(HULIS) by ENVI-18, HLB, XAD-8 and DEAE sorbents: Elemental

Chemosphere 93 (2013) 1710–1719
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Chemosphere
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Comparative study for separation of atmospheric humic-like substance
(HULIS) by ENVI-18, HLB, XAD-8 and DEAE sorbents: Elemental
composition, FT-IR, 1H NMR and off-line thermochemolysis
with tetramethylammonium hydroxide (TMAH)
Xingjun Fan a,b, Jianzhong Song a,⇑, Ping’an Peng a
a
b
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China
Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
h i g h l i g h t s
HULISs isolated by ﬁve SPE methods show similar characteristics and are comparable.
Methods ENVI-18, HLB-M, and XAD-8 are preferable for characterization of HULISs.
The caution is required when using DEAE and HLB-N methods for characterizing HULISs.
a r t i c l e
i n f o
Article history:
Received 5 February 2013
Received in revised form 13 May 2013
Accepted 14 May 2013
Available online 15 June 2013
Keywords:
Humic-like substances
Solid-phase extraction
Elemental analysis
FTIR
1
H NMR
Thermochemolysis
a b s t r a c t
Humic-like substances (HULIS) are signiﬁcant constituents of aerosols, and the isolation and characterization of HULIS by solid-phase extraction methods are dependent on the sorbents used. In this study,
we used the following ﬁve methods: ENVI-18, HLB-M, HLB-N, XAD-8 and DEAE, to isolate atmospheric
HULIS at an urban site. Then we conducted a comparative investigation of the HULIS chemical characteristics by means of elemental analysis, Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy and off-line thermochemolysis with tetramethylammonium hydroxide. The results
indicate that HULIS isolated using different methods show many similarities in chemical composition and
structure. Some differences were however also observed between the ﬁve isolated HULIS: HULISHLB-M
contains a relatively high content of OACAH group, compared to HULISENVI-18 and HULISXAD-8; HULISXAD-8 contains a relatively high content of hydrophobic and aromatic components, compared to
HULISENVI-18 and HULISHLB-M; HULISDEAE contains the highest content of aromatic functional groups, as
inferred by 1H NMR spectra, but a great amount of salts generally present in the HULISDEAE and thereby
limited the choices for characterizing the materials (i.e., elemental analysis and TMAH thermochemolysis); HULISHLB-N has relatively high levels of H and N, a high N/C atomic ratio, and includes N-containing
functional groups, which suggests that it has been altered by 2% ammonia introduced in the eluents. In
summary, we found that ENVI-18, HLB-M, and XAD-8 are preferable methods for isolation and characterization of HULIS in atmospheric aerosols. These results also suggest that caution is required when
applying DEAE and HLB-N isolating methods for characterizing atmospheric HULIS.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Humic-like substances (HULIS) are a class of water-soluble
organic matter (WSOM) in atmospheric aerosol which present
complex properties (i.e., acidity, UV–Vis absorbance, ﬂuorescence,
FTIR, and NMR characteristics) similar to those associated with
⇑ Corresponding author. Address: Guangzhou Institute of Geochemistry, Chinese
Academy of Sciences, Wushan, P.O. Box 1131, Guangzhou 510640, PR China. Tel.:
+86 20 85291312; fax: +86 20 85290706.
E-mail address: [email protected] (J. Song).
0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.chemosphere.2013.05.045
naturally-occurring humic and fulvic acids (Graber and Rudich,
2006). They are ubiquitous in the atmosphere, comprising up to
60% (in terms of carbon) of the WSOCs (Krivacsy et al., 2001; Kiss
et al., 2002; Duarte et al., 2007; Fan et al., 2012; Song et al., 2012).
HULIS play a signiﬁcant role in atmospheric processes due to their
water solubility and strong surface activity (Kiss et al., 2005; Dinar
et al., 2006; Sun and Ariya, 2006). For example, they may affect
many aerosol properties, particularly those associated with scattering and light absorption (Hoffer et al., 2006; Lukacs et al.,
2007), and can thus have a major inﬂuence on the radiation balance in the atmosphere (Ramanathan et al., 2005). HULIS also have
X. Fan et al. / Chemosphere 93 (2013) 1710–1719
an important effect on the hygroscopicity, surface tension, formation, growth, and critical super saturation of cloud-nucleating
cloud droplets (Kiss et al., 2005; Dinar et al., 2007; Ziese et al.,
2008). They therefore have a substantial inﬂuence on the indirect
climate forcing of atmospheric aerosols (Mircea et al., 2005; Dinar
et al., 2006) as well as the hydrological cycle (Ramanathan et al.,
2005).
Since HULIS encompass a large number of diverse chemical
functionalities, there is no single analytical method can reveal
the actual concentration of these macromolecular organic solutes
in samples. Solid-phase extraction (SPE), an effective and selective
method for the isolation of atmospheric HULIS, is the most frequently-used method for the simultaneous concentration and isolation of HULIS from other water soluble constituents (Graber and
Rudich, 2006; Krivacsy et al., 2008; Baduel et al., 2009; Lin et al.,
2010; Fan et al., 2012; Song et al., 2012). SPE methods can be used
to retain and isolate hydrophobic compounds as well as less polar
macromolecular organic compounds, depending on certain hydrophobic–hydrophilic interactions between organic solutes and sorbents. Among the SPE methods, the commonly-used absorbents
are C-18 (Limbeck et al., 2005; Samburova et al., 2007), HLB (Varga
et al., 2001; Krivacsy et al., 2008; Lin et al., 2010), XAD-8 (Duarte
et al., 2004, 2005; Duarte and Duarte, 2005) and DEAE (Havers
et al., 1998; Baduel et al., 2009). It is expected that sorption of HULIS solutes onto a wide variety of sorbents takes place via different
mechanisms under given chemical conditions. Different adsorbents retain and isolate different compounds, depending on their
chemical properties. In a comparison of HLB and C-18 methods,
Varga et al. (2001) found that HLB could recover more WSOC and
had a lower tendency for irreversible adsorption. Baduel et al.
