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Sensitive and Comprehensive Detection of Chemical Warfare Agents in Air by Atmospheric Pressure Chemical Ionization Ion Trap Tandem Mass Spectrometry with Counterflow Introduction Yasuo Seto,*,† Hiroshi Sekiguchi,†,⊥ Hisashi Maruko,†,⊥ Shigeharu Yamashiro,†,⊥ Yasuhiro Sano,†,⊥ Yasuo Takayama,†,⊥ Ryoji Sekioka,†,⊥ Shintaro Yamaguchi,†,⊥ Shintaro Kishi,†,⊥ Takafumi Satoh,†,⊥ Hiroyuki Sekiguchi,†,⊥ Kazumitsu Iura,†,⊥ Hisayuki Nagashima,† Tomoki Nagoya,† Kouichiro Tsuge,† Isaac Ohsawa,† Akihiko Okumura,*,‡ Yasuaki Takada,‡ Naoya Ezawa,§ Susumu Watanabe,∥ and Hiroaki Hashimoto∥ †

National Research Institute of Police Science, Kashiwa, Chiba 277-0882, Japan Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185-8601, Japan § Defense Systems Company, Hitachi, Ltd., Chiyoda, Tokyo 101-8608, Japan ∥ Hitachi High-Tech Solutions Corporation, Mito, Ibaraki 319-0316, Japan ‡

S Supporting Information *

ABSTRACT: A highly sensitive and specific real-time field-deployable detection technology, based on counterflow air introduction atmospheric pressure chemical ionization, has been developed for a wide range of chemical warfare agents (CWAs) comprising gaseous (two blood agents, three choking agents), volatile (six nerve gases and one precursor agent, five blister agents), and nonvolatile (three lachrymators, three vomiting agents) agents in air. The approach can afford effective chemical ionization, in both positive and negative ion modes, for ion trap multiple-stage mass spectrometry (MSn). The volatile and nonvolatile CWAs tested provided characteristic ions, which were fragmented into MS3 product ions in positive and negative ion modes. Portions of the fragment ions were assigned by laboratory hybrid mass spectrometry (MS) composed of linear ion trap and highresolution mass spectrometers. Gaseous agents were detected by MS or MS2 in negative ion mode. The limits of detection for a 1 s measurement were typically at or below the microgram per cubic meter level except for chloropicrin (submilligram per cubic meter). Matrix effects by gasoline vapor resulted in minimal falsepositive signals for all the CWAs and some signal suppression in the case of mustard gas. The moisture level did influence the measurement of the CWAs.

C

hemical warfare agents (CWAs)1,2 are acute and lethal toxic gases. These agents have been synthesized and used by military forces since World War I.3 Currently, many countries have agreed to accept the Chemical Weapons Convention that prohibits the development, production, stockpiling, and use of chemical weapons, having mandated their destruction in 1997.4 However, there has been an ongoing threat of chemical terrorism through the use of CWAs. The threat of chemical terrorism was first given worldwide publicity by the sarin (GB) nerve gas attack on the Tokyo subway system by the Aum Shinrikyo cult in 1995.5 Most recently, in Syria in 2013, thousands of people were killed through the use of CWAs, including sarin.6 To counter and mitigate the effects of chemical warfare and terrorism, rapid on-site detection and identification of the agents is needed.7 Monitoring of low levels of CWAs is also required in chemical weapons disposal facilities for the health © 2014 American Chemical Society

and safety management of workers and the local population. Accordingly, development of on-site methods for detecting airborne CWAs has been undertaken, with the aim of achieving high sensitivity and specificity at short measurement time scales using compact and easy-to-operate instrumentation. Hand-held or portable equipment, such as the gas detection tube,8 ion mobility spectrometers,9−18 and surface acoustic wave detection arrays,19 have been commercialized for measurement of CWAs. However, performance,20−23 in general, has lacked sensitivity and/or specificity. Gas chromatography (GC) with element-specific detection24 or GC/mass spectrometry (GC/MS)25,26 does provide high specificity for CWA detection. In addition, higher sensitivity Received: January 4, 2014 Accepted: March 28, 2014 Published: March 28, 2014 4316

dx.doi.org/10.1021/ac500042r | Anal. Chem. 2014, 86, 4316−4326

Analytical Chemistry

Article

fulfilled the requirements for counterterrorism application47 (detection of 1/100 median lethal concentration for 1 min of exposure (LC50)). Analytical performance was also sufficient to meet the performance criteria in health and safety management (hygienic regulations or 1/1000000 LC50, within several minutes) of some agents. High specificity of the method was demonstrated by a relative absence of false-positive signals for gasoline vapor for all agents. Gasoline was selected as the test compound for examining matrix effects, because it contains plenty of volatile organic compounds and can be detected in the urban environment. The effect of the moisture level on measurement of the representative CWAs was also examined.

can be achieved when a preconcentration step is performed. However, GC-based methods have drawbacks such as a long measurement time, low sensitivity for gaseous CWAs, and nonapplicability for labile CWAs unless a sample pretreatment or derivatization 27,28 is performed to prevent analyte degradation during GC separation, maintenance, or column and/or sorbent tube cleanup. Moreover, such instrumentation is relatively complex, requiring skilled personnel to operate. Consequently, in recent years, direct air sampling followed by MS has received considerable interest for the measurement of CWAs. The combination of soft ionization29 and tandem MS provides information not only on molecular weight, but also the molecular structure of analytes, enabling highly specific detection without chromatographic preseparation. Ion trap mass spectrometry (ITMS) may be the best choice on account of its high specificity, based on exploiting multiple-stage mass spectrometry (MSn), and its field usability because of the compactness and robustness of the ion trap. Miniature ITMS instruments have been developed and used for the detection of CWAs or their simulants using preconcentration30 or GC preseparation.31 An alternative measurement approach is the combination of soft ionization and high-resolution mass measurement, so-called millimass measurement.32−35 However, instruments for millimass measurement, such as time-of-flight (TOF) MS and FTICR MS, need high vacuum and precise assembly of large-scale components such as the TOF drift tube and are essentially too cumbersome and delicate for field use. Among various emerging MS technologies for ambient ionization, 36 atmospheric pressure chemical ionization (APCI)29 is a traditional simple and efficient soft ionization technique and has been widely used in commercial MS and ion mobility spectrometry instruments. APCI using a corona discharge (CD) for primary ionization is a highly efficient soft ionization technique, as demonstrated for the detection of sarin in positive ion mode.37 However, the sensitivity was relatively poor in negative ion mode because of interfering byproducts such as ozone and nitrogen oxides generated by the CD. Generation of such byproducts was less in APCI using a radioactive source,38 but the radioactive source is undesirable for field use by nonspecialists in view of international regulations. Recently, CD-APCI with a novel counterflow introduction (CFI) configuration,39 which enabled effective removal of byproducts, was developed to realize efficient chemical ionization of various hazardous materials, not only in positive ion mode but also in negative ion mode.40−46 Recently, CFI-APCI-MS was deployed for sensitive detection of four representative volatile CWAs, including positively and negatively ionizable agents (sarin, tabun, mustard gas, lewisite 1).47 Proton transfer reaction MS48,49 and selected ion flow tube MS50,51 generate the selected or purified reactant ion flow (H3O+, O2+, NO+, etc.) to be mixed with sample air for chemical ionization of analytes. This contributes to simplifying the mass spectral pattern and thus improving the sensitivity and specificity. However, the reported detection sensitivity for some CWAs48,52,53 was poorer than that obtained by our instrument. In addition, complicated instrumentation for the generation of purified reactant ion flow may be unsuitable for field use. In current research, the feasibility of CFI-APCI and MSn (n ≤ 3) for the detection of a wide range of CWAs, including not only typical volatile CWAs but also gaseous and nonvolatile CWAs, was investigated. Sensitive real-time detection was achieved for all the agents tested using a commercial field monitoring instrument based on ITMS. The new approach

