ARTICLE pubs.acs.org/ac
Interfacing an Ion Mobility Spectrometry Based Explosive Trace Detector to a Triple Quadrupole Mass Spectrometer Joseph Kozole,*,†,‡ Jason R. Stairs,† Inho Cho,†,‡ Jason D. Harper,†,‡ Stefan R. Lukow,† Richard T. Lareau,† Reno DeBono,§ and Frank Kuja§ †
U.S. Department of Homeland Security, Science & Technology Directorate, Transportation Security Laboratory, Atlantic City International Airport, New Jersey 08405, United States ‡ Nova Research, Inc., 1900 Elkin Street, Suite 230, Alexandria, Virginia 22308, United States § Smiths Detection, Inc., 7030 Century Avenue, Mississauga, Ontario, Canada
bS Supporting Information ABSTRACT:
Hardware from a commercial-off-the-shelf (COTS) ion mobility spectrometry (IMS) based explosive trace detector (ETD) has been interfaced to an AB/SCIEX API 2000 triple quadrupole mass spectrometer. To interface the COTS IMS based ETD to the API 2000, the faraday plate of the IMS instrument and the curtain plate of the mass spectrometer were removed from their respective systems and replaced by a custom faraday plate, which was fabricated with a hole for passing the ion beam to the mass spectrometer, and a custom interface flange, which was designed to attach the IMS instrument onto the mass spectrometer. Additionally, the mass spectrometer was modified to increase the electric field strength and decrease the pressure in the differentially pumped interface, causing a decrease in the effect of collisional focusing and permitting a mobility spectrum to be measured using the mass spectrometer. The utility of the COTS-ETD/API 2000 configuration for the characterization of the gas phase ion chemistry of COTS-ETD equipment was established by obtaining mass and tandem mass spectra in the continuous ion flow and selected mobility monitoring operating modes and by obtaining mass-selected ion mobility spectra for the explosive standard 2,4,6 trinitrotoluene (TNT). This analysis confirmed that the product ion for TNT is [TNT H], the predominant collision-induced dissociation pathway for [TNT H] is the loss of NO and NO2, and the reduced mobility value for [TNT H] is 1.54 cm2V1 s1. Moreover, this analysis was attained for sample amounts of 1 ng and with a resolving power of 37. The objective of the research is to advance the operational effectiveness of COTS IMS based ETD equipment by developing a platform that can facilitate the understanding of the ion chemistry intrinsic to the equipment.
T
he use of improvised explosive devices (IEDs) for terrorist purposes will continue to be a threat to public safety for the foreseeable future.1 To help mitigate this threat, more than 15 000 explosive trace detectors (ETDs) are deployed at security checkpoints worldwide.2 The function of an ETD is to detect the presence of an explosive residue on a subject that has been in contact with an explosive material. Accordingly, the primary characteristics required of an ETD are selectivity and sensitivity. Ion mobility spectrometry (IMS) is the analytical method most commonly implemented in ETD equipment.36 In IMS, the velocity of an ion though a buffer gas in the presence of an r 2011 American Chemical Society
electric field is measured. Because the velocity of the ion is proportional to reduced mass and cross-section, the ion is identified on the basis of size. Moreover, because the ion is formed by efficient atmospheric pressure chemical ionization (APCI) processes, low limits of detection are observed in positive polarity for molecules with high proton affinities and in negative polarity for molecules with high electronegativities. Received: August 3, 2011 Accepted: September 30, 2011 Published: October 21, 2011 8596
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Figure 1. Schematic of the IMS/MS/MS instrument.
