02-The TRAPS Apparatus_ Enhancing Target Density of Nanoparticle Beams in Vacuum for X-ray and Op

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The TRAPS Apparatus: Enhancing Target Density
of Nanoparticle Beams in Vacuum for X-ray and
Optical Spectroscopy
J. Meinen, S. Khasminskaya, E. Rühl, W. Baumann & T. Leisner
To cite this article: J. Meinen, S. Khasminskaya, E. Rühl, W. Baumann & T. Leisner (2010)
The TRAPS Apparatus: Enhancing Target Density of Nanoparticle Beams in Vacuum for
X-ray and Optical Spectroscopy, Aerosol Science and Technology, 44:4, 316-328, DOI:
10.1080/02786821003639692
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Aerosol Science and Technology, 44:316–328, 2010
Copyright © American Association for Aerosol Research
ISSN: 0278-6826 print / 1521-7388 online
DOI: 10.1080/02786821003639692
The TRAPS Apparatus: Enhancing Target Density of
Nanoparticle Beams in Vacuum for X-ray and Optical
Spectroscopy
J. Meinen,1,2 S. Khasminskaya,2 E. Rühl,3 W. Baumann,4 and T. Leisner,1,2
1Institute for Meteorology and Climate Research, Aerosols and Heterogeneous Chemistry in the
Atmosphere (IMK-AAF), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
2Institut for Environmental Physics (IUP), Atmosphere and Remote Sensing, Ruprecht-Karls-Universität
Heidelberg, Heidelberg, Germany
3Physical Chemistry, Institute for Chemistry and Biochemistry, Freie Universität Berlin, Berlin, Germany
4Institute for Technical Chemistry, Thermal Waste Treatment (ITC-TAB), Karlsruhe Institute of
Technology (KIT), Karlsruhe, Germany
We present an experimental setup that allows the injection of
charged nanoparticles in a diameter range of 3–15 nm into a vac-
uum chamber and their storage there in an electrodynamic cage.
The nanoparticle density in the trap is limited by space charge
and can be several orders of magnitude higher than in a free
nanoparticle beam. The setup provides for the first time a tool
for the application of advanced techniques of spectroscopy to free
nanoparticles in this size range. It consists of a combination of (1)
a plasma discharge nanoparticle source that generates a high den-
sity of nanoparticles of various composition suspended in helium
carrier gas at a pressure of about 10–150 mbar, (2) an aerodynamic
lens optimized for small particles (diameter 3–15 nm) that forms a
well-collimated beam of charged nanoparticles and focuses it into
(3) an octopole ion trap operated at low frequencies and filled with
helium buffer gas at 10−2 mbar in order to moderate and store the
nanoparticles at densities of more than 107 cm−3.
1. INTRODUCTION
Nanoparticles in the 5–50 nm diameter range have found
widespread application in science and technology ranging from
atmospheric science over quantum computing applications to
pharmaceutical formulations (Shaw and Liu 2008). Their prop-
Received 5 August 2009; accepted 5 January 2010.
We wish to thank V. Szabo for support with the design and buildup of
the microwave plasma particle source. We also give thanks to ANSYS
and CADFEM for their support and fruitful help with the CFD code
design and Johan Söderström for working with us on the simulation of
aerodynamic lenses. This work was supported through the framework
of FSP 301—FLASH (Förderkennzeichen 05 KS7VHA) of the German
Federal Ministry for Education and Research, Bonn, Germany.
Address correspondence to J. Meinen, Institute for Meteorology
and Climate Research, Aerosols and Heterogeneous Chemistry in the
Atmosphere (IMK-AAF), Karlsruhe Institute of Technology (KIT),
Karlsruhe, Germany. E-mail: Meinen@KIT.edu
erties are often strongly size dependent and in many cases a
result of quantum confinement effects (Kruis et al. 1998). In
many natural condensation processes, the initial embryo parti-
cles are in the 10 nm diameter range. Such nanoparticles are,
therefore, of considerable interest in atmospheric science. More
specifically, nanoparticles formed by the condensation of evap-
orating meteorite material are thought to constitute the nuclei
for the formation of noctilucent clouds in the mesosphere (e.g.,
Gadsen 1982; Rapp and Lübken 2004).
Many of the interesting properties of nanoparticles are only
observable in isolated particles, as the interaction with matrices,
surfaces, or among themselves strongly modifies the optical and
chemical properties of the particles, even if insolating surfac-
tants are used for stabilization. Individual trapped nanoparticles
with diameter >100 nm have recently been characterized with
synchrotron light (Grimm et al. 2006). Smaller nanoparticles
in the gas phase are much more difficult to assess experimen-
tally due to the inherently low target density (Wang et al. 2006;
Ziemann et al. 1995). The lifetime of d = 10 nm airborne
nanoparticles with respect to coagulation and coalescence pro-
cesses falls below 10 s if their density exceeds 108 cm−3. In
order to avoid diffusion driven loss, beams of nanoparticles in
the vacuum have been prepared by expanding aerosol through a
nozzle (Shu et al. 2006b). The particle density in vacuum beams
is much lower than in the primary aerosol, since the gas vol-
ume expands as the pressure drops. If an aerosol containing
106 particles per cm3 is expanded from ambient pressure into
a vacuum of 10−4 mbar, the particle density at the position of
the experiments is limited by the gas expansion to roughly 0.1
cm−3. The conditions become less favorable, if better vacuum or
high target density is required. Therefore, such beams are often
combined with aerodynamic focusing (e.g., Shu et al. 2006a).
Usually, this focusing is limited to the size regime above 50
nm (Shu et al. 2006b). A combination of aerodynamic focusing
316
THE TRAPS APPARATUS 317
of nanoparticles into an ion guide similar to our concept was
described earlier (Wang et al. 2006) and used for nanoparticle
analysis with laser ablation mass spectrometry.
