An Inexpensive Developing Chamber for Radial Chromatography Robeti C. Gadwood Northwestern University, Evanston. IL 60201
Analytical thin-layer chromatography is clearly one of the most valuable and most often'used techniques in chemistry. It is particularly important to the field of organic synthesis, where i t can he used to monitor reaction progress, determine nroduct distributions. and o ~ t i m i z esolvents for HPLC and preparative chromatographcc separations. While the traditional linear elution of TLC plates is still the most common technique, radial (or circular) elution1 has the advantages of better resolution, shorter elution times, and lower operating costs. Despite these advantages, the widespread use of radial TLC has been limited in part by the lack of a convenient, small-scale developing chamber comparable in price to those available for linear TLC. This note details the construction and use of a s i m ~ l eand inexnensive radial TLC chamher that has been employed for several years in this laboratory with excellent results. The central component of this chamber is the conical . ~ holes are ~olvethvleneliner of a size 38 bottle c a ~Several made in the wall of the central tube o i the liner (to allow passage of the solvent), and a solvent wick of appropriate length (about 9 mm) is inserted.3 This assembly is placed into a size 48 hlack plastic screw cap4 from which the cardboard liner has been removed. The length of the solvent wick is adjusted so that the tip of the wick is even with the rim of the hlack plastic screw cap. For TLC elution the polyethylene cap liner is filled with solvent and the TLC plate is nlaced over the solvent wick. adsorbent side down. A cutaway view of the chamber is shown in Figure 1. For chromatography of one to three samples a t a time, the chamber can he used with 2.5-cm squareplates. The samples are snotted in the corner of the d a t e . as shown in Fieure 2. and the plate is placed over the solvent wick so that thk wick contacts the corner of the plate helow the sample spots. During elution, a 5 X 7-cm crystallizing dish is placed over the whole apparatus to maintain a solvent-saturated atmosphere. For four to eight samples, a 5-cm square plate is used, and the samples are spotted around the center of the plate, as shown in Figure 3. The center of the plate is placed over the solvent wick, and elution is allowed to proceed until the solvent front reaches the inner edge of the hlack screw cap. With these larger plates, the use of a crystallizing dish as a cover is unnecessary. The advantages of this simple system over traditional linear chromatography are: 1) low usage of solvent (1.5 mL is sufficient for three to four 2.5-cm-
Figure I. Cutaway view of t h e radial chromatography chamber. A: Solvent wickS:0: polyethylene cap linearz;C: black plastic screw cap.'
Figure 2. illustration of small plate elution (1-3 Jamples).
Figure 3. illustration of large plate elution (4-8 samples).
2) rapid elution times (-1.5 to 2 min) 3) economical use of TLC plates (Sixty-four2.5-cm-square plates
can be cut from one 20-cm-squareplate; the cost per 2.5-cmsquare plate is only a few cents.) 4) better resolution than linear plates' 5) rapid determinationof preparative chromatography solvent systems 6) greater mobility of very low Rr spots. These last two characteristics are due to the relationship between radial Rr (Rr,) and linear Rr (Rr,) which can he expressed as 1
square plates)
' Zlatkis. A..
and Kaiser. R. E., "HPTLC: High Performance Thin Layer Chromatography." Journal of Chromatography Library, Volume 9. Elsevier Scientific Publishing Co.. Amsterdam. 1976, Chapters 2, 3. and 4. Available from Sargent-Welch Scientific Co., Skokie, IL; Catalog No. S-9611-G. Solvent wicks are available from Analtec, Inc., Newark. DE. In verslons for both nonaqueous (catalog no. 60-01)and aqueous (catalog no. 60-02) solvents. Available from Sargent-Welch ScientificCo., Skokie, IL: Catalog No. S-9610-K. 820
Journal of Chemical Education
Thus, a suitable solvent for preparative chromatography is one that moves the bands of interest to the center of the radial TLC plate (RfLof 0.25 = Ry, of 0.5). LOWRy spots have greater mobility with radial elution (e.g., Rf, of 0.01 = Rr, of 0.1), and therefore, very low Rr compounds which may otherwise be indistinguishable from baseline are much better resolved. The system described here has been used most frequently with ronventional silira or alumina TLC d a t e s and orranic C an; re solvents. However, high-performance T ~ plates verse-phase TLC plates may also be used.
