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Broadband Cavity Ringdown Spectrometer Applied to Explosives Detection (revised as of 9/24/02)
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$ 483,542
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24
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Section C NSF Form 1207 (10/99)
0
Section A - Project Summary
Cavity ringdown spectroscopy, or CRDS, is a highly sensitive absorption
technique that can be operated from ultra violet to infrared wavelengths depending on
optical components. With this technique, monochromatic laser light enters a cavity
enclosed by two highly reflective mirrors. Light reflecting between these mirrors creates
an effective path length kilometers in length resulting in high sensitivity. Because of this
phenomenon, ultra low absorption losses have been recorded allowing low parts per
trillion (ppt) concentrations of trace gases to be determined.
However, current CRDS systems only operate in a narrow wavelength window.
The narrow range is a result of common laser sources, such as the Nd-YAG, which only
offers a window of operation of roughly 20nm. Also, the current applications of CRDS
often only require the analysis of a single peak with no tests of interfering molecules or
mixtures of components; therefore a 20nm range is not problematic. These limitations
have stemmed from the technique’s inception in 1988 and extend to the present day.
From an analytical standpoint, CRDS in this format offers considerable sensitivity, but
little selectivity over possible interfering compounds. If this methodology were applied to
a situation where an interferent strongly absorbed at the monitored wavelength, or if
mixtures of several weaker absorbing interferents were present, it would be rendered
useless. This proposal argues that CRDS has not yet been used to its fullest capacity. The
next logical step for the technique is to move out of the sheltered environment of the
laboratory toward simulated real world sampling conditions. Therefore a CRDS system
that encompasses the already attained sensitivity and additional selectivity would
maximize the potential of the CRDS technique considerably.
Through this proposal, CRDS will be combined with infrared spectroscopy, a
technique with a high degree of selectivity. This improved system will use lead salt (Pbsalt) tunable laser diodes for source radiation in the mid-IR region of the spectrum. These
lasers are capable of operating over a wavelength range of 200cm-1, which in the mid-IR
is equivalent to 1µm. Initially, one laser diode will be used. For later experiments, the
simultaneous use of two diodes allowing for significantly wider spectral range will be
attempted. Once constructed, this CRDS system will be thoroughly characterized prior to
use for its intended application. In order to demonstrate the capabilities of the CRDS
system, explosive compounds will be used in a variety of tests. This class of compounds
was chosen since they are traditionally difficult to detect at trace levels in the vapor phase
and would pose a significant challenge. Also, due to recent world events, there is an
urgent need for a reliable and accurate explosive detector, an application for which this
device is well suited. While the immediate application of this system is toward the
detection of explosive vapors, other applications such as atmospheric and trace gas
detection will benefit from such a system. The aims of this proposal will be two-fold:
Firstly, a CRDS system will be constructed that will allow for greater selectivity and
sensitivity than currently published instrumentation. Secondly, the system proposed will
result in an explosive vapor detection device offering detection limits likely to surpass
those found with current methodologies.
1
Section B - TABLE OF CONTENTS
For font-size and page-formatting specifications, see GPG Section II.C.
Total No. of
Pages in Section
Section
Page No.*
(Optional)*
Cover Sheet (NSF Form 1207) (Submit Page 2 with original proposal only)
A
Project Summary (not to exceed 1 page)
1
1
B
Table of Contents (NSF Form 1359)
1
2
C
Project Description (including Results from Prior NSF Support)
(not to exceed 15 pages) (Exceed only if allowed by a specific
program announcement/solicitation or if approved in advance by the
appropriate NSF Assistant Director or designee)
18
3
D
References Cited
5
21
E
Biographical Sketches (Not to exceed 2 pages each)
N/A
N/A
F
Budget
(NSF Form 1030, plus up to 3 pages of budget justification)
4
23
G
Current and Pending Support (NSF Form 1239)
N/A
N/A
H
Facilities, Equipment and Other Resources (NSF Form 1363)
N/A
N/A
I
Special Information/Supplementary Documentation
1
30
J
Appendix (List below)
Include only if allowed by a specific program announcement/
solicitation or if approved in advance by the appropriate NSF
Assistant Director or designee)
N/A
N/A
Appendix Items:
*Proposers may select any numbering mechanism for the proposal. The entire proposal, however, must be paginated. Complete
both columns only if the proposal is numbered consecutively.
NSF Form 1359 (10/99)
46
2
Section C – Project Description
Background
Traditional Fourier transform infrared spectroscopy (FT-IR) thus far has proven
unable to detect trace levels of explosives. Although FT-IR is a very selective technique,
it lacks sensitivity. Hypothetically, if the sensitivity could be significantly increased, IR
spectroscopy would be a very powerful tool for not only trace explosives detection, but
also trace gas detection in general. Cavity ringdown spectroscopy (CRDS) will be used in
this proposal to construct an IR spectrometer capable of achieving high sensitivity. This
system will also implement multiple broadband laser diodes as sources to scan windows
of the IR spectrum to retain the selectivity of IR spectroscopy. Such a CRDS system has
not yet been attempted. The capabilities of this system will be demonstrated on explosive
compounds. These materials were chosen since they are traditionally difficult to detect in
the vapor phase and there is currently and urgent need for an explosive detector that
functions with both high sensitivity and selectivity. These experiments will demonstrate
the system’s sensitivity by determining limits of detection. Additionally, the selectivity
gained by using broadband laser sources will be shown through testing various mixtures
of explosive compounds.
Cavity ringdown spectroscopy has been previously suggested as a means to a high
sensitivity technique in the mid-IR region[1]. CRDS deviates from classical absorption
spectroscopy by measuring the time required for incident light to decay in a cavity rather
than simply the absorption at a given wavelength[2]. The instrumentation can be custom
fabricated to operate at any wavelength from the ultraviolet to far infrared[3].
The optical design for CRDS is relatively straightforward. In a simplified setup,
as shown in Figure 1, the laser source injects monochromatic radiation into an optical
cavity formed with two highly reflective end mirrors. Incident light reflects between the
mirrors creating a large effective path length. Emerging radiation is detected and
digitized with an oscilloscope and sent to a computer for analysis. With each successive
pass, the detected light intensity diminishes exponentially.
Laser
Cavity &
Mirrors
Detector
Oscilloscope
Figure 1. Simplified block diagram of a cavity ringdown experiment. Adapted from Ref 4
The rate of change in intensity with time inside an empty cavity can be expressed
the differential equation
dI Tc
(1 − R )c
I=
=
I
(1)
dt
L
L
where I is the intensity, T is the transmittance of the mirrors, c is the speed of light, L is
the cavity length and t is time. The transmittance can be substituted with the reflectivity,
R, of the mirrors. Here, the per pass intensity loss is given as TI, where the per pass time
is L/c.
3
Solving equation 1 leads to the first order exponential decay
− c (1− R )t
I = Ioe L
(2)
where Io is the incident intensity outside the cavity. The time constant for this equation is
defined as the time taken for the intensity to decay to e-1 of its original value. This time,
called the ringdown time, is expressed as
L
τ=
(3)
c(1 − R )
By determining the ringdown time in an empty cavity, the reflectivity (or
transmittance) of the mirrors can be obtained knowing only the length of the cavity. This
self calibration feature is a large advantage for CRDS over other absorption methods.
Equations 1-3 assume that no absorbing compounds were present and all intensity
losses were due to the transmittance of the mirrors. Once absorbing species are
introduced into the cavity, the intensity decays at a faster rate and the rindgown time is
decreased. Since all cavity losses are additive, modification is straightforward. Equation
2, modified to account for the presence of absorbing molecules, can be written
− c (1− R +αL )t
L
I = Ioe
(4)
where, α is the fractional absorption loss in cm-1. Consequently, the ringdown time can
also be modified from equation 3.
L
τ=
(5)
c[(1 − R ) + αL ]
Absolute concentrations can be determined from the absorption loss by examining
Beer’s Law. The fractional absorption loss is the absorbance occurring per unit length
inside the cavity. Therefore the absorbance, A, is simply this coefficient multiplied by the
cavity length in cm. This quantity can then be set equal to the familiar Beer’s Law
equation
A = αL = εLc conc
(6)
where ε is the molar absorptivity, L is the path length and cconc is the concentration of the
absorbing species. Substituting equation 6 into equation 5 results in an expression which
allows the concentration to be determined directly from the ringdown time knowing the
experimentally determined molar absorptivity beforehand.
