Nitrogen oxides as dopants for the detection of aromatic compounds with ion mobility spectrometry
Nitrogen oxides as dopants for the detection of aromatic compounds with ion mobility spectrometry
Urszula Gaik 0 1
Mika Sillanpää 0
Zygfryd Witkiewicz 0 1
Jarosław Puton 0 1
0 School of Engineering Science, Laboratory of Green Chemistry, Lappeenranta University of Technology , Sammonkatu 12, 50130 Mikkeli , Finland
1 Institute of Chemistry, Military University of Technology , Kaliskiego 2, 00-908 Warsaw , Poland
Limits of detection (LODs) in ion mobility spectrometry (IMS) strictly depend on ionization of the analyte. Especially challenging is ionization of compounds with relatively low proton affinity (PA) such as aromatic compounds. To change the course of ion-molecule reactions and enhance the performance of the IMS spectrometer, substances called dopants are introduced into the carrier gas. In this work, we present the results of studies of detection using nitrogen oxides (NOx) dopants. Three aromatic compounds, benzene, toluene, toluene diisocyanate and, for comparison, two compounds with high PA, dimethyl methylphosphonate (DMMP) and triethyl phosphate (TEP), were selected as analytes. The influence of water vapour on these analyses was also studied. Experiments were carried out with a generator of gas mixtures that allowed for the simultaneous introduction of three substances into the carrier gas. The experiments showed that the use of NOx dopants significantly decreases LODs for aromatic compounds and does not affect the detection of compounds with high PA. The water vapour significantly disturbs the detection of aromatic compounds; however, doping with NOx allows to reduce the effect of humidity.
Ion mobility spectrometry; Dopants; Aromatic compounds; Nitrogen oxides
Ion mobility spectrometry (IMS) is a fast, simple and sensitive
analytical technique used for the investigation of gaseous
samples. The principle of this method is ions separation based on
differences in their movements in the electric field [1, 2]. The
range of applications of IMS is very wide [2–12] thanks to its
many advantages such as a short analysis time, accuracy, low
concentrations detectability, low costs of use or the possibility
of performing real-time analysis  without the necessity of
samples transportation to the laboratory.
IMS method sensitivity and limit of detection (LOD) are
strictly related to analyte ions generation taking place in
the reaction region of the spectrometer. Ionization
processes depend on the composition of drift gas, temperature and
the construction of a reaction region. The introduction of
some substances, called dopants, to the gases flowing
through the detector allows ion-molecule reactions
occurring in the spectrometer to be controlled with high
effectiveness [14, 15]. Dopant molecules form so-called
alternative reactant ions, which interact with the analyte in a
different way than the ions present in the pure carrier gas.
Choosing a proper dopant allows for the generation of
more stable ions containing the analyte molecule  or
an increase in selectivity by shifting peaks in the drift time
spectrum . The most well-known use of dopants
consists of inhibiting the ionization of substances which
disturb the analyte detection in positive mode of IMS. For this
purpose, ketone dopants, which are compounds of
relatively high proton affinity (PA), are used very often [10, 18].
The reactant ions formed by these dopants molecules do
not ionize interfering compounds molecules of low PA
values. Therefore, better selectivity is accomplished by
the significant elimination of cross-sensitivity effects.
There is also a quantitative aspect of the use of dopants.
Their presence may affect the detection efficiency and
LOD. It was stated, however, that in the detection of
organophosphorus compounds (OPC) using ketones as
dopants, the detection sensitivity does not depend on the
presence and concentration of the dopant [19, 20].
The detection of aromatic compounds like benzene,
toluene or xylene (BTX) in the low concentration range
is a very important analytical problem. This kind of
aromatic compounds is commonly used in many branches of
industry and is a serious threat to human health. The
application of IMS in the effective detection of these
substances is difficult, especially in cases where atmospheric
pressure chemical ionization (APCI) is used to ionize the
analyte molecules. The relatively low PA of the
substances is considered the main reason for the low
effectiveness of the detection of aromatics in positive mode of
IMS . Therefore, there have been attempts to apply
detectors with UV ionisation sources [22, 23], where
analyte ions are generated in the photoionization process.
For some aromatic compounds, it is also possible to carry
out measurements in negative mode of IMS. One of them
is toluene diisocyanate (TDI) [24, 25], which is highly
toxic and used in the production of polyurethane foams.
