AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-10-1623-2017The ion trap aerosol mass spectrometer: field intercomparison with the
ToF-AMS and the capability of differentiating organic compound classes via
MS-MSFachingerJohannes R. W.GallavardinStéphane J.HelleisFrankFachingerFriederikeDrewnickFrankBorrmannStephanstephan.borrmann@mpic.dehttps://orcid.org/0000-0002-4774-9380Particle Chemistry Department, Max Planck Institute for Chemistry,
Mainz, 55128, GermanyInstitute for Atmospheric Physics, Johannes Gutenberg University
Mainz, Mainz, 55128, GermanyMax Planck Institute for Chemistry, Mainz, 55128, GermanyStephan Borrmann (stephan.borrmann@mpic.de)27April2017104162316378November20161December20164April20175April2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/10/1623/2017/amt-10-1623-2017.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/10/1623/2017/amt-10-1623-2017.pdf
Further development and optimisation of a previously
described ion trap aerosol mass spectrometer (IT-AMS) are presented, which
resulted in more reproducible and robust operation and allowed for the
instrument's first field deployment. Results from this 11-day-long
measurement indicate that the instrument is capable of providing
quantitative information on organics, nitrate, and sulfate mass
concentrations with reasonable detection limits (0.5–1.4 µg m-3 for 1 h averages) and that results obtained with the IT-AMS can
directly be related to those from Aerodyne aerosol mass spectrometers. The
capability of the IT-AMS to elucidate the structure of fragment ions is
demonstrated via an MS4 study on tryptophan. Detection limits are
demonstrated to be sufficiently low to allow for MS2 studies not only
in laboratory but also in field measurements under favourable conditions or
with the use of an aerosol concentrator. In laboratory studies the
capability of the IT-AMS to differentiate [C4Hy]+ and
[C3HyO]+ fragments at the nominal m/z 55 and 57 via their
characteristic fragmentation patterns in MS2 experiments is
demonstrated. Furthermore, with the IT-AMS it is possible to distinguish
between fragments of the same elemental composition
([C2H4O2]+ at m/z 60 and [C3H5O2]+ at
m/z 73) originating from different compound classes (carboxylic acids and
sugars) due to their different molecular structure. These findings
constitute a proof of concept and could provide a new means of
distinguishing between these two compound classes in ambient organic
aerosol.
Introduction
Despite the fact that atmospheric aerosol particles have an important
influence on air quality, public health, and the climate system, the
knowledge on the influence of individual aerosol particle chemical
components remains limited (Fuzzi et
al., 2015). One main contributor to fine particulate matter is organic
aerosol (Kanakidou et al.,
2005), which includes a large variety of different, to a large part unknown,
organic molecules (Goldstein and Galbally, 2007).
Much work has been invested in the past in order to chemically characterise
the organic material present within the ambient aerosol, mostly using mass
spectrometric techniques (Hoffmann et al., 2011). For offline
analyses of filter samples from atmospheric aerosol particles, mass
spectrometry is often coupled to chromatographic methods for prior
separation of the organic compounds (Pratt and Prather, 2012),
while with the most commonly used online techniques either single particles
(Murphy, 2007; Silva and Prather, 2000) or small ensembles of
particles (Canagaratna et al., 2007) are
analysed without prior separation of substances.
Currently, the most widely used instrument deploying the latter method is
the Aerodyne aerosol mass spectrometer (AMS, Aerodyne
Inc.; Jayne et al., 2000), in which ensembles of particles are
flash-vaporised at typically ∼ 600 ∘C and the
evolving vapour is ionised by electron impact ionisation before mass
spectrometric analysis, usually with a time-of-flight mass spectrometer
(ToF-AMS; Drewnick et al., 2005; DeCarlo et al., 2006).
Since a large number of different molecules are analysed simultaneously, a
mathematical deconvolution algorithm has to be applied to the acquired mass
spectra in order to obtain information on different particle components
(Allan et al., 2004). Due to the thermal
desorption (which already might cause some fragmentation) and additional
“hard” electron impact ionisation, molecules are highly fragmented, which
means a partial loss of the original molecular information (e.g. typically
no molecular ion is observed). At the same time, since different molecules
containing the same substructure will be reduced to the same fragment ions,
this considerably reduces the complexity of information when dealing with
mixtures of a large variety of different organic compounds (Hoffmann
et al., 2011) while some important information on the original molecular
structures are still retained in these common fragment ions, which enables
the determination of “types” of organic particle constituents using
positive matrix factorisation (Zhang et al., 2011).
The differentiation of organic components can be further constrained and
improved using the higher-resolution ToF-AMS, which has a mass resolution of
∼ 2000 or ∼ 4000 in two modes providing higher
and lower sensitivity, respectively (DeCarlo et al., 2006).
With these resolutions, it is possible to distinguish between isobars, i.e.
ion fragments of the same nominal mass-to-charge ratio (m/z) but with
different elemental compositions. For example, a prominent mass spectral
signal at m/z 55 is observed in both hydrocarbon-like (traffic-related) and
cooking-related organic aerosol. While this signal seems to be related
mostly to [C4H7]+ in hydrocarbon-like organic aerosol, an
additional large fraction of [C3H3O]+ at the same nominal
m/z is found for cooking-related organic aerosol
(Sun et al., 2011).
While information on elemental composition of the fragment ions can be
obtained with the ToF-AMS, it does not allow for the differentiation between
fragment ions of the same elemental composition but with different
structural formulas (i.e. isomeric ions). Such information can be obtained
using ion trap mass spectrometers (March, 1997), which
not only allow for measuring a “classic” mass spectrum (MS) but also enable
MSn studies. In such experiments, ions of a single nominal m/z are first
isolated and then fragmented by collisions with buffer gas atoms (collision-induced dissociation, CID). The resulting fragment ions can be mass
scanned, which provides the mass spectrum of the fragment ions (MS-MS, or
MS2), or again ions of a certain m/z can be separated and fragmented, and
so on (MSn). From the fragment ions and fragmentation pathways,
conclusions on the molecular structure of the parent ion can be drawn
(March, 1997; McLafferty and Tureček, 1993). Additionally,
ion–molecule reactions inside the ion trap can be utilised in order to
differentiate between isobaric or isomeric ions (e.g.
Kascheres and Cooks, 1988).