In this contribution, we describe an experimental setup de-
signed to generate very high density targets of very small gas
phase nanoparticles with 3–15 nm diameter, which is suitable for
optical and X-ray spectroscopy. The high density is achieved by
combining (1) microwave driven gas phase synthesis of nanopar-
ticles under low pressure conditions, (2) efficient aerodynamic
focusing of the particles into the vacuum chamber, and (3) mod-
eration and storage of the nanoparticle beam in a gas-filled
octopole ion cage. With the combination of these stages, the
target density could be increased by about seven orders of mag-
nitude compared to state-of-the-art particle production and free
expansion from ambient pressure as assumed in the example
above. The assembly presented here forms a part of the TRAPS
(Trapped Reactive Atmospheric Particle Spectrometer) appara-
tus which is currently under development for spectroscopy of
free nanoparticles under the well-controlled conditions of the
mesosphere. The article is organized as follows: In section 2 the
setup of the three main components of theapparatus is described
and results from simulation calculations that led to their respec-
tive design are presented. In section 3, first benchmarks results
on the performance of the setup are shown and compared to prior
expectations. An outlook on the potential application of such an
assembly for various types of spectroscopy will be finally given.
2. EXPERIMENTAL
An overview over the setup is given in Figure 1. Nanopar-
ticles are produced in the Microwave Plasma Particle Source
(MPPS); they enter the TRAPS device through a Flow Limiting
Orifice (FLO), pass the AeroDynamic Lens (ADL) where the
particle beam is focused at the SKiMmer (SKM). Depending
on the device used, the particles now enter either a Particle
Mass Spectrometer (PMS) or an ElectroDynamic Cage (EDC)
of the TRAPS device where they can be trapped by an electric
potential at the inlet lens (Lin) and the exit lens (Lout). When
the exit lens is switched to ground, the particle beam passes a
quadrupole bender and is detected by one of the Faraday Cups
(FCn). The corresponding levels of pressure are depicted in the
lower graph of Figure 1.
2.1. Particle Generation and Characterization
As detailed below, the nanoparticles have to be suspended
in helium for efficient transfer into vacuum via an aerodynamic
lens. This might appear as limitation of the TRAPS device since
it excludes direct ambient aerosol inlet. However, nanoparticles
with diameter <30 nm exhibit a rather short atmospheric life-
time. For the investigation of small nanoparticles it is therefore
convenient to synthesize the particles instantaneous prior to
examination. Many nanoparticles sources, such as plasma
reactors, spark generators, nebulizers and electrospray sources,
can be operated with helium as carrier gas. Once the particles
are transferred into the vacuum device, they can be exposed to
realistic atmospheric compositions in a subsequent particle trap.
In this contribution a microwave plasma reactor was chosen as
a particle source since it provides 3–15 nm SiO2 and Fe2O3 par-
ticles at high concentrations (109–1012 cm−3 in a 10–150 mbar
helium atmosphere) by plasma decomposition of precursor
gases. Details of this device have been given earlier (Szabo et al.
1997; Szabo and Vollath 1999; Baumann et al. 2006; Mätzing
et al. 2009). In order to produce SiO2 nanoparticles a gas flow
(4500 sccm) of 0.02% SiH4 in helium and 4% O2 in helium was
fed through to a microwave discharge in an homemade resonator
(max power 1250 W at 2.45 GHz). In the microwave plasma
zone particles nucleate and grow to sizes ranging from 3 to 15
nm according to the different parameters of the plasma, precur-
sor pressure and residence time in the reactor tube, temperature
in the plasma zone, and flow rate of precursor and carrier gas.
Nanoparticles produced in the plasma reactor have been char-
acterized by Szabo et al. (1997) and Baumann et al. (2006) by
using transmission electron microscopy, particle mass spectrom-
etry and quartz crystal microbalance measurements. On line
characterization of the nanoparticles is achieved by an electro-
static deflection particle mass spectrometer (PMS, Paur et al.
2005). It operates by deflection of a chopped particle beam in a
homogeneous electric field (Figure 2). By varying the deflection
voltage, particles of different energy-to-charge ratios reach the
Faraday cup, which is located at a fixed off-axis position at the
end of the detection chamber.
The current signal generated at the Faraday cup is propor-
tional to the incoming number of particles multiplied by their
average charge. To convert this spectrum into a particle diameter
distribution, the average number of charges z, the particle speed
v and the material density ρ must be known. The investigations
of Baumann et al. show comparable results obtained from PMS
and Transmission Electron Microscopy (TEM) measurements
(Baumann et al. 2006; Mätzing et al. 2009). The approximate
diameter of the material sampled from the molecular beam was
determined by TEM. However, the sampling and micrograph
analysis of these particles by TEM appear to be difficult which
is due to the poor contrast of amorphous SiO2 particles and over-
lapping of particles. A typical mass spectrum of SiO2 nanoparti-
cles from the microwave plasma synthesis achieved by the PMS
is shown in Figure 3. The M1- peak recorded using the aerody-
namic lens (ADL) (full line) corresponds to a particle diameter
of 5.7 nm with a FWHM of 0.6 nm.
For further particle characterization, the TRAPS apparatus
can be used as a time-of-flight spectrometer, as discussed in
paragraph 3.4.
2.2. Aerodynamic Lens Inlet and Differential Pumping
Stage
One major innovation of the TRAPS instrument is the
efficient particle inlet for small nanoparticles provided by
an optimized, tunable aerodynamic lens. Transferring small,
318 J. MEINEN ET AL.
FIG. 1. Schematic diagram of the experimental setup consisting of a Microwave Plasma Particle Source (MPPS), Flow Limiting Orifices (FLO) Skimmers
(SKM), AeroDynamic Lens (ADL), Particle Mass Spectrometer (PMS) ElectroDynamic Cage (EDC) with entrance and exit lens (Lin, Lout), and Faraday Cups
(FCn). For details see the text. The corresponding levels of pressure are depicted in the lower graph.
airborne particles into vacuum by an aerodynamic lens (ADL)
is well understood for most demands. Liu et al. have provided
pioneering work in focusing fine particles by a series of contrac-
tions and expansions of a gas flow caused by a set of orifices (Liu
et al. 1995a, b). A critical orifice limits the gas flow through the
ADL. It is usually located far enough upstream from the first
orifice to ensure a fully developed laminar input flow for the
ADL.
