B American Society for Mass Spectrometry, 2015DOI: 10.1007/s13361-015-1212-0

J. Am. Soc. Mass Spectrom. (2015) 26:1903Y1910

RESEARCH ARTICLE

Chemical Ionization Mass Spectrometry Using CarbonNanotube Field Emission Electron Sources

Erich J. Radauscher,1 Adam D. Keil,2 Mitch Wells,2 Jason J. Amsden,1 Jeffrey R. Piascik,3

Charles B. Parker,1 Brian R. Stoner,1,3 Jeffrey T. Glass1

1Department of Electrical and Computer Engineering, Duke University, Durham, NC 27708, USA2FLIR Systems, Inc., West Lafayette, IN 47906, USA3Engineering and Applied Physics Division, RTI International, Research Triangle Park, NC 27709, USA

CNT Field Emission Electron Source

Abstract. A novel chemical ionization (CI) source has been developed based on acarbon nanotube (CNT) field emission electron source. The CNT-based electronsource was evaluated and compared with a standard filament thermionic electronsource in a commercial explosives trace detection desktop mass spectrometer. Thiswork demonstrates the first reported use of a CNT-based ion source capable ofcollecting 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 demon-strated an average lifetime of 320 h when operated in constant emission mode under elevated CI pressures. Theability of the CNT field emission source to cycle on and off can provide enhanced lifetime and reduced powerconsumption without sacrificing performance and detection capabilities.Key words: Carbon nanotube, CNT, Field emission, Field emitter, Mass spectrometry, Chemical ionization,Explosives

Received: 29 April 2015/Revised: 27 May 2015/Accepted: 11 June 2015/Published Online: 2 July 2015

Introduction

Mass spectrometry (MS) is a standard laboratory technol-ogy for identifying trace levels of chemical compounds

in complex environments. The selectivity and specificity ofchemical identification via MS across a wide range of speciesarguably exceeds that of other more commonly fielded instru-ments, such as Raman spectroscopy and Fourier transforminfrared spectroscopy (FTIR). For example, the accurate andrapid identification of trace explosives by MS is of greatinterest in the realm of early threat detection for homelandsecurity and defense [1, 2]. The purpose of the work reportedhere was to evaluate the use of carbon nanotube (CNT) fieldemitters as the electron source in a mass spectrometer designed

for security checkpoint screening, and reports the first utiliza-tion of a CNT field emission source for chemical ionizationmass spectrometry (CI-MS).

Ionization of the analyte of interest is a critical step in theidentification process during mass spectrometry [3]. Electronionization (EI) and chemical ionization (CI) are both usedwhenidentifying chemical threats with MS. CI-MS is advantageousfor the selective identification of explosives as they can benegatively ionized more easily than most interferents becauseof the strong electron affinity of the nitro group of thesecompounds [4]. However, gas requirements, power consump-tion, and inadequate lifetime are often limiting factors for theuse of conventional ion sources in portable mass spectrometers.Thus, the development of a low cost, replaceable CI massspectrometer field emission electron source would be benefi-cial for use in mass spectrometers for portable applications,such as security checkpoint screening.

Field emission electron sources have several advantages forportable applications, such as very small size, low power con-sumption, and minimized thermal degradation of the materials.

Electronic supplementary material The online version of this article(doi:10.1007/s13361-015-1212-0) contains supplementary material, which isavailable to authorized users.

Correspondence to: Erich T. Radauscher; e-mail: ejr18@duke.edu

Since first being reported as an electron field emitter in 1995[5–7], carbon nanotubes have proven to be a viable candidatefor electron sources in various applications [8, 9]. CNTs have ahigh aspect ratio, small tip radius of curvature, good conduc-tance, extremely large Young’s modulus (high stiffness), andhigh tensile strength allowing the tip to withstand the extremelystrong fields (several V/nm) needed for field emission [8]. Massspectrometers usually require current levels in the 1–2 mA/cm2

range (10 to 100 s of μA emitted from a filament). Many groupshave shown that CNT field emitters are capable of emissioncurrent exceeding 10 mA/cm2 [10–12], demonstrating they canprovide sufficient electron emission current for MS. Turn-onfields for CNT field emitters are also very low, enabling lowpower operation. Wang et al. reported turn-on fields as low as0.64 V/μm at a current density of 10 μA/cm2 from functional-ized CNT structures [13]. Furthermore, the highly orderedcovalently bonded crystalline structure of the CNTs provideincreased stability compared with other metal field emitterscommonly used, such as tungsten and rhenium. The activationenergy for surface migration of atoms on CNTs is significantlylarger (17 eV) than tungsten (3.2 eV) and rhenium (2.7 eV);thus CNTs are less sensitive to surface migration [8]. CNTs alsopossess a high melting temperature compared with refractorymetals such as tungsten. Additionally, ions impact electronsources during MS operation, thereby sputtering the electrodeand decreasing its lifetime. CNTs are composed of carbon,which has a low sputter coefficient [14], making them robustfield emitters for mass spectrometry. With so many benefits, itis not surprising that the number of CNT field emitter applica-tions have increased in the last decade. However, while CNTshave been used as field emission sources in numerous applica-tions [15], including EI-MS [16–19], there are, to our knowl-edge, no reports that have examined CI using a CNT fieldemission electron source.

