Chromatography Forum

(re)-Developing a method and I've misplaced my notes - frankly feeling a bit silly just now , and a bit humbled as I'm usually the "go-to guy" in my lab for things Analytical, as I really should know what I was doing and just cannot seem to either remember what I was doing nor where/what I did with my notes.

Working on separating some problematic peaks. I was calculating an index based on the relative retention time of the constituents against n-heptane:

I've calculated the relative retention times (RRT) of my other peaks: where c = constituent and t = un-adjusted retention time of the peak - standard stuff.

From there I have

I just do not remember where the second equation came from and we've changed the column from DB-FFAP to a Restek-624, along with changing the length and ID, etc; thus, I'm re-developing/validating the methodology (why the change, because management said so... ); therefor, I am certain that the 0.77936 has to be recalculated based on the new column.

I really don't mind looking this information up, I probably need to refresh my memory over the basics. Can anyone please point me toward the text/reference where this is information is located or have I really lost my mind and have something really goofy going here?

I've tried searching my office, my home library, asking my co-workers (yes - they got a little chuckle here, but that's ok, can't remember everything all of the time), and the internet for hours now without so much as a crumb when this site appeared in the Google results - the power of the internet!

TYIA
-z

What is that second equation calculating? Beyond the 0.77 . . . I'm also curious about the 2.5 value as well.

What is that second equation calculating? Beyond the 0.77 . . . I'm also curious about the 2.5 value as well.

+ Thank you for taking an interest - I was beginning to think I actually pulled this equation out of the air somewhere - been re-reading a lot of journal articles and textbooks without any luck; however, good refresher on the basic methods. I was fairly certain I had pulled this from one of the chemical journals dealing with method development.

+ IRC:: retention index relative to the n-heptane peak as adjusted for temperature program as an indication of the resolution of the constituents against ideal separation.

+ I do believe that 2.5 is the time from the first ramp/hold as this is in part of my worksheet investigating the separation of two closely eluting components.
The method at this point was
T0=25 (room temp) Hold 2.5min
Ramp at 5C/min to 35C hold 1.5min
Then ramping at 10/min thru to the next pair of problems etc...
So in this case the first pair is acetaldehyde (bp=20.2C, rt=2.400min) and methanol (bp=64.7C, rt=2.445) and the n-Hep rt=12.013. Obviously this column wasn't suitable for these two components, we've move to a different column, and the calculation (2.5+(0.77-RRT()) was what sealed that fact with the lab-manager.

Now I was attempting to do this same thing for a different method to qualify, or disqualify as the case may be, the change in column and method.

As for the 0.77936, it may be something dealing the desired resolution/separation ratio of the peaks in question that I may have hand calculated... Normally I put extra notes in the worksheets; however, this time all I entered was "Vol6Pg216MP" (middle of page ) however, as the title implies... I cannot find my lab journal number 6. I don't know if it was tossed with the old documents scheduled for shredding ( ) or sitting on-top of a cabinet camouflaged with dust to look like a rabbit!

TBH: at this point I had to move on with the project and went back to my old standby methods for optimization. Even though, I'd still like to understand what I was doing so many years ago with this equation.

This sounds like a case of "Don't think, measure !" to quote Hans Muller who used to be on the forum.

Inject analytes, look at separation, tweak conditions, repeat.

With retentions that close to the dead time the peak shape will be dominated by inlet effects, not chromatography, and good luck with calculating them !

Peter