An Inverse Analysis Approach to the Characterization of Chemical Transport in Paints

Summary

In this paper, a procedure for quantifying the mass transport parameters of chemicals in various materials is presented. This process involves employing an inverse-analysis based diffusion model to vapor emission profiles recorded by real-time, mass spectrometry in high vacuum.

Abstract

The ability to directly characterize chemical transport and interactions that occur within a material (i.e., subsurface dynamics) is a vital component in understanding contaminant mass transport and the ability to decontaminate materials. If a material is contaminated, over time, the transport of highly toxic chemicals (such as chemical warfare agent species) out of the material can result in vapor exposure or transfer to the skin, which can result in percutaneous exposure to personnel who interact with the material. Due to the high toxicity of chemical warfare agents, the release of trace chemical quantities is of significant concern. Mapping subsurface concentration distribution and transport characteristics of absorbed agents enables exposure hazards to be assessed in untested conditions. Furthermore, these tools can be used to characterize subsurface reaction dynamics to ultimately design improved decontaminants or decontamination procedures. To achieve this goal, an inverse analysis mass transport modeling approach was developed that utilizes time-resolved mass spectroscopy measurements of vapor emission from contaminated paint coatings as the input parameter for calculation of subsurface concentration profiles. Details are provided on sample preparation, including contaminant and material handling, the application of mass spectrometry for the measurement of emitted contaminant vapor, and the implementation of inverse analysis using a physics-based diffusion model to determine transport properties of live chemical warfare agents including distilled mustard (HD) and the nerve agent VX.

Introduction

The mass transport mechanisms associated with contamination of materials by chemical warfare agents are driven by a variety of convolved processes including physical state transitions, chemical interactions between mobile species, and materials interfaces. To develop efficacious decontamination technologies, optimized decontamination procedures, and predictive models, it is vital that the contamination process is well understood, including the transport of contaminants into materials via absorption and the subsequent chemical emission back into the environment. Consequently, it is imperative that approaches are developed that can evaluate subsurface concentration profiles for contaminant-material pairs as a function of environmental conditions. A continuum-scale, physics-based model was developed to predict the concentration distribution of absorbed agent in a contaminated substrate. Experimentally derived mass transport parameters enable the prediction of the vapor emission from the contaminated material post decontamination. An ability to predict the concentration distribution in a material can facilitate the assessment of potential vapor hazards and, in turn, enable accurate diagnoses of toxicological hazards¹. This approach allows for an estimation of contaminant-material pair specific mass transport parameters such as diffusivity and saturation concentration that in turn permit modeling for a other scenarios and conditions. In this study, we have treated the liquid phase contamination of solvent-dispersed, polyurethane paint coatings with chemical warfare agents bis(2-chloroethyl) sulfide (distilled mustard, blister agent HD) and O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (VX), an organophosphate nerve agent.

The developed methodology characterizes gas desorption profiles from contaminated materials, including chemical warfare agents like HD and VX, without many of the restrictions that hamper other approaches^2,3. Time-resolved mass spectrometry measurements of contaminant evolution from contaminated substrates allow for a diffusive transport model with inverse analysis to calculate mass transport parameters for the contaminant in the material, including the absorbed concentration profile for the contaminant starting from the original permeation event. With the establishment of a predictive capability for delineating concentration profiles of contaminants in materials as a function of environmental conditions comes the ability to assess toxicological hazards and ultimately develop routes for efficacious decontamination.

In this paper, the details associated with sample preparation are presented, including work with chemical warfare agent contaminants, as well as experimental data collection from contaminated materials and subsequent modeling⁴. Experimental runs were conducted as described in the chemical contaminant and decontaminant source document⁵ and will be discussed in the next section. A flow chart for sample preparation and analysis steps in included in Figure 1.

Protocol

1. Condition Paint Substrates to the Desired Environment

1. Preset the environmental chamber for substrate conditioning to the specified temperature and relative humidity (20 °C, 50%). Ensure that the substrate conditions are consistently maintained since both temperature and water content can significantly influence absorption rates into materials.
2. Coat 0.32 cm thick, 5.08 cm radius stainless steel discs with a surface area of 20.25 cm² with a layer of paint (MIL-DTL-53039, a solvent-dispersible (SD) aliphatic polyurethane coating system⁶) with a total coating thickness (primer and top coat) of approximately 100 µm. Place the substrates (with desired number of replicates) on stainless steel trays with the test surface to be exposed to a chemical agent facing upwards.
3. Cover the substrates with Petri dishes. Place the trays containing the test substrates into the environmental chamber for at least 60 min but ideally O/N, if possible.

