International Journal of Mass Spectrometry 470 (2021) 116706
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry
journal homepage: www.elsevier.com/locate/ijms
Virtual-slit focusing in a cycloidal mass spectrometer e A proof of
concept
Rafael Bento Serpa a, Elettra L. Piacentino a, Kathleen L. Horvath a, Tanouir Aloui a,
Yuriy Zhilichev b, Charles B. Parker a, Jeffrey T. Glass a, Scott B. Tilden c, Justin A. Keogh c,
Robert Kingston c, Roger P. Sperline c, M. Bonner Denton c, Jason J. Amsden a, *
a
b
c
Department of Electrical and Computer Engineering, Duke University, Durham, NC, 27708, USA
Independent Consultant, Durham, NC, 27708, USA
Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ, 85721, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2021
Received in revised form
14 September 2021
Accepted 16 September 2021
Available online 20 September 2021
This work demonstrates a novel approach to mass spectrometry using the “virtual-slit” created by the
small, localized volume from which ions are generated by localized ionization techniques such as laser
ionization of particles and surfaces, and spark ionization. That is, the volume in which the ions are
generated creates a localized source of ions in much the same way as a slit that allows only ions from a
specific cross sectional area to pass. As demonstrated in this work, the unique perfect focusing properties
of a cycloidal mass analyzer can enable the localized ionization volume to function as a virtual-slit. In this
manuscript, we provide a proof-of-concept (POC) demonstration of a virtual-slit cycloidal mass spectrometer (VS-CMS) consisting of a cycloidal mass analyzer, a laser ionization source, and an ion array
detector. The laser is used to ionize either bulk or thin film samples of Cu, Ti, Ni, and Cr. Results indicate
that the width of the peaks at the detector is the same as the laser spot size, validating the concept of
virtual-slit focusing. Furthermore, the Ti and Cu ablated with each laser pulse is estimated at 60 and
140 fg, respectively, indicating that a very low detection limit is possible given the ability of the cycloidal
mass analyzer to collect ions generated by laser ionization that have wide energy and angular dispersions. The VS-CMS concept has the potential to enable development of fieldable instruments for
chemical, elemental, and isotopic analysis of organic and inorganic samples, that are relatively small
compared to currently used laser ionization time-of-flight and laser ablation inductively coupled plasma
mass spectrometers.
© 2021 Elsevier B.V. All rights reserved.
Keywords:
Cycloid
Mass analyzer
Sector
Array detector
Laser ionization
Single particle
1. Introduction
All mass spectrometers consist of an ion source, mass analyzer,
and detector. In the prototypical implementation, there is an
aperture between the ion source and mass analyzer to localize
where ions enter the mass analyzer. In order to maximize
* Corresponding author. Department of Electrical and Computer Engineering
Duke University Durham, NC, USA.
E-mail addresses: rafael.bento.serpa@duke.edu (R.B. Serpa), elettra.piacentino@
duke.edu (E.L. Piacentino), kathleen.horvath@duke.edu (K.L. Horvath), tanouir.
aloui@duke.edu (T. Aloui), yzhilichev@aim.com (Y. Zhilichev), charles.parker@
duke.edu (C.B. Parker), jeff.glass@duke.edu (J.T. Glass), sbtilden@email.arizona.edu
(S.B. Tilden), jkeogh@email.arizona.edu (J.A. Keogh), kingston@optics.arizona.edu
(R. Kingston), rsperlin@email.arizona.edu (R.P. Sperline), mbdenton@email.
arizona.edu (M.B. Denton), jason.amsden@duke.edu (J.J. Amsden).
https://doi.org/10.1016/j.ijms.2021.116706
1387-3806/© 2021 Elsevier B.V. All rights reserved.
transmission, there are also ion optics to collect and direct ions,
generated in the ion source with various initial positions and velocity vectors, through the aperture. Often, there is also an aperture
between the mass analyzer and the detector. For example, in sector
instruments, there is typically an entrance aperture between the
ion source and the mass analyzer (also referred to as the entrance
slit), as well as an exit aperture (or exit slit) between the mass
analyzer and detector. In sector mass spectrometer instruments,
these entrance and exit slits determine the resolution of the instrument [1]. Inevitably, some of the ions generated in the ion
source will hit the ion optics and aperture and will be blocked
before reaching the mass analyzer, reducing transmission. Loss of
ions is compounded when employing ionization techniques such as
laser and spark ionization that generate ions with large spreads in
initial energy and direction.
R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
This manuscript provides a proof-of-concept demonstration of a
virtual-slit cycloidal mass spectrometer (VS-CMS). First, the perfect
focusing properties of the cycloidal mass analyzer are reviewed and
the virtual-slit focusing concept is introduced. Then, finite element
analysis (FEA) simulations are presented investigating the potential
of virtual-slit focusing in a proof-of-concept (POC) VS-CMS apparatus. Finally, using this apparatus and laser ionization, virtual-slit
focusing with several materials including bulk Cu, and thin films
of Ti, NI, and Cr is demonstrated by showing that the laser spot size
is the same size as the width of the peak on the detector.
