Using high pH mobile phase, the retention of polar bases like morphine will be increased. Signal response does depend upon ionizations state, so if we use high pH, we are actually de-ionizing our bases and should loose sensitivity. But this doesn’t happen actually. 

When we increase the retention of these polar bases using high pH, they elute later in the gradient run and in a high percentage of organic (acetonitrile or methanol).  For example, when using formic acid, the morphine peak is eluting in about 5-10% acetonitrile, but when we switch to pH 10, the morphine peak is coming off in about 30-40% acetonitrile. The increase % of acetonitrile in the source is what allows the ions to be better desolvate and ionized thereby giving you a better response in ESI-MS.