Introduction
Organosulfur compounds (OSCs) such as methanethiol (CH3SH, MTO),
dimethyl sulfide (CH3SCH3, DMS), dimethyl disulfide
(CH3SSCH3, DMDS), and dimethyl trisulfide (CH3SSSCH3,
DMTS) have been measured in air (Nguyen et al., 1983; Andreae et al., 1985; Andreae,
1990; Andreae et al., 1993; Aneja, 1990; Bates et al., 1992; Watts, 2000; de Bruyn et al., 2002;
Xie et al., 2002; Jardine et al., 2015). In marine environments, DMS is the major
organosulfur compound emitted from phytoplankton decomposition and algae
activities. Organosulfur compounds have also been reported from terrestrial
biogenic sources including wetlands, soils, vegetation, and biomass burning
(Goldan et al., 1987; Bates et al., 1992; Kesselmeier et al., 1993; Crutzen et al., 2000; Watts,
2000; Meinardi et al., 2003; Geng and Mu, 2006; Yi et al., 2008). In addition to these
biogenic sources, several recent studies report organosulfur compound
emissions from anthropogenic agricultural and composting activities and
from animal waste (Burnett, 1969; Williams et al., 1999; Filipy et al., 2006;
Mayrhofer et al., 2006; Kim et al., 2007; Shaw et al., 2007; Trabue et al., 2008; Feilberg et al., 2010;
Papurello et al., 2012; Meinardi et al., 2013; Zhang et al., 2013). Atmospheric mixing ratios of
OSCs range from a few ppt to hundreds of ppbs. Typically, ppt levels are
reported in pristine marine environments and the free troposphere
(Nguyen et al., 1983; Andreae, 1990; Crutzen et al., 2000; Watts, 2000; de Bruyn et al.,
2002), with ppb concentrations in coastal ecosystems and wetlands
(Watts, 2000). Concentrations of tens to hundred of ppt have been
reported above vegetation, soil, and rice paddy fields (Crutzen et al., 2000;
Geng and Mu, 2006; Yi et al., 2008; Jardine et al., 2015) with some measurements above the
canopy reaching a few ppb for DMS in some cases (Kesselmeier et al., 1993).
Finally, much higher concentrations have been reported from local
anthropogenic activities, such as livestock and animal and domestic waste,
with levels reaching a few ppb to hundreds of ppb (Williams et al., 1999; Watts,
2000; Filipy et al., 2006; Kim et al., 2007; Feilberg et al., 2010; Papurello et al., 2012).
In the atmosphere, OSCs have short lifetimes with respect
to OH radicals during the day and NO3 radicals at night, leading to the
formation of sulfur dioxide (SO2) (and ultimately sulfuric acid,
H2SO4) and methanesulfonic acid (MSA, CH3SO3H)
(Hatakeyama and Akimoto, 1983; Grosjean, 1984; Barnes et al., 1988, 1994, 2006; Berresheim et al.,
1990; Yin et al., 1990a, b; Tyndall and Ravishankara, 1991;
Davison and Hewitt, 1994; Vandingenen et al., 1994; Capaldo and Pandis, 1997;
Patroescu et al., 1999; Finlayson-Pitts and Pitts Jr., 2000; Zhu et al., 2006;
Berndt and Richters, 2012). For example, under a typical daytime OH
concentration of 5 × 106 cm-3, the lifetime of DMS in air
is about 8 h, and for a typical nighttime NO3 of 5 × 108 cm-3 it is
∼ 30 min. In the presence of
water, amines/ammonia, H2SO4 and MSA are known to form new
particles in air (Kulmala et al., 2004; Bzdek and Johnston, 2010; Smith et al., 2010;
Dawson et al., 2012; Zhang et al., 2012). These newly formed particles can ultimately grow by uptake of organic vapors, amines/ammonia, and/or water to sizes capable of scattering sun light and impacting clouds, thus influencing the
Earth's energy balance and climate (Finlayson-Pitts and Pitts Jr., 2000;
Kulmala and Kerminen, 2008; Hallquist et al., 2009; Zhang et al., 2012; Kulmala et al., 2013). In
addition, atmospheric particles have been previously linked to negatively
affect health and visibility (Dockery et al., 1993; Hinds, 1999; Pope III et al., 2002;
Pope III and Dockery, 2006).
Because of their key role in the formation of new particles in air, it is
critical to account for all sources of OSCs. Several
sample collection strategies have been applied over the years to the
measurement of OSCs in air including the use of
Tedlar® chambers (Hansen et al.,
2011), metal canister or glass vessel-based methods (Kesselmeier et al., 1993;
Williams et al., 1999; Simpson et al., 2001; Meinardi et al., 2003; Blunden et al., 2005; Trabue et al., 2008;
Beyersdorf et al., 2010; Guo et al., 2010; Khan et al., 2012; Meinardi et al., 2013; Zhang et al., 2013), solid
sorbents (Filipy et al., 2006) or sorptive metal
(Andreae et al., 1985), solid-phase microextraction
(Xie et al., 2002; Lestremau et al., 2004), and cryotraps (Hofmann et al., 1992; de Bruyn et al.,
2002). Subsequent analysis of the collected sample is typically performed by
gas chromatography (GC). However, quantifying these species is quite challenging
due to their oxidation and loss on surfaces (Kuster and Goldan, 1987;
Devai and Delaune, 1994; Katoh et al., 1995; Wardencki, 1998; Sulyok et al., 2002;
Bashkova et al., 2003; Lestremau et al., 2004; Kim et al., 2006; Andersen et al., 2012). More recently,
proton-transfer reaction mass spectrometry (PTR-MS) has been applied to the
measurement of these species from various sources (Crutzen et al., 2000;
Hayward et al., 2002; Aprea et al., 2007; Shaw et al., 2007; Feilberg et al., 2010; Kai et al., 2010; Hansen et al.,
2012a, 2013; Papurello et al., 2012; Koga et al., 2014) including breath
(Taucher et al., 1996; Herbig et al., 2009) and food (Aprea et al., 2007). This technique
provides a fast response, high sensitivity, and generally relatively low
fragmentation. It has been increasingly applied to the measurement of
volatile organic compounds; however, it is sensitive only to molecules that
have a proton affinity higher than that of water, and several classes of
compounds are subject to fragmentation (Buhr et al., 2002; Tani et al., 2003; Maleknia et al.,
2007; Kim et al., 2009; Brown et al., 2010; Gueneron et al., 2015), which complicates attributions
of peaks in complex mixtures. In addition, despite the fact that sample collection is
not required, uptake or displacement on sampling lines can occur for some
compounds (Christian et al., 2004; Mikoviny et al., 2010; Freshour et al., 2014).
