Chemical compositions of secondary organic aerosol from the ozonolysis of cyclohexene in the absence of seed particles

Article abstract of DOI:10.1246/cl.2005.1584

The composition of the aerosol from the ozonolysis of cyclohexene in the absence of seed particles has been investigated by laboratory chamber experiments. The aerosol collected on filters was analyzed by a liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry. Low-molecular weight products, i.e., dicarboxylic acids, oxocarboxylic acids, and hydroxydicarboxylic acids, as well as oligomers with molecular weights more than 200 were found in the aerosol. Copyright

Full text of DOI:10.1246/cl.2005.1584

1584
Chemistry Letters Vol.34, No.12 (2005)
Chemical Compositions of Secondary Organic Aerosol from the Ozonolysis
of Cyclohexene in the Absence of Seed Particles
Kei Sato
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506
(Received August 30, 2005; CL-051114)
The composition of the aerosol from the ozonolysis of cy-
Toledo, AG285) before and after the sampling in order to meas-
ure the mass of the collected aerosol. The average of aerosol
yield with the error of the standard deviation was 24 ꢀ 8 wt %
for eight experiments, showing agreement with the literature
values (19.1–21.1 wt %).9
clohexene in the absence of seed particles has been investigated
by laboratory chamber experiments. The aerosol collected on fil-
ters was analyzed by a liquid chromatography/atmospheric pres-
sure chemical ionization-mass spectrometry. Low-molecular
weight products, i.e., dicarboxylic acids, oxocarboxylic acids,
and hydroxydicarboxylic acids, as well as oligomers with molec-
ular weights more than 200 were found in the aerosol.
The pretreatment and the analysis were performed just after
the sampling. The filter was sonicated in 3-mL dichloromethane
for 10 min. The extract was evaporated to nearly dryness under a
gentle stream of nitrogen. The concentrated sample was dis-
solved in 1.0-mL acidic methanol water (0.043% formic acid,
15% methanol). 50 mL of the solution was loop-injected into
an LC/APCI-MS instrument (Shimadzu, LC8000ꢀ) equipped
with an Inertsil ODS-3 C18 column with 3-mm particle size
(GL science, 150 ꢂ 3:0 mm) kept at 35 ꢃC. Methanol and
0.05% aqueous solution of formic acid was used as the mobile
phases. The concentration of methanol was programmed to be
15, 15, 90, and 90% at retention times of 0, 5, 25, and 30 min,
respectively. The total flow rate of the mobile phase was set
to 0.4 mL minꢁ1. The separated analytes were detected by the
APCI mass spectrometer run in negative ion mode. As standard
mass spectra, the spectra of dodecanedioic acid, octanedioic
acid, heptanedioic acid, hexanedioic acid, pentanedioic acid,
butanedioic acid, 4-oxobutanoic acid, and 2-hydroxy-2-methyl-
butanedioic acid were measured. The quasi-molecular ions were
detected without serious fragmentations for all the molecules
examined.
The ozonolysis of cyclohexene is one of typical gas-phase
reactions leading to formation of condensable products, and its
reaction mechanism may be analogous to the monoterpene ozo-
nolysis contributing potentially to formation of atmospheric sec-
ondary organic aerosol.1 As the molecules contained in aerosol
from the title reaction, multifunctional species such as dicarbox-
ylic acids, oxocarboxylic acids, and hydroxydicarboxylic acids
have been known from previous studies using gas chromatogra-
phy–mass spectrometry.2,3
Very recently, Tolocka et al. applied liquid chromatography
(LC) combined with matrix-assisted laser desorption ionization
(MALDI), electrospray ionization (ESI), and desorption ammo-
nia chemical ionization (CI)-mass spectrometry (MS) for aerosol
from the monoterpene ozonolysis and found oligomers in aero-
sol with the soft ionization techniques, i.e., MALDI and ESI.4
Gao et al. used an LC/ESI-MS to analyze aerosol from the
ozonolysis of six cyclic alkenes and found that oligomers were
contained in late eluent of reverse phase chromatography for
all aerosol.5
In this work, an atmospheric pressure chemical ionization
(APCI) method was employed for the LC/MS analysis of aero-
sol from the ozonolysis of cyclohexene. APCI is generally a
powerful soft ionization technique for molecules with moderate
polarities6 and may provide high sensitivities for analytes
included in late eluent for reverse phase chromatography. A
purpose of this work is to test APCI-MS to probe oligomers in
aerosol.
Figure 1 shows mass spectra of a flow-injected aerosol sam-
ple at the methanol concentration of 15%. The highest peak at
mass to charge ratio (m=z) of 145 is attributed to adipic acid with
molecular weight (MW) of 146. Mass peaks appeared also in a
region from m=z ¼ 146 to ca. m=z ¼ 500. Especially, the group
of peaks in the m=z range between 200 and 300 showed regular
mass differences of m=z ¼ 14, 16, and 18. Such a regular struc-
ture is typical for oligomers.10
100
The experiments were conducted with a 6-m3 laboratory
chamber7,8 without using seed particles at 298 ꢀ 1 K. Prior to
each experiment, the chamber was filled with 760-Torr dry puri-
