Analytical methodology for determination of organic aerosol functional group distributions

Article abstract of DOI:10.1021/ac3028728

Secondary organic aerosol (SOA) particles result from the condensation of oxidized volatile organic compounds (VOC) and consist of a complex mixture that is not conducive to complete compositional analysis. We present a simple methodology for determining the quantities of aldehydes and ketones, alcohols, and carboxylic acids in such samples via derivatization and high-performance liquid chromatography (HPLC) with ultraviolet-visible absorption detection and tandem mass spectrometry (MS/MS). Useful concentration ranges are presented with an examination of the specificity of these compounds in multiply substituted systems. Such data are valuable in modeling the formation of laboratory-generated aerosols and in identifying point sources of field-collected aerosol samples. Calibration curves on standard samples, MS/MS transitions, including collisionally induced dissociation (CID) products, and a quantitative examination of the specificity of the derivatization reagents toward multiple functional groups are presented for a series of aliphatic standard samples. These methods are then applied to filter extracts from SOA derived from 1-iodooctane photolysis at 254 nm to demonstrate the methodology on a complex system. Ultimately, this methodology enables the measurement of variations in the chemical nature of the oxygen within an SOA particle, providing a distribution of functional group concentrations. This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society.

Full text of DOI:10.1021/ac3028728

Article
pubs.acs.org/ac
Analytical Methodology for Determination of Organic Aerosol
Functional Group Distributions
Alicia J. Kalafut-Pettibone and W. Sean McGivern*
Chemical Science Division, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8320, Gaithersburg, Maryland
20899, United States
S
* Supporting Information
ABSTRACT: Secondary organic aerosol (SOA) particles
result from the condensation of oxidized volatile organic
compounds (VOC) and consist of a complex mixture that is
not conducive to complete compositional analysis. We present
a simple methodology for determining the quantities of
aldehydes and ketones, alcohols, and carboxylic acids in such
samples via derivatization and high-performance liquid
chromatography (HPLC) with ultraviolet−visible absorption
detection and tandem mass spectrometry (MS/MS). Useful
concentration ranges are presented with an examination of the
specificity of these compounds in multiply substituted systems.
Such data are valuable in modeling the formation of
laboratory-generated aerosols and in identifying point sources
of field-collected aerosol samples. Calibration curves on standard samples, MS/MS transitions, including collisionally induced
dissociation (CID) products, and a quantitative examination of the specificity of the derivatization reagents toward multiple
functional groups are presented for a series of aliphatic standard samples. These methods are then applied to filter extracts from
SOA derived from 1-iodooctane photolysis at 254 nm to demonstrate the methodology on a complex system. Ultimately, this
methodology enables the measurement of variations in the chemical nature of the oxygen within an SOA particle, providing a
distribution of functional group concentrations.
econdary organic aerosols (SOA) represent a significant
fraction of the overall condensed-phase atmospheric carbon
a need for more detailed chemical information call for
additional complementary techniques for determining func-
tional group distributions.
S
load and are formed from gas-phase oxidation of volatile
organic compounds (VOC) and subsequent condensation of
the oxidation products. These reactions lead to complex
mixtures of oxidized organic species, which undergo further
transformation through atmospheric aging. The measurement
of the composition of SOA, both from laboratory and field
samples, is an area of vigorous study. However, the extreme
complexity of even the simplest SOA renders a complete
chemical analysis of the composition intractable.1
Numerous approaches to functional group analysis have been
used to probe aerosol samples. Fourier transform infrared (FT-
IR) spectroscopy has been utilized extensively to provide a
“semiquantitative”2 distribution of functional groups in aerosol
samples.2−12 Blando et al. have examined the use of FT-IR to
probe functional group distributions in standards, laboratory-
generated particles, and ambient samples.2 They found that FT-
IR was capable of determining functional group concentrations
on aerosol particles as a whole without bias from filtration and
extraction. However, these whole-aerosol techniques were
observed to be sensitive to sampling geometry (e.g., particle
distributions on impactors). In addition, knowledge of the
relative absorptivity of the moiety of interest relative to that of
an ion used to determine absolute concentration chromato-
graphically is required. Such uncertainties in utilizing FT-IR and
High-performance liquid chromatography (HPLC) studies of
bulk aerosols can provide data on the molecular composition of
the oxygenated species in SOA, and numerous research groups
have utilized these methods, often coupled to mass
spectrometry and in tandem with other techniques.13−17 In
general, however, these HPLC methods focus on a single class
of compounds, and the extreme complexity of the mixtures in
SOA limits the ability to cleanly identify individual compounds.
