for mix (2M+NH4)+ acetonitrile+water. The amount of water added affects the response of 118 m/z, with higher amounts yielding a greater response and vice versa. You can add water up to the amount of 5% (v/v).

Hi, please read the attachment, I hope it can help you.

1. You have to check the mobile phase background.

2. Capillary 

Thank you for your clues.

EN Kabachkov, I will chceck this. Actually, I used as one eluent 5mM ammonium formate in acetonitrile/water 90/10 (v/v).

Hi, Tomasz, indole, valine or betaine all have m/z 118 for [M+H]+.Check their potential contamination.

Thank You.