Introduction: Miniaturization of mass spectrometers for field and space applications is a growing trend,

with a distinct emphasis on Micro-Electro-Mechanical Systems (MEMS), also referred to as

mass spectrometers on a chip. But miniaturization is often associated with performance

limitations, such as lower resolving power, smaller mass range, and lower sensitivity as

compared to full laboratory and desktop systems. Those limitations are clearly a heavy

price to pay to provide the transportable, low-cost, robust, and light-weight systems that

are desired for field applications. This article describes an evaluation of a magnet-assisted

EI source as a means of improving sensitivity while keeping the instrument's size and

resolving power unchanged. This rather conventional cross-beam magnetic EI source

(CBM-EI), which is based on the 1947 design by Nier1, is currently used in many if not most

commercial gas chromatograph/mass spectrometers. But this work, with support from

both 3D modeling and experimental data, shows that the CBM-EI source shapes,

concentrates, and amplifies the ion beam in a way that is especially compatible with the

object-slit-based ion trajectory and beam geometry of the SFMS design. Thus, the CBM-EI

provides benefits for SFMS-based instruments that are unavailable from non-magnetized

CB-EI sources. In addition, 3D modeling of an axial-beam magnetic EI source (ABM-EI) for

the SFMS is briefly introduced to highlight its potential extra benefits and to support a

planned future experimental evaluation of an ABM-EI source in the SFMS.

High-Efficiency Cross-Beam Magnetic Electron-Impact Source (CBM-EI) for Boosted Miniature Sector-Field Mass Spectrometer (SFMS) Performance

x y

z

0.1 mm slit plate 1.5 mm air gap

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y z

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m-1

)

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normalized to electrons

normalized to ions

N+ 2 n

orm

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ign

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SB/S

(B=

0))

B-field (G)

Experimental setup: The strength of the non-scanning SFMS design is that it allows ions of different m/z to be

detected simultaneously and continuously, i.e., with a quasi-100% duty cycle, when a solid-

state array detector is used. However, most of the pixelated array detectors, including the

IonCCD used in the work reported here, offer virtually no inherent signal gain, which

places them at an inherent disadvantage relative to secondary electron-multiplier type

detectors, even though much of the resulting sensitivity deficit can be offset by the

simultaneous and continuous nature of non-scanning SFMS detection.

Figure1: CBM-EI and MH-MS based system. a) Photograph of the 9 in. long SFMS system. b)

Zoomed view of the CBM-EI showing the magnet frame around the EI source and the cross-beam

(CB) geometry. c) Geometry and magnetic field modeling (Dexter Magnetic Technologies, Hicksville,

NY) of the source magnetic assembly. The magnetic field analysis is illustrated in a 4 mm-diameter

circle defining the ionization volume.

10 20 30 40 50 60 70 80 90 100

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4.2 W, 227 , 4.2 nA

3.3 W, 132 A, 2.2 nA

Io

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nal

(d

N)

18(5/7)Re

2+

m/z (u)

1 kV-1 T, 7*10-7 torr, 0 G vs. 450 G cross beam magnetic EI source

100 frames averaged @ 100 ms integration time/frame

Theoretical results: 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

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

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

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

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

appreciation for the electrostatic forces involved. The illustrated view plane was also selected to appropriately visualize the geometry of the dual-filament-equipped

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).

Figure 2: Potential diagram view showing the 3D simulation of the electron trajectories in

CBM-EI (top) and CB-EI (bottom) ion sources. The X-axis (not shown in the coordinate

diagram) is the electrical potential axis. The view is the source cross-section that is

perpendicular to the ion-extraction axis. The electrons are emitted from the left side and

reflected or trapped at the opposite side. The slender yellow rectangle (0.1 x 1.5 mm) shows

the ionization area that yields optimum transmission through the SFMS system.

Figure 3: 3D bar plot showing the simulation results of the electron density calculated

at the center of the yellow rectangle in Figure 2 at line of sight of the IonCCD detector.

The electron density is plotted as a function of the electron repeller bias voltage and the

B-field strength. The simulation predicts that maximum electron density occurs at a 0

V repeller bias and a 500- to 550 G magnetic field strength. The electron trap was

biased at 2 V.

Experimental results: 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

function of the B-field strength, normalized to the corresponding CB-EI data and EI source parameters (figure 5).

Figure 4: Mass spectrum of the residual gas composition of the SFMS instrument (7*10-7 Torr). The orange- and blue-

filled spectra were produced with the CB-EI and CBM-EI ion sources respectively. The inset values are the ion source

conditions with respective B-field values.

Figure 5: Ratio of CBM-EI to CB-EI nitrogen signals (i.e., the "normalized" CBM-EI signal) plotted as a function of the

CBM-EI source magnetic field strength. The data are further normalized to equivalent levels of electron emission current

(solid) and extracted ion current (dashed) for the two ion sources.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 700.01

0.1

1

10

Ion t

ransm

issi

on (

%)

m/z (u)

BvqFB

Limitation for Proton Detection Previous work showed that a redesign of the magnetic-sector allows for high dynamic mass range

(HDMR magnet)2, where simultaneous detection of proton (unit mass) to mass 70 u is achieved.

Solution to the problem: The low-mass efficiency loss described above may be virtually eliminated by using an axial-beam

magnetic EI source (ABM-EI)3. The deflecting magnetic force above is eliminated in the ABM-EI

source, since the ion velocity (extraction axis) is collinear with the B-field. Moreover, the ABM-EI

should yield even further improvement in the SFMS sensitivity as a result of the alignment of the

ion-extraction axis with the long axis of the ionization volume (see figure bellow). This alignment

will place essentially all of the ionization volume in the line of sight of the 1.5-mm air gap of the

magnetic sector and the IonCCD detector that is situated at the instrument's focal plane.

However thermal ions will be subject

to magnetic deflection in the SFMS

analysis plane. This deflection should

be stronger and more observable for

masses below about 14 u. This effect is

certainly not desirable for hydrogen

applications where signal loss is

strongly amplified. Simulation shows

that protons would be detected with

100-fold less transmission efficiency

than that for high-mass (>14 u) ions.

Figure 7: 3D modeling for the next generation axial

beam magnetic electron-impact source (ABM-EI) source

from which encouraging results were observed. We have

made plans to construct and evaluate an ABM-EI source

at our facility in the near future.

Conclusion and Applications: 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.

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.

5. Confidence gained for moving forward with the proton-discrimination-free ABM-EI.

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

sniffing) and corrosive samples and carrier gases.

Acknowledgement The authors acknowledges the financial support of OI Analytical for this research. The principal author would like to thank Michael Devine and Chun Li from Dexter Magnetic Technologies for the modeling and fabrication of the source magnetic assembly and for 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.

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). 4. O. Hadjar, W. K. Fowler, G. Kibelka and W. C. Schnute, Journal of American Society of Mass Spectrometry 23 (2), 418-424 (2012).

Figure 6: Simulation of ion transmission at 400G CBM-EI for the mass range [1, 68] u.

a)

b) c)

Omar Hadjar, Bill K. Fowler, Gottfried Kibelka, Chad Cameron, Scott Kassan and Ken Kuhn

OI Analytical, 2148 Pelham Pkwy, Bldg. 400, Pelham, AL 35124