Tandem mass spectrometric detection (MS/MS) can be
effective, but ionization of many compounds in SOA is poor,
often yielding no significant signal from aldehydes, ketones, and
alcohols and only a weak negative [M − H]− signal for
carboxylic acids.
Numerous groups have applied size-resolved aerosol mass
spectrometry via volatilization/electron impact ionization18 to
oxidized products in both field19,20 and chamber21−27 samples.
However, these size-resolved experiments are necessarily
limited in their ability to identify individual compounds or
Received: October 3, 2012
Accepted: March 5, 2013
Published: March 5, 2013
This article not subject to U.S. Copyright.
Published 2013 by the American Chemical
Society
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compound classes. In general, these experiments can provide
elemental ratios on nonrefractory particle samples in aerosols,
the most important of which for SOA are O/C and H/C.28
In general, derivatization can be used to enhance the
chromatographic separability and detectability of these
compounds. Kowalewski and Gierczak describe a methodology
to probe functional group distributions by derivatizing
carbonyls, alcohols, and carboxylic acids to enhance volatility
for analysis in a gas chromatograph.29 This technique is
effective but requires difficult-to-handle silyl reagents and will
ultimately be limited by the vapor pressures of the analytes.
Similar gas-phase derivatization studies have been performed by
Jaoui et al.,30 Li and Yu,31 and Jang and Kamens32 with similar
advantages and disadvantages. Liquid chromatography methods
offer the ability to probe low-volatility species, and numerous
derivatization methodologies for HPLC analysis of SOA
samples have been described in the literature. However, these
methods require extremely difficult-to-handle reagents or
numerous steps prior to analysis.33−38 We have developed
and characterized a simpler methodology to examine the
functional group distribution of compounds typically observed
from SOA based on derivatizations that have been utilized
previously in the biochemical community39,40 and in gas-phase
atmospheric studies.41,42 The derivatization reactions, which are
specific to aldehydes and ketones (referred to as “carbonyls” in
the remainder of this work), carboxylic acids, and alcohols,
enhance both the ultraviolet absorption and the electrospray
ionization (ESI) of compounds containing these three
functional groups. In this work, we utilize three separate
derivatization reactions to probe a series of oxygenated aliphatic
standard compounds similar to those found in newly formed
SOA and characterize their ultraviolet responses. We have
chosen singly and doubly substituted standard compounds to
determine useful concentration ranges for these species by
measuring absorption response linearity and specificity of these
derivatization agents toward their target functional groups. In
particular, we present relative ultraviolet absorption responses
for the derivatization reagents and calibration curves with
appropriate limits of detection and linearity ranges. This
methodology utilizes acetonitrile (ACN) as a solvent and is
conducive to the analysis of ACN filter extracts. Finally, the
applicability of these reagents to more complex systems is
shown via the analysis of SOA formed from the photolysis of 1-
iodooctane in air.11
on systems broadly consistent with SOA constituents. (Certain
commercial materials and equipment are identified in this paper
in order to specify adequately the experimental procedure. In
no case does such identification imply recommendation of or
endorsement by the National Institute of Standards and
Technology, nor does it imply that the material or equipment
is necessarily the best available for the purpose.)
Derivatization. The derivatization reagents were chosen
from the literature and subsequently optimized for coreagent
type, relative concentrations of reagents and samples, and
temperature using model compounds similar to those found in
SOA. Emphasis was placed on finding reagents that would
enhance both ESI ionizability and UV absorption response. All
reagents and coreagents and their respective concentrations
were required to maintain compatibility with standard HPLC
columns, show good separation from other reagents and the
analytes, and require mild conditions with minimal handling
requirements. The reagents and coreagents chosen used are
shown in Table 1 with their respective functional group targets.
a
Table 1. Summary of Derivatization Reagents and Targets
reagent
target
coreagents (equivalents)
dansylhydrazine (DnsHz)
R1(CO)R2
RCHO
trifluoroacetic acid (0.2)
4-(dimethylamino)benzoyl
chloride (DMABC)
ROH
pyridine (5, 1)
2-picolylamine (2-PA)
RCOOH
triphenylphosphine (1)
dipyridyldisulfide (1)
a
Coreagent quantities are provided as molar equivalents of
derivatization reagents.
Aldehydes and ketones are analyzed via condensation of the
sample with DnsHz to form hydrazones (Scheme 1).43,44,47
A
Scheme 1. Ketone/Aldehyde Derivatization To Form
a
Hydrazone with DnsHz
a
CID of the molecular ion results in cleavage of indicated bond and a
product ion with m/z 170.