fied air. 1.0–2.5-ppmv ozone was introduced into the chamber
from an ozone generator (Nippon Ozone, Type-012). Equimo-
lecular gaseous cyclohexene was collected in a calibrated bulb
and was then injected into the chamber with a nitrogen carrier
gas. Ozone and cyclohexene were monitored by an FTIR spec-
trometer (Nicolet, Nexus 670) with an optical length of 221.5
m. The reaction was almost concluded in 40 min. After the con-
clusion of the reaction, the aerosol was collected on a Teflon
membrane filter with 47-mm diameter and 0.45-mm pore size
(Sumitomo Electric, FP-045) at a flow rate of 10 L minꢁ1 for
40 min. The filter was weighed with an electric balance (Mettler
5
50
0
200
300 400
m/z
500 600
0
100
200
300
m/z
400
500
600
Figure 1. Mass spectra of aerosol from ozonolysis of cyclo-
hexene (Intensity at m=z ¼ 145 is set to 100).
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.12 (2005)
1585
tions occurring in the chamber. Gao et al. suggested that the
oligomers found in the absence of seed particles might be pro-
duced by heterogeneous reactions catalyzed by organic acids,4
while Ziemann proposed gas-phase radical–radical reactions
leading to dimeric products.11 The formation mechanism of
oligomers is still unclear.
TIC
m/z = 145
m/z = 217
The quantifications of dicarboxylic acids were carried out
using the corresponding standards, whereas those of oxocarbox-
ylic acids, hydroxydicarboxylic acids, and oligomers were, re-
spectively, performed by employing 4-oxobutanoic acid, 2-hy-
droxy-2-methylbutanedioic acid, and dodecanedioic acid as sur-
rogate standards. The mass fractions of dicarboxylic acids, oxo-
carboxylic acids, hydroxydicarboxylic acids, and oligomers in
the total aerosol were obtained to be 30 ꢀ 8, 4:8 ꢀ 1:4, 1:1 ꢀ
0:6, and 5:3 ꢀ 3:8 wt %, respectively. Gao et al. reported these
values for dicarboxylic acids, oxocarboxylic acids, hydroxydi-
carboxylic acids, and oligomers to be 24, 4, 3, and 3 wt %, re-
spectively.5 Although their value of hydroxydicarboxylic acids
is slightly higher than the present result, the other results show
agreement with the corresponding present results, respectively.
In conclusion, an LC/APCI-MS method was firstly exam-
ined for the analysis of aerosol from the ozonolysis of cyclo-
hexene, and low-MW and oligomeric components in aerosol
were successfully analyzed. APCI-MS was shown to be one of
powerful detection techniques for oligomers. The formation
mechanism of oligomers in the absence of seed particles is still
unclear. One of possible approaches to study the formation
mechanism is measurements of the concentrations of aerosol
constituents as a function of reaction time.2 Such measurements
are now in progress.
m/z = 231
m/z = 245
0
10
20
Retention time / min
Figure 2. Total ion chromatogram and extracted ion chromato-
grams of ions with m=z ¼ 145, 217, 231, and 245.
Figure 2 illustrates total ion chromatogram (TIC) and ex-
tracted ion chromatograms (EIC) of ions with m=z ¼ 145, 217,
231, and 245 of the aerosol sample. Though overlaps of peaks
were observed in TIC, all peaks were successfully separated in
EIC of each m=z ion. The chromatographic peaks of ions with
m=z < 200 appeared in a retention time region earlier than
15 min. These low-MW components were identified from the
mass spectra and the retention times as dicarboxylic acids
(hexanedioic acid, pentanedioic acid, and butanedioic acid),
oxocarboxylic acids (oxohexanoic acid, oxopentanoic acid, 4-
oxobutanoic acid, dioxohexanoic acid, and dioxopentanoic
acid), and hydroxydicarboxylic acids (hydroxyhexanedioic acid,
hydroxypentanedioic acid, and hydroxybutanedioic acid). Di-
oxohexanoic acid is a newly identified molecule. Cn (n > 6) di-
carboxylic acids and dicarboxylic acid esters reported as minor
components by Gao et al.5 were not found.
The chromatographic peaks of ions with m=z ꢄ 200 ap-
peared in a region latter than 15 min. Strong peaks were ob-
served at m=z ¼ 215, 217, 229, 231, 243, 245, 257, 259, 273,
and 289. This component was tentatively identified as oligomers.
In order to confirm that no oligomer was produced during the
sampling, the pretreatment, and the analysis, the following two
checks were performed: First, oligomer formation during the
ionization were checked by analyzing equal-mass methanol so-
lution of hexanedioic acid, 4-oxobutanoic acid, 2-hydroxy-2-
methylbutanedioic acid, and pentanedial. No oligomer was
found in the mixture solution. Next, oligomer formation during
the sampling and the pretreatment was studied. The mixture so-
lution was dried on a Teflon filter, and 0.1-ppmv ozone/air gas-
eous mixture was then flowed through the filter at 10 L minꢁ1 for
40 min. In the extract of the filter, no oligomer was detected.
These results indicate that the oligomers are produced by reac-
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology and a Global Environment Research
Fund Project from the Ministry of the Environment.
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Published on the web (Advance View) October 27, 2005; DOI 10.1246/cl.2005.1584