Article pubs.acs.org/ac
Sampler for Collection and Analysis of Low Vapor Pressure Chemical (LVPC) Particulates/Aerosols K. J. Ewing,* D. Gibson, and J. Sanghera Naval Research Laboratory, Code 5620, Washington, DC
F. Miklos Sotera Defense Solutions, 2121 Cooperative Way, Herndon, VA S Supporting Information *
ABSTRACT: Detection of low vapor pressure chemicals (LVPCs) such as pesticides and other toxic/hazardous materials on various environmental surfaces as well as LVPC aerosols is a significant challenge for current vapor phase detectors. We describe a novel sampling device which utilizes stainless steel screens coated with a sticky polydimethylsiloxane coating for collecting LVPCs aerosolized off of a surface. Results are presented for the collection and detection of a pesticide simulant, dimethyl methylphosphonate sorbed onto silica gel (DMMP/ SG), using direct analysis in real time-cylindrical ion trap mass spectrometry (DART-CITMS).
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captured biological organisms. For example, the SuperSAS 180 directs the biological-agent-laden air onto an agar plate where the biological agents are captured via impaction. The agar plate is then removed and incubated for identification of the captured biological agent. There are also a number of larger particulate collectors including impactors and cyclone collectors for collection of biological agents and in some cases chemical particulates. For example, a multistage impactor for collection of atmospheric aerosols was developed which enables the fractionation of aerosols by particle size.6 The different fractions were collected onto a polyurethane foam substrate which had to be physically removed for analysis. The system, while being small for aerosol collectors, was still not portable for easy field use. Another sampler used a stacked filter unit with coarsepored nuclepore filters to collect different size fractions of particulates in ambient air.7 In order to maximize particulate collection, the nuclopore filters were coated with Apiezon grease, which minimizes particle bounce and increases the collection efficiency. The goal of our work is the development of a compact, inexpensive, hand-held sampler capable of collecting chemical particulates both as an aerosol and as particulates on environmental surfaces. The sampler interfaces with a mass spectrometer using either thermal desorption of the collected analyte or an atmospheric ionization technique such as the
hemical particulate hazards and low vapor pressure chemicals such as pesticides and energetic materials are significant threats to both the general population as well as the military. These materials can be released into the atmosphere in aerosol form and pose an inhalation threat and, once they settle onto environmental surfaces, pose a long-term contact hazard threat. There are a number of analytical devices/detectors capable of measuring parts per million to parts per billion levels of hazardous chemical vapors in real time such as ion mobility spectrometers, infrared spectrometers, and mass spectrometers.1−4 However, these detectors sample the vapor emitted from the hazardous material, making it difficult to detect low vapor pressure chemicals (LVPC) because of the very low levels of vapor released by LVPCs at ambient temperatures. While it is possible to use preconcentration techniques for detection of LVPCs, the time required to preconcentrate sufficient vapor for detection limits the applicability of preconcentration techniques for LVPCs.5 Therefore, there is a need to develop methods for collection and detection of LVPCs as particulates either in aerosolized form or as particulates on environmental surfaces. There are a number of commercially available hand-held biological agent collectors such as the Cherwell Super SAS 180 (Bioscience International, Rockville, MD), the Biotest RCS (Ashtead Technology, Houston, TX), and the FLIR biocapture (FLIR Systems, Inc., Wilsonville, OR) systems. Each of these collectors is designed for the collection of aerosolized biological agents which are particulate in nature. The collection media used by these systems is designed to capture live biological agents to enable colony growth for identification of the © XXXX American Chemical Society
Received: April 15, 2013 Accepted: September 20, 2013
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Particulate-contaminated surfaces were prepared by weighing out a small mass of the DMMP/SG onto weighing paper. The particulates were then spread across the surface by gently tapping the weighing paper; no effort was made to distribute the particulate evenly across the surface. Three replicate samples of each concentration of DMMP/SG were collected and analyzed. Collection of particulates from the contaminated surface (weighing paper) was accomplished using a Biosciences SAS 1800 bioaerosol collector as the vacuum source to pull particulate-laden air through the sticky screen, as shown in Figure 2. A sampling hood with air vents cut into the sides,
DART-MS or desorption electrospray ionization-mass spectrometry (DESI-MS) for rapid analysis of captured particulates. The effort described in this work investigates a novel sampling approach for chemical particulates on a surface using stainless steel screens coated with sticky polydimethylsiloxane (PDMS) followed by analysis of the collected chemical particulate using DART-CITMS.