(2009) proposed a comparison between DEAE and C-18 + SAX
and claimed that the former was a preferred method for the isolation of HULIS, since it demonstrated a higher recovery and better
reproducibility. In a previous paper we made a number of comprehensive comparisons between the ENVI-18, HLB, XAD-8 and DEAE
methods (Fan et al., 2012). These comparisons, which mainly focused on the quantiﬁcation of HULIS by TOC, UV–Vis absorbance
and ﬂuorescence spectrophotometry, provide a relatively simple
characterization of HULIS by UV–Vis and/or ﬂuorescence spectra.
There is however a need for a comprehensive comparison on the
basis of chemical composition and structure of HULIS isolated by
means of different methods that broadly targeted the same generic
class of WSOC.
In the present work, our goal is to gain further understanding of
the similarities and differences in chemical composition and structure of atmospheric HULIS, isolated by means of ENVI-18, HLB,
XAD-8 and DEAE methods. For this purpose we examined the
chemical composition and structure of isolated ambient HULIS at
an urban site, using various techniques, which include elemental
analysis, Fourier transform infrared (FT-IR), 1H nuclear magnetic
resonance (1H NMR) and off-line thermochemolysis with tetramethylammonium hydroxide (TMAH thermochemolysis). This
comparison will complement our previous studies, as well as other
studies on isolation methods for atmospheric HULIS (Varga et al.,
2001; Duarte and Duarte, 2005; Baduel et al., 2009; Fan et al.,
2012).
1711
hou, China. Aerosol sampling was carried out from 16 August to
15 September, 2011. Each sample acquisition lasted for 24 h and
time interval was set as 24 h and a total of 15 samples were collected. Prior to sampling, ﬁlters were packed in aluminum foil
and pretreated by baking in a furnace for 4 h at 450 °C to remove
any organic contaminants. After sampling, the ﬁlters were stored
in a freezer (20 °C) until further analysis.
2.2. Isolation and fractionation of HULIS
Each ﬁlter was entirely extracted with 200 mL of high purity
water in an ultrasonic bath for 30 min. The slurries that were obtained were then ﬁltered through a membrane ﬁlter of 0.22-lm
pore size to remove the ﬁlter debris and suspended insoluble particles. To obtain a sufﬁcient quantity for comprehensive characterization of HULIS, the extracts of different batches were combined.
The sorbents utilized for isolation of the HULIS samples were as
follows: (1) XAD-8, a non-ionic macroporous resin; (2) ENVI-18,
a non-polar (highly hydrophobic) sorbent, constituted of alkyl
chains of C-18 covalently bonded to a silica substrate; (3) Oasis
HLB, a hydrophilic–lipophilic-balanced reverse-phase sorbent;
and (4) DEAE, a weakly basic cellulose anion exchange resin. The
ﬁrst three resins were used to isolate HULIS according to their
hydrophobicity, while the DEAE was used to isolate HULIS according to their polyacidic characters. Details of the sample isolation
procedures can be found in our previous study (Fan et al., 2012).
Here, we used 40% methanol solution (in water), pure methanol,
and 1 M NaCl solution to elute the HULIS fraction retained on
XAD-8, ENVI-18, and DEAE resins, respectively. The eluents used
for the HLB method included pure methanol and 2% (v/v) ammonia/methanol. The corresponding methods are described as HLBM and HLB-N, respectively. Finally, the HULIS fraction isolated by
ﬁve methods and original WSOC sample were dried by means of
N2 and/or freeze drying. The solid HULIS and WSOC samples were
stored in a freezer until further analysis.
To check for laboratory contamination, we ran blanks using
50 mL milli-Q water for each method through the above procedures. The organic carbon in the blank was negligible for all
methods.
2.3. Analysis
The ﬁve different types of HULIS and the original WSOC were
characterized in terms of their chemical, structural, and molecular
properties, using various techniques that are brieﬂy described
below.
2.3.1. Elemental composition
The elemental compositions (C, H, O and N) of the six samples
were determined by means of an elemental analyzer (Elementar
Vario EL CUBE, Hanau, Germany) following a standard high-temperature combustion procedure. The acetanilide was used as calibrating standard to determine the contents of C, H, N elementals,
and benzoic acid was used to determine the O contents, respectively. The weight of the sample for each determination was
3.0 mg. The ﬁnal data that we report on in this paper were based
on the analyses of triplicates for each sample, for which the calculated relative standard deviation was less than 1%.
2. Experimental
2.1. Sampling
PM2.5 aerosol samples were collected on Whatman quartz ﬁber
ﬁlters (QMA, 20.3 cm 25.4 cm) using a high-volume air sampler
at ﬂow rates of 1.05 m3 min1 (Tianhong Intelligent Instrument
Plant of Wuhan, China) at a representative urban site in Guangz-
2.3.2. FTIR Spectroscopy
The FTIR spectra (4000–400 cm1) of the ﬁve HULIS and the original WSOC were recorded on a Fourier transform FTIR spectrophotometer (Nicolet 6700 spectrometer, USA) for KBr disks prepared
by mixing the sample (1 mg) with 100 mg KBr. To improve the signal to noise ratio, 64 scans were collected, utilizing 4 cm1
resolution.
