oxidation of biogenic compounds12,13 or from field samples.14
Examples of these techniques are atmospheric pressure chemical
ionization source coupled with an ion trap mass spectrometer;12
atmospheric sampling glow discharge ionization coupled with a
quadrupole ion trap mass spectrometer;13 proton-transfer reaction
coupled with a gas chromatograph, which is itself coupled with
an ion trap mass spectrometer;14 and in situ long-path Fourier
transform infrared spectroscopy. The first three techniques are
in development, and isomers are difficult to resolve using these
techniques. In addition, the interpretation of mass spectra is
complicated by the fragmentation of product ions and the
formation of cluster ions. This is further complicated when
complex mixtures need to be analyzed, such as those from field
or chamber experiments, because of the lack of separation prior
to detection.
Current procedures for analyzing POCs are based on single-
step or multistep derivatization techniques. One of the most
common derivatization techniques used in atmospheric chemistry
is based on derivatizing carbonyl groups using O-(2,3,4,5,6-
pentafluorobenzyl)hydroxylamine (PFBHA) or methoxyamine
along with a silylation agent (e.g., bis(trimethylsilyl) trifluoro-
acetamide, BSTFA) to trimethylsilylate carboxylic and hydroxyl
groups simultaneously.8,15 Only the desired derivatives are ex-
pected from these techniques; however, a number of artifacts are
formed.16 When one or more hydroxyl groups coexist in the same
molecule with a carboxylic group, this method leads to ambiguity
between these two different groups. Also, a number of artifacts
are reported from the silylation reaction that can affect product
identification,16 which was also observed in this study. Most of
the artifacts noted in our laboratory are reported by Little16 and
are only briefly discussed here. We refer the reader to this
interesting paper for more information. Scientists should be wary
when using silylation reactions in atmospheric chemistry as a tool
to identify organic compounds bearing OH groups. In fact, current
analytical methods do not allow the complete identification of the
relatively strong acid (BF3 or BCl3), followed by silylation using
BSTFA to derivatize OH groups or PFBHA to derivatize ketone
and aldehyde groups. Special emphasis was given to POCs bearing
both hydroxyl and carboxylic groups. The new method was
successfully tested on 52 model compounds, on samples from
chamber experiments (secondary organic aerosol (SOA) from
photooxidation of biogenic and aromatic compounds such as
R-pinene, â-pinene, and toluene), and on field samples (PM2.5).
Due to lack of space, results from smog chamber experiments
and field samples are reported in a separate paper.17 Several
parameters of the esterification were investigated and optimized,
including derivatizing reagent (BF3-methanol, BCl3-methanol,
and BF3-1-butanol), temperatures, and reaction times. The new
method (BF3-methanol + BSTFA/ PFBHA) was compared to
current derivatization methods (BSTFA, PFBHA, and PFBHA +
BSTFA). Also, several combinations of the three derivative agents
(BF3-methanol, BSTFA, PFBHA) were used, and artifacts are
reported. Note that Kubatova et al.7 used diazomethane to
methylate field samples followed by silylation, and the derivatives
were analyzed only in electron impact ionization (EI) mode.
However, to our knowledge, this is the first time that a derivati-
zation based on a combination of BF3-methanol (or BF3-1-
butanol), PFBHA, or BSTFA coupled with EI and chemical
ionization (CI) mass spectrometry for the identification and
quantification of POC compounds is reported. Tandem mass
spectrometry (MS/ MS) spectra were also obtained for the
molecular or fragment ions of each compound, which were used
for further identification of the reaction products and to elucidate
dissociation pathways.
EXPERIMENTAL METHOD
Chemicals and Solvents. All chemicals of analytical reagent
grade were purchased from Aldrich Chemical Co. (Milwaukee,
WI) except dimethyl esters, which were purchased from Acros
Organics (Geel, Belgium). All chemicals were purchased at the
highest purity available and used without further purification. All
solvents were from Burdick and Jackson (Muskegon, MI) and
specified as GC2 quality. Reagent agents used for the different
derivatizations (BF3-methanol, BCl3-methanol, BF3-1-butanol,
BSTFA 1% trimethylchlorosilane, TMCS, and PFBHA) were
obtained from Aldrich Chemical Co. Pinonaldehyde was synthe-
sized in our laboratory.18 Glassware was washed with soap, rinsed
with hot water, and dried overnight at 200 °C. Just before use,
the glassware was rinsed three times with acetone and three times
with methylene chloride and dried at 120 °C.
Model Compounds. A total of 52 standard compounds (see
Tables 1 and S-1 (Supporting Information)) that exist commercially
or were synthesized in our laboratory were selected to optimize
the analytical method developed in this study. Five groups of
compounds shown in Table 1 were used: mono- and multicar-
boxylic acid compounds (1 -2 0 ), hydroxycarboxylic acids (com-
pounds 2 1 -2 7 ), carboxylic acid with carbonyl groups (2 8 -3 1 ),
hydroxy/ oxo aldehydes or ketones (3 2 -3 4 ), and hydroxy
compounds (3 5 -4 1 ). These 41 analytes were derivatized using
the procedures described below and analyzed in EI and CI modes.
Also, MS/ MS spectra were recorded for most of these standards
organic compounds comprising PM2.5
.
Recently, Edney et al.8 identified a number of POCs in field
samples using the PFBHA + BSTFA double-derivatization tech-
nique. However, it was difficult for these authors to assign an
unambiguous structure to compounds bearing more than one
hydroxyl or one carboxylic group simultaneously. The primary
motivation of the present study is (1) to investigate the possibility
of identifying compounds bearing hydroxyl, ketone, and carboxylic
groups simultaneously that current derivatization methods are
unable to unambiguously differentiate and (2) to extend the
capability of current analytical techniques used for aerosol
analysis. In fact, a new analytical method was developed for the
characterization of aerosol oxygenated organic compounds bear-
ing one or more of the following groups: hydroxyl (-OH),
carboxylic (>COOH), and ketone or aldehyde (>CO). This
approach is based on derivatizing carboxylic groups using
methanol or 1-butanol as derivatizing agents in the presence of a
(12) Warscheid, B.; Ku¨ ckelmann, U.; Hoffmann, T. Anal. Chem. 2 0 03, 75, 1410-
1417.
(13) Dalton, C.; Jaoui, M.; Kamens, R. M.; Glish, G. Submitted to Anal. Chem.
(14) Warneke, C.; De Gouw, J.; Kuster, W. C.; Goldan, P. D.; Fall, R. Environ.
Sci. Technol. 2 0 0 3 , 37, 2494-2501.
(15) Yu, J.; Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 1 9 9 8 , 32, 2357-
2370.
(17) Jaoui M.; Corse, E. W.; Kleindienst, T. E.; Lewandowski, M.; Offenberg, J.;
Edney, E. O. Submitted to Environ. Sci. Technol.
(16) Little, J. M. J. Chromatogr., A 1 9 9 9 , 844, 1-22.
(18) Jaoui, M.; Kamens, R. M. Atmos. Environ. 2 0 0 3 , 37, 1835-1851.
4766 Analytical Chemistry, Vol. 76, No. 16, August 15, 2004