Reduction of Ionization Suppression in Water Samples when Using LC-ESI-MS in Negative Ion Mode

Experimental modifications that inhibited ionization suppression when using LC-ESI-MS (liquid chromatographyelectrospray ionization-mass spectrometry) in negative ion mode are described for two classes of materials: steroid estrogens and acidic herbicides. Introduction Ionization suppression is a problem when using electrospray ionization (ESI) in negative ion mode. It leads to an apparent reduction in analyte recovery from different matrices when, in fact, extraction efficiencies are very similar. Ionization suppression effects are mostly due to the competition between matrix components and the analytes of interest to become ionized in the electrospray source, and their release from the droplets formed. This suppression is compounddependent and appears to occur mainly in highaqueous mobile-phase conditions. There are several approaches to reduce the effects of ionization suppression and hence improve the apparent extraction efficiency of the analyte of interest. Reduction of Ionization Suppression in Water Samples when Using LC-ESI-MS in Negative Ion Mode Application • Matrix clean-up: removal of unwanted material from the sample extract. It may be possible to tailor a chemical clean-up procedure for a small suite of compounds, but this may not be possible when dealing with a larger suite of analytes due to their different physical and chemical properties. For instance, some of the more polar compounds may not be retained on a solid phase cartridge or eluted from the cartridge during the washing step. • The use of extracted standards from the matrix of interest. This approach would work, providing only one matrix was being analyzed. If different matrices were being analyzed in small numbers, this would lead to low-throughput. • Use of internal standards (ISTDs). This is a very useful approach when dealing with individual analytes (assuming a deuterated equivalent is available). However, when dealing with large suites of compounds, this could involve many ISTDs, adding complexity and cost to the method. • Explore other ionization modes (if available) during method development; for example, atmospheric pressure chemical ionization (APCI) and/or atmospheric pressure photo ionization (APPI). Both of these sources are less prone to matrix suppression effects, but they may not be suitable for all analytes in an extended suite of compounds. Some analytes may be thermally sensitive, while others may not ionize. The use of positive/negative switching was shown to be useful when dealing with ionization suppression [1]. For example, imazapyr (an herbicide) showed varying recoveries Environmental