Chromatography Forum

Dear chromatographers,

I found after my peak be eluted, the baseline was not meet its initial state. Is it included in tailing phenomenon? Or is there any explanation about it?
I have used 0.5% TEA in the mobile phase.
pH of mobile phase: 6.5
pKa of substance: 8.91

... next, additional question:
What should I do to attach the chromatograph?

Best regards,
SYX

How do you adjust the pH of the TEA to 6.5?

Dear chromatographers,

...
What should I do to attach the chromatograph?

Find a place to host a graphic, there are numerous sites that allow this if you do not have any web friendly storage space (villagephotos, aol etc).

Then use a FTP client to upload graphic file (gif or jpg) if there is no uploading utility on the site from where you wish to store photos (FTPexpress or other freebie).

Then paste the link to the photo here, highlight it and click on the Img button above.

Try to make sure that the graphic file is not huge. Nobody wants to wait for several MB to download or have to scroll all over a 2000x2000 pixel photo.

GL

How do you adjust the pH of the TEA to 6.5?

I use 0.1M ammonium acetate, + TEA (final conc 0.5%), adjust with acetic acid to pH 6.5.
Mobile phase composition: MeOH - buffer (55 : 45)
Work conc: 100 ppm (20 uL)

For DR, thanx a lot for the guidance. Here is the chromatogram:


Regards,
SYX

If it is not bad column (significant void in the top of the packing) you have a typical example of bad tailing. You stationary phase has an active sites that strongly retain your analyte. Since even TEA does not help much it is probably a polyamino compound. Try to use diethylamine instead of TEA. Try to use lower pH mobile phase. If this does not help try to use well desactivated column (you can find them from any major vendor now). Another solution, which is probably most efficient, to chose a RP column with positively charged surface at pH of your MP such as Primesep B2. (see example here: http://allsep.com/makeChr.php?chr=Chr_065

Yury,

Edit your reply by deleting the parenthesis and point at the end of your link, otherwise it does not working...

Note added by admin: problem fixed.

Thank you Kostas
The link should be this http://allsep.com/makeChr.php?chr=Chr_065
And another mistake: instead of diethylamine I wanted to say diaminoethane.

Thanx Mr. Yuri,
I have experience with low pH and found what a fat peak. Very huge ... and never met the initial state of baseline. These days I try higher pH mobile phase and other mobile phase additive as you suggest.

... I used higher pH: 7.0 and the peak is split!!
I have reserved the column, change the guard column, change the column, ... and found it is still doubled.

I watch closely to its structure the substance must be a chiral. Could it be the reason of split peak?

... I used higher pH: 7.0 and the peak is split!!
I have reserved the column, change the guard column, change the column, ... and found it is still doubled.

I watch closely to its structure the substance must be a chiral. Could it be the reason of split peak phenomenon?

Strong tailing, fronting and fat peaks can be caused by more than one reason. The first thing that could be suspect is packing with the good old silanols interacting with a strongly basic compound. This usually gets better at acidic pH. Another possibility is a mismatch between the mobile phase and the injection solvent. This can be tested quickly by making a sample in mobile phase to see if the peak shape improves.

More strange things (such as interconverting conformers) are possible too, but I first look for the simpler possibilities above.

Dear Dr. Neue,

I have tried lower pH of mob phase but the peak is worst than pH 6.5+. In lower pH, the baseline behind the peak is higher than initial state. When I use pH 7.0 peak shape and baseline behind the peak is significantly restored to good condition, but the peak is split. I will try higher pH to see whether the peak(s) will be separated or not.
I always use mobile phase for diluting the substance as soon as possible.

How to delete a message? I have double message here ...

To Syx
Can you post you splitted chromatogram here?

Can you show the structure?

This is the image at pH 7.0 mobile phase (MeOH 85%).


R1-CH(CH3)-R2
It has piperidine ring.