Application of a High Surface Area Solid-Phase Microextraction Air

Aug 1, 2013 - After allowing 3 days for system equilibration, the 10 cm length DIMP ... Lower concentrations of DIMP (3.5, 1.0, and 0.75 ppbv) were ...
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Application of a High Surface Area Solid-Phase Microextraction Air Sampling Device: Collection and Analysis of Chemical Warfare Agent Surrogate and Degradation Compounds Michael E. Stevens, Jr.,†,∇ Christopher A. Tipple,‡ Philip A. Smith,§,○ David S. Cho,∥ Robert V. Mustacich,⊥ and Brian A. Eckenrode*,‡ †

Visiting Scientist Program, Oak Ridge Institute for Science and Education, Counterterrorism and Forensic Science Research Unit, Federal Bureau of Investigation Laboratory, Quantico, Virginia 22135, United States ‡ Counterterrorism and Forensic Science Research Unit, Federal Bureau of Investigation Laboratory, Quantico, Virginia 22135, United States § Department of Preventive Medicine and Biometrics, Uniformed Services University of the Health Sciences, Bethesda, Maryland, United States ∥ Oak Ridge Institute for Science and Education, Counterterrorism and Forensic Science Research Unit, Federal Bureau of Investigation Laboratory, Quantico, Virginia 22135, United States ⊥ Agilent Technologies, Incorporated, Santa Clara, California 95051, United States S Supporting Information *

ABSTRACT: This work examines a recently improved, dynamic air sampling technique, high surface area solid-phase microextraction (HSA-SPME), developed for time-critical, high-volume sampling and analysis scenarios. The previously reported HSA-SPME sampling device, which provides 10-fold greater surface area compared to commercially available SPME fibers, allowed for an increased analyte uptake per unit time relative to exhaustive sampling through a standard sorbent tube. This sampling device has been improved with the addition of a type-K thermocouple and a custom heater control circuit for direct heating, providing precise (relative standard deviation ∼1%) temperature control of the desorption process for trapped analytes. Power requirements for the HSA-SPME desorption process were 30-fold lower than those for conventional sorbent-bed-based desorption devices, an important quality for a device that could be used for field analysis. Comparisons of the HSA-SPME device when using fixed sampling times for the chemical warfare agent (CWA) surrogate compound, diisopropyl methylphosphonate (DIMP), demonstrated that the HSASPME device yielded a greater chromatographic response (up to 50%) relative to a sorbent-bed method. Another HSA-SPME air sampling approach, in which two devices are joined in tandem, was also evaluated for very rapid, low-level, and representative analysis when using discrete sampling times for the compounds of interest. The results indicated that subparts per billion by volume concentration levels of DIMP were detectable with short sampling times (∼15 s). Finally, the tandem HSA-SPME device was employed for the headspace sampling of a CWA degradation compound, 2-(diisopropylaminoethyl) ethyl sulfide, present on cloth material, which demonstrated the capability to detect trace amounts of a CWA degradation product that is estimated to be less volatile than sarin. The rapid and highly sensitive detection features of this device may be beneficial in decision making for law enforcement, military, and civilian emergency organizations and responders, providing critical information in a contaminated environment scenario when time is of the essence.

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based methods used by emergency responders are not likely to provide the necessary specificity and sensitivity required due to interferences by other chemicals in a sample mixture.2−5 Traditional industrial hygiene air sampling methods can

ilitary and emergency response teams tasked to handle accidental or intentional releases of toxic chemicals would benefit from improvements in the speed and sensitivity of field collection techniques for chemical compounds. These response teams often rely on chemical collection and analysis methods which are not sufficiently effective to meet known requirements. Colorimetric (e.g., length of stain tubes) methods can lack the necessary sensitivity to provide a margin of safety for human health.1 Ion mobility spectrometry (IMS)© XXXX American Chemical Society

