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Methods to Control the Polymorphic Phase of RDX-Based ExplosivesBrittney Argirakis

DHS-HS STEM Summer Intern, Transportation Security Laboratory, William J. Hughes Technical Center, Atlantic City, NJ 08405

Abstract

The effect of mass loading and solvent deposition on the polymorphic phase of RDX was

examined using Raman spectroscopy. Direct depositions of quantified RDX and C4 solutions in

THF and ACN were made onto PTFE, or Teflon®, substrates at various mass loadings and

imaged with Raman microscopy to determine the initial phase. The effect of dry transfer was

examined with Raman measurements. An additional set of samples of C4 were made with

aerosol spray deposition at known masses and imaged with Raman microscopy to determine the

effect of sample preparation.

Raman spectra of the samples revealed minimal differences between C4 and RDX

deposits, as well as solvent choice for RDX. Spectra of bulk RDX and C4 showed that α-RDX is

the dominant phase observed in bulk material. The solution deposits confirm that α- and β-RDX

were present at higher mass loadings, while only β-RDX was present at low mass loadings.

Results showed that direct deposit alone is incapable of producing α-RDX. However, the data

indicate that to successfully create the desired bulk phase (α-RDX), the depositions of RDX or

C4 must be dry transferred onto a substrate.

1. Introduction

The Transportation Security Lab (TSL) aims to certify detection equipment by making

standards for fingerprints of explosives. Creating accurate and precise standards from

fingerprints is impossible due to the variability in mass and concentration from consecutive

fingerprints. With this in mind, the trace laboratory at the TSL uses techniques to mimic the

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different metrics of the fingerprint, including aspects, such as particle distribution, particle size,

adhesion, mass loading, and isomorphic state.

The bulk component of the plastic C4 explosive is 1,3,5-trinitro-1,3,5-triazine (RDX).

This explosive exhibits polymorphism. Specifically, RDX has two polymorphs, α and β, that can

exist in ambient conditions as conformational polymorphs of one another. The α polymorph

commonly is found in bulk, or visible,

quantities of C4 and has been determined

through previous studies to have an axial,

equatorial conformation of the NO2 groups

around the triazine ring (Figure 1).1,2 The α-

phase is stable at room temperature and

atmospheric conditions. The β-phase is more kinetically stable than the α-phase. β-RDX can

remain on a surface for long periods of time but will revert back to α-RDX if subjected to

mechanical disturbances. The β-phase contains all axial NO2 groups around the triazine ring.3

Characterizing the conversion from α- to β-RDX polymorphs using various solvents,

mass loadings, and deposition techniques will provide insight toward accurate imitation of C4

fingerprints. For example, if a specific deposition technique only produces β-RDX, use of those

artificial fingerprints for the test and evaluation of detection technologies, such as those based on

Raman emission, which is sensitive to polymorphs, will not provide realistic test results. The β-

phase may alter additional metrics important in trace test standards production, such as adhesion,

particle size, and distribution, thereby decreasing the standards validity. Additionally, it is

imperative that all field detection devices are able to identify all trace amounts of RDX and/or

C4, not just the more common α form.

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β-RDXα-RDX

Figure 1. AAE and AAA conformations of α and β-RDX.5

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Previous studies have concluded that the

polymorphic state of RDX is affected by

deposition technique and mass loading.4,5 To

further such discoveries, this study aims to

explore various solvent depositions at different

mass loadings and their effects on the

polymorphic state of RDX via Raman

spectroscopy.

The techniques to be used are “direct deposit,” “dry transfer,” and aerosol spray

deposition. Direct deposit involves depositing a known volume and concentration of the threat in

solvent onto a substrate. The solvent is allowed to air dry until completely evaporated,

mimicking the random distribution of particles in a fingerprint. Dry transfer uses the direct

deposit method onto PTFE (Teflon®) strips. Subsequently, the residue is rubbed onto a new

substrate to illustrate the transfer of a fingerprint onto a surface. Finally, aerosol spray deposition

involves a constant spray of a known volume and concentration of an explosive in solvent onto a

substrate.

2. Experimental

Direct Deposit:

Two RDX stock solutions were made in acetonitrile and tetrahydrofuran. One C4 stock

solution was made in tetrahydrofuran at a concentration of 20 mg/mL. Solution concentrations

underwent quality control procedures to verify concentrations and then diluted down to 10, 1,

and 0.1 μg/μL. Deposits of 10 μL of each concentration were made in triplicate onto PTFE strips

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Figure 2. Photograph of a RamanRXN™ Microprobe with an optical microscope.Figure 2. Photograph of a RamanRXN™ Microprobe with an optical microscope.Figure 2. Photograph of a RamanRXN™ Microprobe with an optical microscope.Figure 2. Photograph of a RamanRXN™ Microprobe with an optical microscope.

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using a calibrated micropipette (Eppendorf, P/N: 1691078) to provide samples at 100, 10, and 1

µg. The solvent then was allowed to evaporate at room temperature.

