In the Laboratory
Paint Analysis Using Visible Reflectance Spectroscopy
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An Undergraduate Forensic Lab Erin M. Hoffman and Douglas J. Beussman* Department of Chemistry, St. Olaf College, Northfield, MN 55057; *
[email protected] Paint analysis is an important aspect of forensic science. Paint samples from cars or interior walls are routinely submitted to crime labs where they must be compared to exemplars. Paint is usually composed of three elements: a solvent, a pigment, and a binder. The solvent allows the paint to be applied smoothly as a liquid and the binder undergoes polymerization giving the paint mechanical strength allowing it to adhere to a surface. The pigment component gives paint its distinctive color and is the focus of this experiment. Samples may appear visually similar, but may actually be chemically different with measurable differences. One of the first steps a scientist can take in comparing samples that appear similar is using reflectance spectroscopy. This method records the quantity of light reflected from an irradiated surface, such as paint. Since most paints absorb some light, not all of the incident light is reflected. Depending on the hue, chroma, and lightness of the paint, the quantity of light reflected from the paint varies. In 1976, the Commission on Illumination (Commission International d’Eclairage, CIE) created a standard method of analyzing color, known as the 1976 CIEL*a*b* (or just 1976 CIELab) (1, 2). Figure 1 illustrates the characteristic colors that L, a, and b refer to. A more detailed description of how these values are obtained is given in the Supplemental Material.W All color samples, if nonidentical, have an individual set of L, a, and b coordinates and therefore a distinct location within the CIELab color space system. For example, the more blue in a sample, the more negative the b value is. As this lab was initially designed for a nonmajors forensic science course, a classroom discussion of basic spectroscopy precedes the laboratory experiment. This includes the concepts of photons, wavelengths, energy levels, and the differences between absorbance, emission, and reflectance. Students in the lab are asked to act as forensic scientists in the
Figure 1. Diagram of the color space according to the 1976 CIELab standards.
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trace evidence unit of the lab and are given submitted paint evidence and 2–3 exemplars for reflectance spectroscopy analysis. Other labs have previously been developed that utilize various spectroscopy techniques for forensic analyses. These include visible spectroscopy for the analysis of alcohol in breath (3) and for carbon monoxide in blood (4), atomic absorption spectroscopy for the analysis of gunshot residue (5, 6) and arsenic poisoning (7), and infrared spectroscopy for the analysis of breath alcohol (8) and fibers (9). Reflectance spectroscopy, mainly using infrared wavelengths, has previously been used in forensic investigations of paint samples (10, 11). In this article, we present an experiment using visible reflectance spectroscopy for the forensic analysis of paint in a teaching lab setting. Experimental Methods Students are given what appear to be two or three identical cans of paint with premixed percentages of blue and white paint in the cans. The cans, presumably collected from one or more suspects, should contain different percentages of the mixed paints but should appear visually identical when the paint is applied to a surface and should be labeled so as not to indicate that there are differences in the paints. The students also need to have some “evidence” from the crime scene to compare to the exemplars. Playing the role of forensic scientists, the students are charged with determining whether any of the paint collected from the suspects could have been the source of the paint smear found at the crime scene. To accomplish this task, they are given access to a reflectance spectrophotometer (S.I. Photonics, Inc., model 440), which they use to quantitatively examine the different paint samples. The instrument consists of a UV–vis spectrophotometer and a reflectance probe. The probe contains a flexible fiber-optic bundle, consisting of a single large fiberoptic cable, surrounded by several small cables, inside a metal cap. The small fiber-optic cables transmit the source light onto the sample, and the reflected light is collected and returned to the spectrophotometer via the large cable. The cap creates a seal around the area to be sampled and prevents stray light from entering the instrument. Although instructors are free to choose other ratios of white and colored paint, we have found that adding 0%, 1%, 5%, and 10% (by weight) of white latex flat-finish paint to blue latex flat-finish paint provides paints that are visually similar but can be distinguished using reflectance spectroscopy. Students need to stir each paint sample for approximately 2 to 3 minutes with a paint stick or a glass stirring rod to mix the paint well. Students next apply two coats of blue paint onto pre-primed sheet rock. Each paint sample should be applied to at least three different spots, each ap-
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In the Laboratory Table 1. Average Values and Standard Deviations for the CIELab Terms for Blue Paint Samples White Paint Added (%)
L
a
b
00
50.22 ± 0.14
4.89 ± 0.06
᎑34.68 ± 0.07
01
50.90 ± 0.18
4.82 ± 0.07
᎑34.57 ± 0.11
05
53.15 ± 0.15
4.29 ± 0.07
᎑34.35 ± 0.09
10
55.92 ± 0.12
3.93 ± 0.08
᎑33.77 ± 0.11
NOTE: Each term value is the average of nine measurements for the sample blue paint, with three replicate samples taken on each of three replicate paint deposition spots.
