mixing 100% water with 100% ACN

[firework1]

I am developing an HPLC method for an eazy compound. I put 100% water in solvent line A, and 100% ACN in solvent B and was about to play with the weight of A and B for a good rentention time.

I did this many many times before, it was all right and nobody told me I was doing wrong...until today, one of our felas said I should not let the pump mix pure water and pure ACN, instead, I should put 100% water in A and 90/10 ACN in B otherwize I would get lousy baseline or air bubbles in the system.

Is there a rationalization in what she said? How come I did not know something so basic in all my three years LC experience?

[skunked_once]

Some HPLC systems have problems pumping 100% acetonitrile. See the post below on "Acetonitrile and HPLC gradients".

[Mark Tracy]

If you have a low-pressure mixing gradient pump with a functional degasser, there will be no trouble. A high-pressure mixing pump may (or may not) have trouble pumping 100% MeCN, and some models don't include a degasser. If you want to transfer this method to another lab or another instrument, you might want to avoid 100% MeCN just to be safe. Of course if you want the ultimate in retention time reproducibility and transferability, you weigh the solvents.

[Consumer Products Guy]

We have pumped 100% acetonitrile from one of our reservoirs on all our HPLC systems (8) for over 25 years, no issues. We use Agilent 1050s and 1100s, seven with quat pumps and one with binary.

[Mark Tracy]

And if you transfer the method to a lab using Brand X HPLC equipment?

[firework1]

Thanks, guys

[danko]

Some premixing (e.g. 90 % water in A and 90 % ACN in B) is always a good idea. I’m not going to list all the good reasons for this, because they are too many.

How come I did not know something so basic in all my three years LC experience?

You are funny Some of us have worked with chromatography for 20 – 30 years and we are still learning.

Best Regards

[annaand]

Hey, I didn't know that neither!
Usually I put 100 water (with buffer or TFA) to one pomp and ACN or MeOH (with TFA) to the other. But in my case I did this only to choose the mobile phase composition (ACN:H2O, MeOH:H2O) and then I prepared the mobile phase in separate bottle. I'm doing frontal analysis and I have only 2 working pomps in my instrument. That's why I was forced to mix mobile phase myself.
My friend has similar problem like you firework1.
I have to say that this forum is great! So many things I can learn!
Anna