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X. Fan et al. / Chemosphere 93 (2013) 1710–1719
2.3.2.1. 1H NMR Spectroscopy. The HULIS and WSOC samples were
analyzed by 1H NMR spectroscopy according to the method of
Tagliavini et al. (2006). The weighted samples (20 mg) were redissolved with 0.7 mL D2O containing sodium 3-trimethylsilyl2,2,3,3-d4-propanoate (TSP, 0.24 mM) as an internal standard.
The 1H NMR spectra were obtained at a frequency of 400 MHz on
a Bruker AVANCE AV 400 spectrometer. Data were acquired 1024
times, with a recycle time of 11.6 s for a condensed water sample.
A sweep width of 5000 Hz was employed with a digital resolution
of 0.15 Hz/point for a 16 K data set.
2.3.3. TMAH thermochemolysis
Samples were weighted (1.0 mg) and placed in a glass ampoule with a measured amount (100 lL) of TMAH (25% in methanol). The methanol was evaporated under vacuum and the
ampoule sealed. The sealed ampoules were then placed in an oven
at 250 °C for 30 min. After cooling to room temperature, the tubes
were cracked open and all inside surfaces washed with methylene
chloride (3 1 mL). The extracts were combined and reduced to
dryness under a stream of nitrogen. The sample was then diluted
with a known volume of methylene chloride (100 lL). The diluted
sample (1 lL) was detected by Trace DSQ Quadrupole MS (Thermo
Finnigan, Austin, TX, USA) operated at an electron impact energy of
70 eV. A HP-5 capillary column (Chrompack, 30 m length, 0.32 mm
ID, 0.25 lm ﬁlm thickness) was employed to separate the pyrolytic
compounds, using helium as the carrier gas. The column temperature was kept initially at 40 °C for 5 min, then was increased at
3 °C min1 to 290 °C, and maintained for 20 min. The pyrolytic
compounds were identiﬁed by their mass spectra and relative
retention times. Most peaks were identiﬁed by comparison with
the NIST library and some were conﬁrmed by comparison with
authentic standards.
od by Salma et al. (2007). In this study, the OM/OC of HULIS isolated using the four methods display some differences. The OM/
OC ratios of HULISXAD-8 and HULISHLB-N were higher than those of
HULISENVI-18 and HULISHLB-M.
Only a limited amount of information on the different HULIS
can be obtained from the elemental composition data, but an
examination of the molar ratios (H/C, O/C and N/C) allows for some
qualitative assessment. As shown in Table 1, the H/C molar ratios
are 1.38, 1.31, 1.72 and 1.40 for the HULIS samples isolated by
ENVI-18, HLB-M, HLB-N and XAD-8, respectively, which is substantially higher than similar ratios obtained from the standard fulvic
acids of the International Humic Substances Society (0.82–1.01).
The higher H/C atomic ratios in the HULIS samples indicate a higher aliphatic character of the HULIS fractions, compared to those of
aquatic FA. The O/C molar ratio for these HULIS samples ranged
from 0.50 to 0.64, indicating that oxygen-containing functional
groups were abundant in the isolated HULIS samples. The N/C molar ratio of the four HULIS samples isolated by the ENVI-18, XAD-8,
HLB-M, and DEAE methods are very similar, which dropped in a
borrow range of 0.05–0.07. The N/C molar ratio of the HULISHLB-N
was however 0.16, which is signiﬁcantly higher than those
obtained from the other four HULIS samples.
The HULIS samples isolated by the ENVI-18, HLB-M, and XAD-8
methods were generally very similar in terms of chemical composition. However, the HULIS isolated when using the HLB-N method
exhibited higher H, N, and H/C, O/C, and N/C atomic ratios than the
HULIS isolated by the HLB-M method despite the same sorbent
were used. These differences may be related to the introduction
of ammonia into the methanol eluent. The higher H/C and N/C values may be a direct effect of the ammonia-containing eluent. The
higher O/C atomic ratio of HULISHLB-N may indicate that it is more
oxidized than the other HULIS samples.
3.2. FTIR Spectroscopy
3. Results and discussion
3.1. Elemental compositions
The elemental compositions (C, H, N, O) for the isolated HULIS
samples are shown in Table 1. It was noted that the HULISDEAE contained a large quantity of NaCl, which was useful for absorption of
moisture. The high proportion of NaCl and the low C content in
these materials would affect the H/C and O/C molar ratios as well
as the OM/OC mass ratio. For this reason, these values for
HULISDEAE are not included in Table 1.
As shown in Table 1, carbon, hydrogen, oxygen and nitrogen are
major elements in the HULIS samples isolated from atmospheric
aerosols (excluding HULISDEAE), which account for 45–54%, 5.5–
6.5%, 35–40% and 3.0–8.2% of total mass, respectively. Carbon
and oxygen are the dominant elements in these HULIS samples,
the sum of which account for 83–92% of the total mass. These data
are similar to the results reported in previous studies (Krivacsy
et al., 2001; Kiss et al., 2002; Duarte et al., 2007; Salma et al.,
2007; Song et al., 2012). It should be noted that the contents of
N and H in HULIS, isolated by means of the HLB-N method, are
higher than that of the other three HULIS samples.
