Abstract

In recent years, the seizure of money obtained from drug trafficking has been associated with the arrests of drug dealers.  The presence of drugs on seized banknotes has been used as evidence in drug trafficking cases.  Until now, the method of drug analysis on paper currency has involved considerable sample handling and is destructive to the evidence, hence preventing future reanalysis.  Our project focuses on the detection of drug surrogates and common cutting agents on paper currency using Raman microspectroscopy.  This method is non-destructive and no sample processing is required.  We have investigated the detection of individual drugs as well as heterogeneous mixtures (drugs and cutting agents) on one-dollar bills.  Our results indicate that it is possible to determine the identity of single crystals in a mixture on paper currency.

Introduction

Drug consumption has become a major societal problem in most countries as it has been linked to high crime rates as well as other social disruptions.  Given the magnitude of this problem and its rapid spread, measures are being taken to curb drug trafficking.  Some countries have even adopted laws that allow seizure of money during the arrests of drug dealers (Sleeman 2000).  The evidentiary importance of the seized currency relies on the detection of drugs on the banknotes.  Though the relevance of this evidence is still being questioned, recent research has been focused on the detection of the presence of drugs on money.

Various analytical techniques have been used in the detection of illicit compounds such as cocaine and 3,4-methylenedioxymethamphetamine, most commonly known as ecstasy (Bell 2000).  Such techniques involve mass spectrometry, chromatography and infrared spectroscopy.  A method for detection of drugs on single bank notes as well as on bundles of money has been recently reported.  This technique involved washing the drugs off the money, followed by gas chromatography-mass spectrometry analysis (Sleeman 2000).  Unfortunately, these methods have proved to be somewhat time consuming, as extensive sample preparation is required (Angel 1999). 

Raman spectroscopy has been used as an alternative forensic tool in the detection of street drugs which are a mixture of drugs and cutting agents (Hodges 1989).  Raman spectroscopy uses a light from a laser to probe vibrational modes of molecules.  This spectroscopic technique offers several advantages over the aforementioned methods.  Raman spectroscopy can be used on any transparent cell, hence allowing analysis of substances present in glass vials or sealed clear plastic bags.  No sample handling is required, hence preventing contamination or destruction of the analyte (Hodges 1989).  Two major shortcomings have however limited the use of Raman spectroscopy for qualitative analysis.  Fluorescence is one major problem as even low levels of fluorescence can mask the Raman signal.  Hence the use of Raman in the investigation of colored samples or highly fluorescing analytes is difficult.  The second problem arises when complex samples are studied in bulk.  It is challenging to determine the identity of each compound when multiple peaks from several compounds are present in the same spectral region.  We hope to overcome some of these challenges by using Raman spectroscopy to look at single crystals, rather than bulk samples.

Recently, the use of Raman spectroscopy has been applied to the analysis of pure samples of illicit drugs, as well as street drugs.  Hodges et al. have used Fourier transform Raman spectroscopy to study bulk samples of amphetamine sulfate, cocaine hydrochloride, heroin as well as actual street samples (Hodges 1989).  Unfortunately, this technique is limited to well characterized samples where individual spectral signals can be identified from the mixture.  Our project aims to apply the selectivity of Raman microspectroscopy which allows us to discriminate between signals originating from individual crystalline particles, hence enabling identification of single crystals.  The microscopic nature of our instrument permits us to investigate the qualitative composition of heterogeneous mixtures without any of the spectral processing necessary for the work of Hodges et al.

Instrument

The Raman microscope was assembled from individual components.  The total cost of the home-built instrument did not exceed $30,000.  The Raman system used is shown in a top view in Figure 1a and in a side view in Figure 1b.  The excitation source is a 10mW HeNe laser at 632.8 nm (Uniphase).  Light from the laser was focused on the sample using an assembly of lenses and mirrors. The laser beam is reflected by a dichroic filter (Omega Optical) and then focused onto the sample by adjusting the 20X ULWD MS-Plan 2 microscope objective, mounted on an Olympus BH2 microscope.  Raman scattering of the sample is back collected through the same objective and is reflected by a 90° glass prism.  Lenses 1 and 2 and the pinhole are used to spatially filter the signal.  The holographic notch filter (Kaiser) is important as it eliminates any Rayleigh scattering present in the signal[1] 

The monochromator (Acton Research Corporation (ARC) SpectraPro-150 with imaging dual grating) used in data acquisition was used with the grating of 600 grooves/mm, which yielded a range of 0-4000 cm^-1 with HeNe laser excitation.  A thermo-electrically cooled charge coupled device (CCD) (SpectruMM CCD Detector, ARC) was used as the detector and data was acquired using ARC SpectraSense CCD (v. 3. 00 ARC) software.  The integration time for spectral acquisition was adjusted in order to optimize the signal to noise ratio.

