Introduction
The Earth's atmosphere contains thousands of inorganic and organic species
that, through complex free radical and multiphase chemistry, play a vital
role in air quality and climate change (Finlayson-Pitts and Pitts, 2000;
Seinfeld and Pandis, 2006; Goldstein and Galbally, 2007). Characterizing the
identity and abundance of many of these species in the atmosphere is
essential for understanding their atmospheric processes and subsequent
environmental and climate impacts. As a result, there is a critical interest
in the development and application of the state-of-art analytical instruments
for the analysis of atmospheric composition (Noziere et al., 2015).
As a sensitive, selective, and soft-ionization measurement technique,
chemical ionization mass spectrometry (CIMS) has received significant use in
the real-time in situ measurement of atmospheric trace species (Huey et al.,
1995; Fortner et al., 2004; Hearn and Smith, 2004; Smith et al., 2004;
Crounse et al., 2006; Huey, 2007; Veres et al., 2008; Kercher et al., 2009;
Zhao et al., 2010). The recent coupling of chemical ionization to high-resolution time-of-flight mass spectrometers (HR-ToF-MS) enables the
simultaneous determination of the abundance and molecular composition of a
wide array of atmospheric inorganic and organic compounds with fast time
response and high sensitivity (Junninen et al., 2010; Bertram et al., 2011;
Yatavelli et al., 2012; Aljawhary et al., 2013; Lee et al., 2014;
Lopez-Hilfiker et al., 2014, 2016a; Brophy and Farmer, 2015, 2016; Yuan et
al., 2016). The use of HR-ToF-CIMS has allowed for groundbreaking progress in
atmospheric organic chemistry, such as the observation of highly oxygenated
molecules (HOMs) formed by monoterpene oxidation (Ehn et al., 2014; Jokinen
et al., 2015; Berndt et al., 2016; Lee et al., 2016). Very recently, a newly
developed proton-transfer reaction (PTR) time-of-flight instrument (PTR-3)
has enabled sensitive detection of a wide range of organic compounds
including HOMs (Breitenlechner et al., 2017).
In CIMS, the analyte molecule reacts with a specific reagent ion via one or
more mechanisms, including ligand switching reaction forming an ion–molecule
adduct (Huey et al., 1995; Kercher et al., 2009; Aljawhary et al., 2013; Lee
et al., 2014; Brophy and Farmer, 2015, 2016), proton addition (abstraction)
forming a protonated (de-protonated) ion (Nowak et al., 2002; Veres et al.,
2008; Yatavelli et al., 2012; Aljawhary et al., 2013; Brophy and Farmer,
2015, 2016; Yuan et al., 2016), or by direct charge transfer forming a
molecular ion (Huey et al., 1995; Kim et al., 2016). The reagent ions used
mainly include I-, NO3-, acetate, CF3O-, and
SF6- for negative ion CIMS, and H3O+, NO+, protonated
ethanol, and benzene cation for positive ion CIMS. Choosing an appropriate
reagent ion is essential for the comprehensive characterization of a specific
class of molecules while having selectivity to avoid unnecessary congestion
of the mass spectrum with unwanted components. For example, previous studies
using NO3- CIMS have reported a very low yield of HOMs from OH
oxidation of monoterpene (Jokinen et al., 2015). However, a recent study
using acetate CIMS found a significantly higher HOMs yields from the same
system (Berndt et al., 2016). The reason for this difference is presumably a
lower sensitivity of NO3- to HOMs formed in OH oxidation of
monoterpene than that of acetate (Berndt et al., 2016). On the other hand,
many atmospheric organic systems consist of a wide range of organic compounds
with different functionality and polarity. Therefore, multiple complementary
ionization schemes are needed to obtain a broad view of these systems
(Aljawhary et al., 2013; Praplan et al., 2015).
Some advantages of CIMS are that it is direct, online, reproducible, and
inherently quantitative in that the kinetic theory of gases allows a robust
upper limit ionization efficiency, and thus instrument response, to be
calculated knowing only the pressure and interaction time of reagent ions and
analyte molecules. However, the need for gas-phase reagent ions limits the
suite of usable reagent ions to those for which a safe and stable gas-phase
precursor exists and which produce the desired reagent ion cleanly at a high
yield when ionized. As such, certain reagent ions such as metal cations
(e.g., Li+, Na+, and K+) and NH4+, which are commonly
used for detection of atmospheric organic compounds in offline techniques
like electrospray ionization (ESI)-MS (Nizkorodov et al., 2011; Laskin et
al., 2012; Witkowski and Gierczak, 2013), have remained largely unavailable
for CIMS (Fujii et al., 2001). Compared to I-, NO3-, and
acetate, which are generally more sensitive to more oxygenated organic
compounds than to less oxygenated ones (Aljawhary et al., 2013; Lee et al.,
2014; Hyttinen et al., 2015; Iyer et al., 2016; Berndt et al., 2016), these
metal cations are expected to be able to sensitively detect both less
oxygenated (e.g., compounds containing only carbonyl groups) and highly
oxygenated multi-functional organic species (Gao et al., 2010; Nguyen et al.,
2010; Nizkorodov et al., 2011; Laskin et al., 2012; Witkowski and Gierczak,
2013; Zhao et al., 2015, 2016; Tu et al., 2016; Zhang et al., 2017), and to
form more strongly bound ion adducts. In addition, at present most CIMS
techniques use a radioactive ion source such as 210Po to produce the reagent
ions, although more recently some utilize X-ray radiation, electrical
discharge (Hirokawa et al., 2009; Yuan et al., 2016), or electron impact
(Inomata and Hirokawa, 2017). Safety regulations for the transport and use of
radioactive materials may limit the deployment of the instrument with a
radioactive ion source in the field, while other methods may be less intense
or lead to higher backgrounds.