L
τ=
(7)
c[(1 − R ) + εLc conc ]
The largest advantage gained with the use of CRDS is its sensitivity. Equation 6
shows that the absorbance is linearly proportional to path length. Although the actual path
length of the cavity is not exceptionally large, the effective path length is very large. For a
given concentration, a large increase in the path length will result in a larger measured
absorbance. Therefore, because of the long path effective path lengths observed with this
technique (often exceeding several kilometers) very low absorptions can be detected[4].
This proposal will address the growing concern that current CRDS experiments
involve conditions that do not allow the technique to be used to its fullest capabilities.
The current literature is filled with reports of CRDS used to examine only a single
absorption with very high sensitivity. Because this is the case, typical narrowband laser
sources such as the Nd-YAG, which offers a tunable range as low as 20nm, poses no
4
problem[5]. Laser diodes which have a typical tunable range of several hundred
wavenumbers, translates into a mere 25-30nm range in the visible where they are most
often used[6]. It is the opinion of some in the spectroscopic community that CRDS offers
much potential as a spectroscopic technique and confining it to such a small operational
spectral window is limiting its progress. Attempts have been made to create broadband
sources through optical parametric oscillator and amplifier assemblies (OPO/OPA),
which alter the wavelength based on the angle of incident light to the OPO crystal, and
other novel techniques[1,7-9]. Although larger spectral ranges were produced, the
increase was not significant and applications were still restricted to monitoring a single
peak. This proposal will diverge from these attempts and provide a true broadband CRDS
system by using two tunable laser lead salt diodes each capable of covering 1µm in the
mid-IR range. Such a wide optical range for CRDS in the mid-IR has not yet been
reported in the literature. The proposed instrument would offer greater selectivity by
having the capability to detect several absorption bands over a wide range rather than a
simply a single absorption. The use of multiple diode sources has been accomplished
previously. However, both diodes were nearly the same wavelength and had severely
limited tunability[10].
CRDS was first developed in 1988 by O’Keefe and Deacon and rapidly found
applications in trace gas detection[11-13]. Initially, CRDS was most widely used in the
UV and visible regions of the spectrum since laser sources were widely available and
most reliable in these wavelengths. CRDS in the mid-IR emerged soon after the
technique was first developed[8]. Trace gas analysis in this region by CRDS produced a
much wider array of analytes since most gases of small molecules absorb strongly at
these wavelengths. Several low-molecular weight atmospheric and potentially toxic
molecules have been detected at sub-part per billion (ppb) levels in the mid-IR[14-18].
However, the application of CRDS to the detection of explosives in the mid-IR has yet to
be reported.
Initially, CRDS implemented pulsed lasers as sources. However, the use of
continuous wave (CW) lasers provides narrow linewidths which often results in lower
detection limits over pulsed sources. Estimates of the minimal detected absorption loss
are at least an order of magnitude less with CW lasers than with pulsed sources[19]. The
use of CW lasers has also allowed the implementation of less expensive but quite reliable
laser diodes as sources. Semiconductor diode lasers have been used widely in the field of
spectroscopy since they have micrometer dimensions, provide high resolution and output
power, and provide a relatively wide tunable range[20,21]. They have only recently been
introduced to the mid-IR range with the advent of the lead-salt (Pb-salt) configuration.
With this diode, the composition of an alloy of lead and other elements such as tin,
selenium and sulfur determine the output wavelength of the laser[22]. IR laser diodes are
advantageous sources since they produce highly monochromatic light at the center
wavelength and are tunable over a 200cm-1 range. Although very versatile sources, these
Pb-salt laser diodes only are able to produce ~0.25mW of output energy. Cryogenic
cooling to liquid nitrogen temperatures is also required in order to reduce their
conductivity. Despite these negative qualities, they have found widespread applications in
the spectroscopic determination of trace gas analyses in non-CRDS spectroscopy
[17,23,24]. This proposal will be the first to implement Pb-salt laser diodes as sources
for a CRDS system.
5
In general, CW laser diodes have been used in the visible and near IR ranges for
CRDS[25,26], but they have not yet been utilized in the mid-IR. Other laser sources have
been used for this region. The use of the Nd-YAG pulsed laser source combined with
OPO and OPA assemblies are often used to generate broadband mid-IR radiation
sources[1,8]. Unfortunately, these assemblies are complex and require significant space
for their operation. CO2 lasers have also been reported to be used[27], but their limited
tunability makes them less attractive. Quantum cascade lasers have recently been
introduced in the literature as high power laser sources for IR-CRDS often generating
upwards of 15mW in the mid-IR[15,16,18]. However, they offer a small operation range
(15-20nm), require cryogenic cooling for continuous wave operation, and are
extraordinarily expensive since they are not yet widely commercialized. In comparison,
Pb-salt diode lasers are highly tunable, occupy minimal space with cryogenic cooling and
offer light intensities adequate for spectroscopy.
Explosives Detection
The chemistry behind the detection of explosive vapors has received much
attention due to recent events around the globe. While several of the most widely used
explosive compounds were discovered over a hundred years ago[28], their detection in
trace amounts still poses a significant challenge to most current analytical equipment.
Therefore, there is an urgent need for instrumentation that can detect commonly used
explosives at trace levels, with a high degree of accuracy, within a small timeframe, and
without significant cost. This proposal will address these issues through the modification
of existing high resolution spectroscopic techniques.
Explosive molecules, organic and inorganic, exist in many forms with most
containing multiple nitro groups[29]. Additionally, they exhibit extremely low vapor
pressures, creating a dilemma for vapor detection systems. Table 1 shows structures and
vapor pressures of three common organic explosives used for this proposal. These three
were chosen since they are currently used in the vast majority of military plastic
explosives[30]. Their low vapor pressures create difficulties in sampling and detection by
providing small vapor sample sizes[31-33]. Particle sampling is equally as difficult since
these materials are likely to be concealed. It has been estimated that the simple wrapping
of explosives will cause the vapor pressure to decrease by as many as three orders of
magnitude[29]. The significant challenge in explosive sampling is realized by the fact
that the saturated vapor pressure of RDX at room temperature is nearly equal to the
detection limit of many commercially available devices[34]. However, since explosive
compounds have strong adhesion properties, the likelihood of surface contamination is
high[35]. It can also be seen from Table 1 that the vapor pressures increase dramatically
with temperature in accord with previously determined vapor pressure equations[36].
Here, a 50°C increase results in a vapor pressures increase of two to three orders of
magnitude. Therefore, sample sizes can be significantly increased from surface
contamination and higher temperatures.
The most common fragment technique available is ion mobility spectrometry
(IMS)[37-41]. Despite high sensitivity, IMS has been shown to lack selectivity[31,42].
Mass spectrometry has also been widely and has produced excellent results in detection
6
sensitivity[43]. However, it also has been reported to have low selectivity since unique
fragmentation patterns are not found with small nitrate ester explosives such as
PETN[29]. In order to increase the selectivity of these techniques, gas chromatographic
(GC) separation is used prior to fragment detection[44,45]. This adds cost and consumes
significantly more space and power than the original instruments alone. Fragment
techniques including IMS, MS as well as GC/IMS and GC/MS are capable of detecting
low picogram quantities for commonly used explosives. GC may be used alone, though
detection limits are often found to be one to two orders of magnitude higher[46].
Table I - Names, Structures, and vapor pressures for explosive compounds used in this proposal
Vapor Pressure
298K (ppbv )
Vapor Pressure
348K (ppbv )
2,4,6 Trinitrotoluene
9.5
4.2x103
Pentaerythritol
tetranitrate
1.8x10-2
56
1,3,5-Trinitro-1,3,5triazacyclohexane
6.0x10-3
7.9
Acronym
Name
TNT
PETN
RDX
Structure
Vapor pressures calculated from ref 36.
Other detection methods have been attempted with success such as surface
acoustic wave devices[47,48], and fluorescence detection[49]. However, these methods
have only detected higher vapor pressure explosives such as mono and dinitrotoluenes
and are not relevant for comparisons to data regarding the explosives used in this study.
Spectroscopic explosive detection has produced several attempts at trace
detection[50-55]. Analysis in the mid-IR with surface enhanced Raman spectroscopy has
resulted in picogram detection limits for TNT[56]. FT-IR, modified with an extended
path length of 13m, has been used to detect TNT and other explosives in soil
samples[57]. However, the detection limit for TNT was reported as 80µg – far too high
for trace detection.