2. Contamination of Preconditioned Substrates

1. Don personal protective equipment such as lab coat, safety glasses, and gloves.
2. CAUTION: Obtain the chemical contaminants from cold storage and allow primary container vial to equilibrate to RT. The chemicals used in this protocol are Chemical Agent Standard Analytical Reference Material (CASARM, 98.0% purity) grade HD and CASARM (89.0% high purity) grade VX (both liquid phase). Obtain purity information from either nuclear magnetic resonance or gas chromatography/mass spectroscopy analyses and maintain on file.
   NOTE: CAUTION: The handling of chemical warfare agents should only be performed by trained personnel at an approved facility using applicable safety, security, and surety precautions.
3. CAUTION: Fit the agent delivery tool with a pipette tip and configure the tool to deliver a droplet of agent (e.g., 1 µl). Uncap the contaminant vial, and place the cap on the hood surface, threads facing up. Pick up the pipette, and slowly lower the tip into the contaminant solution. Load the agent delivery tool with agent in accordance with the manufacturer’s directions. Gently place the loaded pipette onto the hood working surface, and recap the contaminant vial.
4. CAUTION: Contaminate the test substrates.
   1. Remove the trays containing the test substrates from the environmental chamber.
   2. Remove the Petri dishes from the test substrates, and set on the hood surface. Digitally photograph each test substrate to record the appearance of each substrate before contamination.
   3. Pick up the loaded pipette and deliver a single droplet (e.g., 1 µl) of agent onto the first test substrate. Cover the contaminated material with a polystyrene Petri dish to minimize evaporation while contaminating additional substrates. Repeat for each substrate; reload the pipette tip (Step 2.3) as necessary.
      NOTE: Collect pipette confirmation samples before and after dosing of substrates for analysis via chromatography to confirm contaminant mass delivery.
   4. Remove the Petri dishes from the test substrates and set on the hood surface. Digitally photograph each test substrate to record the initial contaminant-material interactions.
   5. Cover the test substrates with the Petri dishes.
   6. Place the tray of substrates back into the environmental chamber.

3. Contaminant-material Interaction Aging Period

1. Prior to aging the samples, vent the stainless steel high vacuum experimental chamber to prepare it for use in Section 4.
2. Allow the contaminated substrates to age in the environmental chamber for the specified duration, which can be varied based on the chemical-material-environment interaction dynamics (e.g., 60 min).
3. Remove substrates from the environmental chamber and place on the working surface of the hood. Remove the Petri dishes from the test substrates and digitally photograph each substrate to record the post-aging contaminant-material interactions.
4. Double-contain the test samples (e.g., multiple airtight containers) and transfer to the working hood with high vacuum chamber.

4. High Vacuum Vapor Emission Chamber Measurement

The high vacuum chamber is a small volume vessel pumped by a turbo molecular drag pump and a diaphragm backing pump (Figure 2). A quadrupole mass spectrometer is mounted on a port that directly faces a temperature-controlled substrate holder and is used to measure real-time gas evolution from contaminated substrates under high vacuum conditions. Full details on the vacuum chamber specifications and materials are included in reference 4.

1. Ensure that the chamber is properly vented.
2. CAUTION: Unpackage the samples prepared in Section 3, and remove Petri dish from the test substrates. Place one test substrate into each temperature-controlled substrate holder using stainless steel tweezers.
3. Seal the vacuum chamber and begin the pump down sequence. Perform this step such that the chamber is vacuum sealed at the specified age time (e.g., 60 min) in relation to when the substrate was contaminated.
4. Begin recording selected mass fragment channels as a function of time (direct measure of mass flux) identified with specific mass per unit charge values (m/z). Measure specific background gas species in addition to the primary mass fragments from the molecules of interest (VX: m/z = 114; HD: m/z = 109) in real time at <0.25 Hz until the contaminant partial pressure drops below detection limits of the mass spectrometer (10⁻⁸ Pa).
5. Collect the emission curves for the duration of the emission of contaminant from the substrate.
6. Stop recording emission curves with the mass spectrometer once the contaminant mass flux has decreased to the chamber pressure baseline.
7. Vent the high vacuum chamber to atmospheric pressure.

