Identification of products containing -COOH, -OH, and -C=O in atmospheric oxidation of hydrocarbons

Article abstract of DOI:10.1021/es980129x

Atmospheric oxidation of hydrocarbons by hydroxyl radicals and ozone leads to products containing -COOH, -OH, and -C=O functional groups. The high polarity of such compounds precludes direct GC-MS analysis. In addition, many such compounds often exist in a single sample at trace levels. An analytical method has been developed to identify compounds containing one or more functional groups of carbonyl, carboxy, and hydroxy in atmospheric samples. In the method, -C=O groups are derivatized using O-(2,3,4,5,6- pentafluorobenzyl) hydroxy amine (PFBHA), and -COOH and -OH groups are derivatized using a silylation reagent N,O-bis(trimethylsilyl)- trifluoroacetamide (BSTFA). The derivatives are easily resolved by a GC column. The chemical ionization mass spectra of these derivatives exhibit several pseudomolecular ions, allowing unambiguous determination of molecular weights. Functional group identification is accomplished by monitoring the ions in the electron ionization mass spectra that are characteristic of each functional group derivative: m/z 181 for carbonyl and m/z 73 and 75 for carboxyl and hydroxy groups. The method is used to identify products in laboratory studies of ozone oxidation of α-pinene and Δ³-carene. Among products from ozone oxidation of α-pinene, we have detected pinonaldehyde, norpinonaldehyde, pinonic acid, norpinonic acid, C₁₀ hydroxy dicarbonyls, pinic acid, 2,2-dimethyl-3-(formylmethyl)-cyclobutane-formic acid, and a product that has a molecular weight of 156 and contains a C=O and a COOH/OH group. The latter two products have not been reported previously. Δ³- Carene is structurally analogous to α-pinene in that both have an internal unsaturated bond where ozone oxidation takes place. We have also identified the corresponding analogous products, of which all but caronaldehyde are reported for the first time. An analytical method was developed to identify compounds containing one or more functional groups of carbonyl, carboxyl and hydroxyl in atmospheric samples. -C-to-O double bond groups are derivatized using 0-(2,3,4,5,6-pentafluorobenzyl)hydroxyl amine, and -COOH and -OH groups are derivatized using a silylation reagent N,O-bis(trimethylsilyl)-trifluoroacetamide. The derivatives are resolved using a gas chromatography column coupled with mass spectrometry. The method identified products in laboratory studies of ozone oxidation of α-pinene and Δ³-carene.