Frequency modulation spectroscopy, a high sensitivity derivative of IR
spectroscopy, has been used by Riris and coworkers to monitor the trace gases NO, N2O,
and NO2 with three tunable Pb-salt diode lasers, each centered on one absorption
peak[58]. Using this technique, 5 pg of RDX was detected. Similar to common CRDS
experiments, only certain wavelengths were monitored each within a small range since
the application was so specific. Since data was given for RDX only, it is assumed that no
mixtures and hence, no tests dealing with selectivity were conducted.
7
Only once has CRDS been used to detect explosives. Usachev and coworkers
used pulsed UV CRDS to detect TNT at a 1 ppbv, or 7.5ng/L, detection limit[59].
Additionally, since this experiment was performed in the UV region, certain fundamental
flaws were encountered. First, the number of explosive analytes is limited in the UV
range since only aromatic nitro compounds such as TNT experience sharp absorptions,
whereas aliphatic compounds exhibit significantly weaker absorptions in the same
region[36]. Generally, absorption features in the UV are featureless and broad, where IR
absorptions are sharp, well defined and characteristic of molecular structure (not
aromaticity), allowing for greater ease in identification. Secondly, the reflectivity values
of the cavity mirrors are relatively poor in the UV region[2] with values rarely exceeding
99.5%. Mirrors intended for IR-CRDS achieve reflectivity values of 99.99% regularly[8].
Similar to the previous experiment involving explosives, no tests with mixtures or other
interferents were undertaken. Although the absorption cross section of the nitro group is
larger in the UV than in the IR (1x10-16 vs. 4x10-17 cm2[58]) resulting in greater
sensitivity according to Beer’s Law, the relatively poor mirror reflectivity values
contribute to an overall less sensitive instrument compared to the proposed design. This
will be further discussed in the experimental section.
Aside from these two experiments, spectroscopic measurements of explosives
have not been studied in detail. Current experiments demonstrate instrument sensitivity
and neglect to mention selectivity in detail, if at all. If spectroscopic determination of
explosives will ever be conducted in real world applications, steps must be taken to try
and improve upon current methodologies. This proposal will address this concern. This
proposal will combine the selectivity from traditional IR spectroscopy with the sensitivity
of CRDS to construct an instrument capable of trace explosives detection.
This proposal will accomplish the following goals:
1. The construction of a cavity ringdown spectroscopy system more versatile
than currently published instruments by providing a wide operational window
2. Detection of the explosives TNT, RDX, and PETN with the CRDS system at
trace levels and subsequent selectivity tests with interfering compounds
Experimental
Optical Design
The optical setup of the CRDS experiments to be used for detection limit tests is
shown in Figure 2. Further tests demonstrating selectivity will be tested in a similar
system using two diode laser sources. Pb-salt laser diodes produce continuous wave laser
light over a range of 200cm-1, or 1µm in the mid-IR, at 0.25mW. Since these diodes
require cryogenic cooling for semiconductor operation, a liquid nitrogen dewar with a
software controlled temperature controller is required. The controller offers high
precision temperature stabilization within 0.1mK; necessary since the diode wavelength
output is proportional to temperature. The output beam of semiconductor laser diodes is
8
elliptical [22] due to the small dimensions of the active layer compared to the emission
wavelength [60]. A gold coated parabolic mirror placed just outside the laser dewar
corrects the divergence and produces a collimated beam approximately 14mm in
diameter. At the center wavelength, the Pb-salt laser output beam is monochromatic.
However, lasing may occur at other wavelengths. In order to avoid this multi-mode
output, a miniature monochromator, situated directly after the laser dewar will ensure
monochromatic output over the entire diode range. The monochromator is also software
controlled and designed to operate in conjunction with the dewar, allowing synchronous
operation.
Mode Matching
Optics
MCT
Detector
Cavity
Oscilloscope
PZT
5x
Telescope
Focusing
Lens
Gas Inlet/Outlet
PC
Monochromator
Trigger
Circuit
AOM
Diode Laser
LN2 Dewar
Focusing
Lens
Figure 2. Optical Design to be used for this proposal. Setup includes a single Pb-salt diode
source. Further experiments will employ a second source with accompanying hardware.
A CaF2 plano convex lens then focuses the large beam diameter into the 1mm
aperture of an acousto-optic modulator (AOM) which creates a pulse train, essentially
transforming the CW laser to pulsed operation. The AOM also functions as a switch,
capable of disrupting the laser beam periodically, preventing laser light from entering the
cavity. The operating premise of CRDS requires the incident laser light be interrupted to
allow the measurement of clean ringdown events[61]. The AOM causes this interruption
by driving an acoustic wave through a high refractive index germanium crystal. The
propagating wave causes the refractive index to sinusoidally change, diffracting the
incident light and allowing periodic laser pulses to enter the cavity[62]. These devices
have become standard components for CW CRDS instrumentation[63-65]. A 60MHz
AOM will be used for this proposal, which will have 16nsec between deflections.
Flat gold mirrors will redirect the beam from the AOM across the optical table.
The laser light emanating from the AOM will be divergent since it was focused down to a
small diameter relative to its incident size. In order to correct for this aberration, a
Galilean 5x telescope composed of a plano-concave lens and a plano-convex lens will
collimate the beam. This expander both enlarges the diameter and reduces the divergence
angle of the beam by a factor of five. After collimation, an examination of the beam will
reveal several cross sectional energy profiles or transverse modes[66]. It is beneficial to
mode match the lowest order transverse electric mode (TEM00) for this application since
9
higher order modes have larger energy profiles which experience increased diffraction
losses[67]. Mode matching is a process that isolates a given mode from others present.
This will be accomplished with two 100mm focal length CaF2 plano-convex lenses, and a
100µm diameter pinhole. The incident light is focused though the pinhole by the first
lens. Higher order modes do not pass through the pinhole since their high energies are
found at larger diameters. Figure 3 indicates the TEM00 mode has higher energies at
locations where higher order modes exhibit nodes. Once through, the laser beam will be
expanded and re-collimated with the second lens. Since no further optical manipulation
of the beam is required, the 5mm diameter beam then enters the laser cavity.
Once mode matched, the beam enters the laser
cavity through the rear of one of the cavity high
reflectivity mirrors. These spherical mirrors are composed
of as many as 20 layers of alternating high and low
refractive index dielectric materials each one quarter of a
target wavelength thick[67]. The rear of the first mirror
will be coated with an antireflective layer of magnesium
fluoride to prevent incident light from reflecting. Although
these mirrors possess reflectivities higher than any metal
Figure 3. Cross-sectional energy mirror in the IR range, the main disadvantage of these
profiles of transverse modes. dielectric mirrors is that they have a limited range of 15%
Taken from Ref 67
about the center wavelength[13]. The custom made mirrors
will be one inch in diameter, possess a six meter radius of curvature, and will possess
reflectivites ≥99.97%, thus resulting in a stable resonator cavity[68].
The laser cavity itself, or optical resonator, will be custom built with stainless
steel capable of withstanding a vacuum in the milliTorr range. The cavity will have a
50cm length, an internal volume of 7L, and ports for gas inlets and outlets as well as for
pressure measurement. Temperatures inside the cavity will be controlled with two large
heating tapes ensuring that the majority the cavity surface will be covered and thus heated
in a uniform manner. The cavity will have the capability to hold two sets of mirrors side
by side. Each end mirror will be mounted on a piezoelectric transducer (PZT) which will
continuously scan the mirror back and forth in order to acquire an axial mode or cavity
resonance. These modes are obtained when the incident light travels an exact number of
half wavelengths from one end of the cavity to the other, creating a standing wave inside
the cavity[67]. When this occurs, a significant amplification of the light intensity is
observed given by the reciprocal of the transmittance of the mirrors. A steady state is
reached during a cavity resonance where the incident light into the cavity is the same
intensity as the exiting light. Normally, the incident light is reduced by the transmittance
of the first mirror and again by the second. For a cavity where both mirrors have a
transmittance of 0.001, incident light is reduced by six orders of magnitude before
reaching the detector. In the case where a cavity resonance is reached, no such decease is
observed.
