Preparation of 20 mmM Ammonium Acetate
Now there is a matter of preparing 20 mM Ammonium Acetate (20 dollars\ mM $Ammonium Acetate), and the method needs to be detailed.

First take an appropriate amount of acetic acid (Acetic Acid) and place it in a clean container. The amount of acetic acid, according to stoichiometry, when accurately measured, is the basis for preparation. Take an appropriate amount of ammonia solution and slowly drop it into the vessel containing acetic acid. When dripping, pay attention to the reaction state, see that the reaction in the solution is smooth and there is no overreaction.

The reaction formula of the two is: $CH_3COOH + NH_3\ cdot H_2O\ longrightarrow CH_3COONH_4 + H_2O $. This reaction is mild, but the operation should not be sparse. After the two are mixed, stir to make the reaction sufficient. When stirring, the rate is moderate, and the solution should not be splashed out too quickly, nor should it be too slow to make the reaction uneven.

After the reaction is completed, if there are impurities in the solution, it can be removed by filtration. Choose a suitable filter paper, place it in a funnel, pour the solution in, so that the impurities remain in the filter paper, and the clear liquid flows to the container below.

Then, concentrate or dilute the resulting ammonium acetate solution as appropriate. If you want to concentrate, you can place the solution on a water bath and heat it slowly to evaporate the water. However, the temperature should not be too high to avoid the decomposition of ammonium acetate. If it needs to be diluted, add it in distilled water or deionized water according to the required concentration.

Finally, a 20 millimolar ammonium acetate solution is obtained, and its concentration is accurate, which can be used for subsequent experiments. The whole preparation process requires rigorous operation, accurate weight injection and sequence of steps to obtain satisfactory results.