separation factor vs selectivity factor, and holdup time

[nelidag]

Hi. This is the first time i post some questions. I have been recently told that one should not use the term separation factor in chromatography, that it is not appropriate. Is that true and why? Also, I was told that to estimate the selectivity factor (separation factor, alpha), if the tm was not observed from the chromatogram (peak of a unretained compound) then the tm should be obtained by the geometry of the capillary column. since i got this comment, I have been reading article after article without finding any that could tell me how to use the geometry of the capillary column to calculate the hold-up time, so that in that way i can properly estimate the selectivity... can someone help?!?!?!?!

thanksssssssss!!!!!

[tom jupille]

Re terminology: you say "po-tay-to", I say "po-tah-to"

There are several "standard" lexicons for chromatography (ASTM, USP, etc.) and they are not always consistent. For example:

• "retention factor" = "capacity factor" = k' = k
  "holdup time" = "dead time" = "to" = "tm"
  "separation factor" = "alpha"

The holdup time is just what it sounds like: the time required for one column volume of gas to pass through the system. If the capillary column is sufficiently short and wide that you can neglect compressibility effects, then you simply divide the volume of your column by the carrier gas flow rate.