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The color of a substance is determined by its absorption spectrum in the vis-ible region (~400–700 nm). Colorants are generally classified as to whether they are organic or inorganic, natural or synthetic, and whether they are dyes or pigments. Further classification is done according to their method of application onto a substrate (such as textile dyes), according to the chemical class of the chromophore, or on the basis of their color. According to Mayer, pigments may be classified according to their color, use, permanence, and so on. It is customary, however to classify them according to their origin, as follows (Mayer, 1991, p. 31):

I. Inorganic (mineral)

a. Native earths (ocher, raw umber, etc.)

b. Calcined native earths (burnt umber, burnt sienna, etc.) c. Inorganic synthetic colors (cadmium oxide, zinc yellow, etc.) II. Organic

a. Vegetable (gamboges, indigo, madder) b. Animal (cochineal, Indian yellow, etc.) c. Synthetic organic pigments

With regard to classification, Mattiello takes a slightly modified approach, first assigning the broad general classes: colored, white, black and metallic element and alloy followed by further delineation according to composition and hue (Mattiello, 1946, p. 3):

I. Colored a. Organic

i. Synthetic—chemically manufactured ii. Natural

b. Inorganic

i. Synthetic—chemically manufactured ii. Natural

II. White a. Opaque

i. Synthetic—chemically manufactured ii. Natural

b. Nonopaque extender

IV. Metallic element and alloy a. Inorganic

i. Synthetic—chemically manufactured

Organic pigments are carbon hydrogen derivatives while inorganic pigments are not formed from carbon and hydrogen, but contain metal atoms. To change the solubility of organic molecules, metals can be added to the mol-ecule. Often, these molecules retain their classification as organic although a metallic component has been added (Mayer, 1991, p. 63). An example of this classification is the phthalocyanines.

Dyes are coloring materials that are partially or completely soluble in a variety of liquids, and can be applied to a variety of substrates based on their affinity to a particular substrate. Dyes are those that dissolve in liq-uids and impart their color effects to materials by staining or being absorbed (Mayer, 1991, p. 29). Classification of dyes can be based upon the method of application or their structure (Gettens and Stout, 1966, p. 112). They consist in structure of a chromophore group and a salt-forming (anchoring) group (Gettens and Stout, 1966, p. 112). Pigments are small, dry particles that are virtually insoluble in liquids and the media in which they are applied. When a pigment is mixed or ground in a liquid vehicle to form a paint, it does not dissolve but remains dispersed or suspended in the liquid (Mayer, 1991, p. 29). The same is true for inks. Dyes and pigments are similar with respect to chemistry and structure and tend to differ in application and usage.

Lomax and Learner define synthetic organic pigments as referring to manufactured colorants that have carboxylic ring skeletons as part of their structure, with many of the ring systems being aromatic in nature, as well as potentially consisting of a variety of functional groups and metal ions (Lomax and Learner, 2006, p. 108). The authors add that synthetic organic pigments, true pigments and not dyes, must be distinguished from those natural organic pigments that are now synthesized; as such, they define syn-thetic organic pigment as referring specifically to those pigments that have no counterparts in nature, but are manufactured in a laboratory to achieve a specific color or application (Lomax and Learner, 2006, p. 108).

Organic pigments are carbon-based, polycyclic compounds that contain one or more characteristic functional groups called chromophores. The shades

and physical properties of organic pigments depend on the nature and num-ber of chromophores and salt forming groups within the molecular species and the relative disposition of these functional groups within the molecule.

Chromophores are classified as either chromogens, which are color generating functional groups, or auxochromes, which are color augmenting functional groups. Molecules referred to as chromogens possess the potentiality for devel-oping color even if they are not in themselves intensely colored (Patterson, 1967, p. 26). Auxochromes produce shifts in the absorption bands of the molecule, which can develop color in the molecule (Figure 7.1). Hypsochromic, or blue shifts are those in which the absorption band moves to a shorter wavelength and bathochromic, or red shifts are those in which the absorption band moves to a longer wavelength (Patterson, 1967). Color-displaying molecules vary in the amount and type of functional groups they contain, which will impact the color they exhibit. The production of color is related to the resonance by delo-calization of the π electrons from the presence of conjugated unsaturated sys-tems. The intensity of the color can depend on the width of the band and other colors may display more than one band in an absorbance spectrum.

