Standoff Raman measurement of nitrates in water
S. Sadatea1, , A. Kassub, C. W. Farleya, A. Sharmaa, J. Hardistyc2,Miles T.K. Lifson d2
a
Department of Physics, Alabama A&M University, 4900 Meridian Street, Normal, AL 35762
Department of Technology, Alabama A&M University, 4900 Meridian Street, Normal, AL 35762
c
College of Engineering, University of Oklahoma, 660 Parrington Oval, Norman, OK 73019
d
Joint Science Department, Claremont McKenna College, 925 N. Mills Ave., Claremont, CA 91711
b
ABSTRACT
The identification and real time detection of explosives and hazardous materials are of great interest to the Army and
environmental monitoring/protection agencies. The application and efficiency of the remote Raman spectroscopy system
for real time detection and identification of explosives and other hazardous chemicals of interest, air pollution
monitoring, planetary and geological mineral analysis at various standoff distances have been demonstrated. In this
paper, we report the adequacy of stand-off Raman system for remote detection and identification of chemicals in water
using dissolved sodium nitrate and ammonium nitrate for concentrations between 200ppm and 5000ppm. Nitrates are
used in explosives and are also necessary nutrients required for effective fertilizers. The nitrates in fertilizers are
considered as potential sources of atmospheric and water pollution. The standoff Raman system used in this work
consists of a 2-inch refracting telescope for collecting the scattered Raman light and a 785nm laser operating at 400mW
coupled with a small portable spectrometer.
Keywords: Explosives, standoff Raman measurement, sodium nitrate, ammonium nitrate, water pollution
1. INTRODUCTION
In order to improve the national security in aviation, environmental monitoring and protection and other areas of
concern, much effort has been focused on direct and real time detection of explosive and hazardous materials in any form
and states of matter. New techniques and highly sensitive detectors have been investigated and developed to detect and
identify such substances in real time. Standoff Raman spectroscopy is one of such techniques widely investigated for
remote detection and identification is Raman spectroscopy. It is a well known and viable technique which provides a
unique set of optical signature/fingerprint about a molecular structure of interest. Due to its chemical specificity,
capability in analyzing and identifying both organic and inorganic compounds in any state, non-destructive and real time
detection and identification of molecules, Raman spectroscopy is becoming a promising technique for remote in-situ
detection and identification of explosives, environmental monitoring and geological mineral identification and analysis
purposes.
To our knowledge, the application of Raman spectroscopy for standoff measurement and identification purpose was
experimentally demonstrated for the first time by D. A. Leonard [1] and J. A. Cooney [2] independently at the same time
in their papers submitted to the respective journals in September 1967. Leonard reported the experimental results of the
Raman scattering collected from atmospheric oxygen and nitrogen using a pulsed nitrogen UV laser operating at 337.1
nm and a 20 cm diameter, 1.6 m focal length telescope and Cooney on the other hand used a giant pulsed ruby laser
operating at 694.3 nm to measure density profiles of the gaseous atmosphere. After these pioneering works, standoff
Raman spectroscopic technique is employed for remote measurement of atmospheric water vapor profile [3] and realtime identification, measurements and quantitative analysis of air pollutants like SO2 and CO2 molecules [4-6]. During
these periods, due to the giant assembly of the entire system (lasers, spectrometers and telescopes), the standoff Raman
1
2
Corresponding Author: Sandra Sadate ([email protected])
REU summer interns
Remote Sensing and Modeling of Ecosystems for Sustainability VIII, edited by Wei Gao, Thomas J. Jackson, Jinnian Wang,
Ni-Bin Chang, Proc. of SPIE Vol. 8156, 81560D · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.893571
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system was either stationary or transported by truck trailer. After two decades of continuous improvement and
innovation in technology, with regard to development of compact and high power lasers, high performance and more
sensitive detectors and spectrometers, Angel et al., [7], demonstrated the application of portable standoff Raman system
for remote monitoring and detection of highly toxic organic and inorganic materials. Following this report, various
groups proposed an alternative design and construction of portable standoff Raman spectroscopy systems using different
types of lasers, telescope and spectrometers [8-13]. The adequacy of the proposed systems for detection, measurement
and analysis was demonstrated using minerals and explosive materials on planetary and geological surfaces [8-14],
environmental monitoring [10, 11, 15], and in-situ investigation of ancient artifacts [16, 17].
Here in this work, a portable stand-off Raman spectroscopy system is used to detect minute amounts of ammonium
nitrate (NH4NO3) and sodium nitrate (NaNO3) dissolved in water. Both ammonium nitrate and sodium nitrate are soluble
in water and are commonly used as ingredients in production of fertilizers and explosives. Some of the potential sources
of nitrates in water are runoff from fertilizer use, erosion of natural nitrate deposits and leakage from septic tanks.
