Extractive electrospray ionization (EESI) has been a well-known
technique for high-throughput online molecular characterization of chemical
reaction products and intermediates, detection of native biomolecules, in
vivo metabolomics, and environmental monitoring with negligible thermal and
ionization-induced fragmentation for over two decades. However, the EESI
extraction mechanism remains uncertain. Prior studies disagree on whether
particles between 20 and 400 nm diameter are fully extracted or if the
extraction is limited to the surface layer. Here, we examined the analyte
extraction mechanism by assessing the influence of particle size and coating
thickness on the detection of the molecules therein. We find that particles
are extracted fully: organics-coated NH
Atmospheric aerosols are suspended particles in the air ranging from a few
nanometers (nm) to several micrometers (
Online molecular characterization of atmospheric aerosols is required to
resolve the spatiotemporal variability of PM molecular composition and to
identify PM sources. Progress has been made with the development of chemical
ionization interfaces such as the Filter Inlet for Gases and AEROsols
(FIGAERO) (Lopez-Hilfiker et
al., 2014), Thermal Desorption Differential Mobility Analyzer (TD-DMA)
(Holzinger et al.,
2010; Wagner et al., 2018), and Chemical Analysis of Aerosol Online (CHARON)
(Eichler et al., 2015) coupled to a mass
spectrometer. However, these techniques suffer from thermal decomposition of
the analyte prior to ionization and/or ionization-induced fragmentation,
impeding molecular speciation
(Müller
et al., 2017; Stark et al., 2017). To complement these instruments, an
extractive electrospray (ES) ionization time-of-flight mass spectrometer
(EESI-TOF) was developed to enable molecular characterization of organic
aerosol at 1 Hz time resolution with ng m
Several studies on topics such as the extraction of macromolecules from colloidal solution (Chen et al., 2006), electron-transfer-catalyzed dimerization (Marquez et al., 2008), and gas plume mixing in the charged droplets (Cheng et al., 2008) reported that the ionization of EESI mainly happens in the liquid phase via interaction between charged ES droplets and neutral analyte molecules. For clarity, we refer to our analytes (here introduced in aerosol form) as “particles” prior to their interaction with ES droplets and as “analyte-laden droplets” afterwards. If this liquid-phase extraction of EESI occurs via total coalescence between particles and ES droplets, the measured EESI signal should be proportional to the total analyte mass concentration, i.e., full extraction of particles by ES droplets as demonstrated by several studies (Law et al., 2010; Lopez-Hilfiker et al., 2019). In contrast, prior studies suggested that the particles may be only partially probed, limiting the full quantification of the extracted analyte with extractive electrospray ionization (Wang et al., 2012, Kumbhani et al., 2018). Kumbhani et al. (2018) suggested that only the surface of particles with a diameter of approximately 100 nm was extracted by comparing infusion ESI-MS with EESI-MS using coated chemical standards (Kumbhani et al., 2018). Using other techniques such as phase Doppler anemometer, Wang et al. (2012) suggested that the extraction happens via fragmentation of the analyte droplets and ES droplets as the result of droplet–droplet collisions (Wang et al., 2012). Finally, other studies proposed that the EESI extraction efficiency could depend on the analyte volatility and size (Meier et al., 2011; Pagonis et al., 2021). Since all these studies only probed simple systems, i.e., individual chemical standards using one kind of experimental setup and EESI ionization source, these discrepancies could be inherently attributed to their differences of ES ionization geometries, experimental conditions, irreproducible ES Taylor cone conditions and perhaps the choices of chemicals.
