Ink Analysis

Introduction

Chemical and physical analysis of inks on questioned documents provides valuable information regarding their authenticity. Comparison of these chemical and physical properties of two or more inks can determine: (1) if the inks were made by the same manufacturer; (2) in some cases, whether the inks are products of the same production batch; and (3) the first production date of the specific ink formulation involved. When dating tags are detected, it is possible to determine the actual year or years when the ink was manufactured. Dating tags are unique chemicals that have been added to ball-point inks by some ink companies as a way to determine the year the ink was made.
Relative age comparison tests performed on inks of the same formula and written on the same type of paper with the same storage conditions (performed by measuring changing solubility properties of inks) can estimate how long inks have been written on paper. This is done by: (1) comparing the rates and extents of extraction of questioned and known dated inks in organic solvents by thin-layer chromatography (TLC) densitometry; (2) comparing changes in dye concentrations by TLC and TLC densitometry; and (3) comparing the volatile ink components by gas chro-matography-mass spectrometry (GC-MS). In cases where known dated writings are not available for comparison with questioned inks, accelerated aging (heating the ink to induce aging of the ink) can sometimes be used to estimate the age of ink using any or all of the above described techniques. Iron-based inks can be dated by measuring the migration of iron along the fibers of the paper by Scanning auger microscopy.
This article describes state of the art procedures for the chemical and physical comparison, identification and dating of inks on questioned documents.


Composition of Major Types of Writing Inks

Knowledge of the composition of inks is necessary to understand the reasons for the various methods used to analyze inks. Also, knowledge of the first production date for each type of ink or certain ingredients in the inks is useful for dating inks.

Carbon (India) ink

In its simplist form carbon inks consist of amorphous carbon shaped into a solid cake with glue. It is made into a liquid for writing by grinding the cake and suspending the particles in a water-glue medium. A pigmented dye may be used to improve the color. Liquid carbon inks are also commercially available. In the liquid carbon inks shellac and borax are used in place of animal glue and a wetting agent is added to aid in the mixing of the shellac and carbon. Carbon inks are insoluble in water, very stable and are not decomposed by air, light, heat, moisture or microbiological organisms. This class of ink has been available for more than 2000 years.

Fountain pen inks

There are two types of fountain pen inks: (1) iron-gallotannate type and (2) aqueous solutions of synthetic dyes. Modern inks of type (2) contain synthetic blue dyes to provide an immediate blue color to the ink which gradually turns black after oxidation on paper. This explains the origin of the name blue-black fountain pen ink. This class of ink is also very stable. This ink is insoluble in water and cannot be effectively erased by abrasion. The most popular fountain pen ink (developed in the 1950s) consists of an aqueous solution of synthetic dyes. These inks are bright and attractive in color, but they are not nearly as stable as the carbon or blue-black inks. Some of the synthetic dyes used fade and are soluble in water. The most modern inks of this type contain pigmented dyes, such as copper phthalocyanine (introduced in about 1953) which makes these inks much more permanent.

