TRANSFER OF RESIDUES IN FINGERPRINTS

In today’s society, it has become necessary to not only identify a terrorist participating in an attack, but determine what explosive was used and what the locations of the terrorist were prior to the attack. Such knowledge helps law officials determine if there are other credible threats, and link terrorists to each other and thereby identify potential terrorist cells. Much of forensic science depends on Locard’s Exchange Principle, which states that when two objects contact each other there is a transfer of material between them. It is therefore expected that when terrorists handle explosives, explosive materials will be transferred first to their hands, and then to other surfaces their hands touch. If the amount of explosive in these prints can be quantified, it may be possible to determine where a terrorist has been, and the order of events leading up to an attack. This study therefore aimed to quantify the amount of energetic salt residue found in consecutive prints of ammonium nitrate and potassium chlorate on three different surfaces (filter paper, polypropylene, and polyurethane). By collecting and extracting consecutive prints from surfaces, it was possible to quantify the amount of trace explosive material in each print using ion chromatography (IC). The trends in material deposited in consecutive prints were then compared to each other to determine the reproducibility of prints between people and on different surfaces. Results indicate both materials typically leave first prints in the amount of several hundred micrograms. Further, while most trials produced decreasing curves, occasional higher amounts were found in later prints. This indicates that occasionally aggregates of particles form and are deposited during printing. While reproducibility indicates that the roughness of the surface did not significantly affect the rate at which material was deposited, the sorption properties of the energetic salts and the surfaces may play a role in the amount of material deposited. This was determined because the highest amounts of the hygroscopic ammonium nitrate were found on the liquid absorbent filter paper surfaces, while the highest amounts of the ionic powder potassium chlorate were found on the electrostatically chargeable polypropylene surfaces.


Introduction
An important postulate of forensic science is Locard's Exchange Principle, which states when two objects come into contact there is transfer of material between them.
Based on this postulate, individuals who handle explosives are expected to transfer some of the explosives to their hands during the process. Transfer of trace levels of the residue from their hands to handled objects is likely due to natural oils on the hands and the adhesive properties of the explosive particulates. Therefore, trace residue of explosives may be found on common items touched by contaminated hands, such as clothing, lap top computers, and luggage. [1][2][3][4] The amount of residue deposited in successive fingerprints is expected to decrease. However, several studies have shown a great amount of variability from one C4 fingerprint to the next. [2][3][4] The amount of residue will depend on the amount initially present on the finger, the number and type of prior finger contacts, 4 and the force applied by the contaminated finger. 2 The purpose of this study was to quantify the amount of energetic residue deposited by successive fingerprints on three different surfaces. To ensure the safety of all participants, only the energetic salts ammonium nitrate (AN) and potassium chlorate (KClO 3 ) were used.