In this research, we evaluate a custom packaged CNT fieldemission electron source that allows for direct replacement of anexisting filament thermionic emission electron source in a FLIRSystem (BFLIR^) (FLIR Systems, Inc., West Lafayette, IN,USA) explosives trace detection desktop mass spectrometerfor use in CI-MS mode. To our knowledge, the present researchdemonstrates the first use of a CNT field emission electronsource for CI-MS. Electrical characteristics, power require-ments, and lifetime for these devices have been evaluated. Bothnegative and positive chemical ionization (NCI and PCI, re-spectively) are examined. Mass spectra for MS calibrants andtrace explosives have been obtained with this novel CI source.

ExperimentalFabrication of the CNT field emission electron source involveda two-step process: CNTs were grown on prepared substratesand then mounted onto a specially designed printed circuitboard (PCB) field emission platform. CNT substrates wereprepared by performing a 250 μm shallow cut (or trench) witha dicing saw into 500 μm thick conductive silicon wafers to

define 3.6 mm2 die sizes. The trench allowed for separationafter growth of the CNT emitters by carefully snapping thewafer along this trench. A 50 Å film of iron was deposited ontothe substrates by electron beam evaporation to serve as a CNTcatalyst. CNTs were prepared using microwave plasma-enhanced chemical vapor deposition (MPECVD) as previouslyreported [20]. Briefly, the substrates were loaded into thegrowth chamber and treated with a 100 sccm (standard cubiccentimeters per minute) ammonia heat-up cycle for 120 s toreach pretreatment conditions of 850°C, 21 Torr, and 2.15 kW.Next, a pretreatment step consisting of 100 sccm of ammoniafor 360 s de-wet the iron film into catalyst nanoparticle tem-plates with diameters of 50–100 nm. Vertically alignedmultiwalled nanotubes (VA-MWCNTs, hereafter CNTs) ap-proximately 30 μm in height were grown using a 3:1 gas ratioof 150 sccm methane and 50 sccm ammonia for an additional360 s. After growth, the 3.6 mm2 dies were separated bysnapping along the trenches for mounting and testing. A scan-ning electron microscope (SEM) image of the CNT forest isshown in Figure 1a.

A custom PCB platform (Figure 1b) was designed to allowfor direct fit and replacement of the existing filament therm-ionic emission source installed in the prototype desktop massspectrometer. A single die with CNTs was mounted with

10 mm10 mm20 mm

(a)

(b) (c)

10 mm10 mm

Extraction Mesh

CNTSample

Figure 1. Images of the CNT electron source components (a)SEM image of CNT field emitters as grown on the substrate, (b)photograph of the Duke/FLIR CNT electron source platform setinside its wiring harness mount, (c) photograph of CNT fieldemitters mounted into the platform. The source is shown beforeand after placement of extraction mesh

1904 E. J. Radauscher et al.: Chemical Ionization MS Using CNT FE Sources

conductive silver paste to the center electrode; the thickness ofthe paste was approximately 20 μm. A surrounding squareelectrode was then fitted with a custom 750 μm thick copperstandoff fabricated by photochemical machining (Fotofab, Inc.,Chicago, IL, USA). An extraction grid of tungsten woven wiremesh with 36% open area was attached to the top of the copperstandoff, providing an extraction grid for electron field emis-sion. Thinner meshes were investigated but did not providesufficient mechanical stability. Figure 1c shows devices withand without the extraction mesh above the CNT die. The CNTtip to mesh distance was in the range of 195 μm to 205 μmwithan average of 200 μm because of the thickness of the conduc-tive silver paste.

