Development of a compound-specific carbon isotope analysis method for 2-methyltetrols, biomarkers for secondary organic aerosols from atmospheric isoprene

Article abstract of DOI:10.1021/ac100214p

The stable carbon isotope compositions of 2-methyltetrols, biomarker compounds for secondary organic aerosols formed from isoprene in the atmosphere, have been determined by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). In this work, isoprene with various δ¹³C values was used to produce 2-methyltetrols via an oxidation reaction with hydrogen peroxide in sulfuric acid under direct sunlight. The target compounds with different stable carbon isotope compositions were then derivatized by methylboronic acid with a known δ¹³C value and measured by GC/C/IRMS. With δ¹³C values of 2-methyltetrols and methylboronic acid predetermined, isotopic fractionation is evaluated for the derivatization process. Through reduplicate δ¹³C measurements, the carbon isotope analysis achieved excellent reproducibility and high accuracy with an average error of <0.3%. The differences between the predicted and measured δ¹³C values range from -0.10 to 0.29%, indicating that the derivatization process does not introduce isotopic fractionation. The δ¹³C values of 2-methyltetrols could be calculated on the basis of the stoichiometric mass balance equation among 2-methyltetrols, methylboronic acid, and methylboronate derivatives. Preliminary tests of 2-methyltetrols in PM_2.5 aerosols at two forested sites were conducted and revealed significant differences in their isotope compositions, implying possible application of the method in helping us understand the primary emission, photochemical reaction, or removal processes of isoprene in the atmosphere.

Full text of DOI:10.1021/ac100214p

Anal. Chem. 2010, 82, 6764–6769
Development of a Compound-Specific Carbon
Isotope Analysis Method for 2-Methyltetrols,
Biomarkers for Secondary Organic Aerosols from
Atmospheric Isoprene
Qiang Li,§ Wu Wang,*,†,‡ Hong-Wei Zhang,‡ Yang-Jun Wang,*,† Bing Wang,| Li Li,† Huai-Jian Li,†
Bang-Jin Wang,† Jie Zhan,† Mei Wu,⊥ and Xin-Hui Bi@
Institute of Environmental Pollution and Health, Shanghai University, 200444 Shanghai, China, Southern Medical
University, 510515 Guangzhou, China, School of Stomatology, Tongji University, 200072 Shanghai, China, Hefei
Artillery Academy, 230002 Hefei, China, Laboratory of Human Micromorphology, Qingdao University Medical College,
266071 Qingdao, China, and State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry,
Chinese Academy of Sciences, 510640 Guangzhou, China
The stable carbon isotope compositions of 2-meth-
yltetrols, biomarker compounds for secondary organic
aerosols formed from isoprene in the atmosphere, have
been determined by gas chromatography/combustion/
isotope ratio mass spectrometry (GC/C/IRMS). In this
work, isoprene with various δ13C values was used to
produce 2-methyltetrols via an oxidation reaction
with hydrogen peroxide in sulfuric acid under direct
sunlight. The target compounds with different stable
carbon isotope compositions were then derivatized
by methylboronic acid with a known δ13C value and
measured by GC/C/IRMS. With δ13C values of 2-meth-
yltetrols and methylboronic acid predetermined,
isotopic fractionation is evaluated for the derivati-
zation process. Through reduplicate δ13C measure-
ments, the carbon isotope analysis achieved excellent
reproducibility and high accuracy with an average
error of <0.3‰. The differences between the pre-
dicted and measured δ13C values range from -0.10
to 0.29‰, indicating that the derivatization process
Evidence that isoprene contributes to the formation of second-
ary organic aerosols (SOAs) has been obtained from both field
and chamber experiments.1-6 It was demonstrated that isoprene
and its gas-phase oxidation products, methacrolein (MACR) and
methyl vinyl ketone (MVK), could be converted in aqueous
solution into 2-methyltetrols, i.e., 2-methylerythritol and 2-meth-
ylthreitol, through an acid-catalyzed reaction with hydrogen
peroxide, which is formed in the atmosphere via recombination
of hydroperoxy radicals.2 Recent laboratory studies of isoprene
photooxidation showed SOA formation at precursor concentrations
as low as 10 ppb. The aerosol yield of isoprene from a laboratory
study is in the range of 1-5%.7 However, there are significant
uncertainties with respect to the true impact of atmospheric
aerosols on climate and health because of a lack of knowledge of
their sources, composition, properties, and mechanisms of forma-
tion.8
Isotopic composition measurements have been extremely
useful for improving our understanding of the sources, sinks, and
distribution of atmospheric trace gases. It has been shown that
changes in the stable carbon isotope ratios can be used as
does not introduce isotopic fractionation. The δ13
C
(1) Claeys, M.; Graham, B.; Vas, G.; Wang, W.; Vermeylen, R.; Pashynska, V.;
values of 2-methyltetrols could be calculated on the
basis of the stoichiometric mass balance equation
among 2-methyltetrols, methylboronic acid, and
methylboronate derivatives. Preliminary tests of 2-
methyltetrols in PM2.5 aerosols at two forested sites
were conducted and revealed significant differences
in their isotope compositions, implying possible
application of the method in helping us understand
the primary emission, photochemical reaction, or
removal processes of isoprene in the atmosphere.