Without reconciling the discrepancies of these reported EESI mechanisms,
EESI quantification must be regarded as highly uncertain when the technique
is used to probe varying size distributions of particles that exist in
different mixing states and are comprised of different molecular polarity,
volatility and sizes. Here, we took advantage of recent advancements in
particle generation and chemical analysis to evaluate the extraction
mechanism of EESI using three different methods for particle generation and
several online mass spectrometers for aerosol chemical speciation. First, we
characterized the EESI extraction efficiency with particles containing
atmospherically relevant standard compounds and mixtures, size-selected in
the range of 30–500 nm using an aerosol aerodynamic classifier. We
elucidated the influence of ES operating parameters and the residence time
of ES droplets and particles within the ionization source using two
different EESI sources. Second, we assessed whether the EESI extraction
efficiency depends on the analyte chemical composition, by comparing
the EESI-TOF with a chemical ionization (CI) TOF-MS equipped with a Filter Inlet
for Gases and AEROsols (FIGAERO) sampling manifold (FIGAERO-CI-TOF-MS) during
measurements of
Acetonitrile (Sigma-Aldrich, UV grade), sodium iodide (Sigma-Aldrich,
99.7 % purity) and Milli-Q water (18 M
Two designs of the EESI sources with a factor of 2 difference in their
residence time in the electrospray ionization region were used in this work,
coupled to a high-resolution TOF mass spectrometer (HTOF, Tofwerk AG,
Switzerland). EESI source A (Lopez-Hilfiker et al., 2019) and B were
developed initially for Tofwerk TOF and Thermo Scientific Orbitrap mass
analyzers (Fig. S1), respectively, though EESI source B is compatible with
both mass analyzers, as described in detail elsewhere
(Lee et al., 2020). Source A was
used throughout the study, and source B was only used in size-selection
experiments shown in Fig. 2. Two electrospray (ES) solutions were used to
generate charged ES droplets: (1) acetonitrile
Figures S2 and S3 show two experimental setups for the investigation of the
size dependence of the particle extraction efficiency using EESI. Chemical
standards were used in the first experimental setup (Fig. S2). Three
individual aqueous solutions containing 4000 ppm of levoglucosan, sucrose
and ammonium nitrate, respectively, were nebulized separately at 1.4 L min
After particle size selection, the sample was drawn through a multichannel
charcoal denuder to strip gas-phase constituents before entering the
EESI-TOF inlet manifold. The sample was also characterized immediately
upstream of the electrospray region by a nano-scanning mobility particle
sizer (size range 2.5–239 nm, nano-SMPS, TSI Inc., USA), a scanning
mobility particle sizer (size range 16–638 nm, TSI SMPS, TSI Inc., USA)
and an aerosol mass spectrometer equipped with a long time-of-flight mass
analyzer (AMS-LTOF, Aerodyne Research Inc., USA) (Fig. S2). The high
concentration of the chemical solutions ensured that sufficiently high
analyte concentrations (
In the second configuration (Fig. S3), we investigated the size-dependent
EESI sensitivity towards biogenic SOA produced from
A 104 cm long Pyrex flow tube of 7.4 cm inner diameter with a total volume
of approximately 5 L (Molteni et al.,
2018) was used for particle surface coating experiments (Fig. S4). A 1000 ppm NH
Figure 1a shows a typical measurement of the EESI-TOF and SMPS for
size-selected sucrose particles. Two sheath flow rates (5 and 15 L min
Sensitivities of the EESI-TOF towards various standards normalized to
their respective values at 100 nm as a function of the particle volumetric
geometric mean diameter. Blue and yellow markers indicate EESI sources A and
B, which were initially developed for TOF and Orbitrap mass analyzers,
respectively
(Lopez-Hilfiker
et al., 2019; Lee et al., 2020). Different marker types (
We investigated the normalized sensitivities of the EESI-TOF for
levoglucosan, sucrose and NH
Assuming that the detected ions from the size-selected particles by EESI are
generated after coagulation and extraction between the particles and ES
droplets, the normalized sensitivity
It is intuitive that the total coagulated mass for extraction is also dependent on the residence time for coagulation between the particles and the ES droplets during electrospray ionization. A longer residence time would allow for a higher percentage of the particle total mass to be extracted; i.e., the coagulation of smaller particles would saturate, while the coagulation of larger particles would continue, which would result in a smaller range of size-dependent total coagulated mass (shallower size-dependent sensitivity). We examined this hypothesis by using an EESI source B which provides a factor of 2 longer residence time in the electrospray ionization region. As shown in Fig. 2, the sensitivity size dependence resulting from EESI source B (yellow markers), which has twice the residence time as EESI source A, is significantly shallower than the one from EESI source A (blue markers), consistent with our hypothesis. Overall, Fig. 2 suggests that the size-dependent sensitivity (total coagulated mass) is dependent on the Brownian coagulation coefficient, which varies with the ES droplet size (and therefore ES operating parameters), as well as the residence time for coagulation. Such size dependence suggests that the ionization of analyte particles in the EESI proceeds through coagulation at a certain size-dependent efficiency, e.g., partial coalescence between particles and ES droplets, as reported by the previous studies (Wang et al., 2012; Kumbhani et al., 2018; Pagonis et al., 2020).
