Chromatography Forum

Hi everyone

How do you choose a suitable needle wash solvent for your system? I'm using the following conditions.
HPLC = Waters Alliance 2695
Mobile phase = buffer/acn 95:5
Needle wash = water/acn 95:5.

When I inject a blank soln there seems to be some carry over in the blank solution even after a few purges are done before the blank is injected.

Should I use a stronger needle wash solvent?

Many Thanks
Mike

Probably a good idea.

We generally match our needle wash solvent to the compounds in the sample with which we are concerned about carryover. For example, for an analysis of an active that is most soluble in methanol, we use 100% methanol as the wash solvent even though our mobile phase is ACN/buffer. In theory, the needle wash never sees the mobile phase.

JZeszotarski has the right idea, I think. We usually use 100% MeOH since most of the compounds we use in the lab are MeOH soluble. The needle wash doesn't have any contact with the Mobile Phase, or at least it shouldn't. You may find that using the strongest solvent appropriate for your sample(s) and system works the best.

My experience is with Shimadzu. I keep 2 bottles of needle wash - 35-40% organic with the balance water. I switch the wash to match the main organic component in the mobile phase.

Our compounds are water soluble, and I have found that organic content over 50% can precipitate anything left over. Additionally, I have had carryover from high organic on Shimadzu.

The reason I chose about 35% is that it is strong enough to wash off compounds without adversely affecting the separation.

What if we use buffer as the solvent for our sample? Won’t it be precipitated in the washing solvent that contains 100% of solvent organic?
We always use aqueous mixture of methanol, acetonitrile or isopropanol for washing solvent (50 – 10%).

Hi Everyone

Thanks for all the suggestions. My actives are soluble in water therefore I thought a needle wash of water/acn 95:5 should be sufficient. The first few injections of the blank are clean with the later ones showing signs of carryover. I decided to change my needle to MEOH/water 50:50. This seems to produce much less carryover (less than 0.05%). We will be doing further tests to try to improve this method.

Thanks again for all the suggestions.

Mike

We always use 50/50 water/methanol as the washing solution. It works for most of our applications. Good luck

someone at waters suggested a mixture of 1:1:1:1 water/acetonitrile/isopropanol/water containing 1% formic acid.

It worked well in our applications. I was using their Acquity system which uses a strong and a weak solvent wash. The strong used the mixture above, the weak was very similar to the composition of my starting gradient.