proximately 1 in. × by 1 in. After these have dried, the reflectance spectrophotometer can be used to collect the reflectance profiles of each paint sample. Once the spectrophotometer has been turned on and the software started, the instrument needs to be initialized and set up for this analysis. The program should to be set to collect the percent reflection with the observer set to the “1964, 10⬚” probe definitions and the chromaticity coordinate set to the “1976” CIE definitions. These are all accomplished by simple software selects from pulldown menus. Once set up, a blank should be measured by placing the probe firmly on a portion of the sheet rock with only primer and then activating the “blank” button. It is important that the probe be flush with the surface being analyzed so that no exterior light is introduced. After the blank has been collected, the reflectance spectrum of each paint sample can be obtained. The probe should be held flush with the surface until the measurement has been taken and the screen is updated. A minimum of three measurements of each sample spot should be taken for statistical purposes, more if lab time permits; therefore, at least nine measurements should be made for each sample paint can. The values for the CIELab terms (L, a, and b) should be entered into an Excel spreadsheet after each reading. Once all of the sample readings have been obtained, the average and standard deviation can be calculated for each of the nine paint spots, as well as a total average and total standard deviation for each paint can. This allows for differences between different paint spots of the same paint to be observed, as well as observing any differences between the individual paint can samples. Since this lab was developed for nonscience major students, we do not ask them to calculate additional statistical parameters, such as Student’s t tests, but accept conclusions they can draw based on average CIELab terms and the standard deviations. As stated above, ratios other than 0%, 1%, 5%, and 10% white may be used, but adding more than 10% white may yield color differences that are visually apparent. We have chosen to use blue paint as our base color, but other colors could be used. In fact, one variation on the lab would be to start the lab by looking at several noticeably different colors (such as red, green, blue, etc.) and allowing the students to obtain the corresponding CIELab terms for each color. This would provide some experience with the CIELab color space prior to analyzing more similar colors and would give students a feel for how much the CIELab terms vary between significantly different colored paint. www.JCE.DivCHED.org
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Hazards This lab is safe with relatively few hazards. The paint used in this lab is water-based latex paint; therefore, it is safe if exposed to skin, although gloves are provided to limit this exposure. Goggles should be worn and paint should be applied and dried in a hood. The dried paint samples are disposed of in the trash. Any unused liquid paint can be saved in a well-sealed container for the next lab section. Empty paint containers or liquid paint that is to be disposed of is collected and sent to the local Hazardous Waste or Recycling Center. Instructors should determine local requirements for the safe disposal of unused paint or empty paint containers. Results The mean and standard deviation values for the L, a, and b terms for a representative analysis of the four paint samples are shown in Table 1. As can be seen in the table, there was an upward trend in the L value and a downward trend in both the a value and b value as more white was added. As would be expected, the differences are smaller when comparing the paint samples with 0% and 1% white added than when comparing other combinations of white added. In fact, when comparing the 0% and 1% white paint samples, the a and b values are within one standard deviation of each other. The L terms for these two paint samples are not within one standard deviation of each other, allowing for the two paints to be distinguished based on this term. Of the three CIELab terms, the L term shows the most variability among the different paint mixtures. This is not surprising since the L term is a measure of the lightness, which is directly related to how much white paint has been added to the original colored paint. Even though it is difficult to visually distinguish even the 0% and 1% white paint samples from each other, the reflectance spectrophotometer was able to do so, allowing students to compare and contrast each suspect’s paint sample with the paint evidence found at the crime scene. Mixtures differing by less than 1% white were not able to be statistically distinguished from one another. To test instrument drift, ten data points were collected over a five minute period. Over this time, the resulting L, a, and b values differed by less than 1%. The values randomly fluctuated with no noticeable increasing or decreasing trend. The small differences are likely as much a factor of different areas of the paint spots being tested as they are of instrumental changes. This stability is important, since some students choose to analyze one paint sample first before depositing the other samples and letting them dry. While this is a more time consuming way to complete the lab, rather than depositing all paint samples and letting them all dry at the same time, we allow the students to devise their own testing methods. While a more thorough statistical treatment of the data is beyond the scope of the nonscience majors course that this experiment was designed for, such a treatment could be included if a different audience is targeted. Even without more detailed analysis, a simple comparison of average values for the CIELab terms, along with a consideration of associated standard deviations, does allow the students to correlate the “evidence” paint sample with a single paint can obtained from
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a single suspect. In the process, students learn something about spectroscopy, sampling technique, and basic statistical analysis of data. Acknowledgments The authors would like to thank Branden Moriarity and Scott Moriarity for supplies and helpful suggestions. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Bell, S. Forensic Chemistry; Pearson Education, Inc.: Upper Saddle River, NJ, 2006; pp 460–474.
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2. Laden, P. J. Chemistry and Technology of Water Based Inks; Blackie Academic and Professional/Chapman and Hall: London, 1997. 3. Timmer, William C. J. Chem. Educ. 1986, 63, 897–898. 4. Huddle, Benjamin P.; Stephens, Joseph C. J. Chem. Educ. 2003, 80, 441–443. 5. Hern, J. A. J. Chem. Educ. 1988, 65, 1096. 6. Dahl, Darwin B.; Lott, Peter F. J. Chem. Educ. 1991, 68, 1025–1026. 7. Tarr, Matthew A. J. Chem. Educ. 2001, 78, 61–62. 8. Kneisel, Adam; Bellamy, Michael K. J. Chem. Educ. 2003, 80, 1448–1450. 9. Bender, Sharin; Lillard, Sheri J. J. Chem. Educ. 2003, 80, 437– 440. 10. Giang, Yun-Seng; Wang, Sheng-Meng; Cho, Li-Ling; Yang, Chao-Kai; Lu, Chen-Ching. J. Forensic Sci. 2002, 47, 625– 629. 11. Zieba-Palus, Janina. J. Mol. Struct. 2005, 744, 229–234.
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