ADLs for many particle types and different vacuum system
have been designed since then. Huffman et al. give a comprehen-
sive review of the characteristics and performance of most ADL
systems (Huffman et al. 2005). These systems achieve optimal
particle beam focusing in a diameter range between 100 nm
and 1 µm (Lee et al. 2008). In 2006, Wang and McMurry pro-
vided the aerodynamic lens calculator (referred as ALC herein,
Wang et al. 2006) a valuable design tool, that enables to design
an ADL for any particle diameter range and vacuum system
without extended knowledge of aerodynamic simulations. This
FIG. 2. Particle Mass Spectrometer (PMS) without particle inlet.
tool is based on several approximations, which tend to become
inaccurate, when particles with a diameter below 30 nm are
considered (Lee et al. 2008).
Few groups have reported ADLs for smaller nanoparticles: To
our knowledge the first work concentrating on particles <30 nm
was published in Wang et al. (2005). However, that the focusing
performance reported for these lenses is comparatively poor
for particles <20 nm (Wang et al. 2006; Passig et al. 2006).
For example, Passig et al. have only been able to enhance the
FIG. 3. Transmission efficiency of a molecular beam inlet with and without
aerodynamic lens. The graph shows the signal intensity over deflection voltage
of the ion beam measured by the PMS instrument. Data points are connected
by a spline in order to identify the two peaks occurring from the charge states.
Data points measured without the ADL are multiplied by a factor of 100.
THE TRAPS APPARATUS 319
particle transmission efficiency through a skimmer system by
a factor of two for 10 nm particles by using an ADL designed
to the guidelines of Wang et al. (2006). (See Figure 3 in Passig
et al. 2006.)
There are basically three reasons for the difficulty of focus-
ing small nanoparticles: First, the minimum achievable diver-
gence of the beam is dominated by Brownian motion for small
nanoparticles. A high axial speed of the particles reduces this
effect (Wang et al. 2006). Second, the slip motion of the par-
ticles, when the gas streamlines bend strongly downstream an
orifice, decreases with the third power of the particle diameter.
This effect could be compensated for by higher gas flux at lowerpressure, but this is limited by the pumping capacity of the vac-
uum system. A higher gas flux increases the radial the ratio of
the acceleration of the particles towards the center axis and the
mean axial velocity of the particles in the reattachment zone.
As it has been shown by Wang et al., focusing of small
nanoparticles is achievable by combining multiple orifices with
sub-optimum Stokes number (Wang et al. 2006). The drawback
is not only a high gas flux but also an increased sensitivity on
the gas type, pressure as well as the density and diameter of
the particles. This makes it difficult to design a lens capable of
focusing particles smaller than 30 nm in diameter of various
densities and in a broad size range, as it can be done for larger
particles (e.g., for a particle diameter range from 30 nm to 1000
nm (Zhang et al. 2004; Liu et al. 2007). For the work presented
herein, the ALC was used to provide a starting point for an im-
proved lens design which is further optimized by computational
fluid dynamic (CFD) simulations.
2.3. CFD Simulations
ANSYS CFX (ANSYS WorkbenchTM 11.0, ANSYS Europe,
Ltd.) is a commercial computational fluid dynamics package
similar to ANSYS FLUENT (ANSYS Europe, Ltd.). Both pack-
ages are based on the numerical solution of the Navier-Stokes
equations discretized on a user-defined mesh. CFX is able to cal-
culate the gas flow in the continuum regime of fluid dynamics
as long as the Knudsen-Number is near or smaller than one. At
a typical pressure of helium in an ADL of 100 Pa, the mean free
path is λ ∼ 250 µm. A characteristic length of the lens is given
by the orifice diameter do > 1 mm. Thus, the Knudsen number
for the gas flow inside the ADL is smaller than one and solving
the Navier-Stokes equation without any correction is valid. With
respect to the particles in the gas flow, the Knudsen number is
greater than one. In ANSYS CFX, the Cunningham-Correction
(Allen and Raabe 1985) can be applied for the particle drag
force via a user-supplied routine.
A dilute suspension of particles in an ideal gas is used for
the numerical calculations. Particle-particle interactions are ne-
glected and it is assumed, that the presence of the particles does
not influence the gas flow. Under these conditions, the flow field
can be calculated independent from the particle load. Particles
are introduced into the flow field afterwards and their velocities
and trajectories are calculated by simple time-step integration.
The “Schiller and Naumann Drag Model,” implemented in CFX,
can be used for this calculation in the case of spherical, solid par-
ticles (ANSYS 2006). This model is valid in both, the viscous
and the inertial regime, when modified by the Cunningham-
Correction calculated at each node of the mesh from the local
gas flow parameters and the particle diameter.
A critical region of the setup is the Mach zone downstream
from the exit of the lens. CFX can solve supersonic flows when
compressible fluid models are used. However, the calculation of
the particle drag force fails in this region. A correction factor,
comparable to subsonic flows with small Knudsen numbers, is
not available. To our knowledge, this problem remains unsolved
independent from the CFD code used. Prior publications have
followed the argumentation of Liu et al. (1995b) who reasoned
that the expected particle trajectories are close to the axis after
they have passed the ADL and that inside the Mach zone (zone of
silence) the pressure is as low as 1–10−2 Pa. They conclude that
the radial particle momentum will not be significantly affected
by the transit through the zone of silence and the Mach shock
wave. This hypothesis has been validated experimentally by
several groups later (Liu et al. 1995b; Wang et al. 2006; Liu
et al. 2007; Lee et al. 2008).
Brownian motion becomes important for small particles and
can be implemented in CFX by a user-supplied routine, as well.
This turns the steady state problem of an aerodynamic lens into
a transient calculation which is very costly in terms of CPU
time. Wang et al. (2006) reported a decreasing broadening of
the beam due to Brownian motion with increasing axial particle
speed. In order to optimize an ADL, the focusing performance
is usually calculated without Brownian motion. The additional
broadening of the particle beam due to Brownian motion is
subsequently estimated.
The design of an ADL should carefully avoid the occurrence
of turbulences. This was ensured by CFD calculations that in-
cluded a turbulent flow model (k-ε-model of CFX). No internal
source of turbulence, as indicated by the ratio between eddy
viscosity and dynamic viscosity, was found, even when a high
turbulent kinetic energy was introduced at the inlet.
For validation of our simulation methodology, lens “C” of
Wang et al. (2006) with spherical particles of density 1000 kg
m−3 was modeled. Our simulation is consistent with the pub-
lished results within <1% in every single feature, where the
1% deviation is mainly due to the inaccuracy in the read-
ing from Figure 2 in Wang et al. (2006). This demonstrates,
that the CFD code and mesh generation is suitable for ADL
calculations.
