Study of the Performances of Thin Layer Chromatography II. Band Broadening and Plate Height Equation

Abstract

The band broadening in thin layer chromatography can be characterized by a height equivalent to a theoretical plate, when some conditions are fulfilled. The validity of this assumption is discussed together with the relationship between the local plate height and the equivalent plate height. It is shown that the molecular diffusion term controls the plate height in most cases, except for relatively coarse particles. The HETP is smaller for large retention factors (Rf) and for small diffusion coefficients. It increases very rapidly with increasing development length when fine particles (7 μm or less) are used. This increase is slower for larger particles (10–15 μm). For coarse particles the HETP decreases at first with increasing development length and eventually increases again at large development lengths. The high performances obtained in HPTLC using fine particles are possible only when short development distances are used (5 cm or less). At larger distances, the use of coarser particles permits the achievement of larger efficiencies in shorter time. Optimum performances seem to be obtained with particles around 12–15 μm. For example, the use of 15 μm particles, of n-heptane as mobile phase and a development distance of 16 cm can yield a 4,000-plate spot for a compound with Rf = 0.7 and Dm = 5 × 10-6cm2/sec. Narrow size distribution is necessary to obtain these high performances.