Consequent to these properties, IMS equipment operated in positive and negative polarity with ammonia and chlorine reagent chemicals has routinely demonstrated nanogram or better sensitivities for the detection of a suite of peroxide, nitro, and nitrate containing explosives.311 While IMS has the selectivity and sensitivity required to detect trace amounts of explosives under a variety of conditions, opportunities for advancement exist regarding false positive and false negative responses.1214 A false positive is typically attributed to the inability of IMS to differentiate an analyte from an interfering agent of a similar size, whereas, a false negative is typically attributed to the inability of APCI to ionize an analyte in a competitive ionization environment. A practical approach to decreasing the false response rate of IMS equipment is to modify the operating parameters of the instrument, such as buffer gas temperature, ionization source, reagent chemical, electric field strength, and detection algorithm, with the objective of increasing selectivity.8,10,11,1416 These types of modifications are most accurately made using a detailed understanding of the gas phase ion chemistry intrinsic to the instrument that is modified. However, a detailed study of the ion chemistry of commercialoff-the shelf (COTS) IMS based ETD equipment has not been performed. For example, the ion species formed in this equipment during the analysis of typically encountered explosives and chemical interfering agents have not been identified with the certainty of tandem mass spectrometry (MS).8,9 In this manuscript, a method for combining a COTS IMS based ETD with a triple quadrupole mass spectrometer is described. Although IMS and MS have been combined to identify explosives on the basis of size and mass for more than 30 years,1719 the IMS/MS/MS configuration reported here is the first time a COTS IMS based ETD has been interfaced to either a mass or tandem mass spectrometer and therefore is the only platform that can directly characterize the ion chemistry of this particular type of instrument.20 Specifically, this platform eliminates any variations that exist between the commercial IMS instrument and a custom research IMS instrument interfaced to a mass spectrometer, including the materials that the equipment is constructed from, instrument dimensions and voltages, sampling protocol, ionization source, reagent chemical, and internal calibrant. At the same time, this platform has the capability to measure the mass-to-charge ratios, collision-induced dissociation pathways, and reduced mobility values for the ion species formed in the commercial IMS instrument. The objective of the research
is to advance the operational effectiveness of COTS IMS based ETD equipment by developing a platform that can facilitate the understanding of the ion chemistry intrinsic to the equipment. Note that the COTS-ETD/API 2000 configuration described here is not intended for use in security environments but is solely for research purposes.
’ EXPERIMENTAL SECTION Instrumentation. A schematic of the IMS/MS/MS instrument is shown in Figure 1. The instrument is comprised of three main parts: a COTS IMS based ETD, an interface region, and a triple quadruple mass spectrometer. Ion Mobility Spectrometry Based Explosive Trace Detector. The fundamental components of the COTS IMS based ETD (Smiths Detection Inc.) are a thermal desorber and a metal cylindrical housing that contains a reaction region with a 63Ni radiation source, an ion gate, an ambient pressure drift region, and a faraday plate detector.8,9 The parameters for the negative polarity operation of the IMS instrument are summarized in Table 1.8,9 To introduce a sample into the IMS instrument, a polytetrafluoroethylene (PTFE) filter dosed with analyte is inserted into the desorber, the analyte is vaporized, and the vapor is carried into the reaction region using a flow of purified air. In the reaction region, chloride reactant ions, generated by the interaction of 63Ni β radiation with purified air and hexachloroethane reagent, react with the vapor analyte to form product ions. The reactant and product ions are transferred into the drift region in the form of packets by an electrostatic ion gate.9 The drift region is composed of a stacked electrode assembly of several conducting rings and a passive guard grid separated by insulating spacers and high-temperature resistors. In the drift region, the ion packets are directed toward the guard grid by a uniform electric field and separate based on the velocity of the ions through a counterflow drift gas of purified air. After passing the guard grid, the ions are detected using either the faraday plate or the mass spectrometer and the ion intensity is registered as a function of the drift time to produce a mobility spectrum. To calculate reduced mobility, the drift time of the analyte is normalized to the drift time of the internal calibrant 4-nitrobenzonitrile. The purified air used for the sample and drift gases in the IMS/MS/MS instrument was house air treated with a purge gas generator (Parker Balston model 75). The purified air had a 8597
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moisture content of 525 ppm as measured by a moisture analyzer (General Electric moisture monitor series 3) and a carbon dioxide content of 50100 ppm as measured by a carbon dioxide transmitter (Vaisala series GMT 220). The moisture and carbon dioxide content of the purified air in the IMS/MS/MS instrument is comparable to the moisture and carbon dioxide content of the molecular sieve treated air typically used in a COTS IMS based ETD. Note that only the hardware, and not the detection algorithms, for the COTS IMS based ETD was used in the ion chemistry studies presented in this manuscript. Interface Region. The mass spectrometer used in the IMS/ MS/MS instrument is an API 2000 (AB/SCIEX). A detailed illustration of the interface between the COTS IMS based ETD and the API 2000 is shown in Figure 2 with dimensions and voltages summarized in Table 2. To construct the interface, the faraday plate and the portion of the IMS housing located after the guard grid were removed from the IMS instrument. After its modification, the guard grid end of the IMS housing was Table 1. Operating Parameters for the IMS Component of the IMS/MS/MS Instrumenta parameter
value
unit
257
V/cm
760
torr
desorber
205
°C
inlet
245
°C
reaction region/drift region (housing)
115
°C
reaction region/drift region (gas)
111
°C
sample gas
250
cc/min
drift gas
300
cc/min
Electric Field Strengths drift region Pressures reaction region/drift region Temperatures
Gas Flows
a
Purified air (525 ppm water and 50100 ppm carbon dioxide) was used as the sample and drift gases, hexachloroethane was used as the reagent chemical, and 4-nitrobenzonitrile was used as the internal calibrant.
