C10–C12 fraction. The lighter fraction volatilized more quickly and showed considerable carbon isotope fractionation. Plots of F versus vaporization time (Fig. 1a) were used to evaluate the vaporization rates of individual LMWHs: The steeper the curve, the faster the vaporization rate. The plots show that the vaporization rates of the LMWHs were inversely related to their boiling points or carbon number and that the individual components of the lighter fraction rapidly evaporated. The residues of individual LMWHs in the lighter fraction became gradually enriched in 13C during vaporization (Table 1; Fig. 2). Their δ13CR−I values reached up to 0.5‰ (beyond the analytical error of CSIA) once 20%–60% of the compounds were removed, and over 3‰ after the evaporation of about 90% of the component.

Fig. 1
figure 1

Variation of the remaining mass fraction (F) as a function of vaporization time. a The artificial oil; b n-hexane; c n-nonane

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Fig. 2
figure 2

Carbon isotope fractionation of individual LMWHs in the residual artificial oil (δ13CR−I) versus remaining fraction (F) of the corresponding components during progressive vaporization

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The heavier fraction evaporated more slowly (Fig. 1a). After 72-h vaporization, the amounts of n-C10, n-C11, and n-C12 remaining in the mixture compounds sample were 34%, 84%, and 99% of the original, respectively (Table 1; Fig. 2). These compounds showed relatively less isotope fractionations over the entire process of vaporization, particularly n-C11 and n-C12, which showed variations of δ13C of less than 0.6‰ (Table 1). A similar result was reported in a previous study (Wang and Huang 2003): the δ13C values of residual C10, C11, C12, C13, and C14 n-alkanes changed by less than ±0.3‰ when 45%, 29%, 30%, 37%, and 51% of the starting compounds remained in the vial, respectively. Therefore, the δ13C values of the heavier n-alkanes varied little enough to make them useful identifiers of oil that has been evaporated to some extent.

Figure 3 shows the good linear correlation between ln[(δ13CR + 1000)/(δ13CI + 1000)] and lnF for the compounds of the lighter fraction, whose regression coefficients (R 2) were 0.98 (n-C6), 0.99 (n-C7), 0.97 (n-C8), 0.91 (n-C9), 0.996 (MCH), 0.97 (benzene), 0.97 (toluene), 0.91 (ethylbenzene), and 0.93 (o-xylene). These results suggest that the carbon isotope fractionations of these compounds during vaporization from a multi-component system (the artificial oil) followed the Rayleigh fractionation model.

Fig. 3
figure 3

Carbon isotope fractionation of individual compounds, as lnA, versus fraction of residual liquid, presented as lnF, during the progressive vaporization of artificial oil

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The vapor–liquid carbon isotope enrichment factor (ε), also sometimes noted by ∆13Cvapor–liquid, is considered the best way to express the isotope fractionation effect (Hayes, 1993). All the ε values observed here were negative, ranging from −0.87 to −1.74‰ (ε = slope × 1000, Fig. 3), indicating that the progressive vaporization of these compounds was dominated by kinetic fractionation, i.e., the preferential removal of molecules containing the lighter isotope. The same trend was observed by Shin and Lee (2010), who reported enrichment factors for benzene and toluene of −0.58 and −0.41‰, respectively. The magnitude of carbon isotope fractionation during the vaporization of a pure liquid phase appears to be considerably less than that from a multi-component system.

3.2 Carbon isotope fractionations of LMWHs during the progressive vaporization of single pure liquid and diffusion through soil

To understand better matrix effects on the carbon isotope fractionation of individual LMWHs during vaporization from a mixture, vaporization experiments of three pure compounds (n-hexane, n-nonane, and n-dodecane) were conducted under the same conditions as the assessment of the artificial oil. Only 7.4% mass loss and no obvious δ13C variation (<0.5‰, VPDB) were observed for n-dodecane after 72 h of vaporization. Consequently, only n-hexane and n-nonane are discussed.

