HPLC HINTS & TIPS for Chromatographers
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.
> Bill Letter, 04/17/2012
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