Ballpoint inks

The ballpoint pen was developed in Europe about 1939 and was initially distributed in Argentina about 1943. In 1946, several million Reynolds ballpoint pens reached the market in the United States.
Ballpoint inks consist of synthetic dyes (sometimes carbon or graphite is also added for permanence) in various glycol solvents or benzyl alcohol. The dyes in ballpoint inks can consist of up to 50% of the total formulation. Several other ingredients are usually added to the ink to impart specific characteristics. These ingredients consist of fatty acids, resins, surface active agents, corrosion control ingredients and viscosity adjustors. The fatty acids (oleic is the most common) act as lubricants to the ball of the pen and they also help the starting characteristics of the ball point.
Ballpoint inks made before about 1950 used oil-based solvents such as mineral oil, linseed oil, reci-noleic acid, methyl and ethyl esters of recinoleic acid, glycerin monoricinoleate, coconut fatty acids, sorbi-tal derivatives, and plasticizers such as tricresylpho-sphate. Modern ballpoint inks (post-1950) are referred to as glycol-based inks, because of the common use of ethylene glycol or glycol derivatives as a solvent for the dyes. Benzyl alcohol is also commonly used as the vehicle (solvent) by some ink manufacturers. Chelated dyes (introduced commercially around 1953) are stable to light. Red, green, yellow and other colored chelated dyes are now used for various colored ballpoint inks.
Pressurized ballpoint inks were developed about 1968. These pens contain a pressurized feed system instead of gravity flow. The physical characteristics of these inks are quite different from the standard glycol based ballpoint inks. The composition is basically the same, but this ink does not become fluid until disturbed by the rotation of the ball point in the socket. Cartridges containing this ink are under the pressure of nitrogen or some other inert gas. The positive pressure on the ink allows the pen to write in all positions and in a vacuum. These pens are used by astronauts during space travel.

Rolling ball marker inks

Rolling ball marker inks were introduced in Japan in about 1968 and shortly thereafter in the United States. These inks are water based and usually contain organic liquids such as glycols and formamide to retard the drying of the ball point. The dyes in these inks are water soluble or acidic dye salts. The light fastness of these dyes range from good for the metal-ized acid dyes to poor for some of the basic dye salts. Water fastness is usually poor, except that some of these dyes have an affinity for cellulose fibers in paper which produces a degree of water fastness. Water-resistant rolling ball marker inks are also available. These inks are totally insoluble in water and can only be dissolved in strong organic solvents, such as pyridine or dimethylsulfoxide (DMSO).

Fiber or porous tip pen inks

This class of inks was developed in Japan about 1962 and in the United States about 1965. Fiber tip inks are usually water or xylene based and contain dyes and additives similar to those in rolling ball marker inks and fountain pen inks. The water-based inks are obviously water soluble, whereas the xylene-based inks are water resistant and can only be dissolved with strong organic solvents. Formamide or glycol solvents are essential ingredients in fiber tip inks to keep the fiber tip from drying out. Fiber tip inks that contain metalized dyes are light fast.

Gel-pen inks

The most recent development in the writing instrument industry is the introduction of the gel-pen by the Japanese. Four brands of gel-pen inks have been introduced: (1) the Uniball Signo by Mitsubishi; (2) the Zebra J-5; (3) the Pentel Hybrid; and (4) the Sakura Gelly Roll pen. These pens have been marketed by the Japanese since the mid-1980s and a limited supply of the pens was sold in the United States about 1993. Two US manufacturers are now producing these pens.
Gel inks contain completely insoluble colored pigments rather than organic dyes. Writing with this ink is very similar to the appearance of the writing with a ballpoint pen. This ink, which is water based, is a gel and not a liquid. It is insoluble both in water and strong organic solvents. This physical property makes it impossible to analyze (by traditional methods) for the purpose of comparing two or more inks of this type.

Ink Comparisons and Identifications

Inks are usually examined for three reasons:
1. To compare two or more ink entries to determine similarities or differences in inks which can provide information concerning whether entries have been added or altered.
2. To determine if two or more entries were written with the same formula and batch of ink, thus providing a lead as to whether certain entries could have been written with the same pen.
3. To date ink entries to determine whether documents have been back-dated. This section deals with the first two reasons for analyzing inks.
Nondestructive methods of comparison should be carried out first, because chemical analysis causes minor damage to the document by removing ink samples for analysis. Typically, the nondestructive methods include: (1) a visual and microscopic examination of the writing to assess its color and the type of pen used; (2) infrared reflectance and luminescence examinations to determine whether the inks reflect or absorb infrared light and whether the inks luminesce; and (3) viewing the inks under long- and shortwave ultraviolet light to determine if the inks are fluorescent under these wavelengths of light. Often these techniques are sufficient to determine if two or more inks are different. However, if these techniques fail to detect any differences in the inks, then further chemical analysis is necessary to determine if the inks being compared really have the same formula.
The most widely used technique for comparing and identifying inks is TLC. This technique separates the dyes in the ink and the invisible organic components in the ink. This allows a direct comparison of the composition of inks being examined on the same TLC plate. To determine the relative concentrations of dyes present in the ink, the dyes separated on the TLC plate are scanned in a TLC scanning densit-ometer. The method is fast, reliable and inexpensive. High performance liquid chromatography (HPLC) has also been used for comparing inks with some success. Gas chromatography-mass spectrometry (GC-MS) is a very useful technique but the equipment is expensive.