Cafmeyer, J.; Guyon, P.; Andreae, M. O.; Artaxo, P.; Maenhaut, W. Science
2004, 303, 1173–1176
(2) Claeys, M.; Wang, W.; Ion, A. C.; Kourtchev, I.; Gelencse´r, A.; Maenhaut,
W. Atmos. Environ. 2004, 38, 4093–4098
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W.; Claeys, M. Atmos. Environ. 2005, 39, 5281–5289
.
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(4) Surratt, J. D.; Kroll, J. H.; Kleindienst, T. E.; Edney, E. O.; Claeys, M.;
Sorooshian, A.; Ng, N. L.; Offenberg, J. H.; Lewandowski, M.; Jaoui, M.;
Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 2007, 41, 517–527
(5) Ng, N. L.; Kwan, A. J.; Surratt, J. D.; Chan, A. W. H.; Chhabra, P. S.;
.
Sorooshian, A.; Pye, H. O. T.; Crounse, J. D.; Wennberg, P. O.; Flagan,
R. C.; Seinfeld, J. H. Atmos. Chem. Phys. 2008, 8, 4117–4140
(6) Wang, W.; Wu, M.; Li, L.; Zhang, T.; Feng, J.; Wang, Y.; Liu, X.; Sheng, G.;
Claeys, M.; Fu, J. Atmos. Chem. Phys. 2008, 8, 7507–7518
.
.
* To whom correspondence should be addresses. (W.W.) Phone: 86 21
66137731. Fax: 86 21 66137731. E-mail: wangwu@shu.edu.cn. (Y.J.W.) Phone:
86 21 66137751. Fax: 86 21 66137731. E-mail: wyj@staff.shu.edu.cn.
† Shanghai University.
(7) Dommen, J.; Hellen, H.; Saurer, M.; Jaeggi, M.; Siegwolf, R.; Metzger, A.;
Duplissy, J.; Fierz, M.; Baltensperger, U. Environ. Sci. Technol. 2009, 43,
6697–6702
.
(8) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.;
Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.;
Hamilton, J. F.; Herrmann, H.; Hoffmann, T.; Iinuma, Y.; Jang, M.; Jenkin,
M. E.; Jimenez, J. L.; Kiendler-Scharr, A.; Maenhaut, W.; McFiggans, G.;
Mentel, Th. F.; Monod, A.; Prevot, A. S. H.; Seinfeld, J. H.; Surratt, J. D.;
‡ Southern Medical University.
§ Tongji University.
| Hefei Artillery Academy.
⊥ Qingdao University Medical College.
@ Chinese Academy of Sciences.
Szmigielski, R.; Wildt, J. Atmos. Chem. Phys. 2009, 9, 5155–5236.
6764 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
10.1021/ac100214p  2010 American Chemical Society
Published on Web 07/28/2010

indicators of atmospheric processing of volatile organic com-
pounds;9 for example, carbon isotope ratio measurements allow
the calculation of the extent of photochemical processing of
isoprene in the atmosphere.10 Furthermore, Rudolph et al.11
developed a gas chromatography/combustion/isotope ratio mass
spectrometry (GC/C/IRMS) technique to measure the stable
carbon isotope ratios of isoprene and its gas-phase oxidation
products, MACR and MVK, and to gain insight into the atmo-
spheric oxidation of isoprene. Very recently, this group has
reported studies on the stable carbon kinetic isotope effects (KIE)
of the reactions of isoprene, MACR, and MVK with OH radicals,
as well as with ozone in the gas phase.12-14 These data are
valuable for obtaining insight into the role of loss processes in
determining the atmospheric mixing ratios. However, there are
no isotopic studies of isoprene SOA products in the aerosol phase.
