Chemical ionization mass
spectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compounds
in the atmosphere. A major limitation of these instruments is the uncertainty
in their sensitivity to many of the detected ions. We describe the
development of a new high-resolution time-of-flight chemical ionization mass
spectrometer that operates in one of two ionization modes: using either
ammonium ion ligand-switching reactions such as for NH4+ CIMS or
proton transfer reactions such as for proton-transfer-reaction mass
spectrometer (PTR-MS). Switching between the modes can be done within 2 min.
The NH4+ CIMS mode of the new instrument has sensitivities of up
to 67 000 dcps ppbv-1 (duty-cycle-corrected ion counts per second per
part per billion by volume) and detection limits between 1 and 60 pptv at
2σ for a 1 s integration time for numerous oxygenated volatile
organic compounds. We present a mass spectrometric voltage scanning procedure
based on collision-induced dissociation that allows us to determine the
stability of ammonium-organic ions detected by the NH4+ CIMS instrument.
Using this procedure, we can effectively constrain the sensitivity of the
ammonia chemical ionization mass spectrometer to a wide range of detected
oxidized volatile organic compounds for which no calibration standards exist.
We demonstrate the application of this procedure by quantifying the
composition of secondary organic aerosols in a series of laboratory
experiments.
Introduction
Understanding the photochemical oxidation of volatile organic compounds
(VOCs) in the atmosphere is crucial for estimating their contribution to the
formation of secondary organic aerosol
(SOA) and tropospheric ozone, key
components of photochemical smog (Atkinson, 2000; Shrivastava et al., 2017).
Identification and quantification of VOCs have remained an analytical
challenge due to the complexity of multigenerational chemical systems and
high variability in VOC concentrations in the atmosphere.
Chemical ionization mass spectrometry (CIMS) has become an important
analytical tool for measurements of organic molecules in the atmosphere.
Reagent ions are typically produced by glow discharge (Hansel et al., 1995)
or a radioactive ion source (Blake et al., 2004). These ions subsequently
react with analyte molecules by ligand switching, reactive electron transfer,
or proton transfer and form product ions which are later detected by a mass
spectrometer. Many modern CIMS instruments use time-of-flight mass
spectrometers (ToF-MSs) which have high mass resolving power and simultaneous
detection of all ions. Some of the benefits of CIMS instruments include high
sensitivity, fast time response, linearity, and reproducibility. A variety of
reagent ions can be used to detect different classes of VOCs. Nitrate ion
CIMS has been used to detect highly oxidized organic molecules as well as
sulfuric acid (Berresheim et al., 2000; Jokinen et al., 2012). Iodide adduct
CIMS and acetate CIMS (both negative ion polarity) have played a key role in
the measurement of carboxylic acids (Lee et al., 2014; Bertram et al., 2011).
CF3O- CIMS has been used to measure specific classes of VOCs
such as hydroperoxides (Crounse et al., 2006). Protonated water clusters have
been used to detect a broad range of chemical compounds containing oxygen,
nitrogen, and sulfur (Lindinger et al., 1998; Yuan et al., 2017). Recently,
two new proton-transfer-reaction time-of-flight mass spectrometers have been
developed: the PTR3 (Breitenlechner et al., 2017) and the VOCUS PTR-TOF
(Krechmer et al., 2018). Using H3O+ reagent ions, both
instruments show sensitivities exceeding 10 000 cps ppbv-1 (counts
per second per part per billion by volume) for select compounds. Detection
efficiency and sensitivity of CIMS instruments depend critically on both the
reagent ion and the measured sample molecule (Hyttinen et al., 2017).
CIMS instruments have also been used for analyzing submicrometer particulate
organic matter. Hellen et al. (2008) equipped the inlet of the
proton-transfer-reaction mass spectrometer (PTR-MS) instrument with a denuder
to remove the gas-phase organics and a heater to vaporize the aerosol
particles. Similarly, Eichler et al. (2015) introduced the CHARON–PTR-ToF-MS
setup that transmits particles with a 75 %–90 % efficiency.
FIGAERO–HRToF-CIMS (Lopez-Hilfiker et al., 2014) uses a new filter inlet for
thermal desorption of ambient submicron particles.