Konermann et al. (2013) reported that the
electrospray droplet evaporation can be affected by the size and the
polarity of analyte molecules, while Meier
et al. (2011) suggested that the extraction efficiency of EESI can depend on
the volatility of analyte molecules. We investigated the EESI sensitivity
size dependence for a complex mixture of internally mixed
The linear behaviors of different measured species between the EESI-TOF and
the FIGAERO-CI-TOF-MS for
Limited surface extraction, approximately 2–4 nm in depth, of the particles
was reported for some ESI source designs
(Kumbhani
et al., 2018; Wingen and Finlayson-Pitts, 2019). If such an effect were
present in the EESI-TOF design used in the current study, it could also
appear as a size-dependent sensitivity. This would mean that a smaller
fraction of the analyte volume is extracted as the particle diameter
increases and that the EESI sensitivity scales with the particle surface
area rather than the volume. To determine the potential contribution by
surface extraction to the observed sensitivity size dependence, we
investigated the extraction efficiency of NH
If extraction were limited to the particle surface, the EESI signal for
NH
Relative changes of
We explored the dependence of the EESI sensitivity on particle size using
individual chemical standards and chemical mixtures with two different EESI
source designs. We show that the EESI sensitivity decreases as the size of
the particles increases. The sensitivity size dependence correlates with the
Brownian coagulation coefficient and the residence time for coagulation. The
results suggest that the particles undergo coalescence with the ES
droplets as suggested in previous studies
(Law et al., 2010; Wang
et al., 2012), but the efficiency of the coalescence is limited by the
coagulation coefficient, which depends on the particle and ES
droplet sizes. From a comparison with the FIGAERO-CI-TOF-MS online
measurements, we show that the EESI sensitivity size dependence is also
present for internally mixed secondary organic aerosol made of molecules
with volatilities varying by approximately 10 orders of magnitude. While
the total extracted mass is related to the size-dependent Brownian
coagulation coefficient (i.e., not all particles of different size can
coalesce with all the electrospray droplets), coating experiments show that
the volume of particles, once coagulated with the ES droplet, is fully
extracted up to a size of 250 nm for our EESI configuration instead of
limited surface extraction reported by the previous work
(Kumbhani et al., 2018). Future
work should investigate the EESI response to coarse-mode particles (with
Data presented in this study can be obtained at the Zenodo online
repository hosted by CERN (
Extractive electrospray ionization (EESI) enables online characterization of particle with negligible thermal and ionization-induced fragmentation. Our study elucidates the extraction mechanism between the particles and electrospray (ES) droplets of different properties. The results show that the extraction rate is likely affected by the coagulation rate between the particles and ES droplets, causing an increase in sensitivity by 1–3 orders of magnitude as particle size decreased from 300 to 30 nm. This size-dependent sensitivity is especially relevant when EESI is used to probe size-varying particles as is the case in aerosol formation and growth studies with size ranges below 100 nm. However, once coagulated, the particles undergo complete extraction within the ES droplet. For the video supplement, please see
The supplement related to this article is available online at:
CPL and MS designed the experiment. MS, CPL, DW, HL, MW, FA, VH and BL performed the experiments. CPL, MS, MW and FA analyzed the data. CPL, IEH, MS, DW, HL, JD, JGS, UB, DMB, NMD and ASHP interpreted the compiled results. CPL prepared the manuscript. All authors contributed to the discussion and revision of the manuscript.
The authors declare that they have no conflict of interest.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “Simulation chambers as tools in atmospheric research (AMT/ACP/GMD inter-journal SI)”. It is not associated with a conference.
We thank our technician Pascal Andre Schneider for technical support throughout our experiments and Martin Gysel for the scientific discussion. Special thanks to the CLOUD collaboration and CERN facilities for providing us the possibilities and resources to realize our investigation.
This research has been supported by Horizon 2020 (grant nos. PSI-FELLOW-II-3i (grant no. 701647), EUROCHAMP-2020 (grant no. 730997) and CLOUD-MOTION (grant no. 764991)), the Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (grant nos. 20020_172602, BSSGI0_155846 and 20FI20_172622), and the National Science Foundation (grant nos. AGS1801574 and AGS1531284).
This paper was edited by Pierre Herckes and reviewed by three anonymous referees.