The Analytical Approach C. W. Jameson National Institute of Environmental Health Sciences Research Triangle Park, N.C. 27709
R. F. Moseman B. J. Collins N. D. Hooper Radian Corporation Research Triangle Park, N.C. 27709
SPY DUST Detecting â Chemical Tracking Agent:
Recent reports have alleged that 5-(4-nitrophenyl)-2,4-pentadien-l-al (NPPD), the chemical referred to as "spy dust," has been used to track em ployees of the U. S. Embassy in Mos cow. The spy dust is believed to have been sprayed or deposited in places that embassy personnel frequently touched, such as their car seats, steer ing wheels, and doorknobs. The mate rial could be applied in such small concentrations that it was essentially invisible. It would then be transferred to anything the employee encoun tered, leaving a trail that could be used to monitor his or her activities. Originally there was speculation that NPPD might be mutagenic and thus might have the potential to cause cancer. However, no toxicity informa tion was available on this chemical to confirm this speculation. The Toxicol ogy Research and Testing Program of the National Institute of Environmen tal Health Sciences became interested in studying this compound because of the lack of toxicological data and the potential for human exposure. Results of toxicity studies showed that NPPD is not well absorbed through the skin and is not mutagenic in mammalian cells. This result, coupled with the ex tremely low levels of exposure, indi cates that NPPD will not cause cancer. Spot test NPPD is an α,/3-unsaturated aro matic aldehyde, which is a light-yellow crystalline solid. It is essentially insol uble in water but soluble in acetone and alcohol. It is a strong absorber of ultraviolet light but does not fluoresce appreciably. As part of our studies of NPPD we investigated basic qualita tive organic analysis techniques to de velop a quick and simple method to detect very small quantities of NPPD.
0003-2700/86/0358-915AS01.50/0 © 1986 American Chemical Society
The method we developed is based on a standard spot test technique and takes advantage of a color reaction to detect aromatic and α,/3-unsaturated aldehydes. Methanol is used as the solvent, naphthoresorcinol as the color reagent, and HC1 is used to catalyze the color reaction. Naphthoresorcinol reacts with NPPD, which is colorless at low concentrations in methanol, to give a pink to red-violet color propor tional to increasing concentrations of NPPD. A spy dust kit, made up of dropper bottles of methanol, naph thoresorcinol, HC1, and cotton swabs could easily be carried in a person's pocket for detection of the tracking agent in the field. The simple procedure for detecting NPPD consists of wiping the object to be tested with a cotton swab moist ened with methanol and placing the swab in a test tube containing 0.5 mL of methanol. The same volumes of 0.1% naphthoresorcinol in methanol and concentrated HC1 are added to the tube. Following addition of the acid a pink to red-violet color is pro duced if NPPD is present. As an alter native to extraction in the test tube, the cotton swab used to wipe the ob ject can be treated with one drop of 0.1% naphthoresorcinal and one drop of concentrated HC1. The pink to redviolet color on the cotton swab itself indicates the presence of NPPD. We have determined that as little as 150 ng NPPD/100 cm 2 can be picked
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986 · 915 A
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Figure 1. Color reaction of related aldehydes (A) Naphthoresorcinol blank, (B) 3-phenyl-2-propen-1-al (cinnamaldehyde), (C) 3-(4-nitrophenyl)-2-propen1-al, (D) 5-(4-nitrophenyl)-2,4-pentadien-1-al (NPPD), (E) 7-(4-nitrophenyl)-2,4,6-heptatrien-1-al
up on a cotton swab and give a color reaction that is visible to the eye. When the corresponding items were sampled prior to addition of NPPD, no color was produced. By monitoring at 506 nm with a spectrophotometer the sensitivity increased to less than 20 ng/100 cm2. Testing the detection method To test this detection method, ex periments were carried out with NPPD-contaminated counter tops and doorknobs. The chemical was transferred to other clean objects by human touch. In one trial 20 μg of NPPD was applied to a doorknob. Af ter using this doorknob, a second door was opened to enter and a third door was opened to exit a lab. All three doorknobs were sampled, and NPPD was detected each time. The intensity of the color obtained decreased from the first to the third sampling. In a second trial, a doorknob was sprayed with 0.5 mg/mL N P P D in methanol and allowed to dry. There was no visi ble sign of NPPD on the knob. A chemist then exited the laboratory us ing the contaminated doorknob and continued working in another labora tory, noting all items that were touched. Forty-five minutes later NPPD was detected on the chemist's hand and on all items touched during that period. As expected, this method will detect the presence of any aromatic or α,/3-unsaturated aldehydes. However, different colors are produced (Figure 1). For example, citral and nitrobenzaldehyde give a yellow-green color, and furfural and orthovanillin give an orange color. There are very few natu rally occurring aldehydes that would interfere with the assay. All test sur faces were sampled prior to applica tion of NPPD as a control, and no col or response was obtained. Our detec tion method was field tested under
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actual monitoring conditions by sur veying vehicles of U.S. Embassy em ployees in Moscow who were consid ered to be the most likely targets for use of the tracking agent. Five of the vehicles tested gave a positive color re sponse for NPPD. This was further substantiated by additional swipe sampling of these cars and analysis at another laboratory by gas chromatography-mass spectrometry (GC/MS), which verified the presence of the spy dust. Swipe samples from cars that gave no color response were also ana lyzed by GC/MS for the presence of NPPD. No spy dust was detected. We have used our sampling and analysis techniques to evaluate opti mum methods for the removal of NPPD from various surfaces. Because this compound is soluble in alcohol, both methanol and isopropanol were used to clean contaminated counter tops. After two washings with paper towels soaked with these alcohols no ' NPPD was detected. An industrial cleaner was also used full strength on a contaminated counter. Again, after two successive washings no NPPD was detected. Removal of NPPD from the hands was also investigated. After touching an NPPD-contaminated counter with the right hand both hands were sam pled. The left hand gave a negative re sponse; the right hand sampled posi tive for the presence of NPPD. The hands were then washed with tap wa ter and again sampled. The left hand was negative, and the right hand gave a weaker positive response. The hands were then thoroughly washed with soap and water, and no NPPD was de tected on either hand. We recom mend, based on our studies, that soap and water be used to clean NPPD from the hands. For other items, we have found that washing with alcohol or a strong industrial cleaner will re move NPPD effectively.