Common chromogens include: C═C and ─C═C─C═C─, C═O (car-bonyl), C═S (thiol), CH═N (azomethine), N═N (azo), N═N⁺O⁻, N═O (nitroso), NO₂ (nitro), and ringed structures (note the presence of unsatu-rated (double) bond systems, which contribute the π electrons). Common auxochromes include: ─OCH₃, ─OH, ─NH₂, NH─R, NR₂, O─R, halogens,

Hyperchromic

Bathochromic Hypsochromic

Wavelength (nm) Abs.

Hypochromic

Figure 7.1 Shifts observed in UV/Vis spectrophotometry.

salts, phenyls and naphthyls, and other conjugated heterocyclic groups. The carbonyl and vinyl groups have chromophoric properties only when they are present in the molecule in multiple conjugated order (Juster, 1962, p. 598).

The structural features responsible for color are also responsible for absorption in the ultraviolet, as well as the visible, region of the electromag-netic spectrum. Table 7.1 demonstrates the colors of substances showing sin-gle absorption bands in the visible region of the electromagnetic spectrum.

In 1856, with the discovery of mauve, the modern synthetic dye indus-try began. In 1865, with the nature of benzene described,^* more sophisti-cated preparations of natural and synthetic dyes arose (Zollinger, 2003, p. 6). Since this time, millions of colored compounds have been synthesized.

Synthetic organic dyes and pigments are synthesized from five basic raw materials: benzene, toluene, xylene, naphthalene, and anthracene (Mayer, 1991, p. 478). These are aromatic hydrocarbons produced by the distillation of coal tar—a byproduct of the coal gas and coke industry, and from some petroleum residues (Mayer, 1991). As such, colors derived in this manner are collectively referred to as coal tar dyes and pigments (Figure 7.2). According to the Pigment Compendium, “the term coal tar appears to have been used primarily from the late eighteenth through to the earlier twentieth century, aniline colours being an essentially synonymous term. Both are now wholly superseded by the use of the chemical nomenclature of the azo and polycyclic pigment groups” (Eastaugh et al., 2008, p. 117). Fay (1919) provides a detailed description of the process of obtaining coal tar and its products:

When bituminous coal is thoroughly ignited in stoves and furnaces and a draught of air freely circulates through the mass, three principle products

* Described by Kekule; the normal vibration modes of benzene have also been diagrammed by Varsanyi.

Table 7.1 Absorption Characteristics of Colors

Wavelength (nm) Color of Light Absorbed Color of Compound

200–300 — —

300–400 — —

400–435 Violet Yellow–green

435–480 Blue Yellow

480–490 Green–blue Orange

490–500 Blue–green Red

500–560 Green Purple

560–580 Yellow–green Violet

580–595 Yellow Blue

595–605 Orange Green–blue

605–750 Red Blue–green

are formed: one is water vapor, a second carbon dioxide and third the ash.

If coal be heated equally hot, but inside a long cast iron or earthen retort shut off from all contact with the oxygen of the air…[an] operation known as destructive distillation… four chief products result from the destructive distillation of bituminous coal…: coal gas, ammoniacal liquor, coal tar and coke (p. 5)… The constituents of tar may, according to their chemical reac-tions, be divided into three classes: first, the hydrocarbons (composed of car-bon and hydrogen), second, the phenols (consisting of carcar-bon, hydrogen and oxygen), third, the nitrogenous compounds (composed of carbon, hydrogen and nitrogen) (p. 6).

