A commercial PTR-TOF-MS has been optimized in order to allow the measurement
of individual organic nitrates in the atmosphere. This has been accomplished
by shifting the distribution between different ionizing analytes,
H
Organic nitrates are an important species of the reactive nitrogen (NO
In addition, several field studies have shown that both polluted and remote
atmospheres contain a large variety of organic nitrates, which significantly
affects the NO
However, measurements of organic nitrates during field campaigns remain rare, thus
preventing a precise evaluation of their impact on the NO
Table 1 summarizes, without the intent of being exhaustive, some keystone articles concerning the organic nitrates analysis in field and laboratory studies.
Summarized analytical approaches into the organic nitrate analysis.
Two approaches have been applied for the analysis of complex mixtures of
organic nitrates during field and lab experiments. The first is analysis at the
molecular scale, which is a powerful approach to elucidate mechanisms but has
the drawback to be often limited to a low number of species, followed by
functional group analysis, which is very useful to assess global budgets but
brings little information about the understanding of processes. The second
approach allows us to shortcut the complexity of the organic nitrates chemistry
governing their production, transformation and removal processes and assess
directly the sum of peroxynitrates
Historically, measurements of individual organic nitrates have been
conducted by gas chromatography coupled to electron capture detection (GC/ECD), both coupled or not (Atlas, 1988; Blake et al., 1999; Flocke et
al., 2005; Fukui and Doskey, 1998; Muthuramu et al., 1993) to a
pyrolysis/luminol chemiluminescence (CL) detector (Buhr et al., 1990;
Fischer et al., 2000; Flocke et al., 1991; Gaffney et al., 1999; Hao et al.,
1994; Winer et al., 1974), to electron impact mass spectrometry (EI-MS)
(Luxenhofer and Ballschmiter, 1994; Luxenhofer et al., 1994) or to
negative ion chemical ionization mass spectrometry (NI/CIMS) (Beaver et
al., 2012; Tanimoto et al., 1999) using thermal electrons (
Although the chromatographic separation represents an effective analytical
tool, the organic nitrate identification relies mainly on the retention
times, while the dedicated detectors are only able to confirm, in the best case scenario,
the presence of the nitrate functional group in the molecule. Illustrative
examples are given by the studies of Luxenhofer et al. (1994)
and Kastler and Ballschmiter (1999), who succeed to analyze complex
mixtures of alkyl and multifunctional organic nitrates by combining
separation with liquid and gas chromatography and detection using the
intense mass-to-charge (
Besides the poor temporal resolution, another major drawback of this method is represented by the recovery factor decline with longer times in the chromatographic columns. According to Roberts et al. (2002), the sensitivity of this method for PANs is characterized by a diminishing response factor proportional to the compounds retention time through the column. Measurement of functionalized (oxygenated) nitrates appears to be a greater challenge as the lower vapor pressures and stronger surface interactions of these molecules make sampling and chromatographic techniques less appropriate. Additionally, the detection of functionalized nitrates by electron impact mass spectrometry has proven to be difficult, mainly due to the instability and thus the fragmentation of the molecular ion formed (Mills et al., 2016; Roberts, 1990; Rollins et al., 2010).
Lately, the newly developed capabilities in atmospheric pressure/chemical ionization mass spectrometry (AP-CIMS) (Huey, 2007; Perraud et al., 2010; Slusher et al., 2004; Teng et al., 2015) prove to be potentially powerful tools in organic nitrates analysis. Several types of CIMS have been highlighted by the literature, and the technique is currently one of the most promising analytic tools.
The AP-CIMS uses methanol as a proton source in order to generate RH
The thermal dissociation–chemical ionization mass spectrometry (TD-CIMS)
technique has been used for measurement of PANs and other multifunctional
organic nitrates by the means of I
The proton-transfer-reaction mass spectrometry (PTR-MS) can be positioned as a subset of CI. Its use in atmospheric research has expanded rapidly these last years but few studies have tested this technique for the detection of organic nitrates (Aoki et al., 2007; Hansel and Wisthaler, 2000; Inomata et al., 2013).
The recent study of Müller et al. (2012) considers
that the quantification of PAN by PTR-MS is difficult due to fragmentation.