hermetically connected to a custom interface flange, which was constructed from stainless steel and designed to attach to the atmospheric pressure interface of the mass spectrometer. A custom faraday plate, which was constructed from gold plated stainless steel and with a centered hole for passing a portion of the ion beam to the mass spectrometer, was positioned after the guard grid on the mass spectrometer side of the interface flange and was electrically insulated using PTFE. The curtain plate was removed from the mass spectrometer, and the interface flange was attached in its place such that the hole in the faraday plate was positioned immediately before the pinhole in the orifice and the IMS housing and interface flange were electrically grounded. Electric contact was made with the faraday plate using the feedthrough for the curtain plate on the mass spectrometer. Additional details important to the design of the interface region include the dual function of the faraday plate and the source of the drift gas for the IMS instrument. In the design, the faraday plate can be connected to a current preamplifier (Stanford Research Systems model SR 570) to measure a mobility spectrum or to a voltage power supply (Spellman model 230, Hauppauge) to extract the ion beam from the IMS instrument into the mass spectrometer. Also in the design, the purified air for the drift gas is introduced into the interface region between the faraday plate and the orifice using the inlet for the curtain gas on the mass spectrometer. Because a portion of the gas is pulled through the pinhole of the orifice and into the vacuum of the mass spectrometer, the total gas flow introduced into the interface region is 900 cc/min, 600 cc/min of which flows into the mass spectrometer and 300 cc/min of which flows into the IMS instrument and serves as the drift gas.21 Triple Quadrupole Mass Spectrometer. The API 2000 is comprised of a differentially pumped interface which is used to transfer the ion beam from atmospheric pressure to the vacuum of the mass spectrometer and a triple quadrupole mass analyzer which is used to filter and detect the ion beam.22 The parameters for the negative polarity operation of the API 2000 are summarized in Table 2. The differentially pumped interface is divided into two separately pumped stages. The first stage is composed of an orifice with a pinhole, a focusing ring, and a skimmer and is evacuated using a mechanical pump while the second stage is composed of a radio frequency (rf) only quadrupole (Q0) and an interquad lens
Figure 2. Computer aided drawing of the interface region of the IMS/MS/MS instrument. 8598
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Table 2. Dimensions and Operating Parameters for the Interface Region and the Mass Spectrometer of the IMS/MS/MS Instrument mass and MS/MS spectra parameter
value
mass-selected ion mobility spectra
unit
value
unit
Dimensions distance from guard grid to faraday plate
1.4
mm
1.4
mm
distance from faraday plate to orifice distance from orifice to skimmer
4.5 3.3
mm mm
4.5 6.3
mm mm
faraday plate hole size
6.0
mm
6.0
mm
orifice hole size
254
μm
254
μm
skimmer hole size
1100
μm
400
μm
interface flange
ground
guard grid
90
V
90
V
faraday plate/extraction electrode
75
V
75
V
orifice skimmer
5 10
V V
20 15
V V
Q0
12
V
12
V
IQ1
13
V
22
V
1st differentially pumped stage
1
torr
1
torr
2nd differentially pumped stage (Q0 region)
8 103
torr
1 103
torr
mass spectrometer
8 107
torr
4 107
torr
orifice
115
°C
115
°C
Voltages ground
Pressures
Temperatures
(IQ1) and is evacuated using a turbomolecular pump. The ion beam enters the first stage through the pinhole of the orifice and is guided into the hole of the skimmer by gas flow and an electric field gradient. In the second stage (the Q0 region), the trajectory of the ion beam is focused onto the axis of Q0 by collisions with the background gas (i.e., by collisional focusing) and is directed into the mass analyzer by the electric field between the skimmer, Q0, and IQ1.22,23 The skimmer in the differentially pumped interface was modified in two ways. In the first modification, the skimmer was electrically insulated from the chamber of the mass spectrometer using polyimide to allow the electrode to be biased. This modification