In this work, we describe the use of protonated ammonia molecules (ammonium,
NH4+) for soft ionization of analyte molecules. Ammonium has
been previously used as a CIMS reagent ion. Lindinger et al. (1998) showed
that proton transfer reactions can be utilized to softly ionize VOCs,
yielding product ions VOC⚫H+:
NH4++VOC→VOC⚫H++NH3.
The proton transfer Reaction (R1) is exothermic for molecules that have
proton affinities higher than those of ammonia (854 kJ mol-1) and is
therefore more selective than the reaction with traditional hydronium ions as
proton donors (proton affinity of water is 691 kJ mol-1). Blake et
al. (2006) showed that numerous VOCs can be detected through an association
reaction of analyte molecules with ammonium clusters (NH4+ and
NH4+⚫(NH3)):
NH4++VOC+M→VOC⚫(NH4)++M,NH4+⚫(NH3)+VOC+M→VOC⚫(NH3)⚫(NH4)++M,
where M is a third-body molecule. Shen et al. (2009) used
these methods for online detection of the explosive triacetone triperoxide
(TATP).
Most recently, Hansel et al. (2018) showed that ammonium–water clusters can
be utilized for soft ionization of organic compounds via exothermic
ligand-switching reaction:
NH4+⚫(H2O)+VOC⇌NH4+⚫(VOC)+H2O.
Hansel et al. (2018) used a modified version of the
PTR3 instrument (called NH4+-CI3-ToF) to detect first-generation
peroxy radicals and closed-shell products from ozonolysis of cyclohexene and
achieved sensitivities of up to 28 000 cps ppbv-1 for these
compounds. However, the enhanced reaction time and increased pressure (4 ms
and 80 mbar compared to 0.1 ms and 2 mbar for PTR-MS instruments operated
under standard conditions, respectively) raise the probability of reverse
ligand-switching reactions, which make it difficult to estimate sensitivities
of the NH4+-CI3-ToF to species that cannot be calibrated
directly.
In this study, we present a new instrument that is equipped with three
similar corona discharge ion sources and currently can be operated in two
different modes: (1) ligand-switching reactions from adduct ions
NH4+⚫(H2O)n, (n=0,1,2) (NH4+
CIMS) and (2) proton transfer reactions with H3O+⚫(H2O)n, (n=0,1) ions (PTR-MS). The instrument is a modified
version of the PTR3 with a helical tripole reaction chamber and a
long-time-of-flight mass spectrometer (Tofwerk AG, Switzerland), and it can
be used for measurements of organic molecules in both gas and particle
phases. Here we discuss the performance of the new instrument and compare the
two detection modes. We demonstrate a mass spectrometric voltage scanning
procedure which is based on collision-induced dissociation that allows for
the determination of the stability of detected ammonium–organic clusters.
With this technique, we can experimentally estimate sensitivities of the
NH4+ CIMS to the vast array of oxygenated organic compounds
without their direct calibration in a matter of minutes. Finally, we present
how this procedure can be applied to the measurement of organic aerosol
composition in laboratory experiments.
NH4+ CIMS instrument description
The instrument developed in this work is based on the PTR3, which is
described in detail by Breitenlechner et al. (2017). Here, we summarize the
basic operating principle and describe the two major design changes made to
the original design. The schematic drawing of the NH4 CIMS instrument is
shown in Fig. 1.
Schematic drawing of the NH4+ CIMS.
Reagent ions are generated in a corona discharge region and are extracted
using a source drift region as indicated by red arrows in Fig. 1. The
reaction chamber uses a tripole electrode configuration and is operated at
typical pressures between 50 and 70 mbar. Unlike many other PTR instruments,
there is no axial electric field accelerating ions towards the exit of the
reaction chamber. Therefore, the reaction time is exclusively determined by
the flow velocity of the sampled gas in the axial direction, leading to a
typical reaction time of 3 ms. The long time-of-flight (LToF) mass
spectrometer with a mass resolution m/Δm of up to 8000 allows for the
separation of the components with the same nominal mass.