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EXPERIMENTAL SECTION Chemicals. GA (O-ethyl N,N-dimethylphosphonocyanidate, tabun), GB (O-isopropyl methylphosphonofluoridate), GD (O-pinacolyl methylphosphonofluoridate, soman), VX (Oethyl S-(diisopropylamino)ethyl methylphosphonothioate), HD (bis(2-chloroethyl) sulfide, mustard gas), and L1 ((2chlorovinyl)dichloroarsine; lewisite 1) were obtained from the TNO Prins Maurits Laboratory (Rijswijk, The Netherlands) or synthesized in the laboratory of the National Research Institute of Police Science (Kashiwa, Japan). GF (O-cyclohexyl methylphosphonofluoridate, cyclohexylsarin), RVX (O-isobutyl S-(diethylamino)ethyl methylphosphonothioate, Russian VX), DF (methylphosphonodifluoride), HN1 (N-ethylbis(2chloroethyl)amine, nitrogen mustard 1), HN2 (N-methylbis(2-chloroethyl)amine, nitrogen mustard 2), HN3 (tris(2chloroethyl)amine, nitrogen mustard 3), CS ((ochlorobenzylidene)malononitrile), and DM (diphenylamine chloroarsine, adamsite) were synthesized in the same laboratory. All of these compounds were >98% pure. DA (diphenylchloroarsine) and DC (diphenylcyanoarsine) were obtained from Hodogaya Chemical Co. Ltd. (Tokyo, Japan). CN (2-chloroacetophenone) was obtained from Kanto Chemicals (Tokyo, Japan). Capsaicin was obtained from Sigma-Aldrich Chemicals (St. Louis, MO). PS (chloropicrin) (purity 99.5%) was purchased from Mitsui Chemicals (Tokyo, Japan). Other reagents were of analytical grade. Instrumentation. A schematic of the CFI-APCI-ITMS instrument (DS-1000, Hitachi, Tokyo, Japan) is shown in Figure 1. The instrument was 270 kg in mass, 880 mm (height) × 820 mm (width) × 710 mm (depth) in size, and 2 kW in power consumption. It could also be easily moved on casters.

Figure 1. Sectional view of the atmospheric pressure chemical ionization ion trap mass spectrometry instrument (Hitachi, DS-1000) with counterflow introduction. 4317



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Details of the instrument were described in a previous paper.47 Briefly, the corona discharge was generated between a metal needle and a disk electrode with an aperture. The sample air was introduced into the discharge region via the aperture and pumped out from the tip of the needle. Positive or negative reactant ions generated in the discharge region, including hydronium ions (H3O+) and oxygen anions (O2−), were forced through the aperture against the incoming counter airflow and then drawn into the mass analyzer through the inlet electrode by the strong electric field formed between the needle and the inlet electrode. In this counterflow configuration, neutral byproducts generated in the discharge, such as ozone and nitrogen oxides, were effectively removed from the system. As a result, the formation of stable NO3− was suppressed, and instead, a high yield of reactant O2− anions was produced, thus promoting, in negative ion mode, a higher ionization efficiency of the analyte. The ions exiting the inlet electrode passed through two skimmer electrodes, which divided the vacuum system into three chambers for differential pumping, and then a focusing static lens assembly prior to entering a conventional three-dimensional quadrupole ion trap. Helium buffer and collision gas (>99.99995% purity, Tomoe Shokai, Tokyo, Japan) was continuously introduced into the ion trap. Mass analysis was carried out using the mass-selective instability mode of operation. In tandem mass analysis, mass-selective resonant excitation was used for isolation and collision-induced dissociation (CID) of the ion of interest. The excitation voltage and cutoff m/z, or minimum m/z of ion that could be trapped, were tuned, depending on the agent, to efficiently generate a fragment ion with relatively large m/z to allow successive fragmentations in the subsequent stages of MSn. All the other instrumental variables except the polarity of the applied voltages were fixed throughout the experiments. Temperatures for the ion source, the inlet electrode, and the second skimmer electrode were kept at 120 °C. The corona discharge current was 10 μA. The applied voltages were the same as those previously reported.47 The airflow in the discharge region was 0.7 L/min, and the resultant overall airflow into the instrument, measured by a flow meter, was 1.2 L/min. Sample Preparation and Experimental Setup. Standard solutions of GB, GD, GF, GA, VX, RVX, DF, HD, L1, HN1, DA, DC, CN, CS, OC, and PS were prepared in n-hexane (Wako Pure Chemical Industries, Osaka, Japan) at specified concentrations. HN2 and HN3 were dissolved in methanol to give a 1% (v/v) solution and then diluted with n-hexane. DM was dissolved in and diluted with methanol. Figure S-1 (Supporting Information) shows the typical experimental setup for generation of sample vapor and introduction into the CFI-APCI-ITMS instrument. The CWA vapor was generated in a custom-made 10 L heated container (Hitachi Kyowa Engineering Co., Hitachi-naka, Japan) by injecting a drop (2 or 5 μL) of a standard solution. The container was made of stainless steel with the inner surface coated with poly(tetrafluoroethylene) and heated with an electric heating mantle to between 70 and 180 °C, depending on the agent. Blank measurements were performed between the test CWA measurements, which indicated minimal carryover. The temperatures of the container and the parts of the instrument were raised to 180−200 °C for cleaning after data acquisitions. Occasionally, some intense mass spectral peaks appeared in high mass ranges, depending on the agent species tested. These peaks could not be assigned and may be due to thermal decomposition products.