Due to their capability to elucidate the molecular structure of organic
molecules, ion traps are often coupled to “soft” ionisation techniques in
order to preserve the molecular structure of the compounds of interest as
much as possible. For atmospheric applications, for example vacuum ultraviolet
single photon ionisation (Hanna et al., 2009; Schramm et al., 2009),
proton transfer reaction (Thornberry et al., 2009), and
atmospheric pressure chemical ionisation (Vogel
et al., 2013), have been successfully applied in conjunction with ion trap
mass spectrometry. Kürten et al. (2007) introduced an
instrument which represents a synthesis of an AMS ionisation chamber
(thermal desorption/electron impact ionisation) with a quadrupole ion trap
mass spectrometer; a similar instrument was also developed by
Harris et al. (2007). With these systems, a strong reduction in
complexity of ambient organic aerosol mass spectra (which consist of a large
number of different organic molecules) is achieved compared to “soft”
ionisation techniques, while still some additional information, e.g. on
molecular functionality, can be obtained which is not accessible from ToF-AMS
measurements.
Schematics of the IT-AMS. The insets (a)–(c) visualise the major
hardware modifications (compared to Kürten et al., 2007) described in
the main text. (TMP: turbo molecular pump; photographs from Fachinger, 2012.)
Here, we present further design improvements of the ion trap aerosol mass
spectrometer (IT-AMS) originally introduced by Kürten et al. (2007). Technical improvements enable more robust and reproducible
measurements with this instrument since the work of Kürten
et al. (2007) and allowed for the instrument's first field deployment. We
show how the IT-AMS compares with a regular ToF-AMS and demonstrate its
MSn capability up to MS4, also providing an estimate of the
related detection limits. Finally, from MS2 studies on various
compounds, we demonstrate how the IT-AMS is capable of distinguishing
between isomeric fragment ions, which could provide a means to distinguish
between carboxylic acids and sugars in organic aerosol and, with more
extensive calibration, potentially also to quantify their relative fractions
in more complex mixtures. Since sugars and carboxylic acids can be
associated with different aerosol sources (sugars originate e.g. from
biomass burning or primary biological material, while carboxylic acids
originate e.g. from photo-oxidation of organic precursors;
Graham et al., 2003), this would help in further
improving the differentiation of various organic aerosol components and
therefore in source apportionment of atmospheric organic aerosol.
Instrumental developments
The original set-up and the working principle of the IT-AMS are described in detail in Kürten
et al. (2007). Here, only a brief overview of the instrument is given
(Fig. 1). The instrument consists of the same commercially available
particle inlet and ionisation chamber as contained in the AMS
(Jayne et al., 2000), coupled to a home-built quadrupole ion
trap (Kürten et al., 2007). Submicron-sized particles
entering the instrument are focused by a Liu-type aerodynamic lens
(Liu et al., 1995). A shutter located behind the inlet can be
either closed to block the particle beam (measurement of instrumental
background) or opened (particle beam plus background), enabling the
calculation of “difference” mass spectra (open minus closed) originating
purely from the particle beam (Canagaratna
et al., 2007). In the ionisation chamber the particles are flash-vaporised
on a hot tungsten surface (∼ 600 ∘C), and the
resulting vapour is ionised by electron impact ionisation (70 eV). The ions
are extracted through ion optics to the quadrupole ion trap, which consists
of a ring electrode and two hyperbolical end cap electrodes with modified
angle geometry with r0= 1 cm and z0=r0/1.9= 0.725 cm, where r0 and z0 are the shortest distances from
the centre of the trap to the ring and end cap electrodes, respectively. In
the ion trap, the ions can be stored and manipulated; ions leaving the trap
are then detected using a Channeltron (KBL510, Sjuts). High-purity helium
(6.0, Westfalen AG) is used as buffer gas in the ion trap. Mass spectral
resolution depends on the settings (Kürten et al., 2007) and was
∼ 400 for the measurements described here.
While the instrument described in Kürten et al. (2007) was capable of
providing quantitative information on aerosol components, it suffered from a
lack of user-friendliness and from limited long-term stability and
reproducibility of the settings. Therefore, in order to prepare the
instrument for field measurements and regular laboratory applications,
several hardware (electronic and mechanical) as well as instrument control
software modifications were performed, all aiming at a more robust,
versatile, and user-friendly operation of the instrument. The operation
principles for the MS and MSn measurements, e.g. mass range extension,
ion isolation, and ion excitation and fragmentation, were nevertheless kept
the same. We only describe the most important changes in detail here; these
as well as some other, minor modifications are also described in
Fachinger (2012).
Most importantly, three hardware modifications related to the mechanical
design of the instrument were realised (see Fig. 1, inserts):
In the ion trap, originally ceramic washers of 2.87 mm ± 25 µm
thickness were used by Kürten et al. (2007) as spacers
between the ring and end cap electrode, and the electrodes were held by four
threaded bars insulated by ceramic shells with a play of 0.5 mm each. Due to
these rather large allowances for tolerance, the electrodes could not be
assembled reproducibly enough to maintain the geometry (i.e. without
rotation or tilting of the electrodes), and after each re-assembly of the
electrodes the voltage settings for the various operations (e.g. trapping,
scanning, resonant excitation) had to be re-tuned. To avoid this, now four
ruby spheres (diameter 6 mm ± 0.635 µm) sitting in precise
countersinks (1.3 mm ± 10 µm) are used instead of the ceramic
washers for a more defined mounting (Fig. 1b). This allows for a more
reproducible assembly of the electrodes (i.e. invariant geometry) and
consequently more reproducible voltage settings without needing to re-tune
after each re-assembly of the ion trap electrodes.
In order to ensure stable and reproducible ion signal intensities, constant
helium pressure in the ion trap is needed, which is achieved in the IT-AMS
by a constant inlet mass flow of helium and constant pumping speed. In the
original set-up, this helium inlet flow was regulated via a needle valve and
therefore changed with changing ambient (upstream) pressure (over the course
of 4 days, a relative standard deviation of 2 % was found at an
average pressure of 2 × 10-5 hPa measured outside the trap). Now,
constant inlet mass flow is provided by a critical orifice (30 µm
inner diameter) with constant upstream pressure ensured by a
pressure-controlled mass flow controller (Bronkhorst High-Tech B.V.,
EL-PRESS P-502-C and F-004AC with a specified flow rate of ≤ 0.7 L min-1; Fig. 1a). With this system, the pressure inside the
system (7 × 10-5 hPa, measured outside the trap) was found to be
stable within less than 1 % (relative standard deviation of 15 s data)
over the course of 11 days.