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EXPERIMENTAL SECTION Stainless steel screens (mesh 100, 150 μm holes) were configured into a “top hat” configuration with an 8 mm diameter flat sampling/analysis area. A sticky screen holder was designed and fabricated to hold the sticky screen for both sampling and analysis; Figure 1 shows the three components of
Figure 2. Hand-held SuperSAS 180 (Bioscience International, Rockville, MD) sampler fitted with sticky screen for collection of aerosolized particulates from surface. Air drawn in through the vents on the sampling hood aerosolizes the particulates on the surface. Once aerosolized, the particulates are captured by the sticky screen.
Figure 1. The three components of the LVPC sampler are shown: sample screen holder, “top hat” screen (uncoated), and screen retaining ring.
designed to fit onto the end of a Swagelok fitting connected to the sticky screen mount, was used to generate a turbulent air flow over the surface being sampled. The sampling hood is lowered onto the contaminated weighing paper surface and the Biosciences SAS 1800 blower activated for 15 s at an air flow rate of 3.95 ± 0.4 L min−1. Air is drawn through the vents in the sampling hood, causing turbulent flow over the surface sweeping the DMMP/SG particulates up into the air flow. Once in the air flow, the particulates impact the sticky screen and are captured for analysis. The DMMP/SG particulate collection efficiency (CE) was calculated according to: m CE DMMP/SG = c mi (1)
the sample screen holder. Here, the uncoated “top hat” screen is first placed onto a recessed ledge in the end of the sample screen holder. Then, the retaining ring is press-fit into the sample screen holder such that the “top hat” screen is held in place. The mounted “top hat” screen is then spray coated with PDMS, and the PDMS is partially polymerized.8,9 This treatment results in a sticky PDMS coating on the screens capable of capturing particulates either through direct surface contact or by impaction of the particulates onto the sticky screen. The configuration of the sticky screen held in place on the sample screen holder enables reproducible sample introduction. An additional advantage of this sampler is that after sampling and analysis, the used sticky screens and retaining ring can be removed, and a new screen can be placed into the sample screen holder. The analyte studied in this work is composed of the simulant, dimethyl methylphosphonate sorbed into silica gel (DMMP/ SG). The simulant, DMMP/SG, is a free-flowing powder that can be aerosolized or deposited onto a surface as a particulate. Once the DMMP is sorbed into the silica gel, the vapor pressure is significantly reduced, mimicking low vapor pressure hazardous chemicals such as common pesticides where the liquid pesticide is immobilized into an inert carrier. Both DMMP and SG were purchased from Sigma-Aldrich and used as received. Three different concentrations of DMMP/SG samples were prepared containing 10, 20, and 36% (wt) DMMP based on the starting mass of DMMP and SG. The DMMP was loaded onto the silica gel as previously described.10 Samples were prepared and stored in a freezer (T = 10 °C) until use.