Methylboronic acid (MBA) has been reported to determine
natural 13C abundances of monosaccharides by derivatizing
adjacent hydroxyl groups of monosaccharides followed by N,O-
bis(trimethylsilyl)trifluoroacetamide (BSTFA) derivatization of
the remaining single OH groups.15,16 Recently, Boschker et al.17
reported a versatile method for stable carbon isotope analysis of
carbohydrates by high-performance liquid chromatography/
isotope ratio mass spectrometry. To develop a compound-specific
isotope analysis method for 2-methyltetrols, marker compounds
of photooxidation products of isoprene, a technique from our
previous work was adapted.18 MBA was used as the derivatization
reagent prior to gas chromatography/combustion/isotope ratio
mass spectrometry (GC/C/IRMS). The derivatizing C from the
reagent accounts for 29% of the analyte in the boronates, but 70%
in the trimethylsilyl (TMS) derivatives. Therefore, the sensitivity
of the MBA method should be high. The oxidation reaction of
isoprene, atmospheric sampling, accuracy, and reproducibility of
the method will be discussed in detail, and the stable carbon
isotope effects during the procedure will be evaluated. δ13C data
for atmospheric 2-methyltetrols will also be presented to
demonstrate the practical utility of this method.
purchased from ABCR GmbH and Co. KG (Karlsruhe, Germany)
(97% pure) and recrystallized three times from a benzene/acetone
mixture (3:1). MBA of the same lot number was used for all
derivatizations. Anhydrous pyridine (99% pure) was supplied by
Acros Organics (Geel, Belgium). BSTFA [N,O-bis(trimethylsilyl)
trifluoroacetamide] was purchased from Pierce (Rockford, IL). All
solvents employed were HPLC grade.
Preparation of Standard 2-Methyltetrols. 2-Methyltetrols
were made by photooxidation of isoprene, which we conducted
by exposing a 30 mL flask with a mixture of 5 mL of 30% H2O2
and 5 mL of isoprene (0.05 mol, 3.4 g) to sunlight. A few drops
of sulfuric acid (0.1 M) was added until the pH of reaction
mixture was between 1 and 2. The resulting mixture was then
maintained with continuous and vigorous stirring in sunlight
for 4 h;19 15 mg of barium carbonate was added to 1 mL of the
reacted solution for neutralization. After centrifugation, the
supernatant was dried, and a slightly yellow oil (2.5 g, 36% yield)
was obtained. It worth noting that exposure to sunlight is
crucial for the production of 2-methyltetrols.
The purification of crude 2-methyltetrols was performed
according to a procedure reported by Wang et al.20 The purity of
2-methyltetrols was verified by gas chromatography/mass spec-
trometry (GC/MS) after they had been derivatized with BSTFA,
and the δ13C value was determined by elemental analyzer/
isotope ratio mass spectrometry (EA/IRMS).
Derivatization of 2-Methyltetrols. The derivatization tech-
nique of 2-methyltetrols with methylboronic acid was adapted from
Wang et al.;18 100 µL of a solution of 2-methyltetrols (ap-
proximately 1 mg/mL in methanol) was dried under a gentle
nitrogen flow, and then 5 mL of a solution of 1 mg of methylbo-
ronic acid in 10 mL of anhydrous pyridine was added. The molar
ratio of MBA to 2-methyltetrols was ∼10:1. The mixture was
allowed to react at 60 °C for 60 min. It should be noted that the
pretreatment of pyridine with 4 Å molecular sieves in excess is
crucial for a successful MBA derivatization. The δ13C value of
methylboronic derivatives was determined by gas chromato-
graphy/combustion/isotopic ratio mass spectrometry (GC/C/
IRMS).
EXPERIMENTAL SECTION
Measurements of the δ13C Value of Standard Isoprene.
The method for determining the δ13C value of isoprene was as
follows.21 Isoprene (1 mL) was sealed in a 2 mL glass vial with
an open screw cap containing a Teflon-lined silica septum. After
∼1 h for equilibrium, 15 µL gas samples from the glass bottle
were injected into the split/splitless injection port of the gas
chromatograph using a Hamilton gastight locking syringe.
