MSA with LC MS

[kingm3]

I am transfering a HPLC method which has ACN and 0.1 % methanesulfonic acid in gradient mode to an LC MS instrument. I changed the acid to Formic acid and also HFBA at the same percentage. The chromatography using the FA and HFBA is much different than with MSA. I can only see 1 large peak. Is it ok to use MSA with LC MS at this concentration? I am looking at positive mode.

[Kostas Petritis]

Normally you shouldn't use methanesulfonic acid with LC-MS as it is not volatile. I would guess that the most similar to methanesulfonic acid is TFA and not the acids that you used.

Did you confirm that you see only one large peak with formic acid and HFBA by using the same detector you used with the methanesulfonic acid or did you do it with LC-MS?

[kingm3]

I ran the MSA, FA and HBFA on a LC system with UV detector and the FA HBFA on an LC MS system. I am a little reluctant to use TFA on the LC MS system as the last time I did it took some days to wash out all traces. If I were to use TFA what concentration would you reccommend?

[Victor]

I can't really answer your question about the concentration of TFA with confidence but most workers seem to be using 0.05-0.1% TFA. I think the less the better as far as the MS is concerned, and some even go below this range to avoid signal suppression. However, low concentrations may compromise your chromatography.

I'm very interested in your comment about the persistence of TFA and your reluctance to use it. Does anyone share this experience, or know of any literature references which describe the problem?

[kingm3]

I am running my sample at the moment with 0.025 % TFA and it seems to be giving me better results than with the FA and HFBA. The last time I used TFA at 0.1 % it left a high background which remained for quite a few days. I am using an ion trap detector. I also ended up replacing my flow cell in the DAD detector. It gave rise to a lot of green/purple rust.

I don't know of any journal articles that refer to this problem but I would think the less TFA used the better.

[WK]

I've noticed since using 0.1%TFA over the last 3 years I now have "green corrosion" on the outlet of my vacuum degasser. That is the only metal surface I have seen with it - it could be inside some stainless tubing.
Does TFA attack stainless tubing?
WK

[kingm3]

0.025 % TFA slightly improved my chromatography but not significantly to match the chromatography achieved with the MSA.

Does anyone have any other suggestions.....FA, HBFA and TFA are not producing good chromatography. A higher concentration of TFA maybe the answer but I am very reluctant to do this.

Any help at all would be greatly appreciated

Marion

[Victor]

Marion- thanks for your comments-I'd be interested in what kind of samples you are analysing e.g. bases , acids or what and what form you think they are in at the pH values you are using e.g. I guess about pH 2.5 with 0.025% TFA. What is bothering you about the chromatography? Peak shapes? Resolution of sample components from one another?

TFA has a lower pH than formic acid if comparing solutions of the same weight %. This may explain some of the corrosion problems that are claimed here-but I don't know if it's the only reason.

[kingm3]

Hi Victor - the main compoud I am looking at is R-Modafinil (C2H5)2CH-SO-CH2-CO-NH2. I'm trying to separate the impurities associated with this which are an ester, acid, SO2 acid and amide. When this is chromatographed with 0.1% MSA each peak is visible along with some more from the sample matrix, however when I try to mimic the chromatography on the LC MS with FA, HBFA or TFA all I see is a large peak (from the Modafinil) and 1 smaller from the ester. I have ran standards to confirm this. I can't see the other peaks in either the UV trace or the TIC in order to identify the molecular weights. I tried the FA and HBFA on an LC system, which resulted in the same chromatography as on the LC MS thus eliminating any problems with the system.

I am unsure of what to try next.

Thanks,

Marion

[Kostas Petritis]

Marion,

Are you saying that when you inject one by one your standards with FA, TFA and HFBA there are some of them that you do not see at all either with the UV or the MS? What is the wavelength you use for the UV and have you tried to infuse the compounds that you can not see in the MS?

If you are using very low wavelengths for the analysis maybe these peaks are masked from your background due to the absorbance of FA, TFA and HFBA and maybe in the MS they do not ionise? It is strange that you do not see them at all when you inject them one by one...(I can understand co-elution but not complete dissapearence)....

[kingm3]

Hi Kostas,

I have a sample of Methanol which contains a residue af the above mentioned compunds and other unidentified compounds. On a normal LC system I can see about 15 peaks (with MSA). 3 of which I can identify with standards. When I change to the LC MS system in order to identify the others all I can see is 3 peaks, the ones previously identified. The lagest peak of the active seems to increase here.

I have heard of others having problems replicating the ion pair quality of MSA with a volatile acid suitable for LC MS. I have tried FA, HBFA and TFA. Are there any others that can be used?

Marion