A
Optimizing the LC Conditions
LC Solvents and Mobile Phase Additives
Thermo Scientific
Surveyor MSQ Plus Hardware Manual
121
Commonly Used Compatible Additives
The additives that are the most compatible with the Surveyor MS detector are:
• Acetic acid or formic acid
• Ammonium hydroxide
• Ammonium acetate or ammonium formate
• Non-volatile salts
• Ion pairing agents
LC separations can be enhanced by reducing the pH of the mobile phase. Suitable additives
for this are acetic acid or formic acid. (Formic acid is stronger than acetic acid and therefore
less needs to be added to reach a required pH.) Addition of acids can suppress ionization in
negative ion analysis and weakly acidic compounds might not form [M-H]
-
ions in acidic
conditions.
Ammonium hydroxide (ammonia solution) is suitable for increasing the pH of the mobile
phase, which can enhance LC separations. When analyzing weakly acidic compounds, in
negative ion mode, it is unlikely that there will be any suppression of ionization.
Volatile salts such as ammonium acetate or ammonium formate are often used to buffer
mobile phases. Use as little ammonium acetate or ammonium formate as possible, keeping the
concentration below 100 mM. Ensure that the cone wash is running when using high
concentrations.
Table 9.
Summary of suitable additives
Ion Polarity Mode
Additive
Positive ion
• Acetic acid
• Formic acid
• Ammonium acetate (<0.1M)
Negative ion
• Triethylamine (TEA)
• Ammonium hydroxide (ammonia solution)
• Ammonium acetate (<0.1M)
Table 10.
Summary of additives to avoid
Ion Polarity Mode
Additive
Positive ion
• Surfactants
• Trifluoroacetic acid (TFA) (>0.1% v/v)
Negative ion
• Surfactants
• Organic acids such as acetic acid, formic acid, trifluoroacetic
acid (TFA).