The ion trap needs a pulsed ion source so that ions are only generated and
transmitted to the ion trap during the trapping phase but not during the
analysis phase. This was realised in the original set-up by gating of the ion
source cage voltages (Kürten et al., 2007): only during
the trapping phase were electrons accelerated into the ion source cage,
while in the other phases, they were deflected from it. This led to an
instable filament emission current directly after switching the voltages,
potentially due to the build-up of space charges. Now, a more stable
filament emission current is achieved by the use of a modified filament
which allows for pulsed ion source operation. In the original filament, the
emission of the electrons (defined by the filament current) and the voltage
of the filament's deflection plate were electronically coupled in such a way
that emitted electrons always were repelled by the deflection plate and
accelerated away from the filament and towards the ion cage. In the modified
filament the deflection plate is electronically decoupled from the filament,
such that electrons are emitted continuously, but the voltage of the
deflection plate can be set independently and switched from negative to
positive sign. By this means, electrons emitted by the filament are now
either deflected or absorbed by this deflection plate (Fig. 1c), depending
on whether they are needed in the ion source or not. This controlled
absorption of the electrons (instead of only repelling them from the ion
cage) allows for a more defined gating of the electrons and avoids the
potential build-up of space charges.
Organic substances investigated in the laboratory studies; n/a: not
available.
a Sigma-Aldrich Chemie GmbH. b Carl Roth GmbH + Co. KG.
c Alfa Aesar GmbH + Co. KG. d Fluka Chemie GmbH. e Merck KGaA.
f Polyethylene glycol. g Average molecular mass. h Aldrich
Chem. Co.
The IT-AMS is controlled via a program written in LabVIEW (v.8.5, National
Instruments), which is also utilised for data acquisition. The original
software was very rudimentary and did not contain several important features
needed for a routine deployment of the instrument. Therefore this software
was extended and now includes the option for a semiautomatic tuning of
operation parameters, i.e. the instrument is programmed by a user-adaptable
text file to automatically scan the various (five for MS, nine for MS2)
parameters of interest and to save the results, which then can be inspected
to find the optimal set of tuning parameters. The software now also allows
for much more flexibility in the measurement types and their operating
conditions (MS, MS2, MSn>2 (n≤ 5), mass range extension),
programming long series of measurements, and the control of the shutter to
enable automatic switching between open and closed measurements, as described
above. Furthermore, all instrumental settings and parameters are now saved
along with the mass spectra after each measurement cycle.
Laboratory and field measurements
In the laboratory studies performed separately with the IT-AMS and the
ToF-AMS, respectively, particles were typically generated from an aqueous
solution of the respective substance (Table 1) using a nebuliser (model
3076, TSI Inc.). Oleic acid was dissolved in ethanol instead of water (in
case of the ToF-AMS, an aqueous suspension was used), while butyl valerate
in both cases was leaked as vapour directly into the instrument. After
generation, the aerosol was dried using two consecutive silica gel diffusion
driers before sampling with the IT-AMS or high-resolution ToF-AMS
(DeCarlo et al., 2006), respectively. For the determination
of the MSn detection limits using tryptophan (Sect. 4.2), monodisperse
aerosol (130 nm mobility diameter) was generated by classifying the dried
particles using a differential mobility analyser (model 3081, TSI Inc.).
Parallel measurement of particle number concentration using a condensation
particle counter (model 3025A, TSI Inc.) enabled the calculation of the
sampled aerosol mass concentration, assuming spherical particles with the
bulk density of tryptophan (1.34 g cm-3).
IT-AMS operating parameters in the MSn studies. VRF is
the amplitude (zero-to-peak) of the radio frequency drive voltage (1.3 MHz)
applied to the ring electrode; qz is the stability parameter resulting
from the Mathieu equation (March, 1997). The resonance
frequency was experimentally determined.
Continuous measurements of ambient aerosol using the IT-AMS and a high-resolution ToF-AMS were concurrently performed on the Mt. Kleiner Feldberg
(Central Germany) from 29 August to 9 September 2011, within the context of
a larger measurement campaign (Sobanski et
al., 2016). Both instruments were sampling in parallel through two separate
inlets (7 m a.g.l), which were located in about 5 m distance from each
other. Since no local sources were close to the measurement site, only
regional background aerosol was measured, which can be expected to be
homogeneously distributed on this spatial scale. The IT-AMS was measuring
with a time resolution of 30 s (10 s particle beam blocked, 10 s open, with
5 s waiting time between each half-cycle; during each 10 s interval 100 mass
spectra were averaged, each acquired after 50 ms ion accumulation time), the
ToF-AMS with a resolution of 60 s (containing 15 s beam blocked, 15 s open,
and 30 s size distribution measurement).
In all laboratory and field measurements, ions were trapped over 50 ms at a
radio frequency (1.3 MHz) drive voltage of 250 to 700 V amplitude
(zero-to-peak) applied to the ion trap ring electrode. To reach the maximum
m/z of 135 used within this work, in the laboratory experiments – if needed –
the mass range was extended by applying an additional voltage of 400 mV
amplitude (zero-to-peak) with a frequency of 400 kHz to both end cap
electrodes. In MSn experiments, ions of m/z of interest were isolated
(typically within a range of ±5 m/z but sometimes up to ±∼ 15 m/z) by broadband excitation using a filtered noise field
(Julian and Cooks, 1993) before they were fragmented using
CID (see parameters in Table 2).
Measured IT-AMS mass spectral signals were converted to ion rates (number of
measured ions divided by the length of the trapping phase) and integrated to
unit mass resolution (UMR) mass spectra using home-built procedures in IGOR
Pro (v.6.22, WaveMetrics Inc.) and MATLAB (R2006, MathWorks). High-resolution ToF-AMS data were analysed using SQUIRREL v.1.44 and PIKA v.1.04
and higher (SQUIRREL, 2016).
From the ToF-AMS field data, organics, nitrate, and sulfate mass
concentrations were determined using the fragmentation pattern table
(Allan et al., 2004) within SQUIRREL
(v.1.51H), which was adjusted to correct for background effects using
routinely performed measurements of particle-free air (obtained by inserting
a high-efficiency particulate arrestance (HEPA) filter in the sampling line).