Method of chemical analysis

Equipment, materials and solvents

• Merck HPTLC plates (silica gel without fluorescent indicator). The plates should be activated at 100°C for 15 min before use.
• TLC scanning densitometer
• Reagent grade pyridine, ethyl acetate, 1-butanol, ethanol, benzyl alcohol, DMSO, and water
• 1 dram (1.8 g) glass vials with screw caps
• 10 ul and 4 ul disposable micropipettes
• TLC glass developing chamber to accommodate standard 4 in x 8 in (10 cm x 20 cm) TLC plates with cover
• 20 guage syringe needle and plunger (the point of the needle must be filed so that the point is flat)
• 10 ul and 20 ul automatic pipettes
• temperature controlled oven

Procedure

• Using the syringe needle and plunger, punch out about 10 plugs of ink from the written line.
• Place the plugs in the glass vial and add 1-2drops of the appropriate solvent to the vial to dissolve the ink (usually pyridine for ballpoint ink and ethanol and water (1:1) for nonballpoint inks. Water resistant nonballpoint inks may require using pyridine or DMSO). Allow 15 min for the ink to dissolve.
• Note and record the color of the ink in solution and then spot the ink on to the TLC plate using the 10 ul micropipette. Keep the spots small by spotting intermittently and allowing the spots to dry between each spotting.
• Repeat the above for all ink samples to be compared. Up to about 20 samples can be spotted on the same TLC plate. Be sure to analyze a sample of the paper with no ink as a control.
• Place the TLC plate with the spotted inks in a temperature-controlled oven for approximately 10 min at 80°C. Allow the plate to cool to room temperature then place the plate in the developing chamber using a solvent system of ethyl ace-tate:ethanol:water (70:35:30 by vol.). The solvent system should be allowed to equilibrate in the developing chamber for at least 15 min.
• Allow the TLC to develop for 15 min, then remove it from the chamber and dry in the oven for approximately 15 min at 80°C.
• View the developed TLC visually and under ultraviolet light to determine which inks match in terms of the dyes and fluorescent components present.
• Scan the plate in the scanning TLC densitometer to measure the relative concentrations of the dyes present in the inks. The dyes are scanned at 585 nm for blue and black inks if a spectrometer type densitometer is used. Video densitometers see all spots in shades of black and therefore no wavelength setting is needed for this instrument. (If the above solvent system did not adequately separate the dyes in the ink for accurate densitometer readings, repeat the tests using 1-butanol:ethanol:water (50:10:15, by vol.).
• Compare the relative concentrations of the dyes present in the various inks. Failure at this point to detect any significant differences among the inks compared justifies a conclusion that all inks are consistent with being of the same formulation. This statement is based on the finding of no significant differences in the nondestructive tests and the chemical analysis. It should be noted that complete identification of an ink is not possible, because not all of the original ingredients in ink are present in ink dried on paper.
• To identify the manufacturer and specific formulation of questioned inks, standard inks of known manufacture and formulation must be analyzed simultaneous with the questioned inks using the same procedures described above. To do this, however, requires access to a complete and comprehensive collection of standard inks and an analytical method that distinguishes each standard. The strength of any identification is only as strong as the completeness of the standard ink reference collection and the ability to identify its inks. • If the ink is properly identified, it is possible to determine from the manufacturer when that specific formulation of ink was first made. This may determine if a document was backdated.
Although the above procedures are the most commonly used and have withstood the test of the courts for the comparison and identification of inks, other methods are available. For example, gas chromatography (GC) and GC-MS can be used to detect any volatile organic ingredients that might be present in the inks. HPLC can be used to detect volatile and nonvolatile components. Electron microscopy can be used to distinguish carbon from graphite, when these are present in inks. Time and the amount of ink sample available for analysis usually make the use of these techniques impractical.

