Knowledge about mass discrimination effects in a chemical ionization mass
spectrometer (CIMS) is crucial for quantifying, e.g., the recently discovered
extremely low volatile organic compounds (ELVOCs) and other compounds for
which no calibration standard exists so far. Here, we present a simple way
of estimating mass discrimination effects of a nitrate-based chemical
ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass
spectrometer. Characterization of the mass discrimination is achieved by
adding different perfluorinated acids to the mass spectrometer in amounts
sufficient to deplete the primary ions significantly. The relative
transmission efficiency can then be determined by comparing the decrease of
signals from the primary ions and the increase of signals from the
perfluorinated acids at higher masses. This method is in use already for
PTR-MS; however, its application to a CI-APi-TOF brings additional
difficulties, namely clustering and fragmentation of the measured compounds,
which can be treated with statistical analysis of the measured data, leading
to self-consistent results. We also compare this method to a transmission
estimation obtained with a setup using an electrospray ion source, a high-resolution differential mobility analyzer and an electrometer, which
estimates the transmission of the instrument without the CI source. Both
methods give different transmission curves, indicating non-negligible mass
discrimination effects of the CI source. The absolute transmission of the
instrument without the CI source was estimated with the HR-DMA method to
plateau between the
The nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer has been established as an important
tool for atmospheric sciences over the last years. It is capable of
measuring sulfuric acid down to levels of 5
The theoretical framework for deriving concentrations of a certain compound
from ion count rates for the CI-APi-TOF is essentially the same as given by
Eisele and Tanner (1993) for sulfuric acid measurements or by Hansel et al. (1995) for a PTR-MS. Ionization of a compound X via
primary ions P
In this study we show the application of the method of depleting primary ions to the nitrate-based CI-APi-TOF for obtaining transmission curves. We describe the measurement setup and the used compounds as well as the statistical analysis that became necessary due to the high level of clustering of compounds inside the CI-APi-TOF. Additionally we compare this method with a second approach for transmission estimations that uses a high-resolution differential mobility analyzer and an electrometer.
Various parts of the time-of-flight mass spectrometer (Tofwerk AG, Thun,
Switzerland) can cause mass discrimination effects, especially the two
quadrupole units in the APi interface, the orthogonal extraction unit in the
TOF and the multi-channel plate (MCP) detector. The CI source itself
(Kürten et al., 2011) may also cause mass discrimination effects, as
several negative voltages are applied to the source to guide the ions to the
pinhole, which marks the entrance of the vacuum region of the instrument.
After the pinhole, three differentially pumped chambers are installed, two of
which contain a segmented quadrupole operating in RF-only mode, and the last
one contains an Einzel lens system. The main task of these transfer units is
to guide and focus the ion beam to the high vacuum TOF region. Quadrupoles in
RF-only mode act as a high pass filter for ions as the trajectories of ions
below a certain
The orthogonal extraction unit of a TOF shows a systematic discrimination of
lower
Also the multi-channel plate detector in the TOF can show mass discrimination effects. Müller et al. (2014) showed for a PTR-TOF, which utilizes the same MCP as the TOF in this study (PHOTONIS Inc., Sturbridge, MA, USA), that the mass discrimination can change due to aging of the detector and is strongly dependent on the operating voltage of the MCP. Especially at low gain voltages, higher masses are discriminated against more strongly. Gilmore and Seah (2000) showed that the mass discrimination effects of an MCP also vary with the total impact energy of the ions, which is defined by the post accelerating voltage in a TOF. This study found a reduced detection efficiency for heavier ions at lower impact energies.
Finally, even the smallest changes of the inner geometry of the instrument, e.g., caused by shipping, can affect ion beam alignment and thus cause different transmission characteristics. This variety of effects makes it almost impossible to calculate the transmission efficiency, so methods for a measurement of this property are crucial.
Compounds used for HR-DMA method with according monomer, dimer, and trimer. Trimers were not always quantifiable as the HR-DMA also selected multiply charged higher clusters when adjusted for trimers (indicated by italics). This was also the case for the tetradodecylammonium bromide dimer.
The development of high-resolution differential mobility analyzers (HR-DMA)
led to the ability of classifying particles and clusters down to a size of
around 1 nm (Steiner et al., 2010; Fernández de la Mora et al.,
2013).
As this corresponds to a mass range the CI-APi-TOF is able to measure, these
two instruments can be combined for a characterization of the latter. Here, a
version of the so called “Vienna-type” DMA was used (Steiner et al., 2010).
It is a closed loop, medium flow (up to around 700 slpm) size classifier,
with a maximum resolution power of around 20. The resolution power is derived
from the ratio of the electrical mobility
Schematic setup for the three types of experiments conducted.
Measuring only in the configurations given in Fig.1a and b yields the
total transmission curve of the whole setup including tubing effects. For
technical reasons the sampling line to the TOF did not have the same length
as the line to the EM, therefore the losses of ions in the lines cannot be
assumed to be identical. Additionally, the sampling line of the TOF
increases in diameter from 6 mm to
The basic idea of the depletion method is to add different substances to the
instrument one after the other in amounts that lead to a significant
decrease in primary ions (Steinbacher et al., 2004; Huey et al., 1995).
