User guide

Multimode Source for 6100 Series Single Quad LC/MS User Guide 53
Reference 3
Mobile phase considerations when using the multimode source
• In negative APCI mode, the use of acetonitrile has also been linked with a
reduction in response of the analytes.
For electrolytes added to the mobile phase used for mixed mode operation, use
care in selecting the anion as it may affect the response in negative APCI
mode. For example, acetate is preferred over formate as it is more likely to
accept a proton from the neutral analyte to produce a negatively-charged
analyte ion under APCI conditions.
Positive/negative mode switching
• In general, you need an intermediate pH (compromise) so protonated and
deprotonated analyte ions may co-exist in significant concentrations.
• Sensitivity and/or chromatographic peak shape are generally not as good as
when the analysis conditions are optimized using a single polarity.
• 0.2% acetic acid or 1 mM ammonium acetate (not ammonium formate) are
suggested as mobile phase modifiers.