Ionisation efficiency of the ToF-AMS was determined prior to the campaign
applying the established method
(Canagaratna et al., 2007) using dried
NH4NO3 particles of known mobility diameter (400 nm). A collection
efficiency of 50 % was applied following
Canagaratna et al. (2007), which resulted
in good agreement with other co-located measurements (Fachinger,
2012). Comparison of the 1 min averaged time series of PM1 calculated
by summing all ToF-AMS measured species plus independently measured black
carbon (using a Multi-Angle Absorption Photometer (MAAP), model 5012, Thermo
Scientific) with measurements of total PM1 (using an Environmental Dust
Monitor (EDM) 180, Grimm Aerosol Technik GmbH & Co. KG) gave a correlation
of Pearson's R2= 0.91 and a slope of 1.11, i.e. good agreement
within the uncertainty of the ToF-AMS of ∼ 30 %.
Comparison of average difference mass spectra measured with the
IT-AMS and the ToF-AMS during 11-day-long ambient measurements. Shown are
the average mass spectra normalised (after conversion to ion rates) to their
respective mass spectral signal at m/z 28 (a) and the ratio (IT-AMS
to ToF-AMS) of these relative signal intensities, colour-coded for the
dominant species at the respective m/z(b). Note the logarithmic
scaling of the y axes and that the upper panel's y axis only starts at
10-5 (i.e. ion signals smaller than that are not shown).
For the IT-AMS field data, difference mass spectra were calculated from the
UMR mass spectra obtained during beam open and blocked time. From these,
species-resolved total ion rates for organics, nitrate, and sulfate were
calculated using a simplified fragmentation pattern table (due to reduced
signal-to-noise ratio) by summing the mass spectral signal of the m/z
containing the respective species' dominant ions in the m/z range 30–106
(nitrate: m/z 30, 46; sulfate: m/z 48, 64, 80, 81, 98; organics: all other m/z except 32, 39, 40).
Results and discussionMeasurement of ambient aerosol: comparability to ToF-AMS results
Figure 2 shows a comparison of the average mass spectra from the IT-AMS and
the ToF-AMS, acquired during 11 days of field measurement. The m/z 30 and above
are colour-coded for the species (organics, nitrate, sulfate) with which
they are dominantly associated (see Sect. 3). Here, we discuss general
trends on the basis of the campaign average in order to minimise the
statistical uncertainty. The same features are typically also visible when
comparing 1 h averaged mass spectra; in that case the observed trends are
more distinct at higher absolute mass concentration (i.e. at higher signal-to-noise ratio).
Strong differences between the mass spectra are observed in the m/z range below
30. Apart from the potential influence of lower ion transmission or lower
trapping efficiencies for low m/z ions in the IT-AMS, this is probably mostly
due to the strong influence of charge-transfer reactions in the ion trap
during the trapping phase in this m/z range, e.g. involving N+, O+,
N2+, or O2+ (Dotan et al., 1997; Hierl et al., 1997).
m/z 28 (N2+), 32 (O2+), and 40 (Ar+), but also m/z 44
(CO2+), are depleted in the IT-AMS compared to the ToF-AMS mass
spectrum, likely due to formation of more stable ions by charge-transfer
reactions in the ion trap (Ottens et al., 2005).
Furthermore, the ratio of m/z 18 to m/z 19 (H2O+ to H3O+) is
much smaller (by more than 99 %) in the IT-AMS mass spectrum compared to
that of the ToF-AMS, probably caused by proton transfer reactions occurring
in the ion trap (H2O + H2O+→
H3O++⋅ OH; Cole et al., 2003).
In laboratory experiments, we found that with increasing ion accumulation
and reaction times, this ratio decreases exponentially, converging to a
ratio of ∼1:1 at accumulation times ≥ 200 ms. Beside
these differences, a plateau of more or less constant relative signal
intensity is observed from m/z 20 to 27 for the IT-AMS (Fig. 2a)
but not for the ToF-AMS. Kürten (2007) found that this plateau
disappears (i.e. the mass spectral pattern in this m/z range becomes more
similar to that of the ToF-AMS) when an additional reaction time is applied
to allow for collisional cooling (Wu and Brodbelt, 1992). It can
therefore be considered an artefact. In this work, we focus on m/z 30 and
above, since the deconvolution of ion signal below m/z 30 is complicated due to
the contribution of several species to most single m/z
(Allan et al., 2004) and the relatively
strong influence of artefacts in this m/z range.
Comparison of 1 h averaged time series of organics (a),
nitrate (b), and sulfate (c) measured with the IT-AMS
(left and right axes for ion rate and mass concentrations, respectively) and
the ToF-AMS (right axis) during 11-day-long ambient measurements. The inlays
show the correlation of the respective time series and the associated
squared Pearson's correlation coefficient (R2); the IT-AMS detection
limits are marked with red dotted lines. IT-AMS mass concentrations are
calculated using the linear correlation with the ToF-AMS time series
(inlays).
Considering the organics signal at m/z > 30, most strikingly, an
increasing signal ratio (IT-AMS/ToF-AMS) with increasing m/z is observed
(Fig. 2b). This probably is explained by different ion
transmission efficiency curves with respect to m/z for the two instruments,
which can easily be accounted for by calibrations. Importantly, though, the
fragmentation pattern (Fig. 2a) is comparable between both
instruments (Pearson's R2 of 0.78; if m/z 44 – which is influenced by
charge-transfer reactions inside the trap, see above – is disregarded,
R2= 0.90) despite the much longer residence time of the ions in the
ion trap (50 ms accumulation time) compared to the ToF-AMS. Thus results
obtained with the IT-AMS, including MS-MS measurements, are directly
transferable to ToF-AMS measurements. This comparability needs to be
validated also for other accumulation and reaction times.
In contrast, signals of m/z mostly related to sulfate fragments are strongly
depleted in the IT-AMS compared to the ToF-AMS mass spectrum, and also the
fragmentation pattern is different. The signal ratio m/z 48 to m/z 64, which are
dominated by the sulfate ion fragments SO+ and SO2+, is 0.8
in the ToF-AMS mass spectrum, but 1.7 in that of the IT-AMS, despite the
opposing trend of ion transmission efficiency. Both decomposition of the
ions and charge-transfer reactions (with different efficiency for different
ions) could cause this ion depletion and fragmentation pattern changes. This
needs to be further investigated, also as a function of accumulation and
reaction times. Contrarily, for nitrate-related ions, no depletion in the
IT-AMS has been found.