Schematic of the electrospray chemical ionization (ESCI) source
module. Also shown are the orthogonal atmospheric pressure IMR and the
entrance capillary serving as the atmospheric pressure interface between the
IMR and the vacuum chamber of HR-ToF-MS. See text for detailed description of
the source.
We have developed a non-radioactive reagent ion source that deploys a
custom-built electrospray setup within an atmospheric pressure orthogonal
ion–molecule reaction (IMR) chamber. The design of the IMR region is similar
to that of the Cluster-CIMS developed by Eisele and coworkers (Zhao et al.,
2010). The electrospray chemical ionization (ESCI) source is coupled to a
HR-ToF-MS for characterization. We present the design and discuss the
parameters most important for optimal performance of the ESCI source. Then,
we assess its performance using the measurement of formic acid, IEPOX, nitric
acid, and organic mixtures formed by ozonolysis of α-pinene in a
continuous-flow reaction chamber. Our results demonstrate that the ESCI
source provides a potential alternative to radioactive and X-ray ion source
and opens a new avenue for the generation of reagent ions such as Li+,
Na+, K+, and so on that were previously unavailable for CIMS.
Experimental section
Instrument description
A schematic of the ESCI module is shown in Fig. 1. The electrospray setup
contains a 15 µm inner diameter (ID) fused silica spray needle
(PicoTip™) mounted within a cylindrical
evaporation chamber through which a flow of ultra-high purity (UHP) N2
(referred to as the ion source flow) is passed to aid in the evaporation of
the spray droplets and to transport ions into the IMR. Several spray needle
diameters were tried (from 8 to 30 µm), with the 15 µm
giving the best combination of longevity and ion intensity. The emitting end
of the spray needle is located 4 mm from the distal wall of the evaporation
chamber, which consists of a 13 mm ID stainless steel (SS) tube centered on
a circular SS aperture having a 4 mm diameter. The aperture forms the
entrance to the IMR, which is a portion of a 22 mm ID SS tube embedded in a
Teflon block. The ion source flow enters the IMR through the aperture
perpendicularly to the direction of a much larger sample flow, typically 10
to 20 standard liters per minute (standard L min-1) drawn through the
IMR by a dry scroll vacuum pump (IDP-3, Agilent Technologies). Preliminary
fluid dynamic simulations suggest that the mixed sample and ion source flow
in the IMR remains laminar when the ratio of the ion source flow to sample
flow is ≤ 0.2 and the overall Reynolds number for the sample flow is
low (sample flow < 20 standard L min-1).
Ions are driven across the perpendicular sample flow to a SS capillary tube
located on the opposite wall of the IMR by means of a 2–4 kV potential
between the evaporation region lens and the capillary tube. The SS capillary
projects 3.5 mm into the IMR and acts as the atmospheric pressure interface
between the IMR and the vacuum chamber of a commercial HR-ToF-MS (Tofwerk AG,
Thun, Switzerland), effectively dropping the atmospheric pressure to
1.5 Torr in the first quadrupole of the MS, and resulting in a sample flow
of ∼ 270 sccm (standard cubic centimeters per minute) into the MS. The HR-ToF-MS and its data acquisition
procedures have been described in detail previously (Junninen et al., 2010;
Bertram et al., 2011; Lee et al., 2014). The evaporation tube, lens and IMR
tube are electrically connected, while the mass spectrometer entrance
capillary is electrically isolated from the IMR by a ∼ 1 mm thick
jacket of Teflon.
During operation, a dilute salt solution (500 ppm) in HPLC-grade
methanol (MEOH) is biased at the reservoir to ±(2–5) kV depending on
the ion mode by connecting a stainless steel rod immersed into the solution
to a high voltage power supply. At a given reservoir solution voltage
(VR), the voltage applied to the evaporation tube and IMR
(VL) was carefully tuned to get the best ion signals
(Smax), as well as the corresponding VL, referred to
as VL (Smax). In the VR range of
2–5 kV, a larger VR (with a larger VL
(Smax)) gives a higher reagent ion signal. To obtain good ion
signals, for most of the measurements performed in this study, VR
values of 5 kV (corresponding VL (Smax)=2.8 kV)
and -5 kV (corresponding VL (Smax)=-3.9 kV)
were used in the positive ion and negative ion modes, respectively. The
reservoir is maintained at approximately 50 mbar above atmosphere using a
commercial pressure controller (FLUIGENT, model MFCS-EZ) with 0.05 mbar
precision. As a result, the salt solution is pushed through the fumed silica
capillary tube to the spray needle at a flow rate less than
100 nL min-1 by the pressure in the reservoir bottle.
Under laminar flow conditions, the reaction time between reagent ions and
sampled trace gases in the IMR is mainly determined by the electric
field-induced drift velocity of the reagent ions. For instance, for two of
the reagent ions used in this study, NO3- and Na+, the ion–molecule
reaction time (i.e., ion drift time) in the IMR is estimated to be 0.5–1 and
0.4–0.7 ms, respectively, with an ion mobility of
2.37 cm2 s-1 V-1 for NO3- (Ellis et al., 1978) and
3.4 cm2 s-1 V-1 for Na+ (Bohringer et al., 1987) under
typical operation conditions (2–4 kV across the IMR). However, when using
electrospray as a source and sampling ambient air of different humidity, the
reagent ions can be solvated by methanol or water clusters (Horning et al.,
1974; Garvey et al., 1994). As the ion mobility of solvated reagent ions is
likely smaller than that for un-solvated reagent ions, the ion–molecule
reaction time between solvated reagent ions and gas-phase analytes in the IMR
is expected to be longer than that estimated for the un-solvated ions. There
was no evidence of protonated methanol clustering observed when
electro-spraying a methanolic solution of the described salts. Although the
reagent ion is likely solvated by methanol initially, the sensitivity of the
ionization to various trace gases did not appear to be significantly affected
in the present study.
The ion source and sample flow rates can significantly affect the
performance of the ion source. The ion source flow can aid in the generation
and transport of the reagent ions into the IMR, but it may disrupt the
initially laminar sample flow, especially when the sample flow is small.