EXPERIMENTAL SECTION
■
Derivatization for various functional groups was accomplished
using 5-dimethylaminonaphthalene-1-sulfonyl hydrazine (dan-
syl hydrazine, DnsHz), 2-aminomethylpyridine (2-picolyl-
amine, 2-PA), and 4-(dimethylamino)benzoyl chloride
(DMABC) to derivatize carbonyls, carboxylic acids, and
alcohols, respectively.42−46 All derivatizations were performed
in ACN through mixing of the analyte with the active
derivatization reagent and necessary catalysts/coreagents in a
single vial. In all cases, the addition of an aromatic functionality
from the derivatization agent provides strong ultraviolet
absorption, and the presence of the nitrogen, whether as a
heteroatom in the aromatic ring or as part of a dimethylamine
moiety, leads to strongly enhanced ionization in the electro-
spray ionizer. In addition, each reagent is found to show good
specificity to its target functional group. In this work, we have
elected to focus primarily on octyl derivatives as standard
samples in order to minimize interfering signals while testing
mixture of 100 μL of 19 mmol L−1 (5.0 mg mL−1) DnsHz was
added to 100 μL of analyte in ACN. In general hydrazone
formation can be acid- or base-catalyzed; in the present
experiments, we have chosen to add 10 μL of 2.5% (vol)
trifluoroacetic acid (TFA) in ACN (0.2 mol equiv) to maintain
acidic conditions, leading to complete reaction in 5 min at
room temperature. In addition to 2-octanone as a standard,
octanal was also used as an analyte and confirmed that
aldehydes derivatize equivalently, although a complete set of
experiments for the aldehyde including calibration curves and
specificity was not performed. Hydrazone formation is
reversible in aqueous solutions at neutral pH, particularly in
the presence of electron-withdrawing groups.4 We utilize an
acetic acid solution as the aqueous HPLC mobile phase to
maintain a sufficiently low pH to prevent hydrolysis. No
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evidence for the reversible hydrolysis was observed during the
chromatographic analysis.
Carboxylic acids are analyzed from the amide formed via
reaction with 2-PA in the presence of 1.0 mol equiv of two
coreagents (Scheme 2): triphenylphosphine (TPP) and
were used with a 1 cm guard column in front with the same
stationary phase as the main column. A Pinnacle II C18 (Restek
Corporation, Bellefonte, PA) chromatography column with 3
μm particles was used for all analyses described in this work.
The typical chromatography method was a reversed-phase
H2O/organic gradient method ranging from 20% ACN to 90%
ACN over 8 min and held for several minutes (varying
depending on the amount of derivatizing agent). Injections of 2
or 20 μL were used, depending on the concentration of the
standard.
Scheme 2. Carboxylic Acid Derivatization To Form Amide
a
with 2-PA
The entire detector UV−vis absorption spectrum, 190−800
nm, was recorded for all retention times; however, analyses
were typically performed at the wavelength corresponding to
the absorption maximum.
a
CID results in cleavage of the indicated bond in the molecular ion to
yield a product ion with m/z 109.
Mass spectrometric analysis was performed on the output of
the diode array detector and was accomplished with an Agilent
6410 triple-quadrupole mass spectrometer. A dual electrospray/
atmospheric pressure chemical ionization source (Agilent) was
used; source parameters were optimized to the derivatized
standards shown in Table 2. These source parameters were
used for all analyses with the corresponding derivatizing
reagent.
dipyridyl disulfide (DPDS).46 The formation of the amide is
mediated by the oxidation of the TPP to a phosphine oxide and
reduction of DPDS to a cyclic thioketone and has been found
to be extremely rapid in a range of solvents over a large
temperature range.48 To 100 μL of analyte in ACN 50 μL of 19
mmol L−1 solutions of 2-PA (2.1 mg mL−1), DPDS (4.3 mg
mL−1), and TPP (5.0 mg mL−1) were added. Reactions were
allowed to occur for a minimum of 20 min at room temperature
prior to analysis.
Alcohols are detected as esters formed from the reaction of
DMABC in the presence of pyridine (Pyr) in a slight excess.45
The formation of the ester is facilitated by the formation of an
acetylpyridinium ion formed from displacement of chloride ion
with pyridine.49 The basic DMABC solution was colorless;
addition of pyridine turned the solution a deep yellow.
