Each team will need:
Black pens # 1, #2, #3 collected from suspects at the crime scene
Strips of filter paper* (about 3cm by 9 cm) –1 strip per team
Developing Solution (Isopropyl Alcohol – “IPA”)
Clear jars (developing chambers)
1. Draw a line in pencil 1cm from the bottom of each piece of the chromatography
filter paper. This is the origin line.
2. Along the origin line, using the pencil place a small dash in the middle of the line.
4. Using pen #1 draw a thick dot on the origin line at the dash.
5. Repeat this set-up for the other pens.
6. Place the strip in the developing chamber with the ink toward the bottom. Allow
the strip to rest at an angle so that the paper will not stick to the side of the jar.
7. Slowly and carefully add some IPA to the beaker with a transfer pipet so that only
the very base of the filter paper touches the solution. Don’t get the black pen marks
wet during the transfer.
8. You should notice that the developing solution starts to be drawn up the paper
immediately. When it reaches the ink mark it will make the ink bleed and then will
start to separate the different inks that make up the black pen. Because there are
molecules of different sizes and solubility making up the black ink, the molecules of
any one type will travel in the solvent and up the strip at a different rate than other
types of molecules. This causes the separation of the different types of molecules.
We are able to see these separated molecules because they are pigments reflecting
specific wavelengths of light to which our eyes are sensitive.
9. It will take up to 30 minutes for the ink to be completely separated.
10. When the ink is no longer spreading upwards or not changing color anymore,
take it out of the jar and place it on some paper towel to dry. Gently mark with
pencil the highest point the developing solution reached on your filter paper.
11. Sketch the colors of each of the pen’s pigments at the bottom of this page.
12. Calculate the Rf values for each pen’s colors. Record this data in the appropriate
table of the report.
To calculate the Rf value, first measure the total distance the solvent (water)
moved from the origin. This is the Solvent Distance.
Now, for each color band for that pen, measure from the origin to the brightest
area of that color. This is the Solute (color) distance.
The Rf value indicates how far the solute traveled relative to the solvent. It is
calculated as: Rf = Solute (color) distance in mm
Solvent distance in mm
13. Using the results, analyze the samples from the crime scene and compare with
the lab sample from Captain Relish’s note. Determine which pen was used to write
Calculations of Rf values: