Chemical Ionization Mass Spectrometry Using Carbon Nanotube Field Emission Electron Sources
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A novel chemical ionization (CI) source has been developed based on a carbon nanotube (CNT) field emission electron source. The CNT-based electron source was evaluated and compared with a standard filament thermionic electron source in a commercial explosives trace detection desktop mass spectrometer. This work demonstrates the first reported use of a CNT-based ion source capable of collecting CI mass spectra. Both positive and negative modes were investigated. Spectra were collected for a standard mass spectrometer calibration compound, perfluorotributylamine (PFTBA), as well as trace explosives including trinitrotoluene (TNT), Research Department explosive (RDX), and pentaerythritol tetranitrate (PETN). The electrical characteristics, lifetime at operating pressure, and power requirements of the CNT-based electron source are reported. The CNT field emission electron sources demonstrated an average lifetime of 320 h when operated in constant emission mode under elevated CI pressures. The ability of the CNT field emission source to cycle on and off can provide enhanced lifetime and reduced power consumption without sacrificing performance and detection capabilities.
Key wordsCarbon nanotube CNT Field emission Field emitter Mass spectrometry Chemical ionization Explosives
Mass spectrometry (MS) is a standard laboratory technology for identifying trace levels of chemical compounds in complex environments. The selectivity and specificity of chemical identification via MS across a wide range of species arguably exceeds that of other more commonly fielded instruments, such as Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). For example, the accurate and rapid identification of trace explosives by MS is of great interest in the realm of early threat detection for homeland security and defense [1, 2]. The purpose of the work reported here was to evaluate the use of carbon nanotube (CNT) field emitters as the electron source in a mass spectrometer designed for security checkpoint screening, and reports the first utilization of a CNT field emission source for chemical ionization mass spectrometry (CI-MS).
Ionization of the analyte of interest is a critical step in the identification process during mass spectrometry . Electron ionization (EI) and chemical ionization (CI) are both used when identifying chemical threats with MS. CI-MS is advantageous for the selective identification of explosives as they can be negatively ionized more easily than most interferents because of the strong electron affinity of the nitro group of these compounds . However, gas requirements, power consumption, and inadequate lifetime are often limiting factors for the use of conventional ion sources in portable mass spectrometers. Thus, the development of a low cost, replaceable CI mass spectrometer field emission electron source would be beneficial for use in mass spectrometers for portable applications, such as security checkpoint screening.
Field emission electron sources have several advantages for portable applications, such as very small size, low power consumption, and minimized thermal degradation of the materials. Since first being reported as an electron field emitter in 1995 [5, 6, 7], carbon nanotubes have proven to be a viable candidate for electron sources in various applications [8, 9]. CNTs have a high aspect ratio, small tip radius of curvature, good conductance, extremely large Young’s modulus (high stiffness), and high tensile strength allowing the tip to withstand the extremely strong fields (several V/nm) needed for field emission . Mass spectrometers usually require current levels in the 1–2 mA/cm2 range (10 to 100 s of μA emitted from a filament). Many groups have shown that CNT field emitters are capable of emission current exceeding 10 mA/cm2 [10, 11, 12], demonstrating they can provide sufficient electron emission current for MS. Turn-on fields for CNT field emitters are also very low, enabling low power operation. Wang et al. reported turn-on fields as low as 0.64 V/μm at a current density of 10 μA/cm2 from functionalized CNT structures . Furthermore, the highly ordered covalently bonded crystalline structure of the CNTs provide increased stability compared with other metal field emitters commonly used, such as tungsten and rhenium. The activation energy for surface migration of atoms on CNTs is significantly larger (17 eV) than tungsten (3.2 eV) and rhenium (2.7 eV); thus CNTs are less sensitive to surface migration . CNTs also possess a high melting temperature compared with refractory metals such as tungsten. Additionally, ions impact electron sources during MS operation, thereby sputtering the electrode and decreasing its lifetime. CNTs are composed of carbon, which has a low sputter coefficient , making them robust field emitters for mass spectrometry. With so many benefits, it is not surprising that the number of CNT field emitter applications have increased in the last decade. However, while CNTs have been used as field emission sources in numerous applications , including EI-MS [16, 17, 18, 19], there are, to our knowledge, no reports that have examined CI using a CNT field emission electron source.
