Low signal in ESI negative ion mode - WKB64035

SYMPTOMS

• Signal lower than expected for analytical methods in ESI negative ion mode
• QDa - internal calibration signal looks normal

ENVIRONMENT

• ACQUITY QDa

CAUSE

Higher than necessary formic acid concentration in mobile phase

FIX or WORKAROUND

Reduce the concentration of formic acid in the mobile phase to the lowest level workable for the method. A concentration of 0.01% formic acid (v/v) is considered appropriate. 

ADDITIONAL INFORMATION

Formic acid suppresses ionization in ESI negative ion mode. In positive ion mode, it can enhance signal.