The recoveries of HULIS-C from WSOC are ranged from 32% to
68% for the ﬁve methods, as indicated in Table 1. The HULIS mass
could be evaluated through the organic matter to organic carbon
mass ratio (OM/OC), which could be inferred from the results of
elemental analysis. We found that the OM/OC ratios of HULIS samples ranged from 1.86 to 2.22 in the study (Table 1). These results
are generally similar to those in the published data (Krivacsy et al.,
2001; Kiss et al., 2002; Duarte et al., 2007; Salma et al., 2007; Song
et al., 2012). For example, HULISHLB-M present similar OM/OC ratio
to that of HULIS isolated from urban aerosol using the same meth-
The FTIR spectra (Fig. 1), as expected, show some similarities
with those of HULIS isolated from other atmospheric environments
(Havers et al., 1998; Krivacsy et al., 2001; Kiss et al., 2002; Polidori
et al., 2008).
All FTIR spectra are characterized by a number of absorption
bands, exhibiting variable relative intensities (Fig. 1). A strong
and broad band at around 3414 cm1 is generally attributed to
OAH stretching of hydroxyl groups involved in hydrogen links, carboxyl groups and phenol groups and possibly a small contribution
from the NAH stretch absorption band of amines and amides (Kim
and Yu, 2005; Simjouw et al., 2005). The intensity of this band is
lower in the ﬁve isolated HULIS than in the original WSOC, which
may suggest that WSOC contains many AOH groups and this may
be ascribed to the abundance of water soluble compounds such as
carbohydrates (e.g., levoglucosan) in the WSOC samples. There are
also a few absorption bands at 2970–2930 cm1, which are present
in spectra of all HULIS samples but absent from the WSOC spectra.
These bonds are assigned to CAH stretching of methyl (CH3) and
methylene (CH2) groups of aliphatic chains (Duarte et al., 2005;
Esteves et al., 2009; Santos et al., 2009). The strong band near
1720 cm1 is generally attributed to C@O stretching vibration,
mainly of the carboxyl group and, to a lesser extent, of ketones
and aldehydes (Duarte et al., 2005; Esteves et al., 2009; Santos
et al., 2009). The band near 1635 cm1 is ascribed to CAC stretching of aromatic rings and to C@O stretching of conjugated carbonyl
groups in ketones, quinones and amides (Stevenso and Goh, 1971;
Simjouw et al., 2005; Duarte et al., 2007). These two bands are both
present in the spectra of all HULIS samples, but the 1720 cm1
band disappeared in the WSOC FTIR spectra. All samples exhibit
broad bands in the region of 1380–1440 cm1, which are probably
due to OAH deformation and CAO stretching of phenolic OH
1713
X. Fan et al. / Chemosphere 93 (2013) 1710–1719
Table 1
Elemental composition and atomic ratios of HULIS from the present study and previous studies.
Sample type
Size
Isolation
method
Elemental composition (%)
Atomic ratios
C
H
O
N
H/C
O/C
N/C
Urban
aerosol
PM2.5
Alpine
aerosol
Rural aerosol
Urban
aerosol
Urban
aerosol
Rural aerosol
PM2.5
ENVI-18
HLB-M
HLB-N
XAD-8
DEAE
C18
51
54
45
47
0.63
52
5.9
5.9
6.5
5.5
0.13
6.7
35
38
38
40
0.85
38
3.0
3.1
8.2
3.2
0.05
2.5
1.38
1.31
1.72
1.40
1.53
0.50
0.53
0.64
0.64
0.55
PM1.5
PM2.5
HLB-M
HLB-M
52
55
6.2
7
39
35
2.5
3.1
1.43
1.49
TSP
HLB-M
XAD-8
4.4–
6.9
5.6–
6.5
38–
44
32–
37
2.0–
3.9
2.1–
3.8
1.07–1.9
PM2.5
43–
53
51–
58
1.21–
1.42
groups, or to COO stretching and CAH deformation of CH3 groups
(Duarte et al., 2005; Santos et al., 2009). Infrared absorption in this
region may also have the contribution of bending vibrations of nitrate ion or ammonium ion (Maria et al., 2003; Polidori et al.,
2008). The bands in the 1280–1210 cm1 region are attributed to
CAO stretching of esters, ethers and phenols (Duarte et al., 2005;
Kim and Yu, 2005; Esteves et al., 2009). A weak band at around
1044 cm1 was assigned to CAO stretching of carbohydrate moieties and ethers (Stevenso and Goh, 1971; Santos et al., 2009). Bands
at 860–750 cm1 can be assigned to the OH stretching vibration of
carboxylic groups (Kim and Yu, 2005).
The original WSOC sample display some strong absorption
bands about inorganic components. For example, the absorption
bonds at around 615 cm1 and 1153 cm1 indicate WSOC contain
some sulfate ions (Maria et al., 2003; Polidori et al., 2008); the
absorption bands at around 830 cm1 and 1384 cm1 signiﬁcantly
suggest nitrate ion are abundant in the WSOC (Maria et al., 2003;
Polidori et al., 2008). Different to FT-IR spectra of WSOC, the inorganic absorption bands are weak or lack in the FT-IR spectra of HULIS. It is noted that the HULIS isolated by the DEAE method
generally contain a lot of inorganic components (i.e., NaCl), however no strong bonds were observed in the FT-IR spectra. These
can be explained by the fact that NaCl do not display absorptions
in range of 4000–400 cm1.