Prior to testing in the mass spectrometer, the fabricateddevices were characterized in a test bed using two Keithley2410 source-measure units and a custom LabVIEW program.This test bed provided a controlled environment and geometryfor device characterization prior to mounting in the desktopmass spectrometer. The test bed also allowed for failureanalysis and examination of the effects of controlledpressure variation. Current-voltage (I-V) curves weremeasured to determine basic device parameters and mod-el the electrical behavior (hereafter referred to as ex-situcharacterization to distinguish it from the data takenwhile in the mass spectrometer). I-V curves were takenat two pressures, a low vacuum level of approximately1×10–4 Pa (1×10–6 Torr) to verify device operation, and1×10–2 Pa (1×10–4 Torr) of room air, the approximateoperating pressure and reagent gas used in the explosives tracedetection 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 currentreaches 1 nA, and Vth is defined as the voltage required to emit1 μA of electron current. Devices were then subjected to lifetimetesting at a pressure of 1×10–2 Pa of lab air until failure. Emittercurrent was held constant by using a feedback control loop tovary the bias applied to the extraction grid to maintain 5 μA. I-Vmeasurements from the electrodes were collected every second.Devices that were used in the desktop mass spectrometer testsunderwent 1 h of burn-in [21] at 1×10–2 Pa to condition the CNTemitters before installing them into themass spectrometer. Initialtesting of the sources in the mass spectrometer was performedby operating the instrument with the ion source outlet flowrestrictors removed so that the sourcewas effectively performingelectron ionization rather than CI. Once device performance wasconfirmed, flow out of the ion volume was again restricted toachieve source pressures suitable for CI.

The ion source conditions were identical when using eitherthe CNT field emission electron source or the filament therm-ionic emission source. A short capillary restrictor admits thesample into an ion region heated to 180°C with a volume ofabout 1 cm3. Two small orifices communicate with the bulkvacuum chamber of the mass spectrometer. A 200 μm diameterorifice in a 6 mm diameter stainless steel plate serves to admitelectrons from either the filament or the CNT source. A 400 μm

diameter orifice in another plate allows ions created in the ionsource to enter into electrostatic lenses that focus the ion beaminto the mass analyzer. The pressure in the ion source, althoughnot measured directly in these experiments, can be calculated inthe 13 to 133 Pa (0.1 to 1 Torr) range based on the conduc-tance’s of the inlet capillary and the orifices that let gas out ofthe ion volume into the main vacuum chamber. This pressurerange is consistent with most CI source pressures. The only gasadmitted into the ion volume was lab air from directly abovethe sample ticket that also contains the analytes in the gasphase; therefore the Bchemical ionization gas^ consisted pri-marily of nitrogen and oxygen.

Mass spectra were acquired over them/z range of 40 to 700 forthe standard MS calibration compound perfluorotributylamine(PFTBA). Headspace vapor was leaked into the vacuum systemunder control of a flow restrictor and solenoid valve. Spectra fortrace explosives were acquired by dosing aliquots of a solutioncontaining the explosive in an organic solvent (typically metha-nol) onto a surface-sampling ticket, allowing the solvent to dry,and then analyzing the ticket via thermal desorption and transportof the explosive analyte into the CI volume. Mass analysis wasperformed with an ion trap analyzer coupled to the CI volumewith electrostatic lenses for ion transport. Ion detection was via aconversion dynode/electron multiplier assembly.

Results and DiscussionEx-Situ Characterization

Figure 2a displays typical ex-situ I-V characteristics for theCNT field emission electron source. The I-V curve providesboth a turn-on voltage and operating bias for desired emissioncurrent levels. This information is essential to choosing operatingconditions for the explosives trace detection mass spectrometerduring testing. The three I-V plots shown in Figure 2a wereobtained with an increasing sweep voltage applied between theCNTs and the extraction grid electrode. The devices maintainedreliable emission current throughout an operating range of 1 nAto a saturation level of approximately 500 μA. The saturationlevel is 14 mA/cm2, which is more than sufficient.

The Fowler-Nordheim (FN) plot shown in Figure 2b wasgenerated from the 0 to 450 V I-V curve of sweep no. 3 inFigure 2a. To create this plot, the FN equation was rewritten inFN coordinate form (Equation 1), where J denotes the emissioncurrent density, ϕ is the work function of the CNT emitters, Eis the applied electric field, β is the field enhancement factor,and the parameters a and b are universal constants defined inliterature, 1.54×10–6 AeVV–2 and 6.83×103 eV–3/2 Vμm–1,respectively [22]:

lnJ

E2

� �¼ ln a

β2

ϕ

� �−b

ϕ3=2

βEð1Þ

The slopes of the generated FN plots indicate that our CNTfield emission electron sources exhibit the non-linearity that is

E. J. Radauscher et al.: Chemical Ionization MS Using CNT FE Sources 1905

characteristic of CNT field emitters [22]. This non-linear phe-nomenon is not clearly understood. Researchers have attributedit to space charge effects and varying electron transport tunnel-ing mechanisms, with the transition point represented by theBknee^ in the F-N plot [23–25]. The device characterized inFigure 2 has a knee between the low and high field behavior ata field of 1.575 V/μm, which closely matches values reportedin literature [26]. Using the FN plot, slope coefficients wereextracted and field enhancement factors (β1 and β2) werecalculated for both regions using the following equation:

β ¼ −bϕ3=2

slopeð2Þ

For the device whose data is shown in Figure 2, and using atypical MWCNT value of 4.8 eV for ϕ, we observed slopecoefficients 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 divergefrom the reported values by±18%. This is due to thevariation of the distance from the CNTs to the extractionmesh caused by the silver paste thickness. The reported βvalues, inclusive of the variation range, also closely coin-cide with literature as expected [21].