Table 2 lists the progressive vaporization results for n-hexane, both its pure single phase and when in 1 g soil and 2 g soil. The pure single phase lost mass approximately as quickly as when it was mixed with 1 g soil: Both showed mass losses of about 90% after 80 min, with the δ13C of the residue shifted more than 2.0‰ in both cases. n-Hexane in 2 g soil lost about 90% of its mass after 120 min of vaporization, and the δ13C of the residue shifted 3.3‰. Its maximal δ13C shift was 3.5‰ after 125 min with a corresponding mass loss of 92%, which represents a much slower mass loss than observed for the pure liquid and the n-hexane in 1 g soil. Increasing the mass of soil slowed the vaporization rate and lessened the change of δ13C of the residue.

Table 2 Carbon isotopic composition of residual n-hexane evaporated as a free phase and from soil (24 ± 1 °C)
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Figure 1b shows that n-C6 added to 1 g soil volatilized at a similar rate to the pure liquid phase, indicating that diffusion through the 1 g soil did not have a remarkable effect on the vaporization of the n-C6 owing to its relatively high volatility. Both n-C9 in 1 g soil and n-C6 in 2 g soil evaporated less quickly than their respective pure compounds. The results show that a LMWH’s diffusion through soil can slow its rate of vaporization, with the effect depending on the volatility of the compound.

Table 3 lists the progressive vaporization results of n-nonane, both its pure single phase and in 1 g soil. The compound evaporated more slowly in the soil than alone, but it showed a greater δ13C shift in the soil (Fig. 1c).

Table 3 Carbon isotopic composition of residual n-nonane evaporated as a free phase and from soil
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The strong correlations between lnA and lnF for n-C6 (R 2 = 0.998, 0.99, and 0.98 for the pure single phase, in 1 g soil, and in 2 g soil, respectively) and n-C9 (R 2 = 0.99 and 0.99 for the pure single and in 1 g soil, respectively) (Fig. 4) indicate that the vaporization in each case followed the Rayleigh trend.

Fig. 4
figure 4

Carbon isotope fractionation of vaporization for individual n-C6 (a) and n-C9 (b), as lnA, versus fraction of residual liquid, presented as lnF, during progressive vaporization

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3.3 Possible mechanism of carbon isotope fractionation of LMWHs during vaporization

Theoretically, the effects of equilibrium vapor pressure (evaporation-controlled) and kinetics (diffusion-controlled) are the two main factors that influence the fractionation of stable isotopes of organic compounds during vaporization, and the competition between them directly determine the direction of the fractionation. The evaporation-controlled process usually results in “inverse isotope fractionation,” characterizing of enriching 13C in the vapor phase (Baertschi et al. 1953; Balabane and Letolle 1985; Huang et al. 1999; Poulson and Drever 1999; Wang and Huang 2001; Jeannottat and Hunkeler 2012; Xiao et al. 2012), whereas diffusion-controlled vaporization, which depends on the system itself and intermolecular free energy due to the van der Waals attractive forces among molecules, results in the “normal isotope fractionation,” characterizing of enriching 13C in the residual liquids (Shin and Lee 2010; Xiao et al. 2012; Kuder et al. 2009; Bouchard et al. 2008a, b, c; Jeannottat and Hunkeler 2012; Hayes 1993; Wang and Huang 2001).

Table 4 lists the carbon isotope enrichment factors of the LMWHs considered here and in previous studies, along with values calculated using Eq. (5) (Craig 1953; Cerling et al. 1991; Bouchard et al. 2008c; Jeannottat and Hunkeler 2012). All the experimental values of ε (ε e) for the C6–C9 LMWHs are negative, indicating the enrichment of 13C in the residual liquids and “normal isotope fractionation” during vaporization. The vaporization of these compounds is thus diffusion-controlled, and the equilibrium vapor pressure has little effect on their natural vaporization.

Table 4 Comparison of experimental carbon isotope enrichment factors of this work, previous studies, and the theoretical enrichment factors of LMWHs during vaporization
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The n-alkanes in the mixture compounds showed decreasing experimental values of ε with their increasing carbon number (Table 4): Values of −1.21 ± 0.06, −1.29 ± 0.14, −1.14 ± 0.23, and −1.04 ± 0.17‰ were observed with 95% confidence limits for n-C6, n-C7, n-C8, and n-C9, respectively. Benzene, toluene, ethylbenzene, and o-xylene, respectively, showed values of −1.74 ± 0.08, −1.48 ± 0.10, −1.12 ± 0.12, and −0.87 ± 0.09‰ with 95% confidence limits. These results confirm that the isotope enrichment factor of a compound is controlled by its molecular weight and boiling point. Therefore, the intermolecular binding energies (van der Waals attraction forces) are the main factor controlling the isotope fractionation during the vaporization of the mixture compounds and the single compounds (n-C6 and n-C9) both as a pure single phase and when in soil.