The spectra of all ﬁve HULIS samples exhibit very similar shapes
and intensities (Fig. 1). It can be concluded that the HULIS isolated
by different methods (ENVI-18, HLB-M, HLB-N, XAD-8 and DEAE)
contain similar groups, which mainly consist of aliphatic and/or
aromatic groups and other oxygen-containing groups. Certainly,
some differences can be also seen from the spectra of HULIS samples. Apparent differences can be found between HULISHLB-N and
other HULIS samples. In the spectrum of HULISHLB-N, relatively
low and high abundances are associated with the peaks at
1720 cm1 and 1582 cm1, respectively, and a band at 3199 cm1
is also present. Bands at 3199 cm1 and 1582 cm1 can probably
be assigned to the amide NAH functional group and carboxylate
ions, respectively (Simjouw et al., 2005; Duarte et al., 2007; Santos
et al., 2009). These differences are mostly linked to the introduction of 2% ammonia as efﬂuent, which again conﬁrms our previous
observation: that HULISHLB-N has been altered by the treatment of
ammonia. No signiﬁcant differences were observed in the other
four HULIS spectra, whatever the shape or relative intensity of
the absorption bands.
3.2.1. 1H NMR Spectroscopy
1
H NMR spectroscopy is an effective tool for investigating the H
functional structure of HULIS and WSOC in aerosol samples
OM/OC
HULIS-C/WSOC
(%)
References
0.05
0.05
0.16
0.06
0.07
0.04
1.94
1.86
2.22
2.14
1.91
55
51
68
53
32
43–65
Present work
0.58
0.47
0.04
0.05
1.93
1.82
38–72
62
Krivacsy et al.
(2001)
Kiss et al. (2002)
Salma et al. (2007)
0.55–
0.76
0.41–
0.55
0.03–
0.07
0.03–
0.06
1.89–
2.28
1.71–
1.95
36–44
Song et al. (2012)
47–58
Duarte et al. (2007)
(Graham et al., 2002; Decesari et al., 2005, 2007; Samburova
et al., 2007; Ziemba et al., 2011; Song et al., 2012). The 1H NMR
spectra of the ﬁve HULIS samples and the original WSOC were obtained (Fig. 2). These 1H NMR spectra present quite similar patterns. A few sharp signals were a distinct feature in the HULIS
spectra, which account for a minor portion of the total integrated
area of the spectra. Most of the signals appear as a continuous
unresolved distribution, suggesting a complex mixture of substances similar to those reported in earlier studies (Santos et al.,
2012; Song et al., 2012). Despite the large variety of overlapping
resonances, according to the chemical shift assignments, 1H resonance peaks were divided into four representative categories of
functional groups: (1) HAC: aliphatic protons (0.6–1.9 ppm); (2)
HACAC@: aliphatic protons on carbon atoms adjacent to
unsaturated groups such as alkenes, carbonyl or aromatic rings
(1.9–3.2 ppm); (3) HACAO: protons bound to oxygenated aliphatic
carbon atoms (3.4–4.4 ppm), showing that carbohydrates and
ethers are present in the HULIS; and (4) ArAH: aromatic protons
(6.5–8.5 ppm). Among these functional groups, the wide range of
chemical shifts of signals attributed to aromatic protons suggests
the occurrence of highly substituted aromatic rings that cover
shifts of phenols and alkylbenzenes (around 6.5–7.0 ppm), benzoic
acids or esters, and, perhaps, nitroaromatics (>7.7 ppm).
The corresponding normalized distributions of the different
functional groups were estimated from the area of the observed
1
H NMR bands for each sample (Table 2). As seen in Table 2, the
ﬁve HULIS samples and the original WSOC sample exhibit quite
similar patterns in terms of structural characteristics. These are
all characterized by the two relatively high signal bonds: aliphatic
protons (HAC, 36.8–47.5%) and aliphatic protons linked to carbon
atoms adjacent to C@C (aromatic rings or vinylic substituents), or
C@O (carbonyls or carboxyls), which account for 32.9–37.4%. A relatively low content of oxygenated saturated aliphatic protons are
also exhibited in the spectra of all samples, which accounts for
12.6–18.3%. The lowest content of H functional group observed is
that for aromatics (4.3–12.1%). These values are, in general, consistent with the published data for WSOC hydrophobic acid fractions
and HULIS from atmospheric aerosols (Graham et al., 2002; Tagliavini et al., 2006; Decesari et al., 2007), which are characterized by
high aliphatic carbon content and low aromatic carbon content. It
should be noted, however, that the low content of aromatic
(dH = 6.5–8.5 ppm) and the absence of vinylic (dH = 5.0–6.5 ppm)
protons suggests that the hydrogen atoms characteristic of the
unsaturated aliphatic groups are mainly in alpha position to carbonyls and carboxyls. Given the above data, we can conclude that
WSOC and HULIS in atmospheric aerosols are composed of a very
complex mixture of oxygenated functional groups, such as ACOOH,
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X. Fan et al. / Chemosphere 93 (2013) 1710–1719
Fig. 1. FTIR spectra of WSOC and the isolated HULIS samples.
ACH2OH and ACOCH3, and of a minor aromatic compound. This
indicates that macromolecular humic-like organic compounds are
present in WSOC and HULIS, which is consistent with previous
ﬁndings (Graber and Rudich, 2006; Sullivan and Weber, 2006;
Lin et al., 2010).