The CFD simulations consider a laminar flow upon entering
the first focusing orifice. As recommended by Liu et al. (2007),
the nozzle and skimmer was included in the calculation. There
are far too many boundary conditions to optimize from an initial
guess. Therefore the following strategy to design and improve
a suitable lens for focusing of small nanoparticles was adopted.
First, the ALC was used to make a guess for an appropriate lens
320 J. MEINEN ET AL.
TABLE 1
Parameters of an ADL for 8 nm SiO2 particles for the TRAPS apparatus as determined from the aerodynamic lens calculator
(ALC) and resulting from fluid dynamics calculations (CFX)
Orifice Upstream Gas Distance to next Pumping capacity
diameter [mm] pressure [Pa] velocity [m/s] element [mm] [Pa l/s]
ALC CFX ALC CFX ALC CFX ALC CFX ALC CFX
1st orifice 1.22 2.20 1802 900 448 250 45 40
2nd orifice 1.88 2.40 970 745 355 250 40 40
Nozzle 2.02 2.10 686 605 420 1050 28 30 750 850 (25Pa)
Skimmer — 3.00 — 25 — 200 — 15
Octupole inlet — 8.00 — 5 — 10 — 23 — 230 (1Pa)
design. If the CFD simulation of the first guess revealed, that the
focusing performance was not sufficient, the inlet pressure of
the lens was varied. Changing the inlet pressure affects the gas
flow across the orifices and thus the drag force on the particles. If
thereby a sufficient focusing could not be achieved, the diameter
of the orifices was varied. Large particles should be focused by
the first orifice; smaller particles can be focused by one of the
orifices downstream. Finally, the aspect ratio of the nozzle as
well as the size and distance of the skimmer downstream the
ADL was optimized.
Applying this method, several lenses for different purposes
were designed. One of these lenses is projected as an inlet focus-
ing 1–15 nm diameter SiO2 particles into the TRAPS apparatus
and will be described in greater detail below. Requirements for
this lens were a high particle flux (106–108 particles s−1) at mod-
erate exit pressure (∼1 Pa). Therefore, a high pumping capacity
and a large skimmer (d = 3.0 mm) was employed. An initial
guess was made with the ALC restricting only the maximum
pumping capacity. For SiO2, the smallest particle diameter that
can be focused according to the ALC is 8 nm. The particle ax-
ial velocity downstream from the lens, however, is rather small
which results in excessive beam broadening due to Brownian
motion. CFD calculations of that design show a rather poor fo-
cusing performance. By applying the strategy detailed above,
ADL parameters were found that perform considerably better.
The improved lens is able to focus 1–15 nm SiO2 particles dis-
persed in helium. The diameter of the tube between the focusing
orifices is 20 mm. The thickness of the orifices is 0.5 mm. A
comparison between the ALC result and the CFX redesign is
presented in Table 1.
The performanceof the redesigned lens can be compared to
the result from the ALC by simulating both geometries with
CFX. The divergence angle is a measure for the focusing per-
formance and can be calculated from the radial and the axial
velocity component of the particle trajectory. For 1 nm particles
it is 1.53◦ and 2.21◦ for the optimized ADL design and the initial
design from the ALC, respectively. For 8 nm particles the diver-
gence is 0.32◦ and 0.47◦ and for 15 nm particles the simulations
yield 1.26◦ and 4.30◦, respectively. This demonstrates that for
particles in the diameter range of 1–15 nm the beam divergence
is smaller in the redesigned ADL (see Figure 4). Additionally,
the higher particle velocity downstream from the lens (see Table
1) reduces Brownian broadening.
The CFD simulations of this ADL show a focusing of parti-
cles >8 nm at the first orifice (Figure 4b). Smaller particles are
focused at the second orifice. The distance between the orifices
and the tube diameter is chosen not to permit the flow to fully
reattach to the walls of the tube. This minimizes beam broaden-
ing by diffusion at low flow speed. The second nozzle is chosen
to accelerate the particles as much as possible under the given
flow parameters. The pressure drops from 605 to 25 Pa across
this nozzle, thus a Mach zone develops downstream. It was found
to be preferable not to place the skimmer in the zone of silence
of the Mach zone. Since the particles are well focused while
passing the Mach zone, they will not be defocused significantly.
Figure 4 shows trajectories of 1–15 nm SiO2 particles dispersed
in helium. After passing the skimmer and entering the octupole
FIG. 4. Two alternative designs for an ADL focusing spherical SiO2 particles
in Helium. (a) Result from ADL (b) as optimized by CFD calculations (see
text). A: ADL inlet with particles homogeneously dispersed in the carrier gas
(900 Pa). B: first orifice. C: second orifice. D: acceleration nozzle. E: opening
to pump (25 Pa). F: ion guide (5 Pa). M: mach zone. SKM: skimmer. Grey lines
are the streamlines of the gas. Black lines are the trajectories of the particles
with a diameter from 1–15 nm coded. Particles >8 nm are focused at the first
orifice; smaller particles are focused at the second orifice.
THE TRAPS APPARATUS 321
FIG. 5. Static pressure, axial gas velocity and particle axial velocity (particle
diameter 1 nm, 10 nm, 40 nm) along the axis of the nanoparticle lens system for
the TRAPS apparatus. This figure can be compared to Figure 2 in Wang et al.
(2006).
ion guide they have a sufficiently small radial momentum to be
trapped by the pseudo potential of the octupole.
Figure 5 shows the static pressure, axial flow and particle
axial velocity along the axis to be compared to the nanoparticle
lens system “C” as introduced by Wang et al. (2006), Figure
2 therein. Obviously, the main difference is the higher axial
particle speed in the new design which results in less Brownian
broadening downstream from the lens.
From the present CFD simulations it can be inferred, that the
inlet pressure is a sensitive parameter of an ADL. If the pressure
changes within 10% the focusing of a distinct size can be entirely
lost. On the other hand, this sensitivity can be advantageously
used to tune the lens to a specific particle diameter of interest.
