Chromatography Forum

Hello,

I’ve been trying to find alternatives to sodium octanesulfonate as an ion pairing agent. I’ve recently tried hfba however this chemical I can only find in 50 ml bottles and is extremely stinky. I’ve also just recently tried hexafluorphosphoric acid HPF6. To make my mobile phase however and have I ended up preparing the following

40ml hpf6 into 2 l of water adjust the ph to 3.

Mobile phase a: 500ml buffer,450ml water, 50ml acetonitrile

Mobile phase b: 500ml buffer 500ml acetonitrile

My question is, is there anything wrong with HPF6 at such a high concentration especially with damaging the column and hplc/uplc? I’m adjusting the ph to within an acceptable range but I’m still worried that this may be corrosive. Does anyone have any experience with this reagent as an ionpaorong agent?

Hi Geof235,

I have no experience with using hexafluorophosphoric acid, but rather sodium hexafluorophosphate and sodium perchlorate, as chaotropic agents. These salts do not seem to cause any undue damage to reversed-phase materials.

Hi Geof235,

Many years ago I tried using sodium hexafluorophosphate in an acidic eluent (I don't recall how acidic, but I think around pH 3) at elevated temperature (maybe 60 C or so). What I remember clearly is that the silica-based packing material dissolved quickly (10% of the column in a few hours). At the time we assumed this was a result of hexafluorophosphate hydrolysis to form HF, which of course is very corrosive to silica. A quick literature search shows that there is ongoing debate about the hydrolysis, but I am not aware of data that indicate how fast it is under chromatographic conditions.

https://pubs.acs.org/doi/pdf/10.1021/jp903292n

Take care.