Direct and Sensitive Detection of CWA Simulants by Active Capillary Plasma Ionization Coupled to a Handheld Ion Trap Mass Spectrometer

Research Article

Abstract

An active capillary plasma ionization (ACI) source was coupled to a handheld mass spectrometer (Mini 10.5; Aston Labs, West Lafayette, IN, USA) and applied to the direct gas-phase detection and quantification of chemical warfare agent (CWA) related chemicals. Complementing the discontinuous atmospheric pressure interface (DAPI) of the Mini 10.5 mass spectrometer with an additional membrane pump, a quasi-continuous sample introduction through the ACI source was achieved. Nerve agent simulants (three dialkyl alkylphosphonates, a dialkyl phosporamidate, and the pesticide dichlorvos) were detected at low gas-phase concentrations with limits of detection ranging from 1.0 μg/m3 to 6.3 μg/m3. Our results demonstrate a sensitivity enhancement for portable MS-instrumentation by using an ACI source, enabling direct, quantitative measurements of volatile organic compounds. Due to its high sensitivity, selectivity, low power consumption (<80 W) and weight (<13 kg), this instrumentation has the potential for direct on-site CWA detection as required by military or civil protection.

Graphical Abstract

Keywords

Dielectric barrier discharge ionization Handheld mass spectrometer Chemical warfare agent simulants 

Introduction

The detection and identification of chemical warfare agents (CWAs) is essential for military, counterterrorism, and civil protection purposes. Consequently, much effort has been spent on different detection technologies and methods [1]. One central aspect of CWA detection research is the development of handheld and therefore field deployable detection systems. Currently, ion mobility spectrometry (IMS) is the most common technology in this context [2]. Until now, IMS holds most promise for fast screenings and is thus ideal for early warning systems. However, IMS has a limited selectivity, hindering unambiguous CWA identification. In contrast, mass spectrometry is the gold standard off-site tool for accurate and robust CWA analysis because of its sensitivity, specificity, and coupling capabilities to orthogonal methods [3]. Up to now, on-site sampling and subsequent off-site analysis in laboratories with a completely monitored chain of custody is required to prove use of an alleged CWA [4].

Latest trends in instrument development focus on the development of miniaturized and portable MS systems, making them available for possible on-site use [5, 6, 7]. Although, there have been some reports on portable/handheld MS instruments for CWA detection, [8, 9] all these systems suffer from a reduced selectivity and greatly decreased sensitivity compared with laboratory equipment. Some of these instruments feature MS2 operation modes, which can compensate the lower resolution. Rather than improving miniature MS instrumentation, we focus on increasing the sensitivity by utilizing more efficient ionization sources. As such, active capillary plasma ionization (ACI) has already proven its exemplary ionization efficiency enabling direct CWA detection in the gas phase, at ppt levels, on a benchtop MS system [10]. This ionization source features a unique geometry acting as an extension of the inlet capillary of the MS, comparable to a recently published LTP version of Spencer et al [11]. By igniting a dielectrically hindered discharge in this inlet capillary, the flow of analytes is ionized in a soft APCI/DART like mechanism. Since this happens at the MS interface, ion transfer into the system and thus sensitivity is greatly enhanced. In another report, a coupling of the ACI source with a portable MS (Mini 10.5) was presented, here the qualitative detection of only a few CWA related chemicals on forensic matrices was shown. [12].

In this study, we present an advanced combination of the ACI source with the discontinuous atmospheric pressure inlet (DAPI) of the Mini 10.5 instrument with drastically improved sensitivity. The advanced coupling enables continuous, direct gas-phase sampling as well as quantification. Due to reduced toxicity and legal restrictions, nerve agent simulants were used in this study instead of intact CWA. However as mentioned above, the capability of ACI to ionize intact warfare agents was already shown [13]. Here, we focus on the increase in sensitivity of portable MS instrumentation by application of the ACI. As a result, we present the up-to-now most sensitive MS2 based direct detection of CWA-related compounds by a handheld instrument.