Less promising results are also reported by Aoki et al. (2007) concerning
the PTR ionization of C
Hansel and Wisthaler (2000) have measured several
PANs (PAN, MPAN and PPN) with PTR-MS and have reported a low detection limit
(70 pptv, 15s integration time) and an overall accuracy of 15 %. It is
worth noting that the reported analytical sensitivities (15–25 counts ppb
So, it appears useful to better explore the possibility of detecting organic nitrates in real time with PTR-MS. In the present work, the detection of alkyl and multifunctional organic nitrates (PANs, ketonitrates, hydroxynitrates) by this technique was investigated. For this purpose, different organic nitrates were synthesized and mixtures at the ppb–ppm level were generated in a smog chamber. A commercial PTR-TOF-MS instrument was used but the operating mode as well as the chemical ionization reagent have been modified and optimized for each type of organic nitrate in order to gain sensitivity in the detection and to reduce fragmentation of ions. Mass spectra have been carefully interpreted and detection limits have also been determined.
A commercially available, high-mass-resolution PTR-TOF-MS instrument (Kore
Technology “Series II” High Performance PTR-TOF-MS) was used in the
present study. The mass resolution of this instrument is > 5000 M/
The PTR-MS is equipped with the newly designed radio frequency (RF) ion funnel
which is used to focus ions in the drift tube and hence to increase the
detection sensitivity (Barber et al., 2012). This ion funnel
uses a series of electrodes with progressively reducing aperture sizes. In
addition to the standard direct current (DC) electrical field, an
alternative current (AC) electric field is provided at a radio frequency
that creates a strongly repulsive effective potential near the surface of the
electrodes, which combined with the reducing aperture sizes serves to focus
the ions radially. This system has been shown to increase the sensitivity by
1 or 2 orders of magnitude (Barber et al., 2012). It is worth noting that operating in the RF mode modifies the dynamic range over which
the drift tube is operated and the contribution of the radio frequency to the
global effective
The PTR-MS sampling line was designed in order to assure the highest
transfer efficiency of the compounds. This line is made of a 1.5 m long
Silcosteel® coated stainless steel tubing (2.1 mm inner
diameter), which has been shown to be appropriated for the transfer of
low-level polar organic compounds (Smith, 2003). This line was heated at
40
In PTR-MS, proton transfer reactions with hydronium ions (H
Only few available data can be found concerning the PA of organic nitrates,
which were calculated using ab initio quantum mechanical methods
exclusively. The PA of organic nitrates has been estimated to be 740 kJ mol
It is well known that other chemical processes can also occur in the PTR
reactor (Blake et al., 2009; de Gouw and Warneke, 2007). The
H
The NO
Previous studies interrelated the PA of water to the NO
This characteristic offers the opportunity to generate soft ionizations with
little fragmentation of the product ions, which is of high interest for the
present study. Reactions which may occur are
The adduct formation pathway (Reaction 2b) has been previously reported for
the detection of C
A third mechanism, similar to the protonation, involves the hydride, hydroxide, methyl or alkoxy abstraction (Reaction 2c), as reported for aldehydes, ethers, alcohols (Smith and Španěl, 2005), terpenoids (Amadei and Ross, 2011) or several unsaturated alcohols (Karl et al., 2012; Schoon et al., 2007; Wang et al., 2004).
The O
To summarize, several parameters will be tested in order to establish the optimal conditions for the detection of each class of organic nitrates.
A carefully thought-out procedure was performed in order to establish the
optimal measurement conditions for each class of organic nitrates:
First, the instrument has been used in a dual mode, employing alternatively
H The ion intensity and distribution are sensitive to the extraction voltage
used to inject the reagent ions into the drift tube and to the hollow
cathode discharge current. The PTR-MS instrument was thus operated in a wide
range of The sampling line and the inlet system along with the drift tube was
slightly heated (40 The influence of radio frequency ion funnel mode on the detection of the
various organic nitrates has been tested by performing experiments with the
RF mode on and off.
Ideally, little or no fragmentation occurs in the case of a soft ionization process. The large differences in ionization energy of the colliding species enhance the fragmentation of the organic molecule. Therefore the ionizing gas species should correspond, especially for labile molecules, to the analyte of interest in order to achieve the high yield ionizations with low excess energy.
In addition, the soft ionization processes demand the usage of uncommon settings of the PTR-MS device. The various effects of the altered PTR-MS tuning are termed in detail elsewhere (Hewitt et al., 2003; Knighton et al., 2009). Supplementary information relating the ion source discharge current and the extraction voltage influence is discussed in the Sect. 3.
All reported signal intensities took systematically into account the background values, representing in most of the cases negligible low values of a few counts per minute. Background measurements are achieved before every single experiment by sampling the dry synthetic air of the reaction chamber.
In this study, calibrated response factors of PTR-MS are determined for
several types of organic nitrates in both H
Due to the high abundance of the ionizing species, the direct measurement of
H
Gaseous mixtures of organic nitrates at the ppb–ppm level were generated in the simulation chamber at LISA. This chamber comprises a Pyrex reactor of 977 L equipped with a multiple reflection optical system interfaced to a FTIR spectrometer (Vertex 80 from Bruker). Details of this smog chamber are given elsewhere (Doussin et al., 2003).