Laser ionization [2e4] in particular, has been employed in mass
spectrometry in a wide variety of applications including planetary
exploration [5], organic and biomolecule analysis [6], inorganic
trace analysis [7], and in single particle analysis of atmospheric [8]
and biological [9] aerosols, pharmaceuticals [10], and explosives
[11,12]. Using a laser for ionization provides significant advantages
over other ionization techniques for solid samples in that it requires
little sample preparation. However, laser ionization produces ions
with a relatively large kinetic energy dispersion and large angular
dispersion [4,13,14]. Depending on the laser fluence and pulse
duration used, ion energies can range from 0 to 1000 eV with an
angular dispersion of up to 180 in the case of a surface [14], or 360
in the case of a single particle. Only a fraction of the ions generated
travel towards the aperture separating the ion source from the
mass analyzer. To compensate for the low transmission, focusing
ion optics are used to collect and direct as many ions as possible
through the aperture to the analyzer. However, such focusing can
further increase both the ion energy and energy spread. As a result,
most laser ionization mass spectrometers employ reflectron timeof-flight analyzers [4], and there are relatively few examples of
laser ionization coupled with double focusing sector instruments
[7,15] because double focusing can only compensate for a relatively
small spread in initial energy and direction of the ions [16,17].
Double focusing sector instruments are preferred over time-offlight analyzers in applications involving isotope ratio analysis.
Therefore, if there were a method to better compensate for the
large energy and angular dispersion of laser ionization in sector
instruments, it would be advantageous for the applications
described above. Furthermore, a method to better compensate for
large energy and angular dispersions could also potentially result in
a relatively small fieldable alternative to laboratory based laser
ablation multi collector inductively coupled plasma mass spectrometry for trace elemental [7] and isotopic analysis of solid materials for security [18,19] and geochronology applications [20,21].
The cycloidal mass analyzer, a crossed electric and magnetic
field sector mass analyzer introduced by W. Bleakney and J. Hipple
in 1938, exhibits perfect focusing in that the position where ions are
focused on the mass analyzer focal plane is independent of the
initial ion energy and direction that the ion is traveling [16]. Since
focusing does not depend on the initial speed or direction of the ion
after ionization, the cycloidal approach makes it possible to focus
ions with large energy and angular dispersion, such as those
generated from laser ionization or spark ionization, without the
need for additional focusing ion optics. Furthermore, in laser and
spark ionization of surfaces and single particles, where ionization is
localized to the size of the particle or size of the laser spot, the
perfect focusing properties of the cycloidal mass analyzer allow for
the small, localized volume where ionization occurs to function as a
“virtual-slit.” Incorporating a virtual-slit eliminates the need for a
physical aperture that defines the mass analyzer resolution. Since
the small, localized volume of ionization functions as a virtual-slit,
ion optics are not needed to collect and direct ions towards an
aperture, and the cycloidal mass analyzer can potentially enable
development of a small high-resolution instrument for in situ
analysis of organic and inorganic materials, including highprecision isotope ratios. Despite these unique perfect focusing
properties, use of the cycloidal geometry has been limited primarily
by the need for a focal plane ion array detector located inside the
magnetic field to fully take advantage of the perfect focusing
properties. Conventional multiplier and multichannel electron
multipliers suffer from poor performance in a magnetic field precluding their use in the cycloidal geometry. However, the recently
developed capacitive transimpedance amplifier (CTIA) ion array
detectors are not constrained by this limitation, generating
renewed interest in the cycloidal mass analyzer [22e24].
2. Virtual-slit focusing in a cycloidal mass analyzer
The cycloidal mass analyzer, similar to the Wein filter [25],
employs perpendicularly oriented homogeneous electric and
magnetic fields. However, in the Wein filter, the ions are accelerated towards and detected on a plane parallel to both the electric
and magnetic fields, while in the cycloidal mass analyzer ions are
detected in a plane parallel to the magnetic field and perpendicular
to the electric field. Equations 1-9 in Table 1 list the equations of
motion and solutions for an ion traveling in perpendicularly oriented homogeneous electric and magnetic fields [16,26,27]. The
solutions to the equations of motion for an ion traveling in
perpendicularly oriented electric and magnetic fields describe
cycloidal trajectories in a plane parallel to the direction of the
electric field and perpendicular to the magnetic field. Fig. 1 shows
an example of cycloidal trajectories for m/q ¼ 63 ions with energies
of 5 eV or 100 eV, initial angles between ±75 measured from the yaxis, traveling in a magnetic field of 0.7 T (-z direction), and an
electric field of 3.03 V/mm (-y direction).