Aerosol Sampling. Samples were collected in boreal-temper-
ate Changbai Mountain Forest Ecosystem Research Station in Jilin
Province and subtropical Dinghu Mountain Nature Reserve in
Guangdong Province. The details for sampling sites have been
described previously.6 The sampling occurred during the summer
when the meteorological conditions and the maximum solar
radiation, as well as high temperatures, were favorable for the
photooxidation of isoprene. A high-volume PM2.5 air sampler
Materials. Isoprene was obtained from three suppliers: Fluka,
Sigma-Aldrich (>98% pure, M1); Alfa-Aesar (Lancaster, England)
(99% pure, M2); and Toyo Kasei Kogyo Co. (Osaka, Japan) (99%
pure, M3). Hydrogen peroxide (30% in water) was purchased from
Guoyao (Shanghai, China). Methylboronic acid (MBA) was
(9) Rudolph, J.; Czuba, E. Geophys. Res. Lett. 2000, 27, 3865–3868
(10) Rudolph, J.; Anderson, R. S.; Czapiewski, K. V.; Czuba, E.; Ernst, D.;
.
Gillespie, T.; Huang, L.; Rigby, C.; Thompson, A. E. J. Atmos. Chem. 2003,
44, 39–55
(11) Iannone, R.; Koppmann, R.; Rudolph, J. J. Atmos. Chem. 2007, 58, 181–
202
(12) Rudolph, J.; Czuba, E.; Huang, L. J. Geophys. Res. 2000, 105, 29323–29346
(13) Iannone, R.; Koppmann, R.; Rudolph, J. Atmos. Environ. 2008, 38, 4093–
4098
(14) Iannone, R.; Koppmann, R.; Rudolph, J. Atmos. Environ. 2009, 43, 3103–
3110
(15) van Dongen, B.; Schouten, S.; Damste, J. Rapid Commun. Mass Spectrom.
2001, 15, 496–500
(16) Gross, S.; Glaser, B. Rapid Commun. Mass Spectrom. 2004, 18, 2753–2764
.
.
.
.
.
.
.
(19) Santos, L.; Dalmazio, L.; Eberlin, M.; Claeys, M.; Augusti, R. Rapid Commun.
(17) Boschker, H. T. S.; Moerdijk-Poortvliet, T. C. W.; van Breugel, P.;
Mass Spectrom. 2006, 20, 2104–2108
(20) Wang, W.; Vas, G.; Dommisse, R.; Loones, K.; Claeys, M. Rapid Commun.
Mass Spectrom. 2004, 18, 1787–1797
(21) Yu, Y.; Wen, S.; Feng, Y.; Bi, X.; Wang, X.; Peng, P.; Sheng, G.; Fu, J. Anal.
Chem. 2006, 78, 1206–1211
.
Houtekamer, M.; Middelburg, J. J. Rapid Commun. Mass Spectrom. 2008,
22, 3902–3908
(18) Wang, W.; Li, L.; Li, H.; Zhang, D.; Wen, S.; Jia, W.; Wang, B.; Sheng, G.;
Fu, J. Rapid Commun. Mass Spectrom. 2009, 23, 2675–2678
.
.
.
.
Analytical Chemistry, Vol. 82, No. 16, August 15, 2010 6765

(Graseby-Andersen) was operated at a flow rate of 1.13 m3/
min, and Whatman quartz fiber filters (20.3 cm × 25.4 cm) were
used; 24 h samplers were collected. All filters were baked at
550 °C for 4 h to remove organic contaminants. After collection,
the filters were stored at -20 °C until they were analyzed. Part
of the filters was extracted with methanol. The solvent was
then concentrated, filtered, and finally dried completely. All
samples were derivatized using the same procedure as de-
scribed above and measured by GC/C/IRMS.
Analytical Systems. The gas chromatography/mass spec-
trometry (GC/MS) instrument consisted of a Hewlett-Packard
(Fullerton, CA) model 6890 gas chromatograph equipped with a
DP-5MS device (30 m × 0.25 mm inside diameter, 0.25 µm film
thickness), coupled to a Hewlett-Packard model 5975MSD quad-
rupole analyzer. Data were acquired and processed with Chem-
Station (Hewlett-Packard). The temperature program was as
follows: initial temperature at 100 °C held for 5 min, a gradient of
3 °C/min up to 200 °C, a gradient of 30 °C/min up to 290 °C,
held for 2 min. The mass spectrometer was operated in the
electron ionization mode at 70 eV and an ion source temperature
of 150 °C. Full scan mode was used in the mass range of m/z
50-420.
