Competing for limited surface excess charge

Even though there are different mechanisms (IEM and CRM) for gas phase ion formation, both theories agree that initial gas ions are generated from the surface phase of ESI droplets. Both the ion-evaporation model (Equation 4.1) and the partitioning-equilibrium model (Equation 4.12) are used to predict that the ESI response in the presence of other components is based on competition for a fixed amount of ESI current (I) or excess charge (Q) which is controlled by the applied voltage and the flow rate. Since I is proportionally correlated to Q as expressed by Equation 4.4, the competition for I and for Q are equivalent. Furthermore, since the excess charges all reside on the surface of the droplets, competition for the limited charge or competition for the limited surface space are both possible. Based on Equation 4.12, it is clear that when total ion concentration in the droplet exceeds [Q], there will be a competition among the ions for the excess surface charge. Matrix induced ion suppression can be

Electrospray Ionization Images
Figure 4.3 Schematic diagram of possible mechanisms of ionization suppression for electrospray ionization.

explained in part as the competition for the limited excess surface charge. The matrix components with higher K are more surface active, therefore they would be expected to out-compete the low K analytes for the limited excess charge or limited space on the initial droplet surface. The uneven fission process will produce an even more profound ion suppression effect when surface-active matrices are present. The high surface-active matrices will out-compete the low surface-active analyte in each uneven fission process and occupy the droplet surface in each subsequent offspring droplet. In this case, analytes, would be preferentially left in the interior neutral phase of each preceding droplet and therefore became undetectable.

Surfactants are molecules with both polar and hydrophobic regions and known to prefer the air-liquid interface. Due to their high affinity to the droplet surface, surfactants are expected to have high ESI responses. Many experiments have shown that surfactants significantly suppress the ESI response of other analytes.12,14 Therefore, it is not hard to understand that surfactants, such as Tween 80, that are used as dosing excipients to improve the solubility of drug candidates, could cause significant ESI ion suppression for co-eluting analytes in LC-MS/MS assays.34 36

Polymers that are used as co-solvents to improve the solubility of hydrophobic compounds usually have both hydrophilic and hydrophobic parts in the same molecule, therefore these polymers also have high surface activity. The attainment of improved solubility using PEG400 is through the bridging effect of this polymer between the hydrophobic analyte and water. The backbone of polyethelene (—CH2CH2—)n of PEG400 will associate with the nonpolar part of hydrophobic compounds via hydrophobic interaction while the terminal hydroxyl group (—OH) will hydrogen bond with water. At the same time, this hydrophobic backbone of polyethelene also provides for sufficient surface activity of PEG400. Therefore, if PEG400 is contained in the plasma sample and cannot be separated from analytes, matrix effects will often be observed. Several reports have described severe matrix effects from plasma samples obtained from laboratory animals dosed with formulations containing PEG400.34 37

Lipophilic components such as long-chain (C12-C16) fatty acids, glycerophosphocholine lipids, phosphatidylethanolamine, sphyngomylein, and triacylglycerols in plasma and tissue sample all have high surface activity, therefore these components can be part of the cause of ion suppression effects. It was demonstrated that lyso-phosphatidylcholine (C16:0, C18:0, C18:2) present in serum contributed to the matrix effects observed in an assay for verapamil.38 In our laboratory, we have observed that hydrophobic matrix effects are more often observed in tissue samples, especially in brain samples; part of the reason for this effect might be that brain tissue contains more lipid components that are surface active than those found in plasma samples.

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