If you are still having problems with hardness after distillation, etc. you either have a leak in your distillation system somewhere or your  resin in your deionizer is used up allowing break through of calcium and magnesium. Or both.

I have used city water after treatment before, but with a RI detector (very insensitive).  Switch your supply to hot water which is softened and after zeolite resins.  Then distill, deionize, reverse osmosis, and ultrafilter.  There are many good commercial systems out there!

I didn't have this experience, but I think the possible reason is Mg2+ and Ca2+. Although, theirselves don't have Uv-vis absorbance, they might affect the baseline when your sample prepared in solvent which is different from your mobile phase. After boiling the water, the peak disappeared. However, CaCO3 and MgOH2 are produced in the same process. They sit in the column and the back pressure goes up. After boiling your water, you can try to filter your boiled water with nylon membrane filter with 0.45 um pore. See it helps or not. A better way is to remove Mg2+ and Ca2+ using cation exchange resins in the H+ form. You can get it from Dow company or others. The millipore DI water system works using the same principles. 

I would ALWAYS advise to buy HPLC grade water for your buffers as even treated water can affect detection. The presence of metal ions will also affect retention characteristics of some analytes (e.g for nucleic acids). HPLC grade water is purpose tested so should never interfere with HPLC separations.

If you have no option but to use tap water: Maybe using a metal chelator treatment would help. Before treatment put a small amount of EDTA in the water to get rid of those Mg2+ ions etc. Say 0.1 mM to start and test if this makes any difference.

I agree. You should use water with better quality. If you can solve a problem in a easier way, you should do it. EDTA always works, but how to get rid of EDTA after the treatment? EDTA is not just in sodium salt form. There are other cations at ppm levels. If EDTA is kept as part of the mobile phase, I don't know it is good or not. It might be a good idea. 

Hi Aleksandra,

I agree with the things named before, very good ideas. However, since your question is if the remaining water hardness is your problem, I would just buy one or 2 bottles HPLC grade water and test this - this for sure has no ions in solution.

If you boil the water before use, you create CaCO3 precipitate - if you see this at the walls of your beaker, this is just a small fraction of the precipitate - you also produce small preipitate particles which kill the column if used as mobile phase (can be removed under acidic conditions, 0.1 vol-% HCOOH or TFA). Filtration will be best for this case. However, I wonder how you have these ions in solution after you perform bidistillation in your lab...if deionization does not work, consider to maintain the ion exchange unit.

Good luck, Marcus

i think the best solution is to buy HPLC  WATER for your analysis or U should use distilled water 

If you are still having problems with hardness after distillation, etc. you either have a leak in your distillation system somewhere or your  resin in your deionizer is used up allowing break through of calcium and magnesium. Or both.