Figure 2c shows the ex-situ emission lifetime measurementsof 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 continuousoperation. Assuming 10 h of continuous operation per day, thisindicates a required emitter replacement period of approximatelyonce per month. However, assuming a reasonable duty cyclethat accounts for the sample collection and traveler interactionsthat would occur at a security checkpoint as 25% on-time, thenthis 320 h lifetime would equate to 4 months of service. Anotherbenefit of using the CNTs is the ability to cycle the emitter onand off with minimal degradation. A standard tungsten filamentcould not be cycled quickly without dramatically shortening itslifetime. Cold field emission cathodes, including these CNTfield emitters, can be quickly cycled between on and off statesto conserve power without affecting emitter lifetime [27]. Wefound that a 10 s on/off cycle had little effect on CNT cathodeperformance or lifetime in the test bed. Specifics of pulsingperformance will be the focus of a future publication. It shouldbe noted that an air jet directed at the CNTs during CI operationin the explosives trace detection mass spectrometer can reduceon-time, however modifications could be made to the system tocorrect for this. Nevertheless, the ability to power cycle withouta lifetime penalty, coupled with its low power and low temper-ature characteristics, make the CNT field emitters attractiveoptions for further study and a viable candidate for CI and EIion sources.

The increase in the voltage required to maintain 5 μAemission over time suggests gradual CNT field emitter degra-dation (Figure 2c). This degradation effect has been reported

(a)

(c)

(b)

Figure 2. Electrical characteristics of the carbon nanotube electron source; (a) Electron emission I-V characteristics of a deviceshowing three consecutive and increasing voltage sweeps, (b) Fowler-Nordheim plot of I-V curve no. 3 demonstrating field emissionbehavior of the CNT source. The nonlinearity of the slope is attributed to a nonlinear field enhancement factor, (c) Lifetime analysis ofthe device operating at a pressure of 1×10–2 Pa (1×10–4 Torr) of room air and a fixed 5 μA CNT emission current

1906 E. J. Radauscher et al.: Chemical Ionization MS Using CNT FE Sources

previously [28–32] and was related to several distinct causes,including shortening of the CNTs from ion bombardment anddamage to the sidewalls from joule heating at the CNT tipduring electron field emission. The voltage spikes observedin Figure 2c can be attributed to sudden changes in emissioncurrent forcing fast instrument correction to maintain the setcurrent. These sudden changes in emission current could beexplained by CNTs undergoing deflection during the emittingprocess. It has been reported [32] that transients in emissioncurrent can be caused by electrodynamic interactions betweenfield emitting CNTs. Deflected CNTs experiencing these inter-actions can undergo an abrupt pull, resulting in the suddencurrent changes.

Integration into the Desktop Mass Spectrometer

The fabricated CNT field emission electron sources were in-corporated into the explosives trace detection mass spectrom-eter. The schematic in Figure 3a illustrates the position of thedevice with respect to the ionization region, repeller electrode,sample inlet capillary, and the aperture leading to the massanalyzer. As described above, CNTs were grown on a 3.6 mm2

die, which was then mounted to the PCBwith conductive silverpaste. This PCB was designed to be the same size and have thesame electrical connections as the filament assembly used inthe desktop mass spectrometer, allowing the CNT field emis-sion electron source to be a drop-in replacement. Figure 3bshows a wide view image of the explosive trace detectiondesktop mass spectrometer with the device installed; the wiringharness mount protrudes from the top for electrical connection.Figure 3c shows the CNT field emission electron sourceintegrated into the system just above the ionization regionwith electrical extensions from the harness board extendingdown to it.

Power requirements of the CNT field emission electronsource installed in the mass spectrometer were minimal. Oper-ating voltages varied between 500 and 1500 V with 5 to 50 μAof emission current, equating to a maximum source powerrequirement of 75 mW. The mass spectrometer allowed

10 ms between the time the voltage was applied to the sourceand the time ions were accepted from the source by the massanalyzer for emission of electrons to begin. The minimumresponse time of the CNT source was not interrogated, but inall cases emission was observed after this 10 ms period in theform of ion signal. Thus 10 ms serves as a maximum bound forthe turn-on time. The kinetic energies of electrons emitted fromthe CNT field emission electron source were not directly mea-sured, but are initially expected to be similar to the bias voltageapplied before being cooled by the gas in the source.

