The third proposed theory for ion suppression in ESI relates to the presence of non-volatile species which can either cause co-precipitation of analyte in the droplet (thus preventing ionisation) or prevent the contraction of droplet size to the critical radius required for the ion evaporation and/or charge residue mechanisms to form gas phase ions efficiently.
These approaches may require in-depth understanding of the ionisation mechanisms involved in different ionisation sources or may be completely independent of the physical factors involved.
If the chromatographic separation can be modified to prevent coelution of suppressing species then other approaches need not be considered.
Consequently, LC-MS/MS has become the analytical tool of choice for bioanalysis owing to its impressive sensitivity and selectivity over other, more conventional chromatographic approaches.
However, during and after uptake by bioanalytical laboratories world wide, it became apparent that there were inherent problems with detection of relatively small analyte concentrations in the complex sample matrices associated with biological fluids (e.g. Put simply, ion suppression describes the adverse effect on detector response due to reduced ionisation efficiency for analyte(s) of interest, resulting from the presence of species in the sample matrix which compete for ionisation, or inhibit efficient ionisation in other ways.
Thus competition for space and/or charge may be considered as a source of ion suppression in ESI.
Both physical and chemical properties of analytes (e.g.
Once the sample has been injected, a drop in signal intensity (or a negative response) should be observed any time a species is ionised in the ion source.
This should allow the retention time of any such species under the analytical parameters of the assay to be determined.
APCI is less prone to pronounced ion suppression than ESI, an inherent property of the respective ionisation mechanisms.
In APCI, the sole source of ion suppression can be attributed to the loss of efficiency during charge transfer from the corona discharge needle to the analyte of interest in the presence of other sample components.
A higher analyte/matrix ratio can give a reduced effect of ion suppression.