Full text of DOI:10.1021/es980129x

Research
(hydroxy) functional groups. Oxidation of alkanes with a
carbon chain length of gC4, mainly initiated by OH radical
attack, forms hydroxy carbonyl products through isomer-
ization of alkoxy radical intermediates at increasing yields
with increasing carbon numbers. For gC5 n-alkanes, hydroxy
carbonyls are dominant products (1-4). Oxidation of
hydrocarbons containing CdC bonds can be initiated by both
O3 and OH radicals. Studies of simple alkenes have estab-
lished that subsequent reactions lead to the formation of
carbonyls, hydroxy carbonyls, dicarbonyls, carboxylic acids,
and oxocarboxylic acids (2, 5-11).
Identification of Products Containing
-COOH, -OH, and -CdOin
Atmospheric Oxidation of
Hydrocarbons
J I A N Z H E N YU , †
R I C H A R D C . F L A G A N , ‡ A N D
J O H N H . S E I N F E L D * , ‡
Department of Environmental Engineering Science and
Department of Chemical Engineering, MS 210-41, California
Institute of Technology, Pasadena, California 91125
Higher molecular weight compounds containing -OH,
-COOH, and -CdO groups are expected to be important
components of secondary organic aerosols (12). Experi-
mental evidence suggests that compounds with these
moieties are significant fractions of aerosol organics. Allen
and co-workers (13-15) analyzed aerosols formed in pho-
tooxidation of â-pinene, 1-octene, and 1,3,5-trimethylben-
zene, using FTIR coupled with a low-pressure impactor, and
found that functional groups such as -CdO, -OH, -COOH,
and nitrate dominate the products. Aerosol formed from
cyclohexene and methylcyclohexene photooxidation also
revealed dicarboxylic acids and other R,ω-difunctional
oxygenates bearing carboxyl, formyl, hydroxyl, and nitrate
groups as major products (16-18).
Information on these oxidation products of hydrocarbons
is important to elucidate the atmospheric hydrocarbon
reaction mechanism and to understand the formation
mechanism of secondary organic aerosols. However, de-
tecting and identifying compounds with these polar func-
tionalities remain a demanding task. Such compounds are
expected to exist at trace levels. Their polarity hinders
identification and quantification by conventional analytical
techniques, as the compounds are easily lost to the surfaces
of injectors and columns. We report here a two-step
derivatization technique, coupled with electron and chemical
ionization mass spectrometry, to identify and detect com-
pounds with -CdO, -OH, and -COOH moieties. In this
method, -CdO groups react with O-(2,3,4,5,6-pentafluo-
robenzyl)hydroxy amine (PFBHA) to form oxime derivatives,
then -COOH and -OH groups react with a silylation reagent
N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA) to form
trimethylsilyl (TMS) derivatives (Figure 1). The first deriva-
tization step prevents conversion of carbonyls to enols, which
could react with BSTFA and complicate data interpretation.
Atmospheric oxidation of hydrocarbons by hydroxyl
radicals and ozone leads to products containing -COOH,
-OH, and -CdO functional groups. The high polarity
of such compounds precludes direct GC-MS analysis. In
addition, many such compounds often exist in a single
sample at trace levels. An analytical method has been
developed to identify compounds containing one or more
functional groups of carbonyl, carboxy, and hydroxy in
atmospheric samples. In the method, -CdO groups are
derivatized using O-(2,3,4,5,6-pentafluorobenzyl) hydroxy amine
(PFBHA), and -COOH and -OH groups are derivatized
using a silylation reagent N,O-bis(trimethylsilyl)-trifluoro-
acetamide (BSTFA). The derivatives are easily resolved by
a GC column. The chemical ionization mass spectra of
these derivatives exhibit several pseudomolecular ions,
allowing unambiguous determination of molecular weights.
Functional group identification is accomplished by
monitoring the ions in the electron ionization mass spectra
that are characteristic of each functional group derivative:
m/z 181 for carbonyl and m/z 73 and 75 for carboxyl
and hydroxy groups. The method is used to identify products
in laboratory studies of ozone oxidation of R-pinene and ∆3-
carene. Among products from ozone oxidation of
R-pinene, we have detected pinonaldehyde, norpinonalde-
hyde, pinonic acid, norpinonic acid, C hydroxy dicarbonyls,
10
pinic acid, 2,2-dimethyl-3-(formylmethyl)-cyclobutane-
formic acid, and a product that has a molecular weight of
156 and contains a CdO and a COOH/OH group. The
latter two products have not been reported previously. ∆3-
Carene is structurally analogous to R-pinene in that both
have an internal unsaturated bond where ozone oxidation
takes place. We have also identified the corresponding
analogous products, of which all but caronaldehyde are
reported for the first time.
Multistep derivatization techniques have long been used
to detect and identify compounds containing multiple
functional groups of carbonyl, carboxy, and hydroxy (19-
27). Le Lacheur et al. (24) suggested a three-step derivati-
zation technique: PFBHA derivatization of carbonyl groups,
methylation of carboxy groups, and silylation of hydroxy
groups. Also, a two-step derivatization technique that
combines PFBHA derivatization and methylation has been
used to detect oxoacids in biological and aqueous environ-
mental samples (20, 21, 26, 27). Third, a PFBHA/ silylation
two-step derivatization technique has been applied to
determine ketosteroids, oxoacids, and hydroxy aldehydes in
biological samples (19, 22, 23, 25). The substitution of
methylation with silylation enables simultaneous derivati-
zation of carboxy and hydroxy functional groups. Here, we
have modified the PFBHA/ silylation technique for atmo-
spheric samples collected under simulated atmospheric
conditions, and explored electron ionization (EI) and chemi-
cal ionization (CI) mass spectrometry as a tool for identifying
Introduction
Atmospheric oxidation of hydrocarbons by species such as
the hydroxyl radical and ozone frequently leads to products
containing -CdO (carbonyl), -COOH (carboxy), and -OH
* Author to whom correspondence should be addressed. E-mail:
seinfeld@cco.caltech.edu; phone: (626) 395-4100; fax: (626) 585-
1729.
† Department of Environmental Engineering Science.
‡ Department of Chemical Engineering.
9
S0013-936X(98)00129-1 CCC: $15.00
Published on Web 07/03/1998
1998 Am erican Chem ical Society
VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 3 5 7