Evidence for Chemical Ionization

The chemical ionization conditions for this work differ frommore traditional CI investigations. The technique is normallyperformed using methane, isobutane, or ammonia as the CI gas[33–35]. In those cases, protonated molecules and molecularfragments tend to dominate the positive ion chemical ionizationmass spectra. For the data presented here, the chemical ioniza-tion gas (lab air) does not donate a proton, but rather thefollowing simplified mechanism dominates, according to Huntet al. [36].

N 2 þ e− 70 eVð Þ→ Nþ2 þ 2e− ð3Þ

Nþ2 þM → Mþ þ N2 ð4Þ

Mass spectra collection was conducted with the filamentthermionic emission source operating at its standard setting of200 μA and the CNT field emission source at 50 μA. The latterwas limited by the dimensions of the CNT source and theinstrument capabilities. Nonetheless, the lower current is ben-eficial for mobile instruments for portable operation due to itscontribution to lower power. The differences in electron currentcould have an effect on the intensity of the mass spectral signalif the electrons are the limiting reagent in the ionization reac-tion. Thus, this work makes no assertion about the MS signal

CNT PCB

To Mass Analyzer

Ionization RegionDevice Installed Into

Ionization Region

User interface

(a) (b) (c)

Figure 3. Integration of the carbon nanotube electron source into the explosive trace detection mass spectrometer. (a) Schematicof the system showing location of the CNT field emission electron source in respect to the ionization region, (b) a photograph of thedesktop mass spectrometer showing the wiring harness mount installed above the ionization region, (c) a magnified photograph ofthe ionization region showing the CNT field emission electron source installed

E. J. Radauscher et al.: Chemical Ionization MS Using CNT FE Sources 1907

intensity resulting from the CNT versus the filament source.However, the current is not expected to affect the peaks thatappear for each analyte or their relative abundance. Thus, thenearly identical peak structure of the resulting mass spectraindicate that the effect of the electrons from the CNT array,once emitted and cooled by the CI gas, are essentially identicalto the effect of the electrons emitted by the filament.

Prior to collection and examination of mass spectra forexplosive analytes, the instrument was calibrated using a stan-dard MS calibration gas (PFTBA) in EI mode, as well as both

positive and negative chemical ionization modes. Analysis andbrief discussion of the PFTBA spectra can be found inSupplementary Data and provides further evidence for chemi-cal ionization resulting from each source. After spectra werecollected using the calibration gas, amounts consistent withtrace standards in the security field of trinitrotoluene (TNT),Research Department explosive (RDX), and pentaerythritoltetranitrate (PETN) were examined. Identical amounts of traceexplosives were introduced during spectra collection for eachsource. The same emission currents used for PFTBA spectra

TNTCH3

O2N NO2

NO2

RDX

N

N N

NO2

NO2O2N

PETN

O

O

OO

NO2

NO2

NO2

O2N

m/z333

m/z378

m/z315,317

m/z288m/z

272

m/z62

m/z324

m/z268

m/z129

m/z102

m/z227

m/z210m/z

197

m/z227

m/z210m/z

197

m/z324

m/z268

m/z129

m/z102

m/z333

m/z378

m/z315,317

m/z288

m/z272

m/z62

(a)

(b)

(c)

Chemical Ionization Mass Spectra

CNT Field Emission Source Filament Thermionic Emission Source

CNT Field Emission Source

CNT Field Emission Source

Filament Thermionic Emission Source

Filament Thermionic Emission Source

(d)

(e)

(f)

Figure 4. Trace explosive chemical ionization spectra collected using both the CNT field emission (left column) and the filamentthermionic emission (right column) sources in the explosive trace detection desktop MS. The spectra for TNT are shown in (a) and(d), the spectra for RDX are shown in (b) and (e), and the spectra for PETN are shown in (c) and (f)

1908 E. J. Radauscher et al.: Chemical Ionization MS Using CNT FE Sources

collection were also used with the trace explosives (50 μA forthe CNT field emission source and 200 μA for the filamentthermionic source).

The normalized mass spectra for TNT collected using theCNT field emission source and filament thermionic emissionsource are shown in Figure 4a and d, respectively, and correlatewell with each other as well as literature spectra collected withfilament thermionic emitters [35, 37]. Gas molecules in rela-tively high pressure ion sources can collisionally cool electronsto kinetic energies near 0 eV. These energies are low enoughfor attachment to molecules of compounds that have positiveelectron affinities while causing little bond breakage, formingmolecular anions. In the case of TNT, the process is accuratelyreferred to as electron capture (EC). However, the EC processin a gas that has been intentionally added to collisionally coolelectrons is often referred to as negative chemical ionization(NCI) in the literature [38]. This latter convention will beutilized in the present manuscript. The mass locations andassignments for the fragments of TNT include m/z = 197[(TNT – NO)–],m/z = 210 [(TNT –OH)–], and an intense peakappearing at m/z = 227, as expected for NCI. The presence ofthe molecular anion is strong evidence of chemical ionization.Electron transfer from the reagent ions and electron capture canboth result in molecular ions [37].