Drinking water containing nitrates in excess of the maximum contaminant level is believed to have a serious health
hazard [18].
2. EXPERIMENTAL
Standoff Raman measurements of ammonium nitrates and sodium nitrates in water were carried out using a portable
Enwave Optronics EZRaman I system (Irvine, CA) with 785 nm diode laser and a maximum power of 400 mW. The
laser beam was coupled to a 2-inch refracting Nikon AF Nikkor telescope for collecting the Raman signals back via a
100 µm diameter and 0.3 numerical aperture fiber-optics probe. The CCD detector thermoelectrically cooled to -50o C
records the scattered Raman signals in the spectral range of 250 – 2200 cm-1 and with a spectral resolution of 6 cm-1.
Figure 1 shows schematic representation of the portable standoff Raman system used in detection of minute amount of
ammonium nitrate and sodium nitrate in water. Sodium nitrates and ammonium nitrates were obtained from Fisher
Scientific in powder form. Solutions of various ppm level concentrations of both sodium and ammonium nitrates (200,
500, 1000, 1500, 2500, 3000, 4500 and 5000 ppm) are prepared by diluting crystals of these nitrates in distilled water.
Fiber probe
Telescope
Laser integrated
with spectrometer
Mirror
Sample
Figure 1: Schematic representation of the portable standoff Raman system used in detection of ammonium nitrate and sodium
nitrate.
3. RESULTS AND ANALYSIS
Figures 2a and 2b show the spectra of sodium nitrate and ammonium nitrate powder respectively, taken with the samples
kept at 5 meters from the telescope and the spectrometer. These measurements were taken for references. Each of these
chemicals was deposited on aluminum trays with a diameter of 8.8cm and a 5cm depth a 2 inch front coated mirror was
used to reflect the light from the laser onto the sample to facilitate the collection of spectra. Sodium nitrate has a
distinctive peak at 1065cm-1 and ammonium nitrate has one at 1041cm-1. These two chemicals also have not so
prominent peaks at each side of the main one; but because when dissolved in water these bonds disappear, they were not
considered for data analysis.
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Figures 3(a) and 3(b) show the spectra of sodium and ammonium nitrate in distilled water at a concentration of 5000ppm
for distances between 1m and 5m. The solutions were put in a PVC tube and a front coated mirror was used to reflect the
light coming from the laser onto the samples. The strong peak at 1045cm-1 for sodium nitrate and 1047cm-1 for
ammonium nitrate is due to the nitrate ion, NO3- . As can be seen from figure 3, the signal-to-noise ratio for the
measurements recorded at a distance of 1m is higher than that of the measurement at 5m. Figures 4(a) and 4(b) show the
standoff Raman measurements of spectra of ammonium nitrate (NH4NO3) and sodium nitrate (NaNO3) at different
concentrations ranging from 200ppm to 5000ppm. Typically, a 300 second integration time is used to change the TE
cooled CCD exposure time. In this work, the maximum standoff distance at which we were able to detect the 200 ppm
NaNO3 and NH4NO3 is at about 1 meter.
Figure 2: Raman spectra of sodium nitrate and ammonium nitrate in powder form at a standoff distance of 5m for a
60sec exposure time.
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Figure 3: Standoff Raman measurements of the 5000ppm solutions of Sodium (figure (a)) and Ammonium nitrates
(figure(b)) at different distances for an exposure time 0f 300sec respectively. In both figures, the topmost curve
represents the 5000ppm concentration at 1m the intensity is close to 2000 count. The subsequent four curves from top
to bottom represent spectra of the same concentration but at a distance of 2, 3, 4, and 5m.
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Figure 4: Raman spectra of solutions of Sodium (figure (a)) and Ammonium nitrates (figure (b)) respectively at
different concentrations for an exposure time of 300sec and at a distance of 1m. The top most curve on figure (a) is the
spectrum of NaNO3 at 5000ppm, the seven subsequent curves represent the same chemical at 200, 500, 1000, 1500,
2500, 3000, and 4500ppm from bottom to top, the laser power was 400mw and the room lights were off. On figure (b)
the topmost curve is NH4NO3 at 5000ppm, the five subsequent curves are 200 , 1000 , 1500, 2500 , and 3000ppm from
bottom to top.