In this paper, we report a comparison between two techniques for the
measurements of OSCs in air, including direct real-time measurements by
proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) and
offline stainless steel canister sampling coupled to gas chromatography with flame ionization detector (GC-FID). Advantages and
challenges associated with these two techniques are discussed with respect
to sampling complex mixtures.
Materials and methods
PTR-ToF-MS
Measurements of OSCs in air were performed using a high-resolution PTR-ToF-MS (model 8000, Ionicon
Analytik). This instrument has been described previously (Jordan et al., 2009;
Graus et al., 2010) and only the key features related to this particular study are
presented here. The air sample was introduced via heated 1/16′′
PEEK® tubing maintained at 70 ∘C (343 K) at a
constant flow of 150 cm3 min-1. The instrument was
operated under the standard ion drift tube conditions with a total voltage
of 600 V (Udrift) and pressure between 2.10 and 2.15 mbar
(pdrift). Under these conditions, the ratio of the electric field (E)
to the number density (N) of the drift tube buffer gas molecules (E/N) was
kept at values of 130–133 Townsends (Td) (1 Td = 10-17 cm2 V molecule-1)
throughout all measurements, leading to the predominance of
the cluster H3O+ in the ion drift over the higher mass water
clusters (de Gouw and Carsten, 2007). Collisions of the H3O+
ions with a volatile organic compound generally results in a proton transfer
reaction if the compound has a proton affinity (PA) higher than that of
water (PA(H2O) = 691 kJ mol-1). The chemical ionization process is
generally considered “soft” and in most cases generates a parent
ion at [M + H]+. However, many reactive compounds fragment, and all
fragments must be taken into account to estimate the mixing ratios of the
targeted species if they are derived from PTR-ToF-MS parameters rather than
calibration with standards (Kim et al., 2009).
In this work, the mixing ratios of each OSC, Cppb, were quantified based
on Eq. (1):
Cppb=1S×∑Imz-Imz_background(IH3O+106),
where S is a calibration factor (or sensitivity) for the target OSCs expressed
in normalized counts per second per ppbv (ncps ppbv-1), determined
experimentally using pure standards; Imz and Imz_background
are the raw and background ion signal in counts per second (cps),
respectively, for one given OSC mass fragment; and (IH3O+) is the
ion signal in cps for the hydronium ion. The hydronium ion counts were in
the range (0.7–2.90) × 106 cps over the entire period of the
study. In practice, IH3O+ is normalized to 106 cps to yield
normalized counts per seconds (ncps). For each OSC, quantification was
evaluated using the sum of the major fragments, although in principle one
peak would be sufficient when calibrations are carried out independently
using authentic compounds (see Sect. 3.1). It is important to note that
the calibration factor S depends strongly on the operating conditions,
maintenance, and tuning of the instrument and as a result may differ between
studies. Calibrations were performed regularly during the entire period of
the study to ensure proper quantification. Multiple analyses of the same OSC
standard concentration were used to evaluate the day-to-day instrument
variation, from which an uncertainty of ∼ 30 % (2σ)
was derived for any given reported mixing ratio.
An alternative method for determining mixing ratios of volatile organic compounds directly from
the PTR-ToF-MS source parameters (Udrift,pdrift,Tdrift, length of
the reaction chamber, etc.), measured ion transmission efficiencies
(Tr), published values of the kinetic rate constant (k) between H3O+ and the targeted OSC, and the reduced ion mobility of
H3O+ ions μo (= 2.8 cm2 V-1 s-1) was
described previously (de Gouw and Carsten, 2007) (see the Supplement). However, this method requires determining the transmission
efficiencies accurately as well as having evaluated and recommended rate
constants for OSCs with H3O+ ions, for which data are scarce
(Passarella et al., 1987; Arnold et al., 1998; Lindinger et al., 1998; Španěl and
Smith, 1998; Williams et al., 1998; Wang et al., 2004; Zhao and Zhang, 2004; Blake et al., 2009;
Cappellin et al., 2010). As a result, this method typically yields larger
uncertainties on the estimated mixing ratios.
In this work, mass spectra and temporal ion signal profiles were extracted
using the PTR-MS TOF Viewer software (Ionicon Analytik version 1.4.0) and a
custom modified Gaussian function fit for each peak. The PTR-ToF-MS is
equipped with a time-of-flight mass filter with a manufacturer-stated
resolution of ∼ 5000, which allows for accurate mass
determination. The resolution of the instrument for the full mass range of
interest was verified using a mixture of 14 aromatic compounds in nitrogen
(mixing ratios of ∼ 100 ppb; TO-14 mix, Linde) and gave
resolutions higher than 4000 for mass-to-charge ratios ranging from m/z 79
to 181, which is sufficient to resolve the peaks of interest in this study.
The lock masses used for the accurate mass determination were the isotopic
peaks of the protonated water ion at m/z 21.0226 ([H318O]+)
and the protonated acetone [M + H]+ ion at m/z 59.0497, which was
always present in room air sampled at the beginning of each run. For the
source samples, the protonated ion for acetone at m/z 59 could not be used
because it was also found in sufficiently high concentration to saturate the
detector, so the masses m/z 21.0226 ([H318O]+) and m/z 123.946
(a common contaminant peak corresponding to SiO6+ ion) were used
instead.
