Calculating resolution for chromatographic peaks

[Ge_D]

I'm confused about which number of theoretical plates to use (since there are three of them), shouldn't there be only one for the same column?

[Ge_D]

Sorry the image did not show up on the previous post, so I'm reposting it here:

[tom jupille]

Hint: Resolution is defined as the center-to-center separation between two peaks divided by the average of their baseline widths. If you think about it, that should tell you which plate numbers to use.

[Ge_D]

Hint: Resolution is defined as the center-to-center separation between two peaks divided by the average of their baseline widths. If you think about it, that should tell you which plate numbers to use.

So it's the average of their theoretical plate numbers? (ie. For resolution of peaks A-B it's (2450+2500)/2 and (2500+2850)/2 for peaks B-C)??) This is actually a sample exam pratice, so I want to make sure if I'm on the right track. Please reply.

[tom jupille]

You got it!

[M Farooq]

It is a very good question asked by an intelligent student. Which "N" should be used in calculating the resolution equation because the critical pair has different efficiencies and the resolution equation has only one N (plate height)?
It may be okay to use the average for normal (educational) purposes. But advanced chromatographers should be aware of at least 16 different versions of the same resolution equation and none of them uses the average N. The simplified resolution equation assumes that N1=N2, and that why we see no subscript with N in the fundamental resolution equation. This problem used to confuse me as well until I came across Foley's article. I had it saved somewhere, but the excellent reference is here:

Resolution Equations for Column Chromatography, by Foley in the Analyst, 1991, 116, 1275.

Anyway, Rs is usually quoted in two digits so for practical purposes these nuances don't matter.

Regards,
Farooq

[cubane]

Depends on which resolution equation you're using. This site has a good overview of the different resolution equations: http://rozing.com/index.php/resolution-equations.

I'll summarize it for you:

Purnell Resolution Equation
[*] Uses plates and retention factor of second peak (N2, k2).


Knox Resolution Equation
[*] Uses plates and retention factor of first peak (N1, k1).


Said Resolution Equation
[*] Uses the average plates and average retention factor of the two peaks.


Answer: The resolution equation on that image you provided (Purnell Equation) would require N2 and k2.

References:
- J. Chem. Soc. (1960) 1268
- JHRCCC (1979) 193
- J. High Res. Chromatogr. 12 (1989) 82