Materials and Methods
Ammonium nitrate was ground in a coffee grinder resulting in particle sizes ranging from 125µm to 1845µm with an average particle size of 577µm. Unground potassium chlorate (Fisher) particles ranged in size from 82µm to 2382µm with an average of 1359µm. Particle size was measured using a Nikon Eclipse E400 Pol microscope with Mettler Toledo FP90 central processor. Images of particles are in Appendix A.
To observe the affect that different surfaces have on residue transfer, three surfaces were used: filter paper (fp), polypropylene (pp), and fake vinyl polyurethane (pu). A photograph of these surfaces can be found in Appendix A. The filter papers (Whatman qualitative circles, 100mm) were cut in quarters, cleaned via acetone Soxhlet extraction for 24 hours, and oven dried flat at 90°C for 24 hours. The polyurethane (100% polyurethane on 50/50 polyester/cotton backing, 1.3mm thick) and polypropylene (0.2mm thick) were cut into 2 by 2 centimeter squares, washed with distilled water, and air dried.
Participants were categorized by the amount of information and experience they had prior to participating in the fingerprint trials. Participants 1 through 10 received verbal instructions only, while participants 11 through 20 received verbal and written instructions (shown in Appendix B). For trials 21 through 28 two laboratory researchers who were intimately knowledgeable of the testing protocols and goals created prints on one surface, filter paper. Odd number participants or trials used AN; even numbered trials used KClO 3 . Each participant created consecutive fingerprints on each surface; the surfaces were collected and extracted; extracts were analyzed by ion chromatography.
Oils on the fingertips, amounts of initial contamination, pressure applied by the fingertips, and numbers of prior prints affect the amount of trace residue transferred to surfaces; the instructions given attempted to limit these variables. Fingers were cleaned with soap and water and thoroughly dried with paper towels before each trial. False positives caused by hand oils, creams, or soaps were accounted for by having every participant create a blank pre-exposure to the energetic salt. After creation of a blank, the finger was exposed to an energetic salt and printed on ten consecutive pieces of one type of surface. This procedure was repeated for each type of surface.
Energetic salts available to participants were provided as a "reservoir" of approximately 8 mg per trial; the salt was re-weighed after printing to estimate how much adhered to the finger by subtracting the two values. All prints were created by placing the same finger on a surface and placing a 200g steel cylinder on the back of the finger to create a consistent pressure during initial exposure and printing. Initial studies using different explosive powders, indicate that after the tenth fingerprint the prints no longer contained sufficient residue to be distinguished from blanks; therefore, only ten prints (and one blank) were collected per trial.
Each surface was extracted with 10 mL of Millipore water (18.2MΩ/cm), shaken for one hour at 240 rpm, and syringe filtered (5mL Luer-Lok tipped syringe with Millipore Millex -FG phobic PTFE 0.20µm) into individually labeled 0.5mL Dionex PolyVial IC vials. Samples determined to be too highly concentrated to be calibrated against the standard curve were diluted 1:25 in water.
All samples (25µL) were run on a Dionex ICS-2100 RFIC with an IonPac AS19 column. The eluent, potassium hydroxide (20mM), was run at a flow rate of 1mL/min; detection was performed with conductivity suppression (ASRS 300-4mm). Samples were run along with standards (ranging from 50ng/mL to 10000ng/mL) and periodically with Millipore water blanks and 500ng/mL standards. Table 1 and for KClO 3 are shown in Table 2.

Results for AN trials are shown in
Initial amounts of AN and KClO 3 adhering to the participants' fingers are based on the amount of salt missing from the residue reservoir. Generally, participants' fingertips picked up 2 to 3 mg of the energetic salt regardless of the salt used. Participants 1 through 20 made prints on three surfaces, while the two laboratory researchers (trials 21 to 28) printed only on filter paper. Tables 1 and 2 present all data in terms of nanograms of salt deposited per fingerprint. These values have been averaged by print number, but in all cases the standard deviation was of the same magnitude as the average itself. Tables 3 and 4 express the amount of residue as percent residue relative to the first print. In general, the amount of residue in the prints decreased with print number. However, there were several instances where later prints showed high amounts of salt suggesting particulate aggregates were occasionally deposited.

Discussion
All sets of prints generally decreased in a manner which could be fitted to a power law although there were occasional high amounts in later prints. There was a good deal of individual variability. Figure 1 shows exemplary data sets from 14 participants printing on filter paper. Results of each participants' ten-print, 3-surface trial can be found in Appendix C. To observe a general trend of residue deposit regardless of the type of residue or surface, data was normalized by relative percent; the first print in a trial was set to 100%, and all other prints in that trial were a percentage thereof (Tables 3 and 4).
There were several difficulties with that approach. 1) In a few trials the first chlorate prints were lost. 2) In one case the blank contained more chlorate than any of the prints (6 mg), clearly an error. 3) In another case (18), all prints and the blank showed the same amount of chlorate (1.3 µg). 4) In many trials three of the ten fingerprints contained more residue than the initial one (Tables 3, 4). 5) There were many instances where one or two of the subsequent prints showed a sharp increase in the amount of residue, suggesting a chunk of reside was deposited. It is difficult to track the source of these errors. The participant may not have contaminated his hand or contaminated it before performing the "blank." Hand washing may have been skipped between data sets or excess pressure applied during printing. It was thought that humidity had an impact on the amount of residue transferred, particularly in the case of AN, which is highly hygroscopicity and deliquescent (62% RHD). [5][6][7][8][9][10] In fact, AN tends to visibly aggregate and liquefy on hands. The data was handled as follows. 1) For the few trials (4-fp, 2-pu, 4-pu) in which the first print showed no chlorate, the first print average was inserted (in red) to permit processing of the rest of the data. 2) The fact that the blank showed more chlorate than any prints in trial 14-fp suggests participant error; thus, the entire data set was deleted. 3) In the case of 18-fp where all prints and the blank showed 1.3 µg of chlorate, the entire data set was dropped. Dealing with intermittent increases in energetic salt in late fingerprints was a more difficult problem. Several of approaches to analyzing the data in a legitimate, unbiased manner were employed. First, a mathematical program, Sage, 11 was used to test the goodness of each data set. The program fitted the data to the formula y=A*e -x +B, where B is the intercept the percentage of residue as print number approaches infinity, negative A values are shown in Figure 2. Using the "Sage A" discriminator, six AN data sets (3-pp, 15-pp, 19-pp, 5-up, 11-up, 19-up) and nine chlorate data sets (8-fp, 10-fp, 18fp, 8-pp, 20-pp, 12-up, 14-up, 18-up, 20-up) were dropped.