Vapors of hydrogen cyanide (AC) and cyanogen chloride (CK) were generated in a 10 L Pyrex glass container with a double valve (Kusano Science, Tokyo, Japan). For generation of AC vapor, an aliquot of an aqueous solution of potassium cyanide (24 mg/mL) was mixed with 10% (v/v) sulfuric acid to be more than the mole equivalent quantity of potassium cyanide. For generation of CK vapor, an aliquot of an aqueous solution of potassium cyanide (11 mg/mL) was mixed with 0.2 M chloramine-T solution to be more than the mole equivalent quantity of potassium cyanide. The vapor concentrations were measured using gas detection tubes for AC (Gastec Corp., Kanagawa, Japan) and CK (Draeger Safety, Lübeck, Germany). Vapors of phosgene (CG) and chlorine (CL) were generated by a permeator (PD-1B-2, Gastec Corp.) using gas permeation tubes (P10 cm, Permeacal, Gastec) for CG (Pr = 2659 (25 °C), 3854 (30 °C), and 5650 (35 °C) ng/(min cm)) and CL (Pr = 6756 (30 °C) and 8918 (35 °C) ng/(min cm)) incubated at 25−35 °C and diluted with pure nitrogen gas. The concentration of the generated gas (C, ng/mL, mg/m3) was obtained using the equation C = [(Pr)L]/F, where Pr is the permeation velocity, L is the tube length (cm), and F is the dilution gas flow rate (mL/min). The vapor concentrations were measured using gas detection tubes (Gastec Corp.). Two air streams were generated from pressurized clean and dried air (Tomoe Shokai Co. Ltd.) using mass flow controllers (SFC280E, Hitachi Metals Ltd., Tokyo, Japan; KOFLOC 3660, Kojima Instruments Inc., Kyoto, Japan). The two air streams (4 and 3 L/min) were passed though the container and a custommade humidifier (GL Sciences, Tokyo, Japan), respectively, and then mixed. The humidity of the generated air stream was controlled to 10 g/m3. A portion of the sample air was sucked into the instrument at a rate of 1.2 L/min, and the excess was evacuated via a fume hood. The signal for the agent was monitored over 10 min for each test run. The average signal intensity was plotted against the average vapor concentration to establish a calibration plot. The average vapor concentration was calculated by dividing the injected amount of agent by the sample air volume generated in 10 min. The prepared sample gases, AC, CK, CG, and CL, were withdrawn by a 50 mL plastic syringe and infused directly into the instrument inlet using a syringe pump (MSPE-1, AS ONE Corp., Osaka, Japan). The measured gas concentration was varied by varying the infusion flow rate. The limit of detection (LOD) was defined as the concentration giving a signal of 3σBG, where σBG is the standard deviation of the background signal. It was calculated using the sensitivity (S) or the slope of the calibration curve, which was obtained by linear regression of the calibration plot. The background signal was measured about 250 times for the laboratory air. In evaluating the LOD, the time for ion loading into the trap was 300 ms and three successive measurements were averaged to form a single data point. This resulted in measurement cycles of 1.2 and 1.3 s for MS2 and MS3, respectively. The measurement routine for MS2 was run even for AC and CK, which gave no product ions in MS2 to reduce chemical noise by applying a CID voltage as high as possible, but which was ineffective for ejecting monitored ions out of the trap. Ion Assignment of Chemical Warfare Agent-Derived Ions. CWAs were analyzed to assign exact ion formulas by high-resolution MS. GB, GA, RVX, L1, HN1, HN2, HN3, DA, DC, and DM were dissolved in acetonitrile and infused into an LTQ XL-Orbitrap mass spectrometer (Thermo Fisher 4318



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Figure 2. MSn spectra for CWAs. Unlabeled peaks in MS1 spectra are not assigned or attributed to solvent or contaminants. In MS2 and MS3 spectra, only the main peaks are labeled with m/z and peaks labeled with “#” and “*” are residual ions due to imperfect isolation and resultant false fragments, respectively. O2− and (H2O)O2− are from air.

Scientific Inc., Waltham, MA) at a flow rate of 20 μL/min. Mass spectrometric conditions were as follows: electrospray ionization; capillary voltage, +10.0 kV; sheath gas flow rate, 8 L/min; heated capillary temperature, 250 °C; spray voltage, 5 kV; tube lens voltage, 110 V; normalized collision energy, 45 V; FTMS resolution, 15000. On MSn, the precursor ions were isolated and dissociated in the linear trap, and the product ions were analyzed by the LTQ XL-Orbitrap mass spectrometer. In the case of DF, the compound was dissolved in acetonitrile and introduced into the mass spectrometer at a flow rate of 0.2 mL/

min. The following mass spectrometric conditions were adopted: atmospheric pressure chemical ionization; capillary voltage, +10.0 kV; heated capillary temperature, 250 °C; vaporization temperature, 450 °C. Safety Considerations. CWAs are highly toxic and were carefully handled by specially trained personnel. All experiments for CWAs were performed in a specialized facility. Sample vapors were prepared in a fume hood with an alkaline solution scrubber system to prevent personnel from being exposed to vapors and to prevent CWAs from being released 4319


4320

+

+

+

+

+ + + + − + + + + +

+ + + + − − − −

−

GB

GD

GF

GA

VX RVX DF HD L1 HN1 HN2 HN3 DA DC

DM CN CS OC AC CK CL CG

PS

polarity

assignment MH+ [M − C3H5]+ MH+ [M − C6H11]+ MH+ [M − C6H9]+ MH+ MH+ MH+ MH+ MH+ [M + O + H]+ [M − 2Cl + 3O + H]− MH+ MH+ MH+ MH+ MH+ MH+ [M − Cl]+ MH+ MH+ MH+ [M − H]− [M + O − Cl]− M− Cl2− Cl− [M − Cl]−

transition (m/z)