Synthetic organic pigments exhibit a wide range of physical and chemi-cal properties, including light fastness, heat stability, solubility in water or organic solvents, reactivity and stability to recrystallization (Lomax and Learner, 2006, p. 108). Chemical robustness is essential to the quality of syn-thetic pigments. The overall permanence of the pigment can be affected by sunlight (or any exposure to ultraviolet radiation), the atmosphere (such as atmospheric gases and moisture), the medium, and the action of the pigments when mixed together as well as the chemical properties of the pigment itself.

Mayer provides a list of preferred requirements for ideal paint pigments, and these can be applied to the pigments used in tattoo inks as well:

1. Should be a smooth, finely divided powder

2. Should be insoluble in the medium in which it is used

3. Should withstand the action of sunlight without changing color, under conditions in which the painting may be exposed

4. Should not exert a harmful chemical action upon the medium or upon other pigments with which it is to be mixed

5. Should be chemically inert and unaffected by materials with which it is to be mixed or by the atmosphere

6. Should have proper degree of opacity or transparency to suit the pur-pose for which it is intended

Benzene Toluene Xylene

Naphthalene Anthracene

CH₃ CH₃

CH₃

Figure 7.2 Coal tar dyes.

7. Should be of full strength and contain no added inert or loading ingredients

8. Should conform to accepted standards of color and color quality and exhibit all the desirable characteristics of its type

9. Should be purchased from a reliable house that understands and tests its colors, selects them from worldwide sources, and can fur-nish information as to origin, details of quality, and so on^*

Synthetic organic pigments include azo compounds, phthalocyanines, oxazines, and quinacridones. Azo pigments are characterized by the presence of the chromophore azo group ─N═N─. Lomax and Learner further specify that most of the α-keto azo pigments exhibit tautomerism with a ketohydra-zone form (Lomax and Learner, 2006, p. 110), which was also described in Zollinger (2003, p. 327) (Figure 7.3).

The azo pigments are described as the largest class of synthetic pig-ments and are subdivided into the following classes based upon their gen-eral formulae: monoazo (one azo group), disazo (two azo groups), trisazo (three azo groups), and additional polyazo (four or more azo groups) classes.

From the point of view of use, azo coloring matters can be classed into acid, mordant, direct, disperse, azoic, and solvent dyes, as well as into pigments, and a few azo compounds appear among basic and vat dyes (Zollinger, 1961, p. 219). Pigment red 146 (PR 146, 2-hydroxy-3-naphtharylide azo pig-ment), pigment red 170 (PR 170, 2-hydroxy-3-naphtharylide azo pigpig-ment), pigment yellow 3 (PY 3, Hansa Yellow 10G, acetoacetarylide azo pigment), and pigment yellow 151 (PY151, Hansa Yellow H4G) are examples of mono-azo pigments. PR 146 and PR 170 are further characterized as aromatic mono-azo compounds, with the general formula Ar─N═N─Ar′ (Figure 7.4). Pigment orange 16 (PO 16, dianisidine orange), pigment orange 34 (PO 34), and pigment yellow 83 (PY 83, permanent yellow HR) are examples of a disazo pigments (Figure 7.5).

Phthalocyanines, organometallic compounds, consist of dyes and pigments that contain the tetrabenzoporphyrazine (TBP) nucleus (Figures 7.6 and 7.7).

* It should be noted that while stringent practices may be employed by the pigment sup-plier, this is not a guarantee that tattoo ink manufacturers will follow stringent quality control measurements in mixing the pigments with the liquid components.

R N N CH (R’, R”) N N C(R’, R”)

Figure 7.3 Hydroxyazo-ketohydrazone tautomerism, in which the structure on the left represents the α-keto azo form and the structure on the right represents the ketohydrazone form.

The color attributed to the phthalocyanines is due to both the inorganic metal ion of the central copper atom^* and the organic conjugated bond system of the annulated system. The principal metal ion is copper and peripheral substitu-ents are chlorine and bromine. Pigment green 7 (PG 7, phthalocyanine green) and pigment blue 15 (PB 15, phthalocyanine blue) are examples of phthalocya-nines. Pigment blue is characterized by the copper centered within the TBP nucleus (copper-tetrabenzo-tetraazo-porphin), and pigment green is char-acterized by the chlorination of the benzene rings with the copper centered TBP nucleus (chlorinated copper-tetrabenzo-tetraazo-porphin). According to Thomson, the halogen bromine may also be incorporated along with the chlorine, where the more bromine used, the more yellow the resultant shade.