Two sharp signals at dH = 5.45 ppm and dH = 2.81 ppm were
obviously exhibited in the WSOC spectra, suggesting that levoglucosan and methanesulphonate are abundant in WSOC (Decesari
et al., 2000; Suzuki et al., 2001). The presence of levoglucosan is
fairly consistent with results from earlier studies (Matta et al.,
2003; Claeys et al., 2004; Schkolnik et al., 2005). Nevertheless,
these sharp peaks disappeared from this region of the HULIS spectra. The reason is that the levoglucosan compound cannot be collected by the four methods described in this paper. These results
are therefore in agreement with results from previous studies
(Limbeck et al., 2005; Sullivan and Weber, 2006; Baduel et al.,
2009; Lin et al., 2010). On the other hand, some distinct peaks in
the region of 6.5–8.5 ppm were present in the spectra of HULIS,
but these were weaker in the WSOC spectra, which suggest that
HULIS contain more aromatic structures than in WSOC.
Among the ﬁve HULIS samples, 1H NMR spectra of the HULIS
samples isolated by the ENVI-18 and HLB-M, are similar. Some
sharp signals were found in the region of 3.5–4.0 ppm. HULISHLBM however contains a relatively higher content of the OACAH
group and a relatively lower content of the RAH groups than that
obtained for HULISENVI-18. These differences may be due to the fact
that HLB is a hydrophilic–lipophilic-balanced reverse-phase sorbent. The HLB sorbents tend to retain relatively polar O-containing
organic macromolecules. Compared with HULISENVI-18 and
HULISHLB-M, HULISXAD-8 contains a relatively low content of
OACAH and a relatively high content of ArAH groups. This indicates that HULIS isolated with the XAD-8 method contain a
relatively high content of hydrophobic aromatic components.
HULISDEAE contains a signiﬁcantly high content (12%) of aromatic
protons, despite its low signal/noise. This indicates that HULISDEAE
may have a higher abundance of aromatic groups (Baduel et al.,
2009; Fan et al., 2012). As is the case for HULISHLB-M, HULISDEAE
also has a relatively high content of the OACAH group in comparison with HULIS isolated with ENVI-18 and XAD-8 sorbents. This
may be due to the chemical composition of DEAE, which consists
of weakly basic cellulose anion exchange resins that tend to retain
the O-containing macromolecule polyacid anions.
Compared with the above four HULIS samples, HULISHLB-N displayed some distinct patterns. The aromatic band in HULISHLB-N
spectra is towards lower chemical shifts (7.0–7.5 ppm) compared
with that of other HULIS, particularly those in which a distinct
maximum is present at dH = 7.8 ppm, suggesting the occurrence
of aromatic rings carrying more electron-donor groups (for example, the amino group) (Decesari et al., 2000; Suzuki et al., 2001).
This is consistent with the conclusions of a former analysis of elemental and FTIR. In addition, HULISHLB-N contains a relatively low
content of OACAH group, but a relatively high content of RAH
and ArAH groups in comparison with HULISHLB-M, despite having
made use of the same HLB sorbent. These differences can be explained by the observation that relatively high RAH and ArAH of
organic molecules can be eluted by the 2% ammonia/methanol
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X. Fan et al. / Chemosphere 93 (2013) 1710–1719
Fig. 2. 1H NMR spectra of WSOC and the isolated HULIS samples.
solution, despite their retention on the HLB sorbent, when pure
methanol is used as an eluant.
3.3. TMAH Thermochemolysis
The total ion chromatograms (TICs) of the pyrolysis products
obtained from the WSOC and the isolated HULIS samples in the
presence of TMAH are compared in Fig. 3. Because of the very
low signal-to-noise ratio of the pyrograms, HULISDEAE is excluded
from this comparative analysis. Major peaks that have been identiﬁed are summarized in Table 3, some of which are labeled in Fig. 3.
As shown in Table 3 and Fig. 3, the dominant compounds include
the following: (1) aliphatic nonocarboxylic acids such as hexadecanoic acid and 11-octadecenoic acid (Nos. M3, M5); (2) aliphatic
dicarboxylic acids, such as butanedioic acid and nonahedioic acid
(Nos. D1, D8); (3) aromatic acids, such as benzenedicarboxylic acid
Table 2
The proton species and corresponding content percentage for WSOC and the isolated HULIS samples.
Sample
WSOC
HULISENVI-18
HULISHLB-M
HULISHLB-N
HULISXAD-8
HULISDEAE
Functional groups (%)
RAH (0.6–1.9)
@CACAH (1.9–3.2)
OACAH (3.4–4.4)
ArAH (6.5–8.5)
43
46
41
47
44
37
35
33
35
33
37
35
18
15
18
13
13
16
4.3
5.8
5.8
6.7
6.1
12
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X. Fan et al. / Chemosphere 93 (2013) 1710–1719
(Nos. A5, A6, A7), and (4) other compounds, such as levoglucosan
(Nos. O2).
Among the aliphatic acids, monocarboxylic acids (fatty acids)
with even-numbered carbon chains, such as hexadecanoic acid
and octadecanoic acid, were detected in all the samples. These belong to the most common fatty acids in plant and animal lipids
(Gobe et al., 2000) and are often detected in HA samples that have
undergone pyrolysis (del Rio et al., 1998; Li et al., 2006). Unsaturated monocarboxylic acids, such as hexadecenoic acid and
octadecenoic acid, also detected in all the samples, may have derived from vegetation emissions (Lehtonen et al., 2001). A high
abundance of dicarboxylic acid methyl esters homologues was detected in all HULIS samples. It is worth noting that nonahedioic
acid, dimethyl ester (No. D8) exhibits a relatively high intensity
in all HULIS pyrograms. These compounds may derive from the
oxidation of fatty acids or isoprenoid precursors, from direct fossil
fuel emissions, or from biomass combustion (Graham et al., 2002;
Zhao et al., 2012). Monocarboxylic acids and dicarboxylic acids
Fig. 3. Total ion chromatograms of the thermal degradation products obtained following pyrolysis of WSOC and the isolated HULIS samples in the presence of TMAH.