Canister sampling coupled with GC-FID analysis
Samples were collected into evacuated 2 L electropolished stainless steel
canisters. Prior to sampling, the canisters were cleaned and conditioned
according to a procedure described previously (Blake et al.,
1994). It has been shown that in order to increase the stability of certain
compounds in the canister as well as provide reproducible split ratios at
the injection, small amounts of water must be present in the canister prior
to analysis (Colman et al., 2001). Ambient samples always contain
some water; however, laboratory generated standards do not. Thus, for an
appropriate analysis of the standards, 18–20 Torr of water vapor was added
prior to sampling (hereafter referred to as water-doped canisters), but no
water was added to the ambient air canister samples.
At the beginning of an ambient air sample collection, the inlet valve of the
canister was fully opened so that the canister reached its final pressure of
∼ 15 psig (1 atm) in less than 1 min. The canisters were
analyzed the same day as the collection. For analysis of each canister, 1350 cm3
of air sample was concentrated by pumping it through a stainless
steel loop (10 mL) filled with glass beads immersed in liquid nitrogen. This
procedure assures trapping of most of the organic compounds of interest
while more volatile species such as CH4, N2, O2, Ar, etc. are
pumped away. The concentrated sample was then vaporized by heating the loop
with hot water (∼ 80 ∘C) and injected into a
parallel three-GC system (Hewlett-Packard) using He as the carrier gas.
Details of the complete analytical system can be found elsewhere
(Colman et al., 2001). The OSCs of interest were identified by
comparison with standards and quantified using a flame ionization detector.
Gas-phase OSC standards
A gas mixture containing 1.02 ± 0.05 ppm (uncertainty taken as
1σ) of DMS and 0.948 ± 0.047 ppm
(uncertainty taken as 1σ) of DMDS in nitrogen was
obtained from Scott-Marrin and used for calibration (uncertainties provided by
the supplier). In addition, generation of gas-phase OSCs was achieved by
injecting a solution of the pure standards in cyclohexane (Fluka,
Spectranalyzed grade), using a syringe pump (Pump systems Inc., model
NE-1000), into a stream of dry synthetic air (ultra zero air, 99.999 %,
Praxair) following a method similar to that described by Jardine and
coworkers (Jardine et al., 2010) (see Fig. S1 in the
Supplement for details),
hereafter referred to as the dynamic injection system. The standards included
DMS (≥ 99 %, Sigma-Aldrich), DMDS (≥ 99 %,
Sigma-Aldrich),
and DMTS (≥ 98 %, SAFC). The mixing ratios
after dilution were estimated using error propagation analysis (Harris,
1991), with an estimated accuracy of ±10 % (2σ) for the
DMS/DMDS gas cylinder, and ±20 % (2σ) for the mixtures from
the dynamic injection system.
A certified gas mixture containing 4.03 ppm of MTO was
obtained from Airgas. In addition, a pure gas-phase MTO standard from
Matheson (purity 99 %) was used to prepare our own gas mixture in the
laboratory using a glass manifold. Ultra zero grade air was supplied from
Praxair for dilution. The uncertainty in the mixing ratio of MTO in the
primary mixture prepared this way was estimated to be ±1 % (2σ). Further dilution of the primary mixture prepared in a 6 L glass
bulb was used for calibration as described in Sect. 3.2.
Lastly, for a separate series of experiments, generation of gas-phase DMTS,
MTO, and hydrogen sulfide (H2S) standards was performed using
commercially available permeation devices (VICI), which were each enclosed in an individual U-shaped glass tube and maintained at 50 ∘C
using a thermostated water bath (LAUDA, model M20). While an individual
sealed tube (or tubular device) was used for MTO and DMTS, the H2S
liquid standard was enclosed in a stainless steel wafer device (wafer device
type 30F3, VICI), which only permeates through a small opening in a
tetrafluoroethylene membrane located at the bottom of the device to allow a
slow permeation rate to be obtained. A flow of 200 cm3 min-1 dry,
filtered air purified by passing through a Fourier transform infrared purge gas generator (Parker
Balston Model 75-62), carbon/alumina media (Perma Pure, LLC), and an inline
0.1 µm filter (DIF-N70; Headline Filters) served as the carrier gas and
diluent through the U-shaped glass tube. Permeation rates for MTO and
H2S were determined gravimetrically giving values of 337 ± 106 and 133 ± 14 ng min-1, respectively. The uncertainties
represent those from repeated weight measurements and are higher for MTO
than stated by the manufacturer (30 vs. 15 %) and lower for H2S
(10 vs. 25 %). The accuracy of the mixing ratios after dilution were
estimated to be the same as that estimated for the primary gas-phase mixing
ratio, as this is the higher uncertainty in the system. It was not possible
to determine accurately the absolute permeation rate for DMTS due to large
variations in the weight of the tube and the presence of some DMDS in the
outflow; however, even if the gas-phase generation system could not be used
for absolute calibration of DMTS, as described in Sect. 3.3, it was useful
for the stability study in which only relative mixing ratios were needed.
Sampling from a complex high-emission source
To compare the performance of both measurement methods on an urban source,
the headspace above street waste bins from a residential area with many pets
was sampled with both the PTR-ToF-MS and the canister/GC-FID method. Since
it is known that livestock in agricultural areas is a significant source of
OSCs (Burnett, 1969; Williams et al., 1999; Filipy et al., 2006; Kim et al., 2007; Shaw et al., 2007;
Trabue et al., 2008; Feilberg et al., 2010; Papurello et al., 2012; Meinardi et al., 2013; Zhang et al., 2013),
the same might be expected for the bins. Two bins were sampled repeatedly: a
135 L bin and a 21 L bin, the contents of which had varied weights (1–8 lb). Before
each sampling period, the bin was opened to ambient air to clear out the
headspace above the sample. The PTR-ToF-MS inlet was then attached to the
lid of the bin, and the sampling started when the lid was repositioned on
the bin. This approach allowed for the measurement of emission rates of the
sulfur compounds from the bin. After each 20 min sampling period with the
PTR-ToF-MS, one canister was attached to the sampling line of the bin and a
sample was taken for comparison. Blank measurements from the bins themselves
and plastic bin liners were also performed and show no detectable OSC
compounds.