Figure 3 shows the measured organics, nitrate, and sulfate concentration
time series for both instruments. For the IT-AMS, the results are reported
both in ion rates and in mass concentrations, which are calculated using the
linear relationships with the ToF-AMS (Fig. 3, inlays). For all three
species, the time series measured with IT-AMS and ToF-AMS correlate linearly
over the range of observed mass concentrations (Fig. 3), confirming the
capability of the IT-AMS to quantitatively measure species mass
concentrations independently using adequate calibrations for all species, as
demonstrated for nitrate by Kürten et al. (2007). The
squared Pearson's correlation coefficients for 10 min (1 h) averaged time
series are 0.82 (0.88) for organics, 0.68 (0.65) for nitrate, and 0.37
(0.58) for sulfate (Fig. 3). The much lower correlation coefficient for
sulfate compared to the other species is in agreement with the observation
of much lower response (and therefore lower ion rates and signal-to-noise
ratio) of the IT-AMS for sulfate than for nitrate and organics (Figs. 2 and 3). Also the fact that the observed range of mass concentrations for
sulfate was smaller than for organics and nitrate might have added to the
less tight correlation for this species.
From the calibration of the IT-AMS against the ToF-AMS, furthermore relative
ionisation efficiency (RIE) values for sulfate and organics can be
calculated for the IT-AMS. The RIE is a constant
factor with which the ionisation efficiency (determined in calibrations
using ammonium nitrate) is multiplied in order to get the species-dependent
ionisation efficiency. In order to determine these RIEs, the slope obtained
for nitrate from the correlation depicted in Fig. 3 (inlay) is related to
those determined for organics and sulfate. By this means, RIE
values of 0.4 for sulfate and of 1.7 for organics were found. For organics,
this is slightly higher than the RIE value of 1.4 used for the ToF-AMS,
consistent with the slightly higher ion transmission of the IT-AMS for
larger m/z (to which mostly organics are contributing). For sulfate, the RIE
of 0.4 is much smaller than the sulfate RIE value typically used for the
ToF-AMS (1.2), consistent with the depletion of sulfate-related ions in the
IT-AMS, as described above. It also has to be kept in mind that even without
those influences, not exactly the same RIE values as for the ToF-AMS can be
expected due to the use of a simplified fragmentation pattern table (see
Sect. 3).
By calibrating the IT-AMS for nitrate as demonstrated by Kürten et al. (2007), with the RIE values determined here the mass concentrations of
sulfate and organics can be directly obtained from IT-AMS measurements.
Note, however, that RIE values might change with different accumulation and
reaction times and therefore need to be newly measured when changing the
instrumental settings. The uncertainty of the derived mass concentrations
can be estimated to 30 % (which includes the uncertainty due to
ionisation efficiency, relative ionisation efficiency, and collection
efficiency), the same as usually estimated for ToF-AMS measurements. Note
that with the IT-AMS, unlike the ToF-AMS, ammonium mass concentration cannot
be determined due to artefacts in the m/z range < m/z 30, as described
above. Another species typically reported from ToF-AMS measurements,
nonrefractory chloride, should in principle be possible to detect with the
IT-AMS (dominant mass spectral lines at m/z 35 and 36) but has not been
observed during this measurement due to very low mass concentrations
(campaign average of 0.04 µg m-3 found with the ToF-AMS).
The mass concentration time series derived from the IT-AMS measurements
using the linear correlation with the ToF-AMS measurement (Fig. 3, inlays)
within their uncertainty agree well with those of co-located measurements:
the sum of black carbon (from MAAP) with IT-AMS sulfate, nitrate, and
organics and corrected for the missing species ammonium by assuming fully
neutralised aerosol (as expected for regional background aerosol and
validated by the ToF-AMS measurements) correlates well with the total
PM1 mass concentration measured with the EDM (slope = 1.03, R2= 0.64 for 1 h data).
MS2 study on glutathione. Shown are the “classic” MS
(a) and the mass spectrum acquired after isolating (b) and fragmenting
(MS2, c) m/z 130. The signal in (c) in the m/z range ∼ 115–130 originates
from ions remaining after the isolation step (b). The dashed
vertical line in panel (a) divides the m/z ranges for which the left/right
y axes are used, respectively.
Detection limits for organics, nitrate, and sulfate are calculated
following the method by Drewnick et al. (2009).
Detection limits for 10 min averages are (3.7 ± 1.1) µg m-3 for organics, (1.3 ± 0.4) µg m-3 for
nitrate, and (1.3 ± 0.4) µg m-3 for sulfate (for 1 h
averages: (1.4 ± 0.4) µg m-3 for organics,
(0.5 ± 0.2) µg m-3 for nitrate, (0.7 ± 0.2) µg m-3 for sulfate). The higher detection limit for
organics can be explained by the fact that the mass spectral signal is
distributed over a much larger number of m/z (Fig. 2), thereby lowering the
signal-to-noise ratio. For all three species, the detection limits found for
the IT-AMS are 2 to 3 orders of magnitude higher than those reported
for the ToF-AMS (DeCarlo et al., 2006; Drewnick et al., 2009) due to
much lower ion rates (by 3 orders of magnitude). Thus, the IT-AMS is
capable of providing absolute mass concentrations of organics, nitrate, and
sulfate at typical ambient concentrations, e.g. in urban areas (i.e. in
the order of several µg m-3) after application of adequate
calibrations.
Prospects and limitations of MSn studies
While the IT-AMS has lower sensitivity compared to the ToF-AMS, it allows
for more detailed, in-depth studies of the measured aerosol. Using MSn
studies, it is possible to derive structural information on the measured
ions, while with the ToF-AMS only information on their elemental
composition can be obtained.
