Forensic Science Chromatography

Understanding light in terms of physics is very important before discussing chromatography. White light such as sunlight reaching earth is composed of several colored lights. In 1666, Isaac Newton passed narrow beam of sunlight through a prism, and discovered a spectrum, which is arrangement of colors from red to violet as shown in the diagram below. Each color in the spectrum is associated with a different wavelength of light. The wavelength of red light is about 700 nm, and violet is about 400 nm. Rest of colors lie between 400nm to 700nm.

Chromatography Each color in the spectrum is associated with different wavelength of light. The wavelength of red light is about 700 nm, and violet is about 400 nm. Rest of colors lie between 400nm to 700nm.

Various colored light obtained from white light Does not require all colors shown in spectrum to have white light Correct intensities of red, green, blue light are enough to produce white light. When two colored lights are mixed, three new colors of light are formed. –Cyan (red and blue) –Yellow (red and green) –Magenta (blue and green).

Red light, blue light and green light are referred as the primary colors of light, Cyan, yellow, and magenta are called the secondary colors of light. Additive process among primary colors is widely used. For example, a color television tube is made with many pixels of red, blue, and green light. Intensities play a role to create vivid colored images.

–An apple is red because it reflects red light to observer’s eyes when white light falls on it. –Likewise, dye is molecule that absorbs certain wavelengths of light and transmits or reflects others. –A cyan color is observed due to the reflection of red and blue but absorption of green light. –Magenta color is from absorption of red color but not blue and green. –Yellow color is from absorption of blue but not red and green. Absorption and Reflection of Light

What is black color then? Mixture of many colors but not the primary colors When magenta, yellow, and cyan are mixed in certain proportion, black color is observed Black ink is made up with secondary colors Black ink pens vary depending on different manufactures

Thin Layer Chromatography (TLC) Is a method to separate compounds (ink in this case) from one another without altering them. It uses solute, solvent, and a medium that molecule will travel along. In order to get noticeably good results, the right solvent for the specific solute must be used.

Thin Layer Chromatography (TLC) Since we know ink is from organic compounds, ethanol is right candidate to dissolve them. As the solvent moves up, the ink molecules will move with it if they are more strongly attracted to the solvent molecules than the molecule of the medium, paper or silica gel. If the ink molecules are more strongly attracted to the paper than the solvent, they will move slowly or even not at all. Depending on how many different types of molecules are in the solute, all may move at different rates.

R f – Retention Factor Each separated spot can be assigned a Retention Factor (R f ). Shows characteristic of specific dyes associated with it. The R f is a ratio between distance the dye travels over the distance solvent travels. This ratio should be constant under same chromatographic conditions such as medium, and solvent.

Highlights TLC is used to separate compounds without altering them for the purpose of comparison The degree of attraction between the molecules and the solvent affects how far the molecules will move on the TLC paper Comparing R f of samples will help you determine which pen was used