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Forensic Analysis of Shimmer Particles in Cosmetic Samples

Kandyss Najjar1, Robert D. Blackledge2, and Candice Bridge1

1National Center for Forensic Science and the Department of Chemistry, University of Central Florida, College of Sciences, Orlando, FL, USA

2Naval Criminal Investigative Service, Regional Forensic Laboratory, San Diego, CA, USA

1.1 Introduction

Shimmer particles are commonly observed in daily life. From cosmetic products to paint samples, shimmer particles are readily present. However, despite its common presence, it has not been readily considered in the forensic trace evidence community as a form of contact evidence.

In this book's first edition, the chapter on glitter analysis mentioned that glitter and shimmer were often confused with each other [1]. Although both are ingredients in cosmetic products, they are easily distinguished by a cursory microscopic examination. In addition to microscopic analysis, there are fundamental chemical differences that differentiate these items. Regardless of the chemical differences, one property that might be common between these items is the potential to be an ideal contact trace evidence with strong indications of association between two people or a person and a physical item.

The previous glitter chapter in the first edition of this book asked the question: What are the properties of the ideal contact trace? To answer this question, the authors listed seven properties that would make any item an ideal contact trace sample. Based on these criteria, glitter can be considered an ideal contact trace material. The next question is - can shimmer particles be considered an ideal contact trace?

In this chapter, we discuss the chemical properties of shimmer particles, how these particles can be recovered as trace evidence samples, and the most appropriate instrumental methods for analyzing these samples. It will also be discussed how a Questioned and Known shimmer sample can be compared in a forensic casework setting. Through these discussions, it will be demonstrated that shimmer particles can also be considered an ideal contact trace evidence in addition to glitter samples. Although glitter has been demonstrated to be a critical associative evidence in numerous criminal investigations, to date, shimmer has been largely ignored by criminalists.

1.2 What is Shimmer?

Although this chapter focuses on cosmetic shimmer, many other products include shimmer particles. In the automotive paint industry, shimmer is known as an "effect pigment." Kids and adults use them in creating arts and crafts. There are many commercial glue and pen products containing both glitter and shimmer. Moreover, particles have also been incorporated into everyday clothing and shoes, as well as in costumes for special events such as Halloween and Mardi Gras. They are even used for decorating greeting cards and ornaments used during holidays such as Thanksgiving and Christmas.

The focus of this chapter is on the identification of shimmer particles used in cosmetic products and personal hygiene products. Such products include lipstick, foundation, eye liner, hair spray, body lotion, and more. Cosmetic products use mica particles with different layer thicknesses of varying metal oxide coatings to achieve various shades of a certain color [2]. This is done to satisfy the ever-increasing demand for new colors in cosmetics [3]. For instance, some silver pigments are created by coating with titanium dioxide to produce shades ranging from soft silver to dazzling silver. In contrast, other natural or earth-toned colored cosmetics use iron oxide to obtain shades ranging from soft bronze to bright red. Since shimmer can be found in various cosmetic products, and most are intended for everyday wear rather than just for special occasion, as a result, shimmer particles may potentially transfer during close personal assaults and can be used as a form of trace evidence.

1.2.1 Shimmer versus Glitter

Many people assume that glitter and shimmer are the same, when, in fact, they are fundamentally and compositionally different. Cosmetic glitter is a man-made product that is usually composed of either tiny pieces of aluminum foil, plastic without a metallic coating, or plastic that has an aluminum layer. It typically starts off with polyester sheets [1], such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) that may have been painted with pigments approved by the U.S. Food and Drug Administration (FDA). These sheets are then cut into tiny pieces with typical shapes of hexagons, squares, or rectangles. Hexagonal shapes are most common, followed by the squared shape. Other manufacturers may create unique glitter shapes such as stars or moons [1].

In her article from The New York Times, What Is Glitter?, Caity Weaver described glitter as "aluminum metalized polyethylene terephthalate" [4]. Glitterex Corporation was presented as one of the largest glitter manufacturers in the United States (Cranford, NJ), which sells glitter that is mainly composed of thin Mylar PET film. To achieve a rainbow-like colored glitter, i.e., holographic glitter, a fine layer of vapor-deposited aluminum is placed onto the polyester film and then embossed with a diffraction grating pattern so that light reflects at different directions simultaneously. Finally, some of their glitter is composed of multi-layered plastic of various refractive indices, with each layer more than 230?nm thick, to achieve different colors at different incident angles [4].

On the contrary, shimmer particles are primarily comprised of mica substrates, which are complex aluminosilicate crystals that readily separate into thin flat sheet-like layers. Mica is a naturally occurring mineral but can also be synthesized commercially. Cosmetic shimmer is primarily comprised of mica particles that have been coated with different types and thicknesses of metal oxides to generate different colors or effects. Furthermore, although shimmer particles may fall into a certain size range, their shape is totally irregular and random.

Glitter has been used previously as associative evidence in real-world cases [1, 5, 6], however, shimmer has not been evaluated despite its potential value as associative evidence as well. Little research has been conducted on shimmer analysis as a means of trace/associative evidence. The examination and evaluation of shimmer may indeed expand the scope of forensic particle analysis that is currently available by providing another type of associative evidence to evaluate and a means to compare known and unknown shimmer particles that may have transferred during a close personal contact.

1.2.2 Shimmer Composition and Use

Albeit being mainly overlooked by criminalists, shimmer is very well known by chemists in various industries, including the cosmetics industry, the craft industry, and the automotive industry; and new and improved forms of shimmer are steadily being introduced into the commercial marketplace. The wide variety of shimmer applications increases its potential value as trace evidence, and the fact that many commercial cosmetic products contain several different types of shimmer particles at different relative amounts makes samples with shimmer easier to distinguish from one another.

Shimmer mainly consists of pieces of mica of a certain size range that have been coated with titanium dioxide (TiO2) of uniform thickness. This chapter will focus on coated mica as it is the most common type of shimmer.

The most common and abundant forms of mica are muscovite and biotite [7]. Biotite mica has the general formula K(Fe, Mg)3(AlSi3O10)(OH)2, where the potassium (K) can be found bound to either iron (Fe) or magnesium (Mg). Biotite is typically darker in color than muscovite, in usually a black or dark brown color [7, 8]. Biotite is used as a filler or insulator in various electrical and construction applications [7]. The mica used in most cosmetic products is muscovite. Although its general formula is KAl3Si3O10(OH)2, depending on geographic location the formula is variable K(Al, Cr, Mn)3Si3O10(OH)2, where the potassium could be bound to either aluminum (Al), chromium (Cr), or manganese (Mn). When only K is bound to Al, muscovite is most commonly clear in color, sometimes occurring in light shades of brown, green, yellow, or rose [9]. When Al is substituted with Cr, the mica is referred to as Fuchsite or Chrommuscovite and the mineral is generally green in color [9, 10]. Manganese rich muscovite mica, when Mn is in the place of Al, occurs in colors ranging from pink to red and is known as Alurgite [10]. These chemical differences provide the first manner in differentiating mica substrates.

Once the cosmetic sample has been applied, the shimmer particle is intended to lie flat on the surface. The longest dimension may range from as little as a few microns up to several hundred, but their thickness is typically one micron or less. Although quite small, shimmer particles cannot be considered nanomaterials.

Mica particles use the basics of thin film light interference to achieve their color shifting properties [11]. Light interference occurs in one of two ways....