The first major instrument design change consists of replacing the single ion
source with three ion sources, one active at a time. Currently, we use two
sources: one for producing H3O+⚫(H2O)n,
(n=0,1) reagent ions (as PTR-MS, called H3O+ mode) and
another for producing NH4+⚫(H2O)n, (n=0,1,2) reagent ions (as NH4+ CIMS, called NH4+
mode). NH4+⚫(H2O)n ions are produced in the
corona discharge ion source from NH3 and H2O. A constant
flow (20 sccm) of ammonia and water vapour is added to the ion source region
from the headspace of a solution of ammonium hydroxide in water. For our
setup, the concentration of the ammonium hydroxide aqueous solution of
approximately 10 % leads to an optimal NH4+⚫(H2O)n primary ion signal with moderate impurities (Fig. S4 in
the Supplement). At smaller concentrations, excessive H3O+⚫(H2O)n primary ions are produced, while at higher
concentrations NH4+⚫(NH3) becomes more prominent.
Figure 1 shows the instrument in the NH4+ mode with the active
ion source on the left, while the two other ion sources (depicted as a single
ion source on the right) are inactive. The innermost source drift plate of
the active ion source and the innermost source drift plates of both inactive
sources generate an electric field perpendicular to the tripole axis. In
addition, another component of the electric field is generated parallel to
the tripole axis by biasing the electric potential at the secondary orifice
relative to the tripole offset potential. Figure 1 illustrates the resulting
electric field in this transfer region. This geometry allows for effective
ion guiding from the active ion source to the centre of the reaction tripole
chamber. Compared to single-source designs, separate ion sources allow for
faster switching between reagent ion species. As shown in Fig. S3, switching
from the H3O+ mode to the NH4+ mode occurs within
1 min, while the reverse switching from the NH4+ mode to the
H3O+ mode can be done within 2 min.
The second major design change consists of replacing the straight tripole
electrode rods with a helix. Simulations of ion trajectories in the original
tripole showed that ions are lost mostly by exiting the device through spaces
between the rods, rather than by collisions with the rods themselves,
probably due to inhomogeneous effective potentials generated by the tripole
radio frequency (RF) fields (Breitenlechner et al., 2017). The helical
structure effectively averages these inhomogeneities, increasing the ion
transmission efficiency and therefore the overall instrument performance.
The instrument can be used for measurements of organic molecules in both the
gas and particle phases. During particle-phase measurements, sampled air
passes through a gas-phase denuder (Ionicon Analytik GmbH, Austria) that
removes the gas-phase organics and then through a thermal desorption region
heated to 180∘ that vaporizes the aerosol particles. For more details
see the Supplement.
NH4+ CIMS instrument performance
Multiple reagent ions are observed in the mass spectrum of this instrument in
the NH4+ mode, including ammonium–water clusters
NH4+⚫(H2O)n, (n=0,1,2) and ammonium ammonia
dimers NH4+⚫(NH3). Humidity of the sampled air only
slightly affects the distribution of the reagent ions, as shown in Fig. S4.
Most organic molecules are detected as ammonium–organic clusters
NH4+⚫VOC with a few exceptions for which protonated ions
VOC⚫H+ are also observed. The protonated ions could be
produced through proton switching reaction from either H3O+⚫(H2O)n or NH4+. However, for all of these
molecules the intensity of the ammonia–organic cluster is at least 1 order
of magnitude higher than the intensity of the corresponding protonated ion.
Sensitivities and detection limits of NH4+ CIMS for
various VOC species, voltage (V50), and corresponding kinetic energy
(KEcm50) at which half of the ions have dissociated.
A series of laboratory experiments were performed to obtain instrument
sensitivities to various organic compounds as a function of relative
humidity. Table 1 shows sensitivities to 16 compounds measured using a liquid
calibration unit (LCU, Ionicon Analytik GmbH, Austria) at 10 % RH and
20∘. The LCU quantitatively evaporates aqueous standards into the gas
stream. A total of 16 standards were prepared gravimetrically or
volumetrically, depending on the compound, with aqueous volume mixing ratios
of compounds ranging between 2 and 6 ppmv. An amount of
10 µL min-1 flow of each of these solutions was then
evaporated into a humidified gas stream of synthetic air (9 slpm), resulting
in calibration standards containing 1–2 ppbv of each calibrated component.