FIGURE 1. Derivatization reactions. (Top) PFBHA derivatizes a carbonyl group. (Bottom) BSTFA derivatizes an -OH group in acids and
alcohols.
TABLE 1. Model Compounds
compd
molecular formula
MWa
derivative MW
Group I: Mono- and Dicarboxylic Acids
CH3(CH2)4COOH
CH3(CH2)5COOH
CH3(CH2)6COOH
CH3(CH2)7COOH
C6H5COOH
p-CH3C6H4COOH
HOOCCOOH
HOOCCH2COOH
HOOCCH2)CH2COOH
HOOC(CH2)2COOH
HOOC(CH2)3COOH
HOOC(CH2)4COOH
hexanoic acid
116
130
144
158
122
136
90
104
116
118
132
146
188
202
216
230
194
208
234
248
260
262
276
290
heptanoic acid
octanoic acid
nonanoic acid
benzoic acid
p-toluic acid
oxalic acid
m alonic acid
m aleic acid
succinic acid
glutaric acid
adipic acid
Group II: Hydroxy Carboxylic Acids
HOCH2COOH
CH3CH2(OH)COOH
glycolic acid
lactic acid
76
80
220
234
Group III: Carboxylic Acids with Carbonyl group
HC(O)COOH
glyoxylic acid
pyruvic acid
74
88
341
355
457
369
471
485
485
451
CH3C(O)COOH
HOOCC(O)COOH
HC(O)(CH2)2COOH
HOOCCH2C(O)COOH
HOOCC(O)(CH2)2COOH
HOOCCH2C(O)CH2COOH
CH3C(O)C4H4(CH3)2CH2COOH
ketom alonic acid
succinic sem ialdehyde
oxalacetic acid
2-ketoglutaric acid
3-oxoglutaric acid
cis-pinonic acid
118
102
132
146
146
184
Group IV: Hydroxy Aldehydes and Ketones
HOCH2CHO
HOCH2C(O)CH3
HOCH2CH(OH)CHO
HO(C6H4)CHO
glycolaldehyde
hydroxyacetone
glyceraldehyde
3-hydroxy-benzaldehyde
60
74
90
327
341
429
389
122
a
Molecular weight.
unknown compounds. To our knowledge, the application
of the PFBHA/ BSTFA double derivatization technique,
coupled with EI and CI mass spectrometry for identifying
unknown compounds, has not been reported for air samples.
from atmospheric oxidation of two biogenic hydrocarbons,
R-pinene and ∆3-carene.
Experimental Section
Model compounds are examined first for their EI and CI
mass spectra fragment patterns. Examples are then given to
demonstrate how this method is used to identify products
Materials. All standards and PFBHA were purchased from
Aldrich (Milwaukee, WI) and used without purification.
BSTFA was from Pierce (Rockford, IL) and contains 1%
9
2 3 5 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 16, 1998

FIGURE 2. Chromatogram of a derivatized mixture of model compounds bearing -OH and -COOH groups. (Top) Total ion current. (Bottom)
Sum of ion current of m/z 73, 75, 117, and 147 ions.
trimethylchlorosilane (served as catalyst). Methylene chlo-
ride and acetonitrile (glass-distilled grade) were from EM
Science (Gibbstown, NJ). Hexane (optimal grade) was from
Fisher Scientific (Fairlawn, NJ).
solution (a minimum amount of water was used to dissolve
PFBHA‚HCl). The mixture was allowed to stand at room-
temperature overnight (16-24 h), before being blown to
dryness under a gentle N2 stream, followed by reconstitution
with 100 µL of hexane and methylene chloride solvent (1:1)
and addition of 20 µL of BSTFA. The mixture was then heated
at 60 °C for 40 min. After cooling briefly, 1 µL of the mixture
was injected for GC-MS analysis.
A Varian Saturn 2000 gas chromatograph-ion trap mass
spectrometer was used for both EI and methane CI analysis.
The GC temperature was programmed at 60 °C for 1 min,
then to 280 °C at 10 °C/ min, and held at 280 °C for 10 min.
Samples were injected in the splitless injection mode. The
injector was switched to split mode 1 min after an injection
was made. The injector port temperature was programmed
Model Com pound Studies. We have selected four types
of model compounds bearing -COOH, -OH, and -CdO
groups: (1) mono- and dicarboxylic acids; (2) hydroxy
carboxylic acids; (3) oxocarboxylic acids; and (4) hydroxy
carbonyls (Table 1). The model compounds are potential
products from atmospheric oxidation of hydrocarbons.
Model compounds containing carbonyl groups only are not
included here, since their identification and detection by
PFBHA derivatives have been described previously (28).
The model compounds prepared in acetonitrile were
derivatized with 50 µL of 19 mM PFBHA acetonitrile/ aqueous
9
VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 3 5 9

FIGURE 3. Chromatogram of a derivatized mixture of model compounds bearing -CdO and -COOH /OH groups. (Top) Total ion current.
(Middle) m/z 181 ion current. (Bottom) Sum of ion current of m/z 73, 75, 117, and 147 ions.
at 60 °C for 1 min, then ramped to 250 °C at 180 °C/ min, and
held at 250 °C until the end of the analysis. The mass range
was 50-650 amu. A 30 m × 0.25 mm × 0.25 µm DB-5 fused
silica was used for all samples.
Sam ple Collection and Treatm ent. Samples were col-
lected from R-pinene/ O3 and ∆3-carene/ O3 experiments,
conducted in the dark in a 50 m3 Teflon reactor which has
been described previously (29). The reactor was heated
during these experiments to 305-310 K, approximating the
temperatures encountered during summer daytime. Initial
concentrations of R-pinene and ∆3-carene were 44 ppbv and
72 ppbv, respectively. 2-Butanol was also added at ap-
proximately twice the initial hydrocarbon concentration to
scavenge OH radicals produced from the biogenic-O3 reaction
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2 3 6 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 16, 1998