Aerosol Spray:

The C4 stock solution was deposited onto PTFE strips in triplicate at 100, 10, and 1 μg

using an Aerosol Spray Deposition System (ASDS) in 1 cm2 deposits.

Dry Transfer:

The measured Teflon® strips were used to dry transfer onto three-square-inch aluminum

test substrates (ACT, Batch: 50829811). This was accomplished by rubbing the dried PTFE strip

onto the test substrate to transfer the particles.

Microscopy:

Raman spectra of the samples were collected via a RamanRXN™ Microprobe with

optical imaging capabilities (Figure 2). The excitation source used for the measurements was an

Invictus™ 785-nm NIR Laser, powered at 151 mW. The integration time was 5 s/spectra.

Additionally, Raman measurements and white light optical images were taken of each sample at

a magnification of 50x.

3. Results and Discussion

Microscopy:

Raman spectroscopy was chosen to differentiate between α- and β-RDX in this study due

to availability in the lab. The laser detects the inelastically scattered light through the specimen.

The majority of the scattered light is the same energy as the laser source, but when the energy

interacts with the individual vibrations of a molecule, the energy frequency may shift, and the

difference in the energy is detected. The intensity of the shifted energy can be plotted with regard

to frequency, yielding a Raman spectrum of the sample. The spectrum provides information

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about the chemical structure of the specimen. Theoretical spectra of RDX show multiple

characteristic peaks that distinguish α- from β-RDX; however, at low mass loadings, the height

of most of these peaks is too low to distinguish. For that reason, the most prominent of the

distinct α- and β-RDX peaks analyzed is characteristic to the ring breathing mode, or strain,

which is located at 883 cm-1 and 877 cm-1, respectively. 4

The sensitivity of the Raman microscope was proven to display accurate measurements at

the predicted shifts of the peak of interest for β- and α-RDX. The literature value of the ring

breathing mode for α-RDX was verified by comparing it to collected bulk RDX and C4 Raman

spectra (Figure 4.a and Figure 4.b). The collected values were in agreement with the literature

values at 883 cm-1.4

C4 and RDX:

Raman measurements showed minimal differences between the C4 and pure RDX

frequency shifts using the direct deposit method. Results displayed in Table 1 and Table 2 show

that β- and α-RDX are both present at 100 and 10 μg on PTFE strips, and only β-RDX is present

at 1 μg. Minimal difference between C4 and RDX

data is expected. About 91 percent of the

composition of military-grade C4 in the United

States is RDX, giving it the characteristics of pure

RDX.6 The amount of pure RDX directly deposited

with respect to polymorphic change is consistent

with previous studies conducted on glass

substrates.4,5

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Table 1. Polymorphic form of RDX at various mass loadings for pure RDX.

Mass Load Solvent

Direct Deposit

Dry Transfer

Teflon Metal100.a

ACNβ / α α

100.b β / α α100.c β / α α100.a

THFβ / α α

100.b β / α α100.c β / α α10.a

ACNβ / α α

10.b β / α α10.c β / α α10.a

THFβ / α α

10.b β / α α10.c β / α α1.a

ACNα α

1.b α α1.c α α1.a

THFα α

1.b α α1.c α α

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Solvent Effects:

Solvent did not appear to have a significant impact on the polymorphic form of the RDX

crystals when the same threat was deposited. Results compiled in Table 1 conclude that β- and α-

RDX are present at 100 and 10 μg on PTFE strips containing deposited RDX in acetonitrile

and tetrahydrofuran, with the

exception of one replicate of

100 μg of RDX in THF. At 1

μg, only β-RDX was observed

in acetonitrile and

tetrahydrofuran.

Direct Deposit:

Raman spectra showed that direct deposit produced β- and α-RDX at 100 and 10 μg and

only β-RDX at a mass of 1 μg for pure RDX. C4 direct deposit results showed no significant

trends, with a mixture of β and α-RDX at all mass loadings. Results for pure RDX direct deposit

displayed uniformly for all solvents and mass loadings. The presence of both β- and α-RDX can

be explained by first-order kinetics of the β-RDX isomer and the low energy barrier for

interconversion of isomers.4,7

Results for C4 direct deposit were sporadic, showing no trends. This could be due to the

selectivity of the areas imaged by the Raman microscope; in actuality, β- and α-RDX could have

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Table 2. Polymorphic form of RDX at various mass loading for C4 in THF. Cells containing n/o were not observed.

ASDS

Mass LoadDirect

DepositDry

TransferAerosol Spray

Dry Transfer

Teflon Metal Teflon Metal100.a β α α α100.b β / α α β / α α100.c β α α α10.a β / α α β α10.b β α β α10.c β α n/o α1.a β / α α β α1.b β / α α β α1.c β α β α

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been present when only β-RDX was observed. Due to time constraints, further research is

required to confirm this conclusion.

