in clinical and forensic studies, and testing for the presence or absence of
specific substances (limit tests) are additional applications. Tables 1 and 2include
further examples.
Quantitative TLCby the measurement of spot areas or by computerized
scanning reflectance densitometry (Topic E8) is possible, but, unless HPTLC
plates are used, the relative precision attainable is generally only 5–10%. The
principal source of error is in applying sample spots to the plate, although auto-
mated systems can reduce this.
TLChas a number of advantages over GCand HPLC:● the ability to run 10–20 or more samples simultaneously for immediate and
direct comparison with standards, which represents a considerable saving in
time;
● the basic technique is very cheap, versatile and quick;
● all solutes, including those that do not migrate from the origin, are detectable.Disadvantages are the limited reproducibility of Rfvalues due primarly to
changes in the composition of the mobile phase during development of the
plate, and increasing spot diffusion (band broadening) because the flow rate of
the mobile phase slows as it travels across the plate.136 Section D – Separation techniques