The normalized mass spectra for RDX collected using theCNT field emission source and filament thermionic emissionsource are shown in Figure 4b and e. The data show adduct ionsthat are clearly the result of ion–molecule reactions. For exam-ple, m/z 268, observed in both the CNT and filament sourcedata, seems likely to be the result of the reaction:

RDX þ NO−2→ RDX þ NO2½ �− ð5Þ

If EI dominated the ionization mechanism, one would ex-pect no significant signal beyond m/z 222, which correspondsto the molecular ion of RDX. The source of the NO2

– is likelyRDX itself, although some NOx production in an ion sourcethat contains both N2 and O2 may be reasonably expected. Themass locations and assignments for additional fragmentationsinclude m/z = 102 [(C2H4N3O2)

–], m/z = 129 [(C3H5N4O2)–],

and m/z = 324 [(RDX + 102)–]. The above peaks areconsistent between the two sources as well as those ob-served with NCI literature spectra using filament thermionicemitters [35, 37, 39]. Finally, the small peak at m/z 176could be attributed to [(RDX – NO2)

–] [C3H6N5O4]– [35].

The normalized mass spectra for PETN collected using theCNT field emission source and filament thermionic emissionsource are shown in Figure 4c and f. Similar to RDX, the ionsignal for PETN atm/z 378 probably results from the followingreaction:

PETN þ NO−3→ PETN þ NO3½ �− ð6Þ

as the molecular weight of PETN is 316 g/mol, so a peak atm/z378 cannot be the result of electron ionization and must be theproduct of an ion-molecule reaction. Both of these product ions

are clearly shown in previously published, more traditionallygenerated CI spectra [35]. The mass locations and assignmentsfor additional fragments include m/z = 62 [(NO3)

–], m/z = 315and 317 [(PETN – H)– and (PETN + H)–], and m/z = 333[(PETN + OH)–]. The peaks are consistent between the twosources and with those in literature using filament thermionicemitters [35, 40]. The calibration curves were generally linearas well. In the case of TNT, the linear plot (which is included inSupplementary Information for reference) had a slope of 991, ay-intercept of –4,518, and a R2 of 0.963. The CNT fieldemission electron sources show similar overall sensitivity tothe standard system using filament thermionic emission.

ConclusionA custom designed, fully packaged CNT field emission devicehas been developed and used in a prototype commercial massspectrometer as an electron source to obtain chemical ioniza-tion mass spectra in both positive and negative modes. Al-though further study is needed, this source is a candidate forCI and EI mass spectrometry applications. The CNT fieldemission source demonstrated low power, low temperature,and attractive lifetime. A constant-on lifetime of 320 h wasdemonstrated for the CNT devices. This lifetime equates to 4months of operation, assuming a 25% on-time in real-timesecurity checkpoint deployment. The CNT emitters also dem-onstrated the ability to cycle and thus avoid continuous-onoperation, the details of which will be the subject of futurepublication. Spectra were collected for a standard MS calibra-tion material (PFTBA) as well as trace amounts of explosives(TNT, RDX, and PETN). The spectra collected showed similarpeaks for both the CNT field emission source and a traditionalfilament thermionic emission electron source in the same sys-tem. Furthermore, the EI and CI spectra collected with the CNTfield emission source closely matched those observed in liter-ature that were produced with traditional filament thermionicemission sources. Sensitivities for the CNT field emissionsource were similar to filament thermionic emitters used inthe same mass spectrometer.

AcknowledgmentsThis work was performed with support of the US Departmentof Homeland Security Science and Technology Directorate.