In Table 1 we also present sensitivities calculated in duty-cycle-corrected
counts per second per part per billion by volume (dcps ppbv-1,
normalized to m/z=100). The duty cycle correction compensates for the
mass-dependent extraction efficiency into the time-of-flight mass
spectrometer: dcps(i) = cps(i) ⋅100/mi. The
extraction frequency of the ToF was set at 14 kHz. Limits of detection are
calculated for a 1 s integration time as 3 standard deviations of measured
background divided by derived sensitivity. Sensitivity to each compound was
measured at 10 %, 30 %, 50 %, and 70 % RH at 20∘.
There is no strong correlation between the sensitivity to the calibrated
compounds and their molecular weight (R2=0.35, Fig. S5).
Signals of NH4+–VOC clusters decrease as humidity of the
sampled air increases, as shown in Fig. 2. Increased reaction time (3 ms)
and elevated pressure (60 mbar) in the reaction chamber, compared to the
conventional PTR-MS instruments (0.1 ms and 2.3 mbar, respectively),
promote equilibrium between the forward and backward ligand-switching
Reaction (R3). Hence, under humid conditions,
excess water vapour favours formation of ammonium–water clusters, which in
turn reduces the abundance of ammonium–organic clusters
NH4+⚫(VOC) and hence the overall instrument sensitivity to
oxygenated VOCs (OVOCs). Humidity dependence of sensitivity does not show a
strong correlation to cluster stability, as quantified by
KE50cm (R2=0.29, Fig. S6). In addition, correlation
between humidity dependence of sensitivity and polarity of analyte molecules
is relatively weak (R2=0.31).
Humidity dependence curves for the normalized signals relative to
the dryer conditions.
Collision-induced dissociation techniques for constraining sensitivity of
the NH4+ CIMS
When the instrument operates in the NH4+ mode, organic molecules
are detected almost entirely as ammonium–organic clusters. However, kinetic
rate constants of ligand-switching Reaction (R3) from ammonium–water ions to
an organic molecule have only been measured for very few analyte molecules.
In addition, enhanced reaction time in the reaction chamber relative to
conventional PTR-MS instruments increases probability of reverse
ligand-switching reactions. Therefore, effective rate constants for both
forward and backward Reaction (R3) are required for analytical estimation of
the compound sensitivities. To avoid these complications, we constrain the
instrument sensitivities to the detected compounds through an empirically
based collision-induced dissociation (CID) technique similar to the one used
by Lopez-Hilfiker et al. (2016) for constraining sensitivity of iodide adduct
CIMS. This is accomplished by varying the voltage between the ionization
region and vacuum region of the mass spectrometer (Fig. 1), which increases
the electric field, while measuring intensities of detected peaks in the mass
spectrum.
The increase in the collisional kinetic energy of the ammonium–organic
clusters and air molecules leads to collision-induced dissociation of the
clusters. For each analyte ion we determine the voltage value (V50) at
which the peak intensity drops by 50 % relative to the intensity at the
operational voltage value and calculate the ion kinetic energy corresponding
to this voltage (KE50). Therefore, we can experimentally determine the
electric field strength necessary to break each ammonium–organic cluster,
which defines the stability of these clusters and hence the sensitivity of
our instrument to analyte molecules. The value of E/N (E is the electric
field strength and N is the sample gas number density) is a suitable metric
to characterize the motion of ions in the reaction chamber and kinematics of
a chemical ionization reaction (Blake et al., 2006). The electric field
strength E in a particular region of the reaction chamber depends on the
voltage V applied in that region and the effective distance between
electrodes d.
E=Vd
Drift velocity of ions in the reaction chamber vd is determined by the
electric field strength E and the ion mobility μ:
vd=μE.