Aerosol Spray:

Aerosol spray deposition produced a combination of β- and α-

RDX and only β-RDX at 100 and 10 μg and only β-RDX at 1 μg. Table 2

summarizes these findings. Observing only β-RDX below 10 μg proves that the RDX

polymorphic phase is sensitive to mass loading. The effect of mass loading on the polymorphic

state of RDX is theorized to be related to the kinetic stability of trace amounts of β-RDX on the

substrate surface.4 Substrate effects were not examined in this study but should be incorporated

into future research to understand fully the effect of mass loading on the metastable form of

RDX.

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Relative Intensity

Relative Intensity

Y-axis

(microns)

X-axis (microns)

Y-axis

(microns)

X-axis (microns)

Y-axis

(microns)

X-axis (microns)

Y-axis

(microns)

X-axis (microns)

Figure 3. 40x40 point Raman image of 1 µg RDX in ACN pre-dry transfer (a) and post-dry transfer (b) at 877 cm-1; 40x40 point Raman image of 1 µg RDX in ACN pre-dry transfer (c) and post-dry transfer (d) at 883 cm-1.

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Dry Transfer:

Dry transfer uniformly produced only α-RDX after deposition with RDX and C4 at all

mass loadings at the locations that were imaged (Figure 4). This can be observed in Table 1 and

Table 2. It is apparent that only one isomorph is present at a time for the 1 µg deposit of RDX in

ACN pre- and post-transfer (Figure 3). Dry-transfer data for C4 and pure RDX displayed

uniform results at all mass loadings at the locations imaged. Dry transfer converted any β-RDX

to α-RDX. The energy barrier for the conversion from β- to α-RDX has been determined through

previous work to be less than 1 kcal/mol at room temperature.7 The low activation energy

explains the conversion to α-RDX after a small mechanical disturbance has been forced onto the

β-RDX during the dry transfer.

4. Conclusions

Deposits of C4 and RDX in various solutions under several deposition techniques were

examined with Raman microscopy. Results showed that direct deposit will not produce a

polymorphically accurate artificial fingerprint with solely α-RDX at a mass as high as 100 µg.

Additionally, spectra indicated that a switch from α- and β-RDX to only β-RDX can occur below

10 µg. The data conclude that to successfully create a standard for fingerprint production in only

α-RDX, depositions of RDX or C4 must be dry transferred onto a substrate. Substrate effects on

RDX polymorphism should be incorporated into future research to understand fully the effect of

mass loading on RDX phases.

5. Acknowledgements

I would like to thank the Department of Homeland Security (DHS) Science and

Technology (S&T) Directorate Office of University Programs, as well as the Oak Ridge Institute

for Science and Education (ORISE) for supporting my research by appointment with the HS-

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STEM Summer Internship Program. Additionally, I would like to express a vast amount of

gratitude toward Dr. Jason Stairs, Dr. John Brady, Dr. Alex Gordon, and all other TSL

employees for being exemplary mentors and for graciously taking time from their busy schedules

to support my research. I have taken away a remarkable expanse of knowledge, experience, and

appreciation for the significance of the work that the TSL accomplishes. I thank you for inviting

me into your workplace and laboratories and for encouraging my enthusiasm as a scientist.

6. References

(1) Hakey, P.; Ouellette, W.; Zubieta, J.; Korter, T. “Redetermination of cyclo-trimethylenetrinitramine”. Acta Cryst. E 2008, 64, 1428.

(2) Choi, C. S.; Prince, E. “The crystal structure of cyclotrimethylenetrinitramine”. Acta Cryst. B 1972, 28, 2857.

(3) Millar, D. I. A et al. “The crystal structure of beta-RDX-an exclusive form of an explosive revealed”. Chem. Comm. 2009, 2009, 562-564.

(4) Torres, P et al. “Vibrational Spectroscopy Study of β and α RDX deposits”. J. Phys. Chem. B 2004, 108, 8799-8805.

(5) Emmons, E. D et al. “Characterization of Polymorphic States in Energetic Samples of 1,3,5-Trinitro-1,3,5-Triazine (RDX) Fabricated Using Drop-on-Demand Inkjet Technology”. Appl. Sci. 2012, 66, 628-635.

(6) Reardon, Michelle R.; Bender, Edward C. “Differentiation of Composition of C-4 Based on the Analysis of the Process Oil”. J. Forensic Sci. 2005, 50, 1–7.

(7) Vladimiroff, T.; Rice, B. M. “Predictions of Crystal Structures from First Principles”. J. Phys. Chem. A 2002, 106, 10437.

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Figure 4. Representative Raman spectra of bulk RDX (a) and C4 (b); 1 μg direct deposit of RDX in ACN on Teflon pre-dry transfer (c); 1 μg direct deposit of C4 in THF on Teflon pre-dry transfer (d); 1 μg RDX in ACN on bare aluminum post-dry transfer (e); 1 μg C4 in THF on bare aluminum post-dry transfer (f).

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a b

c d

e f