Stand-off and up-close Raman detection of nitrates …people.cas.uab.edu/~mirov/sharma 3.pdfStand-off and up-close Raman detectio n of nitrates buried in sand and soils Carlton Farley

Stand-off and up-close Raman detection of nitrates buried in sand and soils

Carlton Farley IIIa, Sandra Sadatea, Aschalew Kassub, Belther Mononoa, William Wittc, Jonathan Bibba, and Anup Sharmaa

aDepartment of Physics, Chemistry and Mathematics bDepartment of Engineering, Construction Management and Industrial Technology Alabama A&M University, 4900 Meridian Street, Normal, Alabama 35762, U.S.A.

cDepartment of Physics, Auburn University, 206 Allison, Auburn, Alabama 36849, U.S.A.

ABSTRACT

Raman measurements, using a 785nm laser, are taken of Ammonium Nitrate and Sodium Nitrate buried in sand. Nitrate is kept in clear plastic containers and buried underneath sand at various depths. Raman measurements are then taken at distances of 5m and 20m, with the sand being completely dry as well as completely wet. A different set of experiments was conducted with Nitrate buried in sand in a glass container, where no Raman signal was seen in dry sand. Water was then added at the edge of the container and allowed to migrate to the bottom. Raman measurements are then taken at a distance of 7mm over time to detect Nitrates brought to the surface by water as it wicks to the surface. Keywords: Raman, nitrate, sand, stand-off, infra-red, soil

1. INTRODUCTION Raman spectroscopy has proven to be a valuable tool in the detection of numerous chemical compounds 1, 2.

This technique can be used to identify nitrates, a type of chemical used in fertilizers and the manufacturing of explosives. Recently, the US Army has taken interest in ways to find hidden improvised explosive devices (IEDs). Raman spectroscopy provides a promising method to identify the nitrates used in these types of explosives in the field 3. Raman spectroscopy utilizes the inelastic scattering of monochromatic light to identify a sample. Results are usually given as a graph of intensity versus wavenumber shift. The Raman signature for nitrates is defined by a sharp intensity peak at approximately 1040 cm-1. Raman is a desirable technique to use for the detection of nitrate based explosives in the field due to its high sensitivity, long range, and ability to see through some materials.

Surface Enhanced Raman spectroscopy can be used to detect very minute samples of nitrate 4, 5, 6. Nitrate can even be detected against a substrate that possesses its own Raman spectrum. It can detect individual ammonium nitrate particles stuck to clothing that are as small as 5 cubic microns, or 180 picograms in mass 7.

One problem with detecting and disarming IEDs in the field is the need to keep soldiers at a safe distance while the supposed location of an explosive is analyzed. A telescope can be used to extend the range of a Raman spectroscopy setup. Using this technique, measurements of relevant quantities of nitrates can be detected at large distances 8, 9, 10, 11.

Raman can also be used to detect concealed chemicals that are not visible to the naked eye. It has been used to detect nitrates through clothing 12 and inside of opaque plastic containers 13.

2. EXPERIMENTAL A 785 nm Raman system (Enwave Optronics) with a 400 mW diode laser source is used for all experiments

reported in this paper. At stand-off distances of 1m and 5m, the Raman system is coupled with a 2” telescope (Nikon). At stand-off distance of 20m, the Raman system is coupled with an 11” telescope (Celestron). The first set of experiments reported were conducted to detect Sodium Nitrate kept inside a clear plastic container which is buried in sand. A clear plastic container is packed full of Sodium Nitrate, then buried in sand at various depths, while Raman analysis is taken at stand-off distances of 5m and 20m. Measurements are taken with dry sand, then water is added using a dropper so as not to disturb the sand, and more measurements are taken.

