Carbon isotopic fractionation during vaporization of low molecular weight hydrocarbons (C6–C12)
Carbon isotopic fractionation during vaporization of low molecular weight hydrocarbons (C6-C12)
Qian-Yong Liang 0 1 2
Yong-Qiang Xiong 0 1 2
Jing Zhao 0 1 2
Chen-Chen Fang 0 1 2
Yun Li 0 1 2
0 PetroChina Research Institute of Petroleum Exploration and Development , Beijing 100083 , China
1 State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences , Guangzhou 510640, Guangdong , China
2 Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Ministry of Land and Resources , Guangzhou 510760, Guangdong , China
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.
Low molecular weight hydrocarbons; Gas chromatography-isotope ratio mass spectrometry; Isotope fractionation; Vaporization
& Yong-Qiang Xiong
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 (Can˜ipa-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; Can˜ipa-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
(Can˜ipa-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 d13C 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) (D13Cvapor–liquid & ?0.2%) (Harrington et al.
1999), trichloroethylene (D13Cvapor–liquid = ?0.1% to
?0.7%) (Poulson and Drever 1999), chlorinated aliphatic
hydrocarbons (D13Cvapor–liquid = ?0.31% for
trichloroethene and D13Cvapor–liquid = ?0.65% for
dichloromethane) (Huang et al. 1999), and MTBE (tert-butyl
methyl ether, D13Cvapor–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 D13Cvapor–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
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; Ho¨hener et al.
2003; Schu¨th 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
(Schu¨th et al. 2003). However, significant fractionation has
been observed after passing some volatile organic
compounds across alluvial sand (e.g., D13Cvapor–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 d13C 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
singlephase compounds. The third series examined vaporization
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
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
ndodecane) 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 lL; benzene, 700 lL; n-C7, 400 lL;
MCH, 400 lL; toluene, 400 lL; n-C8, 300 lL;
ethylbenzene, 300 lL; o-xylene, 300 lL; n-C9, 200 lL; n-C10, 200
lL; n-C11, 200 lL; and n-C12, 200 lL) to a 4-mL glass vial
capped with an aluminum–rubber seal.
In the first vaporization experiment, aliquots
(approximately 200 lL) 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
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 lL) 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 d13C 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
HPPONA fused silica capillary column (50 m 9 0.20 mm
i.d. 9 0.50 lm), 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
Carbon isotopic compositions of LMWHs were measured
with a gas chromatograph (Agilent 6890) equipped with a
DB-5MS column (50 m 9 0.25 mm i.d. 9 0.25 lm)
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
(NafionTM) with a dry He counter flow. Carbon isotope
ratios were computed by five pulses of CO2 reference gas
with known d13C 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%
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 ¼ Ro Fa 1
d13CR þ 1000 = d13CI þ 1000
where F is the mass fraction of the original compound
remaining, R and Ro are the 13C/12C value of the individual
compound at a specific F (F \ 1) and at F = 1,
respectively, and a (equal to Rvapor/Rliquid) is the vapor–liquid
fractionation factor. d13CR and d13CI are the d13C values of
the residual and initial compound, respectively. For each
compound of the artificial oil, the a values were calculated
by linear regression of lnF versus lnA.
The corresponding isotope enrichment factors can be
calculated according to:
eð&Þ ¼ ða 1Þ 1000
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
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
d13CR-I values of 0.6, 0.4, and 0.2% (within the CSIA
Based on their vaporization rates and variations in d13C
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 d13CR-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.