Dating of Inks

As mentioned earlier in this article, there is a huge demand for the dating of inks on questioned documents. Any time during an investigation when there is some question about the date of preparation of a document, an ink dating chemist is needed. Over the past 30 years, the ability to perform these examinations has become widely known and recognized among forensic scientists, document examiners and attorneys throughout the world. The ink dating procedures that will be described have passed the Frye and Daubert tests on numerous occasions and are therefore routinely accepted in US courts. Testimony has also been admitted using these techniques in Israel and Australia.

First date of production method

After the ink is uniquely/positively identified, the first date of production of that ink or certain ingredients in the ink is determined from the manufacturer of that specific ink formulation. If the ink was not made until after the date of the document, then it can be concluded that the document was backdated. If the ink was available on the date of the document, then the document could have been written on that date.

Ink tag method

If an ink tag is identified in an ink, it is possible to determine the actual year or years when an ink was made. Tags have been added to some ballpoint inks by the Formulab Company since before 1970; however, the use of tags in their inks was discontinued in June 1994. Since the tags are considered proprietary information by Formulab, no further information about the tags can be reported here. Formulab should be contacted directly, if this information is needed.
Ink dating tags are detected and identified by TLC using a solvent system of chlorobenzene and ethyl acetate (5:1, v/v). Standard samples of the tags should be run simultaneously on the same TLC plate as the questioned inks. The tags, if present, are viewed under longwave ultraviolet light and the RF values of the tags present in questioned inks are compared with the RF values of the standard tags. The dates the various tags were used must be obtained from Formulab.

Relative age comparison methods

Dating inks by this procedure is based on the scientifically proven premise that as ink ages on paper, there are corresponding changes in the solubility properties of the inks. Therefore, by comparing the solubility or extraction properties of questioned inks with known dated inks of the same formula on the same type of paper and stored under the same conditions, it becomes possible to estimate how long the ink has been written on the document. Two or more inks of the same formulation can be compared without known dated writings to determine whether the writings were made at the same or different times. This is only true if the inks being compared are still aging (drying), because after the ink has aged out (completely dry), no differences in solubility properties are expected, even if the inks were written at different times. Typically inks will become totally dry (as measured by these procedures) within 6 years; some inks become dry in less than 6 years.
When two or more matching inks are compared without known dated writings, it is still possible to determine the sequence in which the inks were written. This again requires knowing that the inks are still aging and also knowing how the inks age. For example, some inks extract faster and more completely in organic solvents as the ink ages; whereas, others extract more slowly and less completely as they age. To determine which way the ink ages, a sample of the ink is heated at 100°C for 30 min. The rate and extent of extraction of this heated sample into an organic solvent is compared with an unheated sample of the same ink to determine if the heated (totally aged) sample extracted faster and more completely than the unheated sample, or vice versa.