TABLE 2. EI and CI Mass Spectra Patterns of Model Compound Derivatives
% relative intensity (EI)
% relative intensity (CI)
compd
hexanoic acid
heptanoic acid
octanoic acid
nonanoic acid
benzoic acid
p-toluic acid
oxalic acid
m alonic acid
m aleic acid
succinic acid
glutaric acid
adipic acid
73
75
117
147
181
M - 15
M
M + 181
M + 73
M + 1
M - 15
M - 89
M - 197
note
Group I: Mono- and Di- Carboxylic Acids
a
64.6
54.3
55.4
52.0
2.2
98.1
82.4
77.3
71.5
5.7
8.7
4.0
9.0
9.6
61.6
72.4
89.2
97.0
0.1
2.5
0.9
0.6
0.9
0.2
1.6
0.6
1.4
0.0
-
100.0
100.0
100.0
100.0
100.0
100.0
5.3
12.5
22.3
24.4
65.4
1.1
2.0
3.2
4.4
5.8
6.4
0.1
1.9
0.04
2.3
1.1
2.5
-
-
-
-
-
-
-
-
-
-
-
-
27.7
28.3
72.4
9.4
3.9
6.2
28.1
12.6
67.0
56.5
64.3
51.0
60.5
72.3
95.7
58.9
22.5
28.2
47.8
100.0
34.6
12.7
5.5
100.0
100.0
100.0
100.0
100.0
100.0
15.5
13.0
25.6
34.6
35.6
90.0
88.1
87.1
65.6
40.0
44.4
0.0
39.0
100.0
100.0
100.0
100.0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3.2
0.2
51.7
20.8
24.2
20.9
35.8
81.3
100.0
100.0
100.0
100.0
100.0
68.0
100% ) 73
15.5
32.1
100.0
0.5
3.8
17.3
71.2
41.6
29.8
Group II: Hydroxy Carboxylic Acids
glycolic acid
lactic acid
42.6
78.2
4.8
10.7
1.2
41.1
100.0
100.0
-
-
21.2
13.0
0.3
0.4
-
-
23.2
53.7
4.7
4.7
33.1
26.9
0.0
0.3
-
-
100% ) 367
100% ) 191
Group III: Carboxylic Acids with a Carbonyl Group
glyoxylic acid
pyruvic acid
29.8
59.5
89.9
5.5
6.9
3.0
4.6
8.7
0.0
0.2
28.3
0.2
63.8
51.5
26.9
2.1
100.0
100.0
100.0
100.0
75.1
74.3
51.1
52.3
24.3
36.5
41.1
35.9
48.0
57.8
54.1
0.6
5.4
8.8
20.8
11.7
9.0
22.0
14.6
25.6
1.4
100.0
18.6
78.8
15.2
77.5
82.6
12.4
6.6
21.0
9.6
32.3
26.0
3.9
0.9
4.4
5.8
12.1
0.0
20.5
46.8
0.0
100.0
0.8
2.2
35.9
7.4
14.2
0.0
0.0
35.9
0.0
2.7
100% ) 73
100% ) 73
100% ) 73
ketom alonic acid
succinic sem ialdehyde
oxalacetic acid
2-ketoglutaric acid
3-oxoglutaric acid
cis-pinonic acid
35.0
33.7
42.2
50.1
29.6
31.9
43.5
16.2
37.7
35.2
17.8
1.1
37.8
4.6
0.0
100.0
100.0
100.0
100.0
20.7
10.0
6.4
nab
nab
nab
0.0
100% ) 73
100.0
100.0
17.0
11.0
5.9
Group IV: Hydroxy Aldehydes and Ketones
glycolaldehyde
hydroxyacetone
glyceraldehyde
3-hydroxy-benzaldehyde
32.1
62.4
78.2
47.2
9.3
16.6
8.5
4.2
5.8
7.6
3.5
0.0
0.4
37.3
0.4
51.3
56.6
33.9
43.6
100.0
100.0
10.8
0.6
1.5
0.3
36.9
17.2
4.7
6.1
0.0
0.0
0.0
44.8
100.0
23.1
100
98.7
12.8
3.9
41.9
81.7
47.0
0.0
28.5
88.9
0.0
100% ) M - 211
8.3
23.8
100.0
2.7
79.1
100.0
a
b
Not observed due to absence of aldehyde or ketone groups. na, not applicable due to being out of available m ass range.