pm after ionization, the y-coordinate of the ion
At time t ¼ 2qB
pm ¼ y while the x-coorreturns to the original position y t ¼ 2qB
oi
dinate is shifted by an amount that depends on the m/q of the ion
and the ratio of the electric field to the magnetic field squared:
pm ¼ m 2pE þ x (Table 1, Equation 11). The quantity p
p ¼ x t ¼ 2qB
oi
q B2
is referred to as the pitch of the cycloid and is the distance along the
focal plane where ions of a particular m/q are focused after
completing a full 360 revolution. There is a linear relationship
between the pitch p and m/q, so the placement of a suitable array
detector along the focal plane enables the detection of a wide range
of masses simultaneously. Note that the pitch p does not depend on
the initial velocity or direction of the ion. Therefore, ions with a
wide range of initial energies and angles are still focused to the
same spot. As shown in Fig. 1, despite the 20x range of energies and
150 angular spread, all m/q ¼ 63 ions are focused to the same spot
along the focal plane. Furthermore, when ions are generated from a
single particle with laser or spark ionization, their angles can range
from 0 to 360 . With the perfect focusing properties of the cycloid,
nearly all ions can be collected from ionization of a single particle.
However, a double sided detector would be needed. A proposed
double sided detector configuration is presented in a recent
perspective article [23].
Note that while the cycloidal mass analyzer exhibits perfect
focusing in the x and y directions, it does not exhibit focusing in the
z-direction. Ions with initial velocity components in the z-direction
are focused along a line parallel to the z-axis. Therefore, to collect
the maximum number of ions, the detector pixels should extend
along the z-direction to capture ions with initial velocity components in the z-direction. In a future publication, we will discuss
methods to refocus ions with z-velocity components in a cycloidal
mass spectrometer.
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R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
Table 1
!
Equations of motion and their solutions for a charged particle moving in perpendicularly oriented homogeneous electric and magnetic fields. F is the force on the ion; m is the
!
!
!
mass of the ion (relativistic effects are ignored and m is a constant); q is the charge of the ion; E is the electric field; a is the acceleration vector; !
v is the velocity vector; B is
the magnetic field; x, y, and z are the position coordinates; and t is time.
Equations of motion
Initial velocities
Initial positions
Solutions to the equations of motion
Pitch
!
!
!
!
F ¼ m a ¼ q E þ q!
v B
1
d2 x
qB dy
¼ m dt
dt 2
d2 y
qE qB dx
þ
¼ 2
m
m dt
dt
d2 z
¼
0
dt 2
dx
dy
ðt ¼ 0Þ ¼ vx0
ðt ¼ 0Þ ¼ vy0
dt
dt
yðt ¼ 0Þ ¼ yoi
xðt ¼ 0Þ ¼ xoi
qB
U ¼
m
vy0
vy0
E
1
E
cosðUtÞ þ
þ xoi
v sinðUtÞ xðtÞ ¼ t þ
B
U
U x0 B
U
vy0
1 E
E
vx0
vx0 cosðUtÞ þ
sinðUtÞ þ
þ yoi
yðtÞ ¼
BU
U B
U
U
zðtÞ ¼ vz0 t þ zoi
2pm
m 2p E
þ xoi
p ¼x t¼
¼
qB
q B2
2
3
4
dz
ðt ¼ 0Þ ¼ vz0
dt
zðt ¼ 0Þ ¼ zoi
5
6
7
8
9
10
11
Fig. 1. Trajectories for ions traveling in perpendicularly oriented homogeneous electric and magnetic fields. Here, the ions are m/q ¼ 63 in two groups with initial energies of either
5 eV or 100 eV and with a range of initial angles from ±75 measured from the y-axis. The ions are traveling in a magnetic field of 0.7 T directed along the -z direction and an electric
field of 30.3 V/cm directed along the -y direction. The areas 1 and 2, indicated by the black boxes, are shown magnified at the right and highlight that all ions of a single mass to
charge ratio arrive at the same focal point regardless of the initial direction or energy.
The theoretical resolving power Dmm of a cycloidal mass analyzer
of the localized ionization volume, and the exit slit size corresponds
to the detector array element spacing. Virtual-slit focusing in a
cycloidal mass analyzer can enable a high resolving power analyzer
with a relatively small size. For the configuration shown in Fig. 1,
assuming a 5 mm diameter particle of m/q ¼ 63, the resolving power
is m/Dm¼p⁄Dp¼25 mm/0.005 mm ¼ 5000 and only requires an area
of approximately 7 6 cm for the ion trajectories.
is equivalent to Dpp , found by differentiating the pitch (Table 1,
Equation 11) with respect to mass. When combining a cycloidal
mass analyzer and a focal plane array detector, the detector acts as
an array of exit slits, where Dp is the peak width measured at the
detector. With the perfect focusing cycloidal mass analyzer, Dp is
equal to the width of the entrance slit. This means that using
localized ionization techniques, such as laser and spark ionization
with a cycloidal mass analyzer, one can use the size of the localized
ionization volume (e.g., laser spot size for laser ionization of surfaces, or size of the particle for laser and spark ionization of particles), as a “virtual-slit” entrance slit while the detector array
spacing acts as the exit slit. The virtual-slit is determined by the size
3. Virtual-slit cycloidal mass spectrometer validation
The derivations above describing the motion of an ion in a
cycloidal mass analyzer support the potential for focusing ions
generated with localized ionization techniques with large energy
3
R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
shows a top-down, cross-sectional schematic view of the POC
apparatus, and Fig. 2b shows a CAD model of the mass analyzer.