However, at large sample flows, the time for the ions to exit the IMR via
the sample flow may be comparable to the ion drift time across the IMR at a
constant potential. As a result, the sample flow may carry away the reagent
ions as well as ion–molecule clusters, lowering the apparent ionization
efficiency. Therefore, the ion source flow and sample flow need to be
carefully optimized.
For comparison purposes, our prototype source was designed such that it could
incorporate a commercial 10 mCi 210Po inline ion source (NRD LLC) as in
more typical low-pressure CIMS instruments used for atmospheric composition
studies (see introduction). With CH3I in UHP N2 as a reagent ion
source, this setup was able to produce 0.6–1.8×106 counts per second (cps) of
reagent ions at atmospheric pressure using an ion source flow rate of
1–2 standard L min-1 and a sample flow rate of
10 standard L min-1, with > 2 kV potential across the IMR.
Although the commercial 210Po sources are not optimized for ion transmission
at low flow rates and high pressures, this intensity is certainly suitable
for use in field or laboratory studies.
Laboratory characterization
Generation of reagent ions and calibration gas standards
In this study, three negative (i.e., I-, NO3-, and acetate) and
four positive reagent ions (i.e., Li+, Na+, K+, and
NH4+) were generated by electro-spraying their precursor salt
solutions prepared in HPLC-grade MEOH (Fisher Scientific). Sodium iodide
(≥ 99.5 %, EMD), sodium nitrate (≥ 99 %, Mallinckrodt),
potassium acetate (AR(ACS), Macron), ammonium acetate (99.2 %, Fisher
Chemical), and lithium chloride (≥ 99 %, Mallinckrodt) were used to
produce I- and Na+, NO3-, K+ and acetate, NH4+,
and Li+, respectively. All the salts were used as received.
Three calibration gases, i.e., nitric acid (HNO3), isotope-labeled
formic acid (H13COOH), and isoprene epoxydiols (trans-β-IEPOX) were used to calibrate the instrument. Gases of nitric acid and
formic acid were generated using a custom-built PTFE permeation tube
containing respective acid liquids, kept constantly at 40 ∘C. The
permeation rate was determined gravimetrically. IEPOX vapor was generated by
passing a flow of UHP N2 over ∼ 200 µL IEPOX solution in
ethyl acetate kept in a glass bulb at room temperature. The concentration of
IEPOX in the flow exiting the bulb was determined by an iodide-adduct
HR-ToF-CIMS employing a radioactive ion source, for which the sensitivity to
IEPOX was calibrated using the method as described previously (Lee et al.,
2014). These three gases are common in the atmosphere and span a range in
their properties important for CIMS such as acidity, polarity, and size.
Optimization of operation conditions, calibration, and background
determination
The influence of sample flow and ion source flow on the ion signals was
systematically evaluated using I- as the reagent ion. The room air was
directly sampled into the IMR at a flow rate ranging from
2 to 20 standard L min-1. At each sample flow rate, the ratio of ion
source flow / sample flow is varied from 0.02 to 0.2. The HNO3 and
H13COOH gases were added to the sample flow during the optimization.
Dependence of ion signals on the ion source flow and sample flow.
(a) Ion signals observed as a function of ion source flow during the
sampling of humid room air (15 mbar water vapor pressure) containing
H13COOH at a flow of 10 standard L min-1. (b) Ion
signals observed during the sampling of humid room air containing
H13COOH and HNO3 gas flow rates of 2–20 standard L min-1
(the ratio of ion source flow / sample flow is fixed at 1 : 10). The
signals for I (H13COOH)- in (a) and (b) are
magnified 100 times.
Calibrations with HNO3, H13COOH, and IEPOX were performed using
I- reagent ions under optimized sample flow and ion source flow
conditions. Atmospherically relevant concentrations of the calibration gases
were obtained by varying the dilution of the source gas in UHP N2 prior
to delivery in the sample flow. The observed ion signals as a function of gas
concentration allow the determination of the instrument sensitivity. In
addition, the sample flow was humidified to a wide relative humidity range (RH; 0–80 %, corresponding to water vapor pressure, PH2O, of
0–25 mbar) to explore the influence of water vapor on the instrument
sensitivity. The determined sensitivities and the dependence on
PH2O were compared to the measurements by a radioactive
iodide-adduct HR-ToF-CIMS. The background signals of the instrument were
determined routinely by directly sampling dry UHP N2.
Chamber experiments of α-pinene ozonolysis
The capability of the instrument for characterizing atmospherically relevant
complex organic systems was evaluated by measuring the oxidation products
from α-pinene ozonolysis using seven different reagent ions described
above. Experiments of α-pinene ozonolysis were carried out in a
0.75 m3 PTFE chamber operated in continuous-flow mode at the
University of Washington. The chamber was first flushed by
12 standard L min-1 of zero air generated by a Teledyne zero air
generator (model 701) for > 72 h. Ozone, generated by flowing ultra-zero
air (Praxair) at 5 sccm past a
mercury lamp, was delivered to the chamber during the zero air flushing.
α-Pinene was then added by flowing 100 sccm of UHP N2 through a
glass diffusion tube containing pure α-pinene and kept in a methanol
cold trap at -40 ∘C. The initial concentrations of O3 and
α-pinene added in the chamber were approximately 75 and 110 ppbv,
respectively. The oxidation products formed in the chamber were sampled at
10 standard L min-1 by the HR-ToF-ESCIMS after 48 h of chamber
equilibration.
Results and discussion
Ion source and sample flow optimization
Figure 2a shows an example using iodide reagent ions of ion signal dependence
on the ion source flow rate during sampling of humid air (PH2O=15 mbar) at 10 standard L min-1 containing an added
H13COOH standard. As expected, the reagent ion (I- and
I(H2O)-) signals increase with increasing ion source flow. The
increase in the signal for I(H13COOH)- is well correlated with that
of the reagent ions. The positive effect of the ion source flow is likely due
to more efficient evaporation and transport of reagent ions from the spray
evaporation region into the IMR region.