Once the light exits the cavity through the end mirror, it is focused onto a liquid
nitrogen cooled mercury-cadmium-telluride (MCT) photodiode detector with a 0.5mm2
active area. This detector is optimized for operation in the 6-8µm region and has a
response time of 7nsec. Between the detector and the AOM is a trigger circuit which
monitors the detector response due to the scanning of PZT. When an axial mode is found,
10
the detector response increases above a threshold limit and signals the AOM to shut off
and stop the pulse train from entering into the cavity. This allows for a clean ringdown
event to be observed[18,63]. The ringdown signal from the detector is digitized by a twochannel 500MHz oscilloscope and sent to a personal computer for analysis of ringdown
times and subsequently absorption loss data.
Cavity Characteristics
For this optical design with a mirror reflectivity of 0.9997 (T = 0.0003) and a
cavity length of 50cm, the anticipated ringdown time is calculated to be 5.6µsec using
equation 3. Further, knowing that the round trip time for this cavity will be 3.3nsec
(2L/c), 1667 round trips will be made during the ringdown time, producing an effective
path length of 1.6km.
The Pb-salt laser diodes will produce an intensity of 0.25mW. However, this
intensity will not reach the cavity since the beam travels through the AOM. Assuming
that 25% of the photons actually pass through the AOM toward the cavity, 62.5µW will
impinge on the cavity. During a cavity resonance, 1/T of this energy will be observed, or
0.208W. Since a steady state is reached, 62.5µW will exit the cavity toward the detector.
During the recording of a ringdown event, this value will be the initial detected energy.
When the ringdown time ends, a value corresponding to e-1 of the initial energy will be
observed, or 23µW. This value will be the smallest recorded for quantitative purposes,
easily detected by the MCT detector which has a responsivity of 40Volts/mWatt. With
this responsivity, 23µW corresponds to roughly a 1V detector response which is of no
concern to any experiments conducted in this proposal.
The operating procedure calls for a cavity resonance to be found in order to reach
a steady state and record a ringdown event. A buildup of photons is required for this
phenomenon and depending on the reflectivity of the mirrors; the time taken for this
buildup may vary. The change in the number of photons with time can be written as
dN
= F (1 − R ) − N 1 − R 2 c
(8)
dt
where F is the number of photons incident on the cavity, R is the reflectivity of the
mirrors, N is the number of photons in the cavity and L is the cavity length. Setting this
equation to zero (steady state involves no change) and solving this equation for N
(assuming 25% of the laser power from the diode passed by the AOM) gives the number
of photons in the cavity at steady state. A calculation of the time required to buildup the
correct number of photons involved integrating both sides to result in a first order
exponential growth equation. This results in a buildup time of roughly 1 millisecond.
Therefore, hundreds of ringdown events can be run in less than a second to be averaged
in order to improve the overall signal to noise ratio.
Because detection limits for several compounds will be determined through this
proposal and Beer’s Law will be observed, it is critical that the minimal absorption be
determined prior to any experimentation. Because the ringdown time decreases with
increasing absorber concentration in the cavity, (Figure 4) the minimal absorption would
then correspond to the minimal change detected in the ringdown time and can be
determined by the equation
(
11
)
αL = (1 − R )
∆τ
(9)
τ
where ∆τ is the minimal change in the ringdown time than can be quantified[69]. Since
the response time of the MCT detector is 7nsec, the smallest change in the ringdown time
will be this value. Substituting all previously established values and solving for the
absorption loss, α, the value calculated is 8x10-9 cm-1. Converting this to an absorbance
measurement yields 4x10-7 as the minimal detectable absorbance for this CRDS system.
This value corresponds well to those found with other mid-IR CRDS systems [15,18,27].
0.06
0.05
α=0
α = 1e-5
I (mWatt)
0.04
0.03
0.02
0.01
0.00
0.0
5.0e-6
1.0e-5
1.5e-5
2.0e-5
2.5e-5
3.0e-5
Time (sec)
Figure 4. Theoretical ringdown curves for empty cavity (solid) and cavity with absorber with
fractional absorption loss of 1x10-5 (dashed). Curves calculated from equation 2. Horizontal line
indicates intensity at the ringdown time and vertical drop lines indicate the ringdown time value.
FT-IR Investigations of Explosives Spectra
While the IR spectra of explosives have been well studied in the condensed
phases[50,55], such studies in the vapor phase are rare. Only a few accounts in the
literature of low resolution gas phase spectra at ambient and elevated temperatures
exist[70-72]. Additionally, even less information is available regarding the effects of
temperature and pressure. Scans run at higher than optimal temperatures encounter
decomposition of the target molecules, adding unwanted peaks to the spectra where lower
than optimal temperatures result in lower peak heights[72]. Although not as critical as
temperature, the correct pressure must be reached in order to obtain high resolution
spectra. Vapor phase spectra commonly produce more peaks than found in condensed
phase spectra due to the limited molecule interactions common in the gas phase[73].
Reduced pressure allows closely separated peaks that would otherwise be visible as one
collective peak to be resolved. For small molecules, pressures of 0.25atm are routinely
used. However, for heavier molecules, pressures lower than 1 torr are required to bring
about sharper spectra[74].
Because high resolution FT-IR studies of explosive molecules are non-existent,
the first undertaking of this proposal will be to create reference of IR spectra of these
12
molecules detailing all absorptions throughout the mid-IR region. This study will aid in
the selection of wavelengths for both sensitivity and selectivity tests discussed later in
this proposal. A stainless steel heated gas sample cell with a 10cm length and potassium
bromide windows capable of operating under reduced pressures and at temperatures as
high as 250°C will be used. Since the limits of detection are not the focus of these
experiments, the amount of sample used for each compound is not critical but should be
consistent and substantial enough to yield acceptable spectra. Samples several milligrams
in mass will be adequate. Quantities of individual explosives dissolved in acetonitrile will
be placed inside the cell. After solvent evaporation, the temperature will be raised to
increase the vapor pressure of the explosive compound as seen in Table 1. Vapor
pressures at a given temperature can be calculated for each of these species from
predetermined equations.
Although the CRDS experiments will not involve temperatures above 60-80°C
for reasons to be discussed later, FT-IR spectra at temperatures in excess of 100°C will
be collected in order to increase the vapor pressure to observe spectra. FT-IR spectra will
be obtained at temperatures beginning at 60°C and progress to higher temperatures until
decomposition is observed (~150°C). During these tests, the cell will be connected to a
two stage rotary vane pump to observe the effect of lower pressures on the spectra. Since
many vibrational states are predicted to be seen with these relatively large molecules,
pressures lower than 1 torr will likely to be required to bring about these features.
Previous work with IR spectra of high explosive compounds employed pressures as low
as 10-4 torr[72]. It is also entirely likely that these features may present problems to future
selectivity tests since too many absorptions may be observed per molecule. Pressures
higher than 1 torr may be used to simplify the spectra when mixtures are run. The initial
spectrometer resolution will be set at 1cm-1. However, this value may be lowered with the
initial experimental results. A minimum of 32 scans of each sample will be attained to
provide an average result for each scan.
Characterization of the CRDS system
Once the CRDS system is constructed, it will require characterization to confirm
proper working order prior to tests with explosives. Optical alignment will be carried out
with a 0.5mW HeNe laser with a wavelength output at 632nm. The CaF2 optics chosen
for this proposal will be able to accommodate this wavelength without issue. Following
proper alignment, the working range of each mirror will be determined as well as their
reflectivities at each wavelength. Since the manufacturer quotes only the center
wavelength reflectivity and a working range of ±6%, more accurate values are required
for high sensitivity experiments. The reflectivity at a given wavelength can be found
through equation 3. Initially, the ringdown times at each wavelength will be determined
with a cavity filled only with high purity nitrogen by scanning the diode laser through its
entire working range. Because the output of the laser diodes is temperature dependent,
time is required for the liquid nitrogen dewar to reach the appropriate temperatures.
Therefore, several minutes are required for a complete scan of the diode’s working range.
Characterization of the laser diodes will not be required since the manufacturer provides
detailed reports regarding the working range and performance at each wavelength.