DMABC is moisture-sensitive; the use of undried ACN or
lengthy exposure to room air will result in the formation of 4-
(dimethylamino)benzoic acid crystals and deactivation of the
reagent. In practice, individual vials of DMABC solution in dry
ACN (dried for several hours over 3 Å molecular sieves in a
desiccator) were prepared in a glovebox and sealed. For
derivatization, the vials were warmed to room temperature and
briefly opened to room air to allow addition of sample and
pyridine solutions. With this handling, vials of the derivatizing
reagent should be used in approximately 8−12 h. Longer useful
life of the reagent may be obtained by adding anhydrous
pyridine (1 mol equiv) to a DMABC solution in anhydrous
ACN in an inert environment and storing the resulting yellow
mixture below 0 °C for direct transfer to the sample vial.
Reagent should be removed anaerobically, such as with a
Mininert valve (Vici Valco, Houston, TX) and syringe, to
prevent deactivation for quantitative work.
Typically, 400 μL of sample in ACN was added to 400 μL of
38 mmol L−1(7 mg mL−1) solution in ACN with 6 μL Pyr (5
equiv). The vial was heated to 70 °C for 20 min prior to
initiating chromatographic analysis, at which point the solution
is colorless. As noted below, these conditions will also produce
∼30% of an anhydride when reacted with a carboxylic acid.
Chromatographic Analysis. Liquid chromatography was
performed with a Dionex UltiMate 3000 system, consisting of
an HPG-3200SD binary pump, autosampler, column oven, and
DAD-3000RS diode array ultraviolet−visible (UV-vis) detector
with a 10 mm absorption cell. In all experiments, the column
oven was held at 40 °C. Complete UV-vis spectra were
collected at 10 Hz and were measured with the “wide” slit
width setting (yielding a resolution of ∼5 nm). Chromatog-
raphy columns were 2.1 mm in diameter and 5 cm long and
Table 2. Limiting Quantities for Octyl Derivatizations
wavelength/
nm
LOD/
LOQ/
a
b
analyte/deriv. agent
ng
ng
relative abs.
17
2-octanone/DnsHz
2-octanone/DnsHz
1-octanol/DMABC
decanoic acid/2-PA
215
254
308
4.3
4.1
0.5
4.1
7.1
5.9
2.5
5.1
3
5.9 1.1
6.4 1.2
1
254
a
b
Signal-to-noise ratio >3. Signal-to-noise ratio >10.
Chemicals. All reactions were performed in HPLC-grade
ACN (Aldrich CHROMASOLV Plus, ≥ 99.9%). When
necessary for derivatization, ACN was dried over 3 Å molecular
sieves (Aldrich). The derivatizing reagents were obtained from
Aldrich and used without further purification: DMABC (97%
and ≥99.0%), pyridine (≥99.0%), 2-PA (99%), TPP (≥98.5%),
DPDS (≥99.0%), DnsHz (98%), and TFA (99%).
Standard samples were obtained from Aldrich and used as
received: 1-hexanol (99%), 1-octanol (≥99%), 2-octanone
(98%), octanoic acid (≥99%), octanedioic acid (98%),1,6-
hexanediol (99%), 2,5-hexanedione (>96%), 5-hydroxy-2-
pentanone (≥95%, mixture of monomer and dimer), 7-oxo-
octanoic acid (≥98%), 10-hydroxydecanoic acid (technical
grade, up to 15% lactones), and decanoic acid (≥98%).
Liquid chromatography was performed with HPLC-grade
acetonitrile and 0.1% (volume) acetic acid (Aldrich) in 18 MΩ
water (generated with a Water Pro PS filtration system,
Labconco Kansas City, MO). In systems containing a free acid
moiety (Table 3), a 50 mmol L−1 ammonium acetate buffer
with pH 4.0 was used to improve peak shapes after
confirmation of the peak identities with MS/MS using 0.1%
acetic acid solution.
RESULTS AND DISCUSSION
■
Chromatography and Detection. The primary challenge
in analyzing the present single-reactor derivatization products is
the separation of the products of interest from the typically
large excess of derivatizing reagents. Although a second solid-
phase extraction step could remove these larger products, such
an analysis adds both complexity and the possibility of product
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a
Table 3. Specificity of Derivatization Reagents
Scheme 3. Alcohol Derivatization with DMABC To Form
Ester
a
active reagent
DnsHz
2-PA
DMABC
decanoic
acid
1-octanol/1-
hexanol
singular std
2-octanone
functional groups
dual standard
b
c
R(OH)COOH
NA
0.99
NA
>0.4
a
Both H+ and Na+ adducts of the parent mass are observed; CID of
10-hydroxydecanoic acid
R(OH)CO
both results in cleavage of the indicated bond and an m/z 148
daughter ion. The protonated ion also yields an m/z 166 daughter.