Experimental

Chemicals and Solvents

Dimethyl methylphosphonate (DMMP) (97%), diethyl ethylphosphonate (DEEP) (>98%), dichlorvos (DCV) (analytical standard), and diethyl phosphoramidate (DEPA) (98%) were obtained from Sigma-Aldrich Chemie GmbH (Buchs, Switzerland). Diisopropyl methylphosphonate (DIMP) (95%) was purchased at Alfa Aesar GmbH & Co. KG (Karlsruhe, Germany). All chemicals were used without further purification.

DMMP, DEEP, DIMP and DEPA are commonly used to simulate G-series nerve agents because of their structural similarities. The pesticide DCV is structurally related to some suspected “new” nerve agent structures (see Supporting Information STable1 for detailed structures and vapor pressures) [10].

For each substance, individual stock solutions (1000 μg/mL) were prepared in pure HPLC-grade methanol (≥99.9%) purchased at Merck KGaA (Darmstadt, Germany). Depending on the experiments, various methanolic dilutions with different concentrations were generated from these stock solutions.

MS Instrumentation

A Mini 10.5 miniaturized mass spectrometer (Aston Labs, Purdue University, West Lafayette, IN, USA) was used in this study. The mass spectrometer has an operational power consumption <75 W and weighs less than 12 kg with a size of 33 × 21.5 × 21 cm. Due to the absence of an external gas supply, the system utilizes air as collision gas for MS2 experiments. For MS2, a precursor ion isolation is performed first by applying a modulated rf to the trapped ions. Then the isolated species are fragmented by excitation and collision with remaining air molecules. However, this process is not fully comparable to a MS2 experiment on a e.g., bench-top instrument with external gas supply. Further information on the instrument and its operation principle and performance is available elsewhere [14]. A photograph of the instrument in combination with the ACI source is shown in Figure 1. All MS and MS2 spectra were recorded in positive ion mode.
Figure 1

Left side shows a schematic of the experimental setup including the sample preparation device, the ionization interface, and the Mini 10.5 MS system (gas flow paths are represented by the red arrows). Right side shows front and side views of the instrumental setup, labeled according to the schematic

Ionization Interface

The discontinuous atmospheric pressure interface (DAPI) of the Mini 10.5 MS was complemented with a micro diaphragm vacuum pump 1410 V/2.2/E/LC (Gardner Denver Thomas GmbH, Fürstenfeldbruck, Germany). The pump was powered by the 24 V power supply of the instrument and connected to the DAPI inlet by a T-piece. On the other side of the T-piece the ACI source was attached and fixed to the Mini casing. A constant flow of 1 L/min was adjusted by a needle valve between the T-piece and the pump, and controlled by a digital flow switch (PFM 7; SMC Corp. Noblesville, IN, US). Although the general interface continuously samples and ionizes 1 L/min, only a small fraction of these ions are introduced orthogonally into the MS through the DAPI line (sampling for 10–20 ms at 1–0.5 Hz). The DAPI line consists of a pinch valve (ASCO Scientific, Florham Park, NJ, USA), which periodically opens a rubber tube (i.d.: 1 mm, o.d.: 3 mm, length: 20 mm) connected to a stainless steel capillary (i.d.: 0.25 mm, o.d.: 1.59 mm, length: 33 mm) serving as flow limiter between ionization system and vacuum manifold of the mass spectrometer.

Active Capillary Plasma Ionization Source

The general concept of the ACI source has been described in detail elsewhere [10, 15]. The general geometry is similar to that used by Wolf et al. [10], using a quartz capillary (i.d.: 0.7 mm, o.d.: 1.0 mm) as dielectric barrier. The inner electrode consists of a stainless steel capillary (i.d.: 0.5 mm, o.d.: 0.6 mm) embedded in a PEEK casing. Minor modifications were made to the outer copper electrode (i.d.: 1.0 mm), surrounding the quartz capillary, which was mounted on a movable PMMA slider, moving alongside three stainless steel bolts where one contained a thread for precise positioning. This enabled optimization of the electrode position during operation. For plasma ignition, a sine modulated (5.75 kHz) high voltage (1.6 kV) was applied to the electrodes. By drawing the sample through the capillary and, therefore, the plasma, the gas-phase and the analytes are ionized by an APCI like mechanism. According to previous findings [10], this ionization is softer than APCI due to the current limiting effect of the dielectric barrier, yielding mainly protonated molecular species MH+ in positive mode. A schematic of the actual source design is available as SFigure 1 in Supporting Information.