where mi represents the initial mass of DMMP/SG placed onto the weighing paper and mc the mass of DMMP/SG captured on the sticky screen. The mass of DMMP in the collected DMMP/ SG was calculated according to: mDMMP = wt%DMMP/SG × mc
(2)
where wt % DMMP/SG represents the weight percent of DMMP on the silica gel. The average collection efficiency of particulates on the sticky screens for 20 replicate collections is 48 ± 13% (see Supporting Information). In the current work, it is necessary to know the collection efficiency for each set of experiments to calculate the mass of DMMP in each sample; Table 1 reports the collection efficiencies and mass of DMMP in the sample for the three different concentrations of DMMP in SG. Micrographs of the sticky screen before and after particulate collection, shown in Figure 3A, clearly show the DMMP/SG B
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nitrogen as the ionization/carrier gas. The total time per experimental run was 30 min for all experiments. The ChemSense 600 utilizes an internal standard, perfluorotributylamine, which exhibits two peaks at m/z 69 and m/z 131 to allow for real time mass calibration of the system. The DART was operated in the positive ion mode; gas temperature was set at 250 °C with a flow rate of 1 L min−1. Nitrogen gas was used as the desorption gas. The angle of the DART stream was approximately 15 degrees with respect to the sticky screen surface. In this configuration, the DART stream impacts the sticky screen surface and is reflected off the surface transporting the desorbed/ionized DMMP into the CITMS inlet. Thermogravametric analysis (TGA) of the three DMMP/SG samples were performed in order to determine the mass of DMMP desorbed from the DMMP/SG captured on the sticky screen under the experimental conditions.16 The temperature of the sticky screen in the DART stream was determined to be 125 °C by placing a thermocouple onto the sticky screen while it was in the DART-heated gas stream. TGA results were obtained using a TA Instruments, SDT 2960 Simultaneous DSC-TGA. DMMP/SG samples were loaded into a platinum dish and placed into the TGA/DSC, and the furnace was sealed. The sample was then heated at 10 °C/minute to 125 °C under flowing argon gas and then held isothermally at 125 °C for 55 min. A plot of the wt % loss versus time for the 36 wt % DMMP/SG sample is shown in Figure 5. The wt % loss by the
Table 1. Collection Efficiency of Sticky Screens and Average Mass of DMMP in Collected Samples for the Three Different DMMP/SG Standards wt % DMMP/SG
collection efficiency (%)
mass DMMP in collected sample (μg)
3.0 13.0 26.7
44 30 51
107 282 454
Figure 3. (A) Sticky screen before and after collection of DMMP/SG particulate from the surface. (B) Uncoated screen before and after collection of DMMP/SG particulate from the surface.
particulates collected by the sticky screen. In Figure 3B, a noncoated screen before and after sampling the same DMMP/ SG particulate shows that an uncoated stainless steel screen does not collect any of the particulates. After sample collection, the sticky screen holder is placed into the analytical system composed of a DART atmospheric ionization system and a FLIR ChemSense 600 cylindrical ion trap mass spectrometer (CITMS) (FLIR, West Lafayette, IN, 47906), as shown in Figure 4.11,12 The analytical system is
Figure 5. Thermogravametric analysis of the 36% (wt) DMMP/SG sample. Figure 4. DART-CITMS configuration. (A) Entire system configuration and (B) close-up of DART outlet, mounted sample screen, and CITMS inlet.
sample over 30 min, corresponding to the time frame of the DART-CITMS measurement time, is 27% (wt); the wt % loss for the 20% (wt) and 10% (wt) samples (see Supporting Information) were determined to be 13.0 and 3.0% (wt), respectively. The data indicate that the mass delivered to the CITMS is less than expected based on the initial loading of DMMP onto the silica gel. These data were used in generating a calibration curve for DMMP vapor originating from the collected DMMP/SG particulate. The CITMS signal strength for the m/z 79 DMMP ion from samples containing 3, 13, and 27% (wt) DMMP/SG collected on sticky screens was recorded over a 30 min analysis time. Plots of the CITMS signal strength versus time were generated for three replicate analyses of the different DMMP/SG samples. The area under the CITMS signal versus time plot and standard deviation were calculated for each DMMP/SG
unique in that it utilizes two different ionization sources, the DART and the ChemSense glow discharge electron ionization (GDEI), to generate ions. The rational for this analytical approach is that ions produced by the DART must pass through a 25 μm particulate filter, which is designed to stop entrainment of particulates into the CITMS inlet. It has been shown that ions can be neutralized upon contact with a surface; in this case, the 25 μm filter media.13,14 Therefore, a second ionization source was required to reionize any neutralized ions to maximize the number of ions delivered to the CITMS for detection. The ChemSense 600 was run in the constant leak mode without preconcentration, and the GDEI source15 used C
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Figure 6. (A) DART-CITMS of the clean sticky screen. The observed peaks correspond to the mass spectrometer internal standard perfluorotributylamine. (B) DART-ITMS of DMMP/SG (26.7% (wt)) captured on the sticky screen. The major peak in this plot at m/z 79 is due to DMMP vapor desorbed from DMMP/SG on the sticky screen.
