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Carbon Nanotube Electron Ionization Source for Portable Mass Spectrometry Theresa Evans-Nguyen,†,^ Charles B. Parker,‡ Christina Hammock,§ Andrew H. Monica,§ Elena Adams,§ Luann Becker,|| Jeffrey T. Glass,‡ and Robert J. Cotter*,† †
Department of Pharmacology, Johns Hopkins School of Medicine, Baltimore, Maryland, United States Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina, United States § Space Systems Application Group, Johns Hopkins University Applied Physics Laboratory, Laurel Maryland, United States Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, United States
)
‡
ABSTRACT: Cold cathode carbon nanotubes (CNTs) are used in a low-voltage quadrupole ion trap mass spectrometer and shown to be a viable low-power alternative to filament sources for portable mass spectrometry instrumentation. No heating is necessary, and the power consumption depends only on the switching characteristics of the electronics. The CNT electron sources are mounted directly in the ring electrode, and their performance is compared directly with a filament source also mounted in the ring electron. Up to a 5 10 4 Torr CO2 environment, reflecting conditions expected during operation in a Mars atmosphere, the CNT emitters may provide up to 1 μA of current over more than 200 h.
he conventional thermionic filament, though a reliable ionization source in mass spectrometers, typically consumes on the order of 1 W in power. For fielded instruments, such a power sink is undesirable, and alternative sources1 for electron ionization have been explored. In the Rosetta mission aboard comet Churyumov Gerasimenko,2 a silicon nanotip array provides electrons using field emission. Kornienko et al.3 studied silicon whiskers as a cold cathode source in a microfabricated ion trap mass spectrometer. However, most materials that have been evaluated for field emission offer lifetimes limited to a few hours and often require redundant architectures. In field portable mass spectrometers (typically ion traps), which employ low-power vacuum pumps, the relatively high pressure (∼10 3 Torr) can rapidly degrade the emitters, thereby exacerbating the lifetime problem. Discovered in 1991,4 carbon nanotubes (CNTs) have been recognized for their unique properties of strength, thermal stability, and conduction. Their potential widespread applications include conductive fillers for polymeric composites, field effect transistors, atomic force microscopy tips, electrochemical devices, fuel cells, and field emission displays.5 7 Their potential as field emitters has been widely studied.8 12 In mass spectrometry, CNTs have been employed for electron capture dissociation13 and matrix-assisted laser desorption ionization (MALDI).14,15 Notably, Bower et al.16 demonstrated CNT emitter operation with a maximum of 50 μA from a 70 μm 70 μm array in pressure conditions up to 5 10 2 Torr. Recently, Getty and coworkers investigated the estimated electron emission lifetime of CNTs for a miniature time-of-flight mass spectrometer (TOFMS).17 19 CNTs are an attractive alternative to conventional
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r 2011 American Chemical Society
electron ionization filaments because they do not require the power consumption associated with heating. A CNT electron ionization source is evaluated herein for use in an ion trap mass spectrometer as part of the Mars organic molecule analyzer (MOMA) for the 2018 ExoMars mission.
’ EXPERIMENTAL METHODS The instrument employed has been described previously.20 Briefly, ion trap electrodes from a Thermo Finnigan GCQ mass spectrometer were removed from their commercial housing and installed in a custom-built vacuum chamber. A commercial rf generator from MassTech Inc. (Columbia, MD) was operated at ∼240 V0 p and 735 kHz to provide the fundamental rf supply to the ring electrode. Two collinearly drilled holes (Figure 1a) through the ring electrode housed the electron ionization source including the standard GCQ filament assembly and the carbon nanotube assemblies. With the fundamental rf voltage to the ring electrode held constant, a supplemental sinusoidal waveform (2 5 Vp p) was applied to the end-cap electrodes to perform a frequency scan typically from 350 to 15 kHz. The low-voltage waveform provided a roughly mass-linear scan using a PCI-5411 arbitrary waveform generator from National Instruments (Austin, TX) through a balun transformer from North Hills (Syosett, NY). Received: March 13, 2011 Accepted: June 30, 2011 Published: June 30, 2011 6527
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Figure 2. Uncalibrated scan performed with a 765 kHz fundamental frequency at 130 Vpp. (Supplemental dc scanned from 350 to 15 kHz (4 Vpp) in ∼280 ms; chamber pressure (helium) ∼2.7 10 5 Torr; 5 ms ionization; 35 ms cooling; 2000 collected spectra.)