Fig. 2c is a photograph of the electric sector and detector mounted
to the front feedthrough (face-down), which connects to the vacuum manifold. Fig. 2d is a photograph of the complete VS-CMS POC
system, with a blue 30 cm ruler positioned in the lower center of
the image.
The magnetic sector is a C-shaped magnet assembly. The magnetic sector, shown in Fig. 3b, is comprised of two, Nd-Fe-B grade
N52 magnets (black) of dimensions 54.5 130 110
mm3 (h w d). Attached to the surface of each magnet is a light
gray, chamfered, 1020 steel pole piece that forms a 31 mm air gap
where the vacuum manifold is installed. A gray, 1020 steel C-shaped
yoke magnetically connects the Nd-Fe-B magnets and poles pieces.
The yoke enhances the magnetic field strength by redirecting the
and angular dispersion by using the virtual-slit concept. These
derivations assume perfectly homogeneous electric and magnetic
fields, which in reality, are impossible to achieve. In this section, we
use a proof of concept (POC) apparatus to show that virtual-slit
focusing using laser ionization of a surface is possible with fields
that are not perfectly homogeneous. We begin by describing the
POC apparatus, followed by showing charged particle finite
element analysis (FEA) simulations of the POC apparatus and
experimental results from the POC apparatus.
3.1. VS-CMS proof-of-concept (POC) apparatus
The VS-CMS POC developed in this work has six subsystems
including the magnetic sector, electric sector, detector, sample,
vacuum system, and a laser with corresponding optics. Fig. 2a
Fig. 2. (a) Top-down, cross-sectional schematic view of the VS-CMS POC apparatus. The different components are labeled: (1) laser focusing lens, (2) front-feedthrough, (3) laser
window, (4) vacuum manifold, (5) detector mount, (6) electric sector electrodes (the symbols #1 and #27 indicate the numbered order of the electrodes referenced in this text), (7)
laser focal spot and sample position, (8) cycloidal ion trajectories, and (9) rear-feedthrough connecting to the vacuum system. The magnetic field is directed into the page, pointing
towards the -z direction and the electric field is applied in the -y direction. (b) CAD model of the mass analyzer including the magnetic sector, electric sector, vacuum chamber, and
front brass feedthrough. The 30 cm ruler (to the right of the image) is added for scale. (c) Photograph of the electrodes (electric sector) mounted on the front-feedthrough which is
facedown. The set of screwdrivers are behind the electric sector as a size reference. (d) Photograph of the complete VS-CMS proof of concept apparatus. The dotted blue line
represents the path of the laser. The blue 30 cm ruler in the bottom center of the image is included for scale.
4
R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
Fig. 3. (a) Magnetic field map produced by the magnetic sector at the center of the gap. The blue contour lines are increments of 0.05% of the maximum magnetic field value B of
0.7016 T at the central coordinate. The red line shows an increment of 0.1% of the maximum B field. The black lines represent an example of a simulated cycloidal trajectory
superimposed on the magnetic field map. The footprint of the electric sector is outlined in gold. (b) CAD model of the C-shaped magnetic sector with a patch inserted between the
pole pieces showing the position of the field map in (a). The 1020 steel parts are silver in color while the Nd-Fe-B magnets are colored in black. A 30 cm ruler was added for scale. (c)
CAD model of the electric sector with simulated ion trajectories of 63Cu and 65Cu isotopes. (d) Simulated spectra generated from the ion trajectories for several different emission
spot sizes from 10 to 300 mm. Measurements were made by recording the position at which the ions pass through the detector plane and counting the number of particles that
would hit 12.5 mm wide x 3 mm tall pixels of the array detector. The electric field was set to 3.03 V/mm. Isotope ratio values were used from the literature [28]. (e) Peak width
(FWHM) obtained from simulated data shown in Fig. 3 (d).
0:091ði 14Þ=27, where E is the desired electric field (V/m), and i is
the electrode number (Fig. 2a). The 14th electrode is grounded. The
voltages supplied to the electrodes are generated by a voltage
divider that consists of two sets of 13 adjustable resistors connected
in parallel to enable independent voltage changes. Each set of resistors in the voltage divider is connected to a Keithley 2410 source/
measure unit to provide the negative input voltage for the negative
potentials (lower electrodes) and the positive voltage input for the
positive potentials (upper electrodes).
The detector is the same 8th generation 1704 channel capacitive
transimpedance amplifier (CTIA) array detector used in our previous work [22e24]. Each of the 1704 pixels is 12.5 mm wide by 3 mm
tall. Additional information about the detector is described in
Refs. [24,29]. Ideally, the detector should be cooled to 230 K to
reduce thermal noise. However, for this proof-of-concept, detector
cooling was not implemented, and the temperature of the detector
was measured at approximately 320 K, resulting in significant
thermal noise in the experimental data.