In this section, we discuss the potential of MSn studies with the
IT-AMS exemplarily by means of results obtained for two compounds:
glutathione and tryptophan (Table 1). Their “classic” mass spectra,
obtained with the IT-AMS, are shown in Figs. 4a (glutathione) and 5a
(tryptophan). Both show prominent ion signals at m/z 130, which judging from
the structure of the respective parent molecules are likely from
[C5H8O3N]+ (glutathione) and [C9H8N]+
(tryptophan). With the ToF-AMS, these would be possible to distinguish
in the mode of highest mass resolution (∼ 4000) but not in
the more sensitive mode of lower mass resolution (∼ 2000; DeCarlo et al., 2006). With the IT-AMS, they can be
distinguished by means of MS-MS (MS2): CID of m/z 130 yields the fragment
ions m/z 103 ([C8H7]+) and m/z 128 ([C9H6N]+) for
tryptophan (Fig. 5b) but m/z 83 ([C4H5ON]+) and m/z 84
([C4H6ON]+) for glutathione (Fig. 4c), revealing the
different nature of their respective parent ions. It has to be noted that
CID in these experiments is found to potentially affect ions in a range of
±1 around the m/z of interest; i.e. in the case of glutathione at
least parts of the signal at m/z 83 could also originate from CID of m/z 129.
The resulting ions (from MS2) can be further fragmented in order to
obtain more detailed information on their molecular structure, as shown for
tryptophan in Fig. 5c, d: CID of m/z 103 yields m/z 77 ([C6H5]+)
(MS3); the thereby formed ion at m/z 77 can be further fragmented to m/z 50
([C4H2]+) and m/z 51 ([C4H3]+) (MS4). When
trying to fragment these ions (MS5), no ions (neither remaining parent
ions at m/z 50/51 nor any fragments) could be detected. This could be
because the ions are very stable and therefore are removed from the ion
trap, even at very low amplitudes for resonant excitation, or because they
are fragmented to ions of m/z < 20, which were not analysed in the
present study.
MS4 study on tryptophan. Shown are the “classic” MS (a) and
mass spectra acquired after isolating and fragmenting m/z 130 (MS2, b),
m/z 103 (MS3, c), and m/z 77 (MS4, d) from the respective previous step.
The dashed vertical line in panel (a) divides the m/z ranges for which the left/right y axes are used, respectively.
Further information on the nature of the ions observed in the “classic” MS
can be obtained by comparing the CID products of different ions from the
original mass spectrum (i.e. MS2 of these ions) with those of the
fragmentation chain (MS3-5) of larger (parent) fragment ions. While
MS2 of m/z 103, 77, and 50 results in the same fragment ions as observed
in the respective MS3-5 studies of m/z 130 presented above, MS2 of
m/z 51 yields different results. Here, m/z 39 ([C3H3]+) and
m/z 63 ([C5H3]+) are formed, while in the fragmentation chain no
further fragmentation was observed for this ion. Therefore, it can be
concluded that additionally to the fragmentation product of m/z 77 (i.e.
[C4H3]+), the doubly charged ion [C8H6]2+
also contributes to m/z 51 in the “classic” MS of tryptophan to at least
∼ 10 %. The ions [C4H3]+ and
[C8H6]2+ would not be possible to distinguish with the
ToF-AMS.
The detection limits of the tryptophan MSn experiments were calculated
following the method by Drewnick et al. (2009) from
the standard deviations of the signal intensities at the investigated m/z in a
blank MSn measurement, and the signal intensity enhancements at the
same m/z during MSn measurements of known tryptophan mass concentrations.
All detection limits were calculated for averages of 3000 mass spectra,
which correspond to measurement times of 6, 16, 28, and 46 min for the
full MS, MS2, MS3, and MS4 cycles, respectively. The
corresponding detection limits were found to be ∼ 0.6, 7, 13,
and 30 µg m-3 at the given measurement conditions. This suggests
that MS2 studies might be feasible under favourable conditions at
ambient concentrations, while MSn>2 studies are only possible at high
mass concentrations, e.g. in smog chamber studies or by applying an
aerosol concentrator.
Results of studies on (a)m/z 55 and (b)m/z 57 for various compounds.
On the left, the relative contributions of [C3H3O]+ and
[C4H7]+ to m/z 55 and of [C3H5O]+ and
[C4H9]+ to m/z 57 are given (from ToF-AMS measurements; the
difference to 100 % is due to one or several other ions contributing
little to the respective m/z). On the right, the results from IT-AMS MS2
studies on these ions are summarised. For each investigated compound, the
signal intensity of the fragment ions (relative to the signal intensity of
the most abundant fragment) and the ion recovery are given. Not shown are
fragments accounting for less than 1 % and, for the MS2 experiment
on m/z 57, small contributions observed at fragment ion m/z 31 for mannitol
(1 % relative fraction) and at m/z 42 for glucose, oleic acid, and butyl
valerate (3, 1, and 3 % relative fraction).
Differentiation of organic compound classes
In this section, we investigate how the IT-AMS is capable of distinguishing
fragments of the same nominal mass but with different molecular and/or
structural formulas. This makes it possible to distinguish not only
between, for example, hydrocarbon-like and oxygenated organic species (Zhang
et al., 2011) but potentially also between species with different
functional groups, which would allow for the assignment of measured organic
species to different compound classes. This potential is investigated by
means of MS2 studies on various substances from different compound
classes, particularly sugars and carboxylic acids (Table 1).
Differentiation of ions of the same nominal mass but with different
molecular formulas
The differentiation of ions of the same nominal m/z but with different
molecular formulas allows, for example, for the distinction between
hydrocarbon-like and oxygenated organic species. This kind of information is
also accessible with a high-resolution ToF-AMS, given the resolution at the
respective m/z is sufficiently high, and is commonly retrieved from such data
(Zhang et al., 2011). Here, we demonstrate how the IT-AMS is
also capable of providing such information. Figure 6 shows a compilation of
results from MS2 studies on m/z 55 and 57 for a variety of compounds. Also
shown is the relative contribution of the major ions at these m/z measured with
a ToF-AMS (left panels): [C3H3O]+ and [C4H7]+
at m/z 55 and [C3H5O]+ and [C4H9]+ at m/z 57.
When fragmenting the ions at m/z 55 and 57 in the IT-AMS (MS2), clear
differences in the resulting fragmentation patterns are observed between the
different compounds. For sugars (glucose, saccharose, mannitol, and
levoglucosan; reddish/brownish colours in Fig. 6), as well as for succinic and
glutaric acid (carboxylic acids, bluish colours in Fig. 6), fragmentation
occurs predominantly by loss of CO (28 Da, resulting in m/z 27 and 29 ions
from parent ions at m/z 55 and 57, respectively). This points to
[C3HyO]+ as the dominant parent ion for these compounds.
