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Organosulfates from isoprene
the aerosol. Here, we focus on the characterization of
additional isoprene SOA-related organosulfates, which have
not been covered in earlier work or require a revision of their
structures. The mass spectral data obtained for selected
unknown polar organosulfates (OSs) present in ambient fine
aerosols, collected from K-puszta, Hungary, during a warm
summer period, have been interpreted in detail and tentative
structures for them are proposed. The K-puszta site was
selected because isoprene SOA formation is known to be
significant during summer.[16,17] The structures were investi-
gated by LC/(ꢀ)ESI-MS and conversion of carbonyl-containing
OSs into 2,4-dinitrophenylhydrazone derivatives. The use of
(ꢀ)ESI is suitable for the sensitive detection of organosulfates,
because the sulfate group is readily deprotonated during the
ionization process, whereas the combination with linear ion
trap mass spectrometry[18] allows one to obtain good-quality
MS2 and MS3 product ion spectra during the elution of a
chromatographic peak. On the other hand, derivatization of
carbonyl-containing OSs to 2,4-dinitrophenylhydrazones
allows one to confirm the presence of a carbonyl group in
the molecules. Derivatization of carbonyl compounds to 2,4-
dinitrophenylhydrazones has been employed for LC/MS
analysis because these derivatives are suitable for detection
in (ꢀ)ESI.[19–21] As in previous work dealing with the charac-
terization of organosulfates from the oxidation of isoprene,
unsaturated fatty acids, and/or monoterpenes,[9,10,22] we
apply in the present study LC/(ꢀ)ESI-linear ion trap MS
methodology and detailed interpretation of the mass spectral
data to structurally characterize polar OSs. Caution is,
however, required with regard to the structural elucidation
of unknown compounds using the latter approach since the
assignments remain tentative. Therefore, in an effort to unam-
biguously characterize an abundant organosulfate (MW 184)
present in an ambient fine aerosol, we have also synthesized
two candidate molecules and have compared their LC/MS
properties with those of the unknown compound.
(Sigma-Aldrich); ethylene glycol (Fluka, Buchs, Switzerland);
oxalic acid and magnesium sulfate (anhydrous) (Merck);
benzene (TCI, Europe N.V. Zwijndrecht, Belgium); and
potassium carbonate (anhydrous), potassium hydroxide (85%),
and p-toluenesulfonic acid (monohydrate) (Acros Organics,
Geel, Belgium).
Organic synthesis of reference organosulfates
The synthetic procedure leading to the organosulfates of 2,3-
dihydroxybutanal (3 and 3’) and 3,4-dihydroxy-2-butanone
(8 and 8’) are given in Scheme 1, and are briefly summarized
below. The detailed synthetic procedures and the 1HNMR
characterization of reaction intermediates are provided in
the Supporting Information.
Organosulfates of 2,3-dihydroxybutanal (3 and 3’)
To obtain epoxide 2, crotonaldehyde was first converted into
acetal 1, which was then reacted with ethylene glycol in the
presence of anhydrous MgSO4 and catalyzed by oxalic acid
instead of p-toluenesulfonic acid in order to avoid a side
reaction (addition of ethylene glycol to the double bond).[23]
The latter was then epoxidized with m-chloroperoxybenzoic
acid (mCPBA).[24,25] Further reaction of epoxide 2 with con-
centrated sulfuric acid in acetone resulted in the formation
of a mixture of organosulfates 3 and 3’.[26]
Organosulfates of 3,4-dihydroxy-2-butanone (8 and 8’)
Epoxide 6 was synthesized according to literature proce-
dures.[24,27] First, 3-chlorobutan-2-one was converted into its
acetal 4 in a standard reaction with ethylene glycol catalyzed
by p-toluenesulfonic acid. An alkali-induced abstraction of
hydrogen chloride from 4 generated a double bond and gave
rise to alkene 5, which was further epoxidized with
mCPBA.[24,25] Subsequently, epoxide 6 was reacted with con-
centrated sulfuric acid in acetone,[26] leading to ring opening
and deprotection, and resulting in the formation of a mixture
of organosulfates 8 and 8’.
EXPERIMENTAL
Chemicals
Aerosol samples and sample preparation
Methanol (ULC/MS grade), used for sample preparation and
as the LC mobile phase, and acetonitrile (HPLC supra-
gradient grade), used for DNPH-reagent preparation, were
from Biosolve NV (Valkenswaard, The Netherlands);
ammonium formate (analytical grade) and cis-pinonic acid
(purity: 98%) were from Sigma-Aldrich (St. Louis, MO, USA);
acetic acid (analytical grade) was from Merck (Darmstadt,
Germany). 2,4-Dinitrophenylhydrazine (DNPH) was obtained
from Sigma-Aldrich (purity ≥99%, delivered in 50% water)
and the moisture was removed prior to use by a double recrys-
tallization from acetonitrile. The thus obtained DNPH crystals
were used for the preparation of an acidified derivatization
solution that contained 10 mM DNPH and 1.5 M acetic acid
in acetonitrile. High-purity water (18.2 MΩ ꢁ cm; total organic
carbon: 2 ppb), used for redissolving aerosol extracts and
preparing the aqueous LC mobile phase, was supplied by a
Milli-Q water purification system (Millipore, Bedford, MA,
USA). The following chemicals were used for the organic
synthesis of reference organosulfates: crotonaldehyde
(predominantly trans) and m-chloroperoxybenzoic acid (77%)
Archived PM2.5 (particulate matter with an aerodynamic
diameter ≤2.5 mm) aerosol samples collected on quartz fiber
filters were used. The ambient aerosol samples were collected
during the BIOSOL (Formation mechanisms, marker com-
pounds, and source apportionment for BIOgenic atmospheric
aeroSOLs) campaign, which took place from 24 May to 29
June 2006 at K-puszta, Hungary, a rural site located on the
Great Hungarian Plain (46ꢂ58’N, 19ꢂ35’E, 125 m above sea
level), 15 km northwest from the nearest town Kecskemét,
and 80 km southeast from Budapest. The surroundings of
the site are dominated by mixed forest (62% coniferous and
28% deciduous) and grassland (10%). The site is characterized
by intensive solar radiation during summer. The composition
of atmospheric particulate matter during the BIOSOL cam-
paign was studied and it was observed that the campaign
time could be divided into two periods: from the start of the
campaign until 11 June 2006 when it was unusually cold with
daily maximum temperatures between 12 and 23 ꢂC and from
12 June 2006 onward when the temperatures were consider-
[28]
ꢂ
ably higher with daily maxima ranging from 24 to 36 C.
Rapid Commun. Mass Spectrom. 2013, 27, 784–794
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/rcm