Samples included bulk copper and thin films of Cu, Ti, Ni, and Cr.
The bulk Cu (HO2 Cu alloy 110, 99.97% Copper) was fabricated into a
5 mm 19.5 mm sample 0.5 mm thick by Fotofab, Inc. To validate
the isotope ratio of the Cu sample from Fotofab, Inc., two samples
magnetic lines from the Nd-Fe-B towards the center of the air gap
between the pole pieces. The magnet was fabricated by Electron
Energy Corporation, Inc. The magnetic field in the center of the gap
is 0.7 T ± 1% within an area of 80 mm 60 mm parallel to the faces
of the pole pieces. The magnetic field map of this area is shown in
Fig. 3a and b superimposed on a simulation of cycloidal ion trajectories. The magnetic sector weighs 48.4 kg.
The electric sector consists of a stack of 27 rectangular, ringshaped aluminum electrodes of two different sizes: the lower
electrodes, #1 - #14, have an interior open area of 94 mm 18 mm,
and the upper electrodes, #15 through #27, have an interior open
area of 54 mm 18 mm. The upper electrodes are shorter in length
to provide space for the detector to be positioned. The thickness of
all 27 electrodes is 3 mm and they were separated by 0.5 mm,
resulting in an internal height of 91 mm. The aluminum electrodes
are plated with 12e25 mm of high phosphorus nickel and
0.25e0.5 mm of gold to prevent surface charge accumulation
(Epner, Inc. Brooklyn, NY). High (>9%) phosphorous content nickel
was selected as the adhesion layer between the aluminum and the
gold as it eliminates the magnetic permeability of nickel, preserving
the uniformity of the magnetic field. The electric field is generated
by supplying a different potential to each electrode defined by E,
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R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
The simulated electric field was 3.0332 V/mm with a variation of
±0.0004 V/mm due to the periodic nature of the electrodes.
The charged particle simulations generated ions from circular
spots located at the laser focus position with diameters between
10-300 mm. The simulation generated 99,999 ions from random
angles (0e60 from the y-axis) across the surface of the circular
spots with energies of 75 ± 25 eV. The energy and angular spread
were chosen to approximate the energy and angular spread corresponding to laser ionization occurring at a surface with 1 mJ
266 nm laser pulse focused to a 300 mm diameter spot size [14]. The
effects of space charge were neglected in these simulations.
Fig. 3c displays the simulated charged particle ion trajectories
for 65Cu and 63Cu ions, and Fig. 3d shows the corresponding
simulated spectra generated from these ions for several different
emission spot sizes ranging from 10 to 300 mm. The simulated
spectra were generated by recording the position at which the ions
strike the detector plane and counting the number of particles that
hit each of the 12.5 mm wide x 3 mm tall pixels. The inset of Fig. 3e
shows the full width half maximum (FWHM) of the peaks for each
circular emitter spot size. Note that for peak widths of >50 mm, the
FWHM of the peak closely matches the diameter of the emitter.
However, for peak widths <50 mm, the peak width is significantly
larger than the diameter of the emitter spot. The ultimate resolution (or peak width, Dp) of a cycloidal mass analyzer is related to
the electric and magnetic field uniformity according to the re-
were analyzed via a Thermo Scientific NEPTUNE Plus multicollector inductively coupled plasma mass spectrometer (ICP-MS)
from diluted digested stock solution at the Pennsylvania State
University Laboratory for Isotopes and Metals in the Environment
(LIME). This analysis revealed a ratio 65Cu/63Cu ¼ 0.446 with a
d65Cu ¼ 0.15 relative to the CuNIST976 isotope standard from NIST,
"
#
where d65 Cu ¼
ð65 Cu=63 CuÞsample
ð65 Cu=63 CuÞNIST976
1 1000. E-beam evaporation
was used to generate the metal thin film samples: 400 nm Ti film,
100 nm Cr film, 100 nm Ni film, and 100 nm Cu film. The target
metals were evaporated onto a silicon wafer (arsenic doped,
1e10 U/cm2).
The vacuum system is composed of an aluminum vacuum
chamber, brass front and rear feedthroughs, a pressure sensor, and
turbopump backed by a dry mechanical pump. The vacuum
chamber was machined from 6061 aluminum and designed to slide
between the pole pieces of the magnetic sector and house the
electric sector and detector. Brass feedthroughs are attached to the
front and back of the vacuum chamber. The front feedthrough
contains an anti-reflection coated window to transmit the laser
through the vacuum feedthrough and feedthroughs fabricated from
FR4 PCboards are used for the electric sector and detector signals
and are sealed with TorrSeal vacuum epoxy. The rear feedthrough
provides a mount for the pressure sensor (Pfeiffer Vacuum model
IKT 010) and the turbopump (Agilent model TV81 M 80 L/s) backed
by a dry mechanical pump (Agilent Technologies IDP-15 Dry Scroll).
The pressure during data collection was 4 x 107 hPa.