X. Fan et al. / Chemosphere 93 (2013) 1710–1719
Table 3
Typical pyrolysis products of WSOC and the isolated HULIS samples.
Signal no.
Compounds
Monocarboxylic acids
M1
M2
M3
M4
M5
M6
Cyclohexaneacetic acid,2-oxo-, methyl ester
9-Hexadecenoic acid, methyl ester
Hexadecanoic acid, methyl ester
cis-10-Heptadecenoic acid, methyl ester
11-Octadecenoic acid, methyl ester
16-Octadecenoic acid, methyl ester
Dicarboxylic acids
D1
D2
D3
D4
D5
D6
D7
D8
Butanedioic, dimethyl ester
Butanedioic,methyl-, dimethyl ester
Pentanedioic acid, dimethyl ester
Butanedioic,ethyl-, dimethyl ester
Pentanedioic acid,2-methyl-, dimethyl ester
Hexanedioic acid, dimethyl ester
Heptanedioic acid, dimethyl ester
Nonahedioic acid, dimethyl ester
Aromatic compounds
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
A11
A12
Benzyl methyl ether
Benzenemethanamine,N,N-dimethyl
Benzoic acid, methyl ester
N-methyl Phthalimide
1,2-Benzenedicarboxylic, dimethyl ester
1,4-Benzenedicarboxylic, dimethyl ester
1,3-Benzenedicarboxylic, dimethyl ester
Benzoic acid,3,4-dimethoxy-, methyl ester
Benzenesulfonamide,N,N,4-trimethyl
3,4,5-Trimethoxybenzoic acid, methyl ester
2,2-Bis(40 -methoxyphenyl)propane
Dehydroabietic, methyl ester
Others
O1
O2
Butanoic acid,2-ethylhexyl ester
1,6-Anhydro-bD-glucose, trimethyl ester
Table 4
Relative percentages of the major groups of pyrolysis products of WSOC and the
isolated HULIS samples.
Samples
Monocarboxylic
acids
Dicarboxylic
acids
Aromatic
compounds
Others
WSOC
HULISENVI-18
HULISHLB-M
HULISHLB-N
HULISXAD-8
8.8
9.4
19
13
8.3
6.4
22
20
21
21
23
67
56
64
67
62
1.9
5.0
2.3
3.9
have also been detected in water soluble organic compounds in organic aerosol (Gelencser et al., 2000; Subbalakshmi et al., 2000)
and in dissolved organic matter in river water or rainwater (del
Rio et al., 1998). The aromatic compounds identiﬁed in HULIS were
remarkably similar to pyrolysis products of HAs, FAs and lignin
(Martin et al., 1994, 1995; del Rio et al., 1998; Chefetz et al.,
2002; Li et al., 2006; Fukushima et al., 2011). All HULIS pyrograms
present two distinct peaks at 32.8 min and 34.5 min (Nos. A5, A6),
which represent 1,2-benzenedicarboxylic, dimethyl ester and 1,4benzenedicarboxylic, dimethyl ester. The presence and great abundance of these aromatic compounds may indicate that aromatic
units are the building blocks of the HULIS structures. Among the
pyrolysates, methoxylic benzoic acids were detected in all samples,
which may be due to the presence of lignin derivatives. Moreover,
three N-containing aromatic compounds (Nos. A2, A4, A9) are presented in HULIS, which may be mainly formed from N-containing
structures in HULIS and/or it may indicate a secondary reaction between aromatic substructures and TMAH. Benzene sulfonate was
ﬁrst identiﬁed in pyrolysates of HULIS, which directly indicates
the existence of sulfate covalently bound to HULIS. The presence
1717
of benzyl methyl ether in pyrolysis products of HULIS, which account for 5.1–32%, was an important feature. In addition, 1,6Anhydro-bD-glucose were identiﬁed in the pyrolysates, which
may have derived from the combustion of biomass.
To compare the pyrograms of the ﬁve HULIS samples and the
original WSOC samples, the relative abundance of pyrolytic products for each sample was calculated by normalizing each individual peak area to the total peak area for all identiﬁable
compounds. The relative content for each group was summed
and the results are listed in Table 4. The data reveal that the four
HULIS samples and the original WSOC sample exhibited signiﬁcant
differences in terms of products distribution. The four HULIS samples contained a high content of aromatics, with the relative content ranging from 56% for HULISHLB-M to 67% for HULISENVI-18
(Table 4). In addition, relatively low contents of dicarboxylic acids
(20–22%) and monocarboxylic acids (8.3–19%) were found in the
total pyrolysis products. These data suggest that the four HULIS
samples have similar chemical compositions. However, the most
abundant pyrolysis product identiﬁed in the WSOC sample is 1,6Anhydro-bD-glucose, a hydrophilic sugar compound derived from
biomass combustion, which accounts for 61% of the total identiﬁed
compounds. In contrast, it accounts for only 1.2–3.4% of total pyrolysis compounds of the four HULIS samples. This ﬁnding again conﬁrms the presence of levoglucosan in WSOC, which is in agreement
with the FTIR and 1H NMR results, mentioned above. Aromatic
compounds accounted for 23% of the total pyrolysis products of
WSOC, which is much lower than the relative abundance of these
compounds of the four HULIS samples. These results indicate that
the HULIS isolation methods are preferable for retaining and isolating hydrophobic aromatic compounds.