permits the electric field strength between the skimmer and Q0 to be tuned to a value that is larger than the standard configuration of the mass spectrometer. Additionally, this modification permits the electric field between the orifice and the skimmer to be used as an ion gate for the mass spectrometer. For negative polarity, the gate is closed when the skimmer is biased to a potential that is 40 V relative to the orifice and is open when the skimmer is biased to a potential that is 5 V relative to the orifice. In the second modification, the skimmer was modified to have a smaller hole and to be positioned farther away from the orifice, specifically, the hole in the skimmer was reduced from 1100 to 400 μm while the length of the skimmer was shortened by 3 mm. An unmodified skimmer was inserted into the mass spectrometer when an elevated pressure in the Q0 region was desired while the modified skimmer was inserted when a reduced pressure was desired. Reducing the pressure in the Q0 region by inserting the modified skimmer into the mass spectrometer decreases the effect of collisional focusing
and increases the velocity of an ion packet in the differentially pumped interface, decreasing broadening and distortion of the ion packet and permitting a mobility spectrum to be measured using the mass spectrometer. A detailed discussion regarding the effect of collisional focusing on an ion packet in the Q0 region is provided later in this manuscript. The components of the mass analyzer are a mass filter (Q1), a collision cell (Q2), a second mass filter (Q3), and an electron multiplier detector. During the operation of the IMS/MS/MS instrument, the mass analyzer is operated in the scan only mode to obtain a mass spectrum, the product ion scan mode to obtain a tandem mass (MS/MS) spectrum, and the selected ion monitoring (SIM) scan mode to obtain a mobility spectrum for a particular m/z. The mass and MS/MS spectra are recorded using the Analyst software (AB/SCIEX). Operating Modes. Continuous Ion Flow. For this operating mode,6,21,24 both the ion gate in the IMS instrument (the first gate) and the ion gate in the mass spectrometer (the second gate) are constantly held in the open position, permitting the ions formed in the reaction region of the IMS instrument to pass unrestricted into the mass spectrometer. This continuous flow of ions is analyzed by operating the mass analyzer in the scan only or the product ion scan mode. Mass-Selected Ion Mobility Single Gate. For this operating mode,6 the first gate is opened for a finite period of time to allow a packet of ions into the drift region of the IMS instrument while the second gate is constantly held in the open position. The mass analyzer is operated in the SIM mode to detect only the ions with a particular m/z. The ion intensity at the electron multiplier for the ion of interest is registered as a function of the drift time to 8599
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Analytical Chemistry produce a mass-selected ion mobility spectrum. The ion intensity is registered using a high speed digitizer (National Instruments model PXI 5124/512) with custom LabVIEW code (National Instruments). Selected Mobility Monitoring Dual Gate. For this operating mode,6,21,24,25 the first gate is opened for a finite period of time while the second gate is opened after a specified time delay for the same duration as the first gate. The time delay is selected to permit only the ions with a particular mobility to pass into the mass spectrometer. This mobility filtered ion population is analyzed by operating the mass analyzer in the scan only or the product ion scan mode. The time delay between the opening of the first and second gate is controlled using a data acquisition board (National Instruments model PXI 6259) with custom LabVIEW code while the voltage applied to the second gate is generated using an arbitrary waveform generator (TEGAM, Inc. model 2720A) and a voltage-to-voltage amplifier (TEGAM, Inc. model 2340). Sample Preparation. To prepare 1, 10, and 100 ng samples of 2,4,6 trinitrotoluene (TNT), dissolved solutions of TNT (AccuStandard) with concentrations of 1, 10, and 100 μg/mL were deposited onto PTFE filters in 1 μL aliquots and dried under a flow of nitrogen.
