Leonard
F.
Druding
Rutgers University
Downloaded via UNIV OF SUSSEX on July 27, 2018 at 17:29:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Newark, New Jersey
TLC
Separation of Ink Pigments
Separation of pigments of writing inks by the thin layer chromatographic techniques was used quite successfully as a student laboratory exercise and as a rapid lecture demonstration of this recently developed analytical separation method. The experiment is an adaptation of a paper chromatographic separation described in Reilly and Sawyer.1 In our experiment, a thin layer of activated silica gel on a glass plate was used as the absorber, and ethanol was used as the solvent. The inks chosen for this demonstration were four of the Sheaffer’s “Script” “washable” writing fluids, black, blue, green, and purple. In addition to the visible pigments, the manufacturer advertises the presence of “RC-35,” a fluorescent ingredient which makes it possible to determine the original inscription in case of erasure. The separation of inks into their component pigments was chosen for its simplicity in illustrating techniques and choice of solvents for thin layer chromatography; and, in addition, affords the student the opportunity to visually observe the course of the separation. One ml of each ink was added to five ml of 95% ethanol. 5A (0.005 ml.) of this solution was applied to the coated plate. If quantities as large as one drop are used, it is suggested that the ink be diluted further. A series of glass plates, 1 in. X 8 in. were coated with silica gel by one of two methods. In the first method, a homemade applicator was used. This was an aluminum trough sliding on aluminum rails mounted to a flat slab. The aluminum rails were slightly thicker than the glass plates used and were spaced so that eight plates could be coated at one time. To 10 g of Kieselgel (Arthur H. Thomas, Philadelphia, Pa.) was added 23 ml of water, and the mixture stirred until thoroughly dispersed. This slurry was poured into the trough and the trough was drawn over the plates. The second method of coating followed the procedure given by Bekersky2 where the absorber was sprayed onto the plates. This resulted in plates of more uniform coating, and furthermore, allowed the students to study as laboratory exercise the effect of layer thickness on the degree of separation. To spray the silica gel slurry on the plates, either a Fisher atomizer (Fisher Scientific Co., New York) or a dentist’s oral atomizer (L. D. Caulk Co., Newark, N. J., or any dental supply company) was attached to a small compressor or a nitrogen cylinder set at 10 psi. Best results were obtained when 10 g of Keiselgel were mixed with 30 ml of water. The plates were placed on a flat surface, and Sawyer, D. T., “Experiments for Instrumental Analysis,” McGraw-Hill, New York, 1961, pp. 241-244, 2 Bekersky, I., Anal. Chem., 35, 261 (1963). 1
Reilly,
536
/
C. N.,
and
Journal of Chemical Education
the atomizer held approximately 12-18 in. from the plate while coating. One slow pass with the sprayer provided a coating equal to the conventional applicator. After the silica gel was applied, the plates were left in position for 10-15 min for the silica gel to set. The plates were then activated by drying in an oven at 110°C for 1 hr. Until use, the activated plates were stored in a desiccator over calcium sulfate. Five lambda (0.005 ml) of the ink solution was applied to a plate 1.5 cm from the bottom, and a line was inscribed on the plate 10 cm above the uppermost edge of the spot. The plate was then placed in a 250 ml cylinder containing about 5-10 ml of 95% ethanol, and the cylinder was stoppered. Solvent rise was quite rapid, and separation of the components of the ink was immediately noticeable. By shining an ultraviolet light on the plate, separation of the ultraviolet constituent could also be observed. When the solvent front had reached the 10 cm line, the plate was removed from the cylinder and air dried. The distance traveled by each component was measured and the values of R{ calculated. No attempt was made to determine the chemical composition of each constituent; however, by using the black ink, and a variety of solvents, the effect of changing solvent polarity on the degree of separation was investigated in the student laboratory. The table shows the results obtained for four colors of ink with 95% ethanol used as the solvent, for it appeared to give the best separation in the shortest time. In trying other solvent systems, it was found that the silica gel plates could tolerate up to 20% water in the solvent; beyond this, the thin layer would flake off the plate. Best separations were achieved by polar, water-like solvents, while non-polar solvents gave zero values for Rit Values for Constituents of "Script" "Washable” Writing Fluids on Silica Gel Plates, Ethanol Solvent
R{
Ink
Black
Constituent
“RC-35” Yellow Red Blue
Blue
“RC-35” Blue
Dark Blue
Green
Purple'
Blue “RC-35” Yellow Blue Blue Red Purple Dark blue
R
t
0.96 0.94 0.92 0.80 0.93 0.92 0.85 0.00 0.95 0.93 0.78 0.85 0.84 0.74 0.00
Comments
Narrow band Wide band Trace amount
Relatively
narrow
band
Trace amount
Very narrow band Narrow band Wide band Trace amount
Although the ink itself fluoresced under an ultraviolet light, no trace of an ultraviolet constituent could be found for this particular sample on the chromatographic plates. a
