LC/MS and the Cone Wash
LC/MS Considerations ____________________________________________________________________
Ammonium acetate or ammonium formate
•
•
•
These volatile salts 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.
Non-volatile salts
When using non-volatile salts, ensure that the cone wash is running as they
can crystallize in the source, block the entrance cone, and prevent the mass
spectrometer from functioning. The most common non-volatile salts used
are phosphates.
Ion pairing agents
Ensure that the cone wash is running when using ion-pairing agents (for
example, sodium octanesulfonic acid). Many ion-pairing agents suppress
electrospray ionization.
Least Suitable Additives
Least suitable additives are surface-active agents/detergents.
These can suppress the ionization of other compounds. Detergents, by their
very nature, are concentrated at the surface of a liquid. This causes problems
with electrospray, as the ionization relies on the evaporation of ions from the
surface of a droplet. The detergent therefore suppresses the evaporation of
other ions. Use surfactants only when they are being analyzed themselves,
not as additives to HPLC mobile phases.
Other Solvents
Other solvents are:
Normal phase solvents
•
•
•
Normal phase solvents such as dichloromethane, hexane, and toluene are
most suitable for use in APCI.
Propan-2-ol (IPA), 2-methoxyethanol, ethanol, and so on
These have all been used with LC/MS, but their use tends to be application-
specific.
Dimethyl sulfoxide (DMSO)
This solvent is commonly used by synthetic chemists for primary dilution.
3-4
___________________________ MSQ Hardware Manual ____________________________