141 → 99 → 97 99 → 97 → 79 183 → 99 → 97 99 → 97 → 79 181 → 99 → 97 99 → 97 → 79 163 → 135 163 → 135 → 126 268 → 128 → 86 268 → 100 → 72 101 → 99 → 97 175 → 77 185 → 123 170 → 142 → 106 156 → 120 → 92 204 → 106 → 70 265 → 229 → 227 256 → 229 256 → 229 → 227 242 → 167 → 139 155 → 77 → 49 189 → 162 → 127 306 → 137 → 122 26 42 70 → 35 70 → 35 35 128 → 46 170−1700

5.7−29 0.14−2.9 0.14−2.9 0.14−2.9 4.3−71 3.3−670 2.2−220 3.1−310

0.057−2.9 0.57−2.9 0.14−2.9 1.5−30 1.0−9.5 0.13−2.6 0.16−3.3 0.18−3.5 0.3−2.7 0.2−1.25

0.065−3.2 0.065−3.2 0.50−10

0.16−3.1 3.1 0.15−7.3

range (mg/m3)

2

3 5 5 5 5 6 5 7

7 4 5 5 5 5 5 5 5 4

5 1 7 7 7 7 5

point

calibration plot

0.9924 0.9986 0.9931 0.9958 0.9231 0.9903 0.9916 0.996 0.9976 0.9994

0.988 1.000 0.9596 0.999 0.9997 0.9977 0.9968 0.9983 0.9639 0.9852 0.9942 0.9994 0.9840 0.999 0.9996 0.9981 0.9288 0.9952

R2

6.1 0.11 0.43 0.15 2.3 6.6 2.3 23 14 540

0.58 0.0076 4.3 0.29 0.22 0.041 0.028 0.0024 1.7 1.3 1.5 0.63 0.66 0.053 0.11 0.31 0.12 0.030

LODa (μg/m3)

−/700

2900/1500 −/400

5000/11000

−/300

3/0.4 −/3 −/3

0.01/0.001

0.1/0.03

100

1500

b

100000

43

0.4

0.0009

0.02

0.01

0.004

−/0.03 0.05/0.03

0.02

TWAc (ppb)

0.1/0.03

STEL/TWAb (μg/m3)

2200f

4500 11000 21000f

10000 7000 61000

1500 1200−1500 1500 3000 1500 15000 10000

30 40f

300

70−100e

70−100

70−100

LC50d (mg/m3)

ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND 0.0029 ND ND ND ND ND ND ND ND ND ND

false-positive rate by gasoline vaporg

a

Defined as the concentration giving an ion signal of 3σBG, where σBG is the standard deviation of the background signal measured for the laboratory air. Short-term exposure limit/time-weighted average (in ref 57). cTime-weighted average (in ref 55). dFifty percent lethal concentration for 1 min of exposure for humans (in ref 1 except for GF, RVX, CL, and PS). eAssumed to be the same as that for GD. f Reference 54. gFalse-positive rate by gasoline vapor [(μg/m3 of CWA)/(mg/m3 of gasoline)]. ND = not detected.

choking agent

blood agent

lachrymator

vomit agent

precursor blister agent

nerve gas

agent

ion monitored

Table 1. Detection Performance of CFI-APCI-ITMS in Real Time (1 s) for Gaseous, Volatile, and Nonvolatile Chemical Warfare Agents and a Precursor in Air

Analytical Chemistry Article



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be detected are extremely low. Among the nerve gases, O-alkyl methylphosphonofluoridates (GB, GD, and GF) are represented as CH3P(O)(F)OR, where R denotes an alkyl group. As is seen in the MSn spectra for these agents (Figure 2a for GD, Figure S-2a (Supporting Information) for GB, and Figure S-2b for GF), the positive ion mode provided a base peak at m/z 99 (precise mass 99.0007, which was obtained by the LTQ Orbitrap mass spectrometer) in MS1 owing to the occurrence of an identical fragment ion32 expressed as [MH − (R − H)]+ or [CH3P(F)(OH)2]+ and a weak broad peak attributable to MH+, where M denotes the mass of the agent, and the other fragment ions. In the case of GD, an additional intense fragment ion peak at m/z 85 was observed in MS1, which could be assigned as the pinacolyl ion (R+, [CH(CH3)C(CH3)3]+). Although the protonated molecular peak was observed at an m/z position lower than that of [M + 1]+, it was considered that the peak was mass shifted to lower m/z as a result of peak broadening, which possibly occurs in ITMS of fragile ions.58 The MS2 spectrum of the parent MH+ provided a base peak at m/z 99, and in the case of GD, an intense fragment ion peak at m/z 85 was observed. In the case of GB, an intense peak at m/z 81 (precise mass 80.9900) was observed, which could be assigned to [CH3P(O)(F)]+. MS3 analysis revealed the transition of MH+ → m/z 99 → 97 and m/z 99 → 97 → 79 for the base peak in MS1 (as seen in Figures 2a and S-2a,b). The ion at m/z 97 (precise mass 97.0050) can be represented as [CH3P(OH)3]+. This ion can be ascribed to the product of the ion−molecule reaction between the parent ion and residual water.48 The ion at m/z 79 (precise mass 78.9944) can be denoted as [CH3P(O)(OH)]+, which could be produced through loss of H2O. The LOD for GD by monitoring the MH+ → m/z 99 → 97 transition was more than 100 times the TWA.55 By signal averaging 100 times, which took only 2 min, the LOD value was still higher than the STEL or the TWA. Alternatively, monitoring of the base peak ion (m/z 99 → 97 → 79 transition), which was 10 times more abundant than MH+, provided an LOD value sufficiently lower than both the STEL and the TWA; in this case, 2 min was required for signal averaging. For GB, the LOD value was improved by MS3 analysis (MH+ → m/z 99 → 97) compared with MS2 analysis (MH+ → m/z 99),47 but the LOD did not attain the values for the STEL or the TWA. Instead, monitoring of the base peak ion (m/z 99 → 97 → 79 transition) provided an LOD value sufficiently lower than both the STEL and the TWA. For GF, the LOD value obtained by MS3 analysis for monitoring the MH+ → m/z 99 → 97 transition was 1 order of magnitude higher than the STEL or TWA, but monitoring the base peak ion (m/z 99 → 97 → 79 transition) provided an LOD value almost similar to the STEL and the TWA; thus, the sensitivity requirement was readily achieved by signal averaging for several seconds. The relatively high LOD value for GD was possibly caused by low sensitivity as a result of analyte loss through adsorption of analyte molecules onto system surfaces; in this example, the calibration plot exhibited significant concave curvature over a wide concentration range. Although monitoring of the base peak ion (m/z 99 → 97 → 79 transition) would not discriminate these G-agents, measurement would still be of value because the same antidote is used for these agents. Another G-agent, GA, has a molecular structure expressed as (CH3)2NP(O)(CN)OR, where R is C2H5. By analogy with the molecular structures of other G-agents (GB, GD, GF), the MS2 transition from MH+ (m/z 163 → 135)47 may be