The lack of way to effectively ionize the aromatic
compounds by proton transfer reactions does not entirely
limit the possibility of generating stable positive ions for
such analytes. Recently, it was demonstrated that the
efficient ionization of aromatic compounds occurs in
IMS detectors equipped with corona discharge (CD)
ionization sources [26, 27]. Types of positive reactant ions
from CD ionization sources are similar to those
generated by radioactive sources. Nevertheless, the amounts of
specific ions are definitely different. The reason for this
is the generation of nitrogen oxides (NOx) between
electrodes of the corona discharge ionization source .
The nitrogen monoxide (NO) reacting with products of
air ionization is transformed to nitrosonium ion via the
following reactions :
Nitrogen monoxide is present in trace amounts in air or
nitrogen used as drift gases. A small peak representing the
signal generated by NO+ ions is therefore also observed in
the drift time spectra obtained from measurements carried
out with radioactive ionization IMS detectors. Due to the
conditions in reaction regions of the IMS detectors, ions produced
via reactions (1) and (2) are hydrated and form clusters with
the chemical formula NO+(H2O)n. In corona discharge
detectors, these ions might be dominant reactant ions, thus
providing the opportunity for ionization through mechanisms other
than proton transfer . If the analyte molecule’s ionization
energy (IE) is lower than 9.26 eV (the IE of nitrogen
monoxide), then the charge transfer is possible:
Sufficiently low IE characterizes many organic analytes,
among them are most of the aromatic compounds .
However, if the ionization energy of the analyte is higher than
that of nitrogen monoxide, then sample ions may be produced
by forming adducts (Eq. 4) or by hydride abstraction (Eq. 5):
M þ NOþ→ðM‐HÞþ þ HNO
The application of NO+ ions allows the ionization of many
kinds of analytes; the method of ionization, however, depends
on the properties of the molecules.
The CD ionization source employed in IMS detectors may
play a dual role. First, as for other ionization constructions, it
produces an electric charge which is used to sample ions
production and second, the nitrogen monoxide generated might
be applied as a reaction gas with a beneficial effect on the
sensitivity and selectivity of detection. Darzi and Tabrizchi
 proposed the use of the chamber with the CD electrodes
in the gas line before the inlet to the detector. This construction
was solely designed for the efficient production of nitrogen
monoxide. The advantage of such solution is the possibility of
introducing the dopant without attaching gas cylinders or any
other sources of NO. Moreover, it allows the dopant
introduction to be switched on and off quickly. The application of CD
sources as a nitrogen monoxide generator also has some
limitations. It is not possible to produce stable and controlled
levels of dopant concentrations in the drift gas. Attempts to
add nitric oxide from gas cylinders into the carrier gas were
carried out at an early stage of research on IMS . It has
been found that for the ionization of n-octane and
2chloronitrobenzene, the reactivity of NO+ ions is higher than
that of hydronium ions. Investigations of influence of nitrogen
monoxide concentrations on the detection of 2,4-lutidine,
ditert-butyl-pyridine (DTBP) and dimethyl methylphosphonate
(DMMP) were carried out by Eiceman et al. . NO was
added to the drift gas from the gas cylinder and an IMS
detector was coupled with a gas chromatograph and mass
spectrometer. The most crucial result of this research was the
demonstration that there are several mechanisms of ionization
when NO+ is used.
The purpose of this research is to investigate quantitative
dependencies of the effect of nitrogen oxides on the detection
of three chosen aromatic compounds. The analysis of two
compounds of high PA (DMMP and triethyl phosphate
(TEP)) as a comparison was also carried out. The
measurements were conducted using a spectrometer equipped with a
radioactive ionization source. Nitrogen oxides used as dopants
were introduced to the system from gas cylinders. This
provided sufficient accuracy for the quantitative determination of
NO and NO2 concentrations.
Gas generator and IMS detector
The measuring system applied in our research was adapted to
produce gas mixtures containing two components, i.e. analyte
and nitrogen monoxide or dioxide as a dopant. Moreover, in
some of the measurements, the water vapour was also added.
The scheme of the gas generator is presented in Fig. 1.
The analyte vapours were generated with permeation
standards placed in the thermostatic container tmst1. The
standards consisted of glass vials sealed with a polymer
membrane. The material and the thickness of the membrane were
chosen carefully to provide source emission, allowing
measurements to be carried out in a dynamic range of detector
response. The gas containing analyte vapours was mixed with
additional stream of pure air in a system of single dilution. Gas
flows were monitored using 7 mass flow controllers (mfc)
5850 series (Brooks Instruments). Both nitrogen monoxide
and dioxide were introduced from gas cylinders. NO was
added through the mass flow controller mfc5. The NO2
concentration in the gas cylinder was too high, therefore an
Fig. 1 Scheme of the pneumatic
system for the generation of gas
mixtures. Abbreviations: flt—
filter, mfc—mass flow controller,
psr—permeation source, mix—
additional dilution was necessary. Water vapour was added
to the gas from an open vial placed in the thermostatic
container tmst2. The emissions of an analyte as well as water
vapour were determined based on weight loss of the sources.