5. Post-treatment Evaluation for Total Remaining Contaminant

1. Open the high vacuum vapor emission chamber (HVVEC) instrument and remove the substrate from the chamber using stainless steel tweezers.
2. Place the substrate into a glass extraction jar and add 20 ml of extraction solvent to the jar (e.g., isopropyl alcohol: VX; chloroform: HD). Cap the jar and swirl the jar three times. Leave the substrate in the extraction solvent for 60 min.
3. Swirl the jar three times again and then uncap the jar. Using a clean, disposable, glass pipette, transfer approximately 1 to 2 ml of extraction solvent into an analytical vial for analysis via gas or liquid chromatography⁷ to measure the contaminant mass retained by the substrates.

6. Data Analysis and Modeling

1. Convert raw mass spectrometry data (partial pressure derived from measured ion current) to mass flux from the substrate. Use a combination of the Hertz-Knudsen formula for converting partial pressure of detected gas species to incident vapor flux at the detector and include a scaling factor that accounts for the contaminated area on the substrate from the original contamination event.
2. Use inverse methods (e.g., Levenberg-Marquardt algorithm) to determine the saturation concentration and diffusion constant values (key mass transport parameters) for the contaminant transport through the paint coating. Compare the experimentally determined vapor flux to a predicted vapor flux (an analytical solution to Fick’s second law with appropriate boundary conditions applied (see equation 4 from reference 4)).
3. Use the mass transport parameters to predict the concentration profiles for the contaminant-material system.

Results

The top panel of Figure 3 displays examples of the calculated mass flux of VX and HD from SD-painted substrates based on time-resolved mass spectrometry for the main mass fragments of VX and HD (mass-to-charge ratio, m/z = 114 and 109, respectively). A quadrupole mass spectrometer has three main components: an ionizer, a mass analyzer or filter, and a charge detector. Gas species are ionized via electron impact ionization (hot filament style electron source), and the produced ions are i...

Discussion

Mass transport parameters for HD and VX in the paint were determined via the numerical inverse analysis of vapor emission data. With calculated parameters, it was possible to then produce time-dependent concentration gradient maps for contaminant distribution in the paint coating. The inverse analysis results demonstrated that the solubility of HD in the SD paint was higher than VX, but the diffusivity was approximately 5x lower. The results suggest that the HD contamination was highly concentrated at the surface of t...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Stainless Steel Tray	McMaster Carr	4189T1	13-5/8" L x 9-3/4" W, http://www.mcmaster.com/#stainless-steel-trays/=p8dcgp
MIL-DTL-53039 solvent-dispersible aliphatic polyurethane coating system			Substrates supplied by internal source
Environmental Chamber			Custom Design. Full details on vacuum chamber specifications and materials included in reference 4.
bis(2-chloroethyl) sulfide	CASARM		TOXIC
O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate	CASARM		TOXIC
Pipetter	Fisher Scientific	22260201	Range of 1.0 µl to 10 ml
Pipetter Tips	Fisher Scientific	13-683-709	0.1 ml Volume
Stainless Steel High Vacuum Experimental Chamber			Custom Design
Quadrupole Mass Spectrometer	ExTorr	RGA300
Stainless Steel Tweezers	McMaster Carr	5516A15	Any stainless steel tweezers are appropriate.
Glass Extraction Jar	Scientific Specialties	170808	Jar fits a ~5 cm diameter substrate. Different glass jars with teflon lined lids are appropriate for different sized substrates.
Chloroform	Sigma-Aldrich	650498	HARMFUL. The extraction solvent for HD may change depending on the analytical method.
Isopropanol	Sigma-Aldrich	650447	HARMFUL. The extraction solvent for VX may change depending on the analytical method.
Pasteur Pipette	VWR	14673-010	size = 5 3/4"