4. CONCLUSION
Portable standoff Raman spectroscopy is successfully used to detect and measure ppm level amounts of NH4NO3 and
NaNO3 in water. In this work, the detection limit of the nitrates dissolved in water is found to be about 200 ppm at a
standoff distance of about 1 meter. Even though the intensity count for 200ppm for both ammonium nitrate and sodium
nitrate is about 50 which is a little low and the signal-to-noise is not so good, this work could have potential applications
in testing contaminated waters or detecting chemicals with nitrates stimulant as well as explosives made with nitrates. In
most cases, the data was processed to remove the oxygen peak at 1550cm-1 and the high background.
ACKNOWLEDGEMENT
This work is funded by a contract from ARMDEC, Huntsville, AL and by a RISE grant from the National Science
Foundation
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Leonard, D., “Observation of Raman scattering from the atmosphere using a pulsed nitrogen ultraviolet laser,”
Nature 216, 142-143 (1967).
Cooney, J. A., “Measurements on the Raman component of laser atmospheric backscatter,” Appl. Phy. Lett.12(2),
40-42 (1968).
Cooney, J., “Remote measurements of atmospheric water vapor profiles using the Raman component of laser
backscatter,” J. of Appl. Meteorology 9, 182-184 (1969).
Inaba, H., and Kobayasi, T., “Laser-Raman radar for chemical analysis of polluted air,” Nature 224, 170 – 172
(1969).
Kobayasi, T., and Inaba, H., “Spectroscopic detection of SO2 & CO2 molecules in polluted atmosphere by laserRaman radar technique,” Appl. Phys. Lett. 17(4), 139-141(1970).
Hirschfeld, T., Schildkraut, E. R., Tannenbaum, H., and Tanenbaum, D., “Remote spectroscopic analysis of ppmlevel air pollutants by Raman spectroscopy,” Appl. Phys. Lett. 22(1), 38-40 (1973).
Angel, S. M., Kulp, T. J., and Vess, T. M., “Remote-Raman spectroscopy at intermediate ranges using low-power
CW lasers,” Appl. Spect. 46(7), 1085-1091 (1992).
Sharma, S. K., Angel, S. M., Ghosh, M., Hubble, H. W., and Lucey, P. G., “Remote pulsed laser Raman
spectroscopy system for mineral analysis on planetary surfaces to 66 meters,” Appl. Spect. 56(6), 699-705 (2002).
Proc. of SPIE Vol. 8156 81560D-5
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/30/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
Sharma, S. K., Lucey, P. G., Ghosh, M., Hubble, H. W., and Horton, K. A. “Stand-off Raman spectroscopic
detection of minerals on planetary surfaces,” Spectrochimica Acta Part A 59, 2391-2407 (2003).
Carter, J. C., Angel, S. M., Snyder, M. L., Scaffidi, J., Whipple, R. E., and Reynolds, J. G., “Standoff detection of
high explosive materials at 50 meters in ambient light conditions using a small Raman instrument,” Appl. Spect.
59(6), 769-775 (2005).
Carter, J. C., Scaffidi, J., Burnett, S., Vasser, B., Sharma, S. K., and Angel, S. M., “Stand-off Raman detection
using dispersive and tunable filter based systems,” Spectrochimica Acta Part A 61, 2288-2298 (2005).
Sharma, S. K., Misra, A. K., Lucey, P. G., Angel, S. M., and McKay, C. P., “Remote pulsed Raman spectroscopy
of inorganic and organic materials to a radial distance of 100 meters,” Appl. Spect. 60(8), 871-876 (2006).
Misra, A. K., Sharma, S. K., and Lucey, P. G., “Remote Raman spectroscopic detection of minerals and organics
under illuminated conditions from a distance of 10 m using a single 532 nm laser pulse,” Appl. Spect. 60(2), 223228 (2006).
Sharma, S. K., Lucey, P. G., Ghosh, M., Hubble, H. W., Horton, K. A., “Stand-off Raman spectroscopic detection
of minerals on planetary surfaces,” Spectrochimica Acta Part A 59, 2391-2407 (2003).
Sharma, S. K., Misra, A. K., Sharma, B., “Portable remote Raman system for monitoring hydrocarbons, gas
hydrates and explosives in the environment,” Spectrochimica Acta Part A 61, 2404-2412 (2005).
Vandenabeele, P., Weis, T. L., Grant, E. R., Moens, L. J., “A new instrument adapted to in situ Raman analysis of
objects of art,” Anal Bioanal Chem. 379, 137-142 (2004).
Vandenabeele, P., Castro, K., Hargreaves, M., Moens, L., Madariaga, J. M., Edwards, H. G. M., “Comparative
study of mobile Raman instrument for art analysis,” Anal Chim Acta 588, 108-116 (2007).
http://water.epa.gov/
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