The focusing performance of an ADL is governed by the particle
Stokes number Sp. For Sp = 1 a particle will be focused at the
center line of the lens. Particles will pass the centre line at Sp > 1
and diverge. For Sp < 1, particles tend to follow the streamlines
of the gas. The particle Stokes number has been derived by
several groups (e.g., Passig et al. 2006) to be:
Sp = τp · U
df
=
√
2
π · γ 3
g
(
U
M · df
)3 4 · ṁ · dp · ρp
p2 · (8 + π · ϑ)
[1]
In this relation U is the gas velocity in the plane of the orifice,
τp is the time it takes the particle to adapt to the gas veloc-
ity, df is the orifice diameter, γ g is the specific heat ratio of
the gas, M is the Mach number, ṁ is the mass flow of the
gas, ρp is the density of the particle, p is the pressure of the
gas and ϑ = 0.9 is an empiric constant. If pressure and mass
flow are changed, the optimum Stokes number for a distinct
particle diameter will shift to different size. Thereby the lens
becomes tunable. For multistage lenses, Sp will shift differently
in each of the orifices if the inlet pressure in the entire lens is
varied. By this effect, the useful tuning range of the ADL is
limited.
This tuning effect is useful for variable composition gas mix-
tures, as well. For instance, aerosols created by a microwave
plasma reactor are immersed in a carrier gas containing a vari-
able amount of oxygen (O2) in helium depending on the amount
of precursor used for the reaction. Since oxygen and helium have
significantly different viscosity, the particle trajectories are sen-
sitively changed. This can be compensated—at least to some
extend—by varying the inlet pressure of the ADL.
It should be mentioned that the calculations were performed
for an ideal geometry. Non-spherical particles will significantly
vary the beam width (Liu et al. 1995a, b). Nonsymmetric flow
fields induce swirl effects at an orifice (Baker 2000). Roughness
at the edges of the orifices can cause nonstationary effects,
such as fluctuating eddies downstream the orifice, resulting in
turbulence and pulsation (Sigloch 2008).
2.4. Octopole Ion Trap
After passing through the ADL, the particles are decelerated
and accumulated in a gas filled octupole ion guide located inside
the vacuum chamber. An octupole ion guide was chosen because
of its great acceptance angle and high trapping capacity. In the
final development stage of the apparatus, this ion guide will
serve for beam deceleration and shaping only. Tight focusing of
the particle beam is not of primary relevance since in the final
development stage subsequent ion guides are quadrupoles. For
the purposes of this work the octupole ion guide was operated as
a nanoparticle cage to demonstrate the prospects and limitations
of such a device.
Linear octupole ion guides are standard radio frequency mass
spectrometric devices. They are routinely used to guide, mod-
erate, and cool ion beams (Gerlich 2003). Details and typical
applications are given in a recent review (Douglas et al. 2005).
For the trap presented herein, the inner diameter of the oc-
tupole was chosen to be 9.52 mm and the rod diameter is 3.18
mm which is the optimum aspect ratio for octupole ion guides
(Gerlich 2003). In order to adapt an octupole ion guide for the
storage of singly charged nanoparticles, the RF frequency has
to be adjusted to accommodate the high mass-to-charge-ratio of
the particles. For nanoparticles in the 5–10 nm diameter regime,
the RF frequency should range between 100 kHz and 30 kHz
respectively. Tunable RF generators operating at such low fre-
quencies are not commercially available. Some groups have
developed ion traps which employ a digitally switched square
wave pulse chain to drive nanoparticle ion traps (e.g., Ding et al.
2004). Theoretically, any periodic waveform can provide stable
conditions for ion trapping. The drawbacks of this approach are
the higher harmonics, which decrease the maximum trapping
capacity of such traps, and also increase the noise problems to
other sensitive equipment around that is generated by switching
the high voltages. Therefore, a RF generator based on a ferrite
core high voltage transformer driven by a push-pull feedback
circuit was designed. It delivers sinusoidal RF voltages up to an
322 J. MEINEN ET AL.
FIG. 6. Schematic view and operating modes of the trap: (a) trap geometry;
(b) filling/storage mode; (c) storage mode; (d) extraction mode.
amplitude of 2 kV at a tunable frequency between 20 kHz and
150 kHz and was used successfully for the present experiments
(Meinen 2009). A further advantage of this approach is that a
DC offset voltage can be superimposed easily to the sinusoidal
output voltage in order to definethe potential on the axis of the
ion guide.
Applying an inert background gas, frictional forces are added
to the electrodynamic forces. These forces allow to decelerate
and to continuously store the ions within the octupole with-
out the need for opening or closing the trap electrically. In the
case of a light background gas, such as helium, moderating the
much heavier nanoparticles, the action of the background gas
can be regarded purely as friction applying the Cunningham
correction to Stokes law. Filling of the trap and extraction of the
nanoparticles are controlled by the electrostatic entrance and exit
lenses of the trap. Typical operating conditions are depicted in
Figure 6.
In the filling/storage mode (b), the entrance lens is kept at in-
termediate repelling potential which is below the kinetic energy
of the incoming particles, while the exit lens is at a potential high
enough to reflect all particles. By traversing the ion guide forth
and back, the ions loose kinetic energy by collisions with the
background gas. The decelerated returning ions are reflected by
the entrance lens and are bound to travel between the entrance
and exit electrode until their longitudinal kinetic energy is com-
pletely thermalized. In storage mode (c) both, entrance and exit
lens are kept at such high repelling potential that particles can
neither enter nor leave through the lenses. This mode is use-
ful for leakage measurements, spectroscopic measurements, or
chemical reactions. In order to extract the ions, the exit poten-
tial is switched to ground so that the particle can exit the trap
(d). The ejected particle cloud is detected on a Faraday cup ei-
ther directly or after being deflected by 90◦ in an electrostatic
quadrupole bender.
A similar trap was used for laser driven charge reversal spec-
troscopy on small metal clusters. In this application, laser pulses
were used to reverse the charge of the stored particles from
FIG. 7. Extraction potential and extraction particle pulse from octupole ion
trap.
negative to positive. The positively charged photoproducts were
expelled from the trap via the entrance and exit electrodes by the
same voltages that confined the anions, while the trap was con-
tinuously filled (Wolf et al. 1995). In the present experiments,
the nanoparticles were extracted by pulsing the exit electrode to
low potential.