FIGURE 4. EI mass spectra of derivatized adipic acid and 3-oxoglutaric acid.
(2, 30). Finally, ozone was added to the reactor until its
Results and Discussion
concentration reached approximately four times that of the
initial hydrocarbon. As a result of excessive ozone, the
hydrocarbon was consumed entirely at the end of an
experiment. Reactor air was pulled through a 0.5 m × 8 mm
i.d. Teflon line and passed through an impinger (with frits)
containing 200 mL of acetonitrile solvent at a flow rate of
about 2 L/ min for 3 h. No ice bath was used to reduce the
evaporation of collection solvent. At the end of the 3 h
sampling, 200 mL of acetonitrile was reduced to about 100
mL. Immediately after collection of a sample, 50 µL of 19
mM PFBHA acetonitrile/ aqueous solution was added, and
the mixture was allowed to react overnight before it was
reduced to about 5 mL by rotary evaporation. The sample
was then blown to dryness under a stream of N2, reconsti-
tuted, and derivatized with BSTFA as described for those
model compounds.
Derivatization Conditions and GC Separation. The reaction
time for the derivatization of carbonyl groups with PFBHA
is chosen to be between 16 and 24 h for convenience and
completeness of reaction (24, 31). At room temperature, the
model compounds require 6 h to complete the silylation
reactions. At an elevated temperature of 60 °C, the reactions
take only 40 min to complete.
The PFBHA and TMS derivatives are well resolved on a
commonly used DB-5 GC capillary column. Figure 2 shows
a GC chromatogram of the TMS derivatives of standards
bearing one or more of OH/ COOH groups (groups I and II
compounds in Table 1). Figure 3 shows a GC chromatogram
of double derivatized oxocarboxylic acids and hydroxy
carbonyls (groups III and IV compounds in Table 1). All the
compounds are well-separated and exhibit good peak shapes.
PFBHA derivatives often show multiple peaks for a non-
9
2 3 6 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 16, 1998

FIGURE 5. CI mass spectra of derivatized adipic acid and 3-oxoglutaric acid.
symmetric parent carbonyl because PFBHA forms two
geometric isomers due to the nitrogen-carbon double bond,
and the isomers can often be resolved by a GC column (24,
28).
with relative intensity (RI) ranging from 15 to 100%. As a
result, ions 73, 75, 117, and 147 can be used to differentiate
compounds bearing -OH and -COOH from other classes
of organics. Figure 2 shows the GC chromatogram of the
TMS derivatives of carboxylic acids and hydroxy carboxylic
acids, along with pentadecane. The select ion chromatogram
for ions 73, 75, 117, and 147 shows peaks for each TMS
derivative, but not pentadecane. This demonstrates that
these common fragments can serve as indicators of the
presence of -OH or -COOH functionality. A strong m/ z
147 ion may be used to postulate the presence of two of -OH
and -COOH groups.
For compounds containing carbonyl group(s), the con-
version of such groups to CdNOCH2C6F5 renders the resulting
derivatives a strong fragment ion at m/ z ) 181, [C6F5 CH2]+,
RI ranging from 34 to 100% (Table 2). Therefore the 181 ion
can be used to identify compounds bearing carbonyl groups
from the rest of a mixture. Figure 3 shows that, for each
EI Mass Spectra of Model Com pounds. The derivatives
share several common ion fragments resulting from the
moieties that come from the derivatizing agents. Table 2
compiles a list of these fragments and their relative abun-
dance. For all compounds containing -OH and -COOH
groups, substitution of the active H atom in -OH and -COOH
function groups with Si(CH3)3 gives rise to common ion
fragments at m/ z 73 and 75, [Si(CH3)3]•+ and [HOdSi(CH3)2]•+,
respectively. In addition, monocarboxylic acids show a strong
fragment at m/ z 117, i.e., [COOSi(CH3)3]•+, a moiety resulting
from the TMS group and acid functionality; for compounds
with two active H atoms such as dicarboxylic acids, hydroxy
carboxylic acids, and dihydroxy aldehydes, the m/ z 147 ion,
postulated as [(CH3)2SidOSi(CH3)3_]•+ (32), is very strong,
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VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 3 6 3

given in Figure 4, using adipic acid and 3-oxoglutaric acid
as examples.
TABLE 3. Pseudomolecular Ions of PFBHA and TMS
Derivatives in Their Methane Chemical Ionization Mass
Spectra
Molecular ions of TMS derivatives of carboxylic acids
without a carbonyl group are weak or nonexistent (RI ranging
from 0 to 6%) in their EI mass spectra. For carboxylic acids
with carbonyl functionality, both carbonyl and acid groups
are derivatized. The resulting derivatives show molecular
ions of moderate intensity (RI 1-55%). For hydroxy alde-
hydes and ketones, their double derivatized forms exhibit
low molecular ion intensity (RI 0.3-1.5%), 3-hydroxy ben-
zaldehyde being an exception probably as a result of the
presence of a benzene-ring functionality. A pseudomolecular
ion at m/ z M - 15, resulting from loss of a methyl group
from the neutral molecule, is prominent in EI spectra for
every standard examined (5-100%), cis-pinonic acid being
an exception. The loss of a methyl group is so favored for
TMS derivatives of monocarboxylic acids that the M - 15 ion
PFBHA derivatives
fragment
TMS derivatives
ion fragment
ion
M + 181 [M + C6F5CH2]•+
M + 1
[M + H]+
M + 73 [M + Si(CH3)3]•+
M + 1
[M + H]+
M - 181 [M - C6F5CH2]•+
M - 15 [M - CH3]•+
M - 197 [M - C6F5CH2O]•+ M - 89 [M - OSi(CH3)3]•+
compound with both carbonyl and carboxy/ hydroxy groups,
the 181 ion chromatogram has a corresponding peak and
that the 73 + 75 + 117 + 147 ion chromatogram also has a
corresponding peak. In addition to serving as indicators,
the strong intensity of these common ion fragments makes
them suitable for quantitation. Two EI mass spectra are
FIGURE 6. Chromatograms of products from r-pinene/O reaction system, see Table 4 for peak identification. (Top) Products bearing -CdO
3
groups. (Bottom) Products bearing -COOH or -OH groups. [(*) n-Alkanoic acids, present in concentrated acetonitrile solvent.]
9
2 3 6 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 16, 1998