Received: April 21, 2013 Accepted: August 1, 2013

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thermocouple and a computer-controlled circuit board for direct heating. The performance of the new temperaturefeedback control was then evaluated under various conditions. The second purpose was to demonstrate the use of HSA-SPME air sampling to collect trace-level chemical warfare agent (CWA) surrogate and degradation compounds with brief sampling times. The analytical results of the HSA-SPME methods were compared with those obtained using commercial sorbent sampling with thermal desorption over a range of different fixed air sample volumes and different sampling times. Additionally, characterization of the mass uptake by use of a tandem HSA-SPME approach, in which two HSA-SPME devices are joined in tandem to double the sampling surface area, was investigated at low airborne analyte concentrations (parts per billion by volume to parts per trillion by volume). Finally, the analytical results following the rapid headspace sampling of cloth material contaminated with a small amount (50 ng) of a semivolatile liquid CWA degradation compound are reported for the tandem HSA-SPME approach.

provide more precise quantification, but sample collection can take several hours and solvent extraction is typically used.6 A sampling device capable of capturing sufficient analyte mass for trace-level detection (parts per billion by volume to parts per trillion by volume) in only seconds would represent an improvement over current methods. A high-speed, highly sensitive sampling device could also be useful for forensic investigation and covert collections. Forensic response units have historically depended on canines to detect scent (or volatiles) profiles to identify the location of buried human remains, assist in focusing an investigation for the capture of a criminal suspect, or detect unique chemical signatures emanating from illicit drugs or explosives.7,8 However, challenges to canine evidence regarding court admissibility have arisen due to the lack of national certification standards and insufficient scientific understanding regarding how trace human scent is detected by canines.9 The limited number of properly trained canines and associated costs of training and maintenance have further driven the need to look for alternative or complementary chemical detection methods. Additionally, in the event of a hazardous chemical release, the use of canines would not be appropriate for identifying chemical signatures in the affected area(s) due to the obvious health hazards posed to the animals. Inspired by the canine sampling model, development of a high-volume sampling technique for short periods using a high surface area device would enable sufficient chemical mass loading for method sensitivity by increasing mass uptake per unit time. Such a sampling method would allow for a greater level of safety involving hazardous substances, as responders would spend less time in a contaminated environment. Additionally, this approach would allow more efficient use of resources and would enable quick and discrete detection of compounds of interest in a potentially hazardous environment, benefitting law enforcement, military, and intelligence agencies. A recently introduced high surface area solid-phase microextraction (HSA-SPME) air sampler offers the potential to provide highly sensitive and rapid sampling capabilities desired by hazardous material response, law enforcement, and military personnel. As previously described,10,11 the HSA-SPME device consists of a thin wire coated with carboxen/poly(dimethylsiloxane) (carboxen/PDMS) wound in the annular space between two concentric glass tubes. This device has a surface area 10 times greater than a commercially available 1 cm carboxen/PDMS fiber, providing a large trapping area for analytes at relatively high sample flow rates (up to 4 L/min) in a matter of seconds. In the initial study by Ramsey et al.,10,11 trapped volatile organic compounds (VOCs) (e.g., BTEX) were desorbed and reconcentrated on a microtrap and further analyzed by a low thermal mass gas chromatograph employing rapid temperature ramping with a mass spectrometric detector (LTM-GC/MS). Use of this method resulted in an improvement in sensitivity of several orders of magnitude relative to dynamic flow SPME with a commercially available 1 cm carboxen/PDMS fiber. The current research had a dual focus. The first was to improve and evaluate the temperature control of the HSASPME device. As mentioned in the previous work,10,11 heating the HSA-SPME prototype was based upon the amount of time that power was supplied to the device. Therefore, the temperatures in the prototype were only approximations, and this resulted in decreased precision between and within device sets. This issue has been resolved by the addition of a type-K