Figure 2b shows the ion signals for I-, I(H2O)-,
I(H13COOH)-, and I(HNO3)- observed during sampling of humid
air (PH2O=15 mbar) containing H13COOH and HNO3
standards at a sample flow rate ranging from 2 to 20 standard L min-1.
The corresponding ion source flow was controlled to always be 1/10 of the
sample flow. All ion signals increase initially with the increase in the
sample flow, reach maximum values at 12 standard L min-1, and then
decrease slightly with further increase in the sample flow. At low sample
flows, the time for the sample flow to pass through the IMR is long compared
to electric field-induced ion drift time across the IMR region, so the
influence of the sample flow upon ion transit across IMR should be small.
However, the corresponding increase in the ion source flow with the sample
flow can promote the generation and transmission of reagent ions into the
IMR, thus leading to the increase in ion signals. At large sample flows, the
influence of the sample flow on the ion transit across IMR becomes
significant and is no longer compensated by the enhancement in ion signals
due to the increased ion source flow, hence resulting in a decrease in ion
signals. Note that the same measurement was also performed at ion source
flow / sample flow ratios ranging from 0.02 to 0.2. The trend of the ion signal
versus the sample flow at each flow ratio is very similar to that shown in
Fig. 2b, though the absolute ion signal values are different.
For the characterizations and applications discussed below, the sample flow
and ion source flow are kept at 10 and 1 standard L min-1,
respectively, unless otherwise noted, as these are reasonable conditions for use in environmental
simulation chambers and in field measurements. We note that the sample flow
can be extended to up to 20 standard L min-1 without significant loss
of ion signal, and the optimal ion source flow of 2 standard L min-1
is essentially the same UHP N2 flow requirement for current 210Po-based
ion sources (Lee et al., 2014). Further improvements in the spray environment
and associated transfer optics will likely further minimize the ion source
flow.
Evidence of chemical ionization
Electrospray plumes not only ionize solvated analytes, but also are capable
of ionizing gas-phase species (Whitehouse et al., 1986; Chen et al., 1994),
the latter termed secondary electrospray ionization (SESI; Wu et al., 2000;
Tam and Hill, 2004). SESI-MS has been used for the real-time analysis of a
variety of gas-phase analytes, including pharmaceuticals (Wu et al., 2000;
Meier et al., 2012), explosives (Tam and Hill et al., 2004; Aernecke et al.,
2015), human metabolites (Martinez-Lozano et al., 2011; Garcia-Gomez et al.,
2015), electronic cigarette vapors (Garcia-Gomez et al., 2016),
volatile emissions from bacteria cultures (Zhu et al., 2010), food (Bean et
al., 2015; Farrell et al., 2017), and plants (Barrios-Collado et al., 2016).
In SESI, the electrospray plume and incoming sample flow intersect in the
ionization region, and analyte ionization proceeds likely via interactions
with both small charged droplets and electrospray-produced gas-phase reagent
ions (Wu et al., 2000). In the present study, by coupling the electrospray
source to an orthogonal continuous-flow atmospheric pressure IMR via an
evaporation region, we separate the electrospray plume from the incoming
samples to avoid SESI, and instead allow for gas-phase chemical ionization.
Signal ratio of NO3- / I(HNO3)- as a function of
HNO3 concentration under dry and humid conditions observed using iodide as
the reagent ion.
Under typical operating conditions, the sample flow is likely to transport
any un-evaporated droplets away from the effective ionization region in the
IMR, thus largely isolating the electrospray plume from the incoming samples,
making the ESCI source a chemical ionization source rather than secondary or
extractive electrospray ionization (SESI or EESI) source. The evidence of the
ESCI source being a chemical ionization source and not SESI or EESI is
provided by monitoring the signal ratio of
NO3- / I(HNO3)- when sampling gas-phase HNO3 in the
iodide mode. If the direct interaction between electrospray plume and
incoming sample flow is important, HNO3 dissolved in charged droplets
can dissociate forming H+ and NO3-, leading to the generation of
NO3- ions in the negative ion mode. Therefore, a high signal ratio
of NO3- / I(HNO3)- is expected. Figure 3 shows the signal
ratio of NO3- / I(HNO3)- as a function of gas-phase
HNO3 concentration under dry and humid conditions observed in the iodide
mode. The signal ratios of NO3- / I(HNO3)- are
significantly smaller than 0.01 at various HNO3 concentrations,
suggesting that the direct interaction of electrospray plume with incoming samples
is not important in the ESCI source.
Time series of I(HNO3)- observed when
sampling (a, b) 5 standard L min-1 or
(c, d) 10 standard L min-1 humid room air containing some
ambient HNO3 vapor. The ion source flow was 1 standard L min-1.
The dashed line indicates the time at which the HNO3 standard gas was
added or shut off.
Time response of the atmospheric pressure IMR
The time response of atmospheric pressure orthogonal IMR was determined using
nitric acid standard in the iodide mode. HNO3 was delivered from a
permeation tube using a 100 sccm continuous UHP N2 flow through a 3 mm
OD Teflon tube to the inlet of the orthogonal IMR. Figure 4 shows the changes
in ion signal for I(HNO3)- upon placing the HNO3 delivery line
at the opening of a 10 cm length of 2.5 cm OD Teflon tubing serving as the
inlet to the IMR or removing the delivery line from the inlet. Tests were
conducted at an ion source flow of 1 standard L min-1 and sample flow
of 5 or 10 standard L min-1. The increase and decay of
I(HNO3)- signal relative to that from HNO3 in the laboratory air
give an e-folding time of about 1 s for nitric acid under two different flow
conditions. This time response value is comparable to or better than that for
the low-pressure IMR (one to a few seconds).
