Chromatography Forum

Hi.

I recently posted about hexaflurophosphoric acid being used as an ion pairing agent. This was a misunderstanding on my part and recently discovered that it can be used as a chaotropic agent. I’m fairly new to the methodology but I prepared about a 25mM solution of hpf6 (about 5ml of 60% hpf6 in water). Adjusting the ph to about 4 with ammonium hydroxide. I got really great peak shape and separation but then all of a sudden the peak shape started deteriorating and went to hell. After previously posting this people said that a byproduct of Hpf6 is hydrofluoric acid which deteriorated the silica support. I spoke to someone recently and they said to use the salt form sodium hexaflorophosphate.

Isn’t this just going to do the same thing? I originally added hpf6 then adjusted with ammonium hydroxide but that’s forming s salt in solution.

I’ve also heard of sodium perchlorate being used for something similar. Is this just as effective of a chaotropic agent as pf6? Better? Worse? Since it’s perchlorate will it be a little better for my column? I was thinking of tryin 25mM is sodium perchlorate ph 4.

I’m trying to separate an n-oxide of a tertiary amine and have been having a hell of a time with it. I have a vendor method that uses acetic acid ph 4 but they have to go almost 100% aqueous and the main peak tails significantly

Perchlorate acts similarly to hexafluorophosphate. Perchlorate increases the retention of cationic analytes to a somewhat lesser degree than hexafluorophosphate; however, it is more secure to use perchlorate than hexafluorophosphate in terms of stability of a SiO2-based stationary phase.

Some info about perchlorate as an ion-pairing agent:
https://www.shimadzu.com/an/hplc/suppor ... 27lab.html

You can also consider using mixed-mode columns which have ion-pairing reagent attached to the surface of silica gel. This makes all mixed-mode stationary phases MS-compatible and you can avoid all problems related to the use of ion-pairing reagents. There are now several companies who are making these stationary phases. I pioneered new mixed-mode back in 2003 and these columns found their way into a lot of pharma, chemical, food and environmental applications. Commercialized more than 40 new stationary phases in a span of 16 years.

Vlad,

Hi I’ve seen your recommendations before. I am now considering it. I have a thermos acclaims mixed mode wcx column I could try. I thought I’ve also seen PFP columns around. Those arr mixed mode as well correct?

Geoff,

PFP can be considered as mixed-mode in MeOH due to additional pi-pi interaction. All ours also have ion-exchange functionality and we are the only one (as far as I know) who makes core-shell mixed-mode columns.