In this research, we evaluate a custom packaged CNT field emission electron source that allows for direct replacement of an existing filament thermionic emission electron source in a FLIR System (“FLIR”) (FLIR Systems, Inc., West Lafayette, IN, USA) explosives trace detection desktop mass spectrometer for use in CI-MS mode. To our knowledge, the present research demonstrates the first use of a CNT field emission electron source for CI-MS. Electrical characteristics, power requirements, and lifetime for these devices have been evaluated. Both negative and positive chemical ionization (NCI and PCI, respectively) are examined. Mass spectra for MS calibrants and trace explosives have been obtained with this novel CI source.
A custom PCB platform (Figure 1b) was designed to allow for direct fit and replacement of the existing filament thermionic emission source installed in the prototype desktop mass spectrometer. A single die with CNTs was mounted with conductive silver paste to the center electrode; the thickness of the paste was approximately 20 μm. A surrounding square electrode was then fitted with a custom 750 μm thick copper standoff fabricated by photochemical machining (Fotofab, Inc., Chicago, IL, USA). An extraction grid of tungsten woven wire mesh with 36% open area was attached to the top of the copper standoff, providing an extraction grid for electron field emission. Thinner meshes were investigated but did not provide sufficient mechanical stability. Figure 1c shows devices with and without the extraction mesh above the CNT die. The CNT tip to mesh distance was in the range of 195 μm to 205 μm with an average of 200 μm because of the thickness of the conductive silver paste.
Prior to testing in the mass spectrometer, the fabricated devices were characterized in a test bed using two Keithley 2410 source-measure units and a custom LabVIEW program. This test bed provided a controlled environment and geometry for device characterization prior to mounting in the desktop mass spectrometer. The test bed also allowed for failure analysis and examination of the effects of controlled pressure variation. Current-voltage (I-V) curves were measured to determine basic device parameters and model the electrical behavior (hereafter referred to as ex-situ characterization to distinguish it from the data taken while in the mass spectrometer). I-V curves were taken at two pressures, a low vacuum level of approximately 1 × 10–4 Pa (1 × 10–6 Torr) to verify device operation, and 1 × 10–2 Pa (1 × 10–4 Torr) of room air, the approximate operating pressure and reagent gas used in the explosives trace detection desktop mass spectrometer during CI mode.
During the ex-situ characterization, the turn-on voltage (Vto) and threshold voltage (Vth) were measured for all devices. Vto is defined as the applied voltage when the emitted current reaches 1 nA, and Vth is defined as the voltage required to emit 1 μA of electron current. Devices were then subjected to lifetime testing at a pressure of 1 × 10–2 Pa of lab air until failure. Emitter current was held constant by using a feedback control loop to vary the bias applied to the extraction grid to maintain 5 μA. I-V measurements from the electrodes were collected every second. Devices that were used in the desktop mass spectrometer tests underwent 1 h of burn-in  at 1 × 10–2 Pa to condition the CNT emitters before installing them into the mass spectrometer. Initial testing of the sources in the mass spectrometer was performed by operating the instrument with the ion source outlet flow restrictors removed so that the source was effectively performing electron ionization rather than CI. Once device performance was confirmed, flow out of the ion volume was again restricted to achieve source pressures suitable for CI.