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EXPERIMENTAL SECTION Preparation of Diisopropyl Methylphosphonate Air Samples. Air samples of a CWA surrogate compound, diisopropyl methylphosphonate (DIMP),12,13 were generated as follows: National Institute of Standards and Technology (NIST)-traceable permeate flow of DIMP was generated by placing a permeation tube containing the chemical (HRT 01000-5016/100 °C, Kin-Tek, La Marque, TX) into a small, temperature-programmable oven unit contained within a precision gas standards generator (491M-B, Kin-Tek, La Marque, TX). After allowing 3 days for system equilibration, the 10 cm length DIMP permeation tube produced a total permeation of 490 ng/min at 100 °C. The diluent gas flow was adjusted to 5 L/min to provide a concentration of 0.098 ng/mL or 13 ppbv concentration. A 10 L Tedlar bag (DuPont, Wilmington, DE) was attached to the gas standards generator span outlet to collect the effluent. Lower concentrations of DIMP (3.5, 1.0, and 0.75 ppbv) were generated by appropriate dilution of the 13 ppbv sample into dry air in separate 10 L Tedlar bags, using a 2 L airtight syringe (Hamilton Co., Reno, NV). Preparation of the Headspace Sample of Liquid CWA Degradation Product. A degradation product of O-ethyl S(2-diisopropylaminoethyl) methylphosphonothiolate (VX), 2(diisopropylaminoethyl) ethyl sulfide (2-DES),14,15 was synthesized following methods described by Smith et al.16 and diluted in methylene chloride to produce a 25 ng/μL concentration. A 50 ng quantity of this analyte was then volumetrically delivered onto a 1 square inch section of cotton T-shirt material, which was contained in a custom-made glass sampling jar with an internal volume of 250 cm3 (Kontes CGS, Vineland, NJ). The jar was sealed with a PTFE cap insert and screw cap and was equipped with two threaded ports on the screw cap that could be loosened or removed to facilitate sampling. HSA-SPME Sampling Device. The directly heated HSASPME device used in this work was adapted and modified from the previous design.10,11 It consisted of an HSA-SPME element, glass tubes, type-K thermocouple, and electrical wires. The HSA-SPME element was a 10 cm Stablohm 675 nickel alloy wire (60% Ni, 24% Fe, and 16% Cr; California Fine Wire Company, Grover Beach, CA) coated with a 65 μm thick Carboxen/PDMS SPME stationary phase (Supelco, Bellefonte, B

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Figure 1. (a) High surface area solid-phase microextraction (HSA-SPME) device (78.5 mm in length) used in this study. The insulated cable contains the electrical leads necessary for resistive heating and a type-K thermocouple for temperature feedback. (b) Close-up view shows both electrical leads and thermocouple centered within the inner glass tube.

Figure 2. Diagram of the three sampling methods used in this study: (a) nonexhaustive single HSA-SPME sampling, (b) nonexhaustive tandem (in series) HSA-SPME sampling, and (c) direct exhaustive sampling into a sorbent tube within the ACEM 9305 thermal desorption system.

surface area was found by Ramsey et al.11 to achieve significantly higher mass uptake per unit time relative to SPME and previously used dynamic SPME methods. Sampling Methods and Conditions. Three air sampling methods were used in this work (Figure 2). The first method used nonexhaustive HSA-SPME sampling at 4 L/min from a Tedlar bag containing a known concentration of DIMP. The second method, tandem HSA-SPME, was the same as the first method except two HSA-SPME devices were used in series to provide double the surface area relative to the single HSASPME device method. The third method employed direct exhaustive sampling (at a 0.585 L/min flow rate) of DIMP from a Tedlar bag into a thermal desorption tube within the Dynatherm automated chemical environment monitor (ACEM, model 9305, CDS Analytical, Inc., Oxford, PA). This tube was then thermally desorbed into the ACEM focusing trap, and this approach represented a conventional sorbent-based method for comparative purposes. In all three methods, analyte was eventually introduced into the thermal desorption tube within the ACEM system containing 200 mg of Tenax TA sorbent material using active sampling. The use of the ACEM system as