13
All explosive molecules that will be tested in this proposal contain multiple nitro
groups which exhibit very strong absorptions due to the symmetric and asymmetric nitro
stretches occurring at roughly 1349 and 1559cm-1, or 7.4 and 6.4µm, respectively. These
bands are typically the strongest bands in the spectrum of any explosives molecule, with
the symmetric being the stronger of the two, because of the large NO2 absorption cross
section. Previous IR experiments involving explosives detection have used these features
for quantitative purposes[51,57]. However, before exposing explosive vapors to the
cavity, preliminary tests of the CRDS system will be preformed using a relatively
concentrated (part per million) nitrogen dioxide (NO2) standard in high purity nitrogen
gas. Initially, spectra will be obtained using the NO2 to ensure that the CRDS system is
fully operational and performing optimally. Measurements will be initially taken at
ambient pressure with a nitrogen background. A calibration plot will be constructed with
this mixture diluted with additional high purity nitrogen to obtain lower concentrations in
the low part per billion range to gauge the linear range of the CRDS system. Flow rates
for these experiments will be controlled with flowmeter regulators to ensure accurate
partial pressures are used. Continuous gas flow through the laser cavity will be
accomplished with the aid of a two stage rotary vane pump. The resulting spectra will be
compared to the HITRAN molecular absorption database (http://www.hitran.com) for
nitrogen dioxide to confirm the CRDS system is producing accurate results. Following
these results, lower pressures will be used to simulate the experimental conditions for
explosives detection. Since nitrogen dioxide is a small molecule, pressures of ~190 torr
(0.25atm) will be sufficient to observe high resolution vibrational spectra. With reduced
pressure tests, the cavity will be filled with the target gases, pumped down to the
appropriate pressure and measurements will be made. Similar to the FT-IR spectra, these
experiments will also initially use a resolution of 1cm-1.
The theoretical detection limit for NO2 can be calculated by first determining the
minimal number density observed. This figure can be calculated from the equation
N min = σα min
(10)
-3
where Nmin is the number density (molecules cm ), s is the absorption cross section of the
measured molecule (cm2), and αmin is the minimal absorption loss as calculated from
equation 9. Using a cross sectional value of 4x10-17cm2 [58], and an absorption loss of
8x10-9cm-1, Nmin is found to be 2x108 molecules cm-3. Once converted to the number of
moles, the number density becomes 3.32x10-13 moles/L, or 0.015ng/L NO2. Accounting
for the 7L volume of the cavity, 0.10ng can theoretically be detected. Due to this small
detection level, it is doubtful that peaks corresponding to these concentrations will be
directly observed largely due to inaccuracy in dispensing concentrations of such small
magnitude. Therefore, the limit of detection will be calculated based on the value that
corresponds to three times the standard deviation of the signal to noise ratio. However,
the limit of detection will be approached as closely as accurately possible with dilutions
of the nitrogen/nitrogen dioxide mixture.
The linear range of NO2 will be calculated on the basis that the ringdown time
decreases with increasing concentration. At a given concentration, the ringdown time is
faster than the detector response time. Assuming a ringdown time of 10nsec (MCT
detector response time is 7nsec), equation 3 shows the maximum fractional absorption
that can be detected as 3.3x10-3 cm-1. Comparing this to the minimal value gives an
expected linear range of approximately five orders of magnitude.
14
Initially, a blank ringdown time will be acquired. This will be performed through
several repeat scans of a cavity filled only with nitrogen. A minimum of 64 scans
(increasing the signal to noise ratio by a factor of 4) will be recorded and will be
averaged together. According to the buildup time calculated in equation 8, 64 scans will
take significantly less than one second to perform. With a symmetric peak width of
~100cm-1 and an initial resolution of 1cm-1, it will take roughly 7 seconds to record all
ringdown times for the entire symmetric peak assuming the laser diode temperature
ramping is negligible. However, this is not the case. Response times for the liquid
nitrogen cooled dewar are likely to be 1-2 minutes for a 100cm-1 scan. The limiting factor
in the data acquisition rate is the laser diode and not the spectroscopy. Likewise, when
NO2 is introduced into the cavity, the same experimental timeframe will be applicable.
Explosives Detection
The sensitivity of this CRDS system will be tested by determining the minimal
detectable limit of three low vapor pressure explosives shown in Table 1. Only one diode
laser source will be required for this work since only the strongest absorption peak will
be used for such experiments.
Once the CRDS system is fully characterized and operation is deemed optimal,
tests on explosive vapors will be conducted. The data concerning optimal sampling
temperature and pressure obtained from the FT-IR experiments will be applied to each of
the three species. Explosive vapors will be introduced directly into the resonator cavity
through the use of an explosives vapor generator custom built by the Idaho National
Engineering Laboratory (INEL). This system delivers precise vapor quantities of TNT,
RDX and PETN within a range of 50pg to 1ng with a pulsed delivery method and has
been used in previous experiments[58]. Since the output is simply a mass value, the
concentration of the pulse is the pulse mass over the cavity volume. The duration of the
pulse increases with the amount of vapor desired. Each of the three explosives has its
own vapor head to avoid cross-contamination and only one explosive can be dispensed at
a time. The vapor is generated from a reservoir of a known quantity of analyte in a
stainless steel chamber. The chamber is heated or cooled according to a built-in CPU
controller and has a maximum internal temperature of 60°C. In response to the chamber
temperature change, a mass quantity of vapor is generated according to previously
established vapor pressure equations. Although the transfer lines are not silanized, INEL
maintains the continuous flow of carrier gas prevents adhesion.
The cavity will be filled with high purity nitrogen gas and pumped down to the
appropriate pressure. The maximum temperature inside the cavity will be 75°C. If the
temperature were much lower, vapors inside the cavity would likely condense when
emitted from the 60°C generator. Higher temperatures would impede the use of the PZT,
which has a upper limit of 85°C. Prior to filling the cavity with analyte vapor, blank
ringdown times will be obtained at each wavelength identical to the method employed
with NO2. The INEL vapor generator will inject a pulse of vapor into the cavity for
measurement. After a minimum of 64 ringdown events are recorded and averaged for
each wavelength, the pressure will be increased to ambient levels with nitrogen while the
pump simultaneously evacuates the chamber of explosive vapors. A dry ice/isopropanol
15
cold trap will condense explosive vapors to later be dissolved in solvent and disposed of
in waste receptacles.
During all measurements, the cavity will be purged of ambient air and high purity
dry nitrogen will be used to avoid water and carbon dioxide absorption interferences[73].
Calibration curves will be constructed with peak height versus mass or versus
concentration spanning the range of the vapor generator. Since the detection limit is
expected to be below minimum generator output of 50pg for each species tested,
detection limits will require calculation rather than spectroscopic confirmation according
to the method used for nitrogen dioxide.
Detection limit calculations similar to those performed on NO2 can be applied to
these thee explosives. Although larger molecules are studied here, the nitro group is still
the functional group of interest. Therefore, it is assumed that the nitro groups on these
molecules would have the same cross section as if they were separate species. This
results in the same minimal number density for all three explosive species (2x108
molecules cm-3). Conversion to mass per volume units results a theoretical detection limit
of 0.075ng/L for TNT, 0.105ng/L for PETN and 0.074ng/L for RDX. Converting these
three values to ppbv units results in a value of 0.009ppbv assuming a temperature of 75°C
and a pressure of 1atm. This sensitivity value is nearly identical to the vapor pressure of
RDX at these conditions (Table 1).
These results compare very favorably with the two previous spectroscopic
explosive detection experiments listed in the introduction section. Riris and coworkers
published a detection limit of 5pg for their experiment. However, since a cell of only
1cm3 was used, this corresponds to a detection limit of 5ng/L. This value is over 60 times
that value for RDX listed in Table 2. Usachev, using UV CRDS for TNT detection
claimed a detection limit of 7.5ng/L. Because their minimal number density is two orders
of magnitude larger than that calculated for this system, the overall detection limit is also
larger by the same value. According to theoretical predictions, this CRDS system would
offer the most sensitive spectroscopic detection of explosives in the current literature. In
comparison to other detection methods, Zhao published a TNT detection limit with MS of
0.3ng/L[43]. The theoretical value for the mid-IR CRDS is a factor of four lower than
that with MS.
The selectivity of this system will be examined by taking steps toward real world
sampling conditions. With these tests, a second laser diode will be added to the optical
set-up pictured in Figure 2. For a device that is intended to detect explosive vapors, it will
be a very rare occurrence that the sample will contain no interfering compounds in the
wavelengths of interest. Since this is the case, any detection device should be capable of
detecting target compounds even when several other compounds are present. To
demonstrate the selectivity of the CRDS system, initial tests will be competed with
simple tertiary mixtures of the three explosive compounds used for detection limit tests.