Additionally, loss of CH2O (30 Da) is observed for MS2 of m/z 57,
yielding the fragment ion m/z 27 ([C2H3]+), while loss of C2H2 (26 Da) results
in the fragment ion m/z 29 ([COH]+) for MS2
of m/z 55 for some of these compounds. Ion recovery (i.e. the total signal
of all fragment ions detected in MS2 divided by the concurrent loss in
signal of the parent ion) is rather low for these compounds for MS2 of
m/z 55 (< 20 %) and slightly higher for m/z 57 (∼ 40 %).
Results of studies on (a)m/z 60 and (b)m/z 73 for various compounds.
On the left, the relative contributions of [C2H4O2]+ and
[C3H8O]+ to m/z 60 and of [C3H5O2]+ and
[C4H9O]+ to m/z 73 are given (from ToF-AMS measurements; the
difference to 100 % is due to one or several other ions contributing
little to the respective m/z). On the right, the results from IT-AMS MS2
studies on these ions are summarised. For each investigated compound, the
signal intensity of the fragment ions (relative to the signal intensity of
the most abundant fragment) and the ion recovery are given. Not shown are
fragments accounting for less than 1 % and, for the MS2 experiment
on m/z 73, a small contribution observed at fragment ion m/z 61 for glucose
(2 % relative fraction).
In contrast, MS2 studies on oleic acid and butyl valerate reveal the
dominant presence of [C4Hy]+ fragment ions at the nominal
masses m/z 55 and 57, consistent with the structure of the respective molecules
(with long hydrocarbon chains, greenish colours in Fig. 6) and with results
from corresponding ToF-AMS measurements (Fig. 6, left): for m/z 55,
predominantly loss of H2 (2 Da), CH4 (16 Da), and C2H2
(26 Da) is observed. Less abundant is the fragment ion m/z 27, originating from
the loss of C2H4 (28 Da). For m/z 57, predominantly fragmentation to
m/z 41 (loss of CH4, 16 Da) is found. Ion recovery for MS2 on these
[C4Hy]+ fragments was found to be much better (50–100 %) than for the [C3HyO]+ fragments discussed above.
For pinonic acid and PEG200 (polyethylene glycol; see Table 1), no clear
picture is obtained. For both compounds, MS2 studies suggest dominance
of [C4H7]+ at m/z 55, with fragmentation patterns similar to
those of oleic acid and butyl valerate. While this is comprehensible from
the molecular structure for pinonic acid, for which also ToF-AMS results
show a strong fraction of [C4H7]+ at m/z 55, for PEG200,
dominance of [C3H3O]+ would have been expected both from the
molecular structure and from the ToF-AMS measurement (Fig. 6). This needs to
be further investigated; it might be possible that the oligomeric structure
of PEG200 plays a role here, which could lead to multiply charged larger ion
fragments and to ion rearrangements and, therefore, to a different molecular
structure of the ion with the elemental composition [C3H3O]+
and a different MS2 mass spectrum. For m/z 57, however, both
PEG200 and pinonic acid show fragmentation patterns originating from
[C3H5O]+ and only small contributions at m/z 41, which likely
originate from fragmentation of [C4H9]+. This is consistent
with the absence of straight hydrocarbon chains in the molecular structures
of these compounds (Table 1) and with the results from the respective
ToF-AMS measurements (Fig. 6, left).
These findings show that it is generally possible to distinguish between
compounds containing straight hydrocarbon chains and such that do not
contain them by MS2 experiments on m/z 55 and 57. Our results suggest that
combined information from the MS2 fragmentation patterns and the
corresponding ion recoveries could facilitate the quantitative determination
of the relative contributions of the different ions ([C3HyO]+
vs. [C4Hy]+) to these parent m/z. More work with a larger number
of different compounds and under a larger range of operating conditions (in
particular also for different vaporisation/ionisation as well as MS2
fragmentation conditions) is needed in order to test further this assumption
and to develop a robust method for the quantification of the relative
contributions of these ions to nominal m/z 55 and 57.
Differentiation of ions with the same molecular formula but different
molecular structures
While differentiation of ions with the same nominal mass but different
molecular formulas is possible both with the IT-AMS and the ToF-AMS, the
differentiation of ions of the same elemental composition but with
different molecular structures is unique to instruments such as the IT-AMS.
This feature has already been demonstrated in Sect. 4.2 in the
differentiation of singly and doubly charged fragments at m/z 51 in the case of
tryptophan and is further investigated here for ions of the nominal m/z 60 and
73 for a number of carboxylic acids and sugars.
In Fig. 7, results from ToF-AMS measurements and IT-AMS MS2 studies on
m/z 60 and 73 are summarised for a variety of sugars (reddish/brownish colours)
and carboxylic acids (bluish colours). For m/z 73, measurements of the
polyether PEG200 (pink colour) and of pinonic acid are also included in the
figure. ToF-AMS measurements indicate the dominant contribution of the ions
[C2H4O2]+ and [C3H5O2]+ to m/z 60 and
73, respectively, for both sugars and carboxylic acids (Fig. 7, first
column). MS2 measurements with the IT-AMS, however, reveal differences
in the molecular structure of these fragments between the two compound
classes, which can be utilised for the differentiation between the latter.
While the ion at m/z 60 from sugars in MS2 experiments fragments only to
ions of m/z 42 by loss of water (18 Da), for carboxylic acids additionally a
small contribution of the fragment m/z 43 (loss of OH, 17 Da) is observed
(Fig. 7a). This can be explained by different molecular structures of the
parent ions [C2H4O2]+ for the two compound classes,
which can be deduced from the molecular structures of the respective parent
compounds (Table 1). For sugars, the fragment ion
[C2H4O2]+ most likely consists of a diol, with two OH
groups attached to two carbon atoms adjacent to each other. In contrast, in
the fragment ion of the same molecular formula formed from fragmentation of
carboxylic acids, both oxygen atoms most likely are attached to the same
carbon atom, forming a carboxyl group. Alcohols fragment predominantly by
loss of water, while carboxyl groups are known to also fragment by loss of
OH (McLafferty and Tureček, 1993), which is in agreement with our
findings. For all investigated compounds, ion recovery was found to be
100 %.