FIGURE 7. Chromatograms of products from ∆3-carene/O reaction system, see Table 5 for peak identification. (Top) Products bearing
3
-CdO groups. (Bottom) Products bearing -COOH or -OH group. [(*): n-Alkanoic acids, present in concentrated acetonitrile solvent.]
dominates the spectra. While this ion can aid the deter-
mination of molecular weight, it alone does not provide a
confident diagnosis.
The adduct ions [(M + 181)+] and [(M + 73)+] result from
a special principle of the ion trap mass spectrometer. In the
ion trap mass spectrometer, the ion source and the mass
analyzer are combined as one piece of hardware. A radio
frequency field is scanned upward to eject ions of increasing
mass-to-charge ratio for detection. Before the ions are
ejected, secondary ion-molecule reactions are possible. For
the derivatives examined here, fragment ions at m/ z 181 and
73 are abundant in the trap, and they have time to form ion
adducts with neutral molecules before they are ejected out
the trap. Ions with m/ z M - 181, M - 197, M - 15, and M
- 89 result from loss of C6F5CH2, C6F5CH2O, CH3, and OSi-
(CH3)3, respectively, from the neutral molecules. For each
model compound examined, three or more pseudomolecular
ions listed in Table 3 are observed to have RI > 4%, as shown
in Table 2. The multiple pseudomolecular ions make it
possible to reliably determine molecular weights for unknown
Methane CI Mass Spectra of Model Com pounds. Table
2 lists the CI mass spectra data of model PFBHA/ TMS
derivatives. As illustrations, Figure 5 shows the full CI mass
spectra of the TMS derivative of adipic acid, and the PFBHA
and TMS derivative of 3-oxoglutaric acid. PFBHA derivatives
are known to have several pseudomolecular ions of strong
relative intensity in their methane CI mass spectra (28). These
ions are listed in Table 3. TMS derivatives of carboxylic acids
and hydroxy carboxylic acids also exhibit a number of
pseudomolecular ions in their methane CI mass spectra with
strong intensity (Table 3). When both -COOH (or -OH)
and -CdO are present in a compound, one observes
pseudomolecular ions characteristic of the two different types
of functional groups in the original molecules.
9
VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 3 6 5