Figure 1. (a) Ion trap electrode configuration, shown with electron filament assemblies, (b) SolidWorks representation of Xintek cathode incorporation into ion trap electrodes, (c) SEM image of grown CNTs, (d) Duke CNT implementation/assembly, and (e) testing chamber for lifetime studies of APL-grown CNTs.
Ionization was performed by both the conventional filament assembly and by two carbon nanotube sources. In the case of the filament, a dc power supply provided the heating current and was floated with a 60 V bias power supply. In the case of the carbon nanotubes, a high-voltage power supply provided 1 5 kV/mm to the cathode in a pulsed fashion using high-voltage switches from Behlke (Kronberg, Germany). For electron emission comparisons, both the filament assembly and the CNT assemblies were monitored by continuous operation through the ring electrode (disconnected from the rf source). Intermittently between mass scans, current to the “grid” of the electron assembly and the ring electrode, which were in metal-to-metal contact, was manually read through an ammeter tied to ground. Carbon nanotube field emitters used in these studies were obtained from three sources. Commercial CNT electrodes were purchased from Xintek (Research Triangle Park, NC). The electrode was removed from its housing and directly inserted into the electron source opening of the trap’s ring electrode (Figure 1b) using a Vespel spacer to approximate the distance to the ring electrode as 0.8 mm at its closest distance and 1.8 mm at its farthest distance. Second, carbon nanotube wafers were prepared at Duke University using microwave plasma-enhanced chemical vapor deposition. First, the catalyst for CNT growth, in this case
50 Å layer of iron, was selectively evaporated onto the cathode using an integrated shadow mask. The metalized wafers were subjected to ammonia pretreatment at 100 sccm for 180 s at 750 C at 21 Torr, 2.1 kW to dewet the iron film into nanoparticle catalysts (10 100 nm). Multiwalled nanotubes were then grown with the addition of methane to the ammonia for an additional 120 s. A hydrogen plasma treatment was used to clean the amorphous carbon. A scanning electron microscopy (SEM) profile image of the wafer is shown in Figure 1c. The CNT wafer was cut into a ∼2 mm 2 mm piece and mounted in place of the filament in the commercial GCQ filament assembly with doublesided copper tape (Figure 1d). To facilitate the use of smaller fields, shorter distances between the assembly’s exit lens were achieved by metal shims providing an ultimate estimated distance of 0.5 mm. Notably, the exit lens was in direct contact with the ring electrode, and thus its electrical potential was dependently tied in both cases of the standard filament and the manufactured carbon nanotube wafer. Finally, CNTs were additionally produced at the Johns Hopkins Applied Physics Laboratory (APL) and used for lifetime testing. The process for growth began with a similar 5 nm layer of iron over bare silicon. The samples were then heated for 30 min to 730 C in air, then subjected to 15 min of argon gas flow at 1500 sccm, after which hydrogen gas at a rate 1000 sccm was added for 10 min. Argon was then reduced to 1000 sccm, hydrogen was turned off, and a C2H2 treatment of 10 sccm was applied for 15 min. The C2H2 treatment was stopped, argon was increased to 1500 sccm, and the furnace was turned off to allow the samples to cool in the argon atmosphere. Lifetime studies of these CNTs as well as the Dukefabricated CNTs were performed at the Johns Hopkins Applied Physics Lab in a controlled vacuum using a gold-coated silicon wafer as the anode for current measurements (Figure 1e). In these studies, a constant current was set up between the CNT cathode and the Au/Si anode while the voltage was monitored over time.
’ RESULTS Commercial CNTs. Preliminary work was performed as depicted in Figure 1b using the commercially available Xintek 6528
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Figure 3. Uncalibrated PFTBA spectrum from commercial cathode electron ionization with an applied 1600 V/∼0.03 0.07 in. on partial pressure of 5 10 6 Torr of PFTBA.
