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Passive Infrared
Spectroscopy with a Planar Array to Detect Trace Quantities of Gas Phase Chemical
Warfare Agents and Other Toxic Materials
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In order to provide a high surface area membrane to
concentrate gas phase molecules for examination with the IR spectrograph,electrospinning of fibers of various commodity
materials was undertaken. These included poly(styrene)
(PS) and a polyamide (Nylon 6). Both the fiber size and, in the case of
PS, the density of nanopores on the fiber
surface could be varied so as to optimize the surface to volume ratio.
Shown in Fig. 1 is a field emission scanning electron microscope (FE-SEM)
picture of an electrospun fiber membrane of
Nylon 6. The fiber diameters range between 2-3 microns and the fibers
themselves are quite uniform. On the right of Fig. 1 is the FT-IR and
PA-IR spectra of the Nylon 6 fibrous membrane. Although the intensities
of the bands below 1500 cm-1 compare favorably between the two
techniques, the FTIR bands in the amide I (1655 cm-1) and
amide II (1545 cm-1) region appear more intense than their
PA-IR counterparts. This is because the FT-IR spectrum was recorded
through a thicker part of the membrane and Beer's law is no longer valid
at absorbance values > 1.5 a.u. The main
"take away" point is that the PA-IR spectrum was recorded
in seconds compared to tens of minutes for the FT-IR spectrum1,2.
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FE-SEM
of Nylon 6 Fibers
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IR
of Electrospun Nylon 6
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Fig. 1. FE-SEM of electrospun
fiber membrane of Nylon 6 (left) and a
comparison of its FT-IR and PA-IR spectra (right).
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The
speed advantage of the PA-IR technique is also demonstrated in the left
side of Fig. 2 where the spectra of dimethylphosphonate,
(HP(CH3)2(O)) (DMP), obtained in a 10 cm. gas cell
as a function of pumping time is shown. Each spectra is recorded in 8.7 millisec except the two weakest, which were recorded
in 87 milliseconds. The strong υa(P-O-C)
band at 985 cm-1 is clearly observed even at the lowest
concentrations.
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When DMP (a Sarin surrogate)
is adsorbed onto the Nylon 6 membrane, the resulting spectra (blue -
shown on right of Fig.2) can be seen to contain additional bands due the
presence of the DMP adsorbent. Subtracting the Nylon 6 spectrum produces
a spectrum of adsorbed DMP that closely resembles that of the gas phase
except for a shift of many of the bands to lower frequency due to the
interaction of DMP molecules with non- hydrogen bonded C=O and N-H bonds
on the surface of the Nylon 6 fibers. Hence combining the speed of the
PA-IR technique and the amplification of a high surface area electrospun membrane can provide a rapid method for
the detection of chemical warfare agents (CWAs).
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Fig. 2. Gas phase spectra (left) of DMP obtained in
a 10 cm. gas cell as a function of pumping time. Each spectra is recorded
in 8.7 millisec except the two weakest, which
were recorded in 87 millisec. Right-spectra of
Nylon 6 membrane, DMP adsorbed on membrane and difference (bottom)
spectrum.
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Hence this demonstrates that the high surface area electrospun membrane concept is a valid and valuable
strategy for amplifying the presence of gaseous and liquid molecules by
concentrating them on the fiber surface.
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References |
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1)"Design and Performance
of a Planar Array Infrared Spectrograph that Operates in the
3400 to 2000 cm-1 Region", Elmore, D. L.; Tsao, M. W.; Frisk, S.; Chase, D. B.; J.F., Rabolt.
Applied Spectroscopy 2002, 56, 145.
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2) "Performance and Application of a New Planar
Array Infrared Spectrograph Operating in the Mid-Infrared (2000-975 cm1)
Fingerprint Region", C. Pellerin, C. Snively, D. B. Chase, and J. F. Rabolt Applied
Spectroscopy 2004, 58, 639-646.
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