The laser employed for VS-CMS POC was a frequency quadrupled Nd:YAG laser at 266 nm with 10 ns laser pulses at an energy up
to 10 mJ/pulse (Quantel Ultra50 laser by Lumibird). Data from the
laser manufacturer indicates that the beam has a Gaussian intensity
profile. Two laser attenuators, comprised of a quarter waveplate
and polarizer (990-0071-266 Eksma Optics), were used to attenuate the laser to 1 mJ/pulse. Finally, a 150 mm focal length fused
silica lens, positioned in front of the vacuum feedthrough, was used
to focus the laser through the window in the vacuum feedthrough
to a 300 mm spot at the sample. Scanning electron microscopy was
used to determine the laser spot size by measuring the diameter of
the laser ablation spot on the sample (see Fig. 5d).
Dp
, where E and B are the electric
lationships DE ¼ EDp p, and DB ¼ B2p
and magnetic field magnitudes, DE and DB, are the change in the
field magnitudes along the ion trajectory, Dp is the desired peak
width (emitter size), and p is the pitch of the cycloidal mass
analyzer. The relationships for the electric and magnetic field uniformity are found by differentiating for the pitch p with respect to E
and B. To achieve virtual slit focusing for a Dp of 10 mm and a pitch of
26 mm, the required electric field uniformity is 0.04% and the
required magnetic field uniformity is 0.02% over the ion trajectories. Simulations of the POC-VS-CMS electric sector indicate an
electric field uniformity of 0.03%, meeting the 0.04% uniformity
requirement for virtual-slit focusing. The fabricated magnet sector
used in this POC does not meet the calculated uniformity requirement of 0.02%; however, for a 50 mm emitter, the magnetic field
uniformity requirement is only 0.1%. As shown by the red contour
line in Fig. 3a, the majority of the ion trajectories are inside of the
region where the field varies by 0.1%. Therefore, for future prototypes aimed at analyzing particles 10 mm, a more uniform
magnetic field will be required for virtual-slit focusing.
We simulated an alternative magnet with a smaller gap between
the poles and improved field uniformity to test the hypothesis that a
more uniform magnetic field would result in improved virtual-slit
focusing for smaller particles. This magnet with improved field uniformity (Fig. 4a) consists of two Nd-Fe-B grade N52 magnets with
dimensions of 5 mm 170 mm 170 mm (h w d) and a gap of
20 mm. An H-shaped, 1020 steel yoke was used to complete the
magnetic circuit and no pole pieces were used. The field at the center
of the gap is 0.709 T, with homogeneity of 0.06% over the area of the
ion trajectories as shown in Fig. 4b. This magnetic sector is estimated
to weigh 52 kg. Fig. 4c shows simulated trajectories and spectra for a
10 mm emitter using this magnetic sector with improved uniformity.
While this magnetic sector does not meet the 0.02% uniformity
requirement for virtual-slit focusing of a 10 mm emitter, it is significantly more uniform than the magnet sector used in the POC apparatus. The FWHM of the peaks in the simulated spectrum are 15 mm,
validating the hypothesis that a more uniform magnetic field improves virtual-slit focusing, thus enabling the use of smaller emitters.
3.2. Finite element simulations
Finite element analysis simulations of the VS-CMS POC
including ion trajectory calculations were performed using OPERA
FEA (Dassault Systems). OPERA was selected because it allows for
multiple FEA models to be combined into a single simulation. We
simulated the magnetic sector using the magnetostatic solver to
calculate the magnetic field. Then, we applied the results of the
magnetostatic simulations in a charged particle simulation to
model the electric field and the resultant ion trajectories.
The magnetic field for the magnetic sector geometry described
in section 3.1 was calculated by applying constant magnetic properties to the Nd-Fe-B grade N52 magnets (relative
permeability ¼ 1.04 and coercivity ¼ 1.1 106 A/m) and nonlinear
BH curves to the low carbon steel 1020 pole pieces and yoke (taken
from Ref. [30]). Fig. 3a shows the magnetic field map produced by
the magnetic sector at the center of the gap. A simplified model of
the electric sector and detector was used to calculate the electric
field. Features from the fabricated electric sector such as external
holes, fillets, etc. were eliminated in this simplified model because
they are not expected to impact the homogeneity of the electric
field and removing them increases the speed of the FEA simulation.
6
R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
Fig. 4. CAD model of a magnetic sector with improved field uniformity. The Nd-Fe-B magnets are in black and the yoke is in gray. A 30 cm ruler is included for scale. (b) Magnetic
field contour lines of the improved magnet. The blue contour lines represent increments of 0.01% of the maximum magnetic field value of 0.709 T. The black lines represent an
example of simulated cycloidal trajectories superimposed on the magnetic field map. (c) Mass spectrum comparing simulated data for 63Cu and 65Cu isotopes using the current POC
magnetic sector and using the improved magnetic sector (shown in (a)) with a 10 mm diameter emitter.
