Chromatography Forum

The mobile phase specified by OSHA Method 47 is

0.01M ammonium acetate in 50:50 (v/v) ACN/water, adjusted to pH 6 with acetic acid

Does this mean 0.01 moles per liter of eluent, or 50% 0.01M ammonium acetate @ pH 6 and 50% ACN?

If it's the former, I would need to make 0.02M ammonium acetate, adjust the pH, and then add an equal amount of ACN, correct? If it is the latter, then I would make 0.01M ammonium acetate, adjust the pH, and add an equal amount of ACN.

I've never seen an eluent specified this way, and it is IMO ambiguous.

Hi,
I think that: for example, 0.01M ammonium acetate + acetic acid (pH=6) then + ACN = mix

The mobile phase specified by OSHA Method 47 is

0.01M ammonium acetate in 50:50 (v/v) ACN/water, adjusted to pH 6 with acetic acid

Does this mean 0.01 moles per liter of eluent, or 50% 0.01M ammonium acetate @ pH 6 and 50% ACN?

If it's the former, I would need to make 0.02M ammonium acetate, adjust the pH, and then add an equal amount of ACN, correct? If it is the latter, then I would make 0.01M ammonium acetate, adjust the pH, and add an equal amount of ACN.

I've never seen an eluent specified this way, and it is IMO ambiguous.

I suspect from the way it is written that it is the former, 500 mL 0.02 M buffer, 500 mL ACN mix and then adjust the pH.

I agree it is confusing, I have seen 1 example of this type in my carreer where the bulk and not the buffer component was pH adjusted. There was a difference of over 20 minutes in RT depending on the way you made the mobile phase.

However if you're still not sure call OSHA for the definitive answer.

That's why it's important to write stuff like this in an umambiguous way. I'm betting you can't contact anyone at OSHA who can tell you for sure.

following up on JGK's comment, you could try it both ways and see how much difference it makes in this case.

THe OSHA site points to how you can ask them, but the alternative conditions in the same method lists a mobile phase of " 0.01 M ammonium acetate in 37.5/62.5 (v/v) ACN/water adjusted to pH 6 with acetic acid ".

I would assume that it's 0.01M ammonium acetate in the eluant, and would first prepare the appropriate concentration in the aqueous component, and mix to achieve 0.01M in the eluant.

If you ask OSHA, you should also ask them to fix the ambiguous wording. Their analytical method style guide on the WWW doesn't offer any clarification. Also, please let us know if we are wrong

Please keep having fun,

Bruce Hamilton

I didn't ask OSHA, but here is what I did:

Made 0.02M ammonium acetate in 500mL water, adjusted that to pH 6 with glacial acetic acid (2 drops!), then mixed that with 500mL ACN. Degassed with sonication.

I mixed it with the eluent already in the reservoir, which was at pH 5.6. The analyst who made this eluent was out, so I couldn't ask exactly how it was made. All I know is that the analyte retention time shifted by 30 seconds. Luckily everything was baseline resolved, so no further problems were encountered.

After thinking about it some more, my guess was that the ammonium acetate was intended to be dissolved and pH adjusted in the 50:50 mixture. But that's not how I did it

I think the instructions are absolutely clear. Make a solution of 0.01 M ammonium acetate in 50/50 ACN/ H20. You can get to the exact same spot by making a solution of 0.02 M ammonium acetate in water, and then dilute this 1:1 with acetonitrile. THEN you adjust to the desired pH to 6 with acetic acid. It actually makes a pretty decent buffer.

The procedure that you used is definitely wrong, because you measured the pH BEFORE you added the acetonitrile.

I thought the usual correct procedure was to adjust pH before adding organics

It is a better and preferred procedure to adjust the pH before adding the organic solvent. But this is not what it says to do. It clearly and undeniably says that you adjust the previously prepared mix to pH 6. This can be done and there is nothing wrong with that.