The ion mobility depends on reaction pressure and temperature:
μ=μ01013mbarprTr273K,
where μ0 is the reduced mobility, which is estimated for each ion
using its mass (Ehn et al., 2011), pr is pressure in the reaction
chamber (mbar), and Tr is temperature in the reaction chamber
(K). Further, we calculate mean kinetic energy of drifting ions
KEion in the laboratory frame (Lindinger et al., 1998):
KEion=32kBT+Mbuffervd22+Mionvd22,
where kB is the Boltzmann constant, and Mbuffer and
Mion are the masses of the buffer molecule in the air and
reagent ion, respectively. Finally, the kinetic energy of analyte ions in the
centre of mass for ion–molecule collisions is given by (McFarland et al.,
1973)
KEcm=MbufferMbuffer+MionKEion-32kBT+32kBT.
For each ammonium–organic cluster we measure V50 and from this
calculate the corresponding kinetic energy at which half of the ions have
dissociated (KEcm50) using
Eqs. (1)–(5). We show a set of de-clustering scans
for eight organic molecules with different functional groups in Fig. 3.
Intensities of all clusters follow similar sigmoidal shapes when the voltage
is increased. Some clusters (i.e. small alcohols and heterocyclic compounds)
are less stable and are dissociated at lower voltages while other clusters
(i.e. large ketones) show higher stability. These scans can be obtained
within 4 min by steadily increasing the voltage between the ionization
region and vacuum region of the mass spectrometer (Fig. 1).
De-clustering scans of ammonium–organic clusters
NH4+⚫ (VOC) for
calibrated components and NH4+⚫ (H2O) reagent ions.
The relationship between calculated kinetic energy of the
ammonium–organic clusters KEcm50 and measured sensitivity for
calibrated compounds. Molecules characterized by KEcm50
smaller than 0.10 eV (region A) cannot be detected by NH4+
CIMS; for molecules characterized by KEcm50 between 0.10 and
0.19 eV (region B) a linear relationship between KEcm50 and
measured sensitivity is observed; molecules characterized by KEcm50 greater than 0.19 eV (region C) are detected at the “kinetic
sensitivity”.
Figure 4 shows the relationship between the calculated kinetic energy
KEcm50 and measured sensitivity for 16 calibrated
compounds at 10 % RH and 20∘. We observe a linear relationship
(R2=0.61) between calculated KEcm50 and
measured sensitivity for calibrated VOCs. This linear relationship is
observed for molecules with KEcm50 in the range
between 0.10 and 0.19 eV (region B in Fig. 4). Molecules characterized by
collisional kinetic energies KEcm50 smaller than
that of the ammonium–water cluster (0.09 eV, region A in Fig. 4) will show
no significant reaction rate since ligand-switching reactions between such
molecules and NH4+⚫(H2O) are endothermic. Conversely,
the ligand-switching reaction rate cannot exceed the kinetic limit for
ion–molecule collisions, and therefore there is also an upper limit of
observed sensitivities. We calculate this limit by using experimentally
determined pressure and reaction time in the reaction chamber and kinetic
limit of ion–molecule reaction rate. We estimate the reaction time in the
reaction chamber using the instrument sensitivity to specific compounds in
the H3O+ mode. For polar compounds with proton affinity much
higher than water (i.e. acetone), we can assume that reverse proton transfer
reactions do not occur. In this case, the instrument sensitivity to those
compounds is given by (Lindinger et al., 1998)
i(RH+)[R]=iprimary⋅k⋅treact⋅preact1013mbar,
where i(RH+)[R] is the component
sensitivity, iprimary is the primary ion current, k is the rate
constant for the proton-transfer reaction (e.g. k=3.6×10-9 cm3 s-1 for acetone; Cappellin et al., 2012), and
treact and preact are the reaction time and pressure
in the reaction chamber, respectively. By measuring the instrument
sensitivity to acetone in the H3O+ mode, we estimate
treact to be 3 ms. In our case, the instrument sensitivity
cannot exceed 70 000 dcps ppb-1, which is in agreement with the
highest sensitivity measured for calibrated compounds. Therefore, we assume
that all components with KEcm50 greater than
0.19 eV (region C in Fig. 4) will be detected at this “kinetic
sensitivity”. As shown in Fig. 2, the sensitivity of NH4+ CIMS
to many calibrated compounds is RH dependent; thus we observe that the
relationship between the calibrated kinetic energy KEcm50 and
the measured sensitivity also depends on the humidity of the sampled air
(Fig. S7). Therefore, the values of the collisional limit and other
calculated sensitivities reported herein are unique to the instrument setup
(i.e. pressures and voltages in the reaction chamber) and vary with the
humidity of the sampled air.