Since the vapor generator can only supply vapor of one explosive at a time, it
cannot be used for these experiments. Instead, explosives will be exposed to the system in
a manner similar to the method employed by Janni et al[72]. The sample will be place in
a sidearm of the laser cavity. In order to control the amount of sample tested, volumes of
standard solutions will be pipetted onto a glass slide. Glass was chosen because explosive
particles adhere very well to many materials including metals and glass[30,35]. Once the
solvent has evaporated, only the explosive mixture will remain. The slide with the
16
explosive material will then be secured in the sampling arm. Heat will be applied to this
area from a third heating tape allowing for thermal desorption of the explosive
compounds.
Because all three explosives exhibit strong absorptions in the 6.4 and 7.4 µm
regions due to the presence of nitro substituents, both qualification and quantification at
these wavelengths will be challenging. However, regions where each species absorbs
without interference from the other two would aid in quantification. This technique has
been previously used to determine mixture compositions of the three mononitrotoluene
isomers[54]. Condensed phase spectra show that such a region exists for TNT, PETN
and RDX between 9-10µm[50,55]. Although the available gas phase spectra in the
literature indicate that these absorptions remain for RDX and TNT, no data is available
for PETN. Although unconfirmed, it is likely this absorption occurs in the vapor phase.
The feasibility of this technique will be determined by the results of the FT-IR
experiments. However, these bands are far from the strongest in each spectra. This
method would achieve selectivity at the cost of sensitivity. Further, this method would
only give accurate results if no band overlap exists, which is unlikely.
A multivariate chemometic approach will be more suitable to determine the
presence and concentration of each component in the mixture. Classical least squares
(CLS) and inverse least squares (ILS) are common multivariate approaches to
quantitative spectroscopy[75]. CLS uses the entire spectrum for concentration
calculations which allows for higher accuracy over other methods that only utilize
portions of the spectrum. However, all compounds in a given mixture must be identified
and included in calibration runs. This alone precludes CLS from real world sampling
where samples contain unknown components. ILS is not a full spectrum technique, but
can accurately construct models for mixtures without a concentration value for every
mixture component. Partial least squares, or PLS, combines the full spectral analysis of
CLS with the ILS benefit of describing mixtures without full compositional
knowledge[76], hence, it is ideal for samples containing unknown components. Within
PLS, there are two separate algorithms, PLS1 and 2. The PLS1 algorithm has a higher
predictive accuracy than PLS2 since it calibrates each component individually, where
PLS2 calibrates all components simultaneously. With PLS1, each component is assigned
its own set of scores and loading vectors that are specific to that constituent. Due to its
greater accuracy, the mixtures will be analyzed using the PLS1 algorithm from the
MATLAB chemometrics toolbox.
The PLS-1 algorithm has been explained in detail previously[77]. Briefly, during
calibration, the absorbance and concentration data are each decomposed into two smaller
matrices consisting of scores o weighting factors and loading vectors. Loading vectors
correspond to a particular mixture component. When a sample is run, the process is
essentially run in reverse where the loading factors and scores are used to determine
concentrations.
CLS has been used previously to determine CO2, CH4, N2O and CO in
atmospheric samples with FT-IR[78]. In this work, there was little spectral overlap
allowing for a more simplified analysis. CLS in conjunction with FT-IR has also been
used to analyze tertiary mixtures of nitrotoluenes in standard soil samples where both
nitro stretches were the portions of the spectrum used for analysis [57]. Although the soil
samples were uncontaminated soils spiked with explosive molecules, the notion that CLS
17
can model three similar compounds based on a common absorption feature is key. In this
experiment, high temperatures were used with a spectral resolution of 1cm-1. Also,
ambient pressures were used, indicating that pressures of 1 torr, needed to observe
rotovibrational features are not necessary for quantitative analysis.
Experiments here will use the symmetric nitro stretch as one of the spectral
windows. The second window will be either the asymmetric stretch as the other or
another widow such as the 9-10µm window suggested above. All three may not be used
since the wavelengths of these windows are far enough apart that three laser diodes
would be required. The procedure for obtaining spectra will be identical to those used for
the sensitivity experiments where 64 scans will be averaged together to obtain a higher
signal to noise ratio.
Once the tertiary mixtures have been modeled, further experiments with an
“unknown” component will be attempted. Several options for an unknown are likely.
Firstly, a higher vapor pressure explosive such as ethylene glycol dinitrate (EGDN) may
be employed. Secondly, a side product from the synthesis of one of the three explosives
in this study such as 2,4-dinitrotoluene, or one of the mononitrotoluene isomers may be
used. Thirdly, a compound unlike any in the original mixture likely to be found in routine
analysis such as nicotine or caffeine could be used. These experiments will test the
usefulness of the PLS1 algorithm in quantifying explosives with additional compounds in
the mixture.
An initial concern regarding the use of explosive mixtures was the number of
vibrational states would be far too large to discriminate between multiple nitro stretches.
However, this is only foreseen as a problem if rotovibrational spectra are observed under
vacuum. It has been shown that nitro explosives can be modeled accurately with CLS at
atmospheric pressure. Therefore, it is expected that the same can be performed with PLS.
When actual field samples are taken, however, it is doubtful that PLS will be able to
discern nitro explosives over several interferents each at concentrations orders of
magnitude greater than the target species.
Timetable
The first year of funding will be principally used for the construction,
troubleshooting, and characterization of the CRDS system. Several required components
for the CRDS optical system will be custom made including the cavity mirrors and the
stainless steel cavity. It is estimated that the fabrication of these components will take
roughly 2-3 months time. Additionally, the vapor generator was quoted to take 3-6
months for construction since this is not an off-the-shelf item. During the time required
for custom fabrication, the experiments involving the optimal temperature and pressure
sampling values will be completed with FT-IR. Since these are a series of simple
repetitive tests, it is expected that this work will last no longer than the time required for
the CRDS components to arrive. Upon arrival of all components, the CRDS optical
components will be assembled and aligned. Assembly and troubleshooting is expected to
take 2-3 months. Following construction, the system will be thoroughly characterized to
ensure that all components function properly. Characterization of the components (mirror
reflectivities and working ranges) is expected to take 1-2 month’s time. Preliminary tests
of the functionality of the system will be acquired using the nitrogen dioxide gas. It is
18
estimated that the experiments for construction of calibration curves and the
determination of detection limits will take the remainder of the first year of funding.
The tests of sensitivity with three explosives will begin during the second year of
funding. These experiments will be expedited by the FT-IR tests for optimal temperature
and pressure sampling. During the tests for detection limits, the necessary equipment will
be ordered to allow multiple laser diodes sources to be used simultaneously. The
selectivity tests will require significantly more time to complete than the sensitivity tests
due to the sample sizes and the modeling with the PLS algorithm. The remainder of the
second year will be spent focused on this work. Publications and progress reports will be
prepared throughout the proposal funding period.
Impact
This research required to achieve the goals in this proposal will function as an
invaluable experience for graduate students and post doctoral researchers. The
construction and operation of a cavity ringdown system is considerable and will function
as a prime learning experience for students and supply them with problem solving skills
through the troubleshooting of the system once constructed. Since any number of
problems may arise during the time taken to bring the entire CRDS system to full
operation, original thinking and problem solving skills will be fostered. Although the
premise of the experimentation is essentially laid out beforehand, there remain many
details that will require contemplation of new and innovative ideas to accomplish the
goals set out in this proposal. In short, the work carried out for this project will allow
students and researchers to be better equipped for future endeavors in either industry or
academia.
One original aim of this proposal was to increase the usefulness and utility of the
field of CRDS by modifying the currently used instrumentation by essentially taking a
step toward a high sensitivity FT-IR. Since CRDS is so versatile and modular, the
applications of this CRDS system extend well beyond explosives detection. Atmospheric
chemistry and trace gas analysis for atmospheric pollutants and toxic industrial gases will
benefit from such a system. Regardless, the explosive detection and spectroscopic
communities will undoubtedly benefit from this design, since it incorporates the
necessary sensitivity and increased selectivity over other previously published CRDS
systems.
