Chromatography is a method of separating out materials from a mixture.

| September 12, 2019

Chromatography is a method of separating out materials from a mixture. Ink is a mixture of several dyes and therefore we can separate those colors from one another using chromatography. When ink is exposed to certain solvents the colors dissolve and can be seperated out. When we expose a piece of paper with ink on it to a solvent, the ink spreads across the paper when the ink dissolves. Some inks are water-soluble, so you can use water as the solvent. Inks which are not water soluble are often alcohol-soluble and you can use isopropyl alcohol as the solvent to create your chromatograph. Also, you can use acetone. Different ink pens use different types of ink and this is obvious when you expose the ink to a solvent. Follow the instructions below to discover the pattern of each pen. • • Materials o Coffee filters/Paper towels/regular copy machine or printer paper can be used. o Crystal glasses. o Another crystal glass to prepare your solvent (if use isopropyl alcohol) o Mini binder clips o Wooden splints or plastic straws o 4 different black pens and/or markers o Scissors o Pencil o Metric ruler o Water o Plastic wrap The Exercise: o Cut the coffee filter paper/Paper towel/printer paper into strips approximately 2 centimeters wide (cm) by 6.5 cm long. Prepare a total of 4 chromatography strips this way. o Take one of the chromatography strips and use a ruler and pencil to draw a line across it horizontally 1 cm from the bottom. This is the origin line. Repeat this step for the 4 chromatography strips. o Use one of the pens/markers, place a small dot of ink at the center of the origin line of a chromatography strip. This is your spotted sample. o o ▪ Use a pencil to label which pen/marker you spotted on the chromatography strip. Do not use a pen labeling the strips> the ink will run when the solvent passes through the strips. ▪ Repeat this step until you have spotted ink on each chromatography strip. Prepare your solvent. Make a 45% isopropyl alcohol solution to use as your chromatography solvent. ▪ Pur 20 milliliters (ml) of 90% isopropyl alcohol solution into a crystal glass. Add 20 ml of water to the glass, the final volume is 40 ml. Stir thoroughly with the wooden splint/straw. Cover the glass with plastic wrap, so that the solution does not evaporate. ▪ If you decide to use acetone or water as solvent, you don’t need to dissolve the solvent. Depending on the glass you use, pour about 2-3 ml of the solvent into the Crystal glass. Depending on the glass, maybe you can run two prepared chromatography strips in the glass. ▪ Clip two of the prepared chromatography strips to a wooden splint or straw. Make sure the two strips do not touch each other and the bottoms align. Rest the splint/straw on top of the beaker so that the strips hang into the glass and do not touch the sides of the glass. ▪ If necessary, add more solvent to the glass. The goal is to have the end of each chromatography strip just touching the surface of the solvent solution but avoiding touching the ink spots. Add solvent as needed to achieve this goal. ▪ Cover the top of the glass with plastic wrap. o Let the solvent rise up the strip (by capillary action) until it is about 0.5 cm from the top, then remove the strip from the solvent. Keep a close eye on your chromatography strip and the solvent front-if you let it run too long the dye may run off the paper and become distorted. o Use a pencil to mark how far the solvent rose. o Take a picture of your TLCs for each pen. o Allow the chromatography strip to dry, then measure (in centimeters) and calculate the Rf for each pen/marker dye component. Record your results in a table like this one for each pen: o Component Color Component Rf value Total number of components: • Write a lab report with your findings, as samples include the four types of pen. In your procedure indicate which solvent you use. And if you have to change your solvent, explain in procedures and observations. In results include the pictures of your TLC and tables. Discuss in Results if the inks from the different pens/markers separate differently and justify your answer based on the colors and Rf values. • Upload as a single pdf your lab report including your pictures. WRITING CRIME LAB REPORTS A lab report is a neatly written account of your experiment and procedures, as described below. It should be an impersonal account written in third person. The report should be typed. All pages should be of the same size with writing on only one side of each sheet, with no ragged edges and paginated. The report should include all information related to the crime lab and the person performed the analysis. The report should include the following information, with headings to begin each section (see the example). Crime Laboratory, Telephone and Fax Date To: the recipient of the lab report (detective) Lab File Number Agency Case Number Evidence Submitted by: Name of the officer Samples received by Laboratory: Date and time Sample 1 Sample 2 Purpose/Objective: A short statement