Sample Preparation

Defined gas-phase sample concentrations were generated by means of a heat- and pressure-assisted nanospray setup. The setup is similar to the one used in previous studies [10]. Briefly, a sample solution reservoir was connected to the inside of a hollow heating cartridge by a fused silica capillary (i.d.: 40 μm, o.d.: 0.36 mm, length: 214 mm). By applying a defined excess pressure (800 mbar) to the sample reservoir, a constant sample delivery is achieved. Assuming a laminar flow through the capillary, the amount of analyte introduced into the system and evaporated by a preheated air stream (180 °C, 3 L/min) inside the cartridge can be calculated by Poiseuille’s law. The parameters temperature, carrier gas flow, fused silica capillary length and diameter, and overpressure on the reservoir were held constant for all experiments.

Identification and Calibration

In order to achieve a sufficient degree of selectivity for on-site applications, MS2 experiments were performed for all calibrations on the Mini 10.5 instrument. Quantification was performed by measuring an external (five to six point) calibration for each analyte. For every calibration point, a 5-min sampling period was performed in triplicate, resulting in 3 × 150 single scans (scan time: 1.5–2 s). All recorded spectra were averaged and the signal intensity and its corresponding standard deviation (error) was used for calibration. Spectra and chronograms in this report were processed with Griffin System Software (Griffin Analytical Technologies Inc., West Lafayette, IN, USA) [14] and plotted with R (The R Foundation for Statistical Computing, Vienna, Austria). Calculations and automatic data treatment were performed with a customized R-script [16]. Refer to Supplementary Information for details on the calculations and automation procedure.

Results and Discussion

Active capillary plasma ionization has already been used to directly measure and quantify intact CWAs and corresponding simulants on benchtop instrumentation. In terms of coupling to handheld MS instrumentation, solely qualitative MS1 results have been reported in a proof of concept study [10, 12]. In this report, we present an optimized coupling for a handheld mass spectrometer (Mini 10.5), featuring a discontinuous sampling and ionization interface, as well as direct and quantitative MS2 based detection of CWA simulants. Our results therefore enable a more sophisticated evaluation and show the possibilities of actual on-site usability of current handheld MS-instrumentation for CWA detection and identification.

MS and MS2 Spectra

Figure 2 shows the MS and MS2 (insert) spectra of DEPA (85 μg/m3), averaged over 1 min of measurement time. For the MS spectrum the molecular ion MH+ at m/z 154 as well as the dimer 2MH+ at m/z 307 are clearly visible. In contrast to previous findings, a significant amount of in-source fragmentation is present. Here, m/z 98 and 126 represent the main fragments of DEPA, corresponding to the loss of one MH+ – C2H4 and both ethyl chains MH+ – C4H8. Due to the absence of an efficient declustering regime in the DAPI interface, some water adducts are present in the MS spectrum, e.g., m/z 116 corresponding to MH+ – C4H8 + H2O. Further signals at m/z 112 and 130 may be explained by an in-source oxidation product of DEPA MOH+ – C4H10 and the corresponding water adducts MOH+ – C4H10 + H2O. The oxidation as well as the extensive fragmentation may occur because of the long transfer and analysis time (1.5–2 s) of the system at relatively high pressure (>10–3 mbar) by collisions and reactions with reactive oxygen species, e.g., ozone formed within the plasma. This assignment remains speculative because of the limited mass accuracy (roughly ±1 m/z) of the system. Additionally, such reaction products have neither been reported nor observed in the spectra of previous studies.
Figure 2

Averaged (30 scans) MS and MS2 (insert, parent ion 154 m/z, isolation width 1 m/z) spectra of DEPA (85 μg/m3), recorded on the ACI-Mini 10.5 system

With such rich mass spectra, it may be possible to directly identify CWAs in MS mode without the need to run MS2 experiments. However, heavy matrix loads and suppression effects may still demand a more robust MS2 identification. For MS2 spectra (Figure 2, insert) we observed the typical DEPA fragments at 98 and 126 m/z by selecting 154 m/z as precursor ion. This spectrum is in agreement with other MS2 spectra recorded with an analogues system but on a benchtop LCQ mass spectrometer [10].