CNT emitters, primarily marketed for use in electron microscopes. Manufacturer specifications indicated electron emission at electric fields below 4 V/μm fields with a maximum current of 100 μA/mm2. These CNTs were operated at fields of approximately 2.2 V/μm producing a measured current on the ring electrode of 0.5 μA. In Figure 2, with a helium pressure of 2.7 10 5 Torr, background spectra were taken in the low-mass region (12 100 Da) to determine if any ions were produced from the CNTs themselves. With a 130 Vpp applied rf voltage, a dominating ∼14 Da peak can be seen in Figure 2. Addition of a low-mass calibrant proved difficult because the trap rapidly filled with the unknown species. To rule out the possibility of simple immediate ejection of unstable species at the start of the scan, the trapping voltage and thus the low-mass cutoff, was lowered to 10 Da (96.5 Vpp) yet still produced the same ∼14 Da peak (data not shown). Because mass assignments are nonlinear in the lowmass region using the dipolar supplemental frequency scanning technique and also due to the lack of space charge control which can contribute to peak shifting, the identity of this species is unclear. Without a suitable calibrant, we currently presume the contaminant peak to be simply amorphous carbon, C+. The source of this carbon may be due to ion sputtering of the stainless steel of the ring electrode which was used as the anode for the Xintek source as depicted in Figure 1b. Alternatively the contamination source may be from the manufacture of the CNTs themselves. Because the ionic carbon species were speculated to compromise the electron ionization spectra through potential chemical ionization reactions, perfluorotributylamine (PFTBA) was analyzed using the Xintek cathode ionization source. The low-mass cutoff was raised to 29 Da to avoid filling the trap with the low-mass carbon species and to increase the trapping potential well depth for the higher mass species. In Figure 3, the PFTBA spectral peaks of 69 and 131 appear with an estimated CNT emission current of 1 μA. No spurious peaks are observed that are readily attributable to alternative chemical ionization processes. Nonetheless, other CNT sources were tested for further consideration in the operation of MOMA. Custom-Fabricated CNTs. Positioned on opposite sides of the ring electrode (Figure 1b), the Duke-fabricated CNT emitter and the standard filament assembly were operated in an alternating fashion for side-by-side comparison. The spectra obtained from the filament (Figure 4, parts a and c) and the CNT sources (Figure 4, parts b and d) of background air and a hydrocarbon mixture air all appear remarkably comparable. Thus, no obvious chemical reactions could differentiate the two ionization sources from potential reagent species from the CNTs.
Figure 4. Mass spectral comparisons between filament and CNT EI spectra: (a) background filament spectrum with a 5 μA emission current, (b) background CNT spectrum with a 30 μA emission current, (c) filament ionization of a hydrocarbon mixture, and (d) CNT ionization of the same hydrocarbon mixture. To collect the background spectra, short cooling times of 5 ms were used.
Notably, in Figure 4b, to achieve comparable electron ionization (EI) ion signal intensity from background air, the required measured CNT current reached 30 μA, or 6 times higher than that of the filament. The reduced efficiency of ionization may be attributed to lower ionization cross sections at higher electron kinetic energies. For example, the EI cross section of water at 60 eV is 2 3 Å2, but at 2.4 keV, it diminishes to 0.4 Å2.21 For a field of 3 V/μm, PFTBA sample pressure of 1 10 6 Torr, and helium pressure of 1 10 5 Torr, the measured emission current was approximately 35 μA. Replacement of the PFTBA and helium supplies with 1 10 5 Torr of the 1 ppm hydrocarbon mixture in helium yielded an emission current of 120 μA. The increased electron current supports evidence of enhanced emission with hydrocarbon adsorption.22,23 However, in Figure 4c, the corresponding spectra from ionization by a 2 μA filament followed by the 120 μA CNTs show that the increased electron current does not translate to higher signal even after taking into account the lower ionization cross sections. Most of the CNT current presumably does not enter the ion trap volume due to the high kinetic energy of the electrons, space charge 6529
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Table 1. APL CNT Operation Conditions for Lifetime Studies current
size and
voltage
lifetime
(μA)
shape of emitter
(V)
(failure/turned off), h
GEN 2A 100 GEN 2B 10
8 mm 20 μm 8 mm 20 μm
300 f 1.1 kV 390 f 1.1 kV
F, 9 11 F, 25 33
1 mm 1 mm
400 f 1.1 kV
F, 17 27
name
GEN 3
10
(array, 20 μm wide)
Figure 5. (a) Duke CNT (1800 V/0.030 in.) assembly emission current degradation, (b) Duke CNT lifetime study in 5 10 4 Torr of CO2 emitting at 1 μA, and (c) APL lifetime study in 5 10 4 Torr of CO2 emitting at 1 μA.
