This document describes experimental modifications to inhibit ionization suppression when using LC-ESI-MS in negative ion mode for steroid estrogens and acidic herbicides. It introduces the problem of ionization suppression and methods to reduce its effects. The application of matrix clean-up in water samples to improve the apparent extraction efficiency of the analyte of interest when using LC-ESI-MS in negative ion mode is also discussed.
Ionization suppression is a common problem when using electrospray ionization (ESI) in negative ion mode, leading to apparent reductions in analyte recovery from different matrices. This document addresses this issue for two classes of materials: steroid estrogens and acidic herbicides. It highlights that suppression is primarily caused by competition between matrix components and analytes during ionization in the electrospray source, particularly under high-aqueous mobile-phase conditions.
The note details a specific experimental approach combining pH adjustment with the post-column addition of 2-propanol to mitigate these effects. Through a series of tests involving tap water and deionized water, the document demonstrates how these modifications can significantly improve recovery rates for steroid estrogens, reducing ionization suppression while maintaining extraction efficiency.
Ionization suppression is mostly due to the competition between matrix components and the analytes of interest to become ionized in the electrospray source.
Yes, adding ammonia in the mobile phase resulted in a four- to five-fold increase in peak abundances and improved recovery values compared to standard conditions.