outside the facility. The use and synthesis of the registered CWAs (GB, GD, GF, GA, VX, RVX, DF, HD, L1, HN1, HN2, HN3) for this research were approved by the Ministry of Economy, Trade and Industry of Japan (http://www.meti.go. jp/polycy/chemical_management/cwc/200kokunai/ 202horitu_gaiyo.htm).

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RESULTS AND DISCUSSION In a previous study,47 MS1 and MS2 spectra for GB, GA, HD, and L1 were obtained by CFI-APCI-MS. In this research, not only these 4 agents but a further 18 CWAs agents and 1 precursor were analyzed by CFI-APCI-MS using more than two stages of MS. For all of the volatile and nonvolatile CWAs investigated, analysis by MSn was successfully achieved to produce product ion(s) in MS3 from one or more characteristic ions of the agent. Gaseous CWAs, AC and CK, provided no product ions in MS2 from the characteristic ion(s) of the agent. Other gaseous CWAs, CL, CG, and PS, provided product ions in MS2 from characteristic ion(s) of the agent. MSn spectra for six selected CWAs and the other CWAs are shown in Figure 2 and Figure S-2 (Supporting Information), respectively. LOD values in real time or based on a 1 s measurement, which were obtained by monitoring characteristic ions of the target agents by using MSn (n = 1, 2, or 3) analyses, are listed in Table 1. The obtained LOD values were estimated after consideration of the requirements for counterterrorism and for health and safety management compliance.47 Regarding the counterterrorism aspect, detection of a vapor level 1/100 of the median lethal concentration for human exposure (LC50) within 1 min was taken as the measurement criterion. The LC50 values were obtained from the LCt50 (median lethal dosage) values,1 except for those of RVX54 and CL, CG, and PS,55 which were by calculation LC50 = LCt50/t (t = 1 min). The acute human toxicity of GF vapor is similar to that for GD,56 so the LC50 for GF can be regarded as the same as that for GD, or 70−100 mg/ m3. These LC50 values are also listed in Table 1, except for OC and DF, whose LCt50 values could not be obtained. From the health and safety management aspect, detection levels concerning hygiene regulations such as the short-term exposure limit (STEL)57 and time-weighted average (TWA)55,57 were considered, and if not available, levels corresponding to the 1/ 1000000 of the LC50 values were used. The reference values are also listed in Table 1. The STEL value represents a permissible concentration for 15 min of exposure. Therefore, the agent should be detectable within 15 min. The TWA is the average concentration of a chemical that a normal worker can be expected to be continuously exposed to during a normal 8 h work day and a 40 h work week without showing any adverse effects, and therefore, longer periods for detection are permissible. The LOD values when using signal averaging were calculated assuming that the σBG would be inversely proportional to the square root of the number of times for signal averaging. This assumption was verified by experiments on the measurement of selected compounds over several hours. As a result, the LOD values obtained without signal averaging were lower than the 1/100 LC50 values, fulfilling the detection sensitivity requirement for counterterrorism purposes for all agents. However, whether the LOD values met the detection sensitivity required for health and safety management purposes depended on the agent and the method for signal averaging (see later for detailed consideration). Nerve Gases. Nerve gases are highest in acute toxicity among all types of CWAs, and the concentrations required to 4321



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interpreted as [MH − (R − H)]+ or [(CH3)2NP(OH)2(CN)]+ as measured by LTQ Orbitrap MS (precise mass 135.0321). A past study60 also revealed the same MS2 transition, and spectral interpretation indicated a loss of C2H4 with good mass accuracy using high-resolution MS. The MS3 analysis revealed the transition MH+ → m/z 135 → 126 (Figure S-2c, Supporting Information). An ESI-MS study61 gave a mass spectrum for GA with ion peaks at m/z 126 and 135, but without species identification. High-resolution mass spectra by LTQ Orbitrap MS showed that the ion of m/z 126 (precise mass 126.0317) could be assigned to [(CH3)2NP(OH)3]+. This ion can be ascribed to the product of the ion−molecule reaction between the parent ion and water, just as in the case for O-alkyl methylphosphonofluoridates (Figures 2a and S-2a,b). A low LOD for GA by MS2 (m/z 163 → 135) has already been reported,47 which was nearly the same as the STEL62 and the TWA.55 In this study, we compared the detection performance for MH+ by MS3 (m/z 163 → 135 → 126) and MS2, the measurement routines for MS2 and MS3 being run simultaneously in the experiments to obtain calibration plots and σBG values. Use of MS3 significantly improved the LOD to about 1/ 10 that of MS2 as a result of the substantial decrease in σBG (about 1/16 that of MS2) despite a slight decrease (30%) in sensitivity. The LOD value by MS3 analysis (0.0024 μg/m3) was considerably lower than the STEL62 and TWA.55,62 VX and RVX have molecular structures represented as CH3P(O)(OR)SC2H4NR′2, where R and R′ are C2H5 and CH(CH3)2 for VX and CH(CH3)(C2H5) and C2H5 for RVX. MSn spectra for these agents (Figure 2b for VX and Figure S-2d (Supporting Information) for RVX) showed a base peak (m/z 268) due to MH+ in the MS1 spectrum. In MS2 spectral monitoring of the transition from MH+, a base peak (m/z 128 for VX; m/z 100 for RVX), ascribed to [(−CH2CH2−)NR′2]+, was observed. RVX provided an additional MS2 fragment ion peak at m/z 212. From the MSn analysis, the ion could be assigned to [MH − C4H8]+ or [CH3P(OH)2(SCH2CH2N(C2H5)2]+. A subsequent transition by MS3 for the base peak in MS2 provided a base peak (m/z 86 for VX; m/z 72 for RVX) ascribed to [(−CH2CH2−)NHR′]+ and a small peak at m/z 44 (possibly [(−CH2CH2−)NH2]+). VX has the highest toxicity and the lowest permissible concentration among the CWAs. The LOD for VX obtained by monitoring the MS3 transition from MH+ (m/z 268 → 128 → 86) was about 1/180 of the 1/ 100 LC50 value. However, the LOD value was about 1700 times the STEL62 or the TWA,55,62 and analysis by signal averaging was ineffective for achieving the STEL and TWA levels within the defined exposure times. For RVX, the LOD value by monitoring the MS3 transition from MH+ (m/z 268 → 100 → 72) was nearly the same as that for VX (Table 1). Further studies are needed to improve measurement sensitivity for VX and RVX, particularly for use in health and safety management scenarios. Precursors of Nerve Gases. DF is a precursor for synthesis of GB, GD, and GF. It has the molecular structure CH3P(O)F2 and is obtained by substituting OR in the Gagents with F. MSn spectra for DF (Figure S-2e, Supporting Information) provided profiles similar to those for the product G-agents. An intense base peak at m/z 101 due to MH+ was obtained for the MS1 spectrum. MS2 for MH+ provided a base peak at m/z 97 and peaks at m/z 99 and 79. In MS3, the ion at m/z 99 was fragmented into product ions of m/z 97 (base ion) and 79. The ions at m/z 99 (precise mass 99.0007), m/z 97 (precise mass 97.0050), and m/z 79 (precise mass 78.9944)