Two kinds of gas chromatography/combustion/isotopic ratio
mass spectrometry (GC/C/IRMS) systems were used in this
study. The analysis of MBA derivatives was performed on an HP
6890 GC system (Agilent, Santa Clara, CA) equipped with a DP-
5MS device (30 m × 0.25 mm × 0.25 µm), connected to an isotope
ratio mass spectrometer (Isoprime, GV instrument, Manchester,
U.K.). The oven temperature program was as follows: initially at
60 °C, a gradient of 4 °C/min up to 110 °C, held for 2 min, a
gradient of 50 °C/min up to 290 °C, held for 2 min. The injector
was set at 250 °C in splitless mode. Helium was used as the carrier
gas at 1.5 mL/min. CO2 with a known δ13C value (-26.65‰)
was used as the external reference gas. The combustion
furnace containing the CuO catalyst and the reduction oven
containing the Cu catalyst were kept at 940 and 650 °C,
respectively. The temperature of the interface between the GC
and combustion furnace was set at 290 °C. The reproducibility
and accuracy of carbon isotopic analyses were evaluated
routinely every day using 10 laboratory isotopic standards (C12,
C14, C16, C18, C20, C22, C25, C28, C30, and C32 n-alkanes supplied
by Indiana University, Bloomington, IN) with known isotopic
values (-31.89, -30.67, -30.53, -31.02, -32.24, -32.77,
-28.49, -32.11, -33.05, and -29.41‰, respectively). 2-Meth-
yltetrol methylboronates prepared in the laboratory were
analyzed 10 times and used as laboratory standards. For δ13C
analysis of isoprene, the same GC/C/IRMS system described
above and a CP-PoraPLOT Q column (25 m × 0.32 mm ×
10 µm, Varian) were used. The GC was run in a split ratio of
∼40:1, and the injector was set at 150 °C. The initial oven
temperature was held at 120 °C for 1 min, followed by a
gradient of 20 °C/min up to 195 °C, at which point the
temperature was held for 10 min. Standard CO2 (δ13C )
-26.65‰) was used as the external reference gas. CH4 with a
known δ13C value (-36.30‰) was used as the laboratory
isotopic standard to check the reproducibility and accuracy of
the carbon isotopic analysis routinely.22 For both analysis
systems, the sextuple analysis of laboratory isotopic standards
indicated excellent accuracy and reproducibility of carbon
isotopic analysis [the corresponding standard deviation ranged
from 0.13 to 0.30‰, and the deviation between the measured
data and the predetermined data ranged from -0.02 to 0.13‰
(Table 1S of the Supporting Information)].
Elemental analyzer/isotope ratio mass spectrometry (EA/
IRMS) was performed as follows. Standard 2-methyltetrols and
recrystallized methylboronic acid were put into cleaned tin
capsules and weighed, repectively. Capsules containing weighed
samples were placed in the CE (Wigan, U.K.) EA1112 C/N/S
analyzer and burned at 960 °C in an O2 atmosphere in a
combustion tube. Combustion gases were swept through a
reduction oven and entered a GC column where CO2 was
separated from other gases. Then the CO2 passed through a
Conflo III interface (Finnigan, Waltham, MA) and entered a
DELTAplus XL mass spectrometer (Thermo Finnigan MAT,
Bremen, Germany) where it was compared to the reference
CO2 with a known δ13C value (-29.10‰, calibrated against the
NBS-22 reference material with a δ13C value of -29.70‰).
During every batch of analyses, an empty tin capsule was
analyzed as a blank to check the background, and the carbon
black sample with a known δ13C value (-36.91‰) was used to
evaluate the reproducibility and accuracy. The standard devia-
tion of analysis and the deviation between the measured data
and the predetermined data were less than 0.3‰ (Table 2S of
the Supporting Information).
All 13C:12C ratios are expressed in conventional delta (δ)
notation, which is the per mil (‰) deviation from the standard
Vienna Pee Dee Belemnite (VPDB) reference point.
