ARTICLE pubs.acs.org/ac
Quantification of VX Vapor in Ambient Air by Liquid Chromatography Isotope Dilution Tandem Mass Spectrometric Analysis of Glass Bead Filled Sampling Tubes Ronald A. Evans,*,† Wendy L. Smith,‡ Nam-Phuong Nguyen,‡ Kathy L. Crouse,† Charles L. Crouse,§ Steven D. Norman,‡ and E. Michael Jakubowski† †
Analytical Toxicology Branch, RDCB-DRT-T, and ‡Environmental Monitoring Branch, RDCB-DPO-M, U.S. Army Edgewood Chemical Biological Center, 5183 Blackhawk Road, Aberdeen Proving Ground, Maryland 21010-5424, United States § Gunpowder Branch, SAIC, Aberdeen Proving Ground, Maryland 21010, United States ABSTRACT: An analysis method has been developed for determining low parts-per-quadrillion by volume (ppqv) concentrations of nerve agent VX vapor actively sampled from ambient air. The method utilizes glass bead filled depot area air monitoring system (DAAMS) sampling tubes with isopropyl alcohol extraction and isotope dilution using liquid chromatography coupled with a triple-quadrupole mass spectrometer (LC/MS/MS) with positive ion electrospray ionization for quantitation. The dynamic range was from one-tenth of the worker population limit (WPL) to the short-term exposure limit (STEL) for a 24 L air sample taken over a 1 h period. The precision and accuracy of the method were evaluated using liquid-spiked tubes, and the collection characteristics of the DAAMS tubes were assessed by collecting trace level vapor generated in a 1000 L continuous flow chamber. The method described here has significant improvements over currently employed thermal desorption techniques that utilize a silver fluoride pad during sampling to convert VX to a higher volatility G-analogue for gas chromatographic analysis. The benefits of this method are the ability to directly analyze VX with improved selectivity and sensitivity, the injection of a fraction of the extract, quantitation using an isotopically labeled internal standard, and a short instrument cycle time.
V
X (O-ethyl-S-[2-(diisopropylamino)ethyl] methylphosphonothiolate, CAS 50782-69-9) is an organophosphate nerve agent (OPNA) of interest for military organizations and homeland security agencies due to its toxicity, low volatility, and potential for persistence in a contaminated environment. Exposure to VX can cause symptoms such as muscle twitching, miosis, convulsions, hyperglandular secretions, seizures, and death.1 VX is estimated to be lethal to one-half of an exposed human population (LD50) at a percutaneous concentration of 142 μg/kg. Although it has a low volatility, VX is highly toxic by inhalation and has a time-weighted average for daily, 8 h per day, 30 year exposure worker population limit (WPL) of 1 10-6 mg/m3 and a general population limit (GPL) of 6 10-7 mg/ m3 for a time-weighted average 24 h per day lifetime exposure.2 Although VX contamination in the workplace or the environment is rare the need for extremely sensitive and specific air monitoring methods is essential due to its enormous toxicity and persistence. In 2006 an accidental release contaminated a laboratory worker when vaporous VX was generated by oven-drying glassware that had not been completely decontaminated.3,4 r 2011 American Chemical Society
Fortunately, this was a minor exposure which caused only miosis, a slight tightness in the chest, but little to no decrease in red blood cell acetylcholinesterase activity. Air monitoring of the area following exposure confirmed the presence of VX with the highest concentration measured (2.6 10-5 mg/m3) in the room nearest to the drying oven. The rooms immediately adjacent to the exposure site were quarantined immediately after the exposure occurred. The laboratory was decontaminated using vaporous hydrogen peroxide. Air monitoring in the weeks following the contamination using the standard method highlighted the need to investigate new methodologies for trace level air monitoring of VX. Derivatization or conversion of VX with silver fluoride has been the standard approach since the late 1980s.5 Current air monitoring methods are based on the collection of VX that has been passed through a silver fluoride coated fiber pad, converted to the G-analogue of VX (Figure 1), and adsorbed onto a Tenax Received: September 17, 2010 Accepted: December 8, 2010 Published: January 14, 2011 1315
dx.doi.org/10.1021/ac1024683 | Anal. Chem. 2011, 83, 1315–1320
Analytical Chemistry
Figure 1. Structure of VX and the G-analogue of VX, the product formed after it has passed through a silver fluoride pad. This conversion is routinely used to collect the G-analogue of VX with a Tenax TA filled DAAMS tube prior to analysis by thermal desorption.
TA (Supelco, Bellefonte, PA) containing depot area air monitoring system (DAAMS) tube.6,7 The conversion creates the more volatile O-ethylmethylphosphonofluoridate, which is especially amendable to collection and analysis using the standard DAAMS tube. The DAAMS tube is then subjected to thermal desorption and analyzed by gas chromatography/flame photometric detection (GC/FPD) with a phosphorus-selective filter. If the GC/ FPD yields a positive response above the lower limit of quantitation (LLOQ), a second collected tube is then analyzed using a GC column of different polarity coupled with FPD or massselective detector (MSD) for confirmation purposes. Limitations of this method include the indirect analysis by conversion to the higher volatility G-analogue of VX that does not allow for a specific confirmation of VX. The potential for false positives can result from the presence of VX breakdown products, bleach-related products, and other naturally occurring volatile phosphorus-containing compounds. Each sample is consumed during the analysis requiring the use of a paired tube on a second analysis system for confirmation of the presence of VX. The use of external calibration by spiking a series of DAAMS tubes cannot account for analysis variability such as sample transfer efficiency and detector fluctuations. The sensitivity of the method is also limited to an LLOQ of 0.5 WPL for a 24 L sample collection. There are a variety of detection systems available for both near real time and historical detection of OPNAs. Of the accepted methods, only DAAMS and absorption air-sampling systems (also referred to as bubblers) which use a glass bead packed vessel filled with a scrubbing solution are capable of detection at the WPL for VX.8 The bubbler systems can be labor-intensive and are known to be susceptible to agent degradation by hydrolysis if not handled properly. Direct analysis of VX by nonpassivated thermal desorption sorbent tubes has recently been explored and demonstrated on a preliminary basis to work at trace level concentrations. The detection of 45 pg of free VX was shown by solvent spiking onto Tenax TA packed tubes and immediate analysis.9 This would theoretically be equivalent to collecting air samples for 12 h at 500 mL/min (360 L total volume) with a detection limit equivalent to 0.125 WPL. Thermal desorption systems have not been routinely used to directly analyze field samples for native VX due to problems with desorption efficiency, active sites requiring the use of preservatives or system priming, and difficulty in quantitative transfer of the sample from the thermal desorption unit to the gas chromatograph. Glass beads are generally not used as a packing material in general purpose DAAMS tubes. Glass beads offer a small (