’ RESULTS AND DISCUSSION To establish the utility of the COTS-ETD/API 2000 configuration for the characterization of the ion chemistry of COTSETD equipment, the mass-to-charge ratios, collision-induced dissociation pathways, and reduced mobility values for the ion species formed during the analysis of the explosive standard TNT were measured. These measurements were obtained by operating the IMS/MS/MS instrument in the continuous ion flow mode to acquire mass and MS/MS spectra, the mass-selected ion mobility mode to acquire mobility spectra for the ions of interest, and the selected mobility monitoring mode to acquire mass and MS/MS spectra for the individual peaks in the mobility spectra. Continuous Ion Flow Mass and MS/MS Spectra. Mass spectra from the analysis of a blank sample, a 1 ng TNT sample, and a 10 ng TNT sample in the continuous ion flow mode are shown in Figure 3ac. The spectra, along with all other spectra shown in this manuscript, are the result when the signal is averaged for a 10 s period following the start of sample desorption. Included in the spectra are contributions from hexachloroethane at m/z 35, 37, 53, 55, 70, 72, and 74 attributed to [Cl], [Cl + H2O], and [Cl2] as well as contributions from TNT at m/z 226 attributed to [TNT H]. The spectra indicate the product ion for TNT is formed by a proton abstraction reaction with the chloride reactant ions, a finding consistent with the ionization process previously reported for TNT in a purified air and chlorinated hydrocarbon reaction environment.3,8,9,17,26 Moreover, the spectra indicate the sensitivity of the IMS/MS/MS instrument for TNT is better than 1 ng when the instrument is operated in the continuous ion flow mode and the mass spectrometer is operated in the scan only mode for the m/z range 25275. A MS/MS spectrum from the analysis of a 10 ng TNT sample in the continuous ion flow mode is shown in Figure 3d. The parent ion selected was [TNT H] at m/z 226 while the collision energy was scanned from 5 to 40 eV. Included in the spectrum are contributions at m/z 46, 136, 166, 183, and 196 attributed to [NO2], [TNT 3NO], [TNT 2NO],
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Figure 3. Mass and MS/MS spectra obtained in the continuous ion flow mode. Plot a is the mass spectrum for a blank sample, plot b is the mass spectrum for a 1 ng TNT sample, plot c is the mass spectrum for a 10 ng TNT sample, and plot d is the MS/MS spectrum for a 10 ng TNT sample when the parent ion selected is [TNT H] (m/z 226).
[TNT NO2], and [TNT NO]. The spectrum indicates the loss of NO and NO2 from the parent ion is the predominant collision-induced dissociation pathway for [TNT H], a finding similar to the dissociation pathways previously reported for [TNT H],27 verifying the assignment of the product ion at m/z 226 as [TNT H]. Mass-Selected Ion Mobility Spectra. Because the mass and MS/MS spectra shown in Figure 3 were acquired in the continuous ion flow mode, the spectra contain information about the mass and structure of the ion species only. To obtain mobility information, the IMS/MS/MS instrument was operated in the mass-selected ion mobility mode. Mass-selected ion mobility spectra obtained using different operating parameters for the Q0 region of the mass spectrometer are shown in Figure 4. The operating parameters optimized include the potential between the skimmer and Q0 (VS‑Q0), the potential between Q0 and IQ1 (VQ0‑IQ1), and the pressure in Q0 (PQ0). For each spectrum, a 100 ng TNT sample was analyzed, the pulse width for the first gate was 0.2 ms, and the product ion selected was [TNT H] at m/z 226. The pressure in Q0 was decreased by replacing the skimmer in the standard configuration of the mass spectrometer with the skimmer that was modified to have a smaller hole and a shorter length. The resolving power (Rp), which is a quantification of the separation capability of a mobility measurement, was calculated using the equation: R p ¼ ðt d =w0:5 Þ
ð1Þ
where td is the drift time and w0.5 is the temporal peak width at half-height.21,28 The mass-selected ion mobility spectra show that the resolving power of the measurement increases from 4 to 37 as VS‑Q0 is increased from 2 to 25 V, VQ0‑IQ1 is increased from 1 to 10 V, and PQ0 is decreased from 8 to 1 mtorr. An increase in VS‑Q0 above 25 V did not result in an additional increase in the resolving power and caused collision-induced dissociation to occur. The standard configuration of the mass spectrometer did not permit VQ0‑IQ1 to be increased above 10 V. A 1 mtorr pressure was 8600
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Figure 4. Mass-selected ion mobility spectra for [TNT H] (m/z 226) when 100 ng TNT samples were analyzed. The spectra were acquired by varying the potential between the skimmer and Q0 (VS‑Q0), the potential between Q0 and IQ1 (VQ0‑IQ1), and the pressure in Q0 (PQ0). The spectra show how the resolving power (Rp) of the measurement is affected as VS‑Q0 is increased from (a) 2 to (b) 25 V, VQ0‑IQ1 is increased from (b) 1 to (c) 10 V, and PQ0 is decreased from (c) 8 to (d) 1 mtorr.