Fig. 2. Exemplary Plots from Sage 11 showing a positive A (left) and negative A (right)
Tables 3 and 4 express the quantity of salt, AN and chlorate, respectively, found in each print as a percentage of that in the first print. When the percentage was greater than 110%, that datum was dropped. In Tables 3 and 4 data deleted due to the "Sage A" criterion are struck through, and those deleted due to the values exceeding 110% of the first print are marked in highlight. The total number of data ignored to obtain the average percentage of salt in each print is indicated on each Table. It is troublesome that in many cases 40% to 60% of the data were discarded. The final averages from all four Tables is shown and plotted in Figures 3 and 4. Figure 3 was constructed from the full data sets, with no data ignored. Figure 4, where residue is expressed as the percentage of the first print, necessarily dropped outlining points. There are significant differences between the averages derived directly from the full data sets (Table 1 and Table 2) and those derived from the culled data.    Table 5 summarizes the average amount of AN or chlorate found in print one and in print ten. The difference in the amount of AN versus chlorate found in the prints was not significant on polyurethane. However, AN appeared to have adhered to the filter paper slightly better than did chlorate. This may be due to the highly hygroscopic nature of AN. The high values of chlorate on polypropylene are more difficult to explain.
Polypropylene is at the negative end of the triboelectric series, increasing its ability to become electrostatically charged, while paper and polyurethane are more neutral. 12 This may explain the increased transference of the ionic powder KClO 3 to polypropylene.
Nevertheless, data suggests that with either AN or chlorate in the first ten prints several tens of micrograms may be found. Table 5. Nanograms Residue in the Print

Conclusion
While most sets of prints showed a decrease in the amount of residue transferred over successive prints, this decrease was rarely smooth. Although the data could be plotted as a power law or exponential decrease, there were intermittent increases in the amount of energetic salt deposited. These are believed to be due to particulate aggregates which periodically dropped from the finger during printing. The variability of the data was the same regardless of the experience or knowledge of the participantsonly instructed verbally (1 to 10), instructed in writing and verbally (11 to 20), or intimately familiar with the experiment (21 to 28). This suggests that variations in the data were not simply a matter of participant error, but rather likely due to the salt aggregates which at times were deposited on the surfaces.
The variability in results make it impossible to detect a difference in transfer between the AN and chlorate. Nevertheless, it is noticeable that more anomalous data was found in the chlorate sets than in the AN sets even though the amounts of residue found in each study were similar. Differences among the three surfaces were noted. The hygroscopic nature of AN was thought to promote its adherence to filter paper, while chlorate was more attracted to polypropylene.
While it is not possible to determine from the amount of residue in a print the order in which the print was deposited, it is possible to detect whether an object was touched by someone who recently handled ammonium nitrate or potassium chlorate.
Hundreds of micrograms could be expected to be in the first few prints.
Image 3: Surfaces used in trialsfilter paper, polypropylene, and polyurethane (left to right).

TO PARTICIPANTS 11 THROUGH 20
Each time your finger touches a substrate or A.N. the weight must be added. 1. Place weight on finger between tip and first joint. 2. Release so that pressure is on finger but weight does not roll off of finger. Do not push.
To Make Fingerprints: I. Wash your hands with soap and water. Dry. II.
Place clean, dry right index finger on first piece of paper substrate (Blank 1). III.
Place same finger on second piece of paper substrate (Blank 2). IV.
Place same finger on AN tray provided. Remember to add the weight. V.
Place same finger on next piece of paper substrate. VI.
Continue placing the finger on the pieces of paper until there are no paper substrates left. VII. Repeat steps 1 through 6 with the new tray of AN and plastic substrate. VIII. Repeat steps 1 through 6 with the new tray of AN and vinyl substrate. IX.
Please wash your hands before leaving the lab.