0.92
0.97
NA
0.68
d
5-hydroxy-2-pentanone
R(CO)COOH
7-oxooctanoic acid
R(OH)2
1.2
0.69
ultraviolet absorption coefficients of the substituted species to
permit quantitative correction of measured chromatograms. In
determining these limits a 20 μL injection volume was used,
although the increased background resulting from tailing on the
overloaded derivatizing reagent peaks limited the overall
advantage in sensitivity from the larger injection. The
wavelengths shown in the table were chosen to be very near
the absorption maximum above 200 nm. For the single
aromatic ring reagents 2-PA and DMABC, the π → π* B-band
transitions near 254 and 308 nm, respectively, dominate the
UV−vis spectrum. However, the naphthyl moiety absorbs on
the stronger E1 and E2 transitions at wavelengths longer than
200 nm, with the E1 band at 215 nm dominating. We have
included both bands in Table 2 in case interferences preclude
the use of the stronger E1 band wavelength.
NA
0.57(0.64)
NA
1,6-hexanediol
R(CO)2
e
0.82(0.04)
NA
2,5-hexanedione
R(COOH)2
NA
0.97(0.0)
0.49(0.15)
octanedioic acid
a
Values in parentheses are relative amounts of the doubly derivatized
standards to the singly derivatized standards. Values in parentheses are
half of the relative amounts of the doubly derivatized standards relative
to the singly substituted standards and sum to unity for complete
derivatization and linear scaling of the absorption. Concentration
uncertainties are estimated to be 5%; however, systematic uncertainties
may exceed 50% in some cases (see text). Not reported because there
was no reaction toward either functional group. Peak overlap
precludes more detailed analysis. Mixture of monomer and dimers.
b
c
d
The linearity of the ultraviolet response was evaluated with a
series of standard 5× dilutions of 1.000 0.009 mL samples
delivered with an adjustable volume pipettor (Eppendorf, 1000
e
Derivatized product underwent dehydration of OH functionality ([M
− 18 + H]+ observed in MS).
μL) to a 5.00
0.02 mL volumetric flask. In all cases, the
response of the derivatized sample was linear over at least 2
orders of magnitude and extrapolated smoothly to zero
response. Calibration curves and relevant fitting parameters
are provided as Supporting Information.
Specificity and Functional Group Quantitation. The
specificity of these derivatization reagents to their target
functional groups was evaluated by reacting each reagent with
a mixture of the singly substituted standards (Table 2, with
hexanol replacing octanol for better chromatographic separa-
bility) at 2.5 mmol L−1 concentrations. The resulting
chromatograms (UV absorption) are shown in Figure 1. All
loss. The aliphatic standard samples derivatized with DnsHz
and DMABC are typically well-separated on a C18 column with
the method described above. Samples derivatized with 2-PA,
which requires two additional coreagents in excess, showed
good chromatographic separation with a 2 μL injection;
however, a 20 μL injection showed overlap with the heavily
tailing DPDS and the standard 2.5 mmol L−1 octanoic acid
sample on a C18 column. A larger diameter column would
alleviate this problem; however, we have elected to utilize
decanoic acid as the standard compound to eliminate the
overlap. In general, higher concentration standards (≥25 μmol
L−1) were analyzed with a 2 μL injection, which provided
excellent peak shapes and complete separation of all
components for all three systems. At lower sample concen-
trations, with the same amount of derivatization reagent, a 20
μL injection was utilized.
All three derivatization agents contain chromophores that
provide a strong ultraviolet absorption; when combined with
the aliphatic compounds in this study, the absorption
coefficients are expected to vary little from those of the parent
derivatization reagents. As discussed below, this can be used for
approximate quantitation of the relative concentrations of
compounds containing the functional groups.
The derivatization reagents also provide ESI-ionizable
moieties as shown in Schemes 1−3. In most cases, the [M +
H]+ molecular ion is dominant, and multiple reaction
monitoring (MRM) of ([W + 265 − 18 + H]+ → 170), ([W
+ 183 − 36 + H]+ → 148), and ([W + 108 − 18 + H]+ → 109),
where W is the mass number of the analyte, can be used to
differentiate compounds with overlapped peaks for derivatized
samples with DnsHz, DMABC, and 2-PA, respectively.
Limiting concentrations for the derivatization of the standard
octyl compounds are shown in Table 2 with the relative
Figure 1. Chromatograms of derivatized samples of singly substituted
standards. 2-PA and DnsHz are 254 nm bsorption signals; DMABC
signals are taken at 308 nm.
reagents were strongly specific to their target functional groups
with the exception of DMABC, which reacted to near
completion with alcohols but showed a partial yield of
anhydride from reaction with decanoic acid. Limiting the
reaction time of the DMABC to 20 min at 70 °C results in a
yield of ∼30% for the anhydride. Increasing the reaction
temperature led to a greater proportion of unwanted anhydride,
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and decreasing the temperature to 60 °C resulted in a
decreased yield of derivatized samples even after double the
reaction time and was deemed unacceptable. Although this
additional derivatization is not ideal for many SOA samples due
to typically large acid concentrations, DMABC still shows a
strong preference for OH and represents a rare simple
derivatization reagent for alcohols that can be analyzed without
substantial additional sample processing for analysis.
