Introduction

Forensic investigation has been around for more than several decades, even though its prominence with the general population has grown exponentially over the last couple of years with the ever-popular CSI shows. In this experiment you will be acting as a criminalistics technician, analyzing evidence collected at the scene of a hypothetical homocide. In particular, you will use a variety of real-life forensic techniques based entirely on chemical principles to determine the probability of guilt of two suspects. These techniques are Ink Analysis by Thin Layer Chromatography, Fingerprint Development and Analysis and Fiber Analysis.

Ink Analysis

The analysis of inks and dyes is an important technique in the arsenal of the document examiner. Such a technique is used daily in detecting the forgery of documents and the falsification of signatures. In its most sophisticated format, ink analysis has even been used to provide an estimate of the date at which a particular document was signed. Even in today’s society forensic ink analysis can be employed to test for the adulteration of numbers increasing the amount of money being charged, observe if ink used in two parts of a document came from the same pin, or even to compare the writing used in a threatening letter with pens confiscated from a suspect’s office as you will do in this experiment.

Thin Layer Chromatography

Liquid inks used in ballpoint pens contain a mixture of substances including dyes, stabilizers, solvents, and resinous binders. In fact, some manufacturers also add fluorescent markers or heavy metals to aid in the identification of their particular product. The preferred technique for such analysis is thin layer chromatography (TLC). TLC is one member of a family of analytical techniques utilized for separating mixtures such as liquid inks. In particular, this technique is often performed on ‘plates’ which have been uniformly coated by silica, alumina, cellulose, or some other type of absorbant.

Generally, the ink to be sampled by TLC will be punched out of paper evidence and then extracted using pyridine. Once this is accomplished, the sample is then spotted onto the TLC plate by means of a glass capillary as a transfer pipette. The plate is then placed in a glass tank usually containing a layer of solvent at the bottom.  Specifically, this solvent is the chromatographic eluent used to separate the mixture of dyes present in the sample and will move the furthest upon the plate. All the other ‘noticeable’ compounds in the mixture will move independently of one another with the distance moved relative to the solvent front being reported as the Rf value. Idealistically, each sample tested will present a unique separation so that an identification of the type of ink used in the evidence may be determined.

Fingerprint and Fiber Analysis

The two other techniques that will be utilized in the investigation are fingerprint and fiber analysis. For the fingerprint analysis, you will be using Ninhydrin to obtain any latent prints. Specifically, these types of prints can be visible or not dependent upon the particular piece of evidence being analyzed. If the prints are not visible, electronic, physical, and yes, chemical processing allows the print to be visualized. Once visualized, the technician must then provide a comparison report to determine if the prints are a match to a suspect's prints. Print comparison is generally digital, but unlike the depiction on CSI, computer searches only produce some probable suspects. The real identification and comparison is generally done by hand.

The last forensic technique you will be performing in this experiment, and perhaps the easiest to perform, is the fiber analysis. A piece of cloth has been retrieved from the crime scene and may be from an article of the perpetrator's clothing. In order to connect the criminal to the crime scene, the cloth must be identified as to type. Normal fiber analysis would involve the use of a microscope or possibly a gas chromatograph, but as you have neither of these at your disposal, you must be creative in your analyses. A simple and inexpensive way to determine the nature of a fiber is to burn it and make careful observations of the results.

Burn test for fibers:

To identify fabric that is unknown, a simple burn test can be done to determine if the fabric is a natural fiber, man made fiber, or a blend of natural and man made fibers. The burn test is used by many fabric stores and designers and takes practice to determine the exact fiber content. However, an inexperienced person can still determine the difference between many fibers to "narrow" the choices down to natural or manmade fibers. The advantage of this test for forensic purposes is the amount of time it takes to complete it.

WARNING: All fibers will burn! Asbestos treated fibers are, for the most part fire proof. The burning test should be done with caution. Use a small piece of fabric only. Hold the fabric with tweezers, not your fingers. Burn over a watch glass with soda in the bottom or even water in the bottom of the dish. Some fabrics will ignite and melt. The result is burning drips which can adhere to fabric or skin and cause a serious burn.

Identifying the Fibers:

• Cotton is a plant fiber. When ignited it burns with a steady flame and smells like burning leaves. The ash left is easily crumbled. Small samples of burning cotton can be blown out as you would a candle.
• Linen is also a plant fiber but different from cotton in that the individual plant fibers which make up the yarn are long where cotton fibers are short. Linen takes longer to ignite. The fabric closest to the ash is very brittle. Linen is easily extinguished by blowing on it as you would a candle.
• Silk is a protein fiber and usually burns readily, not necessarily with a steady flame, and smells like burning hair. The ash is easily crumbled. Silk samples are not as easily extinguished as cotton or linen.
• Wool is also a protein fiber but is harder to ignite than silk as the individual "hair" fibers are shorter than silk and the weave of the fabrics is generally looser than with silk. The flame is steady but more difficult to keep burning. The smell of burning wool is like burning hair.

A more complete flowchart is available here.

Man Made Fibers

• Acetate is made from cellulose (wood fibers), technically cellulose acetate. Acetate burns readily with a flickering flame that cannot be easily extinguished. The burning cellulose drips and leaves a hard ash. The smell is similar to burning wood chips.
• Acrylic technically acrylonitrile is made from natural gas and petroleum. Acrylics burn readily due to the fiber content and the lofty, air filled pockets. A match or cigarette dropped on an acrylic blanket can ignite the fabric which will burn rapidly unless extinguished. The ash is hard. The smell is acrid or harsh.
• Nylon is a polyamide made from petroleum. Nylon melts and then burns rapidly if the flame remains on the melted fiber. If you can keep the flame on the melting nylon, it smells like burning plastic.
• Polyester is a polymer produced from coal, air, water, and petroleum products. Polyester melts and burns at the same time, the melting, burning ash can bond quickly to any surface it drips on including skin. The smoke from polyester is black with a sweetish smell. The extinguished ash is hard.
• Rayon is a regenerated cellulose fiber which is almost pure cellulose. Rayon burns rapidly and leaves only a slight ash. The burning smell is close to burning leaves.
• Blends consist of two or more fibers and, ideally, are supposed to take on the characteristics of each fiber in the blend. The burning test can be used but the fabric content will be an assumption.

Hopefully at the completion of these techniques you will be able to suggest that the District Attorney prosecute a suspect or continue searching! Now download your casefile and get to work!