References1. Alberici, R.M., Simas, R.C., Sanvido, G.B., Romao, W., Lalli, P.M.,

Benassi, M., Cunha, I.B., Eberlin, M.N.: Ambient mass spectrometry:bringing MS into the Breal world^. Anal. Bioanal. Chem. 398(1), 265–294 (2010)

2. Harris, G.A., Galhena, A.S., Fernandez, F.M.: Ambient Sampling/ionizationmass spectrometry: applications and current trends. Anal. Chem. 83(12),4508–4538 (2011)

3. Gross, J.H.: Mass Spectrometry: A Textbook, 2nd edn. Springer, Heidel-berg (2011)

4. Xu, X., van de Craats, A.M., de Bruyn, P.C.A.M.: Highly sensitive screen-ing method for nitroaromatic, nitramine and nitrate ester explosives by high

E. J. Radauscher et al.: Chemical Ionization MS Using CNT FE Sources 1909

performance liquid chromatography-atmospheric pressure ionization-massspectrometry (HPLC-API-MS) in forensic applications. J. Forensic Sci.49(6), 1171–1180 (2004)

5. Chernozatonskii, L.A., Gulyaev, Y.V., Kosakovskaja, Z.J., Sinitsyn, N.I.,Torgashov, G.V., Zakharchenko, Y.F., Fedorov, E.A., Val'chuk, V.P.:Electron field emission from nanofilament carbon films. Chem. Phys. Lett.233(1/2), 63–68 (1995)

6. deHeer,W.A., Châtelain, A., Ugarte, D.: A carbon nanotube field-emissionelectron source. Science 270(5239), 1179–1180 (1995)

7. Rinzler, A.G., Hafner, J.H., Nikolaev, P., Nordlander, P., Colbert, D.T.,Smalley, R.E., Lou, L., Kim, S.G., Tománek, D.: Unraveling nanotubes:field emission from an atomic wire. Science 269(5230), 1550–1553 (1995)

8. Jonge, N.D., Bonard, J.-M.: Carbon nanotube electron sources and appli-cations. Philos. Trans.: Math. Phys. Eng. Sci. 362(1823), 2239–2266(2004)

9. Nojeh, A.: Carbon nanotube electron sources: from electron beams toenergy conversion and optophononics. ISRN Nanomaterials 2014, 1–23(2014)

10. Zhu, W., Bower, C., Zhou, O., Kochanski, G., Jin, S.: Large current densityfrom carbon nanotube field emitters. Appl. Phys. Lett. 75(6), 873–875(1999)

11. Teo, K.B.K., Chhowalla, M., Amaratunga, G.A.J., Milne, W.I., Pirio, G.,Legagneux, P., Wyczisk, F., Pribat, D., Hasko, D.G.: Field emission fromdense, sparse, and patterned arrays of carbon nanofibers. Appl. Phys. Lett.80(11), 2011–2013 (2002)

12. Chen, Z., Zhang, Q., Lan, P., Zhu, B., Yu, T., Cao, G., den Engelsen, D.:Ultrahigh-current field emission from sandwich-grown well-aligned uni-form multi-walled carbon nanotube arrays with high adherence strength.Nanotechnology 18(26), 1–6 (2007)

13. Wang, Z., Zuo, Y., Li, Y., Han, X., Guo, X.,Wang, J., Cao, B., Xi, L., Xue,D.: Improved field emission properties of carbon nanotubes decorated withTa layer. Carbon 73, 114–124 (2014)

14. Paulmier, T., Balat-Pichelin, M., Le Quéau, D., Berjoan, R., Robert, J.F.:Physico-chemical behavior of carbon materials under high temperature andion irradiation. Appl. Surface. Sci. 180(3/4), 227–245 (2001)

15. Lan, Y., Wang, Y., Ren, Z.F.: Physics and applications of aligned carbonnanotubes. Adv. Phys. 60(4), 553–678 (2011)

16. Kyusung, H., Yuri, L., Dohan, J., Soonil, L., KwangWoo, J., Sang Sik, Y.:Field emission ion source using a carbon nanotube array for micro time-of-flight mass spectrometer. Jpn. J. Applied. Phys. 50(6S), 06GM04 (2011)

17. Getty, S.A., King, T.T., Bis, R.A., Jones, H.H., Herrero, F., Lynch B.A.,Roman, P, Mahaffy, P.: Performance of a carbon nanotube field emissionelectron gun in micro (MEMS) and nanotechnologies for defense andsecurity. Proc. SPIE 6556, 655618-1–655618-12, Micro (MEMS) andNanotechnologies for Defense and Security, 655618 (May 03, 2007)

18. Bower, C.A., Gilchrist, K.H., Piascik, J.R., Stoner, B.R., Natarajan, S.,Parker, C.B., Wolter, S.D., Glass, J.T.: On-chip electron-impact ion sourceusing carbon nanotube field emitters. Appl. Phys. Lett. 90, 124102-1–124102-3 (2007)

19. Evans-Nguyen, T., Parker, C.B., Hammock, C., Monica, A.H., Adams, E.,Becker, Glass, J.T., Cotter, R.J.: Carbon nanotube electron ionizationsource for portable mass spectrometry. Anal. Chem. 83(17), 6527–6531(2011)

