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PAPER CHROMATOGRAPHY
Chromatography is a technique that is used to separate and to identify components of a mixture. This analytical technique has a wide range of applications in the real world since many substances are mixtures of chemical compounds. In this lab, you will explore two applications of chromatography - identification of an unknown ink sample and the separation of food colorings.
In paper chromatography, the sample mixture is applied to a piece of filter paper, the edge of the paper is immersed in a solvent, and the solvent moves up the paper by capillary action. Components of the mixture are carried along with the solvent up the paper to varying degrees, depending on the compound's preference to be adsorbed onto the paper versus being carried along with the solvent. The paper is composed of cellulose to which polar water molecules are adsorbed, while the solvent is less polar, usually consisting of a mixture of water and an organic liquid. The paper is called the stationary phase while the solvent is referred to as the mobile phase. Performing a chromatographic experiment is basically a three-step process: 1) application of the sample, 2) "developing" the chromatogram by allowing the mobile phase to move up the paper, and 3) calculating Rf values and making conclusions.
In order to obtain a measure of the extent of movement of a component in a paper chromatography experiment, we can calculate an "Rf value" for each separated component in the developed chromatogram. An Rf value is a number that is defined as:
distance traveled by component from application point Rf = ---------------------------------------------------- distance traveled by solvent from application point
The distance traveled by the spot is measured to the MIDDLE of the spot.
Pencil Mark
Solvent Front
A
B Rf = A/B
INTRODUCTION
Forensic chemistry, the profession popularized by TV's "Quincy" is the application of chemical techniques to law. Many samples that are received in a crime laboratory are complex mixtures of chemical compounds and chromatography is often ideal for separating and identifying these components. One example of this is the identification of an unknown ink sample. Suppose the authenticity of a signature or the source of a ransom note is in question. Using paper chromatography, it is often possible to identify the source of the inks that were used and thereby provide strong evidence that can be used in the courtroom
In the second part of the experiment, you will separate a mixture of three food colorings using "reverse-phase" chromatography. In this type of chromatography, the stationary phase is a non-polar material and the mobile phase is relatively polar. You will use a commercial C 18 cartridge, which contains very non-polar 18-carbon long chains as the stationary phase. The mobile phase will be water and/or isopropyl alcohol (IPA).
Using chromatography paper strips
AVOID EXCESSIVE HANDLING OF PAPER
- The chromatography paper is cut in about 2.5 x 10 cm strips. Along one of the shorter sides, draw a horizontal line in pencil (lead will not move) about 1.5 cm from the edge of the strip. This will be your “base line”, the starting line where the samples will be spotted.
- Apply a dot of ink from each black felt tip pen on the baseline. Make sure dots are evenly spaced on the baseline and not too close to the edge. It may be helpful to use a pencil to mark the spots before placing the ink on the paper. See the diagram below.
- Label each spot in pencil so that you know what each spot is.
- Stand the paper in the eluting solution. Taping the top of the strips to the side of the beaker may be helpful.
B a s e L in e 1 2 3
TECHNIQUES
- Pour some of the developing solvent in the beaker, using only enough solvent to cover the bottom of the beaker (about 5-10 mL).
- Remember, one lab pair will use 1:2 IPA and the other lab pair will use 2: IPA. G. Place the paper strips into the developing chamber so that the edge near the ink spots is submerged in the solvent. THE INK SPOT MUST BE ABOVE THE SOLVENT LEVEL. You should avoid allowing the paper strips to touch each other or the sides of the beaker. (Taping the top of the strips to the outside of the beaker may be helpful.) Once you have inserted the paper, cover the chamber with a watchglass and make sure the solvent is progressing up the paper. H. When the solvent has risen almost to the top of the paper strip, remove the strip from the chamber and immediately mark the level to which the solvent has risen and circle each spot present on the strip with a pencil. (The solvent front and the spots will continue to move for several minutes after removing the strip from the beaker.) I. Measure the distance each spot traveled and calculate R (^) f values for each spot. Use the circles you drew around each spot with your pencil and not the actual spot! J. Turn your chromatogram in as part of your report.
II. Separation of Food Colorings
A. Remove the piston and insert the LONG END of the C 18 cartridge snugly into the syringe tip. B. Add 5 mL of IPA, insert the piston, and slowly flush at a flow rate not exceeding 5-10 mL per minute. It is important to not flush liquid through the cartridge more rapidly than 10 mL per minute. Repeat this step with another 5 mL portion of IPA. C. Flush with two 5 mL portions of distilled water. D. Add about 1 mL of the food coloring mixture to the syringe, insert the piston, and push the mixture onto the cartridge. As you push the sample through the cartridge, immediately begin collecting the drops in a test tube. The first color comes off quickly.
- Once the 1 mL of food coloring mixture is pushed onto the cartridge, add water to the syringe and push it through the cartridge until all the yellow is gone. Collect the yellow liquid. The process of pushing solvent(s) through a cartridge is called elution. In this case, water elutes the yellow color.
- The remaining colors in the mixture must be removed by eluting with a water/IPA mixture. Prepare a 1% IPA solution by placing 5 mL of water into the syringe and adding 1 drop of IPA. Mix well by shaking the syringe. Push this solution through the cartridge and collect all the colors that come off. If coloring still remains on the cartridge, prepare a 2% IPA solution by adding 2 drops of IPA to 5 mL of water, mix, and elute this solution through the cartridge. Continue to increase the percentage of IPA (3% - 3 drops IPA + 5 mL water, 4% - 4 drops IPA + 5 mL water, etc.) until all or most of the coloring has been removed from the cartridge. A 5% IPA
solution may be the most concentrated solution needed to remove all of the colors from the cartridge.
- Any coloring residue remaining on the cartridge should be removed using a 1:1 IPA:water mixture. This residue may need to be collected.
- Save the separated colored solutions for the next part.
E. See what would happen if several of the separated colored solutions were mixed.
- Try to make a green solution from the colors you separated.
F. Make sure the cartridge is clean before returning it.