Figure 1.  The instrumental set up of the Raman microspectrometer.  (a) illustrates the top view of the instrument, and (b) shows the side view picture.

Data Acquisition

Benzocaine (CAS# 94-09-7), lidocaine (CAS# 137-58-6), isoxsuprine (CAS# 579-56-6) and norephedrine (CAS# 154-41-6) were obtained from Sigma Chemical Co. (St Louis, MO), and used without further purification.

The first part of this study consisted of acquiring the spectra of the homogeneous sample, i.e. pure chemicals.  These spectra were taken on microscope slides as well as on a one-dollar bill.  A small portion of the chemical (~5mg) was sprinkled onto the appropriate sample cell (slide or bill). 

In the second phase of the experiment aimed at investigating heterogeneous mixtures.  These mixtures were made by thoroughly mixing small portions of the four analytes together (~1.25 mg/sample).  Once the mixture is prepared, it was placed on a microscope slide or on a one-dollar bill for analysis.  Spectra of the individual crystals in the mixtures were recorded.  Each round of data acquisition was repeated three times and the peak positions were noted and averaged.

Calibration is required prior to each set of data acquisition to convert our data from, an "intensity vs. pixel" spectrum to an "intensity vs. wavelength" spectrum.  Prior to acquiring each set of data, the instrument is calibrated using a Helium emission lamp.  This is necessary because the charge coupled device (CCD) detector is a multi-channel detector attached to a variable wavelength monochromator.  Each time the monochromator grating is moved, it is necessary to determine the wavelength associated with each detector element[2].  Our calibration is achieved by taking the spectrum of a Helium lamp, and by plotting the known wavelength for each He emission line versus the pixel number for each peak.  A linear fit is carried out to convert from pixel number to wavelength and further calculations are then done to convert the peak positions from wavelength (nm) to wavenumber (cm^-1).

Results and Discussion

Two drug surrogates[3] and two common adulterants were used in this study.  The two drug surrogates were isoxsuprine and norephedrine, respectively a cocaine- surrogate and an amphetamine-related substance.  The two cutting agents were benzocaine and lidocaine.  Cutting agents were included in our analysis as they are local anaesthetic sugars that are commonly found as adulterants in street drugs (Carter 2000). 

On Microscope Slide

The spectra taken for the four chemicals on microscope slides are illustrated in Figure 2.  Information about the position of the peaks and their intensity is extracted from these spectra and used as the spectral signature of the respective analyte.  Major peak positions are reported in Table 1.

Figure 2.  The spectra for the various chemicals on microscope slide: (a) Benzocaine, (b) Isoxsuprine, (c) Lidocaine and (d) shows spectrum of Norephedrine.  The spectrum in (a) was acquired for 6 seconds.  Spectra for (b), (c) and (d) were acquired over 20 seconds.

Table 1.  Position of the peaks for each chemical on microscope slide and on one-dollar bill.

It was observed that each analyte has a different number of peaks.  Even though some of the peaks are overlapping, the pattern of each analyte is distinguishable.  Hence, information about the peaks is unique to each chemical and can be used to determine the identity of the analyte. 

It should be noted that all of the spectra, except for benzocaine, required relatively long integration times.  The low quality of the signal is believed to be due to both inherently poor scattering by the samples and the low power of the laser source. 

On Dollar Bill

The spectra of the four chemicals were then acquired on the dollar bill.  The spectra obtained for each analyte are shown in Figure 3.

Figure 3.  The spectra of drugs and cutting agents on a one-dollar bill: (a) Benzocaine, (b) Isoxsuprine, (c) Lidocaine and (d) Norephedrine.  The spectrum in (a) was acquired for 2 seconds.  Spectra for (b), (c) and (d) were acquired over 5 seconds.

Table 1 compares the positions of the peaks on microscope slides as well as the position of the peaks on dollar bill for each analyte.  Several discrepancies can be noted by comparing the two sets of data for each chemical.  There is some inconsistency in the peak positions.  This difference in wavenumber between the two sets of peaks can be attributed in large part, to the uncertainty in our wavelength calibration (~ ± 5 cm^-1).  Another difference lies in the number of peaks that are observed on the dollar bill as compared to the microscope slide.  For instance, the spectrum of norephedrine on the slide indicates the presence of 6 peaks, while showing only 3 peaks on the dollar bill.  Though the number of peaks has been reduced, the fact that all spectra are distinguishable from each other, allows us to discriminate between the different chemicals, even with a limited number of peaks.  The reduction in the number of peaks is inherently tied to the level of fluorescence produced by the dollar bill.