Infrared Remote Sensing and Instrumentation XXII, edited by Marija Strojnik Scholl, Gonzalo Páez, Proc. of SPIE Vol. 9219, 92190J · © 2014 SPIE

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The next set of experiments reported were conducted to detect Sodium Nitrate buried under soils without the

presence of containers. Three samples were used in this study. Each sample is contained inside a glass vial with a tight lid. Two grams of Sodium Nitrate is placed inside each glass vial. One sample also contains 13 grams of dirt. One sample contains 13 grams of sand. The final contains a mixture of 6.5 grams of dirt and 6.5 grams of sand. The 3 vials were shaken vigorously for 5 minutes so that the samples would be well mixed. Measurements were then taken from outside the vial, with the Raman probe touching the glass. Afterwards, 8mL of water was added to each sample. This was enough water so that the water level would be about one half inch above the soil level. The samples were then shaken vigorously for 5 minutes, and then left to settle, so that the water above the soil would be clear. Measurements were then taken from outside the vial, with the probe touching the glass, of the water above the soil.

For the final experiments reported here, a thin layer of Ammonium Nitrate is placed in the bottom of a glass

container and then covered under 1cm of sand. A Raman probe is kept 7mm above the sand, where it remains for several days. A small amount of water is added to the container and measurements are then taken every day for several days. After some time, a small amount of water carrying nitrate wicks to the surface of the container and gives a clear Raman spectrum for nitrate.

3. RESULTS & DISCUSSIONS For the first set of experiments (Figures 1-2), Sodium Nitrate is placed inside a clear plastic container, then

buried in sand at depths ranging from 1mm to 3mm. With the Raman probe coupled with a 2” telescope and kept 5m above the sand, and using a 60s Integration Time, measurements were taken first when the sand was dry (Figure 1), then water was added to the sand, and more measurements were then taken (Figure 2). The primary peak for Sodium Nitrate is located at about 1065cm-1, while the secondary peak is at about 740cm-1. A silicon peak (sand) is seen at about 460cm-

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Figure 1 – Raman spectra of Sodium Nitrate kept in clear plastic container and buried under different depths of dry sand. Peak at about 1065cm-1 is main peak of Sodium Nitrate. Raman probe was 5m from surface of sand, and Integration Time of 60s was used. 2” telescope was used.

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Figure 2 – Raman spectra of Sodium Nitrate kept in clear plastic container and buried under different depths of wet sand. Peak at about 1065cm-1 is main peak of Sodium Nitrate. Raman probe was 5m from surface of sand, and Integration Time of 60s was used. 2” telescope was used.

For the second set of experiments (Figure 3), Sodium Nitrate is placed inside a clear plastic container, then buried in sand at a depth of 1mm. With the Raman probe coupled with an 11” telescope and kept 20m from the sand, and a mirror was used to reflect the laser light onto the sand. Using a 10s Integration Time, measurements were taken first when the sand was dry, then water was added to the sand, and more measurements were then taken. The primary peak for Sodium Nitrate is located at about 1065cm-1. A silicon peak (sand) is seen at about 460cm-1.

Figure 3 – Raman spectrum of Sodium Nitrate kept in clear plastic container and buried under 1mm of dry sand as well as spectrum after adding water to sand. Peak at about 1065cm-1 is main peak of Sodium Nitrate. Raman probe was 20m from surface of sand, and Integration Time of 10s was used. 11” telescope was used. A mirror was placed at an angle about 20cm from sand and used to reflect beam onto sand.

Next, we buried Ammonium Nitrate under 1cm of dry sand in a glass container. The Raman probe was kept 7mm above the sand, and 13 measurements were taken over the course of several days. After the first measurement was taken, water was added along the side of the container. The water then made contact with the Ammonium Nitrate, where it dissolved nitrate into the water. Over the course of many hours, this water wicks to the surface of the sand, where nitrate is detected by the Raman probe. Figure 4 shows ratios of nitrate peak to silicon peak versus time. Low power is used with an integration time of 40s.

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Figure 4 – Ratio of nitrate peak signal-to-noise ratio to silicon peak signal-to-noise ratio vs Time. Ammonium Nitrate is buried in 1cm of sand. 40s integration time is used.