selected for PQ0 to correspond to a pressure regime where collisional focusing is less prominent.22,23,29 The increase in the resolving power of the mass-selected ion mobility measurement at increased electric field strengths and decreased pressures in the Q0 region is attributed to a decrease in the effect of collisional focusing.29 At lower electric field strengths and higher pressures, the ion packet is slowed to thermal velocities by collisions with the background gas in Q0. While this condition focuses the trajectory of the ions onto the axis of Q0 and increases the transmission of the ions into the mass analyzer, it also causes the distribution of the ions in the packet to diffuse along the axis of Q0. However, when the electric field strength is increased and the pressure is decreased, the ions transit Q0 at an increased velocity and therefore are not readily focused onto and diffused along the quadrupole axis. This increase in velocity and decrease in diffusion is confirmed by the shift of the peaks in the mass-selected ion mobility spectra to shorter drift times and more symmetrical profiles as VS‑Q0 and VQ0‑IQ1 were increased and PQ0 was decreased. The resolving power at a VS‑Q0 of 25 V, a VQ0‑IQ1 of 10 V, and a PQ0 of 1 mtorr is 37, a value comparable to those obtained with the IMS instrument before being interfaced to the mass spectrometer,12 suggesting the broadening and distortion of the ion packet due to collisional focusing under these operating parameters is effectively zero. In addition to decreasing the effect of collisional focusing and optimizing the resolving power, these parameters also decrease the sensitivity of the measurement by a factor of ∼20. This decrease in sensitivity is attributed to the reduced size of the skimmer hole and a decrease in the transmission efficiency of the ion beam into the mass analyzer. Accordingly, the Q0 region is operated with a VS‑Q0 of 2 V, a VQ0‑IQ1 of 1 V, and a PQ0 of 8 mtorr to obtain mass and MS/MS spectra in the continuous ion flow and selected mobility monitoring modes and is operated with a VS‑Q0 of 25 V, a VQ0‑IQ1 of 10 V, and a PQ0 of 1 mtorr to obtain mass-selected ion mobility spectra (Table 2).
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Figure 5. Mobility spectra for 100 mg TNT samples obtained using a benchtop IMS instrument and the IMS/MS/MS instrument. Plot a is the mobility spectrum acquired using a benchtop IMS instrument, plot b is the mobility spectrum acquired using the faraday plate in the IMS/ MS/MS instrument, plot c is a multiple mass mobility spectrum acquired using the IMS/MS/MS instrument, and the plots in part d are the individual mass-selected ion mobility spectra for [Cl] at m/z 35 (red), [Cl + H2O] at m/z 53 (green), [Cl2] at m/z 70 (blue), and [TNT H] at m/z 226 (black) acquired using the IMS/MS/MS instrument.
Mobility spectra obtained using a benchtop IMS instrument (Smiths Detection Inc.) and the IMS/MS/MS instrument operated under similar conditions are shown in Figure 5. Figure 5a is the mobility spectrum acquired using an IMS instrument identical to the IMS component of the IMS/MS/ MS instrument; Figure 5b is the mobility spectrum acquired using the faraday plate in the IMS/MS/MS instrument; Figure 5c is a sum of multiple mass-selected ion mobility spectra acquired using the IMS/MS/MS instrument; and the plots in Figure 5d are the individual mass-selected ion mobility spectra for [Cl] at m/z 35, [Cl + H2O] at m/z 53, [Cl2] at m/z 70, and [TNT H] at m/z 226 acquired using the IMS/MS/MS instrument. For each spectrum, a 100 ng TNT sample was analyzed and the pulse width for the first gate was 0.2 ms. The spectra are plotted as a function of the inverse of reduced mobility (1/K0) to adjust for any variations between the instruments and to adjust for the flight time between the faraday plate of the IMS instrument and the electron multiplier of the mass spectrometer. Reduced mobility, which is the proportionality coefficient for mobility normalized to standard temperature and pressure, was calculated using the equation: cal K 0 ¼ ðt cal d K 0 Þ=t d
ð2Þ
cal where tcal d is the drift time of the internal calibrant, K0 is the reduced mobility of the internal calibrant, and td is the drift time of the ion of interest.30 The reduced mobility of the internal calibrant was calculated to be 1.75 cm2 V1 s1 using the equation:
K0cal ¼ ðL=ðEt cal d ÞÞð273=TÞðP=760Þ
ð3Þ
where L is the length, E is the electric field strength, T is the temperature, and P is the pressure in the drift region of the IMS instrument.6,30 8601
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Analytical Chemistry Two important observations can be made regarding the mobility spectra in Figure 5. The first observation is the mobility spectrum acquired using the benchtop IMS instrument (Figure 5a), the mobility spectrum acquired using the faraday plate in the IMS/MS/MS instrument (Figure 5b), and the multiple mass mobility spectrum acquired using the IMS/ MS/MS instrument (Figure 5c) are quite similar with respect to the number of peaks, ratios between the peaks, reduced mobilities of the peaks, and resolving power of the measurements, indicating that the operating principles of the IMS instrument are unchanged subsequent to being interfaced to the mass spectrometer and that the core ion species formed in the IMS instrument are largely unaltered during transfer from the IMS instrument to the mass spectrometer. However, it should be recognized that clustering and declustering of the ion species with water molecules as well as a small amount of fragmentation to the core ion structures may occur in the differentially pumped interface.6,20 The second observation regarding the mobility spectra is the individual mass-selected ion mobility spectra acquired using the IMS/MS/MS instrument (Figure 5d) determine the K0 values for [Cl], [Cl + H2O], [Cl2], and [TNT H] to be 2.90, 2.88, 2.45, and 1.54 cm2 V1 s1. A K0 value of 1.54 cm2 V1 s1 for [TNT H] is consistent with the K0 value previously reported for [TNT H],3,17,26 indicating that the combination of an internal calibrant, mobility spectra, and mass-selected ion mobility spectra to measure K0 values is a valid protocol. Although not shown, it is important to note the sensitivity of the IMS/MS/MS instrument for TNT when the instrument is operated in the mass-selected ion mobility mode with a gate width of 0.2 ms is ∼1 ng, a value comparable to those obtained using the benchtop IMS instrument.9 Collectively, these observations lead to the conclusion that the COTS-ETD/API 2000 configuration is useful for the accurate and extensive characterization of the ion chemistry of the COTS-ETD equipment. Mass, mobility, and mass-selected ion mobility spectra as well as K0 values for the explosives standards cyclo-1,3,5trimethylene-2,4,6-trinitramine (RDX) and pentaerythritol tetranitrate (PETN) are reported in the Supporting Information. This analysis determined that the product ions for RDX are [RDX + Cl], [RDX + NO2], and [RDX + RDX + Cl] and have K0 values of 1.47, 1.42, and 1.00 cm2 V1 s1, whereas the product ions for PETN are [NO3], [PETN NO2 + H + Cl], [PETN H], [PETN + Cl], and [PETN + NO3] and have K0 values of 2.40, 1.29, 1.27, 1.22, and 1.18 cm2 V 1 s1. The product ions and K0 values reported in this analysis are consistent with the product ions and K0 values previously reported for RDX and PETN.3,8,9,18 Because the product ions formed during the analysis of RDX and PETN are less stable than the product ions formed during the analysis of TNT,3,8,31 the success of this analysis further substantiates the utility of the COTS-ETD/API 2000 configuration, specifically, the conclusion that the ion species formed in the IMS instrument can be characterized by the mass spectrometer without substantial change to the core ion chemistry. An additional detail regarding the mass-selected ion mobility mode is that the loss of low mass ions (m/z < 50) due to scattering is negligible compared to the continuous ion flow mode. This decrease in low mass scattering is attributed to the decreased pressure in Q0 in the mass-selected ion mobility mode and is the reason the ratio of hexachloroethane signal to TNT signal is larger in the mass-selected ion mobility mode (Figure 5) than in
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Figure 6. Mass and MS/MS spectra obtained when a 100 ng TNT sample was analyzed in the selected mobility monitoring mode. Plot a is the mass spectrum when a K0 of 2.90 cm2 V1 s1 was monitored, plot b is the mass spectrum when a K0 of 2.45 cm2 V1 s1 was monitored, plot c is the mass spectrum when a K0 of 1.54 cm2 V1 s1 was monitored, and plot d is the MS/MS spectrum for [TNT H] (m/z 226) when a K0 of 1.54 cm2 V1 s1 was monitored.