20. Cui, H., Zhou, O., Stoner, B.R.: Deposition of aligned bamboo-like carbonnanotubes via microwave plasma enhanced chemical vapor deposition. J.Appl. Phys. 88(10), 6072–6074 (2000)

21. Cheng, Y., Zhou, O.: Electron field emission from carbon nanotubes.Comptes. Rendus. Physique. 4(9), 1021–1033 (2003)

22. Lu, X., Yang, Q., Xiao, C., Hirose, A.: Nonlinear Fowler-Nordheim plotsof the field electron emission from graphitic nanocones: influence of non-uniform field enhancement factors. J. Phys. D Appl. Phys. 39(15), 3375(2006)

23. Zhang, J., Yang, C., Yang, W., Feng, T., Wang, X., Liu, X.: Appearance ofa knee on the Fowler-Nordheim plot of carbon nanotubes on a substrate.Solid State Commun. 138(1), 13–16 (2006)

24. Feng, Y., Verboncoeur, J.: Transition from Fowler-Nordheim field emis-sion to space charge limited current density. Phys. Plasmas (1994 topresent). 13, 073105-1–073105-8 (2006)

25. Barbour, J., Dolan, W.W., Trolan, J.K., Martin, E.E., Dyke, W.: Space-charge effects in field emission. Phys. Rev. 92(1), 45–51 (1953)

26. Bai, R., Kirkici, H.: Nonlinear Fowler-Nordheim plots of carbon nanotubes(CNTs) in vacuum and partial pressures. Proceedings of othe Power Mod-ulator and High Voltage Conference (IPMHVC). 2012 I.E. International,San Diego (2012)

27. Leberl, D., Ummethala, R., Leonhardt, A., Hensel, B., Tedde, S.F.,Schmidt, O., Hayden, O.: Characterization of carbon nanotube field emit-ters in pulsed operation mode. J. Vacuum Sci. Technol. B. 31, 012204-1–012204-6 (2013)

28. Bocharov, G.S., Eletskii, A.V.: Degradation of a carbon nanotube-basedfield-emission cathode during ion sputtering. Technical Phys. 57(7), 1008–1012 (2012)

29. Wang, Z.L., Gao, R.P., de Heer, W.A., Poncharal, P.: In situ imaging offield emission from individual carbon nanotubes and their structural dam-age. Appl. Phys. Lett. 80(5), 856–858 (2002)

30. Dean, K.A., Burgin, T.P., Chalamala, B.R.: Evaporation of carbon nano-tubes during electron field emission. Appl. Phys. Lett. 79(12), 1873–1875(2001)

31. Wei, Y., Xie, C., Dean, K.A., Coll, B.F.: Stability of carbon nanotubesunder electric field studied by scanning electron microscopy. Appl. Phys.Lett. 79(27), 4527–4529 (2001)

32. Mahapatra, D., Sinha, N., Melnik, R.: Degradation and failure of fieldemitting carbon nanotube arrays. J. Nanosci. Nanotechnol. 11(5), 3911–3915 (2011)

33. Field, F.H.: Chemical ionization mass spectrometrv. Acc. Chem. Res. 1(2),42–49 (1968)

34. Westmore, J.B., Alauddin, M.M.: Ammonia chemical ionization massspectrometry. Mass Spectrom. Rev. 5(4), 381–465 (1986)

35. Yinon, J.: Analysis of explosives by negative-ion chemical ionization massspectrometry. J. Forensic Sci. 25(2), 401–407 (1980)

36. Hunt, D.F., Stafford, G.C., Crow, F.W., Russell, J.W.: Pulsed positivenegative ion chemical ionization mass spectrometry. Anal. Chem. 48(14),2098–2104 (1976)

37. McLuckey, S.A., Glish, G.L., Kelley, E.: Collision-activated dissociation ofnegative ions in an ion trap mass spectrometer. Anal. Chem. 59(13), 1670–1674 (1987)

38. Gregor, I.K., Guilhaus, M.: Comparative study of the influence of theelectron-energy moderating gas in electron-attachment reactions underNCI conditions. Int. J. Mass Spectrom. Ion Processes 56(2), 167–176(1984)

39. Yinon, J., Harvan, D.J., Hass, J.R.: Mass spectral fragmentation pathwaysin RDX and HMX. A mass analyzed ion kinetic energy spectrometric/collisional induced dissociation study. Org. Mass Spectrom. 17(7), 321–326 (1982)

40. Boumsellek, S., Alajajian, S.H., Chutjian, A.: Negative-ion formation in theexplosives RDX, PETN, and TNT by using the reversal electron attachmentdetection technique. J. Am. Soc. Mass Spectrom. 3(3), 243–247 (1992)

1910 E. J. Radauscher et al.: Chemical Ionization MS Using CNT FE Sources