Sensitivity to selected trace gases
To assess the performance of the HR-ToF-ESCIMS, we measured the sensitivity to
HNO3, H13COOH, and IEPOX using I- as the reagent ion. The
iodide-based CIMS has been widely used to detect atmospheric inorganic and
organic compounds in previous studies (Huey et al., 1995; Kercher et al.,
2009; Lee et al., 2014, 2016; Brophy and Farmer, 2015; Lopez-Hilfiker et al.,
2016b), though almost exclusively at low pressure (20–80 mbar) as opposed
to the atmospheric pressure (1013 mbar) implementation used here. The
sensitivity of iodide-based CIMS to a given compound mainly depends on the
polarity and hydrogen binding energy of a compound to the I- ion (Lee et
al., 2014; Iyer et al., 2016). In the atmospheric pressure ESCIMS, the ion
molecule reaction time (a few milliseconds) is set by the electric field, and is up to
a factor of 30 or more less than those (30–120 ms) in low-pressure CIMS
instruments (Bertram et al., 2011; Lee et al., 2014, Lopez-Hilfiker et al.,
2016a). The shorter reaction time should linearly lower sensitivities.
However, the ion–molecule collision frequency is more than a factor of 10
higher in the atmospheric pressure ESCIMS for the same ambient concentrations
of analytes. Thus, we would expect the ESCIMS sensitivities to be only
slightly lower than those found in the low-pressure CIMS. It is possible that
adduct formation is further stabilized by third-body effects and that the
ESCIMS could in fact have higher sensitivities for some compounds forming
clusters with high excess energy.
Figure 5 shows the signals of I(HNO3)-, I(H13COOH)-, and
I(IEPOX)- per million reagent ion count rate at different
atmospherically relevant concentrations of the standards under dry and humid
conditions. The signal response is linear within the investigated
concentration range for all three trace gases, with the slope of the linear
fit to the ion signals corresponding to the sensitivity per million reagent
ion count rate. The HR-ToF-ESCIMS exhibits a sensitivity of 11, 2.4, and
10 cps pptv-1 to HNO3, HCOOH, and IEPOX, respectively, under dry
conditions and 9.1, 0.5, and 1.7 cps pptv-1, respectively, under humid
conditions (PH2O=14 or 15 mbar). These sensitivities, and
those that follow are given in per million cps of reagent ion. Lee et al. (2014)
explored the sensitivity of a low-pressure iodide-adduct HR-ToF-CIMS equipped
with a radioactive ion source to a number of atmospheric inorganic and
organic compounds. They reported sensitivities to HNO3, HCOOH, and IEPOX
of 4.0, 2.9, and 0.39 cps pptv-1, respectively, at 0.2 mbar water
vapor pressure in IMR. Using the same instrument as used by Lee et
al. (2014), we have more recently obtained higher values of sensitivities to
HCOOH (7 cps pptv-1) and IEPOX (10 cps pptv-1) in the
laboratory. Thus, the atmospheric-pressure ESCIMS and low-pressure CIMS
approaches are fairly similar in response to the same compounds. The
sensitivity difference in these calibrations is likely attributed to the
differences in instrument parameters, including the configurations and
pressures of the ion source and IMR, and the ion optic settings within the
vacuum chamber that strongly affect ion transmission to the mass
spectrometer.
The sensitivity to (a) nitric acid, (b) formic
acid, and (c) IEPOX under dry and humid (14 or 15 mbar water vapor
pressure) conditions. Signals are normalized by the ratio of observed total
reagent ion count rates to a million ion count rate. The normalized signals
were observed to be a linear function of the delivered concentration. The
slope derived from a linear fit corresponds to the sensitivity per million
reagent ion count rates.
The presence of water vapor can affect sensitivities, either by competing for
I- ions, thus lowering the sensitivity, or by accommodating excess
energy from the collision to stabilize the iodide–molecule clusters, thereby
increasing the sensitivity (Lee et al., 2014; Iyer et al., 2016). Water vapor
may also affect sensitivities by changing the size distribution of reagent
ion clusters and thus their residence time (ion–molecule reaction time) in
the IMR. Moreover, water vapor can affect the transmission of soluble gases
through sample tubing. It is difficult to evaluate the effect of changing
cluster size distribution as the information regarding the distribution and
ion mobility of the reagent ion clusters is currently unavailable. In the
current configuration of the ESCIMS, it is also difficult to isolate the
sample transfer effect experimentally, as done previously in low-pressure IMR
regions by using separate delivery lines for calibrants and water vapor (Lee
et al., 2014). Thus, our results shown here reflect a combination of
ionization efficiency, cluster distribution, and sample transfer aspects, and
the latter could be significant given the ∼ 50 cm length of tubing
used in these tests.
Normalized signal of I(HNO3)-, I(H13COOH)-, and
I(IEPOX)- as a function of water vapor pressure (PH2O) in
the IMR. The signal of iodide–analyte clusters is first normalized by the
total reagent ion (I- and I(H2O)-) signals. The resulting
normalized signal at each PH2O was then normalized again to the
respective value under dry conditions (PH2O=0, dry UHP
N2).