Discontinuous Sampling Interface

Due to the operation principle of the ACI-source, a constant sampling flow is preferred and enhances the reproducibility. In order to combine the source with the DAPI interface of the Mini 10.5 MS, a micro diaphragm pump was installed and connected by a T-piece, ensuring a constant flow through the source. As a result, the ACI source reached high efficiency and reproducibility compared with the previous coupling to a handheld mass analyzer [12]. Figure 3 depicts a typical signal, recorded in MS2 mode for three replicate injections of two DEPA concentrations (24 μg/m3 and 81 μg/m3). The duration of each injection and blank period was 5 min. The dataset was recorded at 0.5 Hz per scan and averaging three scans per datapoint, thus each timepoint reflects 6 s of measurement. As shown, the signal still suffers from fluctuations and slow rise and fall times, especially for low concentrations. The significant signal fluctuation in the untreated signal (black) may not be due to inconsistent ionization because of the high frequency of the discharge (6 kHz), but is more likely caused by the oscillation of the DAPI, which has a mechanically limited accuracy because of the pinch valve and rubber tube opening matching the frequency of the observed fluctuations. To enable automatic signal processing and peak picking, the signal was smoothed over 10 spectra (red curve). After automatic peak selection, the untreated raw data was used for calibration and LOD calculations. According to the MS2 signal shown in Figure 2, a near real-time (<10 s) detection of CWA would be possible with this system. However, quantification at low μg/m3 levels would require averaging over 1–5 min because of the relatively slow rise and fall times (around 30 s) of the signal. As reasons, the lack of heating in the ion source and in the transfer (DAPI) system as well as possible adsorption in the rubber tube of the pinch valve may be noted. In consequence, sufficient protection gear would be required to protect the infield user during on-site investigation and quantification.
Figure 3

Chronogram of three replicate injections of two different DEPA concentrations, with untreated MS signal (black) and smoothed signal (red)

Quantification

To elucidate and compare the performance of this portable system, a MS2 based calibration was performed for five CWA-related chemicals mimicking nerve agents. Therefore, a dilution series of each substance was measured. The results of these experiments are summarized in Table 1 and the according calibration curves and data are included in Supplementary Material. Using the definition of IUPAC [17], the corresponding LODs were calculated and range from 1 to 6.3 μg/m3. Looking at the LCt 50 and the minimum effect dose of e.g., the CWA sarin (GB), which is 35,000 μg/min·m3 and 500 μg/min·m3, respectively [18], the presented detection limits and measurement times would be sufficient for first responders to put on their protective gear. However, the dilution series covered more than 3 orders of magnitude but the linear dynamic range was found to be limited to roughly 1 order of magnitude for most substances. Only the response for DEPA was found to be linear over 2 orders of magnitude. In consequence, the quantification would be limited to a narrow concentration range below the minimum effect dose of most CWA compounds. There may be several reasons for this limitation, however, in former studies with the same ion source, evaporation system, and chemicals the linear dynamic ranges were found to be >3 orders of magnitude [10]. Also, Keil et al. [19] reported a reduced linear dynamic range for the Mini 10 using internal electron impact ionization. Therefore, this limitation may be attributed to the Mini 10.5 MS instrument. An explanation could be a limited size of the ion trap resulting in space charge effects and faster overfilling or the lack of further ion guiding systems.
Table 1

Calibration Parameters and Results for the Investigated Compounds

Compound

Number of points

MS2 transition

Calibration range [μg/m3]

R2

Limit of detection [μg/m3]