were assigned as [CH3P(F)(OH)2]+, [CH3P(OH)3]+, and [CH3P(O)(OH)]+, respectively, similar to the results for alkyl methylphosphonofluoridates. The LOD value (1.5 μg/ m3) by MS3 analysis (MH+→ m/z 99 → 97) was sufficiently low, considering the relatively low toxicity of these precursors. Blister Agents. MS2 analysis for HD and L1 and related LOD values were described in a previous paper.47 Briefly, HD formed the MH+ ion and the protonated molecule of an oxidized HD, [M + O + H]+ or [S(OH)(CH2CH2Cl)2]+, possibly formed by the reaction with ozone generated in the corona discharge. L1 is known to be easily hydrolyzed even in ambient air. It gave intense ion peaks due to the formation of various characteristic ions in negative ion mode. The chloride adduct ion of L1, [M + Cl]−, was observed only at very low humidity, and in MS2, this ion yielded chloride ion inefficiently. Alternatively, one of the ions derived from the decomposition products of L1, the ion at m/z 185 (precise mass 184.8931), assigned to [M − 2Cl + 3O + H]− or [(ClCHCH)As( O)(OH)(O)]−, was monitored for L1. In this study, the feasibility of performing MS3 for HD and L1 was investigated. For HD in MS2, the ion monitored at m/z 77 ([S(OH)(−CH2CH2−)]+) produced from the ion at m/z 175 ([M + O + H]+) could be further fragmented in MS3 into a product ion (m/z 59) (Figure S-2f, Supporting Information), which can be assigned to [S(CCH)H2]+ or [SCHCH2]+. However, use of MS3 decreased S/σBG. Although the sensitivity by monitoring the oxidized HD will be higher in the conventional air introduction configuration where the reaction of HD with ozone will be promoted, the previously reported LOD value47 for MS2 analysis was much lower than the STEL,63 and signal averaging over several seconds would satisfy the TWA criterion.63 For L1 in MS2, the monitored ion at m/z 123 (assigned to [AsO3]−) produced from the ion at m/z 185 ([M − 2Cl + 3O + H]−) could be fragmented in MS3 into a product ion (m/z 107, assigned to [AsO2]−) (Figure S-2g). Use of MS3 analysis only slightly improved S/σBG. The previously reported LOD value47 by MS2 analysis was much lower than the TWA. Nitrogen mustards, HN1, HN2, and HN3, have similar structures expressed as R−N(C2H4Cl)2, where R is C2H5, CH3, and C2H4Cl for HN1, HN2, and HN3, respectively. As seen in the MSn spectra for these agents (Figure 2c for HN1, Figure S2h,i (Supporting Information) for HN2 and HN3), intense base peaks due to MH+ were observed in the MS1 spectra. In the MS2 spectra, MH+ provided various fragment ions, including [MH − 28]+ through loss of C2H4, [MH − 36]+ through loss of HCl, [MH − 64]+ through loss of C2H5Cl, [MH − 98]+ through loss of C2H4Cl2, and [C2H4Cl]+ (m/z 63). MS3 analysis was conducted for a base ion peak in the MS2 spectrum, and a base ion peak in the MS3 spectrum was monitored for detection of the agent. These fragment ions were assigned by LTQ Orbitrap MS. The MS3 transition monitored was MH+ → [MH − C2H4]+ → [MH − C2H4 − HCl]+ (possibly [(ClC2H4)NH(−CH2CH2−)]+) (m/z 170 → 142 → 106) for HN1, MH+ → [MH − HCl]+ (possibly [(ClC2H4)N(CH3)(−CH2CH2−)]+) → [MH − HCl − C2H4]+ (possibly [(ClC2H4)NH(−CH2CH2−)]+) (m/z 156 → 120 → 92) for HN2, and MH+ → [MH − C2H4Cl2]+ (possibly [(ClC2H4)NH(−CH2CH2−)]+) → [MH − C2H4Cl2 − HCl]+ (possibly [N(−CH2CH2−)2]+) (m/z 204 → 106 → 70) for HN3. LOD values were well below the TWA values (for HN1)44 or the 1/ 1000000 LC50 values (for HN2 and HN3). Vomit Agents. DA and DC have the molecular structure Ph2AsX, where X is Cl for DA and CN for DC. As seen in the 4322