Charge provided by the primary ions is therefore shifted to the
Compounds used for depletion method. The perfluorinated acids only
differ in the number of CF
The saturation method works fine for a PTR-MS as this technique is a rather
soft ionization method and there is almost no clustering of a substance with
itself. Therefore the shift of charge from the
The measurement setup has to ensure that sufficient amounts of the desired
compound enter the CI source of the instrument to deplete the primary ions,
but on the other hand one has to be careful not to put too much into the
system to avoid long term contamination. One way to achieve this is to
inject a gas sample from the headspace of the desired compound with a needle
into the sample flow of the instrument through a rubber sealing. This works
in principle, but gives only very short signal spikes as the volume in the
needle is quickly diluted. A better approach is shown in Fig. 1c), where a
Spectra for all compounds used for depletion mode experiments.
Primary ions are NO
The spectra for the different compounds are shown in Fig. 3. It is evident
that the charge provided by the primary ions is not only shifted to one
distinct
To obtain time series of the data we used the MATLAB based tofTools
software. For the monomer intensities of the perfluorinated acids and the
silane compound, all monomer peaks were added up without any further
correction for transmission. The measurement data from the heating
experiments with perfluorinated heptanoic and nonanoic acid (Fig. 4) show a
decrease of the primary ion signal (sum of NO
Measurement examples for
However, as the charge from the primary ions is not only transferred to one
distinct mass, but distributed over a wider range, we cannot apply a linear
fit to the data. We therefore introduce a relation that combines the
measured signal with the theoretical total amount of charge in the system
via weighting factors, which will lead to relative transmission factors:
The relative uncertainty was estimated via the 95 % confidence bonds of
the regression analysis. Additional uncertainty for the relative
transmission factors comes from possible fragmentation of larger clusters to
primary or monomer ions. However, as fragmentation to primary ions
implicates that their signal cannot be fully depleted, and during the
strongest heating the primary ion level goes, e.g., below 4 and 5 % of the
original level for heptanoic and nonanoic acid, respectively, we use 5 %
as upper limit for fragmentation to primary ions. For fragmentation to
monomer ions, we compare the highest dimer signal to the monomer signal at
the same point of time (the right peak of the monomers can be excluded for
this, as no dimer contains NO
We have obtained two transmission curves for the HR-DMA method and one for
the depletion method, which are shown in Fig. 5. The curves were obtained by
applying least-squares fits to a 2-fold Gaussian distribution for the APi-,
CI-mode and depletion data. To ensure reasonably good representation of the
measured data, we had to set lower limits for the two parameters in the
denominator of the exponent (200 for the CI-mode and depletion method and
350 and 500 for the APi-mode fit). The overall transmission for the APi mode
is in the range of 1 % with a steep increase from
The results for the relative transmission efficiency estimated with the
depletion method are shown in Fig. 5b. The agreement between the individual
perfluorinated acids and the silane compound is remarkably good. There is a
clear maximum of transmission efficiency in the range around 600 Th.
Especially the
If we use the results of the regression analysis for the individual acids and correct the corresponding time traces in Fig. 4 for transmission, their sum should yield a constant level over the duration of the experiment. This is the case for both acids shown in Fig. 4 (light blue line). The constant signal confirms our method and represents now the constant ion production in the CI source.
The shape and the position of maximum transmission differ from the two
curves obtained by the HR-DMA method. These differences may be attributed to
mass discrimination effects of the CI source because the HR-DMA method does, in
principle, not account for this. The CI setup shown in Fig. 1b tried to
simulate these effects, but for the price of introducing an additional
potential step of
We have presented an easy-to-use and straight-forward way of estimating the
mass-dependent transmission efficiency of a nitrate-based CI-APi-TOF via
the depletion of primary ions. This method is in use already for PTR-MS, but to
our knowledge has not been applied to a nitrate-based CI-APi-TOF before. The
additional difficulty compared to PTR-MS is a strong clustering of the
tested substances, which is treated with statistical analysis of the
measurement data. We used different perfluorinated acids and one silane
compound to estimate relative transmission factors over the
Here we present the detailed settings of the CI-APi-TOF used in this study. The settings were applied for the depletion method as well as for HR-DMA experiments.
Voltage and flow settings of the CI source. The basic design of
the CI source is given in Kürten et al. (2011); however, the HNO
Voltages and frequencies in the APi section.
Voltages and pressures in the TOF section (operated in negative high sensitivity mode).
We thank Jay Slowik for the fruitful discussion. We also thank the tofTools team for providing tools for mass spectrometry analysis. This work was funded by: EC Seventh Framework Programme (Marie Curie Initial Training Network, MC-ITN “CLOUD-TRAIN” no. 316662), German Federal Ministry of Education and Research (project no. 01LK1222A), the University of Innsbruck Research Grant for Young Scientists and the Austrian Science Fund, FWF (grant no. P27295-N20). Edited by: F. Stroh