The concentrations of all components of the carrier gas were
calculated using emission values and gas flow balances.
Parameters of the IMSD-B detector used for this research
and a schematic of its reaction section are presented in . It
is a precise instrument dedicated for laboratory purposes. The
device is equipped with radioactive ionization source 63Ni
(300 MBq) and 6.1 cm long drift region.
The measurements were carried out at several different
temperatures of the IMS detector. Toluene, benzene and
DMMP detections doped with NO were conducted at 80 °C,
and toluene detection doped with NO2 at 120 and 80 °C.
Measurements of TDI with NO admixture were performed
at 60, 90 and 120 °C. TEP with NO2 introduced to the carrier
gas was tested at 80 °C. Detections of DMMP and toluene
without dopants were carried out at 60 °C.
In all investigations, the carrier and drift gas was air purified
and dried with molecular sieves 13× (Alfa Aesar) placed in a
2-L volume container. Analytes, i.e. benzene and toluene
(Chempur), TDI and TEP (Sigma Aldrich), and DMMP
(Alfa Aesar) were at least of analytical purity. Gas mixtures
containing nitrogen monoxide (20 ppm) and nitrogen dioxide
(500 ppm) were obtained from Air Products.
Results and discussion
There are three kinds of reactant ions in the positive mode of
IMS (see Fig. 2a). The peak of greatest intensity observed in
the drift time spectrum is generated by (H3O+)H2On ions. In
much smaller amounts, there are also NH4+ and NO+ ions
present. Usually, they are also hydrated. Under normal
conditions in the IMS detector, all of the mentioned ions are
hydrated. Introducing substances with high PA to the carrier gas
causes the creation of ion products containing one or two
molecules of the analyte. All three kinds of reactant ions
participate in the ionization process, which results in an even
decrease of peaks amplitudes with an increase of the analyte
concentration. A typical example of the drift time spectrum
showing this scheme of ionization was recorded for the
detection of DMMP (Fig. 2a). The investigation of many aromatic
compounds has shown that these chemicals behave
differently. As an example, the results registered for increasing
concentration of toluene can be shown (Fig. 2b). Decline of the
hydronium reactant ion peak can be clearly observed only for
concentrations higher than 100 ppm. Simultaneously, those
concentration levels of toluene result in the complete
disappearance of the NO+ ions peak. Thus, it can be concluded that
the ionization of toluene molecules with nitrosonium ions is
much more effective than for hydronium ions. The purpose of
further research was to determine the quantitative
characteristics of the detection of chosen analytes in the presence of
nitrogen monoxide and nitrogen dioxide added intentionally
to the carrier gas.
Drift time spectra measured for different concentrations of
NO and NO2 are shown in Fig. 3. Spectra obtained for both
dopants appear very similar. Minor differences are related to
the ammonium ions peak. The presence of a nitrogen
monoxide and nitrogen dioxide admixture decreases the height of the
Research on the detection of toluene, benzene, TDI,
DMMP and TEP using nitrogen oxides as the carrier gas
dopants in the positive mode of IMS was conducted. The
influence of NO and NO2 admixtures on the drift time
spectra recorded for toluene, benzene and TDI is shown
in Fig. 4. In the measurements carried out without
dopants, ions generated as a result of interaction with NO+ are
observed for all analytes. Except for the case of benzene,
the peaks of the analytes ionized using hydronium ions
are also visible. After the introduction of dopants, the
peaks corresponding to the analyte ions produced by
alternative reactant ions are significantly higher than those
obtained with pure carrier gas. In the case of toluene and
benzene, these peaks are shifted towards higher drift times
compared to the ion peaks generated from hydronium
ions. This may indicate that the corresponding analyte
ions are characterized by a greater mass and collision
cross-section. Therefore, it is possible that this type of
ionization of analytes occurs through the reaction of
Fig. 2 Drift time spectra of DMMP (a) and toluene (b) measured for the
carrier gas without the dopant at 60 °C
adduct formation between the analyte molecule and the
dopant ion. The described shift causes a complete
separation of the peaks derived from the reaction of hydronium
ions and ions of the analyte. This allows more accurate
determinations of the tested substances to be conducted
and, in the case of benzene, its detection to be performed
because the values of the mobilities of hydronium ions
and the benzene ions generated by them are very similar.