A result from a typical trapping experiment is depicted in
Figure 7. The trap was kept in filling/storage mode for 0.5 s and
then the exit lens was switched open at time t = 0 to extract
the particles. The exit lens potential and the detector current
are displayed in Figure 7 as a function of time. After a certain
flight time, a large particle peak is measured, which reaches its
maximum after 1.4 ms. After 1.8 ms the trap is emptied and
the particle current reaches its cw value which can hardly be
seen in the figure (the octupole trap works as an ion guide)
until the exit lens is closed again after 4 ms. Integrating the
particle peak yields the total charge stored in the trap, which
reaches 2·107 elementary charges in this case. From the cw
value one can obtain the load current of the trap. Here, it is 70
pA corresponding to 4·107 particles per second. Thus, for this
configuration the particle density in the trap was enhanced by a
factor of (66 ± 3). From the time interval between the opening
of the ion trap and the signal on the Faraday cup, the diameter of
the particles and their spatial distribution in the trap (cf. below)
can be inferred.
3. RESULTS AND DISCUSSION
3.1. Characterization of the Plasma Particle Source
The plasma particle source and the ADL were characterized
by SiO2 nanoparticles. For this purpose, an ADL was designed
to fit at the existing Particle Mass Spectrometer (PMS) described
in section 2.1. This lens consists of one focusing orifice with
df = 2.10 mm and an accelerating nozzle with df = 2.13 mm,
THE TRAPS APPARATUS 323
only. SiO2 particles where produced by the microwave plasma
reactor using SiH4 and O2 in He at a total flow of 2.5 10−3 Pa L/s.
The inlet pressure was 400 Pa for helium as a carrier gas and the
pressure downstream the nozzle was 10 Pa. The flow limiting
orifice was positioned close to the reactor in order to avoid
coagulation of the particles. An aerodynamic skimmer with an
opening of df = 1.00 mm was positioned 15 mm downstream
the nozzle in order to differentially pump to 0.01 Pa in the mass
spectrometer.
Particles carrying one or two charges of both polarities have
been detected by the PMS in comparable fractions. For the
subsequent analysis, only singly (NP−) and doubly (NP2−) neg-
atively charged particles were used. The total current carried by
the beam was about 350 pA; the current assigned to NP− and
NP2– was 225 pA and 125 pA, respectively. By means of PMS,
the diameter of the particles has been determined to be dp =
(5.7 ± 0.8) nm. The axial velocity of the particles downstream
from the skimmer has been measured to be (380 ± 40) ms−1 by
using a chopped beam lock-in technique.
In order to compare the performance of this ADL inlet, the
lens was replaced by a classical molecular beam inlet consisting
of a flow-limiting orifice in front of an aerodynamic skimmer
with an opening of df = 0.50 mm. This skimmer was placed
15 mm upstream the nozzle. Figure 3 shows the signal intensity
of the deflected particle beam for two inlet systems, the ADL
(boxes) and the molecular beam (circles) inlet. Note that the
signal recorded using the molecular beam inlet is multiplied by
a factor of 100 in order to facilitate a comparison between both
experiments. The total current carried by the beam was about
350 pA and 2.5 pA for the ADL and the molecular beam inlet,
respectively. The particles produced for the experiment with the
molecular beam inlet (circles) are about 0.5 nm smaller than
the particles used for the ADL inlet experiment (boxes). This is
due to a slightly higher pressure within the microwave plasma
reactor in the molecular beam inlet case. As the mass flow of
the precursors has been carefully controlled, it can be assumed
that more particles of smaller size have been produced in the
plasma in order to transform all reactants. For this reason the
amplification of the transmitted particle beam caused by the
ADL might be even higher.
These experiments demonstrate that the microwave plasma
source is a reliable source for intense beams of small nanoparti-
cles in a carrier gas at moderate pressure. A carefully designed
aerodynamic lens enhances the fraction of particles that can be
transferred into a vacuum system by about a factor of 100.
3.2. Characterization of the Aerodynamic Lens Inlet
The aim of this work is to design an ADL for use as a
nanoparticle inlet for the TRAPS device, as depicted in Figure 1.
The parameters of the ADL used for the following experiments
are listed in Table 1. The skimmer with an opening of df = 3.0
mm is positioned 15 mm downstream the nozzle. The distance
between skimmer and inlet lens of the octupole is 23 mm. The
FIG. 8. Transmission of the aerodynamic lens in dependence of the inlet
pressure of the lens probed with (6.0 ± 0.4) nm diameter SiO2 particles. Open
boxes represent the detector current. In order to have a relative measure for
the transmission efficiency of the ADL the data was corrected for the gas flow
increasing with inlet pressure.
length of the octupole is 220 mm and the distance between
octupole and electrometer is 162 mm, whereas the detection area
has a diameter of 17 mm. SiO2 particles with dp = (6.0 ± 0.7)
nm were produced by microwave plasma synthesis at 10 kPa and
characterized with the TRAPS-TOF device (see section 3.4).
It was shown above that the inlet pressure of the ADL is the
key parameter for the focusing performance of the lens since it
defines the pressure and velocity of the carrier gas inside the lens.
Figure 8 shows an experiment demonstrating this relationship.
A rotary pump is connected to the tube between flow limiting
orifice and first focusingorifice. The pressure, which is referred
to as inlet pressure, is adjusted by a valve and controlled by a
capacitive pressure gauge. Figure 8 clearly shows a maximum
signal at 860 Pa which is close to the pressure the lens was
designed for (900 Pa, see Table 1). An increase of the inlet
pressure leads to an increase of the mass flow through the ADL.
Therefore, the measurement is corrected for the change of the
mass flow (filled spheres in Figure 8). The corrected signal
can be interpreted as a transmission efficiency of the whole
inlet system consisting of ADL, skimmer, and octupole. CFD
calculations indicate that the peak at 473 Pa might be induced by
particles with larger aerodynamic diameter. A similar effect was
reported by (de la Mora 1996) who noticed a stepped collection
efficiency of aerodynamic particle impactors for different gas
velocities. De la Mora was able to address this effect to varying
classes of particle sizes. Our CFD simulations show a high
skimmer transmission of 12 nm diameter particles at 473 Pa but
a complete loss of particles at 860 Pa, so it can be assumed that
the second peak in Figure 8 can be assigned to agglomerates of
primary particles.