RESULTS AND DISCUSSION
δ13C Analysis of Standard 2-Methyltetrols. Figure 1
shows a typical total ion current (TIC) GC/MS chromatogram
from isoprene oxidation products derivatized by BSTFA. Identi-
fication of these two major compounds was made by comparison
of the retention time and mass spectra with those published in
the literature.1,6,20 Figure 2 presents the TIC GC/MS chromato-
gram of 2-methyltetrol-MBA derivatives. The mass spectra of
these three compounds are very similar, as could be expected
for diastereoisomers. The mass spectrum of compound 2 reveals
a tiny molecular ion (m/z 184). Characteristic ions at m/z 99 and
85 correspond to the cleavage of the C-C bond at the C2 position.
Other ions at m/z 69, 57, and 43 can be explained by the further
fragment of the ion at m/z 85 or 99.
The δ13C values of methylboronic acid and standard 2-me-
thyltetrols were determined by EA/IRMS to be -24.96 0.06‰
(eight measurements) and -32.40 0.14‰ (n ) 6, from M1),
-29.84 0.12‰ (n ) 6, from M2), and -32.36 0.18‰ (n )
6, from M3) (Table 1).
δ13C Analysis of MBA Derivatives. In derivatization, the
preparation of 2-methyltetrol-MBA derivatives alters the original
stable isotope compositions of the 2-methyltetrols. To correct for
the introduction of carbon during derivatization, it is necessary
to assess the isotopic reproducibility of the derivatization method.
(22) Wen, S.; Feng, Y.; Yu, Y.; Bi, X.; Wang, X.; Sheng, G.; Fu, J. Environ. Sci.
Technol. 2005, 39, 6202–6027
.
6766 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010

Figure 1. GC/MS total ion chromatograms obtained for the reaction mixtures of isoprene with H2O2 and sulfuric acid derivatized by BSTFA and
spectra for products (insets) 1 (2-methylthreitol) and 2 (2-methylerythritol).
Figure 2. GC/MS total ion chromatogram obtained for 2-methyltetrols derivatized by MBA and mass spectra for the products (insets). Compounds
1 and 2 correspond to 2-methylerythritol-MBA derivatives, and compound 3 corresponds to a 2-methylthreitol-MBA derivative.
Table 1. Stable Carbon Isotopic Compositions of Isoprene, 2-Methyltetrols (2-MT), and MBA Derivatives Measured
and Predicted in the Derivatization Reaction
δ13Ca
g
supplier
measured isopreneb,c
measured 2-MTb,d
measured methylboronatesb,c,e
calculated methylboronatesb,f
∆
M1
M2
M3
-24.54 0.18 (n ) 7)
-23.72 0.23 (n ) 7)
-25.19 0.19 (n ) 7)
-32.40 0.14 (n ) 6)
-29.84 0.12 (n ) 6)
-32.36 0.18 (n ) 6)
-29.98 0.22 (n ) 6)
-28.08 0.15 (n ) 6)
-30.35 0.16 (n ) 6)
-30.27
-28.24
-30.25
0.29
0.16
-0.10
a Stable carbon isotopic compositions reported in per mil relative to PDB. b Arithmetic means and standard deviations. c δ13C values determined
d
e
by GC/C/IRMS. δ13C values determined by EA/IRMS. Derivatized by methylboronic acid with a δ13C value of -24.96
0.06‰ (eight
measurements) determined by EA/IRMS. f On the basis of mass balance relationship eq 1. Calculated δ13C - measured δ13C.
g
The reproducibility of the carbon isotope composition for three
2-methyltetrols (with different δ13C values) was evaluated. Their
δ13C values and those of the corresponding MBA derivatives
are listed in Table 1. The analytical errors (standard deviation)
obtained for six EA/IRMS analyses of 2-methyltetrols produced
from isoprene from the same supplier ranged from 0.12 to 0.18‰,
with an average of 0.15 0.03‰, while for GC/C/IRMS analyses
of MBA derivatives, the analytical errors were from 0.15 to 0.22‰,
with an average of 0.17 0.04‰. The reproducibility compares
well with those obtained in the derivatization of fatty acids.23
Isotopic Effects of the Method. According to eq 1, the
theoretical δ13C values of methylboronates can be calculated
according to stoichiometric mass balance and reflect the
relative contributions of carbon from 2-methyltetrols, methyl-
boronic acid, and their respective δ13C values:
13
13
δ13Cmethylboronate ) f
δ CMBA + f
δ C2-methyltetrols
MBA
2-methyltetrol
(1)
where fMBA and f2-methyltetrol are the molar fractions of carbon
in methylboronate arising from underivatized 2-methyltetrol
and MBA reagent, respectively. Here, fMBA
f2-methyltetrols
5/7. The stable carbon isotopic compositions
obtained for 2-methyltetrols and derivatives are listed in Table
)
2/7, and
)
(23) Abrajano, T. A.; Murphy, D. E., Jr.; Fang, J.; Comet, P. A.; Brooks, J. M.