Furthermore, because of the low proton affinity of this
analyte, its ionization by hydronium reactant ions is very
inefficient. Studies of toluene detection using nitrogen
dioxide introduced to the carrier gas were also carried
out. The influences of two admixtures are compared in
Fig. 4b. It is interesting that the position of the peaks
corresponding to the ions containing toluene molecules
is the same for both dopants.
Ionization of TDI using nitrogen oxides is different from
previous cases and the created peak is shifted towards the
shorter drift times. This means that the product ions exhibit
higher mobility than the ions produced using (H3O+)H2On
ions, which generally also indicates a lower collision
crosssection and/or weight of such ions. The ionization mechanism
in this case probably differs from that for benzene and toluene.
Two kinds of product ions are observed in the drift time
spectra measured for TDI. The ions of lower mobility, with the
drift time of 14.2 ms, are dominant for ionization without a
NO dopant. Thus, it can be assumed that the fragmentation of
a compound occurs when detection with IMS is carried out
using nitrogen oxides as the carrier gas dopants.
The influence of nitrogen oxides as dopants on the
detection of compounds with high proton affinity, such as DMMP
or TEP, which are easily ionized by standard reactant ions,
was also investigated. DMMP detection was conducted using
nitrogen oxide, and TEP using nitrogen dioxide. The results of
these studies are shown in Fig. 5a, b, respectively. The drift
time spectra are presented in such a manner that allows to
show the set of peaks with the same mobilities and intensities
to be shown which are observed irrespective of the dopant
presence in the carrier gas. Therefore, it can be concluded that
the efficiency of ionization and thus the detection of these
analytes is not affected by the use of nitrogen oxides dopants.
The results of research conducted by Eiceman et al.  using
IMS coupled with mass spectrometry (IMS/MS) confirmed
that both the analyte monomer and dimer ions have the same
mass, regardless of which reactant ions were used (NO+ or
Fig. 4 Drift time spectra of benzene measured in the presence of NO at
80 °C (a), toluene measured in the presence of NO and NO2 at 80 °C (b),
and TDI measured in the presence of NO at 60 °C (c). Peaks marked 1, 2
and 3 in the panel a correspond to (NH4+)H2On, (NO+)H2On and
(H3O+)H2On ions, respectively
Fig. 5 Drift time spectra of DMMP measured in the presence of NO (a),
and TEP measured in the presence of NO2 (b) at 80 °C. Peaks marked 1, 2
and 3 in the panel a correspond to (NH4+)H2On, (NO+)H2On and
(H3O+)H2On ions, respectively
(H3O+)H2On). It is very interesting that the same analyte ions
are obtained for different reaction mechanisms available for
ionization by these two kinds of reactant ions. A similar
phenomenon has already been observed for DMMP and TEP
detected in the presence of ketone dopants . However, this
only concerned the conservation of dimer ions peaks
intensities, while the monomer ions peaks were different depending
on whether ketone dopants or pure carrier gas were used. In
the case of NOx doping, the peaks of both the monomer and
the dimer ions of the analyte are the same.
Quantitative assessment of the effectiveness of doping with
nitrogen oxides is possible by comparing the calibration curves.
Selected calibration curves are presented in Fig. 6. In general,
calibration dependencies for IMS are non-linear. The limited
linear dynamic range of the IMS detectors can cause the
difficulties in quantitative analyses . In the case of toluene, the
signal from MH+ monomer ions, generated by hydronium ions,
is small over the entire range of analyte concentrations. In
contrast, the signal from toluene ions produced by the alternative
reactant ions, regardless of whether nitrogen oxide or dioxide
was applied, is much higher. Despite the lack of measurements
for comparable concentrations of admixtures, it can be seen that
doping with nitrogen dioxide may be less effective.