R-Ratio (rate of extraction) method and percent (extent) extraction method

• Using the syringe and plunger, remove 10-15 plugs of ink and paper and place them into 1 dram glass vials. Cap and label the vial with the sample number. Repeat for each sample to be analyzed.
• Set the timer to 10 min.
• Using the automatic 20 ul pipette, add 20 ulofa weak solvent to the vial containing the ink sample and start the timer immediately. (For almost all ballpoint inks, 1-butanol is a good weak solvent.)
• Stir by rotating the vial containing the ink and weak solvent immediately after adding the weak solvent and just before each aliquot is removed for spotting.
• Spot 4 ul aliquots of dissolved ink in one continuous application on a TLC plate at 0.5, 1.5, 3 and 10 min intervals. Place these spots side by side at one end of the plate approximately 1 cm apart. (It may be necessary to use tweezers to remove the pipette from the vial.) Note: If a nonballpoint ink is being analyzed, it may be necessary to spot the 4 ul aliquots intermittently to prevent the spot from getting too large. The spot should be no larger than 0.3 cm in diameter
• Repeat the above procedures for each sample to be analyzed.
• Evaporate the solvent remaining in the vials in an oven at 80°C (about 15 min).
• Remove the vials from the oven and allow them to cool to room temperature.
• Using the automatic pipette, add 10 ul of a strong solvent to each vial and allow to extract for 15 min. (Benzyl alcohol is the solvent of choice for ballpoint inks and some nonballpoint inks. Some non-ballpoint inks may require using ethanol:water (1:1) or DMSO for water resistant nonballpoint inks.)
• Spot 4 ul of the ink extracted with the strong solvent adjacent to the weak solvent spots. (If benzyl alcohol is used for the strong solvent, spot in one continuous application of the pipette to the plate. If pyridine is used, spot intermittently to keep the spot from getting too large.)
• Repeat the above steps for each sample.
• Dry the spots on the TLC plate at 80°C for about 15 min.
• Remove the plate from the oven and allow to cool to room temperature.
• Scan the plate in the scanning TLC densitometer along the path of the four weak solvent spots and the one strong solvent spot and read the relative concentrations of the five spots.
• Repeat the scan described above for each sample.
• Calculate the various R-ratios for each sample by letting the percent of ink extracted in the weak solvent at 10 min equal one. Then calculate the R-ratios for each time interval of 0.5, 1.5, 3 and 10 min. This gives a normalized curve.
• To obtain R-ratio curves, plot R-ratios vs. time of extraction (Fig. 1). Since all samples are being compared in the same manner, it is not necessary to correct for volume changes caused by successive aliquots removed from the vials.
• Compare the R-ratio curves of all inks tested of the same formulation. To estimate the age of the questioned inks, compare the R-ratio curves of the questioned inks with known dated inks.
• Calculate the percentage or extent of ink extracted in the weak solvent at the various time intervals, by dividing the reading for each weak solvent spot by the total amount of ink extracted in the weak and strong solvent, then multiply by 100. Figure 2 shows the amount of ink extracted in 10 min.
A simplified percent extraction procedure can be performed by extracting each sample for just 1 min in the weak solvent, then after spotting this 1 min extract, the strong solvent is added directly to the weak solvent remaining in the vial. Then after allowing the strong solvent to extract for 15 min a second aliquot is spotted. This procedure produces just two spots to measure in the densitometer. Although R-ratios cannot be determined by this procedure, accuracy and reproducibility of the percent extraction measurements are improved by reducing the number of steps in the procedure and increasing sensitivity. If this procedure is followed, only 10 ul of weak solvent is needed and sample size can be reduced to less than 10 plugs of ink.
 R-Ratio curves (rates of extraction) for Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. The matching curves for the 1992 and 1994 inks means that the ink became totally dry after 4 years, because there was no change in rate of extraction after this time.
Figure 1 R-Ratio curves (rates of extraction) for Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. The matching curves for the 1992 and 1994 inks means that the ink became totally dry after 4 years, because there was no change in rate of extraction after this time.
Percent extractions for Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. As with the R-ratios in Fig.1, the age of this ink could only be distinguished between 1994 and 1998. Notice more ink was extracted from the older inks than the newer inks. This is an example of a reverse extraction rate and extent, because most inks extract more from newer inks.
Figure 2 Percent extractions for Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. As with the R-ratios in Fig.1, the age of this ink could only be distinguished between 1994 and 1998. Notice more ink was extracted from the older inks than the newer inks. This is an example of a reverse extraction rate and extent, because most inks extract more from newer inks.