([CH3P(O)OCH3]+ + H2O]).11 However, the DMMP spectrum in Figure 6B exhibits only one strong peak at m/z 79 and none of the other peaks reported in reference 11. A study of atmospheric plasma decontamination of DMMP on surfaces reported that the mass spectrum of the decomposition products exhibited the DMMP m/z 79 peak as the most intense peak.17 The authors assigned the m/z 79 peak to the PO3 species, which resulted from the decomposition of the DMMP by the plasma. Our data suggest that the DART is decomposing the DMMP to the PO3 species, resulting in a single strong peak at m/z 79 similar to the result reported in reference 17. A plot of the DMMP peak intensity (m/z 79) versus time is presented over the experimental time frame of 30 min for both the screen blank and DMMP/SG particulates captured on a screen in Figure 7. The plot shows the m/z 79 DMMP peak intensity increases rapidly after introduction of the sample into the DART stream, reaching a maximum, and then decreasing to
concentration. A calibration curve was generated by plotting the CITMS signal area for the m/z 79 DMMP peak versus the mass of DMMP delivered by DART desorption/ionization. The detection limit was calculated using the calibration equation and 3 times the standard deviation (3σ) of the m/z 79 peak area for the 3% (wt) DMMP/SG sample for the peak. Three unknown samples were generated by collecting 58% lower mass samples of the 27% (wt) DMMP/SG sample. This resulted in 58% less DMMP delivered to the CITMS, representing a true unknown sample. The calibration curve was used to calculate the mass of DMMP in the unknown sample.
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RESULTS The ability of a novel collection media, sticky PDMS-coated screens, is evaluated for collection of chemical particulates aerosolized from a surface with subsequent analysis using DART-CITMS. The sticky screen holder (Figure 1) is designed to enable collection of aerosolized particulates by drawing air through the sticky screen. Particulates are drawn from a surface and impact the sticky screen becoming trapped onto the surface, as shown in Figure 3. The screen containing the particulates is then analyzed using a DART atmospheric ionization system, which enables rapid and reproducible analysis of the captured particulates with virtually no sample preparation. The mass spectrum of a blank sticky screen and a sample containing 27% (wt) DMMP/SG particulates are presented in Figure 6A,B, respectively. The mass spectrum of the blank screen in Figure 6A show two peaks at m/z 69 and 131, which are due to the internal standard perfluorotributylamine used in the FLIR ChemSense 600 ITMS; there is no evidence of characteristic DMMP peaks in the blank mass spectrum. The mass spectrum of the DMMP/SG particulates, shown in Figure 6B, exhibits a single intense peak for DMMP at m/z 79; the parent ion peak at m/z 124 is not observed. Published data for the mass spectrum of DMMP using the ChemSense 600 mass spectrometer employing only the GDEI source to ionize the DMMP was reported to generate the following DMMP peaks in decreasing intensity, m/z 125 (MH+), m/z 79 ([MH+ − CH3OCH3]+, and m/z 111
Figure 7. Plot of the DMMP (m/z 79) peak intensity versus time for DMMP/SG collected on a sticky screen and a blank sticky screen. Note that the blank screen data correspond to the y-axis on the right side of the plot, and the DMMP data correspond to the y-axis on the left side of the plot. D