Results and Discussion
Fragmentation pattern of the OSCs in the PTR-ToF-MS
Signal response and fragmentation patterns in the PTR-ToF-MS were
investigated from the analysis of the pure OSC standards (Fig. 1).
DMS, DMDS, and MTO have known proton affinities (Lide,
1994) of 830.9, 815.3, and 773.4 kJ mol-1,
respectively, well above the proton affinity of water (691 kJ mol-1). As a result, these compounds are expected to be efficiently
ionized in the PTR-ToF-MS and no relative humidity dependence of the signal
is expected for these compounds. There are no reported values for the proton
affinity for DMTS, but this compound is expected to behave similarly to DMS
and DMDS.
Individual PTR-ToF-MS mass spectra for each organosulfur
compound: (a) methanethiol (MTO) from the laboratory-generated gas-phase
standard, (b) dimethyl sulfide (DMS) and (c) dimethyl disulfide (DMDS) from
injection of the individual pure liquid standards into air in a 100 L Teflon
chamber, and (d) dimethyl trisulfide (DMTS) from the dynamic injection
system. In spectra (b) and (c), the gray peaks correspond to ion fragments
resulting from the ionization of background species in the Teflon chamber;
in (d), the gray peaks correspond to ion fragments resulting from the
ionization of cyclohexane, which is used here as the solvent.
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Accurate mass and elemental composition of the major fragments
observed for the analysis of standard organosulfur compounds by PTR-ToF-MS.
Accurate
Intensity
Elemental
Exact
Absolute mass
mass (Da)
(%)
composition
mass (Da)
difference (mDa)a
Methanethiol
49.0106
100
[CH3SH + H]+
49.0112
-0.6
(MTO)
Dimethyl sulfide
63.0262
100
[CH3SCH3+ H]+
63.0268
-0.6
(DMS)
Dimethyl disulfide
94.9984b
100
[CH3SSCH3+ H]+
94.9989
-0.5
(DMDS)
78.9667b
38
CH3SS+
78.9676
-0.9
49.0102
4
CH3SH2+
49.0112
-1.0
Dimethyl trisulfide
126.9718c
23
[CH3SSSCH3+ H]+
126.9710
+0.8
(DMTS)
92.9836c
31
CH3SSCH2+
92.9833
+0.3
80.9806c
28
CH3SSH2+
80.9833
-2.7
78.9680c
100
CH3SS+
78.9676
+0.4
61.0122
13
CH3SCH2+
61.0112
+1.0
49.0108
19
CH3SH2+
49.0112
-0.4
44.9797
9
CHS+
44.9799
-0.2
a All reported data are within the 3 mDa acceptable mass difference
defined by the Journal of Organic Chemistry (Greaves and Roboz,
2013).b Fragments used for quantification of DMDS.c Fragments used for quantification of DMTS.
As seen in Fig. 1a and b, MTO and DMS give one major peak corresponding
to their respective protonated [M + H]+ ions at nominal masses m/z 49
and 63, respectively. Accurate mass determination shows very good agreement
with the expected elemental composition for the protonated ion within -0.6 mDa of the
expected masses (Table 1 and Fig. S2). In addition, the isotopic
distribution for both parent ions agrees well with the presence of one
single sulfur atom in the molecule with an 34S/32S isotopic ratio
of ∼ 4 % (Berglund and Wieser, 2011).
The mass spectrum of the DMDS standard (Fig. 1c) shows a base peak at
nominal mass m/z 95 corresponding to the [M + H]+ ion and a fragment at
m/z 79. Accurate mass determination (Table 1 and Fig. S2) confirmed the
identity of the parent ion at m/z 94.9984 (-0.5 mDa away from
[CH3SSCH3+ H]+ exact mass), and the ion at m/z 78.9667 was
attributed to the CH3SS+ ion fragment (-0.9 mDa mass difference).
Under our experimental conditions, the peak intensity at m/z 79 was
∼ 38 % of the base peak (m/z 95). In the PTR-ToF-MS, the
fragmentation of one species is generally governed by the electric field
strength (E/N) applied to the drift tube (Tani et al., 2003; Brown et al., 2010;
Gueneron et al., 2015). Our experiments were carried out at an E/N of 130–133 Td.
Schuhfried et al. (2013) studied the fragmentation of
DMDS at different E/N values using density functional calculations and
reported a value for m/z 79 contribution between 20.7 % at E/N = 127 Td
and 66.3 % at E/N = 140 Td. Our work is in good agreement with these
calculations (Fig. S3). In addition, a very small fragment at m/z 49 from
the DMDS standard was observed with an intensity of ∼ 4 % of
the base peak (m/z 95), which is in good agreement with the value of 5 %
predicted by Schuhfried et al. (2013).
Compared to the smaller OSCs, DMTS shows much more fragmentation in
PTR-ToF-MS. Figure 1d shows the mass spectrum obtained when sampling the DMTS
standard from the dynamic injection system. Peaks corresponding to the
protonated ion [M + H]+ and CH3SS+ ions were observed at m/z 126.9718 (+0.8 mDa mass difference from [CH3SSSCH3+ H]+
exact mass) and m/z 78.9680 (+0.4 mDa mass difference from
CH3SS+ exact mass), respectively. Five additional peaks, previously
reported as fragments in the DMTS mass spectrum (Mockel and
Weiss, 1980), were observed at nominal masses m/z 93, 81, 61, 49, and 45 and
were assigned to CH3SSCH2+, CH3SSH2+,
CH3SCH2+, CH3SH2+, and CHS+, respectively.
These assignments are supported by the excellent agreement with the exact
masses (see Fig. S2 and Table 1). It is important to note that the peak at
m/z 80.9806 corresponds here to a fragment and is not due to the isotopic
distribution of the major fragment at m/z 78.9680 (see Fig. S2). The
intensity observed at m/z 80.9806 is 28 % of that of the peak at nominal
mass m/z 79. This is much larger than the isotopic distribution expected for
sulfur containing compounds, which would be 8.9 % for a
[CH3-32S-34S]+ fragment (Berglund and Wieser, 2011).