This phthalocyanine is known as pigment green 36 (PG 36, phthalo green) (Thomson, 1977, p. 447). Phthalocyanines are subject to modifications through substitutions, and reports of α, β, γ, δ, ε, π, ρ, and R crystal forms of metal

* Described in the crystal field theory developed by Bethe and van Vleck.

Pigment red 146 Pigment red 170

Pigment yellow 3 Pigment yellow 151 CI

Pigment orange 16

phthalocyanines have been described. Pigment blue 15 is the primary blue pigment in the phthalocyanines class, with nine forms of copper phthalocya-nine being reported, including four crystal forms of copper phthalocyaphthalocya-nine.

According to the Pigment Compendium,

PB 15 is an unstabilised (against crystal modification), non-halogenated cop-per phthalocyanine with the α-crystal modification, 15:1 is the same but with 0.5–1 chlorine substitutions, 15:2 is a non-flocculating version of 15:1, 15:3 is an unsubstituted β-copper phthalocyanine, 15:4 is a non-flocculating version of 15:3, and 15:6 is a stabilized, unsubstituted ε-modification form. Pigment blue 16 designates a metal-free phthalocyanine and Pigment Blue 7 is a cobalt phthalocyanine. Pigment Green 7 describes a copper phthalocyanine with 14–15 chlorine substitutions and Pigment Green 36 is a copper phthalocyanine with 4–9 bromine and 8–2 chlorine substitutions (Eastaugh et al., 2008, p. 305).

Oxazines are characterized by the presence of the chromophore oxa-zine ring, which forms the center of three condensed rings (Figure 7.8). The oxazines are subdivided into mono oxazines (one auxochrome is a free or

Pigment green 7 Pigment green 36

Pigment blue 15

substituted amino group), dioxazines (two oxazines are condensed together), and oxazones (the auxochromes are hydroxyl groups) (Color Index). Pigment violet 23 (PV 23) is an example of a dioxazine, a linear system of five anel-lated rings. According to The Pigment Compendium, PV 23 is used to shade phthalocyanine pigments, to counteract the yellowish cast of titanium dioxide whites and to shade carbon-based blacks that have a brownish cast (Eastaugh et al., 2008, p. 309). Polymorphisms of PV 23 exist as α and β polymorphs.

Quinacridones are a ring system with the following general structure (the linear, trans form is shown in Figure 7.9, at left). Color modification of the quiacridones can be done by substitution of side groups, crystal modifi-cation, and particle size (Patterson, 1967, p. 61). Pigment red 122 (PR 122), 2, 9-dimethyl-quinacridone, is an example of a quinacridone. It is described as a linear cis-quinacridone and is reported to have α, β, and γ forms.

An extensive amount of literature concerning the chemical analysis of pigments can be found within a vast array of scientific journals, with the focus of these articles addressing color science, forensic science, and art con-servation science. A study by Palenik et al., published in 2011, was conducted in an effort to classify and identify pigments using Raman spectroscopy specifically in the context of forensic investigations. Using excitation wave-lengths of 785 and 514 nm, the authors analyzed pigments from a reference

Quinacridone general structure Pigment red 122

H O

Oxazine ring Oxazine ring bound into a

three ringed structure Pigment violet 23

N C

collection which includes “approximately 1,200 pigment and >5,500 dye samples … includ(ing) 300 unique organic and inorganic pigments” (Palenik et al., 2011, p. 13). In addition, the authors “verified pigment identity” using energy dispersive spectroscopy (EDS), XRD, FTIR, and polarized light microscopy (PLM) (Palenik et al., 2011, p. 28).