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MSn spectra (Figure 2d for DA and Figure S-2j (Supporting Information) for DC), ion peak(s) due to MH+ and a base peak, at m/z 229, ascribed to fragment ion Ph2As+,64 were observed. In MS2, fragmentation of the MH+ ion provided a base ion peak at m/z 229 due to loss of HX, and in MS3, monitoring the transition MH+ → m/z 229 provided a base peak at m/z 227 due to loss of H2. The structure of the ion at m/z 227 (precise mass 226.9841) was assumed to be benzene ring-bridged. The base peak in MS1 at m/z 229 provided the transition m/z 229 → 227 (loss of H2) → 152 as seen in Figure S-2j. The ion at m/z 152 (precise mass 152.0623) was assigned to [C12H8]+. This ion might be assumed to be produced from the transient fragment ion at m/z 154 (biphenyl ion) by sequential loss of hydrogen atoms in the ion trap.65 DM (Figure S-2k) provided an intense base peak at m/z 242 ascribed to [M − Cl]+ in the MS1 spectrum. In the MS2 spectrum, fragmentation of the [M − Cl]+ ion provided a base peak at m/z 167 ([C12H9N]+, precise mass 167.0730), and in the MS3 spectrum monitoring the transition m/z 242 → 167 provided intense ion peaks at m/z 139 ([C11H7]+, precise mass 139.0544) and 140 ([C10H6N]+, precise mass 140.0497). The structures of these ions are considered to be heterocyclic.66 For DA, the LOD by MS3 for MH+ was lower than 1/100 of the 1/ 1000000 LC50 value. For DC, the LOD by MS2 for MH+ was also lower than 1/100 of the 1/1000000 LC50 value. The LOD for DM by MS3 ([M − Cl]+ → m/z 167 → 139) was lower than the 1/1000000 LC50 value. Lachrymators. As seen in the MSn spectra (Figure S-2l, Supporting Information), CN provided intense base peaks at m/z 155 and 157 ascribed to MH+ in the MS1 spectrum. In MS2, the MH+ ions provided fragment ion peaks at m/z 77 and 79 ascribed to [C(O)CH2Cl]+, which could be further fragmented in MS3 to product ions ascribed to [CH2Cl]+ with m/z of 49 and 51. CS (Figure S-2m) provided positive ion peaks at m/z 189 and 191 ascribed to MH+ in MS1. In MS2, the ion corresponding to MH+ at m/z 189 provided fragment ion peaks at m/z 162 (ascribed to [M − CN]+ or possibly [Cl(C6H4)(CHCCN)]+) and m/z 153 ([M − Cl]+, possibly [(C6H4)(CHC(CN)2)]+). In MS3, the transition m/z 189 → 162 provided a base peak at m/z 127 ascribed to [M − CN − Cl]+, possibly [(C6H4)(CHCCN)]+. CS also provided a negative molecular ion, M−. Although the intensity of M− was 3 times that of MH+, fragmentation of M− by MS/MS was not efficient. The LOD values for MH+ in MS3 were substantially lower than the TWA55,67 for CN and the 1/1000000 LC50 value for CS. OC (Figure S-2n) provided a mass-shifted broad peak attributed to MH+ (m/z 306) and a base peak at m/z 137 in the MS1 spectrum. The MH+ ion provided a dominant base ion at m/z 137 (ascribed to [CH3O(HO)C6H3CH2]+ in MS2, and in MS3 the transition m/z 306 → 137 provided a dominant base peak at m/z 122 through loss of CH3. Gaseous Agents. Analyses of gaseous agents provided characteristic negative ions and no detectable positive ions. AC (Figure S-2o, Supporting Information) provided a base peak at m/z 26 ascribed to CN− and a small peak at m/z 42. CK provided a base peak at m/z 42 and very weak peaks at m/z 35 and 37 ascribed to Cl− (Figure S-2p). Fragmentation of the m/ z 42 ions from AC and CK by CID was unsuccessful. The m/z 42 ion may possibly be OCN− generated through oxidation of agents by ozone similarly to the formation of oxidized HD, as mentioned above. The LOD value for AC by monitoring m/z 26 (CN−) was well below both the STEL57 and the TWA.55,68 The LOD value for CK by monitoring m/z 42 (CNO−) was

lower than the 1/1000000 LC50 value. CL (Figure S-2q) provided molecular ion (Cl2−) peaks at m/z 70, 72, and 74 and peaks at m/z 35 and 37 ascribed to Cl− in the MS1 spectrum. In MS2 Cl2− provided Cl−. The LOD value for CL in MS2 (Cl2− (m/z 70) → Cl− (m/z 35)) was considerably below the STEL69 and the TWA.69,70 CG provided no evidence of molecule-related ions, in both positive and negative ion modes. and instead, intense ion peaks due to Cl− at m/z 35 and 37 and weak peaks due to Cl2− at m/z 70, 72, and 74 (Figure S-2r) were observed. The LOD values by monitoring Cl− (m/z 35) in MS1 and Cl2− in MS2 (m/z 70 → 35) were also considerably below the TWA.59 As seen in Figure S-2s, PS provided peaks at m/z 128, 130, and 132 ascribed to [M − Cl]− and an intense fragment ion peak at m/z 46 ascribed to NO2− in the MS1 spectrum. MS2 for [M − Cl]− revealed a transition to NO2− (m/z 46, base peak ion) and Cl2−. The LOD value by monitoring the ion of the [M − Cl]− → NO2− transition at m/ z 128 was lower than the TWA.55,71 Interference by Gasoline Vapor. The potential matrix effect of gasoline vapor on the detection of GB was examined. Gasoline vapor was prepared in the heated 10 L container by injecting liquid gasoline (Esso Regular, Exxon Mobile, Irving, TX, purchased at a local gas station) with and without GB vapor and introduced into the instrument. MS3 transitions for the MH+ (m/z 141) and [M − C3H5]+ (m/z 99) ions were monitored. The signal intensity for GB (3 μg/m3) fell to the background level when the concentration of gasoline vapor was increased to 60 mg/m3. This was probably caused mainly by a space charge effect;72 hence, the ion-loading time was reduced by 1/10 to 30 ms. Figure S-3 (Supporting Information) shows the monitored signal intensity for MH+. Gasoline vapor of 6, 60, and 220 mg/m3 concentration without GB vapor gave no observable signal for the MS3 transition of MH+. Meanwhile, the signal for the MS3 transition of [M − C3H5]+ also showed no observable increase. The signal intensity for GB at 3 μg/m3 increased and then decreased as the gasoline vapor concentration increased from 0 to 220 mg/m3. The decrease of the signal intensity was possibly caused by the abovementioned space charge effect in addition to a competing protonation by the gasoline vapor. The reason for the increase of the signal intensity is not clear, and further investigations are needed. This may be a matrix effect of n-hexane, the solvent used in preparation of the standard solutions. The n-hexane vapor also gave an increase in the signal intensity for GB (3 μg/ m3), but the signal increase was estimated as being equal to only several percent at the n-hexane vapor concentration where the LOD was evaluated, implying overestimation of the LODs may be insignificant. The possibility of false-positive interference by gasoline vapor on all of the agents tested was checked. Liquid gasoline was directly infused into the heated inlet of the instrument using the syringe pump at a rate of 0.01, 0.1, or 1 mL/min to generate a gasoline vapor concentration of 6.5, 65, or 650 mg/m3, respectively. The gasoline infusion at the specified flow rates was performed over 10 min, during which time simultaneous measurements for a set of four to six agents were carried out. When the average signal intensity exceeded 3 times the standard deviation of the background signals, it was deemed that a false-positive signal had occurred. The results are listed in Table 1. The gasoline vapors gave no false signals for all the CWAs investigated, including the precursor agent DF, except for GA, where gasoline vapor had a false-positive effect in MS2 4323