Finally, although a peak at nominal mass m/z 96 was also observed in the
mass spectra of DMTS, accurate mass determination precluded the assignment
to an S3+ fragment (Fig. S4) and the identity of this minor
fragment remains unknown. This fragment was previously reported by Mockel
and Weiss (Mockel and Weiss, 1980) from DMTS chemical ionization mass
spectrometry analysis; however, the study was done using a quadrupole mass
spectrometer which cannot provide exact mass information. In the present
study, the relative intensities of the fragments observed, taking the base
peak at m/z 79 to be 100, are 9:19:13:100:28:31:8:23 for m/z 45, 49, 61, 79,
81, 93, 96, and 127, respectively.
Quantification of OSCs by PTR-ToF-MS
Calibration of the PTR-ToF-MS for DMS, DMDS, and DMTS was performed using
successive dilutions of the 1 ppm certified gas cylinder for DMS/DMDS and
the outflow of the dynamic injection system for DMTS. The dynamic injection
system was also used with DMS and DMDS standards to validate the technique.
Very good agreement between the gas cylinder and dynamic injection system
was observed for DMS/DMDS (Fig. S5), supporting its application to DMTS
calibration. Measurements showed a linear dynamic range from 0 to ≥ 250 ppb for all three sulfides (Fig. S5). Analytical limits
of detection were estimated as 3σ of the baseline noise, where
peak-to-peak baseline variation was taken as 5σ (Skoog and
Holler, 2007). Limits of detection (LODs) for DMS and DMDS were both 49 ± 15 ppt, while the LOD for DMTS was
81 ± 24 ppt.
Calibration for MTO was more difficult to achieve due to its loss and
reactivity on surfaces. For example, losses on metal surfaces were observed
when placing a ∼ 20 cm stainless steel or copper tubing in the
sampling line between the certified 4.3 ppm MTO gas cylinder and the
PTR-ToF-MS inlet. Figure S6 shows a drastic loss of MTO in both cases as
soon as the metal tube is inserted. In addition, PTR-ToF-MS analysis from
the certified gas cylinder revealed that DMDS was formed inside the
regulator, which precluded the use of this standard for calibration (Fig. S6). As an alternative, we chose to perform the PTR-ToF-MS calibration using
our own laboratory generated gas-phase mixture of MTO in clean dry synthetic
air from a gas cylinder of pure MTO. Once extracted from the cylinder, the
gas was never in contact with any metal tubing or connectors. Known amounts
of the pure standard were transferred into a previously evacuated 5 L glass
bulb that was pumped on overnight. Successive dilutions in clean dry
synthetic air were then made using a glass manifold to reach a final mixing
ratio of 4.08 ± 0.04 ppm MTO. This mixture was stored in a separate
previously evacuated 6 L glass bulb overnight to make sure the mixture was
well mixed in the bulb prior to its use. During preparation of the mixture,
there was no evidence for MTO loss on the glass surfaces, consistent with
the observation of Devai and Delaune (1994) who reported a 90 % recovery after 24 h
for MTO samples prepared in dry air in a 125 mL glass bulb. Calibration mixtures were prepared by
diluting a flow of 50 cm3 min-1 of the bulb contents with a flow
of 1 to 7.5 L min-1 of dry synthetic air. No evidence of any other OSC
was observable in the PTR-ToF-MS spectra, apart from MTO. A linear dynamic
range was observed from 0 to 200 ppb (Fig. S5) and an LOD of 65 ± 20 ppt was determined.
GC-FID chromatogram for (a) DMS (∼ 21 ppb) and DMDS
(∼ 20 ppb) and (b) DMTS (∼ 22 ppb) standards
from the dynamic injection system. The cyclohexane peak is due to the
solvent used in the injection system. Analysis followed immediately after
canister samples were prepared.
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Comparison of quantification of DMS, DMDS, and DMTS using GC-FID
(blue markers) and PTR-ToF-MS (red markers). Standards were sampled from
either dilutions of a certified gas cylinder (DMS and DMDS; filled symbol)
and/or from the dynamic injection system (DMS, DMDS, and DMTS; open
symbols). Errors on the mixing ratios measured by the GC-FID method were
taken as the 95 % confidence interval (±20 %) as reported by Simpson
et al. (2001) and errors on the mixing
ratios measured by the PTR-ToF-MS were taken as ±30 % corresponding to the
day-to-day instrument variation. The 1:1 line corresponds to the line of
perfect agreement between the mixing ratios measured by either the
PTR-ToF-MS or the GC-FID and the expected value. The errors bars for the
expected mixing ratios after dilution were estimated as ±10 %
(2σ) for the DMS/DMDS gas cylinder and as ±20 % (2σ)
for the mixtures from the dynamic injection system, based on error
propagation analysis.
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Recovery of DMS, DMDS, and DMTS in electropolished stainless steel
canisters after 1 week. Data include measurements made from the dynamic
injection system (mottled bars; [DMS] ∼ 21 ppb; [DMDS] ∼ 20 ppb; [DMTS] ∼ 33 ppb) and from a gas-phase
source (gray bars; [DMS] ∼ 140 ppb; [DMDS] ∼ 131 ppb; [DMTS] undetermined). The asterisks correspond to DMTS samples where
DMDS was observed in the canister after 1 week, suggesting that DMTS
decomposed on the surface of the canister.
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GC-FID chromatograms from the analysis of stainless steel canisters containing pure MTO standard mixtures prepared
in the laboratory (a) under dry conditions ([MTO] = 1.49 ppm), (b) with water present in the
canister ([MTO] = 1.39 ppm), and (c) under the same conditions as (b) except the canister
was analyzed 24 h later.
![]()
GC-FID chromatograms from the analysis of a glass sampling vessel containing pure MTO standard mixtures prepared
in the laboratory. Analyses were performed via (a) the conventional pre-concentration method ([MTO] = 1.80 ppm) and
(b) via a fast injection method ([MTO] = 12 ppm).