Org. Geochem. 1994, 21, 611–617
.
1. The measured data for methylboronates were compared with
Analytical Chemistry, Vol. 82, No. 16, August 15, 2010 6767

Figure 3. GC/C/IRMS chromatogram of 2-methyltetrol-MBA derivatives: (a) standard derivative and (b) extract of the fine size fraction of a
24 h Hi-Vol aerosol sample collected at Dinghu, China, on August 2, 2006. Compounds 1 and 2 are 2-methylerythritol-MBA derivatives, and
compound 3 is the 2-methylthreitol-MBA derivative.
those predicted by eq 1. The predicted and measured δ13C values
of methylboronates agreed well with each other.
2-methyltetrols, MBA, and methylboronates, respectively. In
this work, SMBA was 0.06‰ and Smethylboronate was 0.17 0.04‰
(0.15-0.22‰). According to eq 2, the standard deviation of the
measured δ13C value of 2-methyltetrol ranged from 0.21 to
0.31‰. The results imply that the method is promising and
introduces no isotopic fractionation in the derivatization pro-
cess, as confirmed by Table 1. The stable carbon isotopic
determination exhibited high precision and good reproducibility.
On the other hand, precursor isoprene was approximately
6.12-7.86‰ more enriched in 13C than 2-methyltetrols (Table
According to Rieley’s discussion of the kinetic isotope effect,24
when a bond containing the carbon atom under consideration is
changed in the rate-determining step, the primary isotopic effect
is the most significant. If no carbon bond changed in the rate-
determining reaction or if no carbon-containing bond is involved
in this step, there should not be a primary isotope effect on the
δ13C value. In this work, four hydroxyl groups from two
molecules of MBA react with four hydroxyl groups of 2-meth-
yltetrol and eliminate four molecules of water. No other carbon-
containing bonds, except C-O bonds of 2-methyltetrols, are
involved in the reaction. The difference between measured and
calculated δ13C values of methylboronic derivatives ranged from
-0.10 to 0.29‰ (Table 1). Accuracy was well within the isotope
technical specification ( 0.5‰). It should be pointed out that the
analytical error of the calculated data for underivatized 2-meth-
yltetrol (usually expressed as the standard deviation, S) could be
calculated by the following equation:
1
). This might reflect isotope fractionation during the photochemi-
cal oxidation reaction. Rudolph et al.12 reported that the stable
carbon isotope fractionation for the reaction of isoprene with OH
radicals in the gas phase was 6.94 0.80‰, very similar to the
value in this work.
To fully evaluate the usefulness of relating precursor isoprene
δ13C values to those measured in aerosol, one must investigate
the potential for the primary kinetic isotope effect (KIE)
involving the carbon-carbon bond cleavage products (such as
methylglyceric acid, glyoxal, and methylglyoxal) to influence
the δ13C value of 2-methyltetrols. This is especially true since
the reaction pathways for formation of 2-methyltetrols remain
unclear. Gas-phase and mixed-phase mechanisms involving the
2
2
S2-methyltetrol ) (1/f2-methyltetrol)2Smethylboronate
+
2
(fMBA/f2-methyltetrol)2SMBA (2)
-3
aqueous aerosol phase have been considered.1
where fMBA and f2-methyltetrol are the same as in eq 1 and S2-methyltetrol
,
Measurements of Atmospheric Samples. The target com-
pounds in samples were well-separated (Figure 3), while the
2-methylerythritol derivative gave rise to double peaks, due to the
SMBA, and Smethylboronate are the analytical standard deviations of
(24) Rieley, G. Analyst 1994, 119, 915–919
.