All experiments were conducted in the dark at 298
Before and after every single experiment, a cleaning procedure was applied
in order to avoid memory effects from an experiment to the next one. It
consists of vacuum clean-up down to 10
The simulation chamber was filled with dry synthetic air generated with
N
The alkyl nitrates considered in the present study (n-propyl nitrate – 97 % Janssen Chimica; AlkC3 and isobutyl nitrate – 96 % Sigma Aldrich; AlkiC4) were used as commercially available, without further purification.
Ketonitrates were synthesized using Kames' method, (Kames et
al., 1993): a liquid/gas-phase reaction in which the corresponding
hydroxy ketone is reacted with NO
The hydroxynitrate was synthesized starting from the commercially available
3-bromo-1-propanol (97 % Sigma Aldrich). Its conversion to the analog
iodide was performed by a Finkelstein reaction with sodium iodide
(Baughman et al., 2004) and the subsequent mild conversion
with AgNO
The PAN-type compounds were generated in the simulation chamber from the
gas-phase oxidation of corresponding aldehydes by NO
The presence of impurities as synthesis byproducts may add supplementary signals in the PTRMS mass spectra. These signals are, however, systematically at lower masses than those in the main ionization processes of the organic nitrates and therefore will not interfere in the main frame of the discussion.
As already termed, intensities and mixing ratios of the ionizing species are
dependent on the
Typically recorded distributions over a relevant range of
Typical ionizing species distribution as a function of the
The NO
Organic nitrates normalized sensitivity as a function of the ionization mode.
A series of tests has been conducted in order to seek the optimum operating
conditions for the measurement of alkyl nitrates with H
The
Recorded mass spectrum of AlkC3 (black bars) at the lowest extent of
fragmentation of the molecular ion at
The only other study (Aoki et al., 2007) describing a tentative of alkyl
nitrate detection with PTR-MS used
The presence of the
At higher abundances of the water clusters, expected to occur in our study
due to the usage of low
Results obtained with the RF mode are equally shown in
Fig. 2 for the AlkC3 for comparison. The
ionization pattern of the analyte is slightly different from the one
obtained without RF mode and enables, adjacent to the formation of the
protonated alkyl nitrate, the identification of other specific signals like
the adduct AlkC3
The alkyl (R
To summarize, the use of H
In Table 2, the characteristic signals of each organic nitrate studied here as well as their detection limits are listed.
The performances of the NO
The recorded mass spectra of the analyte of interest in terms of relative
peak intensities as a function of the
For both alkyl nitrates, the adduct formation (Reaction R2b) appears to be
the main ionization mechanism under these given conditions, leading to
intense peaks at
An intense peak corresponding to the alkyl fragment (R
In conclusion, the NO
As for alkyl nitrates, the measurement of hydroxynitrates by PTR-MS has been
tested with H
When the RF mode is on, the same signals (43, 104, 122, 139) were observed
but with different relative abundance. The influence of
The
The water clusters are probably abundant in the PTR reactor due to the low
The mass spectrum illustrated by Fig. 5a
was recorded in absence of the RF funnel and is dominated by fragments like
The above described water adduct formation (
Recorded mass spectrum of 1OH3C3 (black bars) at the lowest extent
of fragmentation of the molecular ion (
In RF mode, the mass spectrum obtained for 1OH3C3 at
To conclude, it has been observed that protonation of the hydroxynitrate is
a minor process in comparison to fragmentation and to adduct formation. The
use of the RF mode significantly reduces the fragmentation for the benefit
of the M
The detection of hydroxynitrates in NO
Results obtained with RF mode are shown in Fig. 4. The main signals that have been observed are 151 and 167, which have been
attributed to M
As expected, the intensity of the signal 43 increases with increasing
Besides the abovementioned characteristic signals, other specific mechanisms
could be associated to the spectral signature of 1OH3C3. The mechanism is
reviewed and suggested by Harrison (1999) for the particular
case of NO
The detection limit obtained in NO
Two distinct ketonitrates were synthesized and characterized in the current study: 3-nitroxy-2-propanone (KnC3) and the 3-nitroxy-3-methyl-2-butanone (KnC5).