PA). The oxidation-resistant nickel alloy wire was coiled in a loose spiral (4.4 mm between coils) around the outer surface of a borosilicate glass tube which was contained within an outer glass tube. The new component added to the current design was the type-K thermocouple used for temperature-feedback control. The modified HSA-SPME device is shown in Figure 1. The specifics of the design were deliberate, as explained by Ramsey et al.,10,11 to take advantage of SPME mass uptake principles when the active flow boundary layer surrounding the polymer is reduced.17 The length of the wire allows for 10-fold greater polymer surface area relative to a commercial SPME device (81 vs 8.1 mm2),10,11 and the outer borosilicate glass tube (145 mm length × 4.8 mm o.d. × 3.0 mm i.d.) confines airflow past the wire into the zone directly adjacent to the coated wire where the boundary layer exists and interaction between the sorbent layer and the sampled chemical compounds occurs.18 As Bartelt and Zilkowski,19 as well as Ramsey et al.11 and Wheeler13 demonstrated, dynamic flow using faster flow rates over a SPME coating can increase mass uptake per unit time into the fiber coating. The use of fast flow over the larger HSA-SPME C

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desorption temperatures (200 and 300 °C) were also measured and found to be similar to those for 250 °C. By comparison, ballistic heating with the prototype21 was appealing for its simplicity; however, the actual final temperature reached was sensitive to environmental variations in convective boundary conditions (especially forced flow rates) and ambient temperature. In addition, small variations in the resistance of the device along with variations in the construction (e.g., spiral spacing) would further add to the variability of performance. Considering these together, temperature variation of the prototype in Celsius was in the range of 10−20%. ACEM Module Operating Conditions. For all analyses, the standard temperatures used for air sampling into a traditional sorbent tube within the ACEM module were 40 °C for the sample inlet, 200 °C for the valve oven, and 40 °C for the sample tube. Connection between the HSA-SPME device and the heated inlet consisted of approximately 3 cm of Teflon tubing. A new piece of tubing was used for each desorption/sample run to prevent any desorbed chemical residue potentially contained on the unheated tubing from biasing subsequent sample runs. While not measured, this unheated tube connection from the device to the ACEM system might have been a potential source of sample loss. The system was loaded at the maximum flow rate where the sample pumping rate through the sample tube matched the pump flow rate set point (0.585 L/min). Analytical Instrumentation. The ACEM preconcentrator module was coupled to an LTM-GC/MS system (RVM Scientific, Santa Barbara, CA), retrofitted to a standard Agilent 6890N GC oven through an insulated, 3 m, statically heated (225 °C) transfer line. A DB5-MS capillary column (30 m length × 0.25 mm i.d. × 0.25 μm film thickness; J & W Scientific, Folsom, CA) was used for all analyses. The GC temperature programming was initiated at 40 °C (30 s hold time) with subsequent temperature ramping of 5 °C/min to a final temperature of 130 °C. High-purity helium was used as carrier gas, with a constant flow rate of 1.0 mL/min. A 5973 quadrupole MS detector (Agilent Technologies, Wilmington, DE) was used with electron ionization (70 eV). Mass spectra were collected in scan mode over the range of m/ z 95−180 when analyzing DIMP and m/z 70−180 when analyzing 2-DES. Extracted ion chromatograms (m/z 97, 123, and 165 for DIMP and m/z 72, 89, and 114 for 2-DES) were examined following analysis. Sample retention characteristics and mass spectra were stored and analyzed using Agilent Chemstation software (version D.00.00.38). Statistical Analysis. A two-way analysis of variance (ANOVA) was used in the determination of comparative significance between the two different sampling methods when using four different fixed sampling times, as well as when using the two sampling methods with three fixed sample volumes. A two-way ANOVA was used to ascertain significance in capture efficiency for the three different sampling methods used in tandem HSA-SPME analysis. If a significant interaction was noted among the factors, then pairwise comparisons by Tukey’s adjustment were used. The statistical analyses were performed using the Statistical Analysis Software program (SAS, SAS Institute, Inc., Cary, NC). In cases of unequal variance among means, average abundance levels were adjusted to natural log scale (with subsequent antilog of the results) to better conform to the assumptions of equal variance when using ANOVA.

