求大家帮忙看看这个流动相该怎么配置啊？

The mobile phase A was 10 mM ammonium acetate solution in 90% LC-MS-grade acetonitrile/0.1 formic acid (v/v), with the pH 3.25; mobile phase B was 100% water LC-MS-grade 10 mM ammonium acetate/0.1%formic (v/v) acid.为什么流动相A还说是90%的乙腈呢，另外的10%是什么呢？