[oksanah]

i was just wondering... what are the main differences between gas chromatography and TLC? I know that GC has a retention time versus TLC which has a retention factor instead, but I don't understand the main principles that differ these two.

[stewie griffin]

GC can give you info such as relative amounts. If you integrate the peak areas it can give you an idea as to whether the ratio between some reaction products is 1:1, 2:1, 5:1, etc...
Normal phase TLC separates compounds based on polarity and only gives qualitative info (how many possible reaction products, is the starting material all gone, etc). GC can (usually) do whatever a TLC can do, but most people use TLC since it's faster and not everything is going to vaporize easily.

[johnmalkinson]

In addition to stewie griffin's comments I would say that the most important differences are in terms of the types of sample that can be separated and the mechanism of separation. GC is only useful for volatile compounds - the compound can only migrate along the GC column when vaporised (in the gas phase) as the mobile phase is gaseous. TLC is more flexible and can be used for involatile (and polar) samples as well. Most importantly TLC differentiates compounds primarily by polarity, whereas GC differentiates mainly on the basis of molecular mass (low molecular mass compounds elute earliest).