Figure 6 shows the dependence of the instrument sensitivities to HNO3,
H13COOH, and IEPOX on the PH2O of the sample flow. The
sensitivities to HNO3, H13COOH, and IEPOX increase initially with
the addition of water vapor at lower PH2O, reach the maximum
values at 4.1, 2.2, and 2.2 mbar, respectively, and then decrease with the
further increase in PH2O. Compared to HNO3 and
H13COOH, the positive water vapor effect on the sensitivity at low
PH2O for IEPOX is significantly smaller. Lee et al. (2014)
investigated the effects of water vapor on the sensitivity of a low-pressure
iodide-adduct HR-ToF-CIMS in the PH2O (water vapor pressure in
IMR) range of 0–0.8 mbar, and found a positive water vapor dependence for
the sensitivity to HNO3 and an approximately inverse U-shaped dependence
for the sensitivity to HCOOH. In general, the trends for the sensitivities to
HNO3 and HCOOH versus PH2O observed by Lee et
al. (2014) are consistent with those at
PH2O<5 mbar observed in the present study. In addition,
recent measurements using the same low-pressure iodide-adduct HR-ToF-CIMS in
our lab show that the addition of water vapor with PH2O of
0.26 Torr has no significant impacts on the sensitivity to IEPOX, consistent
with the relatively weak humidity dependence of the sensitivity to IEPOX at
low PH2O observed in the present study. The sharp decrease in
the sensitivities at higher PH2O as seen in Fig. 6 is therefore
likely a result of the competitive consumption of I- ions by water
vapor, which dominates over the kinetic stabilization effect of water for the
ion–molecule clusters, as well as a larger wall partitioning in the sampling
tube under these conditions.
High-resolution mass spectra collected when sampling
(a) UHP N2 and (b) UHP N2 containing HNO3,
H13COOH, and IEPOX gases.
Instrument backgrounds and detection limits
The background signals for the instrument arise mainly from the impurities in
the electrospray solvent and the salts used for the generation of reagent
ions, as well as the desorption of gas species adsorbed onto the wall of the
sampling tube and IMR. The instrument backgrounds were routinely measured by
sampling UHP N2. Figure 7 shows a typical high-resolution mass spectrum
in the I- mode recorded when sampling UHP N2. The spectrum recorded
during the addition of HNO3, H13COOH, and IEPOX to the UHP N2
flow is also displayed for comparison. The typical backgrounds for HNO3,
H13COOH, and IEPOX were measured to be 800, 240, and 50 cps,
respectively. It is noted that the instrument backgrounds can be reduced by
using higher purity electrospray solvents and reagent ion precursor salts, or
by using a larger sample flow that can dilute the background concentration of
the species desorbed from the wall. Moreover, many experiments adding large
concentrations of these standards to the sampling tube had been performed
over months, and thus it is likely that these backgrounds are anomalously
high.
Assuming the uncertainty in the signal and background follows Poisson
counting statistics, the signal-to-noise (S/N) ratio can be determined from
Eq. (1) (Bertram et al., 2011):
SN=Cf[X]tCfXt+2Bt,
where Cf is the instrument sensitivity; [X] is the concentration for a
trace gas; B is the background count rate; t is the integration time. We
define the detection limit of the HR-ToF-ESCIMS for a trace gas as the
concentration that gives rise to an S/N ratio of 3. Using the measured
instrument sensitivities and backgrounds, we calculate a detection limit of
4.9, 12.5, and 1.4 pptv for HNO3, H13COOH, and IEPOX,
respectively, for 5 s averaging, in the I- mode. These limits of
detection are comparable to those for a low-pressure iodide-adduct HR-ToF-CIMS
in our lab (Lee et al., 2014).
High-resolution mass spectra obtained during ozonolysis of α-pinene in a steady-state chamber in (a) I- and
(b) NO3- modes. For NO3- mode, the chemical formulae
of organic ion clusters are shown without the corresponding NO3-
adduct for clarity as, unlike I- mode, organic ions without a
NO3- adduct were negligible components of the spectrum.
High-resolution mass spectra of α-pinene ozonolysis products
in (a, c, e, g) monomer and (b, d, f, h) dimer regions
observed in (a, b) Li+ mode, (c, d) Na+ mode,
(e, f) K+ mode, and (g, h) NH4+ mode. The
chemical formulae of the detected organics are given for major peaks observed
in the mass spectra. To allow direct comparison, the reagent ion adduct has
been removed from the detected cluster in each spectrum.
Comparisons of mass defect plots derived in (a) I- and
Na+ modes and (b) NO3- and Na+ modes during
ozonolysis of α-pinene in a steady-state chamber. To compare the mass
defect plot obtained in two different ion modes, the reagent ions in observed
clusters are excluded for the mass defect calculation, and the signals are
normalized to the corresponding pinic acid intensity in each mode (see text
for details). The purple circles do not necessarily mean such ions were
undetected in the negative mode as they may have very small signal
(< 5 cps) and be excluded from the high-resolution fitting.
Application to chamber studies of α-pinene ozonolysis
Raw mass spectra
Gas mixtures formed by ozonolysis of α-pinene in a steady-state
chamber were used to assess the capabilities of this technique for
characterizing complex organic systems of atmospheric relevance. Three
negative ions (i.e., I-, NO3-, acetate) and four positive ions
(i.e., Li+, Na+, K+, NH4+) were used as reagent ions
for measurements. High-resolution peak fitting was performed and reasonable
molecular formulae were assigned for detected ions that have intensity higher
than 5 cps in all seven ion modes. Many ions are present at < 5 cps,
which were excluded from the high-resolution fittings to ease the number of
identifications required for comparison of several different reagent ion
spectra. Although these lower signal ions might be of importance to various
mechanisms of particle growth or organic radical chemistry, identifying their
compositions was deemed beyond the scope of this paper. Overall, the results
show that the ions observed in NO3- and four positive ion modes are
in the form of ion–molecule clusters, whereas those observed in I- and
acetate modes are either ion–molecule clusters or molecular ions. The iodide
clusters can be easily distinguished from iodide-free molecular ions due to
the large negative mass defects of iodide (Lee et al., 2014), although this
advantage weakens at sufficiently high masses (>∼ 500 m/Q for a
resolution of 5000). In contrast, broadly distinguishing between
acetate-neutral clusters and de-protonated organic ions in the acetate mode
remains a challenge when using non-isotopically labeled acetate and operating
the instrument in a cluster-transmitting mode with no comprehensive voltage
scanning experiments (Lopez-Hilfiker et al., 2015; Brophy and Farmer, 2016),
as is the case in the present study. As a result, the high-resolution ions
observed in the acetate mode cannot be confidently assigned to
α-pinene ozonolysis products and are excluded from further
discussions.