DMABC-derivatized moiety formed sodium adducts, whether
the carbonyl moieties were in the underivatized ketones or
acids or in a doubly derivatized (diester) species. For distantly
spaced functional groups, such as in octanedioic acid or 10-
hydroxydecanoic acid, [M + Na]+ is the primary ion formed.
For more closely spaced moieties, such as in 1,6-hexanediol
(doubly derivatized) both [M + Na]+ and [M + H]+ ions are
formed. Finally, the α-hydroxy acid 2-hydroxyoctanoic acid
(not shown in Table 3) showed no evidence for a sodium
adduct, despite showing two distinct retention times,
presumably corresponding to derivatization of the hydroxy
moiety or the OH in the carboxylic acid. The collisionally
induced dissociation (CID) of any [M + H]+ species leads to
both m/z 148 and m/z 166; that from [M + Na]+ species leads
to only m/z 148. On the basis of these observations we have
chosen to recommend m/z 148 as the primary diagnostic peak
for identifying CID products of DMABC derivatization, despite
the more obvious choice of m/z 166 based on the typical
reaction scheme (Scheme 3).
The relatively large deviations from unity (up to 20%) found
in the table are most likely due to deviations from linearity in
the ultraviolet absorption of variously substituted derivatized
analytes. In addition, particularly with DMABC but to a lesser
extent with DnsHz, the freshness and water content of the
standard solutions had an effect on the completeness of the
derivatization. Better results would be obtained if all of these
reactions were done strictly in an inert environment. However,
the emphasis in this work is on ease and speed of analysis at the
possible expense of uncertainty. Within such a framework,
variations on this order are certainly acceptable, and these
compounds provide good overall specificity toward their target
functional groups.
Quantitation from Ultraviolet Absorption. As discussed
previously, the absorption coefficient of a chromophore from
the derivatization reaction was assumed to be unaffected by the
analyte chain length or presence of other functional groups.
The detection wavelengths, shown in Table 2, were chosen to
provide maximum signal with minimum interference. Under
these conditions, the doubly derivatized products in Table 3 are
expected to exhibit twice the molar absorption as the singly
derivatized products. HPLC spectra of derivatized hexanoic and
decanoic acids are shown in Figure 2. Both of these systems,
which are purely aliphatic beyond the derivatization sub-
stitution, yield very similar peak sizes at 254 nm, where the
absorption derives solely from the aromatic chromophore.
Under this assumption, individual concentrations of com-
pounds containing OH, COOH, and CO (i.e., a functional
Additionally, we evaluated the behavior of the derivatization
agents on molecules with more than one functional group.
Derivatization was carried out on a series of mixtures each
consisting of one singly substituted and one doubly substituted
analyte as indexed by the columns and rows of Table 3,
respectively. The derivatized mixtures were then analyzed
chromatographically to determine their relative UV absorption
signals. The results are presented in Table 3. The UV
absorption coefficient for the chromophore added as a result
of derivatization was assumed to be unchanged due to the
presence of the second functionality; the veracity of this
assumption is examined in the following section. Different
chain lengths were used as shown in the table to ensure good
chromatographic separation for the various components. In
constructing Table 3, we have attempted to minimize
interactions between the functional groups by using widely
spaced functionalities. 2-Hydroxyoctanoic acid was also
derivatized with 2-PA and DMABC. These results were
compared to singly substituted standards decanoic acid and
1-octanol, respectively, to evaluate the effect of closely spaced
moieties. In the 2-PA case, the ratio of derivatized products
from 2-hydroxyoctanoic acid/decanoic acid was 0.9 0.1 with
the α-hydroxy acid adduct showing a broad double peak (vide
infra). For DMABC, the ratio of derivatized products of 2-
hydroxyoctanoic acid/1-octanol was found to be 1.1 0.2.