In a diffusion-limited vaporization system, it is well known that the higher the vapor saturation is above that of volatilizing water, the lower the isotope effects (Craig and Gordon 1965). The results of this study combined with previous results indicate that this rule holds for the vaporization of LMWHs. The carbon isotope enrichment factor of volatilizing pure single-phase n-hexane was −0.95 ± 0.04‰ (95% confidence limit, Table 4). While that for pure n-hexane volatilizing across 1 g soil, 2 g soil, and a soil column became gradually higher with the progression along the series of matrices in which the vapor space become increasingly unsaturated and the vapor pressures gradually decreased. A similar trend was observed during the vaporization of n-C9. This demonstrates the effects of the matrix: diffusion materials like soil can decrease the vapor saturation and make the remaining liquid enriched in 13C, further increasing the isotope enrichment factor.

n-C10, n-C11, and n-C12, on the other hand, showed little change in isotopic composition with mass loss (or lack thereof) during vaporization. The isotopic composition of the residual liquids varied almost within the CSIA error for these compounds (Table 1; Fig. 1a). This may be because (1) these hydrocarbons are heavier than those in the lighter fraction, and their strong intermolecular binding energies reduced their evaporation rates and (2) the vapors of these hydrocarbons approached close to saturation, thus greatly impeding their vaporization, which resulted in them showing greatly lower mass loss than the lighter fraction.

4 Conclusions

The effect of vaporization on the carbon isotopic compositions of LMWHs was investigated through three series of experiments examining a mixture of compounds and pure compounds both alone in a single state and when diffusing across soil. Most of the mixture compounds showed obvious mass loss during vaporization, with the rate of vaporization decreasing with the increasing carbon number of the compounds, indicating that molecular weight and boiling point were the main regulator of that vaporization.

Isotope analysis showed that the vaporization patterns of the C6–C12 LMWHs could be classified into two types: one for the lighter C6–C9 fraction and another for the heavier C10–C12 fraction. The remaining portion of the lighter fraction was significantly enriched in 13C by vaporization, with the vaporization fractionation of each hydrocarbon following the Rayleigh model, indicating that kinetic isotope effects controlled the natural vaporization of the molecules and their diffusion through soils. Additionally, significant isotope enrichments (more than 3‰) were apparent in the δ13CR−I values of the corresponding compounds when more than 90% of each components of the lighter fraction had evaporated. In contrast, the heavier fraction remained isotopically consistent due to its lower mass loss during the vaporization, indicating that the isotopic characteristics of these heavier hydrocarbons could be extremely useful for identifying the source of a given oil sample, even one slightly evaporated.

Comparison of all the series of studies conducted here suggests that both mixing a given hydrocarbon and its diffusion through soil could slow its vaporization and increase its carbon isotope enrichment factors, because both the mixture and the soil decreased the vapor pressure in the vapor–liquid system. The values of carbon isotope enrichment factors for the LMWHs are quite close to those calculated from theory and reflect a diffusion-controlled vaporization process during the natural vaporization of the LMWHs.

The C6–C12 LMWHs are widely used to identify the source of oil samples, to assess the thermal maturity of oils and condensates, to determine the source allocation of mixed oils, to identify various secondary alterations of crude oils, and to trace the source of petroleum-related contaminants. This study shows that there is significant isotope fractionation during the natural vaporization of the lighter fraction of these hydrocarbons, which means that isotope monitoring using the C6–C9 LMWHs should be used carefully. However, as natural vaporization has little influence on the isotopic compositions of the heavier hydrocarbons in a short time (i.e. within 72 h), these molecules can provide reliable carbon isotope data better than C6–C9 LMWHs for use in petroleum and environmental science.