Prior to the Palenik et al. (2011) study, Scherrer et al. (2009) appeared to have one of the most comprehensive Raman spectral reference collections of synthetic organic pigments to date, which included approximately 120 pig-ments. The majority (“about 90%”) of samples were analyzed using a 785 nm excitation wavelength while the remaining samples were analyzed using an excitation wavelengths of 514 and 633 nm. The pigments analyzed included in Table 7.2 (Scherrer et al., 2009, p. 508). A 2008 article by Schulte et al. con-tains reference spectra of 23 pigments, included in Table 7.3 (Schulte et al., 2008, p. 1457).

A 2000 article by Vandenabeele et al. contains reference spectra of 21 azo pigments. This database is limited to red and yellow pigments, and the excitation wavelength used was 780 nm. The pigments compromising the database are included in Table 7.4 (Vandenabeele et al., 2000, p. 510). A 2010 article by Colombini and Kaifas contains reference spectra of 23 orange and yellow organic pigments (in addition to fluorescent orange and yellow pigment samples), which were analyzed using 514 and 785 nm excitation

Table 7.2 Pigments Analyzed by Scherrer et al. (2009)

Pigment reds 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 16, 18, 48:3, 49, 49:1, 49:2, 53, 53:1, 57, 57:1, 68, 83:1, 112, 122, 144, 146, 149, 166, 170, 179, 185, 187, 188, 214, 242, 254, 255, 264

Pigment oranges 5, 13, 34, 36, 43, 48, 49, 62, 73

Pigment yellows 1, 1:1, 2, 3, 5, 10, 16, 65, 73, 74, 81, 83, 93, 95, 97, 109, 111, 120, 129, 139, 150, 151, 154, 155, 175, 181, 194

Pigment greens 7, 9, 36

Pigment blues 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 60 Pigment violets 5, 19, 23, 32, 36, 37

Pigment browns 23, 25

Table 7.3 Pigments Analyzed by Schulte et al. (2008)

Pigment reds 2, 49, 83:1, 88, 122, 123, 146, 176, 179, 181

Pigment oranges 13, 34, 43

Pigment yellows 3, 83, 109, 110

Pigment greens 7

Pigment blues 15:3

Pigment violets 1, 2, 5:1, 19

wavelengths. The pigments compromising the database are included in Table 7.5 (Colombini and Kaifas, 2010, p. 17). Ropret et al. (2008) presented a refer-ence library of 21 yellow pigments, which were analyzed with an excitation wavelength of 785 nm. The yellow pigments analyzed include PY 6, 73, 75, 97, 111, 213, 100, 55, 81, 16, 155, 95, 128, 151, 154, 129, 153, 109, 110, 173, and 139 (Ropret et al., 2008, p. 488).

Using pigment spectral data, such as those publications reported above, as well as conducting computer searches of existing pigment libraries, Miranda conducted a preliminary evaluation of the pigments present in select tattoo inks (Miranda, 2012b). Following preliminary evaluation of pigment compo-sition based on reported data in the literature, it is essential to obtain pigment standards for comparison and subsequently analyze the pigment standards under analytical conditions similar to that of tattoo inks. When these condi-tions are met, accurate determination of pigment composicondi-tions can be made with confidence. More detailed examination with microscopy as well as the use of a variety of instrumental techniques can provide better conclusions;

and finally, interpretation of spectral data to identify specific atomic and molecular features can enable the analyst to explain any effects that may be due to pigment mixtures, contaminants, matrix effects, and so on.

Table 7.5 Pigments Analyzed by Colombini and Kaifas (2010) Pigment oranges 34, 36, 43, 48, 49, 59, 61, 65, 73

Pigment yellows 1, 3, 16, 24, 74, 83, 109, 129, 138, 139, 151, 154, 173 Table 7.4 Pigments Analyzed by Vandenabeele et al. (2000) Pigment reds 3, 8, 9, 17, 22, 23, 49:1, 52:1, 53:1, 57:1, 112, 170 Pigment yellows 1, 3, 12, 13, 14, 17, 83, 65, 74

161

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The Chemical Analysis