The ion source conditions were identical when using either the CNT field emission electron source or the filament thermionic emission source. A short capillary restrictor admits the sample into an ion region heated to 180°C with a volume of about 1 cm3. Two small orifices communicate with the bulk vacuum chamber of the mass spectrometer. A 200 μm diameter orifice in a 6 mm diameter stainless steel plate serves to admit electrons from either the filament or the CNT source. A 400 μm diameter orifice in another plate allows ions created in the ion source to enter into electrostatic lenses that focus the ion beam into the mass analyzer. The pressure in the ion source, although not measured directly in these experiments, can be calculated in the 13 to 133 Pa (0.1 to 1 Torr) range based on the conductance’s of the inlet capillary and the orifices that let gas out of the ion volume into the main vacuum chamber. This pressure range is consistent with most CI source pressures. The only gas admitted into the ion volume was lab air from directly above the sample ticket that also contains the analytes in the gas phase; therefore the “chemical ionization gas” consisted primarily of nitrogen and oxygen.
Mass spectra were acquired over the m/z range of 40 to 700 for the standard MS calibration compound perfluorotributylamine (PFTBA). Headspace vapor was leaked into the vacuum system under control of a flow restrictor and solenoid valve. Spectra for trace explosives were acquired by dosing aliquots of a solution containing the explosive in an organic solvent (typically methanol) onto a surface-sampling ticket, allowing the solvent to dry, and then analyzing the ticket via thermal desorption and transport of the explosive analyte into the CI volume. Mass analysis was performed with an ion trap analyzer coupled to the CI volume with electrostatic lenses for ion transport. Ion detection was via a conversion dynode/electron multiplier assembly.
Results and Discussion
For the device whose data is shown in Figure 2, and using a typical MWCNT value of 4.8 eV for ϕ, we observed slope coefficients of –16.67 and –33.49 for the high and low regions, respectively, which yielded β values of 4309 and 2144, respectively. We observed that the computed β can diverge from the reported values by ± 18%. This is due to the variation of the distance from the CNTs to the extraction mesh caused by the silver paste thickness. The reported β values, inclusive of the variation range, also closely coincide with literature as expected .
Figure 2c shows the ex-situ emission lifetime measurements of a device at a steady state pressure of 1 × 10–2 Pa (1 × 10–4 Torr). The device functioned for more than 320 h of continuous operation. Assuming 10 h of continuous operation per day, this indicates a required emitter replacement period of approximately once per month. However, assuming a reasonable duty cycle that accounts for the sample collection and traveler interactions that would occur at a security checkpoint as 25% on-time, then this 320 h lifetime would equate to 4 months of service. Another benefit of using the CNTs is the ability to cycle the emitter on and off with minimal degradation. A standard tungsten filament could not be cycled quickly without dramatically shortening its lifetime. Cold field emission cathodes, including these CNT field emitters, can be quickly cycled between on and off states to conserve power without affecting emitter lifetime . We found that a 10 s on/off cycle had little effect on CNT cathode performance or lifetime in the test bed. Specifics of pulsing performance will be the focus of a future publication. It should be noted that an air jet directed at the CNTs during CI operation in the explosives trace detection mass spectrometer can reduce on-time, however modifications could be made to the system to correct for this. Nevertheless, the ability to power cycle without a lifetime penalty, coupled with its low power and low temperature characteristics, make the CNT field emitters attractive options for further study and a viable candidate for CI and EI ion sources.
The increase in the voltage required to maintain 5 μA emission over time suggests gradual CNT field emitter degradation (Figure 2c). This degradation effect has been reported previously [28, 29, 30, 31, 32] and was related to several distinct causes, including shortening of the CNTs from ion bombardment and damage to the sidewalls from joule heating at the CNT tip during electron field emission. The voltage spikes observed in Figure 2c can be attributed to sudden changes in emission current forcing fast instrument correction to maintain the set current. These sudden changes in emission current could be explained by CNTs undergoing deflection during the emitting process. It has been reported  that transients in emission current can be caused by electrodynamic interactions between field emitting CNTs. Deflected CNTs experiencing these interactions can undergo an abrupt pull, resulting in the sudden current changes.