the chromatography inlet provided a versatile approach to focus analytes for GC/MS analysis from direct air sampling, from resistive heating desorption of a HSA-SPME device (external to the ACEM system), or from quantitative injections of analyte in a solvent. Sampling for HSA-SPME was performed by drawing air at a set flow rate over the coated HSA-SPME element(s) using a small, battery-powered personal air sampling pump (GILAIR-5, Sensidyne/Gilian, Clearwater, FL) connected by Tygon tubing to the outlet of the device(s). Flow rate calibration was performed before and after collecting each sample using a primary standard calibrator (Bios International Corporation, Butler, NJ). Additionally, headspace sampling of 2-DES using the custom-made sampling jar described earlier was performed to demonstrate the versatility of the HSA-SPME device. After placing 50 ng of the degradation compound onto a 1 square inch section of cotton T-shirt material and allowing the solvent to evaporate for 20 min, the jar containing the T-shirt material was sealed and placed in a digitally controlled heating block (Electrothermal Barnstead, Dubuque, IA) at 30 °C for 15 min prior to sampling. The HSA-SPME device was connected with Tygon tubing to a threaded fitting that passed through the cap and extended nearly to the bottom of the jar. Sampling the small jar at the relatively high flow rates of the HSA-SPME method required pressure equalization from the surrounding atmosphere to allow for sampling flow. Slightly unscrewing the cap of the jar permitted makeup airflow into the jar, allowing the operation of the HSA-SPME device at the 4 L/min flow rate. HSA-SPME Temperature Control Module and Desorption. Previous HSA-SPME devices maintained temperature control based on the application of constant current.11 This caused temperature variability from one device to the next, and temperature control was improved by utilizing a calibrated type-K thermocouple centered inside the innermost tube, in conjunction with an electronic circuit board for feedback control of temperature. The software-adjustable, heater-control circuit board provided highly reproducible temperature programming of the HSA-SPME nickel alloy heating wire.20 Initial conditioning of the HSA-SPME device was performed by heating the element to 300 °C for 30 min with high-purity helium (99.999%, AirGas Specialty Gases, Radnor, PA) passed through the device at 200 mL/min. Blanking of the HSA-SPME device was performed between analyses and at the beginning of each day of use. For analyses, trapped analytes were desorbed at 250 °C from an HSA-SPME device into the heated inlet of the ACEM 9305 preconcentration module. The heating power for desorption of the HSA-SPME device was provided by a +12 V dc power supply with a 5 A current capacity. The estimated peak power (during the rapid rise to target temperature) and steady-state power required to hold the temperature of the HSA device at 250 °C were 6.9 and 3.0 W, respectively. These values are significantly lower than power consumption by conventional sorbent-bed-based desorption devices, such as the ACEM 9305 module, under focusing trap desorption conditions which required 213 W for peak and 65 W for steady-state operation. The lower HSA-SPME power requirements make this device more amenable to field sampling. With feedback temperature control, rise times were in the range of 1−3 s, and temperature overshoots ranged from 6.6 to 15.7 °C, which represents only 3.6% deviation on average (in Celsius) from the target temperature. Profiles at other D

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Figure 3. Results from fixed volume comparison of the HSA-SPME (single device) and the ACEM methods (13 ppbv DIMP). As expected, the sorbent tube/ACEM approach displays greater capability than HSA-SPME at fixed volumes, as it represents exhaustive sampling (p < 0.0001). Relative standard deviation for the ACEM method ranged from 1.9% to 7.3% and from 1.9% to 10.5% for the HSA-SPME method. GC/MS peak areas were for triplicate averages of the sum of m/z 97, 123, and 165 ion current traces.

Figure 4. Results for HSA-SPME (single device) and ACEM methods with fixed sampling times (13 ppbv DIMP). Sampling by HSA-SPME is observed to produce significantly higher mass uptake per unit time (p value