![]()
Intercomparison between GC-FID and PTR-ToF-MS measurements of (a) DMDS and (b) DMS from the bin source samples. The dotted lines correspond to
the 1:1 line (line of perfect agreement) and the black lines correspond to
linear regression fits with y= 0.89 +1.27x for DMS (r2= 0.997) and y=29+2.00x
for DMDS (r2= 0.956).
![]()
Normalized mixing ratios and signals obtained for all four
organosulfur compounds from the source samples including (a) MTO, DMTS, and
H2S; (b) DMS; and (c) DMDS in different bins. Canister samples were not available
for bins 2 and 6. The lines between symbols of individual bins are
simply a visual aid.
![]()
Left panel: GC-FID chromatograms of (a) the H2S standard in dry synthetic
air (∼ 460 ppb), (b) the MTO standard in dry synthetic air
(∼ 816 ppb), and (c) a mixture of the MTO and H2S standard
([MTO] ∼ 408 ppb; [H2S] ∼ 230 ppb) sampled
using a water-doped stainless steel canister. Right panel: Corresponding PTR-ToF-MS
spectrum for each sample.
![]()
Analysis of OSC standards using the canisters and GC-FID
Quantification of DMS and DMDS (but not DMTS) using electropolished
stainless steel canisters has been previously reported (Colman et al., 2001;
Simpson et al., 2001; Meinardi et al., 2003, 2013; Beyersdorf et al., 2010; Guo et al., 2010).
In this study, identification of the retention times for the three sulfides
was performed by running standards (Fig. 2). The responses obtained from
the FID were converted from area units into mixing ratios based on a
per-carbon response factor (PCRF) as described previously
(Simpson et al., 2001). Because the three sulfides contain
two methyl carbons, and are thus likely to have the same FID response, we
assigned a single PCRF to these compounds based on the PCRF for ethane, with
the addition of a correction factor for the presence of sulfur atoms
(Mockel, 1976). It is important to note that Mockel (1976)
reported that the number of sulfur atoms does not alter the FID response and
thus a single factor was used for DMS, DMDS, and DMTS. The LOD for the three
sulfide compounds was 20 ppt for the analysis of 1350 cm3 from the
canister (Meinardi et al., 2013).
A direct intercomparison between the PTR-ToF-MS and GC-FID methods was
performed for DMS/DMDS using a 1:100 with dilution and a 1:7 dilution of the
certified gas cylinder, as well as sampling a mixture of DMS
(∼ 21 ppb) and DMDS (∼ 20 ppb) generated using
the dynamic injection system. A separate experiment was performed for DMTS,
using the dynamic injection system (∼ 22 ppb). Mixing ratios
of DMS, DMDS, and DMTS analyzed by GC-FID immediately after sampling were
generally in good agreement with the values reported by the PTR-ToF-MS
within experimental errors as seen in Fig. 3.
The stability of the three sulfides in the water-doped canisters was also
investigated. The study was performed by analyzing the canisters the same
day of the standard sample collection and again after 1 week. First, the
outflow of the dynamic injection system for all three sulfides was collected
in two separate canisters with one canister analyzed on the same day, while
the second was stored at room temperature for 1 week. Results are shown in
Fig. 4, as the percentage of the mixing ratios measured after 1 week to
that on the first day. Recoveries ranged from 53 to 68 % for OSCs
generated using the dynamic injection system. This could be due to two
factors: the canisters might not have had the exact same initial
concentration, and/or the presence of cyclohexane used in generating the
calibration mixtures may induce artifacts in the canister.
A second set of tests made using direct gas-phase standards (certified gas
cylinder for DMS and DMDS and the permeation tube for DMTS) shows much
better recoveries, ranging from 85 to 92 %. However, in all tests, the
presence of DMDS was observed in the DMTS-doped canister analyzed after 1
week, suggesting that DMTS decomposes on surfaces to yield DMDS. Dimethyl
disulfide was also seen as an impurity in the DMTS generated with the
permeation tube by PTR-ToF-MS and is likely due to reaction in the
permeation tube.
Methanethiol proved to be a challenging compound to analyze using the
offline canister/GC-FID approach. A 1.5 ± 0.02 ppm mixture of the
pure gas-phase standard in dry synthetic air was prepared in the laboratory
and analyzed via the conventional method using the usual electropolished
stainless steel canister without water added prior to sampling. The
resulting FID chromatogram (Fig. 5a) shows no MTO but instead a significant
peak for dimethyl sulfoxide (DMSO; (CH3)2SO) and DMDS. The
presence of acetone was always observed as an impurity in those samples.
Interestingly, when 18–20 Torr water was added to the canister prior
addition of MTO mixture the DMSO/DMDS ratio was reversed (Fig. 5b), with a
higher contribution from DMDS than DMSO. This water-doped canister was
re-analyzed after 24 h, and the DMSO peak vanished, while the DMDS peak
increased (Fig. 5c). To the authors' knowledge, this is the first time that
DMSO has been observed as an artifact from MTO sampling. Conversion of MTO
into DMDS has been previously reported to occur on surfaces, such as old
SilcoCan canisters (> 6 years old), due to possible cracks on the
inert coating of the canister that exposed the metal surface
(Khan et al., 2012), various solid sorbents (Katoh et al., 1995;
Bashkova et al., 2003; Lestremau et al., 2004; Andersen et al., 2012; Hansen et al., 2012b), and on solid-phase microextraction fibers (Haberhauer-Troyer et al., 1999; Lestremau et al.,
2004). The presence of metal ions and/or a thermal oxidation was suspected
to be the source responsible for the reaction. The mechanism of formation of
DMSO is not known but may involve the reaction of MTO with the metal oxide
surface whose catalytic sites become covered when water is present in the
canister.
A new mixture of 1.8 ± 0.02 ppm MTO was made in a glass sampling
vessel instead of a stainless steel canister and analyzed by GC-FID
immediately. Results are shown in Fig. 6a for a dry mixture. A significant
amount of DMDS is still present but MTO can now be observed in the
chromatogram. The ratio of the MTO peak area to that of DMDS was 0.65.
