Training Lab - Identifying Writing Pens/Ink Using Paper Chromatography
NAME ________________________
Background: Chromatography is a technique used in labs to separate mixtures of chemicals.
There are many kinds of chromatography that can be useful; however, in this training lab you
will use paper chromatography. In this method, you place the mixture of chemicals to be
separated at one end of a strip of paper. This end of the paper strip is then placed in a solvent.
The solvent moves up the paper strip, however, as it travels upward it dissolves the mixture of
chemicals and pulls them up the paper too. The chemicals in the mixture become separated as the
chemicals that dissolve best in the solvent move up the paper strip further than chemicals that do
not dissolve well. So, how does this help solve a crime?
Black ink is a strange thing - it is made by mixing together several different colored chemicals.
Paper chromatography can separated this mixture of colored chemicals so you can see what
black ink is actually made from. Each pen company produces their black in using their own
secret mixtures of colored chemicals. This means is you find some ink left behind at a crime
scene (for instance, on a ransom note), you can use paper chromatography to identify what kind
of pen was used to write the note. This could help you identify a suspect and solve the crime!
1. You will be trained to use paper chromatography to separate the chemicals found in ink
samples.
2. You will be trained to construct a reference library of ink samples and use this reference
library to identify the pen used to write a note.
Procedures:
Part 1 - Paper Chromatography of ink samples
1. Pick up three chromatography tubes. Use a test tube holder to hold your tubes so they
won’t get knocked over.
2. Pour about 1 cm of alcohol into a chromatography tube Alcohol is the solvent that will
move up the paper strip and dissolve the colored chemicals in the ink.
3. Cut a strip of chromatography paper so that it is a little longer than the chromatography
tube.
4. Cut one end of the chromatography paper to form a point.
5. Pick up a pen to test. Record the BRAND AND MODEL of the pen being tested on your
Reference Library Page.
6. Write the brand and model of the pen on the top of the chromatography paper.
7. Use the pen to make a good-sized dot of ink about 1.5 cm up from
the pointed end of the paper.
8. USE A PENCIL AND MAKE A MARK ON THE PAPER STRIP
BESIDE THE INK DOT!
9. Lower the pointy end of the paper strip into the solvent BUT make
sure the ink dot stays ABOVE the solvent level.
10. The solvent should immediately start moving up the paper strip
carrying the ink chemicals with it. While waiting for the solvent to
rise toward the top of the paper you can set up your other chromatography tubes and test
two more pens.
11. When the solvent has moved more than half way up the paper strip you can remove
the paper from the chromatography tube.
12. Immediately MARK WITH A PENCIL THE HIGHEST POINT THE SOLVENT
TRAVELED UP THE PAPER STRIP.
13. Pour the solvent back into its storage container; it can be reused.
14. Let the strip dry out some, then tape it on your Reverence Library Page under its correct
pen name
Part 2 – Calculating Rf Values and Describing Your Ink Samples
1. You have probability noticed that each pen uses a different combination of chemicals to
produce black ink. It should be fairly simple for you to test an unknown ink sample with
chromatography and use your Reference Library to match colors and determine which
pen was used to write the note.
2. Look closely at each pen’s chromatography strip and determine how many different
chemicals were present in each ink sample. Each chemical should appear as a different
colored smear on your paper strip; however, some chemical smears may be overlapping.
Record the Total Number of Colored Chemicals present for each ink sample on your
Reference Library Page.
3. Look closely at each pen’s chromatography strip and record the colors that are visible in
the order that they appear. Begin with the original ink dot and list the colors as you move
up the paper. Each color represents a different chemical present in the ink. Record the
colors you observe on your Reference Library Page (Colored Chemical #1 = gray color,
Colored Chemical #2 = yellow color, ect.)
4. To prove that an ink sample belongs to a certain pen
you will also need to calculate the Rf values of the
different colored chemicals present in the ink. Rf is
a simple calculation that compares the distance the
solvent traveled up the paper strip to the distance a
chemical traveled up the same strip.
5. First, look at a chromatography strip and measure
the distance in millimeters from the original ink dot
to the final point the solvent traveled (this is why
you asked to make those pencil marks on the strip!)
The distance you just measured is the SOLVENT
DISTANCE MEASUREMENT. Record this
measurement in the correct location on your
Reference Library Page.
6. Next measure, in millimeters, from the original ink
dot to the highest point the first colored chemical
(Colored Chemical #1) traveled up the strip. This is
the CHEMICAL DISTANCE MEASUREMENT
FOR Chemical #1. Record this measurement in the
correct location for colored Chemical #1
7. If there is a Colored Chemical #2 present on the strip measure, in millimeters, from the
original ink dot to the highest point this second colored chemical (Colored Chemical #2)
traveled up the strip. This is CHEMICAL DISTANCE MEASUREMENT Chemical
#2. Record this measurement in the correct location for Colored Chemical #2.
8. If there is a Colored Chemical #3 present repeat the chemical distance measurement for
Chemical #3.
9. Calculate the Rf calculate for each colored chemical in you ink sample using the simple
formula below:
Rf Value =
chemical distance measurement
solvent distance measurement
10. Record the Rf Values for each colored chemical in the ink on your Reference Library
Page.
11. Repeat these measurements and Rf calculations for each of the pens/inks tested.
12. Turn in your Reference Library Page with the lab so it can be included in your class’s
Reference Library, so that you can use it to determine what kind of pen was used at a
crime scene. You may just crack the case with this evidence!
Questions – Identifying Writing Pens/ink Using Paper Chromatography
1. What is chromatography?
2. Do chemicals that dissolve well in the solvent end up lower or higher on the strip of
chromatography paper than chemicals that do not dissolve well?
3. Why is it very important that you use a PENCIL to put a mark next to the ink dot?
4. What is your favorite type/brand of pen to write with?
5. What is your favorite ink color to write with?
6. Did any of the pens you tested contain any of the same ink chemicals? Fill out the chart
below to prove that two pens contained the same ink chemical. Remember, if it is the
same ink chemical it will be the same color AND it will have about the same Rf value!
Pen's Name
Color of Ink chemical
Rf Value of Ink
Chemical
7. Below is the information for my favorite kind of pen. Using this information, draw the
correct locations and colors of the colored chemicals as they would appear on the empty
chromatography strip to the right if I had put a dot of my favorite pen’s ink on the paper
strip.
Colored Chemical #1 Colored Chemical #2
Color = yellow
Color = Purple
Rf value = .3
Rf value = .8