1. High-Efficiency Cross-Beam Magnetic Electron-Impact Source (CBM-EI)
for Boosted Miniature Sector-Field Mass Spectrometer (SFMS) Performance
Omar Hadjar, Bill K. Fowler, Gottfried Kibelka, Chad Cameron, Scott Kassan and Ken Kuhn
OI Analytical, 2148 Pelham Pkwy, Bldg. 400, Pelham, AL 35124
Introduction: Theoretical results: Limitation for Proton Detection
Miniaturization of mass spectrometers for field and space applications is a growing trend, The 3D simulation of the electron trajectories was performed by SIMION 8.0, a widely used software package for charged-particle optics simulation. In order to simulate Previous work showed that a redesign of the magnetic-sector allows for high dynamic mass range
with a distinct emphasis on Micro-Electro-Mechanical Systems (MEMS), also referred to as the performance of the CBM-EI ion source, we started electrons from a 100-µm-diameter filament as shown in figure 2. For optimally realistic results, the electrons were (HDMR magnet)2, where simultaneous detection of proton (unit mass) to mass 70 u is achieved.
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mass spectrometers on a chip. But miniaturization is often associated with performance simulated to start from an area 1-mm long along the length of the filament and centrally located on the filament, with 180° emission solid angle and a Gaussian energy However thermal ions will be subject
limitations, such as lower resolving power, smaller mass range, and lower sensitivity as distribution centered at 0.5 eV with 0.3 eV FWHM. Every simulated data point in this paper is the statistical result of 4000 electrons. For every run, the magnetic field as to magnetic deflection in the SFMS
Ion transmission (%)
compared to full laboratory and desktop systems. Those limitations are clearly a heavy well as the electron-trap and-repeller voltages were varied systematically. Figure 2 illustrates the electron trajectories in a potential energy view, to elicit a better analysis plane. This deflection should
1
price to pay to provide the transportable, low-cost, robust, and light-weight systems that appreciation for the electrostatic forces involved. The illustrated view plane was also selected to appropriately visualize the geometry of the dual-filament-equipped be stronger and more observable for   
are desired for field applications. This article describes an evaluation of a magnet-assisted CBM-EI. The XYZ axes of Figure 2 were maintained from Figure 1 to keep a clear 3D view of the electron-injection axis (Z) with respect to ion-extraction axis (X). masses below about 14 u. This effect is FB  q  v  B
EI source as a means of improving sensitivity while keeping the instrument's size and certainly not desirable for hydrogen 0.1
resolving power unchanged. This rather conventional cross-beam magnetic EI source 60
applications where signal loss is
-1
)
ron density (m
(CBM-EI), which is based on the 1947 design by Nier1, is currently used in many if not most 50 strongly amplified. Simulation shows
commercial gas chromatograph/mass spectrometers. But this work, with support from that protons would be detected with 0.01
40 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
both 3D modeling and experimental data, shows that the CBM-EI source shapes, 100-fold less transmission efficiency m/z (u)
concentrates, and amplifies the ion beam in a way that is especially compatible with the
30 Figure 6: Simulation of ion transmission at 400G CBM-EI
than that for high-mass (>14 u) ions.
for the mass range [1, 68] u.
elect
object-slit-based ion trajectory and beam geometry of the SFMS design. Thus, the CBM-EI 20
provides benefits for SFMS-based instruments that are unavailable from non-magnetized 10 Solution to the problem:
CB-EI sources. In addition, 3D modeling of an axial-beam magnetic EI source (ABM-EI) for 0 The low-mass efficiency loss described above may be virtually eliminated by using an axial-beam
ele
0
the SFMS is briefly introduced to highlight its potential extra benefits and to support a -2
ctr
-5 magnetic EI source (ABM-EI)3. The deflecting magnetic force above is eliminated in the ABM-EI
o
-10
nr
planned future experimental evaluation of an ABM-EI source in the SFMS. -15 60 700 source, since the ion velocity (extraction axis) is collinear with the B-field. Moreover, the ABM-EI
epe
-20 40 500 0
z -25 30
l
0
y should yield even further improvement in the SFMS sensitivity as a result of the alignment of the
ler
20 0
10
Experimental setup: (G)
-30 0 0
0
-field
(V
B ion-extraction axis with the long axis of the ionization volume (see figure bellow). This alignment
)
The strength of the non-scanning SFMS design is that it allows ions of different m/z to be will place essentially all of the ionization volume in the line of sight of the 1.5-mm air gap of the
Figure 2: Potential diagram view showing the 3D simulation of the electron trajectories in Figure 3: 3D bar plot showing the simulation results of the electron density calculated
detected simultaneously and continuously, i.e., with a quasi-100% duty cycle, when a solid- magnetic sector and the IonCCD detector that is situated at the instrument's focal plane.
CBM-EI (top) and CB-EI (bottom) ion sources. The X-axis (not shown in the coordinate at the center of the yellow rectangle in Figure 2 at line of sight of the IonCCD detector.
state array detector is used. However, most of the pixelated array detectors, including the
diagram) is the electrical potential axis. The view is the source cross-section that is The electron density is plotted as a function of the electron repeller bias voltage and the
IonCCD used in the work reported here, offer virtually no inherent signal gain, which
perpendicular to the ion-extraction axis. The electrons are emitted from the left side and B-field strength. The simulation predicts that maximum electron density occurs at a 0
places them at an inherent disadvantage relative to secondary electron-multiplier type
reflected or trapped at the opposite side. The slender yellow rectangle (0.1 x 1.5 mm) shows V repeller bias and a 500- to 550 G magnetic field strength. The electron trap was
detectors, even though much of the resulting sensitivity deficit can be offset by the
the ionization area that yields optimum transmission through the SFMS system. biased at 2 V.
simultaneous and continuous nature of non-scanning SFMS detection.
a) z x Experimental results:
y In addition to the foregoing simulation, we also studied experimentally the effect of the CBM-EI relative to the CB-EI on SFMS performance. The first of two experiments
was carried out in closed capillary mode at high electron emission regime (i.e., high filament current) while monitoring the whole mass spectrum (figure 4). The second
experiment, which was more extensive, was carried out in open capillary mode while monitoring the nitrogen peak-area signal at moderate electron emission as a Figure 7: 3D modeling for the next generation axial
function of the B-field strength, normalized to the corresponding CB-EI data and EI source parameters (figure 5). beam magnetic electron-impact source (ABM-EI) source
from which encouraging results were observed. We have
-7
10000 1 kV-1 T, 7*10 torr, 0 G vs. 450 G cross beam magnetic EI source made plans to construct and evaluate an ABM-EI source
0.1 mm slit plate
IonCCD signal (dN)
1.5 mm air gap at our facility in the near future.
1000 3.3 W, 132 A, 2.2 nA
4.2 W, 227 , 4.2 nA 18(5/7) 2+
Conclusion and Applications:
100 Re 1. Theoretical and experimental demonstration of one to two orders of magnitude in signal boost.
2. This outcome is decoupled from MCP results4 hence total improvement of 105 can be expected.
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3. Theory-to-experiment agreements for CBM-EI suggests promising ABM-EI results.
4. SFMS permits conversion of gained sensitivity into higher resolution hence higher mass range.
1
5. Confidence gained for moving forward with the proton-discrimination-free ABM-EI.
y 10 20 30 40 50 60 70 80 90 100
b) c) z x 100 frames averaged @ 100 ms integration time/frame m/z (u) 6. Magnetic confinement will be exploited to differentially pump the filament emission area from
the sample ionization area to allow extending IonCam application to oxidizing (direct air
N2 normalized signal (SB/S(B=0))
Figure1: CBM-EI and MH-MS based system. a) Photograph of the 9 in. long SFMS system. b) normalized to electrons sniffing) and corrosive samples and carrier gases.
Figure 4: Mass spectrum of the residual gas composition of the SFMS instrument (7*10-7 Torr). The orange- and blue- normalized to ions
Zoomed view of the CBM-EI showing the magnet frame around the EI source and the cross-beam 100
filled spectra were produced with the CB-EI and CBM-EI ion sources respectively. The inset values are the ion source Acknowledgement
(CB) geometry. c) Geometry and magnetic field modeling (Dexter Magnetic Technologies, Hicksville, The authors acknowledges the financial support of OI Analytical for this research. The principal author would like to thank Michael
conditions with respective B-field values. Devine and Chun Li from Dexter Magnetic Technologies for the modeling and fabrication of the source magnetic assembly and for
NY) of the source magnetic assembly. The magnetic field analysis is illustrated in a 4 mm-diameter modeling the cylindrical magnet for the ABM-EI. The author would like to thank CMS Field Products for technical support. The
work was performed at CMS Field Products, a subsidiary of OI Analytical, within the Analytics Value Center of Xylem, Inc.
circle defining the ionization volume. 10
References
1. A. O. Nier, Review of Scientific Instruments 18 (6), 398-411 (1947).
2. O. Hadjar, T. Schlathölter, S. Davila, et al. Journal of the American Society for Mass Spectrometry 22 (10), 1872-1884 (2011).
3. C. J. Park and J. R. Ahn, Review of Scientific Instruments 77 (8), 1-5 (2006).
Figure 5: Ratio of CBM-EI to CB-EI nitrogen signals (i.e., the "normalized" CBM-EI signal) plotted as a function of the 1 4. O. Hadjar, W. K. Fowler, G. Kibelka and W. C. Schnute, Journal of American Society of Mass Spectrometry 23 (2), 418-424 (2012).
+
CBM-EI source magnetic field strength. The data are further normalized to equivalent levels of electron emission current
-100 0 100 200 300 400 500 600 700
(solid) and extracted ion current (dashed) for the two ion sources.
B-field (G)