I have been optimizing method for determination of predsolone (360 amu) and dexamethasone (392 amu) using the Agilent 1100 HPLC coupled with the Waters tandem Mass spectrometry with electrospray ionization in positive mode. I´m using masslynx software.

For Mass spectrometry i´m using the cone voltage (v) 10; Source temp 150°C; desolvation temp 450°C; desolvation gas flow 650 L/hr; cone gas flow 110 L/hr; and cappilary voltage 3.4 kv.

My sample was diluted in 95%:5% (HPLC water:methanol). I am using a gradient method with a mobile phase composition of HPLC water and methanol both with 0.01% of formic acid.

I´m done with predsolone where I got 361 m/z as molecular ion peak with high intensity. But for dexamethasone, I didn't got the 393 m/z as molecular ion peak, instead I got 373 m/z with high intensity. The expected molecular ion peak (393 m/z) has very small relative abundance. I have tried to change the cone voltage of 5, 15, 20 and 25 but still no any changes observed instead the intensity decrease. Also I tried to lower the Source temp to 120°C but nothing changed.

Please can you assist me on how to enhance it?