Who Wrote the Note?: Teacher's Notes
- Plastic dish
- filter paper (with diameter larger than diameter of plastic dish)
- filter paper wicks
- ransom note
- plastic brace
- 3 black pens
- tracing ring
Pens - Various pens with water-soluble black ink can be used. Three which give distinctive results are Vis-A-Vis Fine Point, Papermate Flair, and Expresso Medium Point.
This experiment may take up to 30 minutes depending on how fast the pigments separate. Using a fast filter paper, such as Whatman #4 will speed things up. The wick is just 1/8 piece of filter paper which is rolled up from the large curved edge to the point. You can also use coffee filters, either the flat disk type or the basket style. This style must be flattened before use. The ink pigments tend to spread out a little so the chromatogram is not as well defined, but the three pens will still be easy to tell apart. See Figure 1.
When developing the ransom note chromatogram, the wick should be leaning on the piece of ransom note. The note must be supported by the plastic brace. Prepare ransom notes on a piece of filter paper, randomly using any of the different pens. Sign the note in such a way that two areas contain long lines suitable to be cut as samples as shown in the photograph in the procedure section.
Both chromatograms should be removed from the dish as soon as the water is about one-half to one inch from the edge of the paper. This usually takes about 10-20 minutes. Otherwise, the pigments will smear out around the edge of the paper.
Students can try this activity at home using a coffee filter and their own water-based ink pens or colored drink mixes. A discussion on why some pens (non-water based) won't work can follow. A pen of this type could also be included in the experiment and could be part of the experimental results.
More advanced students could be asked to measure the distance the pigments traveled from the starting point. This distance can be related to molecular size and solubility. Pigment color and distance traveled can be used to compare the similarity of ink components.
Pretesting of the pens you plan to use is suggested as a few brands seem to use identical components and their chromatograms cannot be distinguished. This can also be used as a classroom activity and can initiate a discussion of the problems encountered in analysis of unknowns. Evidence can only be positively "excluded", not positively" included" when mass produced items are analyzed.