In order to identify the optimal conditions for the detection of
ketonitrates in H
Several signals which can be attributed to ketonitrates have been detected,
as illustrated in Fig. 6 for KnC3: at low
The
Redistribution processes among the various precursor ions formed into the
glow discharge can equally occur, leading to the formation of auxiliary
hydrated ions such as NO
For intermediate
For high
From these results, the intermediate
For KnC3, the most intense signal corresponds to M
The already discussed Reactions (R5) and (R6) could explain the intense signal
of
Recorded mass spectrum of protonated KnC3 (black bars) for
It is worth noting that due to the low
In the case of KnC5, the M
The intense
In the RF mode, the water cluster distribution is dominated by the
H
The most noteworthy difference in the RF mode resides in the presence of an
intense signal corresponding to a H
With the same PTR entry voltage, the RF mode activation induces the
enhancement of the fragmentation due to higher input energies and the
maximum of the protonated ketonitrate signal glides towards 48 Td. Making
the assumption that the highest sensitivity is related in the two modes to
an analogous
To conclude, it has been observed that protonation of ketonitrates is the
main ionization process in the absence of a RF field. The use of the RF mode
modifies the fragmentation pattern and enhances the mechanism, leading to the
M
For the first time, the detection of ketonitrates using NO
In the given ionization mode and considering the higher IE of ketonitrates, the adduct formation is expected to prevail over the charge transfer, the literature stating that the yield of parent radical cation formation seems to be anticorrelated with the IE of the ketones, the lowest IE analytes, presenting the highest probability for the charge transfer reaction (Smith et al., 2003).
Following the same approach as in the previous case, the
We plot as an example, the KnC5 adduct signal at
Due to the rising incidence of the NO
The KnC3 spectrum was recorded at
In an analog way, in Fig. S4, the mass spectrum of KnC5 is
plotted for the instrumental setup corresponding at the lowest fragmentation
(
The best results of the NO
The detection of PANs with PTR-MS has been tested by generating the
PAN from the well-known NO
The optimal conditions for the detection of PANs in H
In a manner analogous to the Reaction (R1a), the protonated peroxynitrates
are equally expected to form low energy gas-phase ion-dipole complexes
(ROOH
Another likely mechanism which, unexpectedly, may lead to the same
analytical signal is reviewed by Roberts (1990), proposing
the unimolecular decomposition of PANs to form the corresponding C
However the PAN decomposition path is considered to be several hundred times
slower than the bond homolysis channel:
In order to certify the mono-nitrogen-containing analytes for several ion
signals in the spectrum, an analysis similar to the one performed by
Inomata et al. (2013), which consists of subtraction of the
isotopic effect of
As described by Table 2 the normalized sensitivity
for the protonated PANs is weak, spanning few counts of ppb
Unlike the ketonitrates, the PANs present a low sensitivity towards
detection in NO
In the NO
The thermolability of the PANs was equally considered for the weak response
factors of the instrumental setup in both ionization modes, since the
sampling requires slightly heated inlet lines. We considered the
40
Organic nitrates play a key role in atmospheric chemistry as they act as reactive nitrogen reservoir species. The use of PTR-MS for the measurement of volatile organic compounds has expanded a lot in atmospheric research these last years but few studies have investigated the performances of this instrument for the detection of organic nitrates. These studies have shown that this technique exhibits poor performances (high fragmentation and poor sensitivity) when it is run in classical mode.
In the present work, the detection of alkyl and multifunctional organic
nitrates (PANs, ketonitrates, hydroxynitrates) by this technique has been
studied by operating the instrument in the classical mode (H
This study has shown that two complementary ionization modes can be used for
the detection of organic nitrates:
The NO The protonation mode in absence of the RF mode, for an
Although slight variations appear in the optimization of the operational
conditions for each type of compounds, it has been observed that a few unit
shifts of the
From this study, we now better understand how to run PTR-MS to allow the detection of individual organic nitrates during lab studies and this will be very useful for a number of research groups working on mechanisms studies which are equipped with this instrument. In particular, the operational mode of the instrument has been optimized for the detection of different types of organic nitrates and the ionization pattern (fragmentation, adduct formation, charge transfer, etc.) of these species is much better understood now.
A perspective of this work was to test the detection of organic nitrates in ambient air with this method. From the detection limits observed in this study, we can expect that accumulations over longer periods will be necessary to decrease the detection limits.
A crucial aspect to be taken into account in further studies for lab and field measurements using this method is the effect of the humidity of the sampled air. In addition, longer acquisition time, elimination of interfering ionization paths by selective ionization sources and softer ionization sources could improve the technique's performance.
Data available upon request.
The authors declare that they have no conflict of interest.
This work was supported by the French National Agency for Research (Project ONCEM-ANR-12-BS06-0017-01), by the European Union's Horizon 2020 research and innovation programme through the EUROCHAMP-2020 Infrastructure Activity under grant agreement no. 730997 and by the Région Ile de France. The authors also thank Fraiser Reich and Kore Company for their advices in the use of the PTR-MS. Edited by: D. Heard Reviewed by: two anonymous referees