6768 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010

Table 2. Concentrations and Stable Carbon Isotopic Compositions of 2-Methyltetrols at Two Forested Sites
δ13Ca
sampling site
Changbai
sampling date
concentration of 2-methyltetrols (ng/m3)
measured methylboronatesb
calculated 2-MTc
July 27, 2007
July 28, 2007
August 1, 2006
August 2, 2006
76.29
108.8
65.30
83.54
-24.27 0.13 (n ) 3)
-24.79 0.61 (n ) 3)
-27.29 0.10 (n ) 3)
-26.93 0.46 (n ) 3)
-24.00 0.18
-24.73 0.86
-28.22 0.14
-26.99 0.64
Dinghu
a Stable carbon isotopic compositions reported in per mil relative to PDB. b Arithmetic means and standard deviations. Derivatized by methylboronic
acid with a δ13C value of -24.96 0.06‰ (eight measurements) determined by EA/IRMS. δ13C values determined by GC/C/IRMS. c Arithmetic
means of calculated δ13C values based on mass balance relationship eq 1 and standard deviations (S) calculated according to eq 2.
formation of both E and Z isomers during the reaction. From our
previous work,18 the erythro form of tetrols, erythritol, eluted
earlier than the threo form, threitol. Under the same conditions,
is introduced. The δ13C values of atmospheric 2-methyltetrols
were determined for two forest aerosols. Considering the
complex formation pathways of 2-methyltetrols, this technique
will be helpful for full field application and will have potential
in differentiation between homogeneous and heterogeneous
oxidation processes. The application of this method may
provide additional information about the sources and sinks of
atmospheric isoprene. Further experiments on photochemical
oxidation in a smog chamber are warranted.
peaks
1 and 2 were identified as (Z)- and (E)-2-methyl-
erythritol-MBA derivatives, respectively, while peak 3 was the
2-methylthreitol-MBA derivative. The latter derivative gives rise
to only one tiny peak, due to the stereo hindrance of the 2-methyl
group. The δ13C value of the 2-methyltetrol-MBA derivatives
was obtained by defining these three peaks as one using the
integral tools of IRMS.
It could be seen from Table 2 that δ13C values of 2-meth-
yltetrols were distinctly different for the two sites. It has been
documented that there were significant differences in carbon
isotope ratios among C3, C4, and CAM plants, e.g., -35 to
-23‰ for C3 with an average of -26‰, -14 to -10‰ for C4
with an average of -13‰, and intermediate carbon isotope
compositions for CAM.25 The δ13C of 2-methyltetrols might
reveal the relation of atmospheric aerosol with the vegetation
types and be used as an indicator of proportions of C3 to C4
plants in vegetation.
ACKNOWLEDGMENT
This work was funded by the Knowledge Innovation Program
of the Chinese Academy of Sciences (Grant kzcx2-yw-139), the
Natural Science Foundation of China (Grants 20677036, 20877051,
and 40873073), the Innovation Program of Shanghai Municipal
Education Commission (10YZ08), the Shanghai Municipal Health
Bureau (054088), Shanghai Leading Academic Disciplines (S30109),
and the Scientific Research Foundation for Returned Overseas
Chinese Scholars, State Education Ministry, China. We thank the
anonymous referee for insightful comments. We also thank Dr.
Jia Wanglu (State Key Laboratory of Organic Geochemistry,
Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences) for his technical assistance.
CONCLUSIONS
The stable carbon isotope compositions of 2-methyltetrols,
biomarker compounds of isoprene in atmospheric aerosols, were
determined by GC/C/IRMS. MBA was used as the derivatization
reagent which allows only 29% contribution of the analyzed C.
The δ13C values of 2-methyltetrols are calculated on the basis
of the stoichiometric mass balance equation among 2-meth-
yltetrols, methylboronic acid, and methylboronate derivatives.
Excellent reproducibility and accuracy are achieved without
carbon isotopic fractionation. Moreover, photosynthesis of
2-methyltetrols through photochemical oxidation of isoprene
SUPPORTING INFORMATION AVAILABLE
δ13C values (‰) of the Indiana reference n-alkanes measured
by GC/C/IRMS (Isoprime) (Table 1S) and δ13C values of the
reference materials of CH4 and carbon black (Table 2S). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Received for review January 25, 2010. Accepted April 19,
2010.
(25) Cerling, T. E.; Wang, Y.; Quade, J. Nature 1993, 361, 344–345
.
AC100214P
Analytical Chemistry, Vol. 82, No. 16, August 15, 2010 6769