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where P is the vapor pressure of the liquid in the pores, P0 is the vapor pressure of the bulk liquid, γ is the liquid−air surface tension, Vm is the molar volume of the liquid, rpore is the pore radius, and θ is the contact angle made by the liquid on the porous solid material. We conclude that the observed exponential relationship between the DMMP m/z 79 peak area and the mass of DMMP in the sample collected onto the sticky screen is a result of the exponential nature of the evaporation of liquid DMMP from the porous silica gel matrix. Three unknown samples of a 27 wt % DMMP/SG were generated by collecting DMMP/SG samples that were 58% of the mass of DMMP/SG used to generate the calibration curve. This approach will generate less DMMP vapor, enabling an evaluation of eq 1 to determine the mass of DMMP in the unknown samples. The samples were collected onto sticky screen samplers and analyzed as described in the Experimental Section. The area under the DMMP (m/z 79) peak intensity versus time curve was calculated for each unknown sample. The average peak area was calculated, and eq 1 was used to calculate the average mass of DMMP in the sample. The mass of DMMP calculated in the sample was 252 μg, and the actual mass of DMMP was 267 μg, representing a 5.8% difference. The instrumental detection limit for DMMP vapor was calculated using 3 times the standard deviation (3σ) of the DMMP (m/z 79) peak area for the lowest mass DMMP sample. The detection limit for DMMP vapor, calculated using eq 1 and the instrumental detection limit, was determined to be 103 μg. The detection limits for DMMP/SG particulates with different DMMP loadings are presented in Table 2 based on
background after 25 min. The DMMP peak intensity (m/z 79) for the blank screen, shown in Figure 7 as the blue line whose intensity is represented by the y-axis on the right side of the plot, is initially very low and decreases in intensity over the course of the scan. The data demonstrate that the DART desorbs and ionizes DMMP from the DMMP/SG particulates captured on the sticky PDMS-coated screen without interference from the sticky PDMS coating. The area under the DMMP (m/z 79) peak intensity versus time curve was calculated for three replicate samples of DMMP/SG at DMMP loadings of 3.0, 13.0, and 26.7% (wt). It was observed that the DMMP m/z 79 peak area (PA) varied exponentially with respect to the mass of DMMP in the samples collected on the sticky screens (see Supporting Information). Therefore, to generate a linear calibration curve, the natural logarithm of the DMMP (m/z 79) peak area (ln (PA)) versus mass of DMMP in the DMMP/SG samples collected onto the sticky screens was plotted and is
Table 2. Calculated Detection Limits for DMMP/SG Particulates Containing Different Weight Percent of DMMP
Figure 8. CITMS signal intensity versus mass of DMMP (μg) from DMMP/SG on a sticky screen thermally desorbed and ionized by the DART. The error bars shown correspond to one standard deviation of the signal for each sample.
particulate DL (mg)
3.0 13.0 26.7
3.4 0.8 0.4
the calculated DMMP detection limit of 103 μg. As expected, the detection limit for DMMP/SG particulates is reduced from milligram to microgram levels of particulates as the loading of DMMP increases from 3.0 % (wt) to 27% (wt) in the silica gel particulates.