Fig. 5. (a) Copper mass spectrum taken with the POC instrument and Gaussian fits of the peaks, E ¼ 3.03 V/mm. (b) Comparison between the experimental copper spectrum from
the POC instrument and simulated copper spectrum with a laser spot size of 300 mm. (c) Titanium mass spectrum from the POC instrument and Gaussian fits of the peaks, E ¼ 5.0 V/
mm. (d) SEM images of the laser ablation spots on bulk copper (left) and 400 nm titanium film on silicon (right). All raw mass spectra data was smoothed using Savitzky-Golay
(window ¼ 5, pol: 2nd order) algorithm to enhance visibility. Fitting was done on unsmoothed data. Original raw data is included in the supplementary material.
experimentally validating the virtual-slit focusing concept.
Furthermore, Fig. 5b shows the experimental data for Cu plotted
with the simulated data from Fig. 3b showing close correspondence
between simulation and experiment.
The experimental data contains a significant amount of thermal
noise due to the high operating temperature of the detector in the
POC apparatus. As a result, we have employed a Savitzky-Golay
(win ¼ 5, pol: 2nd order) smoothing algorithm to enhance peak
visibility. All fitting analysis for peak width and the isotope ratio
measurements described below were performed on unsmoothed
experimental data. Furthermore, examples of unsmoothed data are
provided in Figure S 1 e Figure S 4. In Fig. 5a, and in all experimental data, we observed a broad, low intensity peak beneath the
3.3. Experimental validation of the virtual-slit concept
The POC instrument and laser ionization were used to generate
the representative mass spectra in Fig. 5a and c for a bulk Cu target
and a Ti thin film, respectively. Figure S 2 and Figure S 3 in the
supplementary material show representative data from thin films
of Cr and Ni, respectively. Each spectrum is from a single 1 mJ laser
pulse focused on the sample with a 300 mm diameter spot size. As
hypothesized, the FWHM of the peaks corresponding to the various
isotopes for each material are equal to the laser ablation spot size
on the substrate. SEM (Fig. 5d, Figure S 2, and Figure S 3) of the laser
ablated area was measured to further validate the hypothesis that
laser spot size correlates to the peak FWHM, therefore,
7
R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
size, and (iii) improving the magnetic field uniformity, a very low
detection limit is possible.
primary, high intensity peaks (see Figure S 1 e Figure S 4). The
origin of this board, secondary peak is unknown. The peak appears
to be more prominent for higher m/q (Cu, Cr, and Ni), and is less
noticeable at lower m/q (Ti) where the electric field magnitude is
higher. Due to the geometry of the electric sector and positioning of
the laser focal spot close to the boundary of the electric sector, a
significant number of the ions are collected by these electrodes (see
Fig. 3c). This causes a temporary perturbation in the electrode
voltage, and therefore electric field. The broad peak could be a
result of altered ion trajectories due to this temporary perturbation
in the electric field. For lower m/q, where the applied electric field is
higher, the resulting voltage perturbations are smaller. This could
explain why lower m/q ions, like Ti, display a smaller broad peak
compared to higher m/q samples. In future prototypes, shifting the
position of ionization further from the electric sector sidewalls may
reduce or eliminate this broad peak.
The isotope ratios for Cu, Ti, Ni, and Cr, were measured by fitting
the data using Gaussian distributions. All fitting parameters were
free to be adjusted, except that all isotope peaks should share the
same width. This is reasonable due to the fact that they originate
from the same ionization site. Table 2 summarizes the isotope ratio
measurements. For Ti and Ni, our measurements are quite close to
values found in literature. For Cu and Cr, there is less agreement
between our measurements and those measured by ICP-MS in the
case of copper and reported in the literature in the case of Cr. Deviations from reference values are likely due to the significant
thermal noise impacting the performance of the detector and the
presence of the broad background peak that complicates data
analysis. Deviations from reference values could also be a result of
isotope fractionation effects similar to those observed in laser
ablation multi-collector inductively coupled plasma mass spectrometry [17]. Cooling the detector to reduce thermal noise and
eliminating the broad background peak by shifting the ionization
away from the edge of the electric sector as described above should
improve isotope ratio accuracy.
Lastly, the sensitivity of the POC instrument was estimated by
repeatedly shooting the laser at the same spot on Cu and Ti thin
films deposited on silicon substrates, until the copper or titanium
signal disappeared and a silicon signal appeared. For Cu, we started
with a thin film of 100 nm. After 450 1 mJ laser pulses, the Cu signal
disappeared and a silicon signal appeared. From this data, we estimate that the Cu signal in Fig. 5a is from approximately 140 fg of
ablated material. Similarly, for Ti, we started with a film of 400 nm.
