Tip # 117: Techniques To Enhance
Negative Mode ESI in LC/MS
Many sample types
require MS analysis using negative ion electrospray mass spectrometry (ES MS) mode.
Sensitivity can be less in this mode, compared to positive mode, as they employ mostly
non-polar solvents which do not promote ion formation. In the past we discussed how adduct
formation can be employed to enhance ion formation. There are other techniques which can
be used as well. Here are a few tips which can be used to improve the quality of the
signal obtained under negative mode ES MS conditions.
(1) Negative ion formation and signal response can be improved by choosing the right
solution chemistry. One method to improve solvent desolvation and reduce the electrical
discharge (noise) is to add isopropyl alcohol (2-Propanol) to the mobile phase. Reported
in the literature nearly twenty years ago, as little as a 10% solution has been found to
increase the signal level and decrease the noise level under many conditions. The addition
of methanol to the mobile phase also can improve the signal, though usually not to the
same degree as IPA.
(2) Another technique used to improve ionization involves the pH of the solution or mobile
phase used. Higher pH values are often better. Post column addition of a basic solution
which adds proton acceptors to the mix, such as ammonium hydroxide (20 or 30 mM), can also
improve signal response and stability.
(3) APCI mode: If running non-polar to semi-polar small molecular weight compounds,
especially those which are volatile, this is still the source of choice. *Conventional ESI
mode is best for polar to neutral molecules in small to large Mw ranges.
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Bill Letter, 04/17/2012