Examples of high-resolution mass spectra of α-pinene ozonolysis
products derived in I- and NO3- modes are given in Fig. 8 and
the spectra obtained in four positive ion modes are given in Fig. 9. The
iodide-mode mass spectrum of the ozonolysis products obtained here is overall
similar to that obtained using the low-pressure iodide-adduct HR-ToF-CIMS (see
Fig. S1 in the Supplement). It can be seen that peaks assigned to monomeric
products (≤ C10) are apparent in all ion modes, while peaks
associated with dimeric species are evident only in the positive ion mode
(discussed further below). Peak distributions in both monomer and dimer
regions is very similar for Li+, Na+, K+, and NH4+,
suggesting these positive ions likely have a similar selectivity to
α-pinene ozonolysis products. It is interesting to note that in
negative ion modes, ion clusters of precursor salt molecules (e.g.,
I(NaI)- and NO3(NaNO3)n-) were observed with high
intensities. These ions can be used as excellent mass calibration species.
Mass defect plots
To better compare the sensitivity and selectivity between this subset of
negative and positive reagent ions, the mass defects of identified products
are plotted against their exact mass for I-, NO3-, and Na+
modes. Figure 10 shows the comparisons of mass defect plots between I-
(or NO3-) mode and Na+ mode. In the mass defect plots, the
green, yellow, and purple open circles represent the products observed only
in one ion mode and their size is proportional to the signal intensity of
observed clusters. The blue open markers in the plots represent the products
identified in both ion modes of comparison and their size is proportional to
the square root of the pinic acid-normalized signal intensity ratio (R)
between the two ion modes:
R=SA-,i/SA-,PASNa+,i/SNa+,PA,
where, SA-,i and SA-,PA are the signal intensity
of clusters for product i and pinic acid in I- (or NO3-) mode,
respectively; SNa+,i and SNa+,PA are
the signal intensity of product i and pinic acid in Na+ mode,
respectively. As pinic acid (C9H14O4) is among the most
abundant products observed in I-, NO3-, and Na+ modes (see
Figs. 8 and 9), the value of R (i.e., the size of the markers relative to
that for pinic acid; red solid circles) can be an indicator of the relative
sensitivity of I- (or NO3-) and Na+ to the oxidation
products.
In the monomer region of the mass defect plots, the less oxidized products
observed in both modes of comparison generally have a value of R≤1 (the
blue markers have sizes smaller than or close to that of pinic acid). Thus,
Na+ is generally more sensitive to less oxidized species than I-
and NO3-, and most of products observed only in the Na+ mode
show very low oxygen contents (nO≤3). As many of these species
have signal intensities larger than 1000 cps, their absence in I- and
NO3- modes suggests that I- and NO3- are extremely
insensitive to these least oxidized species, in agreement with the
observations in previous studies (Lee et al., 2014; Hyttinen et al., 2015;
Iyer et al., 2016). In contrast, the more oxidized products observed in both
modes of comparison show a wide range of R values (e.g., R≤1 or R≥1, corresponding to the blue markers having sizes smaller or larger than
that of pinic acid). This indicates that I-, NO3-, and Na+
are all sensitive to more oxidized species but have different sensitivities
to a specific species. In fact, some highly oxidized products having high
oxygen contents (nO≥5) are observed only in one of these three
ion modes. Note that most of these products have signal intensities lower
than 50 cps, suggesting that they likely have very low concentrations, which
are below the detection limit in the other two modes.
The selectivity of I- and NO3- toward more oxidized species as
suggested here is consistent with the observations in previous studies (Lee
et al., 2014; Berndt et al., 2016), which showed that these two reagent ions
can have distinct sensitivities to the oxidized species having similar oxygen
contents, depending on the identities and locations of the functional groups.
It is clear in Fig. 10 that some very small species (e.g., CH2O2,
CH2O3, C2H2O3, and C2H4O3) have a
value of R significantly larger than 1, indicating that I- and
NO3- are markedly more sensitive to these small species than is
Na+.
Comparisons of the mass defect plots in the dimers region show a large
difference in the detection of the gas-phase dimers between I- (or
NO3-) and Na+ modes. These dimers have compositions ranging,
for example, from C15H26O3 to C20H32O7. We
note that many of these dimers have been recently detected in the gas phase
using a low-pressure iodide-adduct HR-ToF-CIMS in a boreal forest environment
(Mohr et al., 2017). Thus, while the lower detection efficiency of dimers in
this work using I- or NO3- may be from differences in reagent
ion sensitivities, we suspect that differences in ion optic settings between
negative and positive ion modes that affect ion transmission efficiencies at
large mass-to-charge ratios is a more likely explanation. These settings
were not optimized in this work, and improvements to high mass transmission
in negative ion mode are ongoing. Therefore, we refrain from concluding
about the relative detection efficiency of dimers in negative ion mode using
the atmospheric pressure ESCI.
Figure 11 shows box plots for the O : C ratio of monomeric products from
α-pinene ozonolysis detected in I-, NO3-, and Na+
modes. The O : C values for all the percentiles observed in I- and
NO3- modes are overall similar, whereas the corresponding values
observed in Na+ mode are obviously smaller. In addition, more than half
of products observed in the three modes have a O : C ratio larger than 0.8.
These results are consistent with the observations from Fig. 10, where
I-, NO3-, and Na+ are all sensitive to highly oxygenated
organics, but the former two reagent ions are insensitive to less oxygenated
organics as compared to Na+.
In summary, these comparisons suggest that there is not a reagent ion that
captures all components of α-pinene ozonolysis with equally high
sensitivity. Therefore, to gain a comprehensive view of a complex organic
system, a combination of reagent ions with different selectivity is needed.