Also in MS2 experiments on m/z 73 ([C3H5O2]+),
differences in the fragmentation chains of carboxylic acids and sugars were
observed (Fig. 7b). While m/z 73 from sugars predominantly fragments by loss of
CO (28 Da) to ions of m/z 45, for m/z 73 from carboxylic acids this fragmentation
pathway is less important, and in contrast it is fragmented predominantly by
loss of water (18 Da) to ions of m/z 55. Notably, ion recovery was found to be
very different for the two compound groups, with ∼ 100 %
ion recovery for carboxylic acids but only ∼ 60 % for
sugars, pointing to an additional fragmentation pathway for m/z 73 from sugars
which was not traceable under the given measurement settings. Interestingly,
the polyether PEG200 shows a similar fragmentation pattern of m/z 73 as sugars
but with ∼ 100 % ion recovery, pointing to yet another
molecular structure of the ion with the chemical formula
[C3H5O2]+.
These results indicate that with the IT-AMS, it should be possible to
determine the relative contributions of carboxylic acids and sugars to the
ions m/z 60 and 73 (even though they have the same elemental composition),
given no major contributions from other compound classes are present. For
MS2 on m/z 60, the ratio of relative intensities of the fragment ions
m/z 43 to m/z 42 is a direct indication for the relative contribution of
carboxylic acids to the parent ion: the ratio was found to be (0%/100 %)
for the sugars, but in the range (11 ± 1) %/100 % for the
tested carboxylic acids (Fig. 7a). For MS2 on m/z 73, the ratio of
relative intensities of the fragment ions m/z 45 to m/z 55 was on average
100 %/3 % (range 100 %/(2–5) %) for sugars and 32 %/100 % (range (13–57) %/100 %) for carboxylic acids
(Fig. 7b).
Using these results and the average relative contributions of m/z 60 and 73 to
the mass spectra of sugars and carboxylic acids, in principle the absolute
contribution of these compound classes to the measured total organic mass
concentration could be determined from MS2 experiments on ions of m/z 60
and 73, which would not be possible with the regular ToF-AMS. More work is
needed in order to test this possibility with a large number of compounds,
concentration ranges, range of operating parameters (especially different
MS2 fragmentation conditions and vaporisation/ionisation conditions),
as well as for mixtures of compounds. In first sensitivity studies for
MS2 on m/z 73, we already found the presented differentiation to be robust
for different aerosol loads, ion quantities in the trapping volume, and ion
trapping parameters qz (see March, 1997). Further
characterisation and calibration work is needed in order to fully facilitate
quantification of the presented compound classes with the IT-AMS with more
robust average values and to identify the associated limitations and
uncertainties, like the cross sensitivity observed for the polyether
PEG200 at m/z 73.
Summary and conclusion
An IT-AMS was further developed and
optimised, now allowing for more reproducible and reliable measurements
while the instrument is also more robust and user-friendly and has extended
measurement capabilities implemented in its data acquisition software. This
allowed for the instrument's first, 11-day-long field deployment, during
which it was continuously running in parallel to a ToF-AMS. The results show
that the IT-AMS is capable of providing quantitative information on the
major nonrefractory submicron species organics, nitrate, and sulfate, with
detection limits between 0.5 and 1.4 µg m-3 for 1 h averages.
IT-AMS and ToF-AMS apply the same type of ion source (thermal desorption/electron impact ionisation), and consequently the observed organics
fragmentation patterns were found to be comparable between the two
instruments.
The capability of the IT-AMS to elucidate the structure of the fragmentation
products observed in such mass spectra was demonstrated by an MS4 study
on tryptophan. Detection limits were found to be sufficiently low to allow
for MS2 studies on atmospheric particles under favourable ambient
conditions (7 µg m-3 at a time resolution of 16 min in the given
example), while MSn>2 studies are only feasible at higher
concentrations. Such a situation can be reached either under laboratory
conditions or via aerosol concentrator systems. All in all, we conclude
that the IT-AMS can provide in-depth, quantitative information on total
organic aerosol and on individual ion fragments both in field and in
laboratory studies and that these results are directly relatable to ToF-AMS
results.
In laboratory studies on a variety of compounds, we found that the IT-AMS is
capable of distinguishing [C4Hy]+ and [C3HyO]+
fragments at the nominal m/z 55 and 57 via their characteristic fragmentation
patterns in MS2 experiments. Furthermore, it is possible to distinguish
between fragments of the same elemental composition but with different
molecular structures: different characteristic MS2 fragmentation
patterns were found for the ions at m/z 60 and 73 for sugars and carboxylic
acids, which have the same elemental compositions
([C2H4O2]+/[C3H5O2]+) in both
cases. While the fragment ions at these m/z originating from carboxylic acids
and sugars therefore could not be differentiated with a ToF-AMS, the
observed differences in their MS2 fragmentation patterns could be used
to distinguish between the two compound classes using the IT-AMS. MS2
at m/z 60 yields ratios of average relative intensities of fragment ions m/z 43 to
m/z 42 of 11 %/100 % for carboxylic acids but of 0 %/100 % for
sugars. Similarly, at m/z 73, the ratio of relative intensities of the MS2
fragment ions m/z 45 to m/z 55 was on average 100 %/3 % for sugars but
32 %/100 % for carboxylic acids.
More research is needed in order to further constrain these average values
by testing a larger number of compounds and by also testing substances from
other compound classes for potential cross sensitivities. However, these
results already indicate that instruments like the IT-AMS potentially enable
the distinction between carboxylic acids and sugars in organic aerosol by
means of MS2 experiments, which would further help in organic aerosol
characterisation and source apportionment.
The data are available upon request from the corresponding author.
The authors declare that they have no conflict of interest.
Acknowledgements
We thank T. Böttger and the teams of the electronic and the mechanical
workshops of the Max Planck Institute for Chemistry for their technical
support and valuable help, especially M. Flanz, K.-H. Bückart, and M. Dieterich. We also thank J. Curtius and A. Kürten for their support and
technical help regarding the initial design of the IT-AMS. P. Faber is
gratefully acknowledged for providing some of the ToF-AMS data. Funding was
provided by the Max Planck Institute for Chemistry and by the Institute for
Atmospheric Physics at the Johannes Gutenberg University, Mainz.
The article processing charges for this open-access publication were covered by the Max Planck Society.
Edited by: P. Herckes
Reviewed by: two anonymous referees
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