Interesting conclusions can be drawn based on the set of
calibration curves plotted for TDI measurements carried out at
different temperatures (Fig. 6c, d). Irrespective of whether the
dopant is used, a better sensitivity of the measurements of TDI
carried out at higher temperatures is observed. For the
Fig. 6 Calibration dependencies
for toluene measured at 80 °C in
the presence of NO (a) and NO2
(b), TDI measured in the presence
of NO at 60 °C (c) and 120 °C (d)
and DMMP measured at 80 °C in
the presence of NO. Dashed lines
in e are for pure air and solid for
doped carrier gas
temperature of 120 °C, the addition of nitric oxide dopant only
slightly increases the IMS detector signal. At lower operating
temperatures of the detector, when nitrogen oxide is applied,
the increase of sensitivity is observed in comparison to the
measurements conducted with pure carrier gas. However, the
beneficial effect of dopant decreases with increasing
temperature. Whereas a significant improvement in sensitivity when
using a pure carrier gas, which is observed at higher
temperatures, can be caused by lower hydration of hydronium ions at
higher temperatures. This happens because the phenomenon
of attaching the subsequent water molecules to hydronium ion
clusters is observed at lower temperatures [35, 36]. Moreover,
the proton affinity of water molecules is 691 kJ/mol ;
however, hydronium ions available in the IMS detector are
in the form of hydrated clusters , and the effective proton
affinity of these ions cores is much higher. In order to calculate
its value, it is necessary to add the enthalpies of subsequent
attachments of water molecules to the PA of neutral molecules
[34, 37, 38]. In effect, the PA of hydronium ions cores at lower
temperatures is higher, and therefore, the proton transfer
reaction is possible only in the case of analytes with a very high
PA. On the contrary, at higher temperatures, when the reactant
ions are less hydrated and the corresponding PA of neutral
molecules is lower, the ionization via proton transfer reaction
is also possible for analytes with lower values of proton
affinity. This effect was observed also for other chosen analytes,
however in smaller scale than in the case of TDI.
The calibration curves plotted based on results of
measurements of DMMP (Fig. 6e) confirm the observation that the use
of nitrogen oxides as dopants to the carrier gas in IMS for the
detection of compounds with relatively high PA, like
organophosphorus compounds, does not affect the efficiency of the
ionization of this type of analyte, and thus their detection with
ion mobility spectrometry. Very similar results were obtained
for the measurements of TEP and NO2 dopant.
To evaluate the effect of humidity on the effectiveness of
doping with nitrogen oxide, the study in which the mixed gas
introduced into the detector contained the analyte, NO
admixture and water vapour was conducted. Measurements were
carried out at 80 °C and did not have the quantitative
character, but the concentrations of all substances in the respective
measurements were maintained at around 12 ppm for NO,
20 ppm for toluene and 250 ppm for water vapour.
Comparison of the drift times spectra recorded for these
measurements is given in Fig. 7. The introduction of water vapour
immediately suppresses the NO+ peak in the drift time
spectrum, irrespective of whether nitrogen oxides were contained
in the pure carrier gas or introduced from the cylinder. This
disturbs the detection of toluene in the tests carried out without
the use of a dopant. Its addition, however, although the water
vapour is present in the carrier gas, enabled the detection of
the analyte. The corresponding peak in the drift time spectrum
is much smaller than in measurements carried out without
Fig. 7 Drift time spectra registered for studies of influence of humidity
on detection with IMS carried out with use of NO at 80 °C
water addition. The results are promising and will form the
basis of further research conducted by the authors.
The application of NOx admixtures to the carrier gas in IMS is
interesting from an analytical point of view. The use of these
dopants in positive mode can significantly increase the
sensitivity of the measurements which has great importance,
especially in the case of aromatic compounds analyses. The
sensitivity enhancement effect is observed for those compounds
which are not easily ionized by proton transfer reactions, i.e.,
in case of this research, benzene, toluene and TDI. In higher
temperatures, the ionization processes, with use of both,
nitrogen oxides ions and standard hydronium ions, are more
effective, and thus, the increase of sensitivity is achieved.
Experiments with dopants introduced from the gas cylinder
into the carrier gas showed that the results obtained with the
addition of NO are similar to those obtained using NO2. The
reactions of ionization with NO+ ions are very well described in
many publications. In the case of NO2+, there is little
information available in literature on their participation in ion-molecule
reactions. The results of our research could not explain the
course of ionization with NO2+; therefore, there is a need of
carrying out more profound investigations. It is very well
known and generally observed behaviour that the presence of
water vapour significantly decreases the sensitivity. However,
the use of nitrogen oxides as admixtures to the carrier gas in
IMS allows the observed effect of humidity to be reduced.
The presence of nitrogen oxides does not affect the
detection of substances characterized by relatively high
PA. It was shown for organophosphorus compounds such
as DMMP or TEP.
The use of these dopants may pose a risk related to
introducing nitrogen oxides into the instrument; however, this
destructive effect has not been investigated.
Acknowledgements This work has been financially supported by
Military University of Technology [grant numbers RMN/08-787/2016
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