Apart from the above-mentioned similarities, differences were
observed between the HULIS samples. It is worth noting that the
relative abundance of aromatic compounds in the HULISHLB-M sample is 56%, which is lower than the 67% abundance estimated for
the HULIS samples that were isolated using the ENVI-18 and
XAD-8 methods. The HULISHLB-M sample has a relatively higher
abundance of aliphatic acids (39%) than that estimated for the
HULISENVI-18 and HULISXAD-8 samples. These differences can be explained by the observation that HLB exhibits both hydrophilic and
lipophilic retention characteristics and that some strong hydrophobic compounds were resistant to methanol elution. The relative
content of aromatic compounds was 64% in the HULISHLB-N sample
that was isolated from the same HLB sorbent, which is similar to its
relative content in HULISENVI-18 and HULISXAD-8. These results suggest that the introduction of 2% ammonia resulted in the release of
more hydrophobic aromatic compounds (that are retained in the
HLB sorbent) than was the case when pure methanol eluent was
used.
4. Environmental signiﬁcance
The ﬁve SPE methods, which include ENVI-18, HLB-N, HLB-M,
XAD-8 and DEAE, have been widely used for isolation and quantiﬁcation of HULIS in atmospheric aerosols (Varga et al., 2001;
Duarte and Duarte, 2005; Graber and Rudich, 2006; Samburova
et al., 2007; Baduel et al., 2009; Fan et al., 2012). These methods
have certain advantages, such as being operatively simple and reasonably accurate. Although the sorbents were different, very similar characteristics, in terms of chemical composition, functional
groups, and pyrolysis characteristics, were noted in the HULIS that
were isolated by means of the ENVI-18, HLB-M, HLB-N, XAD-8 and
DEAE methods. This indicates that the HULIS products, that were
isolated using the ﬁve methods, are comparable. These HULIS are
all composed of a number of polyfunctional compounds that are
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X. Fan et al. / Chemosphere 93 (2013) 1710–1719
mainly aromatic structures carrying polar groups such as carboxyl,
hydroxyl and carbonyl.
Some differences can however be identiﬁed by comparative
investigation of the ﬁve methods. It is apparent that the isolation
of HULIS using the ENVI-18, HLB-M, and XAD-8 has certain advantages: the properties of HULIS were not changed during the treatment processes and there was a low level of inorganic impurities in
the products (Varga et al., 2001; Duarte and Duarte, 2005; Fan
et al., 2012). This result may be due to the use of methanol or
methanol/water as eluents. Because different sorbents were used
in the three methods, some slight differences in chemical structures were observed between these HULIS samples. HULISHLB-M
contains a relatively high content of the OACAH group than that
of HULISENVI-18 and HULISXAD-8. We also found that HULISXAD-8 contained a relatively high content of hydrophobic and aromatic components compared to that found in HULISENVI-18 and HULISHLB-M.
The DEAE method has been shown to have certain advantages,
such as direct isolation without the use of any pre-acidiﬁcation. It
may therefore be suitable for the isolation and quantiﬁcation of
aerosol HULIS (Fan et al., 2012). Characterization of HULISDEAE
was however hampered because of a large quantity of inorganic
impurities in the isolated HULIS fraction. In this study, O and H
analysis and TMAH thermochemolysis were excluded because of
interference of inorganic materials or because of very low signals,
and the 1H NMR spectra, which also indicated low signals. Nevertheless, the results did indicate that the HULIS that were isolated
using the DEAE methods contained a relatively high abundance
of aromatic compounds. It is recommended that further improvements, such as a reduction of inorganic interferences in the DEAE
method, should be attempted in the future.
It is further noted that the HULIS that was isolated using the
HLB-N method exhibited some differences from the other four
HULIS. HULISHLB-N contains relatively high levels of H and N, a high
N/C atomic ratio as well as some N-containing functional groups
such as amide NAH, amino and carboxylates, as indicated by elemental, FTIR, H NMR and TMAH thermochemolysis analyses. These
tests also indicated that HULIS isolated by the HLB-N method may
have been altered, due to the introduction of 2% ammonia into the
eluents. These alterations of HULIS may be the results of the interaction of ammonia and HULIS. This could prevent the irreversible
adsorption of high hydrophobic aromatic components (Baduel
et al., 2009; Lin et al., 2010; Fan et al., 2012). Although this method
may be appropriate for quantiﬁcation of ambient HULIS, it may not
be a suitable isolation method for the characterization of HULIS in
atmospheric aerosols.
All these results are only related to the HULIS samples isolated
from atmospheric aerosol of autumn at urban area. However, it is
found that the HULIS samples isolated from different environments
or sources, and from aerosol samples collected in different seasons
can have different properties (Salma et al., 2007, 2010; Wex et al.,
2007; Lin et al., 2010; Claeys et al., 2012; Fan et al., 2012; Song
et al., 2012). Therefore, further efforts will be required in the future
to comparative study the HULIS isolating methods, possibly using
different HULIS samples isolated from rural, marine and biomass
burning aerosol samples, and aerosol samples in different seasons.
The results will be expected to better understand the chemical
composition and structures of HULIS isolated with different
methods.
Acknowledgments
The work was supported by the Natural Science Foundation of
China (Nos. 41173110, 40975090, and 40830745). We greatly
appreciate the assistance of two anonymous reviewers for the
helpful comments that greatly improved the quality of this manuscript. This is contribution No. IS-1686 from GIGCAS.
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