Figure 6. Lifetime study of APL CNT operation in 5 10 4 Torr of CO2 to maintain 10 μA (Gen 2B, Gen 3) and 100 μA (Gen 2A) currents.
effects, and the lack of suitable ion optics. Interestingly, low-mass carbon species attributable to the configuration of the commercial CNTs were not observed in the Duke CNT spectra at typical cooling times (∼30 ms), possibly attributable to the plasma treatment to remove amorphous carbon. Alternatively, because the anode of the Duke CNTs is independent of the ring electrode, the possibility of ion sputtering is greatly reduced, which could account for the absence of the low-mass contaminant observed with the commercial CNT cathode. Over the course of several days of pulsed activation (2.4 V/μm, 2.3 Hz, 5 ms duration), the CNT emitter current degraded considerably in the 1 10 5 Torr helium environment (Figure 5a), but settled around 15 μA. Degradation can be attributed to several possible causes including cathode heating and oxidation. Enhanced study of lifetime effects were studied using the
configuration shown in Figure 1e at APL. With dynamic feedback, the Duke-fabricated CNT cathodes were tested for lifetime stability as shown in Figure 5b within a CO2 environment of 5 10 4 Torr. A constant 1 μA of current could be maintained for almost 200 h of continuous operation without causing runaway voltage adjustments. Likewise, in Figure 5c, one generation of CNTs produced at APL performed in a similar manner for 370 h under the same conditions. A systematic study of CNT lifetime stability of higher currents (10 and 100 μA) was performed with three iterations of the APL CNTs as portrayed in Figure 6 and tabulated in Table 1. In previous work, enduring performance of CNT electron emission has been studied with more than 10 2 Torr of helium for several days.24 However, as shown in Figure 6, an environment of only 5 10 4 Torr of CO2 resulted in runaway failure from degradation of the CNTs. Such behavior has been noted by others22,23,25,26 who attribute the phenomenon to an increased work function of the CNT surface due to adsorbed gas.
’ CONCLUSIONS Cold field-emitting CNTs are shown in this work to be a viable low-power alternative to filament sources for portable mass spectrometry instrumentation. Considering a filament operating at a conservative 0.5 W of continuous power consumption, the field emitters tested herein required considerably lower power on the order of 0.5 mW. No heating is necessary, and thus the power consumption only depends on the switching characteristics of the electronics. Even taking into account a 1% duty cycle in which even the heating current of the filament is pulsed, the power consumed would still be 10 times that of the field emitter. Other carbon-based cold cathode field emitters of note include so-called carbon nanoparticles27,28 or carbon nanopearls29 and have produced EI spectra without mention of ions formed by chemical ionization. Similarly, from the faithful reproduction of filament EI spectra, the CNT EI sources do not appear at first glance to encourage unintended chemical ionization pathways. Curiously, in the commercial variation, presumed amorphous carbon ion species do exist and crowd out the low-mass region but are fortunately not observed in the Duke-fabricated CNTs. Because the Xintek electrodes required low-mass exclusion to avoid overfilling the trap and the MOMA mission objectives may eventually include low-mass observations, the Duke CNTs are better candidates for consideration. Upon further examination of the custom-fabricated CNTs from Duke, we observed enhanced electron current from the hydrocarbon exposure, but interestingly, no corresponding spectral signal intensity appeared. The higher electron current might be attributed to hydrocarbon adsorption, though ionization efficiency clearly suffered. An interesting aspect of this work is the similarity of mass spectra from such high-energy electrons. Although typical EI cross sections for most species are highest in the range of 100 eV, the presumed energy of the electrons produced by these CNTs 6530
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’ AUTHOR INFORMATION Present Addresses ^
Current address: Draper Laboratory, 3802 Spectrum Blvd. Suite 201, Tampa, FL 33612.
’ ACKNOWLEDGMENT This work was supported in part by a NASA Technology Development Project (TDP) contract NNX08AO82G to L. Becker. ’ REFERENCES
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’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on August 10, 2011. Additional minor text corrections were added and the corrected version was reposted on August 31, 2011.
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