After 2000 1 mJ laser shots, the Ti signal disappeared and we began
to see the signal from the silicon substrate. Therefore, we estimate
that the Ti signal in Fig. 5c is from approximately 64 fg of ablated
material. These results imply that with some improvements to the
instrument in terms of (i) increasing the signal to noise by cooling
the detector, (ii) increasing the resolution by reducing the laser spot
4. Summary and conclusion
In summary, this paper presents a proof of concept for virtualslit focusing in which the small, spatial volume from which ions
are generated by localized ionization techniques (e.g., laser ionization of particles and surfaces, and spark ionization) is used
instead of a physical slit. This concept is enabled by the perfect
focusing properties of the cycloidal mass analyzer. This virtual-slit
concept was validated using finite element analysis simulations of
electric and magnetic fields from a POC apparatus. Furthermore,
the virtual-slit concept was validated experimentally with the POC
apparatus where the width of the laser ablation spot was shown to
be the same size as the width of the peak at the detector for a variety of samples including Cu, Ti, Ni, and Cr. Finally, the capability of
the POC apparatus to measure isotope ratios was demonstrated and
a sensitivity of 60e150 fg was estimated. These results show the
potential for the unique focusing properties of the cycloidal mass
analyzer when coupled with an array detector to enable relatively
small, high-resolution, high sensitivity instruments using laser
ionization of surfaces, samples residing on surfaces, and single
particles. These proof of concept results suggest that with some
improvements to the POC instrument in terms of increasing signal
to noise by cooling the detector and increasing resolution by
reducing the laser spot size, a very low detection limit is possible.
Therefore, the VS-CMS concept has the potential to enable development of relatively small instruments for in situ chemical,
elemental, and isotopic analysis of organic and inorganic samples,
compared to currently used laser ionization time-of-flight and laser
ablation inductively coupled plasma mass spectrometers.
Credit roles (see: https://www.elsevier.com/authors/policiesand-guidelines/credit-author-statement)
Rafael Bento Serpa: Formal analysis; Investigation; Data Curation; Writing e original draft; Visualization, Elettra L. Piacentino:
Investigation; Writing e Review & Editing, Kathleen L Horvath:
Investigation; Writing e Review & Editing, Tanouir Aloui: Investigation; Writing e Review & Editing, Yuriy Zhilichev: Investigation;
Writing e Review & Editing, Charles B. Parker: Data Curation;
Writing e Review & Editing; Funding Acquisition, Jeffrey T. Glass:
Writing e Review & Editing; Funding Acquisition, Scott B Tilden:
Software; Writing e Review & Editing, Justin Keogh: Investigation;
Formal analysis, Software; Writing e Review & Editing, Robert
Kingston: Software; Writing e Review & Editing, Roger P. Sperline:
Conceptualization; Investigation; Writing e Review & Editing, M.
Bonner Denton: Conceptualization; Formal analysis; Investigation;
Writing e Review & Editing; Supervision; Funding acquisition,
Jason J. Amsden: Conceptualization; Formal analysis; Investigation;
Writing e Review & Editing; Supervision; Project administration;
Funding acquisition.
Table 2
Summary of the reference and experimentally measured isotopic ratios.
Element
Reference Isotopic Ratio
Measured Isotopic Ratio
Copper *
Chromium #
65
65
Nickel #
Titanium #
Cu/63Cu ¼ 0.446
50
Cr/52Cr ¼ 0.07
53
Cr/52Cr ¼ 0.31
54
Cr/52Cr ¼ 0.11
60
Ni/58Ni ¼ 0.38
62
Ni/58Ni ¼ 0.06
46
Ti/48Ti ¼ 0.11
47
Ti/48Ti ¼ 0.07
49
Ti/48Ti ¼ 0.08
50
Ti/48Ti ¼ 0.09
Cu/63Cu ¼ 0.53
Cr/52Cr ¼ 0.05
53
Cr/52Cr ¼ 0.11
54
Cr/52Cr ¼ 0.02
60
Ni/58Ni ¼ 0.38
62
Ni/58Ni ¼ 0.05
46
Ti/48Ti ¼ 0.11
47
Ti/48Ti ¼ 0.10
49
Ti/48Ti ¼ 0.07
50
Ti/48Ti ¼ 0.07
50
Declaration of competing interest
The authors declare the following financial interests/personal
relationships which may be considered as potential competing
interests: M. Bonner Denton, Jason Amsden, Rafael Serpa, Elettra
Piacentino, Charles Parker, Justin Keogh, Roger Sperline, and Scott
Tilden has patent VIRTUAL-SLIT CYCLOIDAL MASS SPECTROMETER
pending to Duke University.
* Externally measured (LIMA at Penn State University).
# [28].
8
R.B. Serpa, E.L. Piacentino, K.L. Horvath et al.
International Journal of Mass Spectrometry 470 (2021) 116706
Acknowledgements
[13]
The information, data, or work presented herein was funded in
part by the Office of the Director of National Intelligence (ODNI),
Intelligence Advanced Research Projects Activity (IARPA), via
2019e19031300004. The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government or any agency thereof. The authors would also like to
thank Matthew Fantle and Sara Kimming of the Penn State University Laboratory for Isotopes and Metals in the Environment for
analysis of the copper isotope ratios with ICP-MS, and Mike Gehm
for use of the optical table in his laboratory.
[14]
[15]
[16]
[17]
Appendix A. Supplementary data
[18]
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.ijms.2021.116706.
[19]
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