Application to secondary organic aerosols
To demonstrate the application of the procedure described above, we performed
a series of laboratory chamber experiments. A complex mixture of organic
compounds in both gas and particle phases was generated by the oxidation of
3-methylcatechol (C7H8O2), a second-generation oxidation
product of toluene and other anthropogenic aromatics, by hydroxyl (OH)
radicals in an environmental chamber. Details of the chamber operations are
given by Hunter et al. (2014), so we include only a brief description here.
Photochemical oxidation occurred in a 7.5 m3 temperature-controlled
Teflon chamber by OH radicals generated through the photolysis of nitrous
acid (HONO). In the experiment described here, 65 ppbv of 3-methylcatechol
(Sigma-Aldrich, 98 % purity) was injected in the chamber and further
oxidized in the presence of ammonium nitrate seed aerosol at 20∘ and
low humidity (3 % RH). Secondary organic aerosol particles produced in
this experiment were detected using an Aerodyne aerosol mass spectrometer
(AMS; DeCarlo et al., 2006) and the described CIMS instrument operating in
both the H3O+ and NH4+ modes, equipped with the
thermal desorption unit described above. High-resolution mass spectra of
3-methylcatechol oxidation products derived in the NH4+-mode in
the gas and particle phases are given in Fig. S8. In this experiment, we
identified 202 peaks in the NH4+ mode mass spectra and grouped
them based on the calculated KEcm50 as shown in
Fig. 5. Among those 202 OC⚫NH4+ peaks, 125 analyte
formulas were also detected as OC⚫H+ in the H3O+
mode. We plot the relationship between the detected signals in both modes of
our instrument in Fig. 6. We use the de-clustering technique described above
to calculate volume mixing ratios of organic molecules detected as
ammonium–organic clusters in the NH4+ mode. In the
H3O+ mode, we apply the calibrated acetone sensitivity to
calculate volume mixing ratios of OVOCs. Breitenlechner et al. (2017) showed
that due to the enhanced reaction time and the increased pressure in the
reaction chamber the equilibrium between the forward and reverse proton
reactions can be achieved. Hence, many compounds require careful calibration
over a broad humidity range. Since PTR3 has the highest detected sensitivity
to ketones, we use the acetone sensitivity to calculate the lower-limit
concentration of OVOCs. Volume mixing ratios of organic compounds detected by
both modes are in excellent agreement with a slope of 0.94 as shown in Fig. 6
(R2=0.78). In addition to 125 peaks measured by both modes, there are
peaks that are detected solely by either the H3O+ or
NH4+ mode. In Fig. 7, we plot 34 identified
CxHyOz⚫H+ peaks detected by the
H3O+ mode and 17 identified
CxHyOz⚫NH4+ peaks detected
by the NH4+ mode on the carbon number-oxidation state diagram.
Two modes cover different areas on this diagram: while the NH4+
CIMS is able to detect larger and more functionalized molecules, PTR-MS is
better at detection of smaller organic compounds (some of them can be formed
as a result of fragmentation during ionization). Hence, the two modes
complement each other and allow for the detection and quantification of a
broader range of oxidized organic molecules. Similar observations about the
selectivity of NH4+ CIMS and PTR-MS have been reported in the
previous studies. Aljawhary et al. (2013) showed that H3O+⚫(H2O)n primary ions are more selective to the detection of less
oxidized water-soluble organic compounds (WSOCs) extracted from alpha-pinene
SOA comparing to acetate CH3C(O)O- and iodide water clusters
I-⚫(H2O)n used as primary ions. Zhao et
al. (2017) demonstrated that multiple positive reagent ions
(NH4+, Li+, Na+, K+) have higher
selectivity for a wide range of highly oxygenated organics with higher
molecular weights formed from ozonolysis of alpha-pinene, while negative
reagent ions (I- and NO3-) are more selective towards
smaller species (e.g. CH2O2, CH2O3,
C2H2O3, and C2H4O3).