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(MH+ → m/z 135), but the signal response in MS3 (MH+ → m/z 135 → m/z 126) was unaffected. The potential matrix effect of gasoline vapor on the detection of the selected CWAs (GD, GA, VX, HD, HN1) was also studied. The instrument inlet was split in two, and liquid gasoline was infused into one of the inlets while a standard solution of HD (100 ppm) was infused into the other inlet at a rate of 0.1 μL/min. The generated HD vapor concentration was 11 μg/m3. In a separate experiment, a standard solution containing 5 ppm GA, 250 ppm GD, 60 ppm VX, and 40 ppm HN1, which resulted in vapor concentrations of 0.46, 21, 5.0, and 3.7 μg/m3, respectively, was similarly infused. Figure S-4 (Supporting Information) shows the dependence of the signal intensity of the agents on the gasoline vapor concentration. For GD, similarly to GB, the signal increased to about the 120% level and then decreased as the gasoline vapor concentration was increased from 0 to 650 mg/m3, but the signal did not fall below the level for zero gasoline concentration. For GA, VX, HD, and HN1, the signals gradually decreased to about 80%, 60%, 60%, and 10% levels, respectively, as the gasoline vapor concentrations were increased. These results indicated that signal suppression effects by gasoline vapor on the CWA detection should be considered if the present methodology is used in field applications Effect of Moisture on CWA Detection. As in a previous study,47 the effect of humidity on the detection of typical CWAs at specified concentrations was examined (Figure S-5, Table S-1, Supporting Information). The experimental procedure was the same as that reported previously.47 The MS2 transition for MH+ to the base fragment ions (GB, GD, GF, GA, VX, HD, HN1, CN) and negative ion of m/z 185 → 123 (L1) were monitored. As the humidity increased from zero to several tens of grams per cubic meter, the signal intensities for GB and GA first decreased to about 60% and 70% levels and then increased. The signal intensities for GD, GF, and L1 gradually increased. That for VX was almost nonvariant. That for HN1 increased at first and then decreased. That for HD gradually decreased to 30%. The humidity effect was significant for CN. The signal intensity for CN decreased to 1% of that at 10 g/m3, where the LOD was evaluated, at 30 g/m3. For the other agents, the lowest signal intensities were in the range of 25−90% of that at 10 g/m3. Although the LOD values were well below that for 1% of the 1/100 LC50 values for all agents tested, values for some CWAs could become greater than the respective STEL or TWA values, depending on the humidity level. Performance Comparison with the Other Detection Technologies. The detection sensitivities by our method were nearly 1/1000000 LC50 or lower levels for volatile CWAs except for VX and RVX (1/10000 LC50 levels). These were superior to those by the conventional hand-held ion mobility spectrometry instruments, gas detection tubes, and acoustic wave detection instruments (nearly 1/100 LC50 or higher levels),20−23 similar to those by a back-packing GC/quadruple MS instrument,26 and poorer than those by a portable GC instrument with an element-specific detector.24 It should be noted that the performance by our instrument was achieved in real-time measurement, whereas GC-based instruments need time-consuming preconcentration and GC separation steps. In addition, GC-based methods have poor sensitivity for gaseous and nonvolatile CWAs because of poor preconcentration efficiency and strong adsorption onto the GC column, respectively. The detection sensitivities for gaseous and

nonvolatile CWAs by our method were lower than hygienic regulation levels (STEL/TWA).

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CONCLUSION Highly sensitive and specific real-time detection of 5 gaseous, 13 volatile, and 6 nonvolatile CWAs and a CWA precursor in air was achieved using a novel CFI-APCI-ITMS instrument. MSn (n ≤ 3) for a characteristic ion of a CWA, generated by soft ionization, was highly effective in terms of yielding high specificity and low LODs. The LODs in real time (1 s) fulfilled the requirements for rapid detection of lethal concentrations of all the agents for counterterrorism purposes. The LOD values were lower than the STEL and TWA values for most of the agents in real time or after signal averaging for short periods of time. Although the experiments were conducted in the laboratory, the instrument could be used in the field. This is the first demonstration of an on-site mass spectrometer capable of measuring in real time a wide range of CWAs in air, fulfilling requirements for both counterterrorism and health and safety management purposes. High specificity of this method was demonstrated by a relative absence of false positives caused by high concentrations of gasoline vapor. Conversely, signal suppressions of between 10% and 80% were observed for some CWAs in the presence of gasoline vapor, indicating the possible need for refinements in methodology (e.g., matrix matching) for reliable detection and precise quantification in certain practical situations.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +81-4-7135-8001. Fax: +81-4-7133-9173. E-mail: [email protected]. *Phone: +81-42-323-1111. Fax: +81-42-327-7807. E-mail: [email protected]. Present Address ⊥

Hiroshi Sekiguchi, Hisashi Maruko, Shigeharu Yamashiro, Yasuhiro Sano, Yasuo Takayama, Ryoji Sekioka, Shintaro Yamaguchi, Shintaro Kishi, Takafumi Satoh, Hiroyuki Sekiguchi, and Kazumitsu Iura: Tokyo Metropolitan Police Department, Chiyoda, Tokyo 100-8929, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed, in part, under the Effective Promotion of Joint Research with Industries, Academia, and Government research program sponsored by the Special Coordination Funds for Promoting Science and Technology and supported by the Ministry of Education, Culture, Sports, Science and Technology.

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REFERENCES

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