Dye ratios for Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. This method allowed the age of all inks to be distinguished between 1992 and 1998. This is because the relative concentrations of the dyes in this ink changed with age up to 6 years. For this ink the solubility properties as measured by the R-ratio and percent extraction methods did not change after 4 years.
Figure 3 Dye ratios for Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. This method allowed the age of all inks to be distinguished between 1992 and 1998. This is because the relative concentrations of the dyes in this ink changed with age up to 6 years. For this ink the solubility properties as measured by the R-ratio and percent extraction methods did not change after 4 years.
Dye ratio method The same plate used for the R-ratio and percent extraction measurements can be used to calculate the various dye ratios for each ink being compared.
• Develop the TLC plate containing all the spots from the R-ratio and percent extraction measurements in a solvent system of ethyl acetate:ethanol: water (70:35:30, by vol.) for 15 min.
• Dry the plate in an oven set at 80°C for about 10 min, then allow the plate to cool to room temperature.
• Scan each sample in the densitometer along the direction of the dyes separated in each sample and from the densitometer readings calculate all possible dye ratios for each sample. For example, divide dye 3 by dye 1, divide dye 3 by dye 2, and divide dye 2by dye 1. Compare the dye ratios of corresponding pairs of dyes obtained for questioned and known dated inks to estimate the age of the questioned inks (Fig. 3).
Since it has been established that these dye ratios change as ink ages, inks with matching dye ratios are consistent with the inks being written at the same time. Inks with dye ratios that do not match generally means that the inks were written at different times, unless one ink had an unusually large batch variation.
It is important to know that depending on the ink formulation involved and the paper it is on, each of the methods described above may not all have equal ability to discriminate the age of the ink being analyzed. For example, it is not uncommon for one method to detect differences in age, when one or both of the other procedures fail to detect this difference. This fact does not negate the positive results of the one method. Only if the results of one method conflict with the results of another method are the overall results negated.
Accelerated aging In situations where known dated inks are not available for comparison with questioned inks, accelerated aging of a questioned ink can be performed to estimate its age (Fig. 4). The measurement procedures are identical to those described for R-ratios, percent extraction and dye ratios. This test involves just one additional step which is to heat a sample of the questioned ink for 30 min at 100°C, allow it to cool and equilibrate with the temperaure and humidity in the room for 1 h and then compare the results of the various measurements (using any or all of the R-ratio, percent extraction and dye ratio methods) with the results obtained from an unheated sample of the same ink.
 Artificial aging of Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. Notice that the newer inks changed more with heat than the older inks. Also, notice that after 4 years, this ink did not change with heat as measured by the percent extraction method. With knowledge of how long it took for this ink to become completely dry, it is possible to estimate the age of questioned Bic black ballpoint inks by determining how much the extraction properties change with heat.
Figure 4 Artificial aging of Bic black ballpoint inks written in 1992, 1994, 1996 and 1998. Notice that the newer inks changed more with heat than the older inks. Also, notice that after 4 years, this ink did not change with heat as measured by the percent extraction method. With knowledge of how long it took for this ink to become completely dry, it is possible to estimate the age of questioned Bic black ballpoint inks by determining how much the extraction properties change with heat.
Significant differences obtained by any one of the methods indicates that the ink is still drying and is therefore less than 6 years old, since no inks have been found to take longer than 6 years to become completely dry using these methods. If it is known that the specific ink in question takes only 3 years to dry, then it can be concluded that the questioned ink is less than 3 years old. This method can also be used to determine which of two or more inks is newer than the other. This is done by observing which ink changes more with heat; the larger the change caused by heat, the newer the ink. This can only be done when all inks compared consist of the same ink formulation on the same type of paper and stored under the same conditions. This statement applies to all of the relative age comparison techniques described here.

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