Declustering scans
Ion–molecule clusters, depending on their binding energies, may break apart
due to collision-induced dissociation (i.e., declustering) during
transmission through the ion optics within the vacuum chamber. In general,
clusters with stronger binding energies can more easily survive declustering
in the vacuum chamber, and thus the instrument likely has higher sensitivities to
the corresponding analytes, and the observed sensitivities should be closer
to those calculated by ion–molecule collision rates. Declustering scanning,
which is performed by systematically increasing the voltage difference
(ΔV) between first and the second quadrupole sections of the MS,
provides insights into the binding energies of clusters (Lopez-Hilfiker et
al., 2016a). Figure 12 shows the declustering scans of clusters containing
C10H16O2-8 and C9H14O3-8 products in I-
and NO3- modes. It is clear that, with the increase in electrical
field strength, the cluster signals for products having higher oxygen
contents generally decay more slowly than those having lower oxygen contents.
This is consistent with the fact that I- and NO3- ions generally
bind more strongly to compounds containing more hydroxy or hydroperoxy
moieties (Lee et al., 2014; Hyttinen et al., 2015; Iyer et al., 2016). We
note that the trends of decay for C10H16O2-8 iodide clusters
are in excellent agreement with previous measurements using a low-pressure
iodide-adduct HR-ToF-CIMS (Lopez-Hilfiker et al., 2016a).
Box plots showing the 5th, 25th, 50th, 75th, and 95th percentiles for
the O : C ratio of monomeric products from α-pinene ozonolysis
detected in different ion modes.
Declustering scans of products C10H16O2-8 and
C9H14O3-8 formed by the ozonolysis of α-pinene in
(a) I- and (b) NO3- modes. ΔV denotes
the voltage differences between the end of first and the entrance to the
second quadrupole sections of the mass spectrometer. Signals at each
ΔV are normalized to that obtained at the weakest declustering
strength (i.e., ΔV=2 V).
Declustering scans of the 15 most abundant dimers formed by the
ozonolysis of α-pinene in (a) Li+ mode,
(b) Na+ mode, (c) K+ mode, and
(d) NH4+ mode. ΔV denotes the voltage differences
between the first and second quadrupole sections of the mass spectrometer.
Signals at each ΔV are normalized to that obtained at the weakest
declustering strength (i.e., ΔV=2 V).
Declustering scans in Li+, Na+, K+, and NH4+ modes
show that the cluster signals for the most abundant monomeric products such
as C10H16O2-5 and C9H14O2-5 increase initially
with increasing ΔV and then decrease with further increase in
ΔV. The reason for the initial increase in cluster signals is
unclear, but might involve secondary ion chemistry and/or slight changes in
ion transmission efficiency of the instrument. Here, we use the declustering
scans of dimers instead of C9 and C10 monomers to compare the
binding energies of four positive reagent ions.
As can be seen in Fig. 13, the decay rate of the cluster signals in four
positive ion modes follows the order
NH4+ > K+ > Na+ > Li+. This indicates an
order of Li+ > Na+ > K+ > NH4+ for the
binding energies of the clusters, consistent with expectations from charge
density considerations. In each ion mode, the cluster signals for smaller
dimers generally decay more slowly than those for larger dimers, suggesting
these positive ions can more strongly bind to the smaller dimers, likely due
to the higher polarity or the smaller steric effect for smaller dimers. It is
worth noting that in the Li+ mode, these dimer ions have
ΔV50 values of ∼ 15 V, suggesting they are very strongly
bound, with a binding enthalpy of ∼ 70 kcal mol-1 according to
the relationship between ΔV50 and cluster binding energies
determined by Lopez-Hilfiker et al. (2016a).
Conclusion
We report an electrospray chemical ionization (ESCI) source coupled to a
HR-ToF-MS for the real-time online measurement of atmospheric organic and
inorganic species in the gas phase. The ESCI source is unique in that it
does not rely on radioactive materials or X-ray radiation that are subject
to safety regulations, and allows for the production of reagent ions (e.g.,
alkaline cations) that are not available in current CIMS techniques.
Calibration experiments using nitric acid, formic acid, and IEPOX gas
standards show that the HR-ToF-ESCIMS using iodide reagent ions has
sensitivities and limits of detection comparable to those obtained for a
low-pressure iodide-adduct HR-ToF-CIMS using a radioactive ion source. The
detection of oxidized organic compounds formed from α-pinene
ozonolysis in a chamber using seven different reagent ions (e.g., I-,
NO3-, acetate, Li+, Na+, K+, and NH4+)
shows different selectivities for these reagent ions and expected ion-adduct
binding energy trends. The data demonstrate the capability of this technique
for comprehensively characterizing complex organic systems using a
combination of reagent ions.
The ESCI source presented here is in its early stages of development.
Continued characterization of the sensitivity and selectivity of different
reagent ions, especially their dependence on humidity, is needed. Further
optimizations of the ion source are also required to improve its performance,
especially long-term stability,
which is particularly important for field
applications. Versions of our prototype source allowed 10 to 24 h of
continuous operation before ion signal degraded, which is certainly suitable
for many laboratory experiment durations. A short immersion of the spray tip
into HPLC-grade MEOH was enough to return to the same ion signal for another
10 to 24 h, suggesting the reason was simply salt build-up on the spray
needle tip altering the spray characteristics. Thus, it is likely that more
dilute spray solutions, shorter spray needle tips, a conventional coaxial
sheath gas flow around the needle tip, and off-axis spray geometry would
greatly improve source stability. Moreover, shifting the spray source further
upstream of the entrance capillary would increase ion–molecule reaction times
and thus sensitivity, as in Zhao et al. (2010). Finally, applying a dry UHP
N2 counter flow at the mass spectrometer entrance capillary would
prevent ambient particles and possible charged spray droplets that are not
completely evaporated from entering and blocking the capillary tube. This
counter flow could also prevent free water molecules from entering the vacuum
chamber and promote the dissociation of reagent ion–water clusters, which may
lead to an increase in the instrument sensitivity, especially in positive ion
mode.