DEEP

6

167 -> 139

2.59-25.91

0.9530

1.0

DMMP

5

125 -> 111

0.86-12.95

0.9271

2.4

DCV

5

221 -> 127

8.11-81.05

0.9297

3.1

DIMP

6

181 -> 139

2.59-25-91

0.8540

5.5

DEPA

6

154 -> 126

2.43-202.63

0.9921

6.3

Comparison with other portable systems

As mentioned previously, there have been some studies on handheld CWA detection systems or other related applications. Some of these studies also featured the Aston Labs Mini series mass spectrometers as detectors. Smith et al. [8] investigated CWA simulants, namely DMMP, 2-chloroethyl sulfide, diethyl malonate, and methyl salicylate, on a portable but bigger mass spectrometer (weight: 25 kg, size: 44.45 × 48.26 × 26.35 cm), additionally equipped with a solid phase enrichment system in front of the MS inlet. Using internal electron impact ionization, they achieved LODs of 0.26 to 5.0 ppb. This equates to approximately 0.3 μg/m3 respectively 26 μg/m3. A similar study on the detection of toxic compounds in air (e.g., acrolein, formaldehyde, phosgene, etc.), using a Mini 10 mass spectrometer, was published by Keil et al. [19]. Here, LODs from 0.8 ppbv to 3 ppmv corresponding to approximately 10 μg/m3 to 4000 μg/m3 were achieved by preconcentration. Rapidly heatable sorbent tubes in front of the mass spectrometer inlet were used to enhance sensitivity. In terms of linear dynamic range, they reported 1 to 2 orders of magnitude, which is comparable to our results, further proving this limitation being attributed to the Mini instrument. In 2000, Syage et al. [20] presented a “man-portable” photoionization time of flight mass spectrometer for the detection of chemical warfare agents.

On their portable system, they investigated DMMP and DIMP as simulants and got LODs for direct MS detection of 25 and 28 ppb, respectively. Assuming units of ppbv, this converts to approximately 300 μg/m3. In 2008, a palm portable MS system with dimensions of 8.2 × 7.7 × 24.5 cm and a weight of 1.5 kg was presented by Yang et al. [5] A detection limit of 22 ppm (~280 mg/m3) for DMMP and 6 ppm for toluene was reported. Comparing our LODs of 1 to 6 μg/m3 with the studies mentioned above, taking into account the absence of any enrichment step, we can state that the combination of the active capillary plasma ionization source increased the sensitivity of the portable MS (Mini 10.5) by at least 1 order of magnitude. Furthermore, all our quantifications were performed in MS2 mode enabling discrimination from matrix and background signals encountered in real-world conditions. Since there have been recent advances in portable MS systems (e.g., Mini 11 and Mini 12), even better results could be expected by combining the ACI-source with state of the art [21] equipment.

Conclusion

A combination of active capillary plasma ionization with a portable mass spectrometer (Mini 10.5) is presented for the sensitive detection and quantification of chemical warfare agent related chemicals. The general power consumption of the instrument, including the ionization device, is <80 W (75 W Mini 10.5 MS + 5 W ionization device), and the total weight is <13 kg. In comparison with previous studies, our results show a significant increase in sensitivity with limits of detection ranging from 1 to 6 μg/m3. Therefore, it can be stated that active capillary plasma ionization improves the sensitivity of existing (portable-) MS systems. Additionally, all quantifications were performed in MS2 mode, enhancing selectivity and handling of strong matrix interferences. This instrumentation opens new possibilities in military, civil protection, and industrial hygiene levels monitoring, since the combination of ACI with portable MS instrumentation starts to approach the performance of standard lab based MS systems, especially when considering its interfacing with state of the art equipment.

Notes

Acknowledgment

The authors thank Christoph Bärtschi and Christian Marro of the ETH Workshop for manufacturing the active capillary plasma source and various other relevant parts. This work was funded by the Federal Office for Civil Protection FOCP, SPIEZ LABORATORY (grant 353004332/Stm) that is gratefully acknowledged.

Supplementary material

13361_2016_1374_MOESM1_ESM.docx (1.3 mb)
ESM 1 (DOCX 1281 kb)

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Copyright information

© American Society for Mass Spectrometry 2016

Authors and Affiliations

  1. 1.Department of Chemistry and Applied BioscienceETH ZurichZurichSwitzerland
  2. 2.Federal Office for Civil Protection FOCP, Spiez Laboratory, Analytical Chemistry BranchSpiezSwitzerland

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