Because no conversion of MTO to DMDS in the glass bulb was observed using
PTR-ToF-MS, it is likely that the short time (< 1 min) the sample
stays in the stainless steel pre-concentration system (loop and transfer
tubing) was enough to allow chemistry to convert some of the MTO into DMDS.
To test this hypothesis, a higher mixing ratio of MTO in dry synthetic air
was prepared and analyzed without the pre-concentration step, reducing the
contact time of the sample in the sampling unit to about 10 s before
injection (in this case only 10 mL of the sample could be analyzed). The
resulting chromatogram (Fig. 6b) shows that while DMDS is still present, the
ratio of the MTO peak area to that for DMDS is now ∼ 7.5,
about an order of magnitude larger. Devai and Delaune (1994) previously observed that water influenced the stability of MTO in a glass
sampling bulb, with significant losses of MTO within the first hour in moist air. This
observation strongly suggests that avoiding metal in sampling systems may
not be sufficient for accurate measurement of MTO. Given these issues with
surface reactions of MTO, PTR-ToF-MS is the preferred analytical approach
for this compound.
Application to a complex high-emission source
Sources of OSCs in urban and rural areas include those of non-marine origin
such as human breath (Tonzetic, 1971; Taucher et al., 1996; Van den Velde et al.,
2008, 2009), agricultural activities, and pet waste
(Burnett, 1969; Williams et al., 1999; Filipy et al., 2006; Kim et al., 2007; Shaw et al., 2007;
Trabue et al., 2008; Feilberg et al., 2010; Meinardi et al., 2013), as well as household biowaste
(Mayrhofer et al., 2006; Papurello et al., 2012; Zhang et al., 2013). In this study, the two
sampling and analysis methods, PTR-ToF-MS and GC-FID, were applied to the
investigation of organosulfur emissions from bins in a suburban location
where most of the waste is from pets. This represents a complex mixture that
provides a more realistic test of the applicability of these techniques to
ambient air and sources than the relatively controlled laboratory samples
described above. A typical mass spectrum of a bin sample is presented in
Fig. S7a. There are clearly many compounds in the headspace of the bins,
illustrating the difficulty of assigning all of the peaks based on the
PTR-ToF-MS alone (Table S1 in the Supplement). However, peaks due to DMS and DMDS were clearly
identified and confirmed by GC-FID measurements. A peak at m/z 49 was also
present in the PTR-ToF-MS spectra, which corresponds to MTO. Positive
identification and quantification were based on accurate mass determination
along with the ratios of the different fragments defined for the standards.
In addition, because DMTS shares common ions with DMDS (m/z 79) and MTO (m/z 49), a positive identification of DMTS was recorded only if nominal masses
m/z 127 and m/z 81 (excluding the isotopic peak from m/z 79) were both
present, and the ratio of m/z 79 to 95 was different than that observed for
the DMDS standard, suggesting an additional contribution for m/z 79.
DMS and DMDS were clearly identified in all samples by
PTR-ToF-MS and GC-FID, while DMTS was only detected in the canister samples.
It is important to note that the peak at m/z 79 observed in all bin samples
was exclusively from the DMDS CH3SS+ fragment, as shown in Fig. S7b. No evidence for DMSO or benzene was observed in any bin samples with
PTR-ToF-MS, as indicated by the absence of peaks at m/z 79.0218 and
79.0548,
respectively. Due to sampling and analysis artifacts described above, MTO
was only observed in the PTR-ToF-MS analysis. Mixing ratios of all four OSCs
measured using PTR-ToF-MS and offline canisters/GC-FID are presented in
Table 2.
Results from source samples – intercomparison between PTR-ToF-MS
and GC-FID analysisa.
Bin
Waste weight
DMS (ppb)
DMDS (ppb)
DMTS (ppb)
MTO (ppb)
no.
and Volbinb
GC-FID
PTR-ToF-MS
GC-FID
PTR-ToF-MS
GC-FID
PTR-ToF-MS
GC-FID
PTR-ToF-MS
1
61 ± 12
47 ± 14
350 ± 70
165 ± 50
33 ± 7
n.d.
n.d.
267 ± 80
2c
8 lb,
–
7 ± 2
–
20 ± 6
–
n.d.
n.d.
33 ± 10
3d
135 L bin
15 ± 3 and
10 ± 3
84 ± 17 and
27 ± 8
2 ± 0.4 and
n.d.
n.d.
59 ± 18
14 ± 3
64 ± 13
5 ± 1
4
18 ± 4
13 ± 4
119 ± 23
44 ± 13
15 ± 3
n.d.
n.d.
111 ± 33
5
1 lb,
1.3 ± 0.3
1.2 ± 0.4
14 ± 3
1.5 ± 0.5
8 ± 2
n.d.
n.d.
33 ± 10
135 L bin
6c
1 lb,
–
23 ± 7
–
23 ± 7
–
n.d.
n.d.
e
7
21 L bin
19 ± 4
15 ± 5
120 ± 24
26 ± 8
188 ± 38
n.d.
n.d.
722 ± 217
8
15 ± 3
12 ± 4
97 ± 19
16 ± 5
145 ± 29
n.d.
n.d.
718 ± 215
9
3 lb,
11 ± 2
7 ± 2
27 ± 5
6 ± 2
33 ± 7
n.d.
n.d.
106 ± 32
10
135 L bin
11 ± 2
7 ± 2
26 ± 5
6 ± 2
27 ± 5
n.d.
n.d.
127 ± 38
11
7 ± 1
5 ± 2
23 ± 5
4 ± 1
34 ± 7
n.d.
n.d.
82 ± 25
a Errors were taken as ±20 % for the GC-FID values
(Simpson et al., 2001) and ±30 % for the
PTR-ToF-MS values (day-to-day instrument variation).b The volume of the bin (Volbin) was determined via two methods:
measuring the weight of the container after filling it with water and by
measuring the time to fill the bin with water at the flow rate of
41 L min-1.c Canister measurements were not available for bins 2 and 6.d Two successive canisters were sampled for bin 3.e The signal for MTO saturated the detector.n.d.: not detected.