In general, the derivatization reagents showed strong
preferences for their intended moieties, yielding the same
amount of derivatized product when the doubly substituted
sample contained only one of the target functional groups to
within 20%. In the case of 2,5-hexanedione, a dehydration
reaction of the singly derivatized product was observed wherein
the analyte peak (confirmed to be the [M + H]+ molecular ion
peak with MS) was observed to disappear with a first-order rate
of 0.50 0.07 h−1 with a concurrent increase in a separate peak
(confirmed to be [M − 18 + H]+). The sum of the singly
derivatized dione and the dehydrated derivatized dione showed
only a very slight increase (6% h−1) over the 50 min span used
to obtain the dehydration kinetics. We speculate this
dehydration occurs via intramolecular nucleophilic substitution
of the other carbonyl carbon but have made no further attempt
to characterize the reaction. The doubly derivatized product
peak showed no time dependence. In addition, it is believed
that the low conversion for DMABC derivatization of 5-
hydroxypentanone is due to the presence of dimers in the
standard that are not effectively derivatized.
As indicated in Table 3, analytes containing two of the target
moieties (e.g., a diacid derivatized with 2-PA) showed a mixture
of singly and doubly derivatized products. Doubly derivatized
product signals were scaled by 0.5 under the linear absorption
assumption, and the sum of the parenthesized entries in the
table and their corresponding partners is expected to be unity
in the case of purely linear and complete specific derivatization.
In each case products were confirmed with MS. In the case of
DMABC, the formation of esters favors the formation of [M +
Na]+ ions. Chains containing carbonyls in addition to the
Figure 2. Chromatograms of hexanoic and decanoic acids derivatized
with 2-PA.
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group distribution) can be derived from the base absorption of
individual derivatized products and the appropriate relative
absorptivity scaling (Table 2).
rapidly forms an octyl peroxy radical. As shown in Scheme 4,
under these low NOx conditions, the peroxy radicals are
Although the absorption linearity assumption is quite strong,
several factors favor its applicability. Group additivity for
ultraviolet absorption is effective in estimating molar
absorptivity of molecules,50 and chromophores at various
locations along a carbon backbone tend to be linearly additive.
The linearity assumption can fail dramatically in compounds
containing conjugated double bonds or aromatic moieties. The
first case is unlikely to pose major problems in SOA analysis.
Exposed double bonds will often have undergone reaction
during the particle formation and aging, although SOA has
been found to include some olefinic compounds, such as oleic
acid observed in ambient samples.51 Even in such cases,
however, a significant shift in the UV absorption spectrum will
be present only in vinylic substitutions that lead to conjugation
in the π-system. For reaction with OH, addition across the
double bond would be strongly favored over abstracting the
vinylic hydrogen atom for O2 to initiate oxidation. The second
case involving aromatic compounds could yield drastic
deviations in linearity if the derivatization occurred on a
functional group bound directly to an aromatic. Substitution of
these highly aromatic derivatization reagents will have a large
nonlinear impact on the final molar absorptivities and
absorption maxima and would lead to a very poor estimate of
the functional group distribution. Consideration of the
ultraviolet absorption spectrum of the underivatized product
may provide evidence that such a nonlinearity is present, such
as if an unexpectedly large peak relative to the collected sample
mass or a large and unexpected shift in the absorption
maximum was observed. Overall, the presence of aromatic
species represents an additional challenge that will demand the
use of an alternative technique to ensure quantitative results.
In the end, the goal of this work is not to provide rigorous
absolute measurements but to provide a methodology to enable
the rapid measurement of functional group concentrations with
greater certainty than previous methods. Even measurements
accurate to only a factor of 2× to 3× are improved from
previous work and are expected to offer significant value in
understanding and modeling oxidized VOC volatility and
subsequent SOA formation. Further work to better elucidate
the limitations of the methodology in these areas, particularly
when derivatizing aromatic substituents, is currently underway.
Although this methodology does enhance the separation of
many components, the presence of an unresolved complex
mixture is still a distinct likelihood. In this case, the enhanced
product ionizability and retention time parameterizations
provided by the derivatization reactions can provide much
greater insight into the nature of this unresolved mixture. In
any case, the complex mixture will contribute to the overall
decrease in the derivatization reagent concentration and will be
included in a total functional group distribution analysis.
Application to Synthetic SOA. As a demonstration of the
applicability of this methodology to complex particle systems,
we have utilized it to probe the functional group distribution in
SOA formed from the 254 nm photolysis of iodooctane in a
flow cell. The photolytic generation of iodooctane SOA was
originally described by Kessler et al. and was studied using
aerosol mass spectrometry to determine the O/C and O/H
ratios as well as the average number of oxygen atoms per parent
molecule in the SOA particle.11 Photolysis of the iodide leads to
relatively unreactive iodine atoms and octyl radical, which in air
Scheme 4. Octyl Radical Oxidation to Condensable
Products
a
a
Stable major products are shown in bold.
expected to primarily undergo reaction with other peroxy
radicals to form stable alcohol and aldehyde products or alkoxy
radicals.11 These alkoxy radicals undergo rapid isomerization or
H-abstraction by O2 to form aldehyde and HO2 (not shown in
the scheme) The alkyl radicals formed from isomerization
rapidly react again with O2 to form hydroxy alkyl peroxy
radicals, which undergo similar peroxy radical reactions to form
1-hydroxy-4-octanone, 1,4-octanediol, and a hydroxy alkoxy
radical, which predominately reacts to form 4-hydroxyoctanal.