TABLE 4. Products from r-Pinene/O Reaction System
3
peak no.
in Figure 6
derivative
MW
original
MW
OH or
COOH
RT (min)
14.14
14.38, 14.48
14.78
15.41
15.90, 16.13
16.49
16.61
17.01
17.11
17.14
CdO
identification
1
2, 3
4
5
6, 7
8
9
10
11
12
330
369
383
367
457
423
397
448
462
448
462
437
573
437
437
451
476
490
502
544
558
558
646
646
186
102
116
100
118
156
130
58
xa
x
x
x
x
pinic acidc
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
succinic sem ialdehydec,d
C5 oxoacidb,d
C4 unsaturated oxoacidb
C4 hydroxy oxoacidb
G in Table 6b
x
x
C6 oxoacidb
glyoxalc,d
72
58
72
m ethylglyoxalc
glyoxalc,d
13
13
14
15
17.43
17.43
17.49
17.66
m ethylglyoxalc
170
183
170
170
184
86
100
112
154
168
168
184
184
x
norpinonic acidb
unknown
x
x
x
norpinonic acidb
D2 in Table 6b
16, 17
18, 19
19
20, 21
22
23, 24
25, 26
27, 28
29, 30
31, 32
18.21,18.36
18.53, 18.76
18.76
18.86, 18.94
19.78
21.26, 21.34
22.13, 22.24
22.34,22.44
22.83,23.53
23.64, 23.81
pinonic acidc
C4 saturated dicarbonylb
C5 saturated dicarbonylb
C6 unsaturated dicarbonylb
norpinonaldehydeb
pinonaldehydeb
pinonaldehydeb
C10H16O3, F in Table 6b
C10H16O3, F in Table 6b
x
x
a
b
c
x denotes presence of a functional group. Tentatively identified based on EI and CI m ass spectra. Confirm ed with an authentic standard.
d
Present in concentrated acetonitrile solvent blank.
TABLE 5. Products from ∆3-Carene/O Reaction System
3
peak no. in
Figure 7
derivative
MW
original
MW
OH or
COOH
RT (min)
CdO
identification
1
2, 3
4
5
6
7, 8
9
10
10
11
14.10
14.49, 14.55
14.84
15.84
16.23
16.33, 16.45
16.69
17.06
17.09
17.19
17.19
17.16, 17.36
17.38
17.33, 17.49
17.64
17.88
18.13
18.31
18.46
18.76, 18.93, 19.01
19.29
330
369
383
411
351
337
397
437
448
462
448
423
437
462
437
435
451
437
451
490
550
539
544
558
558
646
186
102
116
144
156
142
130
170
58
xa
x
x
C9H14O4, E′ in Table 6b
succinic sem ialdehydec,d
C5 oxoacidb,d
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
C7 oxoacidb
C10 carbonylb
C9 carbonylb
x
x
C6 oxoacidb
norcaronic acidb
glyoxalc,d
72
58
m ethylglyoxalc
glyoxalc,d
11
11, 12
12
12, 13
14
156
170
72
170
168
184
170
184
100
88
200
154
168
168
184
x
x
G′ in Table 6b
norcaronic acid or D′ in Table 6b
2
m ethylglyoxalc
x
x
x
x
x
norcaronic acid or D′ in Table 6b
2
b
15
16
17
18
C9H12O3
caronic acidb
D′ in Table 6b
2
caronic acidb
19, 21
22
C5 saturated dicarbonylb
C3 hydroxy dicarbonylb
C10 hydroxy oxoacidb
norcaronaldehydeb
caronaldehydeb
x
x
23
24
25, 26
27, 28
29-31
19.81
21.29
21.58, 21.79
22.84, 22.04
22.73, 22.96, 23.11
caronaldehydeb
x
C10H16O3, F′ in Table 6b
a
b
c
x denotes presence of a functional group. Tentatively identified based on EI and CI m ass spectra. Confirm ed with an authentic standard.
Present in concentrated acetonitrile solvent blank.
d
compounds. In addition, unlike EI spectra that often exhibit
numerous fragment ions (Figure 4), CI mass spectra show
fewer fragments, making identification easier.
Detecting and Identifying Products from O3 Oxidation
of r-Pinene and ∆3-Carene. Using combined PFBHA/ BSTFA
derivatization, we have detected a number of products from
the O3 oxidation of R-pinene and ∆3-carene. The presence
of carbonyl-containing products was readily determined by
examination of the reconstructed ion chromatogram for m/ z
181, whereas the presence of products bearing hydroxyl and
carboxyl groups was determined by the reconstructed ion
chromatogram for m/ z 73, 75, 117, and 147 (Figures 6 and
7). Tables 4 and 5 list the products shown in Figures 6 and
7. Many of the products have more than one polar functional
groups. The PFBHA derivative of pinonaldehyde shows peaks
(peaks 25 and 26) in the bottom chromatogram in Figure 6,
where they are not expected, as pinonaldehyde is a C10
dicarbonyl without a COOH/ OH group. A close examination
9
2 3 6 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 16, 1998

FIGURE 8. Methane chemical ionization mass spectra of two double-derivatized products from ∆3-carene/O reaction. (Top) Caronic acid.
3
(Bottom) Norcaronic acid.
of the EI mass spectra showed that ions at m/ z 73, 75, 117,
and 147 have low relative intensity, unlike those with the
TMS moiety that results in these ions of strong intensity.
Ions at m/ z 73, 75, 117, and 147, along with many other small
ion fragments, were generated as a result of the high
concentration of the pinonaldehyde-PFBHA derivative in the
sample.
The molecular weights of most compounds are easily
determined by interpretation of their EI and CI mass spectra.
For example, Figure 8 shows the CI mass spectra of the
derivatives of two products, caronic acid and norcaronic acid,
from the ∆3-carene/ O3 reaction, illustrating the determination
of molecular weights and some structural moieties.
Among biogenic hydrocarbons, R-pinene is the most
frequently studied monoterpene for its atmospheric chem-
istry. At an initial R-pinene mixing ratio of several parts per
million by volume, pinonaldehyde has been clearly identified
as a major product (33-36); Hatakeyama et al. (33) also
identified norpinonaldehyde, pinonic acid, and norpinonic
acid as oxidation products. In a study conducted with mixing
ratios of R-pinene at several hundred parts per million by
volume, additional products, R-pinene epoxide, and three
C10 hydroxy dicarbonyls were reported (37). Table 6 lists the
structures of these products from O3 oxidation of R-pinene.
In our experiment conducted with an initial R-pinene
concentration of 0.044 ppmv, all of the above products, with
the exception ofR-pinene epoxide, which does not have either
a CdO or an OH/ COOH group, were observed using the
PFBHA/ BSTFA derivatization technique. We also detected
three additional products. The first was identified as pinic
acid, a C9 dicarboxylic acid (peak 1 in Figure 6, see Table 6
for its structure), from the EI and CI mass spectra of its TMS
derivative. The identification was confirmed by comparison
with an authentic standard obtained from Aldrich. The
9
VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 3 6 7