The requirements to the beam divergence downstream the
ADL are quite low in our application, since the beam is focused
324 J. MEINEN ET AL.
electrodynamically once it is inside the octupole ion guide.
According to Liu et al. (1995a) the minimum divergence of
a nanoparticle beam exiting an ADL is determined by the
Brownian motion which adds a radial velocity component to
the particle beam exiting from the nozzle. Assuming that up-
stream of the nozzle particles are in perfect thermal equilibrium
with the surrounding gas, the velocity distribution of the par-
ticle thermal agitation can be approximately described by the
Maxwell-Boltzmann distribution. Downstream the nozzle gas-
particle collisions are assumed to be negligible since the pressure
is low. From this it follows for the beam diameter dB,Brown at a
certain distance L downstream the nozzle:
dB.Brown (L) = 3.04
√
12 · kB · T0
π · d3
p · ρp
L
Up∞
[2]
where kB is the Boltzmann constant, T0 is the gas temperature
and Up∞ is the axial particle velocity at the exit plane of the
nozzle. Using the values from CFD simulation for 6 nm SiO2
particles and an ADL inlet pressure of 860 Pa, the particle
beam diameter caused by Brownian motion at the location of
the skimmer entrance plane L = 15 mm is dB,Brown = 0.4 mm.
Neglecting any influences by the skimmer and the flow field
inside the octupole ion guide, the particle beam diameter caused
by Brownian motion at the location of the entrance plane of the
faraday cup L = 390 mm is dB,Brown = 10 mm. This estimate of
the beam diameter inside the high vacuum region of the device
shows that the beam can be focused tightly enough to fulfill the
demands, since the orifice diameters are df = 3 mm and df =
17 mm for the skimmer and the faraday cup, respectively.
Beam profiles have been measured by sliding a sharp edged
copper plate into the beam. The analysis of the Faraday cup sig-
nal over plate position yields the beam profile. Beam diameters
are defined following Liu et al. (1995a) as the width containing
90% of the particles. Figure 9 shows the particle beam diame-
ter of 7 nm SiO2 particles measured 235 mm downstream the
nozzle. CFD simulations with appropriate parameters show a
minimum at the working pressure of the lens and, as expected,
a significantly smaller beam size since the CFD code does not
account for Brownian broadening.
Figure 9 shows the ADL would be able to focus the beam
to 1 mm diameter at the working pressure if Brownian motion
would be absent. Brownian broadening determines the width of
the beam over the full pressure range tested. The beam width
measured at working pressure is about 2.4 mm. Assuming Equa-
tion (2) is valid, this beam diameter corresponds to 12 nm par-
ticle diameter while it was determined by time-of-flight (TOF)
measurement to be dp = (7.0 ± 0.6) nm (cf. below). The the-
oretical Brownian limit for a 7.0 nm particle is calculated from
Equation (2) is depicted as dashed line in Figure 9. This might
be an indication that the assumptions made in the derivation of
Equation (2) are not completely valid. The particles are possibly
not in perfect thermal equilibrium with the surrounding carrier
FIG. 9. Beam diameter 235 mm downstream the nozzle recorded for (7.0 ±
0.6) nm SiO2 with different inlet pressure of the ADL. The profiles are compared
to the results from CFD calculations, which show a minimum at the working
pressure of the lens and an overall lower beam diameter. The Brownian limit of
a 7 nm particle beam according to Equation (3) is depicted for comparison.
gas while passing the acceleration nozzle of the ADL. Further-
more, in the discussion of the octupole ion trap downstream
from the skimmer (cf. below) evidence will be presented that
the drag force of the low background gas pressure inside the ion
guide is influencing the particle trajectories. CFD simulations
show that trajectories of particles with diameter dp < 5.0 nm are
influenced by the gas flow through the exit lens of the octupole
ion guide. This is an indication for an additional focusing effect
of the skimmer and the octupole ion guide geometry.
3.3. Characteristics of the Nanoparticle Trap
The trapping capacity of the octupole ion trap is the key
parameter for being able to do optical or X-ray spectroscopy on
small nanoparticles. Up to 107 particles per cubic centimeter are
required to achieve a measurable optical absorption signal in the
visible for strongly absorbing materials like iron oxide particles.
The spatial distribution of the trapped particles will extend to
larger radii with increasing particle number since space charge
effects add to the pseudo potential of the electrodynamic trap.
These processes are very well characterized by Douglas et al.
(2005); Gerlich (2003); and Terasaki et al. (2009).
The storage characteristics of the trap were assessed by ac-
cumulating particles for various time periods by analyzing the
ion pulses leaving the trap. SiO2 particles from a microwave
plasma source were used. The diameter of the particles was
determined by the TRAPS-TOF device (see section 3.4) to be
dp = (6.4 ± 0.5) nm. The particles were transferred by the
ADL into the vacuum as shown in Figure 1. After passing the
skimmer, the particles enter the octupole ion guide. The back-
ground pressure of the gas inside the octupole is 1.8 Pa. The
THE TRAPS APPARATUS 325
FIG. 10. Particle pulses obtained from the octupole ion trap for various filling
times dt. (a) dt = 0.2 s, (b) dt = 0.5 s, (c) dt = 1.5 s, (d) dt = 10 s. The intensity
of the filling beam was 0.1 nA.
pressure downstream from the octupole was 0.03 Pa with a
helium gas flow of 31 Pa·L·s−1 through the experiment. The
octupole was driven with a sinusoidal waveform at a frequency
f = 50 kHz, an amplitude of 255 V and an DC offset voltage of
–186 V. This provides a beam current of 100 pA as measured by
the electrometer at the straight through Faraday cup. A voltage
of –350 V was applied at the exit lens of the ion guide for trap-
ping the particles in the octupole ion trap. This potential could be
switched to ground by a fast MOSFET-Push-Pull switch (HTS
61-03-GSM, BEHLKE Electronic GmbH, Germany) within 500
ns for a period of 40 ms.
Figure 10 shows exit pulses for increasing filling time. It is
interesting to notice that the pulses first grow in height and then,
for prolonged filling time, they grow preferentially in width.