These reactions lead to a reasonably complex mixture of
compounds that is conducive to functional group analysis.
We have performed a similar experiment to that of Kessler et
al.11 utilizing 12 uncoated fluorescent light bulbs (117 cm, 32
W) for 254 nm photolysis in a quartz laminar flow tube with a
length of 183 cm and an inner diameter of 14.0 cm at a total
residence time of 3.7 min. Photolysis of iodooctane occurred
over the length of the fluorescent bulbs. Under these
conditions, iodooctane concentrations of 2−12 μL L−1 in air
lead to the formation of SOA products, which were monitored
by a scanning differential mobility analyzer (TSI, Inc. model
3080L) and an n-butanol condensation particle counter (TSI,
Inc. model 3775) and collected on quartz microfilters
(Munktell) for a time sufficient to obtain a few hundred
micrograms of sample, 2−24 h. These filters were sonicated in
dry HPLC-grade acetonitrile for 1 h, filtered, and analyzed with
the present methodology.
A typical set of spectra are shown in Figure 3 for the
DMABC (hydroxyl moiety) derivatization of the filter extract.
Numerous peaks, most of which are well-resolved, are present
corresponding to compounds with different OH substitution
patterns. In these spectra, DMABC of higher purity (HPLC
grade, ≥ 99%) was used to minimize background peaks in the
complex chromatogram. A complete analysis of these peaks is
beyond the scope of the present work and will be the subject of
a future publication. Overlaid on the spectra are the peak
integrals. Provided the assumption of linear ultraviolet response
is valid, the total integrals of all peaks should remain identical in
the blank and sample spectra. The measured total integrals
shown in the figure differ by <1%, and the observed decrease in
the DMABC signal at 1.25 min (not shown to allow sample
peaks to be visible on the scale) is fully compensated by the end
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Figure 3. LC-UV (274 nm) chromatograms of SOA derived from iodooctane photolysis derivatized with DMABC to probe hydroxyl functionalities.
The spectra of the sample (solid line) and an extract of a clean filter (dotted line) are plotted relative to the left axis; the integrals of sample and clean
extract are plotted relative to the right axis (dashed and similar dotted lines, respectively).
of the run. Note that this spectrum is taken at 274 nm rather
than the recommended 308 nm (Table 2). In this work,
analysis is typically performed using injection volumes that
yield saturated derivatization peaks to maximize the sample
peak responses, consistent with good peak shape. Under these
conditions, the total integrals of the sample and blank spectra
are not necessarily the same, and we have chosen to present the
lower ultraviolet response at 274 nm, which yielded a fully
unsaturated spectrum that showed both good chromatographic
resolution and linearity of absorption. The 308 nm sample and
blank chromatograms are included as Supporting Information.
In addition, both 2-PA and DnsHz were used to probe the
iodooctane product extracts for carboxylic acids and carbonyls,
respectively. Spectra are included as Supporting Information
and are consistent with Scheme 4 and the results of Kessler et
al.11 Only a very small amount of carboxylic acid is found,
showing three minor peaks in addition to the 2-PA
derivatization agent and coreagent peaks. The spectrum for
the carbonyl functionalities shows a series of single peaks, a
minor broad unresolved component, and a poorly resolved
larger peak. Major products were found to be 1-octanol,
octanal, and a compound with one hydroxyl and one carbonyl
group, presumed to be 4-hydroxyoctanal and 1-hydroxy-4-
octanone. These spectra show the effectiveness of the
derivatization methods on analyzing complex mixtures typical
of SOA.
applied to determine functional group distributions in samples
derived from SOA.
ASSOCIATED CONTENT
* Supporting Information
■
S
Calibration curves and MS/MS transitions for the specific
standards in Table 1 and spectra of 1-iodooctane photolysis
products derivatized with DMABC, DnsHz, and 2-PA. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sean.mcgivern@nist.gov.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support for this work was provided by the NIST Greenhouse
Gas Measurements and Climate Research Program. A.J.K.-P.
acknowledges the National Research Council/NIST Postdoc-
toral Research Associates Program for financial support.
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In this work, we provide a simple methodology for derivatizing
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