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
-46.1 ± 0.02
-45.6 ± 0.03
-44.9 ± 0.21
-44.0 ± 0.08
-42.1 ± 0.00
-24.6 ± 0.17
-24.1 ± 0.22
-23.6 ± 0.03
-22.3 ± 0.03
-20.6 ± 0.17
-39.4 ± 0.12
-39.3 ± 0.10
-39.3 ± 0.07
-39.0 ± 0.04
-38.2 ± 0.07
-37.4 ± 0.24
-36.6 ± 0.04
-33.5 ± 0.13
-29.3 ± 0.07
-29.2 ± 0.07
-29.2 ± 0.03
-29.0 ± 0.09
-28.6 ± 0.07
-27.9 ± 0.01
-27.3 ± 0.07
-25.6 ± 0.05
-29.3 ± 0.25
-29.1 ± 0.23
-28.9 ± 0.09
-28.8 ± 0.12
-28.2 ± 0.14
-27.4 ± 0.18
-26.9 ± 0.16
-24.4 ± 0.05
-45.9 ± 0.15
-45.9 ± 0.10
-45.8 ± 0.13
-45.7 ± 0.11
-45.6 ± 0.01
-44.5 ± 0.25
-44.2 ± 0.00
-43.6 ± 0.06
-39.4 ± 0.00
-28.3 ± 0.18
-28.3 ± 0.09
-28.3 ± 0.05
-28.1 ± 0.03
-28.1 ± 0.06
-27.0 ± 0.25
-27.2 ± 0.05
-26.0 ± 0.05
-25.1 ± 0.15
-28.0 ± 0.13
-28.1 ± 0.08
-28.0 ± 0.16
-28.1 ± 0.00
-28.0 ± 0.08
-27.5 ± 0.21
-27.4 ± 0.03
-27.0 ± 0.03
-24.9 ± 0.00
-49.4 ± 0.04
-49.5 ± 0.19
-49.3 ± 0.21
-49.3 ± 0.06
fractionations over the entire process of vaporization,
particularly n-C11 and n-C12, which showed variations of
d13C of less than 0.6% (Table 1). A similar result was
reported in a previous study (Wang and Huang 2003): the
d13C values of residual C10, C11, C12, C13, and C14
nalkanes changed by less than ±0.3% when 45%, 29%,
-49.4 ± 0.07
-49.0 ± 0.12
-49.4 ± 0.00
-49.3 ± 0.05
-48.4 ± 0.24
-47.3 ± 0.28
-44.5 ± 0.06
-35.6 ± 0.10
-35.8 ± 0.11
-35.4 ± 0.10
-35.7 ± 0.00
-35.5 ± 0.10
-35.3 ± 0.05
-35.0 ± 0.00
-35.0 ± 0.03
-34.9 ± 0.05
-27.8 ± 0.28
-27.6 ± 0.07
-27.6 ± 0.04
-27.7 ± 0.03
-27.5 ± 0.03
-27.7 ± 0.00
-27.6 ± 0.00
-27.5 ± 0.01
-27.5 ± 0.03
-31.9 ± 0.07
-31.9 ± 0.05
-31.7 ± 0.01
-31.8 ± 0.03
-31.7 ± 0.03
-31.8 ± 0.00
-31.9 ± 0.00
-31.7 ± 0.03
-31.7 ± 0.03
Evaporation time, h
Fig. 1 Variation of the remaining mass fraction (F) as a function of
vaporization time. a The artificial oil; b n-hexane; c n-nonane
30%, 37%, and 51% of the starting compounds remained in
the vial, respectively. Therefore, the d13C 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[(d13CR ? 1000)/(d13CI ? 1000)] and lnF for the
compounds of the lighter fraction, whose regression
coefficients (R2) 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
Fig. 2 Carbon isotope fractionation of individual LMWHs in the c
residual artificial oil (d13CR-I) versus remaining fraction (F) of the
corresponding components during progressive vaporization
(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.
The vapor–liquid carbon isotope enrichment factor (e),
also sometimes noted by D13Cvapor–liquid, is considered the
best way to express the isotope fractionation effect (Hayes,
1993). All the e values observed here were negative,
ranging from -0.87 to -1.74% (e = slope 9 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 d13C
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
nhexane, 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 d13C 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 d13C of the residue shifted 3.3%. Its
maximal d13C 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 d13C of the
Figure 1b shows that n-C6 added to 1 g soil volatilized
at a similar rate to the pure liquid phase, indicating that
b Fig. 3 Carbon isotope fractionation of individual compounds, as lnA,
versus fraction of residual liquid, presented as lnF, during the
progressive vaporization of artificial oil
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
nnonane, 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 d13C shift in the soil (Fig. 1c).
The strong correlations between lnA and lnF for n-C6
(R2 = 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 (R2 = 0.99
-46.2 ± 0.08
-46.0 ± 0.04
-45.8 ± 0.08
-45.6 ± 0.05
-45.1 ± 0.05
-44.8 ± 0.09
-44.5 ± 0.12
-44.1 ± 0.08
-46.2 ± 0.05
-45.9 ± 0.12
-45.8 ± 0.07
-45.6 ± 0.04
-45.3 ± 0.11
-45.0 ± 0.02
-44.5 ± 0.06
-44.2 ± 0.11
-44.0 ± 0.05
d13C ± SD (VPDB, %)c
d13C ± SD (VPDB, %)c
-4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5
Fig. 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
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.
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
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 e (ee) 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.
The n-alkanes in the mixture compounds showed
decreasing experimental values of e 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
n-C10, n-C11, and n-C12, on the other hand, showed little
change in isotopic composition with mass loss (or lack
Table 4 Comparison of
experimental carbon isotope
enrichment factors of this work,
previous studies, and the
theoretical enrichment factors of
LMWHs during vaporization
eea (%) 95% CIb
Experimental material atc
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.
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 d13CR-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
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
Acknowledgements We are grateful to Chen H. S. for the technical
assistance. This work is financially supported by the National ‘‘863’’
Project (Grant No. 2012AA0611401) and the program of the Chinese
Academy of Sciences (Grant No. KZCX2-YW-JC103). This is
contribution number IS-2343 from Guangzhou Institute of Geochemistry,
Chinese Academy of Sciences (GIGCAS). We also acknowledge
three anonymous reviewers for their helpful comments and
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