TABLE 6. Products from Ozone Oxidation of r-Pinene and ∆3-Carene
9
2 3 6 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 16, 1998

FIGURE 9. Mechanism for the r-pinene/O reactions. (*) Energy-rich species. The marked letters correspond to those in Table 6.
3
second was identified as 2,2-dimethyl-3-(formylmethyl)-
cyclobutane-formic acid (peaks 16 and 17 in Figure 6, D2 in
Table 6), an isomer of norpinonic acid (D1 in Table 6). The
third has a molecular weight of 156 (peak 8 in Figure 6) and
contains both a carbonyl group and a COOH/ OH group, as
indicated by its EI and CI mass spectra. This product could
be 1-acetyl-2,2-dimethyl-3-hydroxymethyl-cyclobutane, or
2,2-dimethyl-3-formyl-cyclobutane-formic acid (G in Table
6). The second possible structure is similar to but having
one fewer carbon atom than norpinonicacid. Distinguishing
further between the two possibilities is not possible based
on the EI and CI spectra.
At the present, mechanistic pathways for the formation
of these products are largely speculative. On the basis of
known reaction mechanisms for alkene-O3 reactions, it is
possible to propose a reaction scheme, shown in Figure 9,
to account for the observed products. The reaction of
R-pinene with O3 proceeds by initial O3 addition to the CdC
bond to yield an energy-rich ozonide, which rapidly de-
composes to two biradicals. The observed products are
formed from the subsequent reactions of the two biradicals.
The energy-rich biradicals can be collisionally stabilized. The
reaction of water with one of the stabilized biradicals leads
to pinonic acid (C). Elimination of O(3P) from either of the
two biradicals gives rise to pinonaldehyde (A). This channel,
however, accounts for only a small fraction of pinonaldehyde
formed, as O(3P) formation yield is less than 3% (2). As the
amount of 2-butanol used in the experiment was insufficient
to scavenge all OH radicals, a fraction of pinonaldehyde likely
results from OH-initiated oxidation of R-pinene (33). The
formation of C10 hydroxy dicarbonyls (F) is postulated to
result from the Criegee radicals via a hydroperoxide channel,
i.e., Criegee radicals isomerization to enehydroperoxides
through intramolecular hydrogen migration, and the sub-
sequent isomerization to hydroxy dicarbonyls (2, 9-11). The
decomposition of enehydroperoxides leads to products with
fewer carbons than the parent Criegee radicals. This may
account for the formation of norpinonaldehyde (B) and D2.
The formation of norpinonic acid (D1) and pinic acid (E) is
presupposed to result from oxidation of their corresponding
aldehydes, norpinonaldehyde, and D2, respectively. How-
ever, the explicit mechanism is unclear. The two possible
candidates for G are postulated to arise from decomposition
of intermediates I1 and I2. I1 results from decomposition of
enehydroperoxide EN1, and the loss of an OH radical from
enehydroperoxide EN2 produces I2 (2).
Two product studies of ∆3-carene oxidation are available
in the literature (34, 35). Only one product, 2,2-dimethyl-
3-(2-oxopropyl)-cyclopropaneacetaldehyde or caronalde-
hyde (name derived as an analogy to pinonaldehyde), has
been clearly identified. As shown in Figure 7, we have
detected products in addition to caronaldehyde. Since ∆3-
carene and R-pinene both have an internal unsaturated bond
where O3 oxidation takes place (see Table 6 for their
structures), one expects that this structural similarity would
lead to similar oxidation products. In addition to listing the
products observed from R-pinene/ O3 reactions, Table 6 shows
the corresponding products from ∆3-carene/ O3. These
analogous products are all tentatively identified (Figure 7
and Table 5).
Pinonaldehyde, norpinonaldehyde, pinonic acid, and
norpinonic acid have been reported to be particulate products
from R-pinene oxidation (33, 38). Considering that pinic
acid and D2 have lower volatility than the above-known
aerosol phase products, one would not be surprised to find
them in the aerosol phase. Similarly, the corresponding
products from ∆3-carene can be expected to contribute also
to aerosol formation. The products identified in this work
suggest that compounds with multiple polar functional
groups are components of secondary organic aerosols. This
9
VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 3 6 9

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18).
Quantitative determination is possible for products that
have standards available. We have not attempted to quantify
the products, as most of them do not have readily available
standards. On the basis of our preliminary work, we estimate
that the method can detect in the parts per trillion by volume
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