Future Directions
The construction of a high sensitivity IR spectrometer allows several avenues of
future experimentation to be pursued. The CRDS system could be applied to new
analytes such as pollutant gas detection. However, the most likely choice would be to
improve upon instrumentation. In particular, the future development of the quantum
cascade laser is quite promising. One major drawback to the proposed system is the fact
that the Pb-salt laser diodes require cryogenic cooling. The dewars take time to
equilibrate at each wavelength and occupy significant space. Recently, Beck reported on
19
a CW quantum cascade laser able to operate in the mid-IR at room temperature with a
limited wavelength range[79], thus removing all the size and power required for the
cryogenic cooling devices. An ultra-broadband pulsed output quantum cascade laser
covering the range from 6-8µm has also published[80]. Such a laser covering a large
window would decrease the complexity introduced to instrumentation from to multiple
sources. It is possible that within a number of years when these devices are optimized for
use in CW-CRDS and widely available for spectroscopic use, that the size of CRDS
instrumentation may decrease significantly.
Conclusions
The proposed mid-IR CRDS system, through theoretical calculations, is shown to
be able to detect sub part per trillion levels of explosives. In comparison to other
spectroscopic methods, this CRDS system is superior. When compared to a fragment
detection technique such as mass spectrometry, the proposed design offers a detection
limit slightly lower but for all intents and purposes, equivalent. The selectivity of the
system using tertiary mixtures is hypothesized to be successful assuming large numbers
of vibrational bands are not observed due to low pressures. The PLS1 algorithm is
anticipated to successfully model the responses for these mixtures and for those
containing an unknown compound. However, when applied to real world samples, it is
thought that this proposal would not meet the necessary requirements for two main
reasons. Firstly, numerous interferents with nitro substituents are possible in any
environment. Secondly, these compounds will exhibit concentrations large enough to
easily swamp out a signal from an explosive at a part per trillion level.
20
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Dahnke, H., Kleine, D., Hering, P., Murtz, M., Real-time monitoring of ethane in human breath
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42.
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59.
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25
FOR NSF USE ONLY
54
SUMMARY PROPOSAL BUDGET
YEAR 1
ORGANIZATION
PROPOSAL NO.
DURATION (MONTHS)
Tufts University
Proposed
PRINCIPAL INVESTIGATOR/PROJECT DIRECTOR
Granted
AWARD NO.
Stefan Lukow
A. SENIOR PERSONNEL: PI/PD, Co-PIs, Faculty and Other Senior Associates
NSF-Funded
Funds
Funds
List each separately with name and title. (A.7. Show number in brackets)
Person-months
CAL
ACAD
SUM
Requested By
Granted by NSF
1. Stefan Lukow
0.00
2.
3.
4.
5. (0) OTHERS (LIST INDIVIDUALLY ON BUDGET EXPLANATION PAGE)
6. (1) TOTAL SENIOR PERSONNEL (1-6)
0.00
B. OTHER PERSONNEL (SHOW NUMBERS IN BRACKETS)
1. (1) POSTDOCTORAL ASSOCIATES
0.00
2. (0) OTHER PROFESSIONALS (TECHNICIAN, PROGRAMMER, ETC.)
3. (1) GRADUATE STUDENTS
4. (0) UNDERGRADUATE STUDENTS
5. (0) SECRETARIAL - CLERICAL (IF CHARGED DIRECTLY)
6. (0) OTHER
TOTAL SALARIES AND WAGES (A + B)
C. FRINGE BENEFITS (IF CHARGED AS DIRECT COSTS)
TOTAL SALARIES, WAGES AND FRINGE BENEFITS (A + B + C)
D. EQUIPMENT (LIST ITEM AND DOLLAR AMOUNT FOR EACH ITEM EXCEEDING $5,000.)
Explosives vapor generator
Liquid nitrogen laser diode dewar and controller
Miniature monochromator
Custom high reflectivity mirror sets (2) $7,500 ea.
$
(If Different)
3.00
0.00
3.00
13,000
12.00
0.00
32,000
0
20,000
0
0
0
65,000
6,400
71,400
13,000
$
$ 51,000
16,700
14,700
15,000
TOTAL EQUIPMENT
E. TRAVEL
1. DOMESTIC (INCL. CANADA, MEXICO AND U.S. POSSESSIONS)
2. FOREIGN
F. PARTICIPANT SUPPORT
1. STIPENDS
$ 0
2. TRAVEL
0
3. SUBSISTENCE
0
4. OTHER
0
TOTAL NUMBER OF PARTICIPANTS (0)
G. OTHER DIRECT COSTS
1. MATERIALS AND SUPPLIES
2. PUBLICATION/DOCUMENTATION/DISSEMINATION
3. CONSULTANT SERVICES
4. COMPUTER SERVICES
5. SUBAWARDS
Proposer
0.00
97,400
3,000
0
0
TOTAL PARTICIPANT COSTS
52,102
500
0
0
0
0
52,602
222,402
6. OTHER
TOTAL OTHER DIRECT COSTS
H. TOTAL DIRECT COSTS (A THROUGH G)
I. INDIRECT COSTS (F&A) (SPECIFY RATE AND BASE)
Modified Total Direct Costs (55% of base = 127,002)
TOTAL INDIRECT COSTS (F&A)
J. TOTAL DIRECT AND INDIRECT COSTS (H + I)
K. RESIDUAL FUNDS (IF FOR FURTHER SUPPORT OF CURRENT PROJECT SEE GPG II.D.7.j.)
L. AMOUNT OF THIS REQUEST (J) OR (J MINUS K)
69,581
294,253
0
294,253
Stefan Lukow
$
$
AGREED LEVEL IF DIFFERENT: $
DATE
FOR NSF USE ONLY
INDIRECT COST RATE VERIFICATION
9/24/02
ORG. REP. TYPED NAME & SIGNATURE*
DATE
NSF Form 1030 (10/99) Supersedes All Previous Editions
*SIGNATURES REQUIRED ONLY FOR REVISED BUDGET (GPG III.C)
M. COST SHARING: PROPOSED LEVEL $0
PI/PD TYPED NAME AND SIGNATURE*
Date
Checked
Date of Rate
Sheet
Initials-ORG
FOR NSF USE ONLY
54
SUMMARY PROPOSAL BUDGET
YEAR 2
ORGANIZATION
PROPOSAL NO.
DURATION (MONTHS)
Tufts University
Proposed
PRINCIPAL INVESTIGATOR/PROJECT DIRECTOR
Granted
AWARD NO.
Stefan Lukow
A. SENIOR PERSONNEL: PI/PD, Co-PIs, Faculty and Other Senior Associates
NSF-Funded
Funds
Funds
List each separately with name and title. (A.7. Show number in brackets)
Person-months
CAL
ACAD
SUM
Requested By
Granted by NSF
1. Stefan Lukow
2.
3.
4.
5. (0) OTHERS (LIST INDIVIDUALLY ON BUDGET EXPLANATION PAGE)
6. (1) TOTAL SENIOR PERSONNEL (1-6)
B. OTHER PERSONNEL (SHOW NUMBERS IN BRACKETS)
1. (1) POSTDOCTORAL ASSOCIATES
2. (0) OTHER PROFESSIONALS (TECHNICIAN, PROGRAMMER, ETC.)
