By Anonymous on Tuesday, April 22, 2003 - 08:58 am:
I am seeing two peaks for a standard of 4-bromothiophenol at widely different retention times (~3min and 17min). These don't appear in other spectra and do not correspond to any other sample I am running. I cannot think of any definite answer to this problem, so far a) oxidation to the disulfide by hydrogen peroxide possibly present due to the reaction of TFA with water (does this happen?) or oxygen in soln, or b) by nuc attack of the thiol on TFA to give the trifluro ester, also not sure if this is a viable reaction.
Column Luna C18 150 x 46, isocratic elution at 90:10 MeCN/H2O + 0.05% TFA v/v for 20min, UV detection.
All help is gratefully recieved
By Tom Mizukami on Tuesday, April 22, 2003 - 04:48 pm:
I know that 4-bromothiophenol is a S-nucleophile, so I would guess b). I would test it by getting rid of the TFA and substitute with dilute phosphoric acid. Good luck.
By HW Mueller on Wednesday, April 23, 2003 - 12:34 am:
You mean "other chromatograms"? You have not run spectra? Are you shure that the 3 min peak really is a regular peak? Your column has a 4.6 mm diameter? Now, I have never heard of TFA producing H2O2, but the air oxidation that you mention might be possible, but expectedly before it contacts the acidic mobile ph. (air oxidations are usually slow in acid). The esterification is normally reversible in acid, one would epxect a smeared peak (unless the reaction is slow relative to the chrom time, of course, you could get a reaction if sample is dissolved and kept in mobile ph. long enough). Did you collect any peak and reinject it? What gifs if you do?
By B.Buglio on Wednesday, April 23, 2003 - 06:17 pm:
Is your std really pure i.e. can you be seeing
one of the other isomers of 4-bromothiophenol: the
2- or 3?
By bill tindall on Wednesday, April 23, 2003 - 07:05 pm:
I have not worked with this sulfide, but the ones I have easily oxidize to disulfides, which of course would elute at much longer retention time. Blow a bunch of air through sample and see if peak 2 gets bigger.
The reaction of a phenol with TFA to make an ester is not going to be very favorable in a partially aqueous mobile phase, or sample solution.
By Anonymous on Thursday, April 24, 2003 - 03:29 am:
The standard was made up from commercially available material (Lancaster) in EtOH so i assume it is pure as the 2 and 3 isomers are much more expensive.
I am thinking that the oxidation to disulfide is the most likely reason-will try gassing std to see what happens.
Has anyone else seen this effect in HPLC of thiols?
By k.tanabe on Tuesday, June 8, 2004 - 02:28 am:
When I analyzed thiophenol by HPLC, I also found two peaks.
Thiophenol form complex with iron easily, so I guessed second peak was complex of thiophenol.