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presented in Figure 8. The line equation and the correlation coefficient for the plot in Figure 8 are: ln(PA) = 0.0217mDMMP + 6.17 (R2 = 0.9935)
DMMP concentration (wt %)
CONCLUSIONS The current work investigates the use of sticky screens to capture and analyze aerosolized LVPC particulates. Sticky screens enable the rapid collection of LVPC particulates via impaction onto the sticky screen surface, resulting in collection efficiencies in the range of 30−50%. Once collected, the LVPC particulates are analyzed using a DART atmospheric ionization source coupled to a CITMS with a 25 μm filter on the inlet to stop particulates from being transported into the mass spectrometer interior. This analytical system uses the DART to desorb/ionize DMMP from the DMMP/SG particles captured on the sticky screen. Because the DMMP ions that are produced must pass through the 25 μm filter, it is likely that a fraction of the ions produced by the DART will be neutralized prior to entering the CITMS. Therefore, the DMMP vapors are reionized by the CITMS system using a GDEI source to maximize the number of DMMP ions injected into the CITMS. Results indicate that the area under the DMMP peak intensity (m/z 79) versus time curve varies exponentially with respect to
(1)
where PA is the peak area for the DMMP m/z 79 ion and mDMMP is the mass of DMMP in the sample. A plot of the area under the TGA curve versus the mass of DMMP in the sample (see Supporting Information) also exhibits exponential behavior similar to the data in Figure 8, suggesting that the observed exponential behavior of the calibration data is a function of the physical nature of DMMP sorbed onto the porous silica gel. It has been demonstrated that the evaporation rate of a liquid from a porous material is exponential due to the reduction of the vapor pressure of a liquid contained within the narrow pores of a porous solid.18,19 In such a system, the liquid vapor pressure is described by the Kelvin equation:20,21 ⎛P⎞ 2γVm ln⎜ ⎟ = − rpore P ⎝ 0⎠ RT cos θ
( ) E
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(11) Smith, J. N.; Keil, A.; Likens, J.; Noll, R. J.; Cooks, R. G. Rapid Commun. Mass Spectrom. 2011, 25, 1437−1444. (12) Gao, L.; Sugiarto, A.; Harper, J. D.; Cooks, R. G.; Ouyang, Z. Anal. Chem. 2008, 80, 7198−7205. (13) Willerding, B.; Heiland, W.; Snowdon, K. Phys. Rev. Lett. 1984, 53 (21), 2031−2034. (14) Gologan, B.; Green, J. R.; Alvarez, J.; Laskin, J.; Cooks, R. G. Phys. Chem. Chem. Phys. 2005, 7, 1490−1500. (15) Gao, L.; Song, Q. Y.; Noll, R. J.; Duncan, J.; Cooks, R. G.; Zheng, O. Y. J. Mass Spectrom. 2007, 42, 675−680. (16) Tiwari, R. R.; Khilar, K. C.; Natarajan, U. Appl. Clay Sci. 2008, 38, 203−208. (17) Moeller, T. M.; Alexander, M. L.; Engelhard, M. H.; Gaspar, D. J.; Luna, M. L.; Irving, P. M. IEEE Trans. Plasma Sci. 2002, 30 (4), 1454−1459. (18) Beverley, K. J.; Clint, J. H.; Fletcher, P. D. I.; Thubron, S. Phys. Chem. Chem. Phys. 1999, 1, 909−911. (19) Pedron, I. T.; Mendes, R. S.; Buratta, T. J.; Malacarne, L. C.; Lenzi, E. K. Phys. Rev. E 2005, 72, 031106−1−031106−5. (20) Thomson (Kelvin), W. Philos. Mag. 1871, 42 (282), 448−452. (21) Skinner, L. M.; Sambles, J. R. Aerosol Sci. 1972, 3, 199−210.
the mass of DMMP in the SG. This is a result of the exponential nature of the evaporation of liquids from porous materials, where the vapor pressure of the liquid in the porous material is described by the Kelvin equation. The mass of DMMP in an unknown sample of DMMP/SG was determined using the calibration equation and resulted in a value of 252 μg of DMMP, which was 5.8% different from the actual DMMP concentration in the SG. The detection limit for DMMP vapor from the DMMP/SG particulates was determined to be 103 μg; this translates into DMMP/SG particle detection limits of between 0.4 to 3.4 mg of particulate for the highest and lowest DMMP loading, respectively. The detection limit for DMMP is relatively high and is thought to be a function of the sticky screen geometry and the interaction of the DART stream with the sticky screen. We are investigating the effect of different DART−sticky screen geometries and how they affect the detection limit for DMMP vapors originating from DMMP/SG particulates.
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ASSOCIATED CONTENT
S Supporting Information *
This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully thank the Joint Program Executive Office for Chemical and Biological Defense/Joint Project Manager for Nuclear, Biological, and Chemical Contamination Avoidance and Dr. Angela Ervin, DHS S&T, Contract No. HSHQDC-11-X-00568 for supporting this work.
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REFERENCES
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