Integration into the Desktop Mass Spectrometer
Power requirements of the CNT field emission electron source installed in the mass spectrometer were minimal. Operating voltages varied between 500 and 1500 V with 5 to 50 μA of emission current, equating to a maximum source power requirement of 75 mW. The mass spectrometer allowed 10 ms between the time the voltage was applied to the source and the time ions were accepted from the source by the mass analyzer for emission of electrons to begin. The minimum response time of the CNT source was not interrogated, but in all cases emission was observed after this 10 ms period in the form of ion signal. Thus 10 ms serves as a maximum bound for the turn-on time. The kinetic energies of electrons emitted from the CNT field emission electron source were not directly measured, but are initially expected to be similar to the bias voltage applied before being cooled by the gas in the source.
Evidence for Chemical Ionization
Mass spectra collection was conducted with the filament thermionic emission source operating at its standard setting of 200 μA and the CNT field emission source at 50 μA. The latter was limited by the dimensions of the CNT source and the instrument capabilities. Nonetheless, the lower current is beneficial for mobile instruments for portable operation due to its contribution to lower power. The differences in electron current could have an effect on the intensity of the mass spectral signal if the electrons are the limiting reagent in the ionization reaction. Thus, this work makes no assertion about the MS signal intensity resulting from the CNT versus the filament source. However, the current is not expected to affect the peaks that appear for each analyte or their relative abundance. Thus, the nearly identical peak structure of the resulting mass spectra indicate that the effect of the electrons from the CNT array, once emitted and cooled by the CI gas, are essentially identical to the effect of the electrons emitted by the filament.
Prior to collection and examination of mass spectra for explosive analytes, the instrument was calibrated using a standard MS calibration gas (PFTBA) in EI mode, as well as both positive and negative chemical ionization modes. Analysis and brief discussion of the PFTBA spectra can be found in Supplementary Data and provides further evidence for chemical ionization resulting from each source. After spectra were collected using the calibration gas, amounts consistent with trace standards in the security field of trinitrotoluene (TNT), Research Department explosive (RDX), and pentaerythritol tetranitrate (PETN) were examined. Identical amounts of trace explosives were introduced during spectra collection for each source. The same emission currents used for PFTBA spectra collection were also used with the trace explosives (50 μA for the CNT field emission source and 200 μA for the filament thermionic source).
If EI dominated the ionization mechanism, one would expect no significant signal beyond m/z 222, which corresponds to the molecular ion of RDX. The source of the NO2– is likely RDX itself, although some NOx production in an ion source that contains both N2 and O2 may be reasonably expected. The mass locations and assignments for additional fragmentations include m/z = 102 [(C2H4N3O2)–], m/z = 129 [(C3H5N4O2)–], and m/z = 324 [(RDX + 102)–]. The above peaks are consistent between the two sources as well as those observed with NCI literature spectra using filament thermionic emitters [35, 37, 39]. Finally, the small peak at m/z 176 could be attributed to [(RDX – NO2)–] [C3H6N5O4]– .
A custom designed, fully packaged CNT field emission device has been developed and used in a prototype commercial mass spectrometer as an electron source to obtain chemical ionization mass spectra in both positive and negative modes. Although further study is needed, this source is a candidate for CI and EI mass spectrometry applications. The CNT field emission source demonstrated low power, low temperature, and attractive lifetime. A constant-on lifetime of 320 h was demonstrated for the CNT devices. This lifetime equates to 4 months of operation, assuming a 25% on-time in real-time security checkpoint deployment. The CNT emitters also demonstrated the ability to cycle and thus avoid continuous-on operation, the details of which will be the subject of future publication. Spectra were collected for a standard MS calibration material (PFTBA) as well as trace amounts of explosives (TNT, RDX, and PETN). The spectra collected showed similar peaks for both the CNT field emission source and a traditional filament thermionic emission electron source in the same system. Furthermore, the EI and CI spectra collected with the CNT field emission source closely matched those observed in literature that were produced with traditional filament thermionic emission sources. Sensitivities for the CNT field emission source were similar to filament thermionic emitters used in the same mass spectrometer.
This work was performed with support of the US Department of Homeland Security Science and Technology Directorate.
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