For the experiments reported in Figures 5-9, 13g soil samples are placed inside glass containers, and measurements with 10s integration times are taken of each soil sample, including sand, dirt, and a 50/50 mixture of sand and dirt (Figure 5). 2g of Sodium Nitrate was then added to each sample, and shaken for 5 minutes to ensure proper mixing. More measurements were then taken (Figure 6). While the primary nitrate peaks were easy to see (about 1065cm-1), we wanted to improve upon them if we could. To this end, we added 8mL of water into each sample, which was enough water so that the water level was above that of the soil, and shook each sample for 5 minutes. We then allowed the soils to settle, while taking measurements of the water from the outside of the glass container (Figures 7-9). As you can see, the intensity of the primary nitrate peaks are much greater than before the water was added, especially after allowing the samples to settle for 60 minutes after shaking. With such a high signal-to-noise ratio at the primary nitrate peak, we are certain that this technique would be useful for testing soil samples for nitrates, and possibly other chemicals, in smaller concentrations.

Figure 5 – Raman Spectra of soil samples kept inside glass containers. Raman probe was touching the glass container. Integration time of 10s was used.

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Figure 6 – Raman Spectra of soil samples (each 13g) mixed with 2g of Sodium Nitrate. Raman probe was touching the glass container. Integration time of 10s was used.

Figure 7 – Raman Spectra of Dirt sample (13g dirt and 2g Sodium Nitrate) after adding 8mL of water then shaking for 5 minutes. Sample is then allowed to rest as measurements are taken at various times. Raman probe was touching the glass container above the elevation of the dirt, so as to take a measurement of the water. Integration time of 10s was used.

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Figure 8 – Raman Spectra of Mixture sample (6.5g dirt, 6.5g sand, and 2g Sodium Nitrate) after adding 8mL of water then shaking for 5 minutes. Sample is then allowed to rest as measurements are taken at various times. Raman probe was touching the glass container above the elevation of the dirt, so as to take a measurement of the water. Integration time of 10s was used.



Figure 9 – Raman Spectra of Sand sample (13g sand and 2g of Sodium Nitrate) after adding 8mL of water then shaking for 5 minutes. Sample is then allowed to rest as measurements are taken at various times. Raman probe was touching the glass container above the elevation of the dirt, so as to take a measurement of the water. Integration time of 10s was used.

4. CONCLUSIONS

It is found that Sodium Nitrate can be detected under thin layers of both dry and wet sand at stand-off distances

of 5m and 20m. Samples with wet sand yield a better signal-to-noise ratio because of a reduction of scattered light due to water acting as an index-of-refraction matching agent. It is also found that while Sodium Nitrate can be detected in different dry soils, it is much easier to detect in wet soils. This is especially true if soil samples can be taken in a clear glass container, then water added to the sample in the lab. If Raman is taken of the wet soil, the water will act as an index-of-refraction matching agent to give a better signal-to-noise ratio. If enough water is added so that Raman can be taken of only water, then the signal-to-noise ratio is much higher due to nitrate dissolving in water. We also show that the presence of loose nitrates buried in sand may be detected after the addition of water.

5. ACKNOWLEDGEMENT

This work is supported by funding from National Science Foundation and AMRDEC Huntsville, Alabama.

6. REFERENCE

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[5] Kneipp, K., Wang, Y., Kneipp, H., Perelman, L. T., Itzkan, I., Dasari, R. R., and Feld, M. S., “Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS),” Phys. Rev. Lett. 78(9), 1667–1670 (1997).

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[6] Nie, S. and Emory, S. R., "Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering," Science 275, 1102-1106 (1997).

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[9] Gaft, M. and Nagli, L., “Standoff Laser-Based Spectroscopy for Explosives Detection,” Proceedings of SPIE, 6739, (2007).

[10] Wolf, S., Wrzesinski, P. J., and Dantus, M., “Standoff Chemical Detection Using Single-Beam CARS,” OSA Technical Digest, (2009).

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[13] Canal, C. M., Saleem, A., Green, R. J., and Hutchins, D. A., "Remote Identification of Chemicals Concealed behind Clothing Using near Infrared Spectroscopy," Analytical Methods, 3(1), 84, (2011).



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Stand-off and up-close Raman detection of nitrates …people.cas.uab.edu/~mirov/sharma 3.pdfStand-off and up-close Raman detectio n of nitrates buried in sand and soils Carlton Farley