giving the objective to be achieved by performing this experiment. Procedure: The procedure is an impersonal set of directions on how you perform the analysis. detailed directions. It should include Observations: Some labs require certain observations like if the sample was consumed during the analysis. observations should be recorded. Such Results: A detailed description of the results of the analysis. It must follow the same order as the procedures and samples received. This section must include tables/graphs and figures and sometimes statistical analysis to improve the understanding of the results. Conclusions: Itemized summary of the results. Forensic Scientist Signature and Date CHS 3501L   Anything that has mass and occupies space Classified by its physical state:  Solid  Liquid  Gas (vapor)  Categorized into two broad groups:  Organics ▪ Contain Carbon, usually with H, N, Cl, or P  Inorganics ▪ Other chemical substances not based on C  Evidence received by crime labs includes:  Drugs of abuse  Synthetic fibers  Petroleum products  Paint binders  High-order explosives  Ink evidence Enable forensic scientists to identify or compare matter  Types of analyses:   Qualitative ▪ Identifies the material—What is it?  Quantitative ▪ Precise measurement of a component of a mixture—How much is there?  Spectrophotometry  Study of absorption of light by a chemical substance  Requires material to be in a relatively pure state  Chromatography  Means of separation & identification of the components in a mixture    Based on the observation of substances escaping into the surrounding environment when dissolved in a liquid or when absorbed on a solid surface A mobile phase is made to move continuously in one direction over a fixed phase or stationary phase Materials that prefer the mobile phase move faster than those that prefer the stationary phase  Gas Chromatography (GC)  High-performance liquid chromatography (HPLC)  Thin-layer chromatography (TLC) Gas chromatography The mobile phase, a carrier gas, flows through a column (stainless steel or glass)  The column is coated in a fixed, stationary phase consisting of a thin film of liquid  Carrier gas flows through the column & carries with it the components of the mixture  Substances with greater affinity for the gas phase travel more quickly      Carrier gas (nitrogen or helium) flows through the column at a constant rate Sample is injected as a liquid & immediately vaporized Sample is carried by the carrier gas through the column—interactions with the stationary phase cause the sample to slow down  More interactions = longer time spent in column   As components emerge, enter a detector Signal is recorded as a function of time  Called a chromatogram     High-performance liquid chromatography A liquid mobile phase is pumped through a column filled with solid particles, or stationary phase Separation is also based on the interaction of the components with the stationary phase Good for materials that are sensitive to high temperatures (certain drugs & organic explosives) Thin-layer chromatography Uses a solid stationary phase & a liquid mobile phase The plate is prepared by coating a glass plate with a thin film or silica gel or aluminum oxide (the film on the plate serves as the stationary phase as it does not move)  Can directly analyze liquid samples or must dissolve solids in appropriate solvents  Placed upright in a chamber containing the mobile phase solution (a solvent moves and carries the materials across the stationary phase)    Liquid (mobile phase) slowly rises up the plate (stationary phase) by capillary action  As liquid moves past sample spot, components become distributed between the two phases   Based on solubility, polarity, size Those that have the highest affinity for the mobile phase travel the fastest and furthest  May require an ultra-violet light to visualize some components   Must run unknown sample along standard on the same plate for comparison purposes  Always use a pencil for any markings (ink interferes)   Compare migration distances by calculating Rf values Not a source of absolute identification  Compounds with the same color & Rf value may be the same but is not definitive  Rf values will not change for a given component in the same solvent system SPOTTING THE PLATE RUNNING THE PLATE     Rf = (distance traveled by component) ÷ (distance traveled by solvent) Measured from origin Calculated for each component Rf values don’t have units  Must be between 0.0 and 1.0  You calculated wrong if not within this range   Ideally, should be between 0.3 and 0.6 Affected by:      Temperature Solvent system Film thickness Amount of spot Other compounds present Rapid & sensitive method  Requires less than 100 μg of material  Cost-effective & no large space requirements  Can analyze numerous samples on one plate   Inks  Drugs  A) Black  A) Cocaine  B) Yellow  B) Heroin  C) Green  C) Methamphetamine  D) Red  D) Unknown  Explosives  Stabilizers in propellants & high explosives  Saferstein’s Criminalistics, 9th ed., Chapter 5 …
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