A comparative set of spectra of isoxsuprine on a microscope slide and dollar bill is shown in Figure 4.  These two spectra clearly illustrate the high level of fluorescence on the dollar bill spectrum.  In fact, all four chemicals under investigation yielded significantly higher levels of fluorescence on the dollar bill than on the microscope slide.  Acquisition time for the spectra on the dollar bill was much lower than on the microscope slide, as saturated peaks are observed at long acquisition times on the dollar bill.  Fluorescence, thereby, hinders the observation of the weaker signals.

Figure 4.  Spectrum of isoxsuprine on microscope slide over an acquisition time of 20s shown in (a).  (b) indicates the spectrum of isoxsuprine on one-dollar bill, acquired over 5 s.

Faces of the Dollar Bill

Our first attempt in understanding the effect of the dollar bill on the level of fluorescence observed was directed to probing the two faces of a one-dollar bill.  Spectra of the four analytes were taken separately on the dollar bill face-up (front) and on the dollar bill facedown (back).  The spectra obtained for benzocaine crystals are used to illustrate our findings.

As shown in Figure 5, a net difference in the spectra from the front of the bill and the back of the bill can be noted; the level of fluorescence is significantly greater on the back of the dollar bill.  This was observed for all four analytes.  The difference in the fluorescence cannot be fully explained.  However, it is possible that this discrepancy in the two spectra might be due to the presence of watermarks for counterfeit protection on the dollar bill.

Figure 5.  Spectrum of benzocaine : (a) on the face and (b) on the back face of the dollar bill.  Both spectra were acquired over a period of 2 seconds.

Colored Regions of the Dollar Bill

We have also directed some effort to the investigation of the spectrum on the different colored regions of the bill.  We have specifically focused on the study of the green colored portion, the light colored part and the dark regions of the bill.  Our motivation relied on the knowledge that deeply colored substances are expected to yield more fluorescence.  Figure 6 illustrates our findings.

The spectra of benzocaine on the green and light-colored region of the bill both show a significant increase in the level of fluorescence compared to the dark region of the bill.  Though the high amount of fluorescence was expected, the fluorescence yielded by the light region was also considerably higher than that from the green region.  The significantly lower level of fluorescence on the darker region of the bill is also unexpected, as dark objects are expected to fluoresce to a greater extent.  The color pigmentation present in the bill is still considered to be responsible for the difference in the spectra, though a clearer understanding of what pigments are present is needed.  One possible explanation for the observed spectra might lie in the complexity of the chemical composition of the lighter and dark colored dyes, which do not fluoresce as expected.

Figure 6.  Comparison of different color regions of dollar bill. Spectra of benzocaine are shown on (a) green region, (b)light region, and (c) dark region of a one-dollar bill. All spectra were taken at an acquistion time of 2 seconds.

Heterogeneous Mixtures

In the course of our project, we have also simulated a street drug sample by mixing the drug surrogates and the adulterants.  The bright field images of such mixtures were taken on the microscope slide and on the dollar bill, and are illustrated in Figure 7.  From these figures, it is apparent that the analytes are of different particle sizes.  The Raman microspectrometer allows us to focus on any individual crystal present in the mixture.  The spectra of every single crystal present in our mixture can be acquired.  The peak positions of each crystal are compared to the spectral signature previously obtained for our four analytes.  We are thus able to identify the presence of the specific drugs and cutting agents on microscope slides as well as the banknote, even when they are present in a heterogeneous mixture.

In the course of this study, we have encountered several challenges for our technique including fluorescence and weak signals.  Our future steps will include attempts to resolve these problems and to improve our analytical method.  We have noted that the smaller sized particles seem to yield poorer signal.  Since we link this particular problem with the large focus of our laser and subsequently large collection volume, we also plan to modify the optical components of our instrument to maximize collection efficiency.  In the future, we also plan to tackle the major problem of fluorescence.  We intend to replace our HeNe laser with a diode laser.  It is believed that the longer wavelength of such a laser will help eliminate or attenuate the levels of fluorescence.  We also plan study paper currency coming from other countries to investigate the fluorescence associated with colored foreign banknotes.  Finally, in order to evaluate the application of our method in actual drug cases, we are also planning to apply for a Drug Enforcement Agency (DEA) license, so that we can investigate illicit drugs.

[1] Raman scattering is approximately a million fold less intense than Rayleigh scattering.  Holographic optical elements allow elimination of a narrow band of wavelengths very efficiently.  Our holographic notch filter allows us to eliminate  99.99% of Rayleigh scattering.

[2] Readers interested in learning more about the CCD should refer to the article published in the third issue of JYI by Courtney Peterson: "How it works - the charged coupled device, or CCD"  http://www.jyi.org/issues/issue3/features/peterson.html

[3] A drug surrogate is a compound which is similar in structure to the illicit drugs e.g. isoxsuprine is similar in structure to cocaine but not pharmacologically active and hence is not illegal to possess without a DEA license.