1) When preparing an ammonium acetate 5mM buffer solution with pH=3.3, which is better to use to adjust the pH? An inorganic acid (HCl) or the corresponding organic acid (CH3COOH)?
2) Is it necessary to prepare the buffer solution using ultra pure water or distilled water is tolerated? because it will be used for an HPLC C18 column injection.
Aliphatic carboxylic diacids, such as glutaric and succinc acids, will be injected.
1) since you are preparing ammonium acetate buffer it is better to use organic acid such as CH3COOH to adjust the pH
2) for hplc better to use ultra pure water
to add an answer for your second question :
It is always necessarry to use hplc grade water, as distilled water may contain particles may damage the pump and injector and also could plug the column. Organic contamination of water may also affect the chromatograms.
1) Use the corresponding organic acid ( acetic acid, as you mentioned). Using another acid would create yet another buffer.
2) use the high quality water. If there are organic impurities in the water, they may be adsorbed on the column and concentrated such that "ghost peaks" occur as the concentration of the organic solvent increases to the point that they elute. They are called "ghost peaks" because whether they appear depends on the time of equilibration at a low organic concentration, and whether a gradient is run that causes them to elute. They may also have a retention such that they appear in one run, but not reappear until several runs later. Both of these characteristics make "ghost peaks" difficult to troubleshoot.
For the preparation of buffers you have to use weak acid & its conjugate base. There are many good guides online utilizing the Henderson Hasselbalch equation.
The water you want to use is ultra-pure grade water, you can either buy this or make it yourself. If you are doing many experiments it is much more economical to buy a water filtration setup made for this purpose. For many modern water filters, deionized water can be filtered directly (not tap water!). In our lab we distill the DI water and then pass it through a filtration system. This produces the industry standard 18 megohm water.
DI water still carries a substantial amount of ions in it and will interfere with your chromatography and can cause inorganic solid buildup in your instrument.
1) since you are preparing ammonium acetate buffer it is better to use organic acid such as CH3COOH to adjust the pH
2) for hplc better to use ultra pure water
It's better to use organic acid (CH3COOH)
For your second question, you should prepare all your solutions with HPLC grade water.
HPLC grade water is optimized for HPLC, but not for ions. Means that should work for your application. But I would recommend to use fresh ultrapure water from an equipment using as UV lamp. This ensures that the water is free of ions and organics.
to prepare a buffer you definately need a weak acid + corresponding base, as patrick said. so HCl is not an option in any case. buffer capacity is best within pH = pKa +/-1. thus, buffer capacity of a 5mM acetate buffer at pH 3.3 might be limited. if the buffer capacity should be insufficient for your application, try using a higher concentration (e.g. up to 50mM) or use formic acid/formate instead.
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