the continuous ion flow mode (Figure 3) when similar samples are analyzed. Because the loss of high mass ions due to scattering is minimum in all operating modes,22,23,29 the mass-selected ion mobility mode provides a more accurate representation of the relative abundances of the ion species formed in the IMS instrument. This statement is supported by the similarity in the ratios between the peaks in the mobility spectra acquired using the benchtop IMS instrument (Figure 5a) and the IMS/MS/MS instrument (Figure 5c). Selected Mobility Monitoring Mass and MS/MS Spectra. To verify the mass, structure, and mobility information obtained in the continuous ion flow and mass-selected ion mobility modes, mass and MS/MS spectra for the individual peaks in the mobility spectra in Figure 5 were acquired by operating the IMS/MS/MS instrument in the selected mobility monitoring mode. The mass spectra for the mobility peaks with K0 values of 2.90, 2.45, and 1.54 cm2 V1 s1 are shown in Figure 6ac while the MS/ MS spectrum for the mobility peak with a K0 value of 1.54 cm2 V1 s1 and the parent ion at m/z 226 is shown in Figure 6d. The spectra are the result when a 100 ng TNT sample was analyzed, and the pulse width of the first and second gates was 1.0 ms. The mass spectra indicate the mobility peaks with K0 values of 2.90 and 2.45 cm2 V1 s1 consist of [Cl], [Cl + H2O], and [Cl2] from hexachloroethane while the mobility peak with a K0 value of 1.54 cm2 V1 s1 consists of [TNT H] from TNT. The MS/MS spectrum indicates the collision-induced dissociation pathway for the product ion with a K0 value of 1.54 cm2 V1 s1 and a m/z of 226 is the loss of NO and NO2, providing additional evidence that the mobility peak with a K0 value of 1.54 cm2 V1 s1 consists of [TNT H]. This example demonstrates that the capability to acquire MS/MS spectra in the selected mobility monitoring mode provides mass, structure, and mobility information in a single measurement, allowing identifications to be attained with higher confidence.25 A gate width of 1.0 ms for the first and second ion gates was chosen for the selected mobility monitoring mode because it 8602
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permitted mass and MS/MS spectra with adequate signal-tonoise ratios to be obtained. While a factor of ∼5 was gained in signal intensity by increasing the gate width of the first and second ion gates from 0.2 to 1.0 ms, a factor of ∼3.5 was lost in resolving power, increasing the probability that ion species with similar mobilities were not completely separated. This decrease in resolving power is the reason [Cl] and [Cl2] are observed in both the mass spectrum for the mobility peak with a K0 of 2.90 cm2 V1 s1 (Figure 6a) and the mass spectrum for the mobility peak with a K0 of 2.45 cm2 V1 s1 (Figure 6b).
’ ACKNOWLEDGMENT J.K. was funded through an interagency agreement between the Department of Homeland Security Science and Technology Directorate and the Naval Research Laboratory under IAA No. HSHQDC-10-x-00444. The authors acknowledge Bruce Thompson, David Atkinson, Robert Ewing, and Jill TomlinsonPhillips for insightful discussion, Pamela Beresford for editing the manuscript, and Darryl Mendillo, Nichole Lareau, and Michael Bell for assistance in preparing the computer aided drawings.
’ CONCLUSIONS A COTS IMS based ETD has been interfaced to an API 2000 triple quadrupole mass spectrometer. The important operating capabilities of the COTS-ETD/API 2000 configuration include the acquisition of mass and MS/MS spectra in the continuous ion flow and selected mobility monitoring operating modes and the acquisition of mass-selected ion mobility spectra. To acquire mass-selected ion mobility spectra, the mass spectrometer was modified to increase the electric field strength and decrease the pressure in the differentially pumped interface, causing a decrease in the effect of collisional focusing. To acquire mass and MS/MS spectra in the selected mobility monitoring mode, the mass spectrometer was modified to include ion gating capabilities in the differentially pumped interface. The utility of the COTS-ETD/API 2000 configuration was established by identifying the product ion formed during the analysis of TNT to be [TNT H], the predominant collisioninduced dissociation pathway for [TNT H] to be the loss of NO and NO2, and the reduced mobility value for [TNT H] to be 1.54 cm2 V1 s1, findings consistent with previous experiments. Moreover, this analysis was attained for sample amounts of 1 ng and with a resolving power of 37, values comparable to those obtained with the IMS instrument before being interfaced to a mass spectrometer. Consequent to the success of this analysis, as well as the success of the analysis for RDX and PETN reported in the Supporting Information, it is concluded that the COTS-ETD/API 2000 configuration is useful for the accurate and extensive characterization of the ion chemistry of the COTS-ETD equipment. A future effort will involve using COTS-ETD based IMS/MS/ MS instrumentation to characterize the ion species formed during the analysis of typically encountered explosives and chemical interfering agents. It is anticipated that an enhanced understanding of the ion chemistry intrinsic to COTS IMS based ETD equipment will lead to the advancement of its operational effectiveness by facilitating modifications to the instrument parameters with the objective of increasing selectivity. Moreover, it is anticipated that the new information uncovered by the COTS-ETD based IMS/MS/MS instrumentation regarding atmospheric pressure chemical ionization of explosive vapor will be applicable to the development of the next generation ETD equipment, whether IMS or MS based.
’ REFERENCES
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
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’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on October 21, 2011. A text correction was made to the caption of Table 1, and the corrected version was reposted on October 27, 2011.
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