3. (1) GRADUATE STUDENTS
$
(If Different)
0.00
3.00
0.00
0.00
3.00
13,390
0.00
12.00
0.00
32,960
0
20,600
0
0
0
66,950
6,592
73,542
4. (0) UNDERGRADUATE STUDENTS
5. (0) SECRETARIAL - CLERICAL (IF CHARGED DIRECTLY)
6. (0) OTHER
TOTAL SALARIES AND WAGES (A + B)
C. FRINGE BENEFITS (IF CHARGED AS DIRECT COSTS)
TOTAL SALARIES, WAGES AND FRINGE BENEFITS (A + B + C)
D. EQUIPMENT (LIST ITEM AND DOLLAR AMOUNT FOR EACH ITEM EXCEEDING $5,000.)
Liquid nitrogen laser diode dewar and controller
Miniature monochromator
Custom high reflectivity mirror set (1)
Proposer
0.00
13,390
$
$ 16,700
14,700
7,500
TOTAL EQUIPMENT
E. TRAVEL
1. DOMESTIC (INCL. CANADA, MEXICO AND U.S. POSSESSIONS)
2. FOREIGN
F. PARTICIPANT SUPPORT
1. STIPENDS
$ 0
2. TRAVEL
0
3. SUBSISTENCE
0
4. OTHER
0
TOTAL NUMBER OF PARTICIPANTS (0)
G. OTHER DIRECT COSTS
1. MATERIALS AND SUPPLIES
2. PUBLICATION/DOCUMENTATION/DISSEMINATION
3. CONSULTANT SERVICES
4. COMPUTER SERVICES
5. SUBAWARDS
6. OTHER
TOTAL OTHER DIRECT COSTS
H. TOTAL DIRECT COSTS (A THROUGH G)
I. INDIRECT COSTS (F&A) (SPECIFY RATE AND BASE)
38,900
3,000
0
TOTAL PARTICIPANT COSTS
0
17,980
500
0
0
0
0
20,480
135,925
Modified Total Direct Costs (55% of base = 97,025)
TOTAL INDIRECT COSTS (F&A)
J. TOTAL DIRECT AND INDIRECT COSTS (H + I)
K. RESIDUAL FUNDS (IF FOR FURTHER SUPPORT OF CURRENT PROJECT SEE GPG II.D.7.j.)
L. AMOUNT OF THIS REQUEST (J) OR (J MINUS K)
53,364
189,289
0
189,289
Stefan Lukow
$
$
AGREED LEVEL IF DIFFERENT: $
DATE
FOR NSF USE ONLY
INDIRECT COST RATE VERIFICATION
9/24/02
ORG. REP. TYPED NAME & SIGNATURE*
DATE
NSF Form 1030 (10/99) Supersedes All Previous Editions
*SIGNATURES REQUIRED ONLY FOR REVISED BUDGET (GPG III.C)
M. COST SHARING: PROPOSED LEVEL $0
PI/PD TYPED NAME AND SIGNATURE*
27
Date Checked
Date of Rate Sheet
Initials-ORG
FOR NSF USE ONLY
54
SUMMARY PROPOSAL BUDGET
TOTAL
ORGANIZATION
PROPOSAL NO.
DURATION (MONTHS)
Tufts University
Proposed
PRINCIPAL INVESTIGATOR/PROJECT DIRECTOR
Granted
AWARD NO.
Stefan Lukow
A. SENIOR PERSONNEL: PI/PD, Co-PIs, Faculty and Other Senior Associates
NSF-Funded
Funds
Funds
List each separately with name and title. (A.7. Show number in brackets)
Person-months
CAL
ACAD
SUM
Requested By
Granted by NSF
1. Stefan Lukow
0.00
2.
3.
4.
5. (0) OTHERS (LIST INDIVIDUALLY ON BUDGET EXPLANATION PAGE)
6. (1) TOTAL SENIOR PERSONNEL (1-6)
0.00
B. OTHER PERSONNEL (SHOW NUMBERS IN BRACKETS)
1. (1) POSTDOCTORAL ASSOCIATES
0.00
2. (0) OTHER PROFESSIONALS (TECHNICIAN, PROGRAMMER, ETC.)
3. (1) GRADUATE STUDENTS
4. (0) UNDERGRADUATE STUDENTS
5. (0) SECRETARIAL - CLERICAL (IF CHARGED DIRECTLY)
6. (0) OTHER
TOTAL SALARIES AND WAGES (A + B)
C. FRINGE BENEFITS (IF CHARGED AS DIRECT COSTS)
TOTAL SALARIES, WAGES AND FRINGE BENEFITS (A + B + C)
D. EQUIPMENT (LIST ITEM AND DOLLAR AMOUNT FOR EACH ITEM EXCEEDING $5,000.)
Explosives vapor generator
Liquid nitrogen laser diode dewar and controller (2) $16,700 ea.
Miniature monochromator (2) @14,700 ea.
Custom high reflectivity mirror sets (3) $7,500 ea.
$
(If Different)
3.00
0.00
3.00
26,390
12.00
0.00
64,960
0
40,600
0
0
0
131,950
12,992
144,942
26,390
$
$ 51,000
33,400
29,400
22,500
TOTAL EQUIPMENT
E. TRAVEL
1. DOMESTIC (INCL. CANADA, MEXICO AND U.S. POSSESSIONS)
2. FOREIGN
F. PARTICIPANT SUPPORT
1. STIPENDS
$ 0
2. TRAVEL
0
3. SUBSISTENCE
0
4. OTHER
0
TOTAL NUMBER OF PARTICIPANTS (0)
G. OTHER DIRECT COSTS
1. MATERIALS AND SUPPLIES
2. PUBLICATION/DOCUMENTATION/DISSEMINATION
3. CONSULTANT SERVICES
4. COMPUTER SERVICES
5. SUBAWARDS
Proposer
0.00
136,300
6,000
0
0
TOTAL PARTICIPANT COSTS
72,085
1000
0
0
0
0
73,085
360,327
6. OTHER
TOTAL OTHER DIRECT COSTS
H. TOTAL DIRECT COSTS (A THROUGH G)
I. INDIRECT COSTS (F&A) (SPECIFY RATE AND BASE)
Modified Total Direct Costs (55% of base = 224,027)
TOTAL INDIRECT COSTS (F&A)
J. TOTAL DIRECT AND INDIRECT COSTS (H + I)
K. RESIDUAL FUNDS (IF FOR FURTHER SUPPORT OF CURRENT PROJECT SEE GPG II.D.7.j.)
L. AMOUNT OF THIS REQUEST (J) OR (J MINUS K)
123,215
483,542
0
483,542
Stefan Lukow
$
$
AGREED LEVEL IF DIFFERENT: $
DATE
FOR NSF USE ONLY
INDIRECT COST RATE VERIFICATION
9/24/02
ORG. REP. TYPED NAME & SIGNATURE*
DATE
NSF Form 1030 (10/99) Supersedes All Previous Editions
*SIGNATURES REQUIRED ONLY FOR REVISED BUDGET (GPG III.C)
M. COST SHARING: PROPOSED LEVEL $0
PI/PD TYPED NAME AND SIGNATURE*
28
Date
Checked
Date of Rate
Sheet
Initials-ORG
Section F - Budget Justification
A. Proposer requests summer funding to aid in the research for this proposal.
Funding amount is based on three ninths of an average academic starting salary
and adjusted for 3% increase the second year.
B. Funding is requested for one post-doctoral researcher with knowledge in optical
mechanics – preferably with previous cavity ringdown experience. One graduate
student will also be funded.
C. Fringe benefits were calculated as an 8% rate for both proposer and one graduate
student for June through August only, and at a 15.5% rate for a post-doctoral
researcher for the full calendar year.
D. Several pieces of equipment are vital for this project. Multiple diode laser liquid
nitrogen dewars with controllers and miniature monochromators are required. An
explosives generator, custom built from the Idaho National Engineering
Laboratory, and three sets of ultra high reflectivity dielectric mirrors will also be
needed.
E. Funds for travel to conferences are requested. Yearly conferences such as the
Pittsburgh Conference and the American Chemical Society National Meetings
will be excellent opportunities to present research findings.
G. To complete the optical setup listed in the proposal, funding is requested to
purchase all optical components required for the CRDS system. Additionally, an
optical table, sample materials, gasses, a heated FT-IR cell, and an explosionproof refrigerator will be required.
29
Updated - Some required corrections
Sampling system – How to get selectivity and sensitivity
This proposal addressed the identification of explosives based in the nitro
stretches in infrared spectroscopy. However, one complaint was that the instrument
proposed did not address selectivity issues. All colognes and perfumes contain nitro
groups in undoubtedly higher concentrations than would be detected for explosives. How
would this instrument be able to detect explosives in an airport screening situation when
someone has on cologne? By using the physical properties of the interferents as an
advantage the selectivity can be overcome. If there are going to be highly volatile
aerosols in the mix with explosives (perfumes and colognes), bring in a sample of air into
a sampling chamber prior to the cavity. Drop the temperature in the chamber or maintain
a low temperature. This will lower the vapor pressure of all compounds inside. The
explosives will solidify since their vapor pressures are so low. The perfume vapor
pressure is so high that a drop in temperature will not change its state and it can be swept
through or vacuumed out. Inside the box is also a substrate that will “catch” the
explosives once the temperature is lowered. Once the perfumes are evacuated, thermally
desorb the explosives into the gas phase and then enter them into the cavity for analysis.
30