Application of the collision-induced dissociation techniques for
measurement of SOA composition produced during photooxidation of
3-methylcatechol in a laboratory experiment. A total of 202 peaks are detected in
NH4+ mode and binned based on their KEcm50. Molecules with
KEcm50 smaller than 0.10 eV cannot be detected by
NH4+ CIMS (region A); sensitivities of molecules characterized by
KEcm50 between 0.10 and 0.19 eV (region B) can be calculated using
the linear fit presented in Fig. 4; molecules with KEcm50 greater
than 0.19 eV are detected at the “kinetic sensitivity” (region C).
Comparison of volume mixing ratios of SOA components
detected by the CIMS instrument in both
H3O+ and NH4+ modes in the photooxidation
experiment of 3-methylcatechol.
Identified SOA components detected in both
H3O+ and NH4+ modes (125 peaks), uniquely in the
H3O+ mode (34 peaks), and uniquely in NH4+ mode (17 peaks)
plotted on the nC-OS‾C diagram.
The gold star corresponds to the precursor of the photooxidation experiment,
3-methylcatechol. The size of the dots is proportional to the logarithm of
the volume mixing ratio of each compound produced at the end of the
experiment.
Figure S9 shows a comparison between the total mass loading of all organic
components measured by the AMS with the sum of masses of all organic
compounds measured by our instrument in both H3O+ and
NH4+ modes. The sum of signals of all components detected in the
NH4+ mode account for 65 % of the total aerosol organic mass
measured by the AMS as shown in Fig. 8. This discrepancy can be explained by
a combination of the following factors: (1) uncertainties in the
sensitivities obtained using the presented technique and in the AMS
measurements; (2) thermal fragmentation of organic molecules in the thermal
desorption unit, which leads to lower observed masses in the mass spectrum;
(3) low NH4+ CIMS sensitivity to certain compounds of organic
aerosols if ligand-switching reactions between these molecules and
ammonium–water clusters are endothermic (e.g. small organic acids); and
(4) wall losses of less volatile organic molecules in the NH4+
CIMS inlet. Although the NH4+ CIMS does not detect all organic
compounds to explain the total organic mass measured by the AMS, it gives
valuable insight into the composition of SOA as shown in Fig. 8.
SOA produced during photooxidation of 3-methylcatechol in
a laboratory experiment. The total organic aerosol mass is measured by an AMS.
OVOCs detected by NH4 CIMS are binned in four groups.
Conclusions
In this study, a new CIMS instrument is described based on the recently
introduced PTR3. The instrument can be operated in both NH4+ and
H3O+ modes as NH4+ CIMS and PTR-MS, respectively,
while switching between the two modes can be done within 2 min. Compared to
the H3O+ mode, the NH4+ mode is able to detect more
functionalized and larger organic molecules. In the NH4+ mode,
the instrument has sensitivities in the range of
80–65 000 dcps ppbv-1 and detection limits in the range of
1.5–60 pptv for a 1 s integration time (2σ). We present a
procedure based on collision-induced dissociation that allows us to estimate
the stability of detected ammonium–organic clusters and therefore to
constrain the sensitivities of hundreds of compounds detected by the
NH4+ mode of the new instrument without their direct calibration
within several minutes.
Data availability
Data used
within this work are available upon request. Please email Alexander Zaytsev
(zaytsev@g.harvard.edu).
The supplement related to this article is available online at: https://doi.org/10.5194/amt-12-1861-2019-supplement.
Author contributions
MB and AZ designed and built the CIMS instrument. AZ and MB developed the
methodology with contributions from ARC and FNK. AZ, ARC, and MB performed the
laboratory experiments. AZ and MB provided data and analysis for the CIMS
instrument. CYL and JCR provided data and analysis for the AMS instrument. AZ
prepared the paper with contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This work was supported by the Harvard Global Institute and the NSF award
AGS-1638672. Martin Breitenlechner acknowledges support from the Austrian
science fund (FWF), grant J-3900. Abigail R. Koss acknowledges support from
the Dreyfus Postdoctoral Program.
Review statement
This paper was edited by Keding Lu and reviewed by two
anonymous referees.
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