Emission rates from bins for DMS, DMDS, and MTO determined by
PTR-ToF-MS in molecules cm-3 s-1 a.
Bin no.
Waste
DMS
DMDS
CH3SH
weight and
(molecules cm-3 s-1)
(molecules cm-3 s-1)
(molecules cm-3 s-1)
bin volume
1
1.32 × 109
4.37 × 109
7.12 × 109
2
8 lb
1.35 × 108
3.74 × 108
6.51 × 108
3
135 L bin
2.37 × 108
5.74 × 108
1.36 × 109
4
2.92 × 108
9.64 × 108
2.51 × 109
5
1 lb,
2.45 × 107
2.78 × 107
7.12 × 108
135 L bin
6
6.36 × 108
7.16 × 108
4.02 × 1010b
7
1 lb
6.18 × 108
1.09 × 109
3.01 × 1010
8
21 L bin
4.85 × 108
5.66 × 108
2.95 × 1010
9
3.17 × 108
2.65 × 108
4.83 × 109
10
3 lb
2.94 × 108
2.70 × 108
5.18 × 109
11
21 L bin
1.88 × 108
1.75 × 108
3.22 × 109
a Errors on these values are typically ±4 % taken as the
95 % confidence interval.b The emission rate for MTO was determined using the beginning of the
sampling (from 0 to 10 min), where the signal was not saturating the
PTR-ToF-MS detector.
As can be seen in Fig. 7a and Table 2, DMDS mixing ratios measured by GC-FID
(ranging from 14 to 350 ppb) were systematically higher compared to the
PTR-ToF-MS measurements (ranging from 1.5 to 165 ppb), with an average
factor of two (Fig. 7a). Note that individual measurements may differ by
more than this average value as indicated in Table 2. This is likely due to
the presence of MTO and its conversion to DMDS as seen in the studies using
individual compounds described earlier. Methanethiol measured by PTR-ToF-MS
ranged from 33 to about 720 ppb. Thus the excess DMDS measured in the
canisters was attributed to the conversion of MTO to DMDS on surfaces.
For DMS, the GC-FID measurements were on average 27 ± 3 % larger
than the PTR-ToF-MS derived mixing ratios. This could be due to differences
in sampling: while the PTR-ToF-MS pulls a constant flow of 150 cm3 min-1
from the top of the bin, the canister pulls a faster flow in a
shorter period of time which could affect the flow dynamics in the bin, thus
altering the mixing ratios somewhat.
Lastly, although DMTS was not observed in the PTR-ToF-MS spectra, this
compound was observed by GC-FID. It has been previously reported that MTO
can be converted to DMTS in the presence of H2S and metals
(Chin and Lindsay, 1994). Hydrogen sulfide was also observed in
the source sample measured by PTR-ToF-MS at nominal mass m/z 35 (exact mass
determination was performed to confirm the identity of the compound).
Hydrogen sulfide is a relatively difficult compound to quantify by
PTR-ToF-MS due to its low proton affinity (705 kJ mol-1) (Lide,
1994) so that its signal depends on the relative humidity of the sample
(Feilberg et al., 2010; Hansen et al., 2012a; Li et al., 2014). Calibration of H2S was
beyond the scope of this study, but the signal observed in the mass spectra
was normalized to its highest value to see whether it was correlated with the
DMTS signal. As seen in Fig. 8a, DMTS was the highest for samples no. 7 and
8, where MTO and H2S were also high. Thus, it is possible that DMTS
was formed in a reaction of MTO with H2S on the surface of the canister
and/or sampling lines rather than being emitted directly from the sample.
Figure 8b and c show the corresponding DMS and DMDS normalized mixing
ratios, which exhibit a very different pattern. This suggests that DMS and
DMDS are not involved in DMTS formation. However, it is noteworthy that DMDS
by GC-FID is highest for samples no. 7 and 8, supporting the reaction of
MTO on surfaces as a source of DMDS.
To test whether MTO and H2S react to form DMTS, a separate set of
experiments was conducted where the outflow of a permeation device
containing gas-phase MTO was mixed with the outflow of a second permeation
device containing gas-phase H2S and analyzed by both PTR-ToF-MS and
GC-FID. Figure 9 shows the results of the analysis for both techniques.
While the PTR-ToF-MS only shows a peak at nominal masses m/z 35 and m/z 49
characteristic of H2S and MTO protonated ions, respectively, the
canister GC-FID measurements show that DMTS is formed when both MTO and
H2S are present. In short, it is clear that MTO and H2S react on
metal surfaces to form DMTS and that MTO alone forms DMDS.
Lastly, because PTR-ToF-MS allows sampling in real time, it was possible to
determine emission rates for DMS, DMDS and MTO directly emitted from the
bins. Between each sample, the bins were aired out, and a new waste sample
was introduced. The lid on the bin was then closed and the increase in the
OSC mixing ratios in the headspace was measured as a function of time.
Results are presented in Table 3. Those values were integrated into a 3-D
airshed model in a separate study to evaluate the importance of such
continental sources on the formation of the OSC oxidation products
methanesulfonic acid and sulfuric acids in a large coastal urban area
(Perraud et al., 2015). In that study, there were a number
of potential sources of atmospheric OSCs that have not been yet quantified,
and these techniques could be useful in the future.
In conclusion, gas-phase OSCs are challenging to measure, especially in complex
mixtures characteristic of air. While PTR-ToF-MS provides real-time sampling
capability, fragmentation of parent ions in such mixtures dictates caution
in assigning peaks to specific compounds without additional data such as
GC-FID. In addition, species such as H2S that have proton affinities
close to that of water are not as easily measured due to the dependence on
relative humidity. Canister sampling with GC-FID provides excellent
sensitivity but can suffer from reactions on metal canister and analysis
surfaces. The latter is also an issue if metal sampling lines are used in
conjunction with PTR-ToF-MS.