This indicates that the trap seems to be gradually filled begin-
ning from the exit lens toward the inlet. Obviously, the particles
are concentrated at the exit of the octupole by the drag force of
the gas flow through the octupole. The average distance between
two particles in a maximum loaded trap is about 30 µm. Thus
a Coulomb repulsion of 3·10−19 N is acting between the two
particles. A drag force of the same order of magnitude causedby the gas flowing through the ion guide is acting on the parti-
cles and pushes them towards the exit of the ion guide. Figure
11 shows the total number of particles trapped as a function of
the filling time. A maximum trap capacity of 5·108 particles is
reached at a filling time of about 10 s with a beam of 6.2·108
particles per second entering the trap. If one assumes that the
particle cloud extends to the octupole rods, the particle density
is 4·107 particles per cubic centimeter. This is in perfect agree-
ment with the theoretically predicted maximum trap capacity of
3.7·107 particles per cubic centimeter (Douglas et al. 2005).
The maximum trap capacity is limited by the number of
particles leaking out of the trap. The leakage is due to particle–
FIG. 11. Storage capacity of the octupole ion trap. (6.4 ± 0.6) nm SiO2
particles have been filled into the octupole ion trap with a current of 0.1 pA
singly charged ions.
particle Coulomb interaction in near collisions, which diphase
the particle motion from the RF field. It can be measured by
varying the storage time. If a constant leakage factor Cl is
assumed, the decay of the particle concentration N (t) can be
described by
∂N
∂t
= −N · Cl → ln
[
N (t)
N0
]
= −Cl · t. [3]
Figure 12 shows the decrease of the stored particles in the
trap as a function of storage time. A linear fit to ln[N (t)/N0]
over t yields a leakage factor of C = 8.5·10−2 s−1. Thus the
mean residence time of the particles in the almost full octupole
ion trap is 12 s. This experiment demonstrates the stability of
rigid, non-volatile particles inside the trap. But it is known from
experiments with flexible bio-molecules that even very labile
particles can be trapped in an electrodynamic potential for hours
(e.g., Iavarone et al. 2006). If the trapped particles have semi-
volatile components, the storage time of the particles might
become limited by their evaporation rate.
The interpretation that the gas flow inside the trap determines
the spatial distribution of the particles is supported by the obser-
vation that under normal operating conditions no potential at the
entrance lens of the octupole is needed in order to trap particles.
Obviously they are held inside the trap by the drag force exerted
by the gas flow.
3.4. Time of Flight Measurements
From the time interval between the opening of the ion trap
and the signal on the Faraday cup, the diameter of the parti-
cles and their spatial distribution in the trap can be inferred
(cf. below). For these measurements the trap is filled with a
comparatively small number of particles in order to minimize
326 J. MEINEN ET AL.
FIG. 12. Particle retention rates of the octupole trap in storage mode as a
function of storage time. The experiment was performed with a beam of 0.1 pA
(=1.2 109 particles s−1) singly charged (6.4 ± 0.6) nm diameter SiO2 particles.
space charge effects and to obtain sharp exit pulses. By vary-
ing the offset potential of the octupole trap, the kinetic energy
of the exiting particles can be varied. In Figure 13 extraction
pulses are shown that were obtained by filling a total number
of 105 particles into the trap before opening the exit lens for
4 ms. The DC offset potential UDC of the octupole rods rela-
tive to ground was –179 V and –155 V for the first and second
pulse, respectively. A shift in time occurring from the different
acceleration of the ions with the mass m and the charge q is
evident. Since the initial velocity v0 of the particles is a pri-
ory unknown, a series of flight times ti at different accelerating
offset potentials was measured in order to eliminate the initial
velocity from the mass analysis. The kinetic energy is assumed
to be the sum of initial kinetic energy and acceleration by UDC
(Equation (5)).
Ekin,i = m
2
v2
0 + q · UDC,i . [4]
If UDC is plotted as a function of t−2
i , a linear relation is expected
(Equation (6)) which allows m and v0 to be determined.
UDC,i = L2
eff
2
m
q
[
1
t2
]
i
− m
q
v2
0
2
. [5]
Leff is the effective length of the flight distance which can
be deduced from the geometry of our setup. If the particles
are assumed as homogeneous spheres carrying a single charge,
the particle diameter can be calculated. The analysis of the full
dataset, Figure 13 was extracted from, yielded values of dp =
(6.8 ± 0.5) nm and v0 = (312 ± 34) ms−1. The particle diameter
and standard deviation corresponds to values achieved by PMS
measurements (compare Figure 3). The initial velocity seems to
FIG. 13. Exit lens potential and time of flight signals for SiO2 particles start-
ing from different potentials. The left peak is achieved by UDC = −179 V and
yields a flight time of 0.61 ms. The right peak is achieved by UDC = −155 V
and yields a flight time of 0.74 ms.
be a reasonable value when compared to the calculated particle
velocity shown in Figure 5.
TOF measurements of NaHCO3 particles have been
compared to particle diameters determined by SMPS (DMA
Classifier: TSI model 3085, CPC: TSI model 3025) in order
to verify this new method. Figure 14 shows a particle diameter
distribution of NaHCO3 particles in a helium atmosphere
produced by an atomizer at atmospheric pressure. The diameter
distribution measured by the SMPS peaks at 9.8 nm and has
a FWHM of about 3 nm. The mean diameter measured by
the TOF is dp = (11.7 ± 2.2) nm. This indicates that both
FIG. 14. Size determination of NaHCO3 particles in helium produced by the
atomizer. SMPS measurements provide a particle diameter distribution. TOF
gives a mean particle diameter with measurement error.
THE TRAPS APPARATUS 327
approaches yield virtually the same results, indicating the
reliability of the present work.
4. CONCLUSION AND OUTLOOK
We have developed a novel apparatus that allows gas-born
nanoparticles in a diameter range from 3 to 15 nm to be effi-
ciently transferred into a vacuum apparatus and to store them
there for a prolonged time for further analysis. We were able
to store up to 5 108 particles in a 20 cm long ion guide with a
diameter of about 9 mm. This target density suffices for direct
absorption measurements in the visible, UV-, and X-ray regime.
A simple time of flight detection scheme allows us to estimate
the particle diameter.
This apparatus will be transformed in the future into a com-
plete nanoparticle spectrometer including a cooled nanoparticle
trap and a second mass selective nanoparticle mass spectrometer
named TRAPS. This should allow assessing chemical reactions
of free nanoparticles, including ice nucleation or halogen acti-
vation reactions.
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